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Regulation of the order–disorder phase transition in a Cs 2 NaFeCl 6 double perovskite towards reversible thermochromic application

Product: RESCU
Date: 2021
Authors: Li W,Rahman NU,Xian Y,Yin H,Bao Y,Long Y,Yuan S,Zhang Y,Yuan Y,Fan J

Stabilities and novel electronic structures of three carbon nitride bilayers

Product: RESCU
Date: 2019
Authors: Lin W,Liang SD,He C,Xie W,He H,Mai Q,Li J,Yao DX
Journal: Scientific Reports

Three new novel phases of carbon nitride (CN) bilayer, which are named as alpha-C$_2$N$_2$, beta-C$_2$N$_2$ and gamma-C$_4$N$_4$, respectively, have been predicted in this paper. All of them are consisted of two CN sheets connected by C-C covalent bonds. The phonon dispersions reveal that all these phases are dynamically stable, since no imaginary frequency is found for them. Transition path way between alpha-C$_2$N$_2$ and beta-C$_2$N$_2$ is investigated, which involves bond-breaking and bond-reforming between C and N. This conversion is difficult, since the activation energy barrier is found to be 1.90 eV per unit cell, high enough to prevent the transformation at room temperature. Electronic structures calculations show that they are all semiconductors with indirect band gap of 3.76 / 5.22 eV, 4.23 / 5.75 eV and 2.06 / 3.53 eV by PBE / HSE calculation, respectively. The beta-C$_2$N$_2$ has the widest band gap among the three phases. From our results, the three new two-dimensional materials have potential applications in the electronics, semiconductors, optics and spintronics.

Moiré Valleytronics: Realizing Dense Arrays of Topological Helical Channels

Product: RESCU
Date: 2018
Authors: Hu C,Michaud-Rioux V,Yao W,Guo H
Journal: Physical Review Letters

Electronic structure of aqueous two-dimensional photocatalyst

Product: RESCU
Date: 2021
Authors: Kang D,Kong X,Michaud-Rioux V,Chen YC,Mi Z,Guo H
Journal: npj Computational Materials

The electronic structure, in particular the band edge position, of photocatalyst in presence of water is critical for photocatalytic water splitting. We propose a direct and systematic density functional theory (DFT) scheme to quantitatively predict band edge shifts and their microscopic origins for aqueous 2D photocatalyst, where thousands of atoms or more are able to be involved. This scheme is indispensable to correctly calculate the electronic structure of 2D photocatalyst in the presence of water, which is demonstrated in aqueous MoS2, GaS, InSe, GaSe and InS. It is found that the band edge of 2D photocatalysts are not rigidly shifted due to water as reported in previous studies of aqueous systems. Specifically, the CBM shift is quantitatively explained by geometric deformation, water dipole and charge redistribution effect while the fourth effect, i.e., interfacial chemical contact, is revealed in the VBM shift. Moreover, the revealed upshift of CBM in aqueous MoS2 should thermodynamically help carriers to participate in hydrogen evolution reaction (HER), which underpin the reported experimental findings that MoS2 is an efficient HER photocatalyst. Our work paves the way to design 2D materials in general as low-cost and high-efficiency photocatalysts.

Raman enhancement on ultra-clean graphene quantum dots produced by quasi-equilibrium plasma-enhanced chemical vapor deposition

Product: RESCU
Date: 2018
Authors: Liu D,Chen X,Hu Y,Sun T,Song Z,Zheng Y,Cao Y,Cai Z,Cao M,Peng L,Huang Y,Du L,Yang W,Chen G,Wei D,Wee AT,Wei D
Journal: Nature Communications

textcopyright 2018 The Author(s). Graphene is regarded as a potential surface-enhanced Raman spectroscopy (SERS) substrate. However, the application of graphene quantum dots (GQDs) has had limited success due to material quality. Here, we develop a quasi-equilibrium plasma-enhanced chemical vapor deposition method to produce high-quality ultra-clean GQDs with sizes down to 2 nm directly on SiO 2 /Si, which are used as SERS substrates. The enhancement factor, which depends on the GQD size, is higher than conventional graphene sheets with sensitivity down to 1 × 10 -9 mol L -1 rhodamine. This is attributed to the high-quality GQDs with atomically clean surfaces and large number of edges, as well as the enhanced charge transfer between molecules and GQDs with appropriate diameters due to the existence of Van Hove singularities in the electronic density of states. This work demonstrates a sensitive SERS substrate, and is valuable for applications of GQDs in graphene-based photonics and optoelectronics.

Electronic Structure and Band Gap Engineering of Two-Dimensional Octagon-Nitrogene

Product: RESCU
Date: 2018
Authors: Lin W,Li J,Wang W,Liang SD,Yao DX
Journal: Scientific Reports

A new phase of nitrogen with octagon structure has been predicted in our previous study, which we referred to as octagon-nitrogene (ON). In this work, we make further investigations of its stability and electronic structures. The phonon dispersion has no imaginary phonon modes, which indicates that ON is dynamically stable. Using ab initio molecular dynamic simulations, this structure is found to be stable up to room temperature and possibly higher, and ripples that are similar to that of graphene are formed on the ON sheet. Based on the density functional theory calculation, we find that single layer ON is a two-dimension wide gap semiconductor with an indirect band gap of 4.7 eV. This gap can be decreased by stacking due to the interlayer interactions. Biaxial tensile strain and perpendicular electric field can greatly influence the band structure of ON, in which the gap decreases and eventually closes as the biaxial tensile strain or the perpendicular electric field increases. In other words, both biaxial tensile strain and a perpendicular electric field can drive the insulator-to-metal transition, and thus can be used to engineer the band gap of ON. From our results, we see that ON has potential applications in many fields, including electronics, semiconductors, optics and spintronics.

Theoretical Design of Topological Heteronanotubes

Product: RESCU
Date: 2019
Authors: Hu C,Michaud-Rioux V,Yao W,Guo H
Journal: Nano Letters

Calculated carrier mobility of h-BN/$gamma$-InSe/h-BN van der Waals heterostructures

Product: RESCU
Date: 2017
Authors: Kang P,Michaud-Rioux V,Kong XH,Yu GH,Guo H
Journal: 2D Materials

textcopyright 2017 IOP Publishing Ltd. Recent experiments reported excellent transport properties of two-dimensional (2D) van der Waals (vdW) heterostructures made of atomically thin InSe layers encapsulated by two hBN capping layers (ISBN). The carrier mobility of the ISBN films exceeded $mu$∼1.2×10 4 cm 2 V -1 s -1 at low temperature, much higher than that of pristine InSe films. It has been puzzling why the relatively inert hBN capping layer could so drastically enhance mobility of the ISBN composite. Using a state-of-the-art first principles method, we have calculated phonon limited carrier mobility of 18 different ISBN films and 6 pristine InSe films with different thicknesses, the largest system containing 2212 atoms. The hBN capping layer significantly alters the elastic stiffness coefficient as compared with pure InSe - thus the acoustic phonons in the ISBN composite - giving rise to the observed large mobility of ISBN films. Of the 18 calculated ISBN films, the ones with no strain at the hBN/InSe interface possess the highest electron mobility, reaching 4340 cm 2 V -1 s -1 at room temperature, which could easily go over 10 4 cm 2 V -1 s -1 at low temperatures. We conclude that the mechanical properties of the composite 2D vdW ISBN material play the crucial role for inducing the large carrier mobility, a principle that could be applied to many other 2D vdW heterostructures.

Twistronics in tensile strained bilayer black phosphorus

Product: RESCU
Date: 2020
Authors: Kang P,Zhang W,Michaud-Rioux V,Wang X,Yun J,Guo H
Journal: Nanoscale

In this work, by performing state-of-the-art first-principles methods combined with molecular dynamic (MD) simulation, we theoretically investigate the electronic and mechanical behaviours of small-angle twisted bilayer black phosphorus (tbBP) under uniaxial tensile deformation. Twistronics, namely the regulation of electronic properties by Moiré physics, is demonstrated as the gene-the most crucial factor dominating not only electronic behaviour but also mechanical behaviour of tensile deformed tbBP. Compared to untwisted few-layer black phosphorus (utBP) with strong electronic sensitivity to geometric deformation, the existence of Moiré patterns in tbBP leads to spatial electronic localization, giving rise to the conservation of direct band gaps and stability of phonon limited carrier mobility under tensile deformation along the armchair direction. Moreover, during the fracture failure process, the nucleation of micro-cracks is preferentially detected at the transitional pattern boundary areas in tbBP, which could be attributed to the intra-layer maldistribution of mechanical strengths in Moiré superlattices. The explorations of twistronics in tensile strained bilayer black phosphorus contribute to the better understanding of such Moiré superlattice structures and provide insights for the design of new 2D van der Waals heterostructures in flexible nano-electronic devices.

Valley filtering effect of phonons in graphene with a grain boundary

Product: RESCU
Date: 2019
Authors: Chen X,Xu Y,Wang J,Guo H
Journal: Physical Review B

Controllable Cs x FA 1- x PbI 3 Single-Crystal Morphology via Rationally Regulating the Diffusion and Collision of Micelles toward High-Performance Photon Detectors

Product: RESCU
Date: 2019
Authors: Wang H,Wu H,Xian Y,Niu G,Yuan W,Li H,Yin H,Liu P,Long Y,Li W,Fan J
Journal: ACS Applied Materials and Interfaces

CsxFA1–xPbI3 single crystals are expected to provide more excitement in optoelectronic applications, including photodetector, laser, light-emitting diode, etc. Herein, we aim to gain an in-depth un...

A polar Bi2CaB2O7 photocatalyst: Synthesis, properties and photocatalytic mechanism

Product: RESCU
Date: 2021
Authors: Zhao W,Liu JJ,Wang XX,Huang Y,Liu JJ,Yu J,Hao B,Wang XX,Wang XX,Zhang M
Journal: Chemical Physics Letters

Bi2CaB2O7 (BCBO) photocatalyst, which belongs to the typical polar borate, had been successfully prepared. It has direct transition optical band gaps of 3.06 eV. XPS results showed that there are oxygen vacancies on BCBO surface, which can capture photoinduced electrons and further prevent the recombination of photogenerated charge carriers. Photodecomposition experiments demonstrated that BCBO exhibits a good activity to photodegrade Rhodamine B under UV light irradiation. Excellent photocatalytic performance was mainly attributed to the promotion effect on the separation and migration of photogenerated carriers, which was caused by the internally polarization electric field perpendicular to the exposed surface.

Dirac electrons in Moiré superlattice: From two to three dimensions

Product: RESCU
Date: 2017
Authors: Hu C,Michaud-Rioux V,Kong X,Guo H
Journal: Physical Review Materials

Band engineering of large scale graphene/hexagonal boron nitride in-plane heterostructure: Role of the connecting angle

Product: RESCU
Date: 2021
Authors: Li Y,Feng Z,Ma Y,Tang Y,Ruan L,Wang Y,Dai X
Journal: Physica E: Low-Dimensional Systems and Nanostructures

The modulation of the interface topography in two-dimensional (2D) lateral heterostructure (LHS) is significant for adjusting their electronic properties. However, previous theoretical studies were usually based on dozens of atoms and ignore the irregular interface configurations. In this work, we constructed a series of graphene/hexagonal boron nitride (G/h-BN) LHS of various misorientation angles, with thousands of atoms and uniform atomic proportion. The influence of connecting angles on the electronic properties has been investigated in detail by using real-space density functional theory. Results show that the bandgap can be modified efficiently by changing the connecting angle between graphene and h-BN in LHS, on account of the mixture of diverse interfacial atomic configurations. The wave function characteristics of different interfaces can be interpreted by the strong quantum confinement and gauge field for the edge states. These findings provide a theoretical basis for elucidating the relationship between the atomic construction and electronic properties of planar G/h-BN heterostructures, which could pave a way for further controllable and tunable 2D electronic devices.

RESCU: A real space electronic structure method

Product: RESCU
Date: 2016
Authors: Michaud-Rioux V,Zhang L,Guo H
Journal: Journal of Computational Physics

In this work we present RESCU, a powerful MATLAB-based Kohn-Sham density functional theory (KS-DFT) solver. We demonstrate that RESCU can compute the electronic structure properties of systems comprising many thousands of atoms using modest computer resources, e.g. 16 to 256 cores. Its computational efficiency is achieved from exploiting four routes. First, we use numerical atomic orbital (NAO) techniques to efficiently generate a good quality initial subspace which is crucially required by Chebyshev filtering methods. Second, we exploit the fact that only a subspace spanning the occupied Kohn-Sham states is required, and solving accurately the KS equation using eigensolvers can generally be avoided. Third, by judiciously analyzing and optimizing various parts of the procedure in RESCU, we delay the O(N3) scaling to large N, and our tests show that RESCU scales consistently as O(N2.3) from a few hundred atoms to more than 5000 atoms when using a real space grid discretization. The scaling is better or comparable in a NAO basis up to the 14,000 atoms level. Fourth, we exploit various numerical algorithms and, in particular, we introduce a partial Rayleigh-Ritz algorithm to achieve efficiency gains for systems comprising more than 10,000 electrons. We demonstrate the power of RESCU in solving KS-DFT problems using many examples running on 16, 64 and/or 256 cores: a 5832 Si atoms supercell; a 8788 Al atoms supercell; a 5324 Cu atoms supercell and a small DNA molecule submerged in 1713 water molecules for a total 5399 atoms. The KS-DFT is entirely converged in a few hours in all cases. Our results suggest that the RESCU method has reached a milestone of solving thousands of atoms by KS-DFT on a modest computer cluster.

A Method of Real Space Spin Density Functional Theory

Product: RESCU
Date: 2016
Authors: Zhou C

In Situ Regulating the Order–Disorder Phase Transition in Cs 2 AgBiBr 6 Single Crystal toward the Application in an X-Ray Detector

Product: RESCU
Date: 2019
Authors: Yuan W,Niu G,Xian Y,Wu H,Wang H,Yin H,Liu P,Li W,Fan J
Journal: Advanced Functional Materials

Novel topological valleytronics in moiré heterostructures

Product: RESCU
Date: 2020
Authors: Hu C

Size dependence in two-dimensional lateral heterostructures of transition metal dichalcogenides

Product: RESCU
Date: 2019
Authors: Jin H,Michaud-Rioux V,Gong ZR,Wan L,Wei Y,Guo H
Journal: Journal of Materials Chemistry C

Lateral heterostructures (LHSs) of semiconductors can give rise to novel electronic and optoelectronic properties, which may open up unforeseen opportunities in materials science and device physics.Lateral heterostructures (LHSs) of semiconductors can give rise to novel electronic and optoelectronic properties, which may open up unforeseen opportunities in materials science and device physics. However, due to the high computational cost, previous theoretical studies are usually limited to small size LHSs, which fail to demonstrate the intrinsic features of the large size LHSs. Here, by using state-of-the-art real-space density functional theory, we study the LHSs of two-dimensional (2D) monolayer semiconductors consisting of transition metal dichalcogenides (TMDs) with a length up to 4234 Å, which for the first time gives the same order of magnitude as compared with the experiments. The numerical calculation shows that the electronic properties of the LHSs are highly dependent on their size. In particular, for the zigzag boundary we find that the band gap decreases monotonously from 1.70 eV to 0 eV with increasing LHS size. Such behavior can be interpreted by the properties of the size dependent edge states resulting from the deformation gauge field and the corresponding effective pseudo-spin–orbit coupling. Consequently, one may precisely control and design the electronic and optoelectronic properties of 2D TMD LHSs by tuning their size. Our investigation could provide an interesting strategy for designing novel electronic and optoelectronic devices.

Band engineering of GaSbN alloy for solar fuel applications

Product: RESCU
Date: 2017
Authors: Shi Q,Chen YC,Chowdhury FA,Mi Z,Michaud-Rioux V,Guo H
Journal: Physical Review Materials

RESCU: extending the realm of Kohn-Sham density functional theory

Product: RESCU
Date: 2017
Authors: Michaud-Rioux V,Guo H

First-Principles Quantum Transport and Linear Response Modeling of Nano-devices and Materials

Product: RESCU
Date: 2017
Authors: Bohloul S

NOPAIN: A Method for Efficient Evaluation of Quantum Nonlocal Operators with Applications to Solids

Product: RESCU
Date: 2019
Authors: Chen YC

Moiré impurities in twisted bilayer black phosphorus: Effects on the carrier mobility

Product: RESCU
Date: 2017
Authors: Kang P,Zhang WT,Michaud-Rioux V,Kong XH,Hu C,Yu GH,Guo H
Journal: Physical Review B

Author(s): Peng Kang, Wan-Ting Zhang, Vincent Michaud-Rioux, Xiang-Hua Kong, Chen Hu, Guang-Hua Yu, and Hong GuoMoiré patterns on two-dimensional van der Waals heterostructure can give rise to unique electronic and transport properties. In this work we report a theoretical investigation of Moiré patterns on twisted bilayer black phosphorus (tbBP). It is found that the Moiré pattern has extraordinary effects a...[Phys. Rev. B 96, 195406] Published Mon Nov 06, 2017

Efficient evaluation of nonlocal operators in density functional theory

Product: RESCU
Date: 2018
Authors: Chen YC,Chen JZ,Michaud-Rioux V,Shi Q,Guo H
Journal: Physical Review B

Electron transport through Al-ZnO-Al: An ab initio calculation

Product: NanoDCAL
Date: 2010
Authors: Yang Z,Wan L,Yu Y,Wei Y,Wang J
Journal: Journal of Applied Physics

The electron transport properties of ZnO nanowires coupled by two aluminum electrodes were studied by ab initio method based on nonequilibrium Green's function approach and density functional theory. A clearly rectifying current-voltage characteristics was observed. It was found that the contact interfaces between Al-O and Al-Zn play important roles in the charge transport at low bias voltage and give very asymmetric I-V characteristics. When the bias voltage increases, the negative differential resistance occurs at negative bias voltage. The charge accumulation was calculated and its behavior was found to be well correlated with the I-V characteristics. We have also calculated the electrochemical capacitance which exhibits three plateaus at different bias voltages which may have potential device application. textcopyright 2010 American Institute of Physics.

Spin polarized I-V characteristics and shot noise of Pt atomic wires

Product: NanoDCAL
Date: 2011
Authors: Wang B,Wang J
Journal: Physical Review B - Condensed Matter and Materials Physics

We report a first-principles investigation of spin polarized transport properties of Pt atomic chains in contact with two semi-infinite Pt slabs along the (111) direction. Our approach is based on the nonequilibrium Green's function coupled with density functional theory so that the Coulomb interaction is included in the calculation of current and shot noise on the Hartree level. For Pt atomic chains with different numbers of Pt atoms, we calculate the spin polarized I-V curve and shot noise. Our results show that the current increases almost linearly with bias for all Pt atomic structures. The calculated Fano factors are comparable to the recent experimental data and show sub-Poissonian behavior. textcopyright 2011 American Physical Society.

Helical states of topological insulator Bi2Se3

Product: NanoDCAL
Date: 2011
Authors: Zhao Y,Hu Y,Liu L,Zhu Y,Guo H
Journal: Nano Letters

We report density functional theory analysis of the electronic and quantum transport properties of Bi2Se3 topological insulator, focusing on the helical surface states at the Fermi level EF. The calculated Dirac point and the tilt angle of the electron spin in the helical states are compared quantitatively with the experimental data. The calculated conductance near EF shows a V-shaped spectrum, consistent with STM measurements. The spins in the helical states at EF not only tilts out of the two-dimensional plane, they also oscillate with a 3-fold symmetry going around the two-dimensional Brillouin zone. The helical states penetrate into the material bulk, where the first quintuple layer contributes 70% of the helical wave functions. textcopyright 2011 American Chemical Society.

SymGF: A symbolic tool for quantum transport analysis and its application to a double quantum dot system

Product: NanoDCAL
Date: 2011
Authors: Feng Z,Sun QF,Wan L,Guo H
Journal: Journal of Physics Condensed Matter

We report the development and an application of a symbolic tool, called SymGF, for analytical derivations of quantum transport properties using the Keldysh nonequilibrium Greens function (NEGF) formalism. The inputs to SymGF are the device Hamiltonian in the second quantized form, the commutation relation of the operators and the truncation rules of the correlators. The outputs of SymGF are the desired NEGF that appear in the transport formula, in terms of the unperturbed Greens function of the device scattering region and its coupling to the device electrodes. For complicated transport analysis involving strong interactions and correlations, SymGF provides significant assistance in analytical derivations. Using this tool, we investigate coherent quantum transport in a double quantum dot system where strong on-site interaction exists in the side-coupled quantum dot. Results obtained by the higher-order approximation and HartreeFock approximation are compared. The higher-order approximation reveals Kondo resonance features in the density of states and conductances. Results are compared both qualitatively and quantitatively to the experimental data reported in the literature. textcopyright 2011 IOP Publishing Ltd.

Conduction modulation of $pi$-stacked ethylbenzene wires on Si(100) with substituent groups

Product: NanoDCAL
Date: 2012
Authors: Smeu M,Wolkow RA,Guo H
Journal: Theoretical Chemistry Accounts

For the realization of molecular electronics, one essential goal is the ability to systematically fabricate molecular functional components in a well-controlled manner. Experimental techniques have been developed such that p-stacked ethylbenzene molecules can now be routinely induced to self-assemble on an H-terminated Si(100) surface at precise locations and along precise directions. Electron transport calculations predict that such molecular wires could indeed carry an electrical current, but the Si substrate may play a considerable role as a competing pathway for conducting electrons. In this work, we investigate the effect of placing substituent groups of varying electron donating or withdrawing strengths on the ethylbenzene molecules to determine how they would affect the transport properties of such molecular wires. The systems consist of a line of p-stacked ethylbenzene molecules covalently bonded to a Si substrate. The ethylbenzene line is bridging two Al electrodes to model current through the molecular stack. For our transport calculations, we employ a first-principles technique where density functional theory (DFT) is used within the non-equilibrium Green's function formalism (NEGF). The calculated density of states suggest that substituent groups are an effective way to shift molecular states relative to the electronic states associated with the Si substrate. The electron transmission spectra obtained from the NEGF-DFT calculations reveal that the transport properties could also be extensively modulated by changing substituent groups. For certain molecules, it is possible to have a transmission peak at the Fermi level of the electrodes, corresponding to high conduction through the molecular wire with essentially no leakage into the Si substrate.

Conductivity of an atomically defined metallic interface

Product: NanoDCAL
Date: 2012
Authors: Oliver DJ,Maassen J,El Ouali M,Paul W,Hagedorn T,Miyahara Y,Qi Y,Guo H,Grütter P
Journal: Proceedings of the National Academy of Sciences of the United States of America

A mechanically formed electrical nanocontact between gold and tungsten is a prototypical junction between metals with dissimilar electronic structure. Through atomically characterized nanoindentation experiments and first-principles quantum transport calculations, we find that the ballistic conduction across this intermetallic interface is drastically reduced because of the fundamental mismatch between swave-like modes of electron conduction in the gold and d wave-like modes in the tungsten. The mechanical formation of the junction introduces defects and disorder, which act as an additional source of conduction losses and increase junction resistance by up to an order of magnitude. These findings apply to nanoelectronics and semiconductor device design. The technique that we use is very broadly applicable to molecular electronics, nanoscale contact mechanics, and scanning tunneling microscopy.

Robustness of helical edge states in topological insulators

Product: NanoDCAL
Date: 2012
Authors: Wang XF,Hu Y,Guo H
Journal: Physical Review B - Condensed Matter and Materials Physics

Topological insulators (TIs) are materials having an energy band gap in the bulk and conducting helical electronic states on the surface. The helical states are protected by time-reversal symmetry and thus are expected to be robust against static disorder scattering. In this work we report an atomistic first principles analysis of disorder scattering in two-probe transport junctions made of three-dimensional TI material Bi 2Se 3. The robustness of the device against disorder scattering is determined quantitatively. Examining many different scattering configurations, a general trend emerges on how strong is the perturbing potential and how it is spatially distributed so that it can derail the helical states on the Bi 2Se 3 surfaces. textcopyright 2012 American Physical Society.

Organic magnetic tunnel junctions: The role of metal-molecule interface

Product: NanoDCAL
Date: 2012
Authors: Liang SH,Liu DP,Tao LL,Han XF,Guo H
Journal: Physical Review B - Condensed Matter and Materials Physics

We report a first-principles theoretical investigation of spin-polarized quantum transport in organic magnetic tunnel junctions (OMTJs) to provide a microscopic understanding on the sign of the tunnel magnetoresistance ratio (TMR). We consider two different OMTJs, formed by sandwiching 1-stearic acid radicals (1-SAR) or 1,18-stearic diacid radicals (1,18-SDR) between two Ni electrodes. Even though the main difference between them is only on one of the Ni/molecule contacts, such a structure difference is found to induce a significant sign change of the TMR. The TMR is negative for 1-SAR at -19.6%, but is positive for 1,18-SDR at 13.7%. By investigating the concept of scattering density of states (SDOS), we found that scattering processes of p electrons at the Ni/molecule interface determines the sign of TMR. Based on spin polarization of the SDOS, we extend the Julliere model to explain both the sign and the value of the TMR qualitatively and semiquantitatively. It is concluded that understanding spin-polarized quantum transport in organic magnetic tunnel junction requires a comprehensive knowledge of the electronic structures of the molecule, the metal electrode, and the metal-molecule contacts. textcopyright 2012 American Physical Society.

Electronic and transport gaps of graphene opened by grain boundaries

Product: NanoDCAL
Date: 2012
Authors: Zhang J,Gao J,Liu L,Zhao J
Journal: Journal of Applied Physics

The electronic and transport properties of graphene grain boundaries (GBs) are studied using density functional theory and nonequilibrium Greens function method. Most GBs preserve the semi-metal properties of perfect graphene; however, some GBs can open a moderate band gap up to 0.5 eV, which provides a potential way for band engineering of graphene-based materials. Nonequilibrium calculations of transmission coefficients showed that the conduction channels for transport electrons at Fermi level can be totally blocked or reduced due to existence of GBs. Moreover, the detailed defect arrangements have some influence on the transport behavior of graphene GBs. textcopyright 2012 American Institute of Physics.

Oxygen vacancy filament formation in TiO 2: A kinetic Monte Carlo study

Product: NanoDCAL
Date: 2012
Authors: Li D,Li M,Zahid F,Wang J,Guo H
Journal: Journal of Applied Physics

We report a kinetic Monte Carlo (kMC) investigation of an atomistic model for 3-dimensional structural configurations of TiO 2 memristor, focusing on the oxygen vacancy migration and interaction under an external voltage bias. kMC allows the access of experimental time scales so that the formation of well defined vacancy filaments in thin TiO 2 films can be simulated. The results show that the electric field drives vacancy migration; and vacancy hopping-induced localized electric field plays a key role for the filament evolution. Using the kMC structure of the filaments at different stages of the formation process, electronic density of states (DOS) are calculated by density functional theory. Filament induced gap states are found which gives rise to a transition from insulating behavior to conducting behavior during the filament formation process. By varying kMC simulations parameters, relations between vacancy diffusion, filament formation, and DOS in the TiO 2 thin film are elucidated. textcopyright 2012 American Institute of Physics.

Tailoring thermopower of single-molecular junctions by temperature-induced surface reconstruction

Product: NanoDCAL
Date: 2012
Authors: Hsu BC,Lin CY,Hsieh YS,Chen YC
Journal: Applied Physics Letters

Recent experiments revealed that surface reconstruction occurs at around 300-400K in the interface of C60 adsorbed on Cu(111) substrate by scanning tunneling microscope techniques. To understand effects of such reconstruction on thermopower, we investigate the Seebeck coefficients of C 60 single-molecular junctions without and with surface reconstruction as a function of temperature at different tip-to-molecule heights from first-principles. Our calculations show that surface reconstruction can enhance or suppress Seebeck coefficients according to junctions at different tip heights. We further observe that the Seebeck coefficient of the junction at d = 3.4 Å may change from p- to n-type under surface reconstruction. textcopyright 2012 American Institute of Physics.

Si cluster based spintronics: A density functional theory study

Product: NanoDCAL
Date: 2013
Authors: Huang YQ,Hao CH,Zheng JM,Ren ZY
Journal: Wuli Xuebao/Acta Physica Sinica

A kind of spintronics is designed by doping the transition metal into Si clusters. Their spin-polarized electron transport properties are investigated by using the first principle analysis. Calculation shows that Fe, Cr and Mn atom doped clusters give the largest spin-polarized transmission coefficients in all the clusters. From Sc to Ni doped clusters, spin filter efficiencies of the systems increase gradually, and the maximal spin filter coefficiency appears in the Fe doped system. The ability to induce the spin-polarized electron transport of the cluster in junction is not cosistent with the magnetic moment of cluster under isolated states. textcopyright 2013 Chinese Physical Society.

Quantum transport modeling from first principles

Product: NanoDCAL
Date: 2013
Authors: Maassen J,Harb M,Michaud-Rioux V,Zhu Y,Guo H
Journal: Proceedings of the IEEE

In the past two decades, significant progress has been achieved in the large-scale fabrication of nanostructures where quantum transport properties of charge and spin are closely coupled to the discreteness of the device material. Multitudes of emerging device concepts and new materials with interesting application potential have been discovered. In order to understand the experimental data and device physics of nanoelectronics, an important task is to develop appropriate theoretical formalisms and associated modeling tools which are capable of making quantitative and material specific predictions of device characteristics without any phenomenological parameters. Here we review the atomistic modeling method based on carrying out density functional theory (DFT) within the nonequilibrium Green's function (NEGF) formalism. Since its original implementation ten years ago, the NEGF-DFT technique has emerged as a very powerful and practically very useful method for predicting nonlinear and nonequilibrium quantum transport properties of nanoelectronics. Recent new developments concerning nonequilibrium disorder scattering will also be presented. Large-scale and scalable computations have allowed NEGF-DFT to model Si structures reaching the present day realistic channel sizes. textcopyright 1963-2012 IEEE.

Active control of thermal transport in molecular spin valves

Product: NanoDCAL
Date: 2013
Authors: Lee MH,Dunietz BD
Journal: Physical Review B - Condensed Matter and Materials Physics

Active control of heat flow is challenging. We demonstrate that molecular spin valves offer a unique opportunity for achieving this goal. Our first-principles calculations of the transport of electrons and phonons in nickel-benzenedithiol-nickel junctions show that when the magnetization direction of the electrodes is changed from parallel to antiparallel the junctions become thermally insulating. Our findings, therefore, suggest a novel avenue for actively controlling thermal transport via the spin degree of freedom. textcopyright 2013 American Physical Society.

The influence of the coupling strength on the electron transport through the benzene-1,4-dithiolate molecular junction

Product: NanoDCAL
Date: 2013
Authors: Yu Y,Li Y,Wan L,Wang B,Wei Y
Journal: Modern Physics Letters B

The electronic transport properties of one benzene-1,4-dithiolate molecule coupled by two aluminum metal leads were investigated by using first-principles method. The influence of the coupling distance between the molecule and the electrodes on I-V curve was studied thoroughly. Our calculations showed that when the system is in the most stable configuration, where the system total energy is the lowest, and the electron transport is in off-resonant state. Starting from the most stable configuration, when we gradually increase the distance between the molecule and electrodes and so decreasing the coupling strength of the molecule and electrodes, the conductance, as well as the I-V curve, does not decrease immediately but increase quickly at first. Only when we separate the molecule and electrodes far enough, the current begins to drop quickly. The total scattering charge density was presented in order to understand this phenomenon. A one-level quantum dot model is used to explain it. Finally, negative differential resistance was observed and analyzed. textcopyright 2013 World Scientific Publishing Company.

Single-molecule conductance through chiral gold nanotubes

Product: NanoDCAL
Date: 2013
Authors: Sen A,Lin CJ,Kaun CC
Journal: Journal of Physical Chemistry C

Using first-principles calculations based on the density functional theory and the nonequilibrium Green's functions approach, we demonstrate that single-molecule junctions can be constructed by chiral single-wall gold nanotubes, which display different transmission spectra from the ones based on achiral gold nanowires. The character of the molecule (viz. $sigma$-or $pi$-type) features the main conduction channel, determining the distribution of local density of states, which can be controlled further by the chirality of the electrodes. Calculated conductance values being in good accord with the available measured data indicates that our analysis can shed light into the viable junction geometries and their conduction mechanisms. textcopyright 2013 American Chemical Society.

Spin transport of polyacetylene chains bridging zigzag graphene nanoribbon electrodes: A nonequilibrium treatment of structural control and spin filtering

Product: NanoDCAL
Date: 2013
Authors: Saraiva-Souza A,Smeu M,Terrones H,Souza Filho AG,Ratner MA
Journal: Journal of Physical Chemistry C

We investigate spin transport properties in a junction composed of a polyacetylene chain bridging two zigzag graphene nanoribbon (ZGNR) electrodes with antiferromagnetic (AF) and ferromagnetic (FM) ordering. The transport calculations are carried out using a nonequilibrium Green's function (NEGF) technique combined with density functional theory (DFT). Previous studies have demonstrated that the ZGNRs exhibit a special AF ordering and half-metallicity at edge states, both of which can be destroyed by applying a strong external electric field. Moreover a stable FM state can be found in ZGNRs under an electric field. Here we demonstrate that the connection between the molecular bridge and nonequivalent carbon atoms (A/B) in the graphene sublattice of ZGNRs may occur in two bonding arrangements and can produce either metallic or semiconducting systems depending on the local coupling. By considering the carbon ring where the chain is attached, one connection resembles a para-linkage in benzene while the other connection is similar to a meta-linkage. This results in different conductances for these configurations, which may be controlled by field-effect gating. Finally, the spin filter efficiency as a function of electric field for these systems, which exhibit intrinsic AF ordering coupled to FM electrodes, is discussed. textcopyright 2013 American Chemical Society.

Effect of anchoring groups on single molecule charge transport through porphyrins

Product: NanoDCAL
Date: 2013
Authors: Li Z,Smeu M,Ratner MA,Borguet E
Journal: Journal of Physical Chemistry C

Controlling charge transport through individual molecules and further understanding the effect of anchoring groups on charge transport are central themes in molecule-based devices. However, in most anchoring effect studies, only two, or at most three nonthiol anchoring groups were studied and compared for a specific system, i.e., using the same core structure. The scarcity of direct comparison data makes it difficult to draw unambiguous conclusions on the anchoring group effect. In this contribution, we focus on the single molecule conductance of porphyrins terminated with a range of anchoring groups: sulfonate (-SO3-), hydroxyl (-OH), nitrile (-CN), amine (-NH 2), carboxylic acid (-COOH), benzyl (-C6H6), and pyridyl (-C6H5N). The present study represents a first attempt to investigate a broad series of anchoring groups in one specific molecule for a direct comparison. It also is the first attempt, to our knowledge, to explore single molecule conductivity with two novel anchoring groups sulfonate (-SO3-) and hydroxyl (-OH). Our experimental results reveal that the single molecule conductance values of the porphyrins follow the sequence of pyridyl > amine > sulfonate > nitrile > carboxylic acid. Electron transport calculations are in agreement that the pyridyl groups result in higher conductance values than the other groups, which is due to a stronger binding interaction of this group to the Au electrodes. The finding of a general trend in the effect of anchoring groups and the exploration of new anchoring groups reported in this paper may provide useful information for molecule-based devices, functional porphyrin design, and electron transfer/transport studies. textcopyright 2013 American Chemical Society.

Negative differential resistance in graphene-nanoribbon-carbon-nanotube crossbars: A first-principles multiterminal quantum transport study

Product: NanoDCAL
Date: 2013
Authors: Saha KK,Nikolić BK
Journal: Journal of Computational Electronics

We simulate quantum transport between a graphene nanoribbon (GNR) and a single-walled carbon nanotube (CNT) where electrons traverse vacuum gap between them. The GNR covers CNT over a nanoscale region while their relative rotation is 90â̂̃, thereby forming a four-terminal crossbar where the bias voltage is applied between CNT and GNR terminals. The CNT and GNR are chosen as either semiconducting (s) or metallic (m) based on whether their two-terminal conductance exhibits a gap as a function of the Fermi energy or not, respectively. We find nonlinear current-voltage (I-V) characteristics in all three investigated devices - mGNR-sCNT, sGNR-sCNT and mGNR-mCNT crossbars - which are asymmetric with respect to changing the bias voltage from positive to negative. Furthermore, the I-V characteristics of mGNR-sCNT crossbar exhibits negative differential resistance (NDR) with low onset voltage V NDRâ‰0.25 V and peak-to-valley current ratio â‰2.0. The overlap region of the crossbars contains only â‰460 carbon and hydrogen atoms which paves the way for nanoelectronic devices ultrascaled well below the smallest horizontal length scale envisioned by the international technology roadmap for semiconductors. Our analysis is based on the nonequilibrium Green function formalism combined with density functional theory (NEGF-DFT), where we also provide an overview of recent extensions of NEGF-DFT framework (originally developed for two-terminal devices) to multiterminal devices. textcopyright 2013 Springer Science+Business Media New York.

Dynamic response of silicon nanostructures at finite frequency: An orbital-free density functional theory and non-equilibrium Green’s function study

Product: NanoDCAL
Date: 2013
Authors: Xu F,Wang B,Wei Y,Wang J
Journal: Journal of Applied Physics

Orbital-free density functional theory (OFDFT) replaces the wavefunction in the kinetic energy by an explicit energy functional and thereby speeds up significantly the calculation of ground state properties of the solid state systems. So far, the application of OFDFT has been centered on closed systems and less attention is paid on the transport properties in open systems. In this paper, we use OFDFT and combine it with non-equilibrium Green's function to simulate equilibrium electronic transport properties in silicon nanostructures from first principles. In particular, we study ac transport properties of a silicon atomic junction consisting of a silicon atomic chain and two monoatomic leads. We have calculated the dynamic conductance of this atomic junction as a function of ac frequency with one to four silicon atoms in the central scattering region. Although the system is transmissive with dc conductance around 4 to 5 e2/h, capacitive-like behavior was found in the finite frequency regime. Our analysis shows that, up to 0.1 THz, this behavior can be characterized by a classic RC circuit consisting of two resistors and a capacitor. One resistor gives rise to dc resistance and the other one accounts for the charge relaxation resistance with magnitude around 0.2 h /e2 when the silicon chain contains two atoms. It was found that the capacitance is around 5 aF for the same system. textcopyright 2013 AIP Publishing LLC.

Large magnetoresistance of paracyclophane-based molecular tunnel junctions: A first-principles study

Product: NanoDCAL
Date: 2013
Authors: Tao LL,Liang SH,Liu DP,Han XF
Journal: Journal of Applied Physics

We report a theoretical study of magnetoresistance and spin-polarized transport of a series of paracyclophane-based molecular tunnel junctions. We predict that the molecular tunnel junction using [2.2]-paracyclophane barrier has the desired low resistance area product in combination with high magnetoresistance ratio. In addition, we find the spin-polarized conductance decreases exponentially with increasing the molecular length, indicating a nonresonant tunneling mechanism. In particular, the characteristic decay constant can be theoretically evaluated from the complex band structure of periodic paracyclophane molecule. The spin-polarized transport mechanism is systematically analyzed. textcopyright 2013 AIP Publishing LLC.

Digitized charge transfer magnitude determined by metal-organic coordination number

Product: NanoDCAL
Date: 2013
Authors: Yang HH,Chu YH,Lu CI,Yang TH,Yang KJ,Kaun CC,Hoffmann G,Lin MT
Journal: ACS Nano

Well-ordered metal-organic nanostructures of Fe-PTCDA (perylene-3,4,9,10- tetracarboxylic-3,4,9,10-dianhydride) chains and networks are grown on a Au(111) surface. These structures are investigated by high-resolution scanning tunneling microscopy. Digitized frontier orbital shifts are followed in scanning tunneling spectroscopy. By comparing the frontier energies with the molecular coordination environments, we conclude that the specific coordination affects the magnitude of charge transfer onto each PTCDA in the Fe-PTCDA hybridization system. A basic model is derived, which captures the essential underlying physics and correlates the observed energetic shift of the frontier orbital with the charge transfer. textcopyright 2013 American Chemical Society.

Hole defects and nitrogen doping in graphene: Implication for supercapacitor applications

Product: NanoDCAL
Date: 2013
Authors: Luo G,Liu L,Zhang J,Li G,Wang B,Zhao J
Journal: ACS Applied Materials and Interfaces

One great challenge for supercapacitor is to achieve high energy capacity and fast charge/discharge rates simultaneously. Porous graphene with large surface area is a promising candidate for electrode materials of supercapacitor. Using first-principles calculations and non-equilibrium Green's function technique, we have explored the formation energies, mechanical properties, diffusion behaviors and electrical conductance of graphene sheets with various hole defects and/or nitrogen doping. Interestingly, graphene sheets with pyridinic-like holes (especially hexagonal holes) can be more easily doped with nitrogen and still retain the excellent mechanical properties of pristine graphene that is beneficial for the long cycle life. Porous graphene electrode with moderate hole diameter of 4.2-10 Å facilitates efficient access of electrolyte and exhibit excellent rate capability. In addition, doping with nitrogen as electron donors or proton attractors leads to charge accumulation and generates higher pseudocapacitance. Transmission coefficients of N-doped graphene sheets with pyridinic-like holes are only moderately reduced with regard to that of pristine graphene and are insensitive to the detailed geometry parameters. Overall, N-doped graphene with pyridinic-like holes exhibits exciting potentials for high performance energy storage in supercapacitor devices. textcopyright 2013 American Chemical Society.

Generation and transport of valley-polarized current in transition-metal dichalcogenides

Product: NanoDCAL
Date: 2014
Authors: Zhang L,Gong K,Chen J,Liu L,Zhu Y,Xiao D,Guo H
Journal: Physical Review B - Condensed Matter and Materials Physics

In two-dimensional crystals of transition-metal dichalcogenides (TMDC) having strong spin-orbit interaction such as monolayer WSe2, quantum states can be labeled by a valley index $tau$ defined in the reciprocal space and the spin index s. We developed a first-principles theoretical formalism to both qualitatively and quantitatively predict nonequilibrium quantum transport of valley-polarized currents. We propose a WSe2 TMDC transistor to selectively deliver net valley- and spin-polarized current I$tau$,s to the source or drain by circularly polarized light under external bias. Due to the lack of translational symmetry of the real-space device, we predict a depolarization effect that increases with the decrease of the channel length of the transistor.

Electrical contacts to monolayer black phosphorus: A first-principles investigation

Product: NanoDCAL
Date: 2014
Authors: Gong K,Zhang L,Ji W,Guo H
Journal: Physical Review B - Condensed Matter and Materials Physics

We report first-principles theoretical investigations of possible metal contacts to monolayer black phosphorus (BP). By analyzing lattice geometry, five metal surfaces are found to have minimal lattice mismatch with BP: Cu(111), Zn(0001), In(110), Ta(110), and Nb(110). Further studies indicate Ta and Nb bond strongly with monolayer BP causing substantial bond distortions, but the combined Ta-BP and Nb-BP form good metal surfaces to contact a second layer BP. By analyzing the geometry, bonding, electronic structure, charge transfer, potential, and band bending, it is concluded that Cu(111) is the best candidate to form excellent Ohmic contact to monolayer BP. The other four metal surfaces or combined surfaces also provide viable structures to form metal/BP contacts, but they have Schottky character. Finally, the band bending property in the current-in-plane (CIP) structure where metal/BP is connected to a freestanding monolayer BP, is investigated. By both work function estimates and direct calculations of the two-probe CIP structure, we find that the freestanding BP channel is n type.

Electric control of spin in monolayer WSe2field effect transistors

Product: NanoDCAL
Date: 2014
Authors: Gong K,Zhang L,Liu D,Liu L,Zhu Y,Zhao Y,Guo H
Journal: Nanotechnology

We report first-principles theoretical investigations of quantum transport in a monolayer WSe2field effect transistor (FET). Due to strong spin-orbit interaction (SOI) and the atomic structure of the two-dimensional lattice, monolayer WSe2has an electronic structure that exhibits Zeeman-like up-down spin texture near the K and K′ points of the Brillouin zone. In a FET, the gate electric field induces an extra, externally tunable SOI that re-orients the spins into a Rashba-like texture thereby realizing electric control of the spin. The conductance of FET is modulated by the spin texture, namely by if the spin orientation of the carrier after the gated channel region, matches or miss-matches that of the FET drain electrode. The carrier current in the FET is labelled by both the valley index and spin index, realizing valleytronics and spintronics in the same device.

Giant magnetoresistance and spin Seebeck coefficient in zigzag $alpha$-graphyne nanoribbons

Product: NanoDCAL
Date: 2014
Authors: Zhai MX,Wang XF,Vasilopoulos P,Liu YS,Dong YJ,Zhou L,Jiang YJ,You WL
Journal: Nanoscale

We investigate the spin-dependent electric and thermoelectric properties of ferromagnetic zigzag $alpha$-graphyne nanoribbons (Z$alpha$GNRs) using density-functional theory combined with non-equilibrium Green's function method. A giant magnetoresistance is obtained in the pristine even-width Z$alpha$GNRs and can be as high as 106%. However, for the doped systems, a large magnetoresistance behavior may appear in the odd-width Z$alpha$GNRs rather than the even-width ones. This suggests that the magnetoresistance can be manipulated in a wide range by the dopants on the edges of Z$alpha$GNRs. Another interesting phenomenon is that in the B- and N-doped even-width Z$alpha$GNRs the spin Seebeck coefficient is always larger than the charge Seebeck coefficient, and a pure-spin-current thermospin device can be achieved at specific temperatures. This journal is

Molecular spintronics: Destructive quantum interference controlled by a gate

Product: NanoDCAL
Date: 2014
Authors: Saraiva-Souza A,Smeu M,Zhang L,Souza Filho AG,Guo H,Ratner MA
Journal: Journal of the American Chemical Society

The ability to control the spin-transport properties of a molecule bridging conducting electrodes is of paramount importance to molecular spintronics. Quantum interference can play an important role in allowing or forbidding electrons from passing through a system. In this work, the spin-transport properties of a polyacetylene chain bridging zigzag graphene nanoribbons (ZGNRs) are studied with nonequilibrium Greens function calculations performed within the density functional theory framework (NEGF-DFT). ZGNR electrodes have inherent spin polarization along their edges, which causes a splitting between the properties of spin-up and spin-down electrons in these systems. Upon adding an imidazole donor group and a pyridine acceptor group to the polyacetylene chain, this causes destructive interference features in the electron transmission spectrum. Particularly, the donor group causes a large antiresonance dip in transmission at the Fermi energy EF of the electrodes. The application of a gate is investigated and found to provide control over the energy position of this feature making it possible to turn this phenomenon on and off. The current-voltage (I-V) characteristics of this system are also calculated, showing near ohmic scaling for spin-up but negative differential resistance (NDR) for spin-down.

Spin-polarized negative differential resistance in a self-assembled molecular Chain

Product: NanoDCAL
Date: 2014
Authors: Chen YC,Hsu SH,Kaun CC,Lin MT
Journal: Journal of Physical Chemistry C

By using first-principles calculations, we investigate the electronic structures and transport properties of a self-assembled Fe2-PTCDA chain. This experimentally observed chain can grow as long as a few tens of nanometers and our calculations suggest that it conducts as a half-metal. Spin-polarized transport properties are attributed to conducting bands of the minority spin near the Fermi energy, raised by the hybridization between orbitals of the molecule and the iron atoms, while a band gap of the majority spin exists. Moreover, this system features a highly spin-polarized negative differential resistance, due to the alignment of conducting bands and the match of their symmetries, which can be integrated to build multifunction spintronic devices.

Magnetic Superatoms Based Spintronics: A DFT Study

Product: NanoDCAL
Date: 2014
Authors: Zheng XL,Guo P,Chen WJ,Zheng JM,Ren ZY
Journal: Journal of Cluster Science

A kind of magnetic superatom is designed by doping transition metal element into Na8 clusters. Their electronic structure and spin-polarized transport properties are investigated using the first principles method. Our calculation shows that electrode materials have notable influence on the superatoms' geometrical stability. Lithium lead is a good choice. Among all the superatoms, TiNa8 and NiNa8 give the highest transmission spin polarization (TSP), for negative and positive values, respectively. Relation between TSP and the magnetic moment of isolated superatom may lead to some promising designs in molecular spintronics devices.

Modeling ion sensing in molecular electronics

Product: NanoDCAL
Date: 2014
Authors: Chen CJ,Smeu M,Ratner MA
Journal: Journal of Chemical Physics

We examine the ability of molecules to sense ions by measuring the change in molecular conductance in the presence of such charged species. The detection of protons (H+), alkali metal cations (M+), calcium ions (Ca2+), and hydronium ions (H3O+) is considered. Density functional theory (DFT) is used within the Keldysh non-equilibrium Green's function framework (NEGF) to model electron transport properties of quinolinedithiol (QDT, C9H7NS2), bridging Al electrodes. The geometry of the transport region is relaxed with DFT. The transport properties of the device are modeled with NEGF-DFT to determine if this device can distinguish among the M+ + QDT species containing monovalent cations, where M+ = H+, Li +, Na+, or K+. Because of the asymmetry of QDT in between the two electrodes, both positive and negative biases are considered. The electron transmission function and conductance properties are simulated for electrode biases in the range from -0.5 V to 0.5 V at increments of 0.1 V. Scattering state analysis is used to determine the molecular orbitals that are the main contributors to the peaks in the transmission function near the Fermi level of the electrodes, and current-voltage relationships are obtained. The results show that QDT can be used as a proton detector by measuring transport through it and can conceivably act as a pH sensor in solutions. In addition, QDT may be able to distinguish among different monovalent species. This work suggests an approach to design modern molecular electronic conductance sensors with high sensitivity and specificity using well-established quantum chemistry. textcopyright 2014 AIP Publishing LLC.

Direct tunneling through high-$kappa$ amorphous HfO2: Effects of chemical modification

Product: NanoDCAL
Date: 2014
Authors: Wang Y,Yu Z,Zahid F,Liu L,Zhu Y,Wang J,Guo H
Journal: Journal of Applied Physics

We report first principles modeling of quantum tunneling through amorphous HfO2 dielectric layer of metal-oxide-semiconductor (MOS) nanostructures in the form of n-Si/HfO2/Al. In particular, we predict that chemically modifying the amorphous HfO2 barrier by doping N and Al atoms in the middle region - far from the two interfaces of the MOS structure - can reduce the gate-to-channel tunnel leakage by more than one order of magnitude. Several other types of modification are found to enhance tunneling or induce substantial band bending in the Si, both are not desired from leakage point of view. By analyzing transmission coefficients and projected density of states, the microscopic physics of electron traversing the tunnel barrier with or without impurity atoms in the high-$kappa$ dielectric is revealed. textcopyright 2014 AIP Publishing LLC.

Spin dependent transport properties of Mn-Ga/MgO/Mn-Ga magnetic tunnel junctions with metal(Mg, Co, Cr) insertion layer

Product: NanoDCAL
Date: 2014
Authors: Liang SH,Tao LL,Liu DP,Lu Y,Han XF
Journal: Journal of Applied Physics

We report a first principles theoretical investigation of spin polarized quantum transport in Mn2Ga/MgO/Mn2Ga and Mn 3Ga/MgO/Mn3Ga magnetic tunneling junctions (MTJs) with the consideration of metal(Mg, Co, Cr) insertion layer effect. By changing the concentration of Mn, our calculation shows a considerable disparity in transport properties: A tunneling magnetoresistance (TMR) ratio of 852% was obtained for Mn2Ga-based MTJs, however, only a 5% TMR ratio for Mn 3Ga-based MTJs. In addition, the influence of insertion layer has been considered in our calculation. We found the Co insertion layer can increase the TMR of Mn2Ga-based MTJ to 904%; however, the Cr insertion layer can decrease the TMR by 668%; A negative TMR ratio can be obtained with Mg insertion layer. Our work gives a comprehensive understanding of the influence of different insertion layer in Mn-Ga based MTJs. It is proved that, due to the transmission can be modulated by the interfacial electronic structure of insertion, the magnetoresistance ratio of Mn2Ga/MgO/Mn2Ga MTJ can be improved by inserting Co layer. textcopyright 2014 AIP Publishing LLC.

Correlation of interfacial bonding mechanism and equilibrium conductance of molecular junctions

Product: NanoDCAL
Date: 2014
Authors: Ning ZY,Qiao JS,Ji W,Guo H
Journal: Frontiers of Physics

We report theoretical investigations on the role of interfacial bonding mechanism and its resulting structures to quantum transport in molecular wires. Two bonding mechanisms for the Au-S bond in an Au(111)/1,4-benzenedithiol(BDT)/Au(111) junction were identified by ab initio calculation, confirmed by a recent experiment, which, we showed, critically control charge conduction. It was found, for Au/BDT/Aujunctions, the hydrogen atom, bound by a dative bond to the Sulfur, is energetically non-dissociativeafter the interface formation. The calculated conductance and junction breakdown forces of H-non-dissociative Au/BDT/Au devices are consistent with the experimental values, while the H-dissociated devices, with the interface governed by typical covalent bonding, give conductance more than an order of magnitude larger. By examining the scattering states that traverse the junctions, we have revealed that mechanical and electric properties of a junction have strong correlation with the bonding configuration. This work clearly demonstrates that the interfacial details, rather than previously believed many-body effects, is of vital importance for correctly predicting equilibrium conductance of molecular junctions; and manifests that the interfacial contact must be carefully understood for investigating quantum transport properties of molecular nanoelectronics.

A computational investigation of topological insulator Bi2Se3 film

Product: NanoDCAL
Date: 2014
Authors: Hu YB,Zhao YH,Wang XF
Journal: Frontiers of Physics

Topological insulators have a bulk band gap like an ordinary insulator and conducting states on their edge or surface which are formed by spin-orbit coupling and protected by time-reversal symmetry. We report theoretical analyses of the electronic properties of three-dimensional topological insulator Bi2Se3 film on different energies. We choose five different energies (−123, −75, 0, 180, 350 meV) around the Dirac cone (−113 meV). When energy is close to the Dirac cone, the properties of wave function match the topological insulator's hallmark perfectly. When energy is far way from the Dirac cone, the hallmark of topological insulator is broken and the helical states disappear. The electronic properties of helical states are dug out from the calculation results. The spin-momentum locking of the helical states are confirmed. A 3-fold symmetry of the helical states in Brillouin zone is also revealed. The penetration depth of the helical states is two quintuple layers which can be identified from layer projection. The charge contribution on each quintuple layer depends on the energy, and has completely different behavior along K and M direction in Brillouin zone. From orbital projection, we can find that the maximum charge contribution of the helical states is pz orbit and the charge contribution on py and px orbits have 2-fold symmetry.

Tunneling magnetoresistance of FePt/NaCl/FePt(001)

Product: NanoDCAL
Date: 2014
Authors: Tao LL,Liu DP,Liang SH,Han XF,Guo H
Journal: Epl

We propose and theoretically investigate an interesting and potentially very attractive magnetic tunnel junction FePt/NaCl/FePt(001) for spintronics. It is attractive because the strain at the FePt/NaCl interface is relatively small and, as a result, spin injection from the FePt metal to the NaCl barrier is significant and thus a potentially large TMR ratio can be obtained. The electronic bands with the symmetry of L10 FePt cross the Fermi level for both the majority-spin and minority-spin channels, and the evanescent state with the symmetry dominates the electron transmission through the fcc NaCl barrier. Very respectable values of the tunnel magnetoresistance ratio are predicted. The microscopic physics of quantum transport in this system is systematically analyzed and understood. textcopyright Copyright EPLA, 2014.

Tunneling magnetoresistance in Fe3Si/MgO/Fe3Si(001) magnetic tunnel junctions

Product: NanoDCAL
Date: 2014
Authors: Tao LL,Liang SH,Liu DP,Wei HX,Wang J,Han XF
Journal: Applied Physics Letters

We present a theoretical study of the tunneling magnetoresistance (TMR) and spin-polarized transport in Fe3Si/MgO/Fe3Si(001) magnetic tunnel junction (MTJ). It is found that the spin-polarized conductance and bias-dependent TMR ratios are rather sensitive to the structure of Fe 3Si electrode. From the symmetry analysis of the band structures, we found that there is no spin-polarized $Delta$1 symmetry bands crossing the Fermi level for the cubic Fe3Si. In contrast, the tetragonal Fe 3Si driven by in-plane strain reveals half-metal nature in terms of $Delta$1 state. The giant TMR ratios are predicted for both MTJs with cubic and tetragonal Fe3Si electrodes under zero bias. However, the giant TMR ratio resulting from interface resonant transmission for the former decreases rapidly with the bias. For the latter, the giant TMR ratio can maintain up to larger bias due to coherent transmission through the majority-spin $Delta$1 channel. textcopyright 2014 AIP Publishing LLC.

The spin transport of the coblt dimers with different directions

Product: NanoDCAL
Date: 2014
Authors: Ren SW
Journal: Applied Mechanics and Materials

In this paper, the spin transport properties of the coblt dimers parrallel to the transport direction and perpendicular to ransprot direction are investigated by using the first principle analysis. Calculation shows that both the coblt dimers parrallel to the transport direction and perpendicular to ransprot direction give obvious spin polarized density of states and current. It is found that the dimer parrallel to the transport direction have larger spin polarization current.The spin polarized efficiency for the parrallel dimer increase steadily with the increase of the bias voltage. But the the spin polarization for the transverse dimer changes greatly. textcopyright (2014) Trans Tech Publications, Switzerland.

Design molecular rectifier and photodetector with all-boron fullerene

Product: NanoDCAL
Date: 2015
Authors: Yang Z,Ji YL,Lan G,Xu LC,Liu X,Xu B
Journal: Solid State Communications

All-boron fullerene B40 is a highly stable molecule, which has been successfully synthesized in recent experiment. In this paper, with Au as two electrodes, the single-molecule device Au-B40-Au was investigated by using density functional theory and non-equilibrium Green's function method. The results show that the device can exhibit large rectification ratio and significant negative differential resistance. More importantly, the photocurrent of the device has different responses in the infrared, visible and ultraviolet regions. The excellent optoelectronic properties ensure that the device can be used as photodetector.

Perfect spin filtering effect and negative differential behavior in phosphorus-doped zigzag graphene nanoribbons

Product: NanoDCAL
Date: 2015
Authors: Zou F,Zhu L,Yao K
Journal: Scientific Reports

On the basis of the density functional theory combined with the Keldysh nonequilibrium Greens function method, we investigate the spin-dependent transport properties of single-edge phosphorus-doped ZGNR systems with different widths. The results show a perfect spin filtering effect reaching 100% at a wide bias range in both parallel (P) and antiparallel (AP) spin configurations for all systems, especially for 6-ZGNR-P system. Instructively, for the AP spin configuration, the spin down current of the 4-ZGNR-P system exhibits a negative differential effect. By analyzing the transmission spectrum and the spin-resolved band structures of the electrodes, we elucidate the mechanism for these peculiar properties. Our findings provide a new way to produce multifunctional spintronic devices based on phosphorus-doped zigzag graphene nanoribbons.

Transport properties of WSe2 nanotube heterojunctions: A first-principles study

Product: NanoDCAL
Date: 2015
Authors: Yu Z,Wang J
Journal: Physical Review B - Condensed Matter and Materials Physics

Using the nonequilibrium Green's function method within the framework of density functional theory, we investigate various transport properties, such as I-V characteristics, shot noise, thermopower, dynamical conductance, of Au- and Na-encapsulated WSe2 nanotube heterojunctions. First-principles transport calculations show that from I-V curves large rectification ratio is found in the (8,0) heterojunction and for shot noise it exhibits sub-Poissonian behaviors under positive biases (on Au-encapsulated tubes) while Poissonian behaviors are found under negative biases. For thermopower, it is found that as one sweeps the Fermi energy, the thermopower can change its sign. For dynamic conductance, the (5,5) heterojunction exhibits capacitivelike behavior. We find that the spin-orbit interaction (SOI) is very important for WSe2 nanotubes. Due to the band splitting originated from SOI, the intrinsic band gap of Au-doped (5,5) nanotube is reduced by about 58% and that of the Na-doped system vanishes, while that of the doped (8,0) nanotubes decreases by about 40%. The reduction of band gap has an important impact on the transport properties. For instance, the transmission gap is decreased by about 48% and 16% in the transmission spectrum of the (5,5) and (8,0) heterojunctions, respectively. The current of the (5,5) heterojunction under small bias is almost doubled and the rectification ratio of the (8,0) heterojunction is enhanced by more than 120% due to SOI.

Conditions for quantized anisotropic magnetoresistance

Product: NanoDCAL
Date: 2015
Authors: Hu C,Teng J,Yu G,Lu W,Ji W
Journal: Physical Review B - Condensed Matter and Materials Physics

Conditions for the appearance of quantized anisotropic magnetoresistance (QAMR) in ferromagnetic conductors have been investigated from the viewpoint of the transmission channels calculated by method of the nonequilibrium Green's function-density functional theory. We demonstrate that the spin-orbital interaction is a precondition and the transverse size is a crucial condition for QAMR to emerge. Furthermore, we show the evolution of QAMR from a stepwise shape (corresponding to small transverse sizes) to a classical smooth cosine shape (corresponding to large transverse sizes). In addition, we prove the strong temperature dependence of QAMR, so a low temperature is necessary. Our research reconciles the different experiments and enlightens experimenters on QAMR.

Switchable valley injection into graphene

Product: NanoDCAL
Date: 2015
Authors: Hu C,Lu W,Ji W,Yu G,Yan Y,Teng J
Journal: Physical Review B - Condensed Matter and Materials Physics

The generation and control of the valley-polarized current play the key roles in valleytronics applications, but still remain as tough challenges. We propose a general idea of switchable valley injection, and predict theoretically that a ferromagnet-covered graphene junction would take the role of a valley injector, whose on/off state can be easily switched only by changing the magnetic direction. This scheme can be extended to other two-dimensional crystals and may lead to an alternative path for valleytronic practical applications.

Variable electronic properties of lateral phosphorene-graphene heterostructures

Product: NanoDCAL
Date: 2015
Authors: Tian X,Liu L,Du Y,Gu J,Xu JB,Yakobson BI
Journal: Physical Chemistry Chemical Physics

Phosphorene and graphene have a tiny lattice mismatch along the armchair direction, which can result in an atomically sharp in-plane interface. The electronic properties of the lateral heterostructures of phosphorene/graphene are investigated by the first-principles method. Here, we demonstrate that the electronic properties of this type of heterostructure can be highly tunable by the quantum size effects and the externally applied electric field (Eext). At strong Eext, Dirac Fermions can be developed with Fermi velocities around one order smaller than that of graphene. Undoped and hydrogen doped configurations demonstrate three drastically different electronic phases, which reveal the strongly tunable potential of this type of heterostructure. Graphene is a naturally better electrode for phosphorene. The transport properties of two-probe devices of graphene/phosphorene/graphene exhibit tunnelling transport characteristics. Given these results, it is expected that in-plane heterostructures of phosphorene/graphene will present abundant opportunities for applications in optoelectronic and electronic devices.

Towards graphyne molecular electronics

Product: NanoDCAL
Date: 2015
Authors: Li Z,Smeu M,Rives A,Maraval V,Chauvin R,Ratner MA,Borguet E
Journal: Nature Communications

$alpha$-Graphyne, a carbon-expanded version of graphene ('carbo-graphene') that was recently evidenced as an alternative zero-gap semiconductor, remains a theoretical material. Nevertheless, using specific synthesis methods, molecular units of $alpha$-graphyne ('carbo-benzene' macrocycles) can be inserted between two anilinyl (4-NH2-C6H4)-anchoring groups that allow these fragments to form molecular junctions between gold electrodes. Here, electrical measurements by the scanning tunnelling microscopy (STM) break junction technique and electron transport calculations are carried out on such a carbo-benzene, providing unprecedented single molecule conductance values: 106 nS through a 1.94-nm N-N distance, essentially 10 times the conductance of a shorter nanographenic hexabenzocoronene analogue. Deleting a C4 edge of the rigid C18 carbo-benzene circuit results in a flexible 'carbo-butadiene' molecule that has a conductance 40 times lower. Furthermore, carbo-benzene junctions exhibit field-effect transistor behaviour when an electrochemical gate potential is applied, opening the way for device applications. All the results are interpreted on the basis of theoretical calculations.

Photogalvanic effect in monolayer black phosphorus

Product: NanoDCAL
Date: 2015
Authors: Xie Y,Zhang L,Zhu Y,Liu L,Guo H
Journal: Nanotechnology

We report a first-principles theoretical approach for analyzing linear and circular photogalvanic effects (PGEs) based on density functional theory within the nonequilibrium Green's function formalism. Using this approach we investigate the PGE phenomena in monolayer black phosphorus (MBP) doped with sulfur atoms. The impurity doping breaks the space inversion symmetry of pristine MBP, leading to a C s symmetry with a mirror reflection plane normal to the zigzag direction of the MBP lattice. Governed by this symmetry, a linear PGE is induced in both zigzag and armchair directions, and a circular PGE is induced along the zigzag direction. A robust broadband photoresponse is found from the near-infrared to the visible range for the MBP device. There is a strong anisotropy in PGE: photoresponse in the zigzag direction can be larger by an order of magnitude than that in the armchair direction. We identify the origin of the observed PGE as the inter-band transitions from the impurity and valence bands to the conduction bands, which involves a transfer of angular momentum from photons to electrons.

Transient dynamics of magnetic Co-graphene systems

Product: NanoDCAL
Date: 2015
Authors: Wang B,Li J,Xu F,Wei Y,Wang J,Guo H
Journal: Nanoscale

We report the investigation of response time of spin resolved electron traversing through a magnetic Co-graphene nano-device. For this purpose, we calculate the transient current under a step-like upward pulse for this system from first principles using non-equilibrium Green's function (NEGF) formalism within the framework of density functional theory (DFT). In the absence of dephasing mechanisms, transient current shows a damped oscillatory behavior. The turn-on time of the magnetic Co-graphene nano-device was found to be around 5-20 femtoseconds, while the relaxation time can reach several picoseconds due to the damped oscillation of transient current for both majority spin and minority spin. The response time was determined by the resonant states below the Fermi level, but does not depend on the chirality of graphene and the amplitude of pulse bias. Each resonant state contributes to the damped oscillation of transient current with the same frequency and different decay rates. The frequency of the oscillation is half the pulse bias and the decay rate equals the lifetime of the corresponding resonant state. When inelastic phase-relaxing scattering is considered, the long duration oscillatory behavior of the transient current is suppressed and the relaxation time is reduced to hundreds of femtoseconds.

Spray-assisted nanocoating of the biobased material urushiol

Product: NanoDCAL
Date: 2015
Authors: Watanabe H,Fujimoto A,Takahara A
Journal: Langmuir

We have demonstrated the spray-assisted coating of the catechol derivative, urushiol. Spraying a mixture of urushiol and iron(II) acetate formed a uniform coating about 10 nm thick, as confirmed by AFM observations. XPS measurements revealed that various substrates, including polyolefins and thermosetting resins, were successfully coated with urushiol. The coating showed good solvent tolerance and coating adhesion after baking at 100 °C for 10 min or after aerobic oxidation for several days. Interestingly, quartz crystal microbalance (QCM) measurements and strain-induced elastic buckling instability for mechanical measurements (SIEBIMM) revealed that density and Youngs modulus of the spray-assisted nanocoatings were higher than those of spray-coated samples. Moreover, the coating was uninvolved in physical properties except surface properties, as demonstrated by several experiments. Because urushiol is a promising biobased material, our unique spray-assisted coating technique could provide a general approach for material-independent surface modification techniques that are environmentally sustainable.

Modulation of Electronic Structure of Armchair MoS2 Nanoribbon

Product: NanoDCAL
Date: 2015
Authors: Zhang L,Wan L,Yu Y,Wang B,Xu F,Wei Y,Zhao Y
Journal: Journal of Physical Chemistry C

We perform first-principles calculations on electronic structures of armchair MoS2 nanoribbons (AMoS2NRs) passivated by non-metal atoms. In contrast to bare AMoS2NR (AMoS2NR-bare) or purely hydrogen (H) edge-terminated AMoS2NR (AMoS2NR-H), it is found that H and oxygen (O) hybrid edge-terminated AMoS2NR (AMoS2NR-H-O) is more stable. AMoS2NR-H-O exhibits a direct band gap of about 1.43 eV, which is larger than those of pristine AMoS2NR (about 0.61 eV) and AMoS2NR-H (about 0.60 eV) and even exceeds the band gap of bulk MoS2 (about 0.86 eV) and is close to that of monolayer MoS2 (about 1.67 eV). The remarkable band gap of AMoS2NR-H-O is attributed to the charge redistribution on the edge atoms of the MoS2 nanoribbon, especially the charges on the edge Mo atoms. Detailed calculations of AMoS2NR-H-O reveal that over 70% of the total density of states (DOS) of the conduction band minimum and the valence band maximum are contributed by the Mo atoms. In particular, edge Mo atoms play a crucial role in modulating the electronic structure. In addition, we have studied a series of functionalized AMoS2NR-H-X with X = S, F, C, N, and P, respectively. It is found that AMoS2NR-H-X with X = S, 2F, C possess remarkable electronic band gaps, whereas AMoS2NR-H-X with X = F, N, P are metallic. Our studies suggest that non-metal atom hybrid passivation can efficiently tune the electronic band gap of MoS2 nanoribbon and open a new route to obtain a MoS2-based practical nanoelectronic device and a photovoltaic device.

Spin-polarized transport through single manganese phthalocyanine molecules on a Co nanoisland

Product: NanoDCAL
Date: 2015
Authors: Hsu CH,Chu YH,Lu CI,Hsu PJ,Chen SW,Hsueh WJ,Kaun CC,Lin MT
Journal: Journal of Physical Chemistry C

We investigate spin-polarized (SP) electronic transport properties and hybrid states of a single manganese phthalocyanine (MnPc) molecule adsorbed on a Co nanoisland, with the SP scanning tunneling microscopy measurements and the first-principles calculations. Our analyses show that the MnPc molecule can pin the Co surface state to the Fermi level, forming hybrid stationary spin resonance states which, with an antiparallel-magnetization tip, give a resonant SP conductance peak. Our calculations further reveal that as the tip approaches the molecule, electronic and magnetic couplings in the junction are tuned as the Zener indirect exchange coupling becomes prominent, which switches the conduction carriers from s to d electrons and leads to the tailored magnetic moments and magnetoresistance.

Conductance of a Single Magnesium Porphine Molecule on an Insulating Surface

Product: NanoDCAL
Date: 2015
Authors: Dou KP,Tai JS,Kaun CC
Journal: Journal of Physical Chemistry C

Using first-principles calculations based on density functional theory and nonequilibrium Green's function formalism, we study the electron transport through a magnesium porphine molecule adsorbed on an ultrathin NaCl bilayer. The conductance of the tip-vacuum-molecule-NaCl-metal junction depends on the orientation of the molecule on the insulating surface and the tip position above the molecule, which is mediated largely by the molecular pz orbital. The movement of molecule results in a perturbation to the spatial extension of these orbitals, leading to different conductions.

Gate controlled electronic transport in monolayer MoS2 field effect transistor

Product: NanoDCAL
Date: 2015
Authors: Zhou YF,Xian HM,Wang B,Yu YJ,Wei YD,Wang J
Journal: Journal of Applied Physics

The electronic spin and valley transport properties of a monolayer MoS2 are investigated using the non-equilibrium Green's function formalism combined with density functional theory. Due to the presence of strong Rashba spin orbit interaction (RSOI), the electronic valence bands of monolayer MoS2 are split into spin up and spin down Zeeman-like texture near the two inequivalent vertices K and K′ of the first Brillouin zone. When the gate voltage is applied in the scattering region, an additional strong RSOI is induced which generates an effective magnetic field. As a result, electron spin precession occurs along the effective magnetic field, which is controlled by the gate voltage. This, in turn, causes the oscillation of conductance as a function of the magnitude of the gate voltage and the length of the gate region. This current modulation due to the spin precession shows the essential feature of the long sought Datta-Das field effect transistor (FET). From our results, the oscillation periods for the gate voltage and gate length are found to be approximately 2.2 V and 20.03aB (aB is Bohr radius), respectively. These observations can be understood by a simple spin precessing model and indicate that the electron behaviors in monolayer MoS2 FET are both spin and valley related and can easily be controlled by the gate.

High-performance giant-magnetoresistance junction with B 2-disordered Heusler alloy based Co2MnAl/Ag/Co2MnAl trilayer

Product: NanoDCAL
Date: 2015
Authors: Li Y,Xia J,Wang G,Yuan H,Chen H
Journal: Journal of Applied Physics

The current-perpendicular-to-plane giant magnetoresistance (MR) devices with full-Heulser Co2MnAl (CMA) electrodes and a Ag spacer have been simulated to investigate the relationship between the transport properties and the structural disordering of electrodes by performing first-principles electronic structure and ballistic transport calculations. The CMA electrode has nearly negligible interfacial roughness in both L21 and B2-types. The transmission coefficient T $sigma$ (E, k → / /) is found strongly dependent on the structures of the trilayers for different structural CMA electrodes. High majority-spin electron conductance in the magnetization parallel configuration turns up in the entire k →-plane and the MR ratio reaches as high as over 90% for the B2-based CMA/Ag/CMA magnetic trilayers. In contrast, the L21-based one has ∼60% MR ratio resulting from much lower bulk spin-asymmetry coefficient ($beta$), which might be caused by the vibrational spin-polarization in each atomic layer adjacent to the interfaces in the corresponding model. The patterns of T $sigma$ (E, k → / /) indicates that B2-based CMA/Ag/CMA magnetic trilayers are promising giant magnetoresistance junctions with high performance.

Large tunnel magnetoresistance ratio in Fe/O/NaCl/O/Fe

Product: NanoDCAL
Date: 2015
Authors: Gong K,Zhang L,Liu L,Zhu Y,Yu G,Grutter P,Guo H
Journal: Journal of Applied Physics

Magnetic tunnel junction (MTJ) is an important device element for many practical spintronic systems. In this paper, we propose and theoretically investigate a very attractive MTJ Fe(001)/O/NaCl(001)/O/Fe(001) as a two-terminal transport junction. By density functional theory total energy methods, we establish two viable device models: one with and the other without mirror symmetry across the center plane of the structure. Large tunnel magnetoresistance ratio (TMR) is predicted from first principles, at over 1800% and 3600% depending on the symmetry. Microscopically, a spin filtering effect is responsible for the large TMR. This effect essentially filters out all the minority spin channels (spin-down) from contributing to the tunnelling current. On the other hand, transport of the majority spin channel (spin-up) having $Delta$ 1 and $Delta$ 5 symmetry is enhanced by the FeO buffer layer in the MTJ.

Electron transport through polyene junctions in between carbon nanotubes: An ab initio realization

Product: NanoDCAL
Date: 2015
Authors: Chen YR,Chen KY,Dou KP,Tai JS,Lee HH,Kaun CC
Journal: Carbon

Using both tight-binding model and ab initio calculations, we investigate a system of polyene-bridged armchair carbon nanotube electrodes to address quantum transport through junctions with multiple conjugated molecules. Both one-polyene and two-polyene cases are considered. The ab initio results of the two-polyene cases show the interference effect in transmission and its strong dependence on the configuration of contact sites. This agrees with the tight-binding model. In addition, the discrepancy brought by ab initio relaxation provides an insight into how the junction's geometry, bonding, and effective potential influence the transmission spectra.

Intrinsic Hydrophobic Cairnlike Multilayer Films for Antibacterial Effect with Enhanced Durability

Product: NanoDCAL
Date: 2015
Authors: Jeong H,Heo J,Son B,Choi D,Park TH,Chang M,Hong J
Journal: ACS Applied Materials and Interfaces

One important aspect of nanotechnology includes thin films capable of being applied to a wide variety of surfaces. Indispensable functions of films include controlled surface energy, stability, and biocompatibility in physiological systems. In this study, we explored the ancient Asian coating material "lacquer" to enhance the physiological and mechanical stability of nanofilms. Lacquer is extracted from the lacquer tree and its main component called urushiol, which is a small molecule that can produce an extremely strong coating. Taking full advantage of layer-by-layer assembly techniques, we successfully fabricated urushiol-based thin films composed of small molecule/polymer multilayers by controlling their molecular interaction. Unique cairnlike nanostructures in this film, produced by urushiol particles, have advantages of intrinsic hydrophobicity and durability against mechanical stimuli at physiological environment. We demonstrated the stability tests as well as the antimicrobial effects of this film.

Two-dimensional germanane and germanane ribbons: Density functional calculation of structural, electronic, optical and transport properties and the role of defects

Product: NanoDCAL
Date: 2016
Authors: Zhao J,Zeng H
Journal: RSC Advances

We have performed first principles calculations combined with non-equilibrium Green's function to study the structural, electronic, optical and transport properties of two-dimensional germanane and germanane ribbons. More importantly, the defect influences on the properties of the germanane-based nanostructures have been investigated. The presence of single hydrogen vacancy induces ferromagnetism to the nonmagnetic pristine germanane according to spontaneous magnetization, while the formation of the dumbbell structure induced by Ge adatom only reduces the electronic band gap. Both H-monovacancy and dumbbell contained defective germanane nanostructures are thermally stable at room temperature. The optical property calculations revealed that the pristine germanane sheet has significant light absorption of the solar spectrum, and the presence of the H-monovacancy and dumbbell defects in the germanane led to redshift and blueshift of the light adsorption peak, respectively. Moreover, both zigzag- and armchair-germanane nanoribbons (zGeNRs and aGeNRs) are nonmagnetic semiconductors with a direct band gap at the $Gamma$-point, and their band gaps are monotonously reduced with increasing width. Our quantum transport calculations have shown different transport behaviors that depend on the GeNRs' edge topology. While the aGeNRs attain a magnetic moment by introducing H monovacancy, it is unlikely to achieve large magnetic moments in germanane via controlling the shape of the H-vacancy cluster since the dehydrogenated nanostructures prefer nonmagnetic characteristics after atomic reconstruction. These calculated results suggest that the germanane has not only suitable transmission gap and light adsorption, but also directionally dependent electron transport, making it an excellent candidate for potential application in the fields of nanoelectronics and optoelectronics.

Chemical substitution assisted ion sensing with organic molecules: A case study of naphthalene

Product: NanoDCAL
Date: 2016
Authors: Min WJ,Hao H,Wang XL,Zheng XH,Zeng Z
Journal: RSC Advances

The chemical modification effects on the electron transport of organic molecules are investigated using first-principles calculations combined with a non-equilibrium Green's function technique, taking the naphthalene (C10H8) molecule as an example. Particularly, one of the -CH groups in each of the benzene rings is replaced by a N atom. It is found that N substitution greatly increases the quantum conductance by ∼80% due to the reduced HOMO (Highest Occupied Molecular Orbital)-LUMO (Lowest Unoccupied Molecular Orbital) gap and the increased charge transfer from the leads to the molecule. More interestingly, the adsorption of different monovalent cations (H+, Li+, Na+, and K+) to the chemically active N site induces a very different electrical response. To be specific, Li+, Na+, and K+ adsorptions result in a very good conducting state, while H+ adsorption gives rise to an insulating state, providing a promising method for H+ sensing or detection.

Spin-dependent Seebeck effects in graphene-based molecular junctions

Product: NanoDCAL
Date: 2016
Authors: Li J,Wang B,Xu F,Wei Y,Wang J
Journal: Physical Review B

We report a first-principles investigation of spin-dependent transport properties in two different graphene-based molecular junctions. By applying different temperatures between two leads without bias voltage, spin-dependent currents are driven which depend on reference temperature T, temperature gradient $Delta$T, and gate voltage Vg. Moreover, pure spin currents without charge currents can be obtained by adjusting T,$Delta$T, and Vg for both molecular junctions. The directions of pure spin currents in these two molecular junctions are opposite, which can be understood by analyzing the transmission coefficients under equilibrium states. Spin thermopower, thermal conductance, and the figure of merit as functions of T,Vg, and chemical potential $mu$ were also investigated in the linear response regime. Large spin thermopower and spin figure of merit can be obtained by adjusting Vg and $mu$ for each junction, which indicates proper application of spin caloritronic devices of our graphene-based molecular junctions.

Enhanced thermoelectric properties of graphene oxide patterned by nanoroads

Product: NanoDCAL
Date: 2016
Authors: Zhou S,Guo Y,Zhao J
Journal: Physical Chemistry Chemical Physics

The thermoelectric properties of two-dimensional (2D) materials are of great interest for both fundamental science and device applications. Graphene oxide (GO), whose physical properties are highly tailorable by chemical and structural modifications, is a potential 2D thermoelectric material. In this report, we pattern nanoroads on GO sheets with epoxide functionalization, and investigate their ballistic thermoelectric transport properties based on density functional theory and the nonequilibrium Green's function method. These graphene oxide nanoroads (GONRDs) are all semiconductors with their band gaps tunable by the road width, edge orientation, and the structure of the GO matrix. These nanostructures show appreciable electrical conductance at certain doping levels and enhanced thermopower of 127-287 $mu$V K-1, yielding a power factor 4-22 times of the graphene value; meanwhile, the lattice thermal conductance is remarkably reduced to 15-22% of the graphene value; consequently, attaining the figure of merit of 0.05-0.75. Our theoretical results are not only helpful for understanding the thermoelectric properties of graphene and its derivatives, but also would guide the theoretical design and experimental fabrication of graphene-based thermoelectric devices of high performance.

Chemically functionalized germanene for spintronic devices: A first-principles study

Product: NanoDCAL
Date: 2016
Authors: Zhao J,Zeng H
Journal: Physical Chemistry Chemical Physics

We have carried out first-principles calculations to explore various chemically functionalized germanene nanomaterials as two-dimensional spintronic devices. The germanene functionalized with O on one side and H on the other side is a ferromagnetic metal, and the phenomenon of negative differential conductance is observed. Moreover, we construct a spin-filter device from it, and about 15% spin filter efficiency is achieved in its ground state by using finite bias. The germanene semi-functionalized with a methyl (-CH3) group is a ferromagnetic semiconductor with a small direct bandgap, and it has highly spin-polarized electronic and transport properties. We proposed that a spin-valve nanodevice with a giant magnetoresistance of up to 107% can be obtained from the semi-methylated germanene nanostructure by introducing an achievable magnetic field to stabilize its metal-like ferromagnetic state. Our findings could be helpful for practical applications of two-dimensional germanane-based nanomaterials in spintronic devices in the future.

Nonequilibrium spin injection in monolayer black phosphorus

Product: NanoDCAL
Date: 2016
Authors: Chen M,Yu Z,Wang Y,Xie Y,Wang J,Guo H
Journal: Physical Chemistry Chemical Physics

Monolayer black phosphorus (MBP) is an interesting emerging electronic material with a direct band gap and relatively high carrier mobility. In this work we report a theoretical investigation of nonequilibrium spin injection and spin-polarized quantum transport in MBP from ferromagnetic Ni contacts, in two-dimensional magnetic tunneling structures. We investigate physical properties such as the spin injection efficiency, the tunnel magnetoresistance ratio, spin-polarized currents, charge currents and transmission coefficients as a function of external bias voltage, for two different device contact structures where MBP is contacted by Ni(111) and by Ni(100). While both structures are predicted to give respectable spin-polarized quantum transport, the Ni(100)/MBP/Ni(100) trilayer has the superior properties where the spin injection and magnetoresistance ratio maintains almost a constant value against the bias voltage. The nonequilibrium quantum transport phenomenon is understood by analyzing the transmission spectrum at nonequilibrium.

Photoinduced valley-polarized current of layered MoS2 by electric tuning

Product: NanoDCAL
Date: 2016
Authors: Yu Y,Zhou Y,Wan L,Wang B,Xu F,Wei Y,Wang J
Journal: Nanotechnology

A photoinduced current of a layered MoS2-based transistor is studied from first-principles. Under the illumination of circular polarized light, a valley-polarized current is generated, which can be tuned by the gate voltage. For monolayer MoS2, the valley-polarized spin-up (down) electron current at K () points is induced by the right (left) circular polarized light. The valley polarization is found to reach +1.0 (-1.0) for the valley current that carried such a K () index. For bilayer MoS2, the spin-up (down) current can be induced at both K and valleys by the right (left) circular light. In contrast to monolayer MoS2, the photoinduced valley polarization shows asymmetric behavior upon reversal of the gate voltage. Our results show that the valley polarization of the photoinduced current can be modulated by the circular polarized light and the gate voltage. All the results can be well understood using a simple kp model.

Carbon-based molecular devices: Fano effects controlled by the molecule length and the gate voltage

Product: NanoDCAL
Date: 2016
Authors: Yang XF,Kuang YW,Liu YS,Zhang DB,Shao ZG,Yu HL,Hong XK,Feng JF,Chen XS,Wang XF
Journal: Nanoscale

Fano effect is an important quantum phenomenon in mesoscopic systems, which arises from an interference between the localized state and the extended state. Here we observe an obvious Fano effect near the Fermi level in an all-carbon molecular device consisting of an acene molecule sandwiched between two zigzag graphene nanoribbon (ZGNR) electrodes. By increasing the length of the molecule, an extended state gradually evolves into a localized state. With the aid of the nearby extended state, a Fano effect is achieved. Using a gate voltage, we can easily tune the Fano effect induced by the single-transmission channel. When the spin degree of freedom is involved, the all-carbon device can show a half-metallic property with positive or negative 100% spin polarization at the Fermi level under the gate voltage; meanwhile the spin thermoelectric effect can also be enhanced.

Giant tunnel magneto-resistance in graphene based molecular tunneling junction

Product: NanoDCAL
Date: 2016
Authors: Wang B,Li J,Yu Y,Wei Y,Wang J,Guo H
Journal: Nanoscale

We propose and theoretically investigate a class of stable zigzag graphene nanoribbon (ZGNR) based molecular magnetic tunneling junctions (MTJs). For those junctions having pentagon-connecting formations, huge tunnel magneto-resistance (TMR) is found. Different from most of the other proposed molecular junctions, the huge TMR in our structures is generic, and is not significantly affected by external parameters such as bias voltage, gate voltage, length of the molecule and width of the ZGNRs. The double pentagon-connecting formation between the molecule and ZGNRs is critical for the remarkable TMR ratio, which is as large as ∼2 × 105. These molecular MTJs behave as almost perfect spin filters and spin valve devices. Other connecting formations of the ZGNR based MTJs lead to much smaller TMR. By first principles analysis, we reveal the microscopic physics responsible for this phenomenon.

Negative differential resistance in GeSi core-shell transport junctions: The role of local sp2 hybridization

Product: NanoDCAL
Date: 2016
Authors: Liu N,Zhang L,Chen X,Kong X,Zheng X,Guo H
Journal: Nanoscale

We report a theoretical investigation of nonlinear quantum transport properties of Au/GeSi/Au junctions. For GeSi semiconducting core-shell structures brought into contact with Au electrodes, a very unusual behavior is that the tunneling transport is on-resonance right at equilibrium. This resonance is not due to the alignment of a quantum level in GeSi to the electrochemical potential of Au, but due to the alignment of very sharp DOS features-hot spots, localized at the two Au/GeSi interfaces of the device. An applied bias voltage shifts the hot spots relative to each other which gives rise to substantial negative differential resistance (NDR). The hot spots localized at the two interfaces were found to be due to the unbonded pz orbital of a sp2 hybridized interface Si atom which is surrounded by three non-sp2 hybridized neighbors. The mechanism of inducing hot spots and NDR by a local structure unit is not limited to the GeSi. The results suggest an interesting scheme for constructing NDR devices by orbital manipulation, to be more explicit, for example, by designing local structural units having unbonded orbitals at the interfaces between electrodes and the central region of the transport junction.

Orientation Dependence of Electromechanical Characteristics of Defect-free InAs Nanowires

Product: NanoDCAL
Date: 2016
Authors: Zheng K,Zhang Z,Hu Y,Chen P,Lu W,Drennan J,Han X,Zou J
Journal: Nano Letters

Understanding the electrical properties of defect-free nanowires with different structures and their responses under deformation are essential for design and applications of nanodevices and strain engineering. In this study, defect-free zinc-blende- and wurtzite-structured InAs nanowires were grown using molecular beam epitaxy, and individual nanowires with different structures and orientations were carefully selected and their electrical properties and electromechanical responses were investigated using an electrical probing system inside a transmission electron microscope. Through our careful experimental design and detailed analyses, we uncovered several extraordinary physical phenomena, such as the electromechanical characteristics are dominated by the nanowire orientation, rather than its crystal structure. Our results provide critical insights into different responses induced by deformation of InAs with different structures, which is important for nanowire-based devices.

Biobased polymer coating using catechol derivative urushiol

Product: NanoDCAL
Date: 2016
Authors: Watanabe H,Fujimoto A,Nishida J,Ohishi T,Takahara A
Journal: Langmuir

We have investigated the mechanism of the superior mechanical robustness of coated thin films of the catechol derivative urushiol. We synthesized hydrogenated urushiol (h-urushiol) by hydrogenating the double bonds in the long alkyl side chain of urushiol, and the physical properties of thin films of mixtures of urushiol and h-urushiol were evaluated. Atomic force microscopy observations revealed that these coated thin films have a homogeneous surface with no phase separation, regardless of the h-urushiol content, arising from the similarity of the chemical structures. The films showed good adhesive properties because the adhesion originates from the catechol structure. In contrast, curing time depended strongly upon the h-urushiol content. The curing of the h-urushiol thin film took 12 h, whereas the urushiol thin film was cured within 10 min. Moreover, the strain-induced elastic buckling instability for mechanical measurements test and the bulge test confirmed that the increase in the h-urushiol content decreased the mechanical strength. Because the double bonds in the urushiol side chain contribute to forming the highly cross-linked structure, the lack of double bonds in hurushiol resulted in the slow curing and low mechanical strength. Interestingly, the mechanical robustness started to increase over 80 mol % h-urushiol. The saturated long alkyl side chain of h-urushiol faced the surface, and the regular structure of the uniform side chain may improve the mechanical properties of the coated film. Our results will help to develop biomimetic catechol-based coatings.

Tunable Thermal Conductivity of Silicene by Germanium Doping

Product: NanoDCAL
Date: 2016
Authors: Guo Y,Zhou S,Bai Y,Zhao J
Journal: Journal of Superconductivity and Novel Magnetism

Silicene possesses excellent electronic properties and low thermal conductivity and hence is a potential material for thermoelectric applications. The key to improve the thermoelectric efficiency of silicene relies on opening a bandgap to enhance the thermopower and suppressing the lattice thermal conductivity. Based on first-principle calculations, we propose germanium doping as an effective way to tailor the thermal conductivity of silicene. The electronic transport properties of silicene is not affected by Ge doping, while the room-temperature thermal conductivity is significantly reduced by 62 % for a doping concentration of 6 %. The depression of phonon transport is attributed to the low-frequency phonon softening and enhanced phonon scattering by Ge doping. Our theoretical results will be beneficial for experimental modulating the thermal and thermoelectric properties of silicene and many other two-dimensional materials.

Strain-Enhanced Spin Injection in Amine-Ended Single-Molecule Magnetic Junctions

Product: NanoDCAL
Date: 2016
Authors: Tang YH,Lin CJ
Journal: Journal of Physical Chemistry C

The first-principles calculation with the nonequilibrium Green's function formalism is employed to comprehensively demonstrate that the mechanical strain and anchoring group are two crucial impacts on spin transport in single-molecule magnetic junctions. For the dissociated amine-ended benzene contacted to cobalt electrodes, we present the strain-enhanced spin injection efficiency, including sign reversal and nearly perfect spin injection under junction stretching process. The underlying mechanism is the strain-assisted movement of pronounced and broad spin-up transmission feature toward the Fermi energy. This intriguing finding reveals the superior spin transfer in amine-ended single-molecule magnetic junction, which is in sharp contrast to the better charge transfer between gold electrodes in traditional thiol-ended molecular junctions. Our calculation results may pave the way for promising tunability of spin injection efficiency under mechanical stimulus of break junction technique.

Two-Dimensional $gamma$-Graphyne Suspended on Si(111): A Hybrid Device

Product: NanoDCAL
Date: 2016
Authors: Saraiva-Souza A,Smeu M,Zhang L,Ratner MA,Guo H
Journal: Journal of Physical Chemistry C

Graphynes (GYs) are a new class of two-dimensional (2D) carbon allotrope materials that are similar to graphene but with C2 units inserted into certain bonds to produce physical properties distinct from those of graphene. In this work, atomic, electronic, and quantum transport properties of $gamma$-GY-a particular type of graphyne-absorbed on the silicon (111) surface are investigated from atomistic first principles. $gamma$-GY possesses an intrinsic direct band gap, and when interacting with the Si(111), interesting subgap electronic structure is induced to mediate charge transport. In particular, the transmission spectra of the $gamma$-GY/Si(111) hybrid device have a high broad peak at the Fermi level due to hybridization. For $gamma$-GY/Si(111) transport junctions having a finite length trench underneath the $gamma$-GY, substantial charge conduction can still occur through the $gamma$-GY bridge. Nonequilibrium calculations suggest that the hybrid 2D $gamma$-GY/Si(111) transport junction is highly controllable by external voltages.

Conductance Superposition Rule in Carbon Nanowire Junctions with Parallel Paths

Product: NanoDCAL
Date: 2016
Authors: Dou KP,Kaun CC
Journal: Journal of Physical Chemistry C

Using first-principles calculations, we investigate conductance of molecular junctions consisted of single and double polyacene (PA) molecules bridging different carbon nanowire electrodes, including armchair carbon nanotubes (CNT) and zigzag graphene nanoribbons (GNR). Doubling the PA molecule enhances the junction conductance, except in the junction where a molecule contacts armchair CNT with nonvertical edges. Elongating the PA molecules change junction conductance. The different conductance scaling behaviors among various junctions are governed by the interface between the molecule and the electrode, the molecular length, and the edge states of zigzag GNR.

Product: NanoDCAL

Product: NanoDCAL

The electronic structure of organic-inorganic hybrid perovskite solar cell: A first-principles analysis

Product: NanoDCAL
Date: 2016
Authors: Pan YY,Su YH,Hsu CH,Huang LW,Kaun CC
Journal: Computational Materials Science

Using first-principles calculations, we investigate the geometric and electronic structures of organic-inorganic hybrid perovskite, MAPbX3 (MA = CH3NH3+, X = Cl, Br, I), the key materials for the highest efficiency solid-state solar cells. The different halide elements in perovskite compound control the electronic characteristics of the materials, such as their orbitals, density of states and spatial distribution of the charges. We identify the orbitals consisting of the conduction and valence bands. Furthermore, we show that MAPbI3 can produce and transfer more electrons than other hybrid perovskite solar cells do.

Tailoring physical properties of graphene: Effects of hydrogenation, oxidation, and grain boundaries by atomistic simulations

Product: NanoDCAL
Date: 2016
Authors: Liu L,Zhang J,Gao H,Wang L,Jiang X,Zhao J
Journal: Computational Materials Science

Graphene, a two-dimensional monolayer of carbon atoms in honeycomb structure, is a research hotspot in multidisciplinary due to its excellent physical properties. To further extend the applications of graphene, various strategies have been proposed to tailor its physical properties. Recently, our group has carried out systematically computational studies on modifying graphene, including hydrogenation, oxidation, and introduction of grain boundaries. Both the hydrogenation and oxidation will convert sp2 hybridized carbons into sp3 configurations, while formation of grain boundaries only makes the sp2 carbon bonds distorted. Employing density functional theory calculations, structures, physical properties and applications of these modified graphene were explored, such as structural phase diagram, mechanical and electronic properties, and photocatalytic applications. It turns out that many physical properties of graphene are tunable, endowing graphene promising applications in various fields. In this review article, we will generally summarize our recent works on the hydrogenated graphene, graphene oxide, and graphene grain boundaries.

Gate-enhanced thermoelectric effects in all-carbon quantum devices

Product: NanoDCAL
Date: 2016
Authors: Liu YS,Shao XY,Shao T,Zhang JY,Kuang YW,Zhang DB,Shao ZG,Yu HL,Hong XK,Feng JF,Yang XF,Chen XS,Wang XF
Journal: Carbon

The possibility to improve thermoelectric performance of carbon-based quantum devices is of fundamental and importance in the fields of energy conservation, environmental protection, and green energy. Here we propose an effective avenue to enhance the thermoelectric figure of merits (TE-FOMs) of an all-carbon quantum device with the help of first-principles methods, and the device is constructed by a zigzag-edged trigonal graphene (ZTG) connected with zigzag-edged graphene nanoribbons (ZGNR) electrodes through the carbon atomic chains (CACs). Using a gate field, the spin-up transmission peak can be tuned from the position below the Fermi level to that above the Fermi level. However, the position of the spin-down transmission peak above the Fermi level is insensitive to the gate field. Therefore, the device can be converted from the p type to n type for the spin-up component by a gate field, while for the spin-down component the device remains n type. Meanwhile, we also find that the charge (spin) TE-FOMs at the Fermi level can be increased to about eight (three) times as compared with the case in the absence of the gate field. These TE-FOMs can also be significantly improved by tuning the incident electron energy and temperature.

Interface characteristics in Co 2 MnSi/Ag/Co 2 MnSi trilayer

Product: NanoDCAL
Date: 2016
Authors: Li Y,Chen H,Wang G,Yuan H
Journal: Applied Surface Science

Interface characteristics of Co 2 MnSi/Ag/Co 2 MnSi trilayer have been investigated by means of first-principles. The most likely interface is formed by connecting MnSi-termination to the bridge site between two Ag atoms. As annealed at high temperature, the formation of interface DO 3 disorder is most energetically favorable. The spin polarization is reduced by both the interface itself and interface disorder due to the interface state occurs in the minority-spin gap. As a result, the magneto-resistance ratio has a sharp drop based on the estimation of a simplified modeling.

Large influence of capping layers on tunnel magnetoresistance in magnetic tunnel junctions

Product: NanoDCAL
Date: 2016
Authors: Zhou J,Zhao W,Wang Y,Peng S,Qiao J,Su L,Zeng L,Lei N,Liu L,Zhang Y,Bournel A
Journal: Applied Physics Letters

It has been reported in experiments that capping layers, which enhance the perpendicular magnetic anisotropy (PMA) of magnetic tunnel junctions (MTJs), induce a great impact on the tunnel magnetoresistance (TMR). To explore the essential influence caused by the capping layers, we carry out ab initio calculations on TMR in the X(001)|CoFe(001)|MgO(001)|CoFe(001)|X(001) MTJ, where X represents the capping layer material, which can be tungsten, tantalum, or hafnium. We report TMR in different MTJs and demonstrate that tungsten is an ideal candidate for a giant TMR ratio. The transmission spectrum in Brillouin zone is presented. It can be seen that in the parallel condition of MTJ, sharp transmission peaks appear in the minority-spin channel. This phenomenon is attributed to the resonant tunnel transmission effect, and we explained it by the layer-resolved density of states. In order to explore transport properties in MTJs, the density of scattering states was studied from the point of band symmetry. It has been found that CoFe|tungsten interface blocks scattering states transmission in the anti-parallel condition. This work reports TMR and transport properties in MTJs with different capping layers and proves that tungsten is a proper capping layer material, which would benefit the design and optimization of MTJs.

Realizing stable fully spin polarized transport in SiC nanoribbons with dopant

Product: NanoDCAL
Date: 2016
Authors: Tao X,Hao H,Wang X,Zheng X,Zeng Z
Journal: Applied Physics Letters

Intrinsic half-metallicity recently reported in zigzag edged SiC nanoribbons is basically undetectable due to negligible energy difference between the antiferromagnetic (AFM) and ferromagnetic (FM) configurations. In this Letter, by density functional theory calculations, we demonstrate a scheme of N doping at the carbon edge to selectively close the edge state channel at this edge and achieve 100% spin filtering, no matter whether it is in an AFM state or FM state. This turns SiC nanoribbon into a promising material for obtaining stable and completely spin polarized transport and may find application in spintronic devices.

Ferroelectricity and tunneling electroresistance effect driven by asymmetric polar interfaces in all-oxide ferroelectric tunnel junctions

Product: NanoDCAL
Date: 2016
Authors: Tao LL,Wang J
Journal: Applied Physics Letters

By constructing asymmetric polar interfaces, all-oxide ferroelectric tunnel junctions (FTJs) are proposed that can achieve a sizable tunneling electroresistance (TER) effect. Based on first-principles quantum transport calculations on a prototypical LaNiO3/BaTiO3/LaNiO3 junction, we predict that TER reaches 103% under a finite bias. Driven by the asymmetric polar interfaces, the resultant intrinsic electric field causes a highly asymmetric electrostatic potential in comparison to that of the FTJ with symmetric polar interfaces. As a result, the tunneling resistance changes significantly upon polarization reversal leading to sizable TER. The physical origin of the TER effect can be well understood in terms of local density of states, transport in momentum space, real-space scattering states and a free-electron tunneling model. Our results provide an insight into the understanding of ferroelectricity and the TER mechanism in FTJs and will be useful for FTJ-based devices design.

Enhanced tunneling electroresistance in multiferroic tunnel junctions due to the reversible modulation of orbitals overlap

Product: NanoDCAL
Date: 2016
Authors: Jiang L,Tao LL,Yang BS,Wang J,Han XF
Journal: Applied Physics Letters

We report a first-principles study of the ferroelectricity and spin-dependent transport through Co/BaTiO3/CoO/Co multiferroic tunnel junctions (MFTJs). We find the coexistence of large tunneling magnetoresistance (TMR) ratio and large tunneling electroresistance (TER) ratio in the MFTJs. The large TMR effect originates from the spin-filter tunneling through the BaTiO3 barrier, while the TER effect is due to the modulation of orbitals overlap by polarization reversal. The microscopic physics of TER are identified and understood through the analysis of metal-oxygen relative displacements, local polarization magnitude, transmission in momentum space and real space scattering states. Our results provide a practical way to achieve the coexistence of large TER and TMR effects in MFTJs.

Spin-polarized quantum transport properties through flexible phosphorene

Product: NanoDCAL
Date: 2016
Authors: Chen M,Yu Z,Xie Y,Wang Y
Journal: Applied Physics Letters

We report a first-principles study on the tunnel magnetoresistance (TMR) and spin-injection efficiency (SIE) through phosphorene with nickel electrodes under the mechanical tension and bending on the phosphorene region. Both the TMR and SIE are largely improved under these mechanical deformations. For the uniaxial tension (ϵy) varying from 0% to 15% applied along the armchair transport (y-)direction of the phosphorene, the TMR ratio is enhanced with a maximum of 107% at ϵy = 10%, while the SIE increases monotonously from 8% up to 43% with the increasing of the strain. Under the out-of-plane bending, the TMR overall increases from 7% to 50% within the bending ratio of 0%-3.9%, and meanwhile the SIE is largely improved to around 70%, as compared to that (30%) of the flat phosphorene. Such behaviors of the TMR and SIE are mainly affected by the transmission of spin-up electrons in the parallel configuration, which is highly dependent on the applied mechanical tension and bending. Our results indicate that the phosphorene based tunnel junctions have promising applications in flexible electronics.

Origins of Dirac cone formation in AB3 and A3B (A, B = C, Si, and Ge) binary monolayers

Product: NanoDCAL
Date: 2017
Authors: Qin X,Wu Y,Liu Y,Chi B,Li X,Wang Y,Zhao X
Journal: Scientific Reports

Compared to the pure two-dimensional (2D) graphene and silicene, the binary 2D system silagraphenes, consisting of both C and Si atoms, possess more diverse electronic structures depending on their various chemical stoichiometry and arrangement pattern of binary components. By performing calculations with both density functional theory and a Tight-binding model, we elucidated the formation of Dirac cone (DC) band structures in SiC3 and Si3C as well as their analogous binary monolayers including SiGe3, Si3Ge, GeC3, and Ge3C. A "ring coupling" mechanism, referring to the couplings among the six ring atoms, was proposed to explain the origin of DCs in AB3 and A3B binary systems, based on which we discussed the methods tuning the SiC3 systems into self-doped systems. The first-principles quantum transport calculations by non-equilibrium Green's function method combined with density functional theory showed that the electron conductance of SiC3 and Si3C lie between those of graphene and silicene, proportional to the carbon concentrations. Understanding the DC formation mechanism and electronic properties sheds light onto the design principles for novel Fermi Dirac systems used in nanoelectronic devices.

Thermal spin current in zigzag silicene nanoribbons with sp2-sp3 edges

Product: NanoDCAL
Date: 2017
Authors: Jiang P,Tao X,Hao H,Song L,Zheng X,Zeng Z
Journal: RSC Advances

Using first-principles calculations combined with non-equilibrium Green's function method, we study thermal spin transport of zigzag silicene nanoribbons (ZSiNRs) with unsymmetrical sp2-sp3 edges under a temperature gradient but no bias. Both in the linear and non-linear response regimes, we have opposite flow directions for different spins, which leads unambiguously to spin current. Most important is that pure spin current can be achieved and basically no tuning of the chemical potential $mu$ is needed since the neutral point is very close to $mu$ = 0 (the chemical potential located at the Fermi level) and this fact holds for a very large temperature range studied (110 ≤ TL ≤ 300 K). The direction of charge current induced by a temperature gradient can be easily reversed by tuning the chemical potential, while the spin current is almost unchanged in the same process, indicating that the spin current is robust and stable. In addition, both spin current and charge current present a thermoelectric diode behavior for TL ≤ 200 K in the nonlinear response regime. These findings suggest that the unsymmetrically sp2-sp3 terminated ZSiNRs are promising materials for spin caloritronic devices.

Spin thermoelectric effects in organic single-molecule devices

Product: NanoDCAL
Date: 2017
Authors: Wang HL,Wang MX,Qian C,Hong XK,Zhang DB,Liu YS,Yang XF
Journal: Physics Letters, Section A: General, Atomic and Solid State Physics

The spin thermoelectric performance of a polyacetylene chain bridging two zigzag graphene nanoribbons (ZGNRs) is investigated based on first principles method. Two different edge spin arrangements in ZGNRs are considered. For ferromagnetic (FM) ordering, transmission eigenstates with different spin indices distributed below and above Fermi level are observed, leading directly to a strong spin thermoelectric effect in a wide temperature range. With the edge spins arranged in the antiferromagnetic (AFM) ordering, an obvious transport gap appears in the system, which greatly enhances the thermoelectric effects. The presence of a small spin splitting also induces a spin thermoelectric effect greater than the charge thermoelectric effect in certain temperature range. In general, the single-molecule junction exhibits the potential to be used for the design of perfect thermospin devices.

First-principles investigation of transient spin transfer torque in magnetic multilayer systems

Product: NanoDCAL
Date: 2017
Authors: Yu Z,Zhang L,Wang J
Journal: Physical Review B

By employing the nonequilibrium Green's function (NEGF) method, the transient current and the transient behavior of the spin transfer torque (STT) of the magnetic layered system are investigated within the framework of density functional theory (DFT). To reduce the huge computational cost of the transient calculation, especially when the dense mesh of k sampling is present for layered systems, the complex absorbing potential (CAP) and the Padé spectrum decomposition are used so that the energy integrals in calculating transient current and STT can be performed analytically using residue theorem, which dramatically reduces the computational complexity of the first-principles calculation of transient behavior. As an application of the NEGF-DFT-CAP formalism, the transient current and current-induced STT of the Co/Cu/Co trilayer system are studied under an upward bias pulse for different angles of magnetization direction between two leads. The transient current shows a damped oscillatory behavior with the oscillation frequency proportional to the applied bias, leading to a relaxation time of hundreds of femtoseconds. The time-dependent STTs show roughly the same profile for systems with different rotating angles. The oscillation behavior is also observed as the transient STT approaches the steady state value. Such oscillations can be attributed to the interface resonant states.

Zigzag C2N nanoribbons with edge modifications as multi-functional spin devices

Product: NanoDCAL
Date: 2017
Authors: Yang XF,Kuang YW,Yu HL,Shao ZG,Zhang J,Feng JF,Chen XS,Liu YS
Journal: Physical Chemistry Chemical Physics

Recently, a holey two-dimensional (2D) C2N crystal with a wide band gap has been successfully synthesized. However, its non-magnetic property largely limits real applications in spintronics. Here we find that edge magnetism can be introduced by tailoring the holey 2D C2N crystal into nanoribbons with zigzag edges. When edge N atoms are bare or passivated by H atoms, the device can be used to design high-performance thermospin devices and thermal rectifiers. This is ascribed to the emergence of a spin semiconducting property with a wide band gap. Moreover, if the edge N atoms are passivated by O atoms, the device shows a half-metallic property; meanwhile an obvious spin Seebeck effect can also be observed when a temperature difference is applied across the device.

Adjustable localized states in perfect and single C-chain doped zigzag AlN nanoribbons

Product: NanoDCAL
Date: 2017
Authors: Tong L,Chen Z,Li J,Zong H,Zhang J
Journal: Physica Status Solidi (B) Basic Research

The localized feature of electronic states is often related to some interesting electronic properties in graphite-like materials. We use the first principles calculations to investigate two important localized states, that is, flat-band states and border states, in zigzag AlN nanoribbons (zAlNNRs). Our results indicate that both localized states can exist in zAlNNRs and the border states have a close relationship with electrical conductivity. It is found that the flat-band states of perfect zAlNNRs result from edge N-pz orbitals, while the border states of doped zAlNNRs are due to $pi$/$pi$* orbitals of C and Al/N atoms. These findings enrich our understanding on localized states in zAlNNRs, showing a potential application in functional nanodevices.

Strain controlled switching effects in phosphorene and GeS

Product: NanoDCAL
Date: 2017
Authors: Li BW,Wang Y,Xie YQ,Zhu L,Yao KL
Journal: Nanotechnology

By performing first principles calculations within the combined approach of density functional theory and nonequilibrium Green's function technique, we have designed some nanoelectronic devices to explore the ferroelastic switching of phosphorene and phosphorene analogs GeS. With the structure swapping along the zigzag direction and armchair direction, band gap transformed at different states due to their anisotropic phosphorene-like structure. From the initial state to the middle state, the band gap becomes progressively smaller, after that, it becomes wide. By analyzing transmission coefficients, it is found that the transport properties of phosphorene and GeS can be controlled by a uniaxial strain. The results also manifest that GeS has great potential to fabricate ferroic nonvolatile memory devices, because its relatively high on/off transmission coefficient ratio (∼1000) between the two stable ferroelastic states.

A light-driven modulation of electric conductance through the adsorption of azobenzene onto silicon-doped- and pyridine-like N3-vacancy graphene

Product: NanoDCAL
Date: 2017
Authors: Zhao J,Liu C,Ma J
Journal: Nanoscale

The ability to modulate the conductance of an electronic device under light irradiation is crucial to the practical applications of nanoscale electronics. Density functional theory calculations predict that the conductance of the photo-responsive graphene-based nanocomposites can be tuned through the noncovalent adsorption of an azobenzene (AB) derivative onto pristine, Si-doped, and pyridine-like N3-vacancy graphene. AB@graphene systems were found to exhibit a visible-light response within the low-frequency region, rendering the trans-to-cis isomerizations of these nanocomposites under the irradiation of solar light. The excellent solar light absorption performances of these hybrids can then be used to modulate the conductance of both N3-vacancy- and Si-doped-graphene AB hybrids effectively through the reversible change of the effective conjugate length of the AB molecule in the photoisomerization. In addition, the solar thermal energy up to 1.53 eV per AB molecule can be stored in the designed nanocomposites with the doped graphene. These findings provide clues for making multifunctional materials with potential applications as both optically controlled nanoelectronics and solar energy storage devices.

Giant magnetoresistance and perfect spin filter effects in manganese phthalocyanine based molecular junctions

Product: NanoDCAL
Date: 2017
Authors: Tao LL,Wang J
Journal: Nanoscale

The spin-filter transport and magnetoresistance effects are of particular interest in the field of molecular spintronics. In this work, based on first-principles quantum transport calculations, we report on the spin-dependent transport properties of a molecular junction made of two manganese phthalocyanine (MnPc) molecules linked by single-walled carbon nanotubes. Owing to the half-metallicity of MnPc around the Fermi energy, a perfect spin-filter effect and a giant magnetoresistance effect are observed in the molecular junction. The current-voltage characteristics show nearly ohmic behavior for the junction in an anti-parallel magnetic configuration, while a very low-bias negative differential resistance effect is observed for the junction in a parallel magnetic configuration. The results are well understood from the analysis of molecular frontier orbitals, scattering states and transmission spectra. Our results provide some fundamental understanding of spin-dependent transport in molecular junctions that are useful for the design of future spintronic devices.

Spin filter and negative differential resistance in carbon-based device with defects

Product: NanoDCAL
Date: 2017
Authors: Gong ZH,Xia TS,Wang YX
Journal: Key Engineering Materials

In this work, we report the electronic transport properties of an atomic carbon chain sandwiched between two ferromagnetic zigzag graphene nanoribbon electrodes with symmetrical nitrogen-vacancy defects using the density functional theory combining with the non-equilibrium Green's function method. Our results show that a perfect spin filter is observed with almost 100% spin polarization. Moreover, we also see the negative differential resistance effect from the spin-up current under a low positive voltage bias. These results may promise potential applications in spintronic devices with multi-function in the future.

Electronic structures and magneto-transport properties of co-based Heusler alloy based magneto-resistance junctions

Product: NanoDCAL
Date: 2017
Authors: Li Y
Journal: Journal of Shanghai Jiaotong University (Science)

A direct link between band structure and the ballistic transport property of full-Heusler alloys based Co2YZ/Al/Co2YZ trilayers (Y = Sc, Ti, V, Cr, Mn and Fe; Z = Al, Si and Ge) has been studied by firstprinciples calculations. It is found that the transport efficiency is determined primarily by three factors related to band structure: the shape of the band crossing Fermi energy EF, the distance d of the two intersection points of Co2YZ and Al at EF, and the absolute maximum of the energy lying in the EF-crossing band, |Emax|. The transmission coefficient distribution patterns imply that the affected factor of magneto-resistance (MR) ratio is attributed to the band features around EF. In general, an intuitively illustrated diagram is proposed to clarify the relationship between the probability of electron transition and the current magnitude.

The tunneling magnetoresistance and spin-polarized optoelectronic properties of graphyne-based molecular magnetic tunnel junctions

Product: NanoDCAL
Date: 2017
Authors: Yang Z,Ouyang B,Lan G,Xu LC,Liu R,Liu X
Journal: Journal of Physics D: Applied Physics

Using density functional theory and the non-equilibrium Green's function method, we investigate the spin-dependent transport and optoelectronic properties of the graphyne-based molecular magnetic tunnel junctions (MMTJs). We find that these MMTJs exhibit an outstanding tunneling magnetoresistance (TMR) effect. The TMR value is as high as 106%. When the magnetization directions of two electrodes are antiparallel under positive or negative bias voltages, two kinds of pure spin currents can be obtained in the systems. Furthermore, under the irradiation of infrared, visible or ultraviolet light, spin-polarized photocurrents can be generated in the MMTJs, but the corresponding microscopic mechanisms are different. More importantly, if the magnetization directions of two electrodes are antiparallel, the photocurrents with different spins are spatially separated, appearing at different electrodes. This phenomenon provides a new way to simultaneously generate two spin currents.

Effect of molybdenum disulfide nanoribbon on quantum transport of graphene

Product: NanoDCAL
Date: 2017
Authors: Gao G,Li Z,Chen M,Xie Y,Wang Y
Journal: Journal of Physics Condensed Matter

Based on the density functional theory method in combination with the nonequilibrium green's function formalism, the quantum transport properties in graphene-MoS2 vertical heterojunction were investigated in this work. The leads are boron doped graphene and seamlessly connect to the graphene nanoribbon in central scattering region. Although there is a weak graphene-MoS2 interaction, molybdenum disulfide can smooth the electrostatic potential and enlarge the transport properties of the whole device. However, another competitive factor is that of the edge states of the MoS2 nanoribbon. When the transport is along the zigzag direction of graphene, the armchair MoS2 nanoribbon simply enlarges the transmission coefficient. Nevertheless, in the armchair transport system, there is an asymmetric electrostatic potential induced by the different atomic potentials of S and Mo atoms at both edges in the zigzag MoS2 nanoribbon, whose potential can lead to obvious scattering from graphene to MoS2 and suppress the transmission probability. Therefore, it also suppresses the influence of zigzag MoS2 nanoribbon on the transmission coefficient. Our first principles simulations provide useful predictions for the application of graphene based emerging electronics, which may stimulate further experimental exploration.

Tuning the electronic and quantum transport properties of nitrogenated holey graphene nanoribbons

Product: NanoDCAL
Date: 2017
Authors: Saraiva-Souza A,Smeu M,Da Silva Filho JG,Girão EC,Guo H
Journal: Journal of Materials Chemistry C

Recently, a new semiconductor two-dimensional (2D) material, namely, holey nitrogenated graphene 2D crystal (C2N-h2D), has been fabricated by using a bottom-up wet-chemical reaction. Using first-principles density functional theory (DFT) combined with the non-equilibrium Green's function (NEGF) technique, we investigate the atomic, electronic and quantum transport properties of porous C2N nanoribbons having both zigzag- and armchair-terminated edges. The zigzag C2N-h nanoribbons (ZC2N-hNRs) are semiconductors with an indirect band gap that decreases as the ribbon width increases. Meanwhile, the armchair C2N-h nanoribbons (AC2N-hNRs) show a metallic behavior for all ribbon widths, except for one of the candidates considered in this study, which presents a small band gap (0.14 eV). Interestingly, non-equilibrium calculations suggest that these structures display edge-dependent electronic transport properties where the armchair C2N-hNRs show a strong negative differential resistance (NDR) behavior with current peak-to-valley ratios that remarkably increase with increasing ribbon width, and non-linear current-voltage characteristics were found for the zigzag C2N-hNRs.

Hard-hard coupling assisted anomalous magnetoresistance effect in amine-ended single-molecule magnetic junction

Product: NanoDCAL
Date: 2017
Authors: Tang YH,Lin CJ,Chiang KR
Journal: Journal of Chemical Physics

We proposed a single-molecule magnetic junction (SMMJ), composed of a dissociated amine-ended benzene sandwiched between two Co tip-like nanowires. To better simulate the break junction technique for real SMMJs, the first-principles calculation associated with the hard-hard coupling between a amine-linker and Co tip-atom is carried out for SMMJs with mechanical strain and under an external bias. We predict an anomalous magnetoresistance (MR) effect, including strain-induced sign reversal and bias-induced enhancement of the MR value, which is in sharp contrast to the normal MR effect in conventional magnetic tunnel junctions. The underlying mechanism is the interplay between four spin-polarized currents in parallel and anti-parallel magnetic configurations, originated from the pronounced spin-up transmission feature in the parallel case and spiky transmission peaks in other three spin-polarized channels. These intriguing findings may open a new arena in which magnetotransport and hard-hard coupling are closely coupled in SMMJs and can be dually controlled either via mechanical strain or by an external bias.

Doping cuprous oxide with fluorine and its band gap narrowing

Product: NanoDCAL
Date: 2017
Authors: Ye F,Zeng JJ,Cai XM,Su XQ,Wang B,Wang H,Roy VA,Tian XQ,Li JW,Zhang DP,Fan P,Zhang J
Journal: Journal of Alloys and Compounds

Phase-pure cuprous oxide (Cu2O) thin films doped with Fluorine (F) have been prepared under thermal diffusion at diffusion temperatures of 1123 K and 1223 K and it is found that higher diffusion temperature leads to larger grain size. F-doping slightly reduces the lattice constant and F-doped Cu2O thin films exhibit p-type semiconductor characteristics. The reduction of band gap occurs due to F-doping induced impurity band, because F-doped samples have larger Urbach tails than that of undoped samples. Theoretical calculation demonstrates that substitutional F-doping makes Cu2O almost metallic because the energy bands of F atoms enter the forbidden gap, and interstitial F-doping narrows the band gap because F atoms contribute to the valence bands. The doped F atoms are very possibly interstial and play the role of acceptors in Cu2O. Phase-pure Cu2O doped with F have smaller resistivity and larger hole concentration, implying potential application in solar cells.

High Tunnel Magnetoresistance in Mo/CoFe/MgO Magnetic Tunnel Junction: A First-Principles Study

Product: NanoDCAL
Date: 2017
Authors: Zhou J,Zhao W,Peng S,Qiao J,Klein JO,Lin X,Zhang Y,Bournel A
Journal: IEEE Transactions on Magnetics

The tunnel magnetoresistance (TMR) ratio in a magnetic tunnel junction (MTJ) is influenced by heavy metal capping layer due to the interfacial effect. We report a systematic first-principles study on MTJ based on CoFe/MgO with capping layer, demonstrate that TMR ratios are sensitive to capping layer material, and show that TMR in Mo-capped MTJ is three times as high as that in Ta-capped MTJ. Besides, TMR in Mo-capped MTJ remains high at finite voltage bias. By analyzing the transmission spectrum and density of scattering states, we found that coherent transmission of $Delta$ 1 state dominates the majority-spin conductance in Mo-capped MTJ, while the resonant tunneling contributes significantly in Ta-capped MTJ. The evolution of TMR for varying MgO and CoFe thickness in Mo-capped MTJ is presented. TMR oscillates as a function of CoFe thickness because of the confined wave function in ferromagnetic layer, while TMR rises with MgO thickness increasing due to the enhanced filtering effect of MgO. This work clarifies the physical mechanism on high TMR in Mo-capped MTJ, which is promising to benefit the design of spintronics device.

First-principles study on the electronic and transport properties of periodically nitrogen-doped graphene and carbon nanotube superlattices

Product: NanoDCAL
Date: 2017
Authors: Xu F,Yu Z,Gong Z,Jin H
Journal: Frontiers of Physics

Prompted by recent reports on 3×3 graphene superlattices with intrinsic inter-valley interactions, we perform first-principles calculations to investigate the electronic properties of periodically nitrogen-doped graphene and carbon nanotube nanostructures. In these structures, nitrogen atoms substitute one-sixth p of the carbon atoms in the pristine hexagonal lattices with exact periodicity to form perfect 3×3 superlattices of graphene and carbon nanotubes. Multiple nanostructures of 3×3 graphene ribbons and carbon nanotubes are explored, and all configurations show nonmagnetic and metallic behaviors. The transport properties of 3×3 graphene and carbon nanotube superlattices are calculated utilizing the non-equilibrium Green's function formalism combined with density functional theory. The transmission spectrum through the pristine and 3×3 armchair carbon nanotube heterostructure shows quantized behavior under certain circumstances.

Tunable band structure and effective mass of disordered chalcopyrite

Product: NanoDCAL
Date: 2017
Authors: Wang ZL,Xie WH,Zhao YH
Journal: Frontiers of Physics

The band structure and effective mass of disordered chalcopyrite photovoltaic materials Cu1-xAgxGaX2 (X = S, Se) are investigated by density functional theory. Special quasirandom structures are used to mimic local atomic disorders at Cu/Ag sites. A local density plus correction method is adopted to obtain correct semiconductor band gaps for all compounds. The bandgap anomaly can be seen for both sulfides and selenides, where the gap values of Ag compounds are larger than those of Cu compounds. Band gaps can be modulated from 1.63 to 1.78 eV for Cu1-xAgxGaSe2, and from 2.33 to 2.64 eV for Cu1-xAgxGaS2. The band gap minima and maxima occur at around x = 0:5 and x = 1, respectively, for both sulfides and selenides. In order to show the transport properties of Cu1-xAgxGaX2, the effective mass is shown as a function of disordered Ag concentration. Finally, detailed band structures are shown to clarify the phonon momentum needed by the fundamental indirect-gap transitions. These results should be helpful in designing high-efficiency photovoltaic devices, with both better absorption and high mobility, by Ag-doping in CuGaX2.

Spin filtering in transition-metal phthalocyanine molecules from first principles

Product: NanoDCAL
Date: 2017
Authors: Niu L,Wang H,Bai L,Rong X,Liu X,Li H,Yin H
Journal: Frontiers of Physics

Using first-principles calculations based on density functional theory and the nonequilibrium Green's function formalism, we studied the spin transport through metal-phthalocyanine (MPc, M=Ni, Fe, Co, Mn, Cr) molecules connected to aurum nanowire electrodes. We found that the MnPc, FePc, and CrPc molecular devices exhibit a perfect spin filtering effect compared to CoPc and NiPc. Moreover, negative differential resistance appears in FePc molecular devices. The transmission coefficients at different bias voltages were further presented to understand this phenomenon. These results would be useful in designing devices for future nanotechnology.

Spin-resolved quantum transport in graphene-based nanojunctions

Product: NanoDCAL
Date: 2017
Authors: Li JW,Wang B,Yu YJ,Wei YD,Yu ZZ,Wang Y
Journal: Frontiers of Physics

First-principles calculations were performed to explore the spin-resolved electronic and thermoelectric transport properties of a series of graphene-nanoribbon-based nanojunctions. By flipping the magnetic moments in graphene leads from parallel to antiparallel, very large tunneling magnetoresistance can be obtained under different gate voltages for all the structures. Spin-resolved alternating-current conductance increases versus frequency for the short nanojunctions but decreases for the long nanojunctions. With increasing junction length, the behavior of the junctions changes from capacitive-like to inductive-like. Because of the opposite signs of spin-up thermopower and spin-down thermopower near the Fermi level, pure spin currents can be obtained and large figures of merit can be achieved by adjusting the gate voltage and chemical potential for all the nanojunctions.

The magnetoresistance effect and spin-polarized photocurrent of zigzag graphene-graphyne nanoribbon heterojunctions

Product: NanoDCAL
Date: 2017
Authors: Li Y,Ma Z,Song X,Yang Z,Xu LC,Liu R,Li X,Liu X,Hu D
Journal: Computational Materials Science

Using density functional theory and non-equilibrium Green's function method, we designed several zigzag graphene-graphyne nanoribbon heterojunction devices and investigated their spin-dependent transport and optoelectronic properties. Our results show that the heterojunctions have outstanding giant magnetoresistance (GMR) effect. The GMR value is as high as 106%. According to the symmetry and connection way, for the ferromagnetic states the heterojunctions may produce two spin currents and exhibit significant rectification behaviors. Furthermore, spin-polarized photocurrents can be generated by irradiating the devices with infrared, visible or ultraviolet light. More importantly, in these heterojunctions we found that the behavior of the spin-up (spin-down) photocurrent for the ferromagnetic state is similar to that of the spin-down (spin-up) photocurrent for the antiferromagnetic state. This novel effect provides an efficient way to control the spin transport in the systems.

Spin-filter effect and spin-polarized optoelectronic properties in annulene-based molecular spintronic devices.

Product: NanoDCAL
Date: 2017
Authors: Ma Z,Li Y,Song XJ,Yang Z,Xu LC,Liu R,Liu X,Hu D
Journal: Chinese Physics B

Using Fe, Co or Ni chains as electrodes, we designed several annulene-based molecular spintronic devices and investigated the quantum transport properties based on density functional theory and non-equilibrium Greens function method. Our results show that these devices have outstanding spin-filter capabilities and exhibit giant magnetoresistance effect, and that with Ni chains as electrodes, the device has the best transport properties. Furthermore, we investigated the spinpolarized optoelectronic properties of the device with Ni electrodes and found that the spin-polarized photocurrents can be directly generated by irradiating the device with infrared, visible or ultraviolet light. More importantly, if the magnetization directions of the two electrodes are antiparallel, the photocurrents with different spins are spatially separated, appearing at different electrodes. This phenomenon provides a new way to simultaneously generate two spin currents.

Adatom-induced local reconstructions in zigzag silicene nanoribbons: Spin semiconducting properties and large spin thermopowers

Product: NanoDCAL
Date: 2017
Authors: Yang XF,Zou XL,Kuang YW,Shao ZG,Zhang J,Hong XK,Zhang DB,Feng JF,Chen XS,Liu YS
Journal: Chemical Physics Letters

Using first-principles methods, we have investigated magnetic properties and thermospin effects of zigzag silicene nanoribbons (ZSiNRs) absorbed by a single Si atom. After a relaxation, a steady dumbbell-like structure is formed, which induces a weaker antiferromagnetic (AFM) coupling between two zigzag edges. Therefore, a band gap is opened, meanwhile the adsorbed ZSiNRs show a spin semiconducting property. A large spin thermopower and weak charge thermopower in adsorbed ZSiNR-based devices can be simultaneously achieved, which is attributed to a nearly perfect mirror symmetry of spin-up and spin-down transmission spectra relative to the Fermi level.

Silicene spintronics: Fe(111)/silicene system for efficient spin injection

Product: NanoDCAL
Date: 2017
Authors: Zhou J,Bournel A,Wang Y,Lin X,Zhang Y,Zhao W
Journal: Applied Physics Letters

Silicene is an emerging 2D material with advantages of high carrier mobility, compatibility with the silicon-based semiconductor industry, and the tunable gap by a vertical electrical field due to the buckling structure. In this work, we report a first-principles investigation on the spin injection system, which consists of a Fe(111)/silicene stack as the spin injector and pure silicene as the spin channel. An extremely high spin injection efficiency (SIE) close to 100% is achieved. The partial density of states of Fe layers in the Fe(111)/silicene stack shows that spin-down states dominate above the Fermi level, resulting in a negligible spin-up current and high SIE. The transmission spectra have been investigated to analyze the spin-resolved properties. The spin injection system based on silicene is promising for the efficient silicon-based spintronics devices such as switching transistors.

First-principles study of MnAl for its application in MgO-based perpendicular magnetic tunnel junctions

Product: NanoDCAL
Date: 2017
Authors: Zhang X,Tao LL,Zhang J,Liang SH,Jiang L,Han XF
Journal: Applied Physics Letters

MnAl, as a prospective candidate of magnetic electrode materials for MgO-based magnetic tunnel junctions, possesses several advantages including the spin polarized $Delta$ 1 band, relatively low Gilbert damping factor, and large perpendicular magnetic anisotropy. Here, we report a thorough first-principles investigation on MnAl/MgO/MnAl-magnetic tunnel junctions (MTJs). It is found that the bulk anisotropy density is 17.39 Merg/cm3, while the interfacial anisotropy contribution is evaluated to be 0.12 erg/cm2 and 0.44 erg/cm2 for Mn- and Al-terminated structures, respectively. The large anisotropy can be attributed to dyz and d z 2 orbits. Furthermore, the formation of a Mn-O bond on the interface of MnAl/MgO is shown to be detrimental for the improvement of perpendicular anisotropy. On the other hand, a giant zero-bias tunneling magnetoresistance ratio is predicted and can be maintained over 2000% even for a bias up to 0.6 V for Mn-terminated MTJs. The in-plane spin transfer torque for Mn-terminated MTJs increases linearly with a bias up to 0.6 V due to the large net spin-polarized current. This work paves the way for the further application of MnAl-based perpendicular magnetic tunnel junctions.

Spin transport through a junction entirely consisting of molecules from first principles

Product: NanoDCAL
Date: 2017
Authors: Wang H,Zhou J,Liu X,Yao C,Li H,Niu L,Wang Y,Yin H
Journal: Applied Physics Letters

Using first-principles calculations based on density functional theory combined with the nonequilibrium Green's function formalism, we studied the spin transport through a single molecular junction which consists of a single 1,4-benzenedithiolate (BDT) molecule and two ferromagnetic electrodes [(Ge5)Fe]∞. A large magnetoresistance ratio (MR) of 21100% was found in the [(Ge5)Fe]∞-BDT-[(Ge5)Fe]∞ molecular junction at small bias voltage, and the MR value decreased with the increase in the applied bias voltage. For the parallel magnetization configuration, the molecular junction showed outstanding spin injection effects. Negative differential resistance was observed for the antiparallel magnetization configuration. Spin dependent transmission spectra at different bias voltages were used to explain the calculated results.

Perfect spin and valley polarized quantum transport in twisted SiC nanoribbons

Product: NanoDCAL
Date: 2017
Authors: Zheng X,Chen X,Zhang L,Xiao L,Jia S,Zeng Z,Guo H
Journal: 2D Materials

The edge magnetism of zigzag SiC nanoribbons in its ground state is half-metallic, but is competed by a non-half-metal state that is energetically extremely close. In this work, we propose and theoretically analyze two-probe transport junctions that overcome this difficulty so that perfect half-metal quantum transport is realized. When two zigzag SiC nanoribbons are connected by a C-Si-C-Si tetramer where the right ribbon is turned by 180° around the transport direction, 100% spin polarization with nearly perfect transmission is obtained due to perfect momentum k-matching across the transport junction. We show such a transport to be independent of the magnetic configurations of the two ribbons. A concomitant property is that 100% valley polarized charge transport is achieved. In principle, the structure of the proposed transport junction turns SiC nanoribbons into a promising, robust, and essentially perfect spin and valley filter.

Ohmic contact in monolayer InSe-metal interface

Product: NanoDCAL
Date: 2017
Authors: Jin H,Li J,Wan L,Dai Y,Wei Y,Guo H
Journal: 2D Materials

As conventional Si-based devices approach their scaling limit, it is of great significance to find new materials for future electronic logic devices. The emerging two-dimensional (2D) materials with atomic thickness have attracted intense interests for their exotic properties. However, the presence of the Schottky barrier limits their applications, which is difficult to control over due to the Fermi level pinning effect. Therefore, searching for low resistance metal contact to 2D semiconductors becomes one of the most important topics. Here, we report that Ohmic contact can be realized in a monolayer InSe–Cu system. Based on the density functional theory combined with the nonequilibrium Green's functions, the geometry, overlapping states, tunneling barrier, Schottky barrier, and band alignment at the interface of group-IB (Cu, Ag, and Au) with InSe monolayer are discussed in details. Our results reveal that Cu, the most common electrode used in the industry, shows great potential to form favorable contact with single layer InSe due to the strong interaction and high orbital overlapping. The calculated drain-source current versus bias voltage (I − V) curve exhibits linear behavior, indicating good Ohmic contact between the Cu electrodes and InSe channel. Our work may pave the way for design of next-generation ultrathin and flexible devices.

Tuning a zigzag SiC nanoribbon as a thermal spin current generator

Product: NanoDCAL
Date: 2017
Authors: Jiang P,Tao X,Hao H,Song L,Zheng X,Zhang L,Zeng Z
Journal: 2D Materials

Quantum transport and spin current in a zigzag SiC nanoribbon device under a thermal gradient are investigated theoretically within the framework of the Landauer–Büttiker formalism using a firstprinciples technique. It is found that the edge state transport channels can be turned off or kept open by specific edge doping, and different spin channels can be controlled separately. Interestingly, by replacing an edge C atom with a B atom and an edge Si atom with a P atom in the scattering region, a Seebeck thermopower with different signs for different spins and a finite conductance for both spins can be obtained in the linear response regime. The subsequent thermoelectric field drives electrons of different spin channels in opposite directions, which leads unambiguously to a spin current. More importantly, by tuning the chemical potential and working temperature, pure spin current can be achieved. This provides a promising twodimensional candidate system for producing pure spin current via the spindependent Seebeck effect.

First-principles study on transport property of molecular device with non-collinear electrodes

Product: NanoDCAL
Date: 2018
Authors: Yan R,Wu ZW,Xie WZ,Li D,Wang Y
Journal: Wuli Xuebao/Acta Physica Sinica

Molecular device is the ultimate electronic devices in the view point sense of scale size. Electron transport in molecular device shows obvious quantum effect, and the transport property of molecular device will be strongly affected by the chemical and structural details, including the contact position and method between the molecule and electrodes, the angle between two electrodes connecting to the molecule. However, we notice that in the existing reports on device simulations from first principles the two electrodes are always in a collinear case. Even for multi-electrode simulations, one usually used to adopt orthogonal electrodes, namely, each pair of the electrodes is in a collinear case. As the electrode configuration will clearly affect the transport property of a device on a nanometer scale, the first principles quantum transport studies with non-collinear electrodes are of great importance, but have not been reported yet. In this paper, we demonstrate the calculations of a transport system with non-collinear electrodes based on the state-of-the-art theoretical approach where the density functional theory (DFT) is combined with the Keldysh non-equilibrium Green's function (NEGF) formalism. Technically, to model a quantum transport system with non-collinear electrodes, the center scattering region of the transport system is placed into an orthogonal simulation box in all the other quantum transport simulations, while one or two electrodes are simulated within a non-orthogonal box. This small change in the shape of the simulation box of the electrode provides flexibility to calculate transport system with non-collinear electrodes, but also increases the complexity of the background coding. To date, the simulation of transport system with non-collinear electrodes has been realized only in the Nanodcal software package. Here, we take the Au-benzene (mercaptan)-Au molecular devices for example, and systematically calculate the quantum transport properties of the molecular devices with various contact positions and methods, and specifically, we first demonstrate the effect of the angle between the two electrodes on the transport property of molecular device from first principles. In our NEGF-DFT calculations performed by Nanodcal software package, the double-$zeta$ polarized atomic orbital basis is used to expand the physical quantities, and the exchange-correlation is treated in the local density approximation, and atomic core is determined by the standard norm conserving nonlocal pseudo-potential. Simulation results show that the chemical and structural details not only quantitatively affect the current value of the molecular device, but also bring new transport features to a device, such as negative differential resistance. From these results, we can conclude that the physics of a transport system having been investigated in more detail and a larger parameter space such as the effect of the contact model having been assessed by a comparison with "ideal" contacts, further understanding of the transport system can be made and more interesting physical property of the device can be obtained, which will be useful in designing of emerging electronics.

Modifying spin current filtering and magnetoresistance in a molecular spintronic device

Product: NanoDCAL
Date: 2018
Authors: Zhao GD,Li LM,Wang Y,Stroppa A,Zhang JH,Ren W
Journal: RSC Advances

The zigzag edged graphene nanoribbon (ZGNR) is excellent for spintronics devices, and many efforts have been made to investigate its properties such as spin filtering, rectification and magnetoresistance. Here we propose a molecular spintronic transport device based on two ZGNR electrodes connected with a dibenzo[a,c]dibenzo[5,6:7,8]quinoxalino[2,3-i]phenazine (DDQP) molecule. By performing first-principles electron transport computations, we found an enhanced spin polarized current-voltage curve, giant spin filter efficiency, magnetoresistance and rectification ratio properties of the device compared to its all-carbon molecular analogue. Our systematic investigation suggests the vital role played in spin polarized electron transport by nitrogen atoms in DDQP, the ZGNR probe's width and terminal geometry, especially the increased spin filter efficiency with higher ZGNR width.

Tunable transport and optoelectronic properties of monolayer black phosphorus by grafting PdCl2 quantum dots

Product: NanoDCAL
Date: 2018
Authors: Sun C,Wang Y,Jiang Y,Yang ZD,Zhang G,Hu Y
Journal: RSC Advances

The electronic, transport, and optoelectronic properties of monolayer black phosphorus (MLBP) are much influenced by grafting PdCl2 groups, demonstrated here by using density functional theory (DFT) and non-equilibrium Green's function (NEGF) as well as the Keldysh Nonequilibrium Green's Functions (KNEGF) methods. We find that the PdCl2 groups prefer to locate over the furrow site of MLBP and form a planar quadridentate structure of . The PdCl2 groups serve as quantum dots by introducing discrete flat levels between the MLBP valence band and the Fermi level (Ef). The conductivity is much lowered after attaching PdCl2 quantum dots, due to the fact that the scattering effect of PdCl2 plays a major role in the process of electron transporting. A threshold voltage is found for the functionalized system with a large density of PdCl2 quantum dots, a valuable clue for exploring current switches. However, no evident threshold voltage is found for the pure MLBP. Electrons permeate easier through the armchair direction compared with the zigzag either in the pure MLBP or in the functionalized composites. More importantly, grafting PdCl2 quantum dots is very beneficial for enhancing photoresponse. The values of photoresponse for the modified species are about 20 times higher than the free MLBP. A significant photoresponse anisotropy is observed for both MLBP and nPdCl2-BP (n = 1, 2, and 4), contrary to the conductivity, the zigzag direction shows much stronger photoresponse than the armchair. All of the aforementioned unique properties make these new two-dimensional (2D) MLBP based materials especially attractive for both electronic and optoelectronic devices.

Substrate-determined exchange interactions between an STM tip and an adatom

Product: NanoDCAL
Date: 2018
Authors: Tao K,Yu Y,Zhang W,Jia C,Xue D,Stepanyuk VS
Journal: Physical Review B

By performing ab initio calculations, we reveal an unexpected behavior of the exchange interactions between the Fe STM tip and the Fe adatom on Cu(001) and on a Cu2N monolayer on Cu(001) surfaces [denoted as Cu2N/Cu(001)]. A tip-adatom distance-dependent antiferromagnetic-ferromagnetic transition and antiferromagnetic exchange interactions between the tip and the adatom are found for these two junctions, respectively. We demonstrate that the different exchange interactions in these systems are determined by the competition between the tip-adatom and the adatom-substrate interactions. Based on transport calculations, we found that the spin polarization and magnetoresistance in the junction on the Cu(001) system and on the Cu2N/Cu(001) system depend on the tip-adatom distance.

Ultrahigh Tunneling-Magnetoresistance Ratios in Nitride-Based Perpendicular Magnetic Tunnel Junctions from First Principles

Product: NanoDCAL
Date: 2018
Authors: Yang B,Tao L,Jiang L,Chen W,Tang P,Yan Y,Han X
Journal: Physical Review Applied

We report a first-principles study of electronic structures, magnetic properties, and the tunneling-magnetoresistance (TMR) effect of a series of ferromagnetic nitride M4N (M=Fe, Co, Ni)-based magnetic tunnel junctions (MTJs). It is found that bulk Fe4N reveals a half-metal nature in terms of the $Delta$1 state. A perpendicular magnetic anisotropy is observed in the periodic system Fe4N/MgO. In particular, the ultrahigh TMR ratio of over 24 000% is predicted in the Fe4N/MgO/Fe4N MTJ due to the interface resonance tunneling and relatively high transmission for states of other symmetry. Besides, the large TMR can be maintained with the change of atomic details at the interface, such as the order-disorder interface, the change of thickness of the MgO barrier, and different in-plane lattice constants of the MTJ. The physical origin of the TMR effect can be well understood by analyzing the band structure and transmission channel of bulk Fe4N as well as the transmission in momentum space of Fe4N/MgO/Fe4N. Our results suggest that the Fe4N/MgO/Fe4N MTJ is a benefit for spintronic applications.

Two-Dimensional Photogalvanic Spin-Battery

Product: NanoDCAL
Date: 2018
Authors: Xie Y,Chen M,Wu Z,Hu Y,Wang Y,Wang J,Guo H
Journal: Physical Review Applied

Pure spin current is of central importance in spintronics. Here, we propose a two-dimensional (2D) spin-battery system that delivers pure spin current without an accompanying charge current to the outside world at zero bias. The principle of the spin battery is rooted in the photogalvanic effect (PGE) and the system has good operational stability against structural perturbation, photon energy, and other material characteristics. The principle of the device is numerically implemented in the 2D material phosphorene as an example, and first-principles calculations give excellent qualitative agreement with the physics of the PGE. The 2D spin battery is interesting as it is both a device that generates pure spin currents and also an energy source that harvests photons. Given the versatile operational space, the spin battery should be experimentally feasible.

Designing lateral spintronic devices with giant tunnel magnetoresistance and perfect spin injection efficiency based on transition metal dichalcogenides

Product: NanoDCAL
Date: 2018
Authors: Zhao P,Li J,Jin H,Yu L,Huang B,Ying D
Journal: Physical Chemistry Chemical Physics

Giant tunnel magnetoresistance (TMR) and perfect spin-injection efficiency (SIE) are extremely significant for modern spintronic devices. Quantum transport properties in a two-dimensional (2D) VS2/MoS2/VS2 magnetic tunneling junction (MTJ) are investigated theoretically within the framework of density functional theory combining with the non-equilibrium Green's functions (DFT-NEGF) method. Our results indicate that the designed MTJ exhibits a TMR with a value up to 4 × 103, which can be used as a switch of spin-electron devices. And due to the huge barrier for spin-down transport, the spin-down electrons could hardly cross the central scattering region, thus achieving a perfect SIE. Furthermore, we also explore for the effect of bias voltage on the TMR and SIE. We find that the TMR increases with the increasing bias voltage, and the SIE is robust against either bias or gate voltage in MTJs, which can serve as effective spin filter devices. Our results can not only give fresh impetus to the research community to build MTJs but also provide potential materials for spintronic devices.

Modified MXene: Promising electrode materials for constructing Ohmic contacts with MoS2 for electronic device applications

Product: NanoDCAL
Date: 2018
Authors: Zhao P,Jin H,Lv X,Huang B,Ma Y,Dai Y
Journal: Physical Chemistry Chemical Physics

The development of MoS2-based electronic devices is dependent on finding electrode materials suitable for forming Ohmic contacts with MoS2. For this purpose, we carried out density functional theory and nonequilibrium Green's function calculations to investigate the possibility of using pristine and modified MXene (Ta2C/Ta2CF2/Ta2C(OH)2) monolayers as the electrode materials in such devices. These calculations indicated the formation of chemical bonds at the MoS2/Ta2C interface, and resulting strong orbital hybridization between the MoS2 and Ta2C components, but relatively weak interactions for MoS2/Ta2CF2 and MoS2/Ta2C(OH)2. Ohmic contacts were observed in all three cases. Transport properties were further simulated by modeling two-probe field effect transistors (FETs) with Ta2C/Ta2CF2/Ta2C(OH)2 as electrodes. Interestingly, these simulations indicated the formation of Ohmic contacts between Ta2CF2/Ta2C(OH)2 electrodes and the channel, but an n-type Schottky barrier for the Ta2C electrode. Furthermore, we found the resistance of the FET based on MoS2/Ta2C(OH)2 to be half of that based on MoS2/Ta2CF2. The results of our study not only revealed promising electrode materials for forming full Ohmic contacts with MoS2 monolayers in FET devices, but also validated the effective role of a small-molecule fragment as a buffer layer in realizing Ohmic contacts between metal and semiconductor.

Remarkable negative differential resistance and perfect spin-filtering effects of the indium triphosphide (InP3) monolayer tuned by electric and optical ways

Product: NanoDCAL
Date: 2018
Authors: Zhang S,Xie Y,Hu Y,Niu X,Wang Y
Journal: Physical Chemistry Chemical Physics

Fully spin-polarized current and negative differential resistance (NDR) are two important electronic transport properties for spintronic nanodevices based on two-dimensional materials. Here, we describe both the electric and optical tuning of the spin-polarized electronic transport properties of the indium triphosphide (InP3) monolayer, which is doped with Ge atoms, by using quantum transport calculations. The spin degeneration of the InP3 monolayer is lifted due to the doping of Ge atoms. By applying a small bias voltage, a fully spin-polarized current can be obtained along both the armchair and zigzag directions. Moreover, a remarkable NDR is observed for the current along the zigzag direction, which shows a huge peak-to-valley ratio of 3.1 × 103, while in the armchair direction, a lower peak-to-valley ratio of 5.5 is obtained. Alternatively, a fully spin-polarized photocurrent can also be generated under the illumination of linearly-polarized light by tuning either the photon energy or the polarization angle.

Tuning the electronic and magnetic properties of InSe nanosheets by transition metal doping

Product: NanoDCAL
Date: 2018
Authors: Wang T,Li J,Jin H,Wei Y
Journal: Physical Chemistry Chemical Physics

Magnetic two-dimensional materials have attracted considerable attention for their significant potential application in spintronics. Here, we systematically study the electronic structures and magnetic properties of a 3d transition metal doped InSe monolayer based on density functional theory (DFT). Our results show that InSe monolayer can be turned into a half-metal when the Ti, Cr, or Ni atom is doped. Further calculations indicate that the Cr-InSe monolayer possesses a robust ferromagnetic ground state due to the effective p-d exchange. The predicted Curie temperature of Cr-InSe is above the room temperature, showing a powerful potential in spintronics. Application in the spin valve is also explored using quantum transport simulations. Our results indicate that the magnetoresistance of a Cr-InSe spin valve attains 100% due to the half-metallic characteristics. These findings may pave the way for designing 2D nano-devices for future spin transport applications.

Pure spin current and phonon thermoelectric transport in a triangulene-based molecular junction

Product: NanoDCAL
Date: 2018
Authors: Wang Q,Li J,Nie Y,Xu F,Yu Y,Wang B
Journal: Physical Chemistry Chemical Physics

The experimental synthesis and characterization of enigmatic triangulene were reported for the first time recently. Based on this enigmatic molecule, we proposed a triangulene-based molecular junction and presented first principles calculations to investigate the electron and phonon thermoelectric transport properties. Numerical results show that the spin polarized electric transport properties of the triangulene-based molecular junction can be adjusted effectively by bias voltage and gate voltage. Through varying the gate voltage applied on the triangulene molecule, the system can exhibit a perfect spin filter effect. When a temperature gradient is applied between the two leads, spin up current and spin down current flow along opposite directions in the system simultaneously. Thus pure spin current can be obtained on a large scale by changing the temperature, temperature gradient, and gate voltage. When the phonon vibration effect is considered in thermal transport, the figure of merit is suppressed distinctively especially when the temperature is within the 10 K < T < 100 K range. More importantly, a large spin figure of merit can be achieved accompanied by a small charge figure of merit by adjusting the temperature, gate voltage and chemical potential in a wide range, which indicates a favorable application prospect of the triangulene-based molecular junction as a spin calorigenic device.

Tunable Schottky contacts in MSe2/NbSe2 (M = Mo and W) heterostructures and promising application potential in field-effect transistors

Product: NanoDCAL
Date: 2018
Authors: Lv X,Wei W,Zhao P,Li J,Huang B,Dai Y
Journal: Physical Chemistry Chemical Physics

The performance of electronic and optoelectronic devices based on two-dimensional (2D) materials could be significantly affected by the electrical contacts. In search of low-resistance contacts with transition-metal dichalcogenides (TMDs), we combine density functional calculations with quantum transport simulations to investigate the structural and electronic properties of the van der Waals (vdW) heterostructures MSe2/NbSe2 (M = Mo and W). The formation of a p-type Schottky contact at the MSe2/NbSe2 interface with small Schottky barriers (0.37 eV for MoSe2/NbSe2 and 0.18 eV for WSe2/NbSe2) is demonstrated. The low Schottky barrier heights indicate a low contact resistance, which is beneficial for electron injection and low-resistance. Remarkably, we demonstrate that the Schottky barrier can be effectively tuned via the application of vertical compressive pressure, an external electrical field and tensile strain. Finally, the results are supported by quantum transport simulation, which further proves the highly transparent contacts and promising application potential in field-effect transistors (FET). Therefore, our formalism and findings not only provide insights into the MSe2/NbSe2 interfaces but also help in the design of MSe2 monolayer-based devices.

First-principles prediction of two-dimensional metal bis(dithiolene) complexes as promising gas sensors

Product: NanoDCAL
Date: 2018
Authors: Liu H,Li X,Shi C,Wang D,Chen L,He Y,Zhao J
Journal: Physical Chemistry Chemical Physics

The recently synthesized two-dimensional metal bis(dithiolene) complex (MDT), a kind of metal-organic framework with a kagome lattice structure, has been found to be a promising material for electronic devices. Here we report the surface adsorption effects of gas molecules on the electronic properties and transport behaviors of two-dimensional MDT (M = Fe, Co, Ni, Pd, and Pt) films. The first-principles results reveal that the MDT nanosheets are selectively sensitive to different adsorbed molecules, such as CO, NO, and O2 molecules. All the studied gas molecules can be chemically adsorbed on the ferromagnetic FeDT and CoDT nanosheets, whereas the non-magnetic PdDT and PtDT films are only sensitive to NO molecules, showing quite weak interaction with CO and O2. The physisorption of CO on PdDT and PtDT originates from the mismatch of energy levels between the metal dz2 orbitals and the CO $sigma$ orbitals. In contrast, the Pd and Pt dxz and dyz orbitals can well align with the NO $pi$∗ orbitals, causing strong chemisorption. More importantly, the adsorption of NO on PdDT and PtDT not only induces a magnetism of 1.0 $mu$B for the two films but also greatly enhances the conductivity. In the case of PtDT, we observe a transition from the semiconducting to the metallic phase on NO adsorption. This significant change in the electronic structure can be understood from the adsorption-induced interfacial charge transfer and the strong orbital hybridization between the metal d states and the NO $pi$∗ states. Our results suggest the potential application of the PdDT and PtDT nanosheets in gas sensing and spintronics.

All-phosphorus flexible devices with non-collinear electrodes: A first principles study

Product: NanoDCAL
Date: 2018
Authors: Li J,Ruan L,Wu Z,Zhang G,Wang Y
Journal: Physical Chemistry Chemical Physics

With the continuous expansion of the family of two-dimensional (2D) materials, flexible electronics based on 2D materials have quickly emerged. Theoretically, predicting the transport properties of the flexible devices made up of 2D materials using first principles is of great importance. Using density functional theory combined with the non-equilibrium Green's function formalism, we calculated the transport properties of all-phosphorus flexible devices with non-collinear electrodes, and the results predicted that the device with compressed metallic phosphorene electrodes sandwiching a P-type semiconducting phosphorene shows a better and robust conducting behavior against the bending of the semiconducting region when the angle between the two electrodes is less than 45°, which indicates that this system is very promising for flexible electronics. The calculation of a quantum transport system with non-collinear electrodes demonstrated in this work will provide more interesting information on mesoscopic material systems and related devices.

Photogalvanic effect induced fully spin polarized current and pure spin current in zigzag SiC nanoribbons

Product: NanoDCAL
Date: 2018
Authors: Chen J,Zhang L,Zhang L,Zheng X,Xiao L,Jia S,Wang J
Journal: Physical Chemistry Chemical Physics

Using nonequilibrium Green's function combined with density functional theory, we investigate the spin-related current generated by the photogalvanic effect (PGE) in monolayer zigzag SiC nanoribbons (ZSiCNRs) by first-principles calculations. Due to its unique atomic structure and band structure properties, we find that 100% spin polarized photocurrent can be easily obtained in a wide range of photon energies by shining linearly/circularly polarized light when ZSiCNRs are in the anti-ferromagnetic (AFM) state. In comparison, when the ZSiCNRs are in the ferromagnetic (FM) state, the spin polarization of photocurrent can vary from 0% to 100% by changing the photon energy or polarization angle. More interestingly, pure spin current can also be generated by changing the circular polarization angle in the FM state. Thus, by properly tuning the photon energy, one can obtain 100% spin polarized current regardless of its magnetic configuration and pure spin current in its FM state. Our numerical findings pave a feasible way for ZSiCNRs' novel applications in spintronics.

Carbon chain-based spintronic devices: Tunable single-spin Seebeck effect, negative differential resistance and giant rectification effects

Product: NanoDCAL
Date: 2018
Authors: Yang XF,Shao ZG,Yu HL,Dong YJ,Kuang YW,Liu YS
Journal: Organic Electronics

Using the density functional theory combined with the Keldysh non-equilibrium Green's function methods, we investigate the thermally (or voltage)-induced spin transport properties of a two-probe device consisting of a carbon atom chain sandwiched between two zigzag graphene nanoribbon (ZGNR) electrodes. When the edge of one ZGNR electrode is partially doped by B atoms, the flowing direction of thermal single-spin current can be reversed in contrast with the undoped case. In addition, when a voltage is applied across the carbon-based device at room temperature, a giant rectification ratio of 104 is observed which mainly originates from the band-structure incompatibility between two ZGNR electrodes in the voltage window. Moveover, in the high-voltage region, a single-spin negative differential resistance is also observed in the carbon-based device. Our findings here suggest that the carbon-based systems can be used to design spintronic devices with multiple functions.

The spin-dependent transport and optoelectronic properties of the 6,6,12-graphyne-based magnetic tunnel junction devices

Product: NanoDCAL
Date: 2018
Authors: Di M,Yang Z,Li J,Bai H,Hao L,Xu LC,Liu R,Liu X
Journal: Organic Electronics

Using density functional theory and nonequilibrium Green's function method, we investigated the spin-dependent transport and spin-polarized optoelectronic properties of the 6,6,12-graphyne-based magnetic tunnel junction (MTJ) devices. The results show that the MTJ devices have prominent dual spin-filtering effect and large tunneling magnetoresistance (TMR); the TMR values of the MTJ devices are as high as 105%. In addition, we found that the spin-polarized photocurrents of the MTJ devices are dependent on the polarization direction of light and the magnetization directions of the electrodes. Two spin-polarized photocurrents can be realized in the MTJ devices and, if the magnetization directions of the two electrodes are antiparallel, the two light-generated spins flow in opposite directions. These interesting phenomena indicate that the 6,6,12-graphyne-based MTJ devices could be used as optoelectronic or opto-spintronic devices.

Multi-functional spintronic devices based on boron- or aluminum-doped silicene nanoribbons

Product: NanoDCAL
Date: 2018
Authors: Liu YS,Dong YJ,Zhang J,Yu HL,Feng JF,Yang XF
Journal: Nanotechnology

Zigzag silicene nanoribbons (ZSiNRs) in the ferromagnetic edge ordering have a metallic behavior, which limits their applications in spintronics. Here a robustly half-metallic property is achieved by the boron substitution doping at the edge of ZSiNRs. When the impurity atom is replaced by the aluminum atom, the doped ZSiNRs possess a spin semiconducting property. Its band gap is suppressed with the increase of ribbon's width, and a pure thermal spin current is achieved by modulating ribbon's width. Moreover, a negative differential thermoelectric resistance in the thermal charge current appears as the temperature gradient increases, which originates from the fact that the spin-up and spin-down thermal charge currents have diverse increasing rates at different temperature gradient regions. Our results put forward a promising route to design multi-functional spintronic devices which may be applied in future low-power-consumption technologies.

H -BN/graphene van der Waals vertical heterostructure: A fully spin-polarized photocurrent generator

Product: NanoDCAL
Date: 2018
Authors: Tao X,Zhang L,Zheng X,Hao H,Wang X,Song L,Zeng Z,Guo H
Journal: Nanoscale

By constructing transport junctions using graphene-based van der Waals (vdW) heterostructures in which a zigzag-edged graphene nanoribbon (ZGNR) is sandwiched between two hexagonal boron-nitride sheets, we computationally demonstrate a new scheme for generating perfect spin-polarized quantum transport in ZGNRs by light irradiation. The mechanism lies in the lift of spin degeneracy of ZGNR induced by the stagger potential it receives from the BN sheets and the subsequent possibility of single spin excitation of electrons from the valence band to the conduction band by properly tuning the photon energy. This scheme is rather robust in that we always achieve desirable results irrespective of whether we decrease or increase the interlayer distance by applying compressive or tensile strain vertically to the sheets or shift the BN sheets in-plane relative to the graphene nanoribbons. More importantly, this scheme overcomes the long-standing difficulties in traditional ways of using solely electrical field or chemical modification for obtaining half-metallic transport in ZGNRs and thus paves a more feasible way for their application in spintronics.

Self-powered photogalvanic phosphorene photodetectors with high polarization sensitivity and suppressed dark current

Product: NanoDCAL
Date: 2018
Authors: Li S,Wang T,Chen X,Lu W,Xie Y,Hu Y
Journal: Nanoscale

High polarization sensitivity, suppressed dark current and low energy consumption are all desirable device properties for photodetectors. In this work, we propose phosphorene-based photodetectors that are driven using photogalvanic effects (PGEs). The inversion symmetry of pristine phosphorene is broken using either application of an out-of-plane gate voltage or a heterostructure that is composed of the original phosphorene and blue phosphorene. The potential asymmetry enables PGEs under illumination by polarized light. Quantum transport calculations show that robust photocurrents are indeed generated by PGEs under a zero external bias voltage because of the broken inversion symmetry. These results indicate that the proposed photodetector is self-powered. In addition, the zero bias voltage eliminates the dark currents that are caused by application of an external bias voltage to traditional photodetectors. High polarization sensitivity to both linearly and circularly polarized light can also be realized, with extinction ratios ranging up to 102. The photoresponse of the proposed phosphorene/blue phosphorene heterostructure can be greatly enhanced by gating and is several orders of magnitude higher than that in gated phosphorene.

Spin injection into graphene from heusler alloy Co2MnGe (111) surface: A first principles study

Product: NanoDCAL
Date: 2018
Authors: Wang YX,Xia TS
Journal: Materials Science Forum

To obtain a larger spin signal for use in graphene-based spintronic devices, the spin injection efficiency needs to be enhanced. Previously researchers can increase the efficiency by inserting a tunnel barrier such as Al2O3 or MgO between ferromagnet and graphene. However, the key value in spin transport is still very low because of the conductance mismatch as well as the limit to fabricate a high-quality tunnel barrier at the junction surface. Here we use a highly spin-polarized ferromagnetic material—Heusler alloy Co2MnGe as a substitutional scheme without the tunnel barrier. The spin injection efficiency of our Co2MnGe (111)/graphene junction can be as high as 73% which is much higher than 1% of ferromagnet/graphene or 30% of ferromagnet/ oxide/graphene using first-principles study. The large spin polarization can be explicated by analyzing the transmission spectrum at the nonequilibrium state.

Realizing fully spin polarized transport in graphene nanoribbons with design of van der Waals vertical heterostructure leads

Product: NanoDCAL
Date: 2018
Authors: Tao X,Jiang P,Kang L,Hao H,Song L,Lan J,Zheng X,Zhang L,Zeng Z
Journal: Journal of Physics D: Applied Physics

Zigzag-edged graphene and h-BN nanoribbons with passivated edges in the ground state are either spin-degenerate or spin-unpolarized systems, which are not directly applicable for spin-polarized transport. In this work, based on density functional calculations, we demonstrate that the combination of them to form van der Waals (vdW) heterostructures can be adopted to realize fully spin polarized transport. As an example, the ballistic transport properties of a zigzag-edged graphene nanoribbon (ZGNR) with six zigzag carbon chains is studied first with each lead as a vdW heterostructure formed by attaching a h-BN monolayer to each side of the ZGNR with AA-stacking. A greatly decreased transmission gap (0.28 eV) in one spin and a greatly increased transmission gap (0.82 eV) in the other spin is achieved in the transmission function, resulting in fully spin polarized transport at low bias. This arises from the stagger potential imposed by the h-BN layers, which not only exists in the leads, but also extends to the channel region. It changes the energy of the edge states of different spins localized at different sublattices in an opposite way in the whole device. The transmission gap and the threshold voltage can be further modulated by applying a vertical pressure to the vdW leads to tune the strength of the stagger potential or simply by changing the ribbon width. Finally, the increase of the channel length will greatly reduce the magnitude of the transmission around the Fermi level and the transmission gap will eventually recover the value of the band gap of the pristine ZGNR. These findings not only provide a novel way for achieving fully spin polarized transport in graphene, but also demonstrate the great importance of vdW heterostructures in the design of spintronic devices.

Photoexcitation Dynamics in Janus-MoSSe/WSe2 Heterobilayers: Ab Initio Time-Domain Study

Product: NanoDCAL
Date: 2018
Authors: Liang Y,Li J,Jin H,Huang B,Dai Y
Journal: Journal of Physical Chemistry Letters

The photoexcitation dynamics plays a key role in determining the properties of van der Waals heterostructures (vdWHs). Based on the time-dependent density functional theory combined with nonadiabatic molecular dynamics, we investigate the charge transfer in Janus-MoSSe/WS2 vdWHs. Ultrafast charge separation is observed, arising from the large overlapping between the donor and acceptor states. While the electron-hole recombination is 2 orders of magnitude slower than the charge separation, this can be understood by the fact that the initial and final states are strictly confined to different materials. Additionally, photoresponsivity performance of the vdWHs is also evaluated using density functional theory combined with the nonequilibrium Green's functions. Simulated results of high photoresponsivity in a broad range of the spectrum endows proposed systems powerful potential in optoelectronic and photovoltaic applications. The atomistic picture revealed in our work provides chemical guidelines and facilitates the design of next-generation devices for light detecting and harvesting.

Large Magnetoresistance in Fe3O4/4,4′-Bipyridine/Fe3O4 Organic Magnetic Tunnel Junctions

Product: NanoDCAL
Date: 2018
Authors: Sun M,Wang X,Mi W
Journal: Journal of Physical Chemistry C

Organic magnetic tunnel junctions (OMTJs) are promising systems thanks to their chemically tunable electronic property, long spin lifetime, and easy functionalizations. Here, the spin-dependent electronic transport properties in Fe3O4/4,4′-bipyridine/Fe3O4 OMTJs are investigated by first-principles quantum transport calculations. Since the transport properties of junctions are sensitive to device details, two types of terminations of Fe3O4 electrodes are considered. The device with tetrahedral Fe termination shows anomalous negative tunnel magnetoresistance (TMR), that is, which has a higher and lower junction resistance in the parallel and antiparallel magnetization configurations, respectively. When the contact termination is octahedral Fe, a large positive TMR of 180% appears. The difference in TMR sign of two OMTJs originates from the electrons transmission mediated by frontier molecular levels coupled differently to Fe d states. Furthermore, TMR can be effectively controlled by applied electrical bias by changing states of octahedral Fe involved in transport, which can reach 22000% at 0.1 V. Moreover, a perfect spin-filter effect is demonstrated irrespective of the contact geometry. The results contribute to a fundamental understanding of spin-dependent transport properties in OMTJs.

Effects of Spin-Orbit Coupling on Nonequilibrium Quantum Transport Properties of Hybrid Halide Perovskites

Product: NanoDCAL
Date: 2018
Authors: Liu YQ,Cui HL,Wei D
Journal: Journal of Physical Chemistry C

The ground-state properties of organic-inorganic hybrid halide perovskites (OHHP) are significantly affected by spin-orbit coupling (SOC). In this paper, we report on an investigation of the nonequilibrium quantum transport properties of MAPbI3 (MA = CH3NH3), considering the cases of noncollinear spin polarization including SOC, noncollinear spin polarization excluding SOC, and no spin polarization, using Keldysh nonequilibrium Green's function formalism combined with density functional theory calculations. Our results indicate that the enhancement of nonequilibrium quantum transport properties is largely determined by SOC. The calculated current density for noncollinear spin polarization including SOC is about 1 order of magnitude higher than that for no spin polarization, resulting from the increase of nonequilibrium transmission coefficient in the bias window and the increase of electron density in the conduction band contributed by the p-state of Pb. The results demonstrate that SOC is essential for understanding the nonequilibrium quantum transport properties of OHHP-based devices.

Thermoelectric Charge and Spin Current Generation in Magnetic Single-Molecule Junctions: First-Principles Calculations

Product: NanoDCAL
Date: 2018
Authors: Kaun CC,Chen YC
Journal: Journal of Physical Chemistry C

We apply first-principles approaches to investigate the spin (charge) Seebeck effects [Ss(c)] and spin (charge) thermoelectric figure of merits [ZTs(c)] of manganese-phthalocyanine spin-polarized scanning tunneling microscopy (MnPc SP-STM) junctions. The magnetic tunneling junctions are N-type junctions because their Sc values are negative. Their Ss and Sc values are sufficiently large for the efficient generation of measurable spin and charge currents. ZTs(c) values strongly depend on the competition between electron and phonon thermal conductances: ZTs(c) ∞ Ss(c)2 for $kappa$ph ≥ $kappa$el, and ZTs(c) ∞ Ss(c)2$kappa$el for $kappa$el ≥ $kappa$ph. Ss changes signs when the spin-valve junction rotates its magnetic structure from the antiparallel (AP) to the parallel (P) configuration. This behavior indicates that spin-current direction can be reversed by alternating magnetic configurations between AP and P states. Spin-current dissipation in the junctions is minimized because the sizes of the junctions are considerably smaller than the lengths of spin-flip scattering and spin dephasing. The low spin-current dissipation of the junctions suggests that they have potential applications in spintronics and renewable energy. The present finding provides a new approach to spin-current generation through the use of SP-STM based on temperature difference and to controlling spin-current direction through magnetic configurations. The integration of numerous single-molecule magnetic junctions as building blocks into a high-density device is a promising strategy for generating a considerable net spin current for applications in molecular spin caloritronics.

Bi2ZnOB2O6: a polar material capable of photocatalytic degradation of Rhodamine B

Product: NanoDCAL
Date: 2018
Authors: Liu J,Zhao W,Wang B,Yan H
Journal: Journal of Materials Science: Materials in Electronics

Bi2ZnOB2O6(BZBO), which belongs to the typical polar borate, has been successfully synthesized by a sintering-hydrothermal two steps method, and further characterized by XRD, TEM and HRTEM. The UV–Vis reflectance spectrum for BZBO was characterized in detail, and it has indirect transition optical band gaps of 3.03 eV. Density functional calculations revealed that the valence band and conduction band of BZBO were composed of hybridized states of the O 2p and Bi 6s or 6p orbitals. Photodecomposition experiments demonstrated that BZBO exhibits a high activity to photodegrade Rhodamine B under UV light irradiation. Through the calculation of dipole moment, it can be found that the polarization field in BZBO, which has a negative c-axis direction, can promote move of photo-induced electron to positive c-axis direction and photogenerated hole to negative c-axis direction. This will greatly improve the separation of photo-induced electron and photogenerated hole and then enhance the photocatalytic activity of BZBO.

A novel electrically controllable volatile memory device based on few-layer black phosphorus

Product: NanoDCAL
Date: 2018
Authors: Zhang L,Yu Z,Zhang L,Zheng X,Xiao L,Jia S,Wang J
Journal: Journal of Materials Chemistry C

Exploiting the unique property of two dimensional material black phosphorus (BP), we theoretically propose a novel volatile memory device based on pure few-layer BP from atomic first principles calculations. When the vertical gate is applied in the system, the semi-conducting few-layer BP can be driven into the metallic phase. By investigating the local density of states (LDOS) under the gate voltage, we find that the metallic electronic states around the Fermi level mostly localize in the topmost or bottom layer depending on the sign of the gate voltage. Through designing a two-probe device with dual vertical gates applied in the electrode region, a metal-semiconductor-metal tunnelling device can be formed. Two states ("ON" and "OFF") are realized by the different tunnelling effects of the metallic states from the topmost layer to the topmost layer or the topmost layer to the bottom layer. Due to the anisotropy property of few-layer BP, the relative current ratios of the two states along the armchair direction and zigzag direction are both large and quite different. We predict a giant "ON/OFF" current ratio that can be over 5000% along the zigzag direction, which indicates that the few-layer BP is a potential candidate in atomically-thin volatile memory devices.

A highly polarization sensitive antimonene photodetector with a broadband photoresponse and strong anisotropy

Product: NanoDCAL
Date: 2018
Authors: Chu F,Chen M,Wang Y,Xie Y,Liu B,Yang Y,An X,Zhang Y
Journal: Journal of Materials Chemistry C

Photodetectors based on two-dimensional materials have shown impressive performance including fast and broadband photoresponse and high responsivity. However, their polarization sensitivity remains to be improved. Here, we propose an antimonene photodetector having a strong polarization sensitivity with a broadband photoresponse, based on quantum transport calculations. A robust photocurrent is generated for almost the whole visible range under small bias, and it saturates at a small bias voltage for most of the photon energies. The photocurrent shows a perfect cosine dependence on the polarization angle, which originates from a second-order response to the electric field of the light. This leads to a strong polarization sensitivity to the linearly polarized light with a large extinction ratio. For a higher photon energy around 3.2 eV, a rather high extinction ratio greater than 100 can be achieved along with a larger photocurrent. Moreover, there is an evident anisotropy between the armchair and zigzag directions, as the photocurrent intensity in the zigzag direction can be approximately 17 times larger than that in the armchair direction at a small bias. These results suggest that antimonene is a promising candidate for anisotropic photodetection in the visible range especially for high frequency visible light.

Novel titanium nitride halide TiNX (X = F, Cl, Br) monolayers: Potential materials for highly efficient excitonic solar cells

Product: NanoDCAL
Date: 2018
Authors: Liang Y,Dai Y,Ma Y,Ju L,Wei W,Huang B
Journal: Journal of Materials Chemistry A

The growth in the area of two-dimensional (2D) crystals offers renewed opportunities for efficient, ultrathin excitonic solar cells (XSCs) beyond those three-dimensional traditional materials. Here, based on first principles, we propose a family of achievable 2D tetragonal-structured titanium nitride halides (TiNX, X = F, Cl, Br) as donor and acceptor materials for XSCs in virtue of their desirable optoelectronic properties, such as a direct bandgap with moderate size, superior optical absorption, ultrahigh photoresponsivity together with very small effective masses and exciton binding energy. More importantly, we find that they can be easily superimposed onto each other to form effective solar cell systems with the type-II heterojunction alignment. We predict that the maximum energy conversion of the designed TiNF/TiNBr, TiNCl/TiNBr and TiNF/TiNCl bilayer solar cells can reach as high as ∼18%, 19% and 22%, respectively, which are far superior to those of typical conjugated polymer or fabricated 2D solar cell systems. Our work suggests these potential bilayer systems are appealing 2D solar cell materials with high efficiency.

Potential of one-dimensional blue phosphorene nanotubes as a water splitting photocatalyst

Product: NanoDCAL
Date: 2018
Authors: Ju L,Dai Y,Wei W,Liang Y,Huang B
Journal: Journal of Materials Chemistry A

Semiconductor photocatalysis via photogenerated electrons and holes can split water into H2 and O2, which is a promising way to utilize solar energy. However, the inefficient generation of oxygen severely restricts the catalytic efficiency for overall water splitting. Thus, it is necessary to find new photocatalysts for the oxygen evolution reaction (OER). In the present study, the electronic structure and related properties of one-dimensional blue phosphorene nanotubes (BPNTs) are systematically investigated using first-principle calculations to explore their photocatalytic activities. Our results demonstrate that the strain energy of BPNTs with a large diameter (larger than 8 Å) is nearly the same as that of carbon nanotubes, indicating that these BPNTs are stable. Due to their perfect band gaps and band energies for photocatalytic activity, BPNTs have potential for application in visible light photocatalysis for overall water splitting. More significantly, p-type zigzag BPNTs with good photooxidation capabilities, low electron-hole recombination, and high hole mobility (1729.53 cm2 V−1 s−1) and photo-response coefficient (20a02 per photon) are promising candidates as photocatalytic materials for OER. Besides, the band gap of BPNTs can be tuned monotonically by reasonable strain. We further report advanced strategies for designing and improving potentially efficient photocatalysts for overall water splitting.

Spin-dependent electronic transport characteristics in Fe4N/BiFeO3/Fe4N perpendicular magnetic tunnel junctions

Product: NanoDCAL
Date: 2018
Authors: Yin L,Wang X,Mi W
Journal: Journal of Applied Physics

Perpendicular magnetic tunnel junctions (MTJs) have attracted increasing attention owing to the low energy consumption and wide application prospects. Herewith, against Julliere's formula, an inverse tunnel magnetoresistance (TMR) appears in tetragonal Fe4N/BiFeO3/Fe4N perpendicular MTJs, which is attributed to the binding between the interface resonant tunneling state and central (bordered) hot spots. Especially, antiferromagnetic BiFeO3 shows an extra spin-polarized resonant state in the barrier, which provides a magnetic-barrier factor to affect the tunneling transport in MTJs. Meanwhile, due to the spin-polarized transport in Fe4N/BiFeO3/Fe4N MTJs, the sign of TMR can be tuned by the applied bias. The tunable TMR and resonant magnetic barrier effect pave the way for clarifying the tunneling transport in other junctions and spintronic devices.

Modulate the direct-current and alternating-current transport properties of magnetic $gamma$-graphyne heterojunctions by chemical modification

Product: NanoDCAL
Date: 2018
Authors: Yang Z,Shen J,Li J,Ouyang B,Xu LC,Liu X
Journal: Journal of Applied Physics

Using density functional theory and the non-equilibrium Green's function method, we theoretically investigated the direct-current (DC) and alternating-current (AC) quantum transport properties of magnetic $gamma$-graphyne heterojunctions. For the DC case, we found that the $gamma$-graphyne heterojunction has rich transport properties such as spin-filtering and magnetoresistance effects. As the marginal H atoms of the heterojunction are replaced by O atoms, an outstanding dual spin-filtering phenomenon appears and the magnetoresistance is enhanced. Meanwhile, after chemical modification, the heterojunction exhibits a noticeable rectification effect. For the AC case, depending on the frequency, the total and spin AC conductances can be capacitive, inductive, or resistive. At some given frequencies, the signs of the imaginary parts of the AC conductances for two different spins are opposite; thus, the two spin currents have opposite AC responses. A significant photon-assisted tunneling effect was found in the heterojunctions at high frequency range. More interestingly, after chemical modification in a wide frequency range, the imaginary part of the AC conductance changes the sign, indicating that the AC transport properties of the $gamma$-graphyne heterojunction can be effectively modulated by chemical methods.

Anisotropic Negative Differential Resistance in Monolayer Black Phosphorus

Product: NanoDCAL
Date: 2018
Authors: Zhang W,Kang P,Chen H
Journal: IOP Conference Series: Earth and Environmental Science

The tremendous potential application in emerging two-dimensional layered materials such as black phosphorus (BP) has attracted great attention as nanoscale devices. In this paper, the effect of anisotropic negative differential resistance (NDR) in monolayer black phosphorus field-effect transistors (FETs) is reported by the first-principles computational study based on the non-equilibrium Green's function approach combined with density functional theory. The transport properties including current-voltage (I-V) relation and transmission spectrum of monolayer BP are investigated at different gate voltages (Vg). Further studies indicate that NDR occurs at a specific gate voltage in the armchair direction rather than in the zigzag direction. The decrease of current in I-V characteristics can be understood from the generation of non-conducting states region moving towards the Fermi level resulting in a reduction of the integration within corresponding energy range in the transmission spectrum. Our results offer useful guidance for designing FETs and other potential applications in nanoelectronic devices based on BP.

Surface and Grain-boundary Effects in Copper interconnects Thin Films Modeling with an Atomistic Basis

Product: NanoDCAL
Date: 2018
Authors: Valencia D,Wang KC,Chu Y,Klimeck G,Povolotskyi M
Journal: International Conference on Simulation of Semiconductor Processes and Devices, SISPAD

As interconnects become smaller, their conductivity increases along with the parasitic effects in MOSFET technologies [1] Therefore, investigating how to model the scattering effects on the nanoscale is important to determine how to engineer interconnects toreduce those parasitic effects. In this work, a fully atomistic method is studied to model the electronic transport properties of copper thin films. For this purpose, a tight binding basis previously benchmarked against first principles calculations [2] is used todescribe surface roughness and grain boundary effects on comparablepper thin films with a thickness comparable to the values suggested by ITRS roadmap [3]. In contrast with traditional models, the results show that the tight binding method can quantify those scattering effects at low temperature without fitting any experimental parameters [4], [5].

First-principles investigation of quantum transport in GeP3 nanoribbon-based tunneling junctions

Product: NanoDCAL
Date: 2018
Authors: Wang Q,Li JW,Wang B,Nie YH
Journal: Frontiers of Physics

Two-dimensional (2D) GeP3 has recently been theoretically proposed as a new low-dimensional material [Nano Lett. 17(3), 1833 (2017)]. In this manuscript, we propose a first-principles calculation to investigate the quantum transport properties of several GeP3 nanoribbon-based atomic tunneling junctions. Numerical results indicate that monolayer GeP3 nanoribbons show semiconducting behavior, whereas trilayer GeP3 nanoribbons express metallic behavior owing to the strong interaction between each of the layers. This behavior is in accordance with that proposed in two-dimensional GeP3 layers. The transmission coefficient T(E) of tunneling junctions is sensitive to the connecting formation between the central monolayer GeP3 nanoribbon and the trilayer GeP3 nanoribbon at both ends. The T(E) value of the bottom-connecting tunneling junction is considerably larger than those of the middle-connecting and top-connecting ones. With increases in gate voltage, the conductances increase for the bottom-connecting and middle-connecting tunneling junctions, but decrease for the top-connecting tunneling junctions. In addition, the conductance decreases exponentially with respect to the length of the central monolayer GeP3 nanoribbon for all the tunneling junctions. I–V curves show approximately linear behavior for the bottom-connecting and middle-connecting structures, but exhibit negative differential resistance for the top-connecting structures. The physics of each phenomenon is analyzed in detail.

Structure and transport properties of amorphous high-$kappa$ Al2O3

Product: NanoDCAL
Date: 2018
Authors: Yu Z,Li J,Wang Y
Journal: Epl

We report the amorphous structure of Al2O3 and the transport properties of the Fe/amorphous Al2O3/Fe magnetic tunnel junction (MTJ) from first principles. The amorphous Al2O3 structure was generated by a heat and quench method via classical molecular dynamics, and the dielectric and transport properties were calculated from first principles. The tunnel magnetoresistance of the Fe/amorphous Al2O3/Fe MTJ is 25% under zero bias and then slightly increases to 28% at the bias of 0.1 V. It then decreases with the increasing bias and becomes negative when the applied bias exceeds 0.4 V due to the rapid increase of the spin-down current in the antiparallel configuration which is originated from the new transmission channel of minority electrons in the left lead introduced by the increasing bias. The spin-injection efficiency (SIE) is also calculated to study the spin-polarized current of the Fe/amorphous Al2O3/Fe system. It is found that the SIE coefficient is around 60% and 15% at equilibrium for the parallel and antiparallel configuration, respectively.

The transport and optoelectronic properties of $gamma$-graphyne-based molecular magnetic tunnel junctions

Product: NanoDCAL
Date: 2018
Authors: Li J,Xu LC,Yang Y,Liu X,Yang Z
Journal: Carbon

By cutting $gamma$-graphyne along different directions, two kinds of $gamma$-graphyne nanodots ($gamma$-GYNDs) can be acquired. In present study, using the $gamma$-GYNDs we theoretically designed and investigated the spin-dependent transport properties of two kinds of molecular magnetic tunnel junctions (MMTJs). Depending on the orientation of $gamma$-GYND and the connection way between $gamma$-GYND and electrodes, our results show that two kinds of MMTJs have different microscopic transport mechanisms. Significant single or dual spin-filtering effects, giant magnetoresistances (reach 108%) and spin negative differential resistances can be observed in the MMTJs. In addition, the spin-polarized optoelectronic properties of the MMTJs have also been discussed, and the results indicate that the spin-polarized photocurrents are dependent on the polarization direction of light and magnetization directions of the electrodes. Especially, two different light-generated spins can be simultaneously produced in the MMTJs if the incident photons have given energies and they flow along opposite directions. The above findings show that the $gamma$-graphyne-based MMTJs can be used as spintronic devices or opto-spintronic devices.

Photoinduced pure spin-current in triangulene-based nano-device

Product: NanoDCAL
Date: 2018
Authors: Jin H,Li J,Wang T,Yu Y
Journal: Carbon

Triangulene has drawn great attention due to its extraordinary chemical and material properties. This magnetic molecule has the ferromagnetic ground state due to its two unpaired $pi$ electrons, making it suitable for spintronic applications. In this work, a triangulene based spin-photovoltaic device is proposed using a two-probe model. The quantum transport and spin current in triangulene device under the light irradiation are investigated based on the density functional theory combined with the nonequilibrium Green's functions. We finds that by adjusting photon energy (Eph) and gate voltage (Vg), the proposed device can produce large pure spin current without charge current. The origin of the pure spin current is also analyzed based on the through-bond mechanism. Our work may provide theoretical reference for design of novel spintronic device with high performance.

Interface characterization of current-perpendicular-to-plane spin valves based on spin gapless semiconductor Mn2CoAl

Product: NanoDCAL
Date: 2018
Authors: Wei MS,Cui Z,Ruan X,Zhou QW,Fu XY,Liu ZY,Ma QY,Feng Y
Journal: Applied Sciences (Switzerland)

Employing the first-principles calculations within density functional theory (DFT) combined with the nonequilibrium Green's function, we investigated the interfacial electronic, magnetic, and spin transport properties of Mn2CoAl/Ag/Mn2CoAl current-perpendicular-to-plane spin valves (CPP-SV). Due to the interface rehybridization, the magnetic moment of the interface atom gets enhanced. Further analysis on electronic structures reveals that owing to the interface states, the interface spin polarization is decreased. The largest interface spin polarization (ISP) of 78% belongs to the MnCoT-terminated interface, and the ISP of the MnMnT1-terminated interface is also as high as 45%. The transmission curves of Mn2CoAl/Ag/Mn2CoAl reveal that the transmission coefficient at the Fermi level in the majority spin channel is much higher than that in the minority spin channel. Furthermore, the calculated magnetoresistance (MR) ratio of the MnCoT-terminated interface reaches up to 2886%, while that of the MnMnT1-terminated interface is only 330%. Therefore, Mn2CoAl/Ag/Mn2CoAl CPP-SV with an MnCo-terminated interface structure has a better application in a spintronics device.

Carbon Nanotubes with Cobalt Corroles for Hydrogen and Oxygen Evolution in pH 0–14 Solutions

Product: NanoDCAL
Date: 2018
Authors: Li X,Lei H,Liu J,Zhao X,Ding S,Zhang Z,Tao X,Zhang W,Wang W,Zheng X,Cao R
Journal: Angewandte Chemie - International Edition

Water splitting is promising to realize a hydrogen-based society. The practical use of molecular water-splitting catalysts relies on their integration onto electrode materials. We describe herein the immobilization of cobalt corroles on carbon nanotubes (CNTs) by four strategies and compare the performance of the resulting hybrids for H2 and O2 evolution. Co corroles can be covalently attached to CNTs with short conjugated linkers (the hybrid is denoted as H1) or with long alkane chains (H2), or can be grafted to CNTs via strong $pi$–$pi$ interactions (H3) or via simple adsorption (H4). An activity trend H1≫H3>H2≈H4 is obtained for H2 and O2 evolution, showing the critical role of electron transfer ability on electrocatalysis. Notably, H1 is the first Janus catalyst for both H2 and O2 evolution reactions in pH 0–14 aqueous solutions. Therefore, this work is significant to show potential uses of electrode materials with well-designed molecular catalysts in electrocatalysis.

KAgSe: A New Two-Dimensional Efficient Photovoltaic Material with Layer-Independent Behaviors

Product: NanoDCAL
Date: 2018
Authors: Wang Q,Li J,Liang Y,Nie Y,Wang B
Journal: ACS Applied Materials and Interfaces

Recent advances in the development of two-dimensional (2D) materials have stimulated people's interest and enthusiasm to discover new kinds of 2D functional materials. In this paper, we propose a novel 2D layered semiconductor KAgSe using the first-principles calculation method, which displays excellent photovoltaic properties with proper direct band gap and significant carrier mobility. By evaluating the cohesive energy, vibrational phonon spectrum, and temporal evolution of the total energy at a high temperature of 500 K, the KAgSe monolayer is proved to be stable. Finite cleavage energy comparable to that of black phosphorus implies the feasibility of mechanical exfoliation of a KAgSe monolayer from the bulk. Layered KAgSe shows a ∼1.5 eV direct band gap, which is roughly independent of the number of layers. Remarkable optical absorption coefficients in the visible light region and significant carrier mobilities reveal a favorable application prospect of layered KAgSe in photovoltaic devices. Especially, the layer-independent optical absorption provides enormous convenience and less difficulty in experimental fabrication of photoelectronic devices which are based on finite layer KAgSe. To further explore the photovoltaic behaviors, the polarization angle-related photocurrent is evaluated for the KAgSe monolayer-based nanodevice by irradiating a beam of linearly polarized light to the scattering region. Moreover, large photon responsivity and external quantum efficiency are also obtained for the KAgSe monolayer.

Perfect spin-filter and negative differential resistance for a carbon chain device with defective graphene nanoribbons connected

Product: NanoDCAL
Date: 2018
Authors: Cao F,Xia TS,Zhao WS
Journal: 2018 14th IEEE International Conference on Solid-State and Integrated Circuit Technology, ICSICT 2018 - Proceedings

In order to study the characteristics of carbon chains and defects in graphene nanoribbons, we studied the electronic transport characteristics for a carbon chain device with defective graphene nanoribbon connected. And density functional theory (DFT) combined with the non-equilibrium green's function (NEGF) is used as the research method. By analyzing the calculations, we found this structure has an excellent spin filtering effect. Meanwhile, we also observed the spin-up current has a negative differential resistance effect (NDR) at a low negative bias. The spin filter and NDR are simultaneously observed in a carbon-based device, which has great significance in spintronic research.

First Principles Simulation of Energy efficient Switching by Source Density of States Engineering

Product: NanoDCAL
Date: 2019
Authors: Liu F,Qiu C,Zhang Z,Peng LM,Wang J,Wu Z,Guo H
Journal: Technical Digest - International Electron Devices Meeting, IEDM

Achieving sub-60 mV/decade FET switching is critical for reducing power dissipation in integrated circuits. Here we propose and theoretically investigate steep slope switching made possible by a 'cold source' that suppresses 'hot' electrons at the thermal tail of the Fermi distribution. We show sub-60 mV/decade switching with: (i) using gapless/gapped graphene as injection source, (ii) introducing a band gap in the source of Si FET. The feasibility and design of the cold source are investigated by first principles on different metals, pocket doping and disorder.

Competition of L21 and L10 ordering in Pd2MnSe, Pd2FeSe, Pd2MnTe and Pd2FeTe Heusler alloys

Product: NanoDCAL
Date: 2019
Authors: Wu M,Han Y,Wang L,Yang T,Kuang M,Chen X,Wang X
Journal: Results in Physics

Based on first-principles method, for Pd2MnSe, Pd2FeSe, Pd2MnTe and Pd2FeTe, we investigated the competition between cubic L21-type and their tetragonal L10-type ordering. Firstly, low-energy tetragonal ground states are found in these alloys during the tetragonal deformation. By regulating the c/a ratio under the fixed volume of equilibrium cubic phase, we obtain two minimums in total energy-c/a curve: a shallow one in c/a 1. Meanwhile the uniform strain is also taken into consideration in this work, and we found that $Delta$EM and Vopt + X%Vopt are negatively correlated, that is, with the volume increases from Vopt − 3%Vopt to Vopt + 3%Vopt, the absolute value of $Delta$EM decreases accordingly. Secondly, the origin of the tetragonal ground states of Pd2 [Fe/Mn][Se/Te] are also be explained by the density of states (DOS) figures. From the valley-and-peak structure, we can find the DOS at or in the vicinity of the Fermi level is much broader and shallower in tetragonal L10 state than cubic L21 state, indicating the phase stability of tetragonal L10 phases of these Pd2-based alloys. Moreover, to further study the competition of L21 and L10 ordering in these alloys and verify the stability of the tetragonal L10 state, elastic constants were introduced in this work, C11, C12 and C44 for cubic state while C11, C12, C13, C33, C44, C66 for tetragonal L10-type structure. According to the judgment basis, all alloys exhibit L10-stable, satisfying our conclusions about the stability of tetragonal states. We hope our work can provide a guidance for researchers to further explore and study new magnetic functional tetragonal materials among the full-Heusler alloys.

Spin thermoelectricity in dualhydrogenated zigzag silicene nanoribbons with surface adsorptions

Product: NanoDCAL
Date: 2019
Authors: Liu Y,Chen X,Yang X
Journal: Physics Letters, Section A: General, Atomic and Solid State Physics

The buckled structure of silicene provides a feasible pathway to influence its electric and magnetic properties via surface adsorptions. Here, we investigate the magnetic and spin thermoelectric transport properties of dual-hydrogenated zigzag silicene nanoribbons (ZSiNRs) without/with the hydrogen adsorption. The band gaps for two spin channels in ZSiNRs under the hydrogen adsorption are shifted near the Fermi level, leading to the appearance of spin Seebeck effect. Using a temperature difference, one can derive the carriers with the different spin index to flow in the opposite direction. Moreover, a large rectification ratio close to 105 at room temperature is achieved for the spin current, and the charge current exhibits a remarkable negative differential thermoelectric resistance (NDTR) behavior. The results presented here are fascinating potential applications in the fields of silicon-based spin caloritronic devices.

Transition metal-containing molecular devices: Controllable single-spin negative differential thermoelectric resistance effects under gate voltages

Product: NanoDCAL
Date: 2019
Authors: Yang X,Tan F,Dong Y,Yu H,Liu Y
Journal: Physical Chemistry Chemical Physics

Based on the non-equilibrium Green function method combined with density functional theory, we investigate the spin-resolved transport through transition metal (TM) (= Cr, Mn, Fe and Ru)-containing molecular devices in the presence of zigzag graphene nanoribbon (ZGNR) electrodes. The wave-function mismatch for the single-spin component results in a perfect spin-filtering property near the Fermi level. Moreover, we also observe Fano and Breit-Wigner resonance peaks in the transmission spectrum. Under a temperature gradient, the single-spin electric current exhibits a remarkable negative differential thermoelectric resistance (NDTR) in the Ru-complex molecular device, which originates from the Fano asymmetry of the single-spin transmission peak near the Fermi level. A gate voltage allows for a precise control of the single-spin NDTR in the Ru-complex molecular device.

A self-powered phosphorene photodetector with excellent spin-filtering and spin-valve effects

Product: NanoDCAL
Date: 2019
Authors: Luo Y,Xie Y,Ye X,Wang Y
Journal: Physical Chemistry Chemical Physics

Spin-filtering and spin-valve effects are fundamental issues of spintronics in two-dimensional materials, where a self-powered nanotechnology is also highly desired for low-power consumption. Herein, we report a self-powered nickel-phosphorene-nickel photodetector driven by photogalvanic effects (PGEs), based on quantum transport simulations. Persistent photocurrent is generated at zero bias due to PGEs induced by vertical illumination with linearly and elliptically polarized light. Moreover, fully spin-polarized photocurrent and large magnetoresistance can be obtained by tunneling the photon energy and light polarization, which indicates both excellent spin-filtering and spin-valve effects. These results suggest a promising application of PGE-driven phosphorene photodetectors in low energy-consumption spintronic devices.

Spin-related thermoelectric transport in wedge-shaped graphene nanoribbon junctions

Product: NanoDCAL
Date: 2019
Authors: Li J,Wang B,Wang J
Journal: Physica E: Low-Dimensional Systems and Nanostructures

Spin-related thermoelectric effect in low-dimensional system has become a very important research filed in nanoelectronics and spintronics. In this work, we report a first principles calculation of spin-resolved Seebeck effect as well as transport properties in wedge-shaped graphene ribbon nanojunctions. A series of spin-resolved transmission dips are formed for the single channel transport due to the quasi-bound states induced by back scattering of electrons near the wedge-shape edges in the center of the junctions. Near the spin-resolved anti-resonant dips, large spin thermopower and spin figure of merits can be achieved. Moreover, by adjusting the chemical potential and temperatures of both leads, pure spin current without charge current can be obtained near the energies of transmission dips for each nanojunction. The proposed wedge-shaped graphene nanojunctions may have potential application as low-dimensional nanoscale thermoelectric devices.

First-principle studies on electron transport properties in four-terminal MoS2 nanoribbons

Product: NanoDCAL
Date: 2019
Authors: Yang Y,Han X,Han Y,Gong WJ
Journal: Physica B: Condensed Matter

We perform the first-principle studies on electron transport through four-terminal MoS2 nanoribbons. It is found that in such structures, insulating bands exist in the linear-conductance spectra of the nanoribbons with straight channels, which are related to the band gaps of the MoS2 nanoribbons. However, nonzero transports are allowed to take place in the insulating-band region in the geometries with curved channels. This phenomenon can be attributed to the formation of edge states in the curved-channel structures. We believe that these results provide useful information for the manipulation of electron transport through MoS2 nanoribbons.

Transport and photogalvanic properties of covalent functionalized monolayer black phosphorus

Product: NanoDCAL
Date: 2019
Authors: Sun C,Wang Y,Jiang Y,Yang ZD,Zhang G,Hu Y
Journal: New Journal of Chemistry

Covalent functionalization is an efficient approach to modulate the nanoelectronic properties, advance the charge separation efficiency, and thus optimize the optoelectronic applications of nanomaterials. Here, the electronic structures, transport properties, and linear photogalvanic effects of two dimensional (2D) monolayer black phosphorus (MLBP) functionalized by PtCl2 groups, (PtCl2)n-MLBP (n = 1, 2, and 4) have been theoretically investigated using density functional theory (DFT), nonequilibrium Green's function (NEGF), and the Keldysh nonequilibrium Green's functions (KNEGF) methods. In the functionalized systems, the valence band maximum comes from the PtCl2 groups while the conduction band minimum originates from the MLBP. Such spatial charge separation hinders electron-hole recombination, suggesting MLBP-based 2D materials as promising candidates for solar cells. The MLBP-based device exhibits opposite responses for the transport properties and linear photogalvanic effects. The transport properties of MLBP along both the armchair and zigzag directions decrease after being grafted with PtCl2 groups. Moreover, the larger the grafting density n, the lower the conductivity. In comparison, the linear photogalvanic effects augment after functionalization, and this phenomenon is more significant with large n. Further, the photoresponse in the zigzag direction is larger by an order of magnitude than that in the armchair direction.

Toward barrier free contact to MoSe2/WSe2 heterojunctions using two-dimensional metal electrodes

Product: NanoDCAL
Date: 2019
Authors: Wang T,Jin H,Li J,Wei Y
Journal: Nanotechnology

In the design of electronic devices based on two-dimensional heterojunctions, the contact between electrodes and different surfaces of two-dimensional heterojunctions may produce different effects. Furthermore, metal-semiconductor contact plays an important role in modern devices. However, due to the Fermi level pinning effect (FLPE), it is difficult to tune the Schottky barrier height between common metals (e.g. Au, Ag, and Cu) and semiconductors. Fortunately, the FLPE becomes weak at the contact between the 2D metal and 2D semiconductor, due to the suppression of metal-induced gap states. Here, we choose monolayer NbS2 as the electrode to be in contact with the MoSe2/WSe2 bilayer. The interfacial properties as well as the stacking dependence are discussed based on the density functional theory, combined with the nonequilibrium Green's functions. Two configurations are considered, i.e. the WSe2/MoSe2/NbS2 and MoSe2/WSe2/NbS2 stacking sequences. Our results show that barrier free contact can be formed in these 2D metal-semiconductor junctions (MSJs). In addition, the transport properties of the proposed devices are sensitive to the stacking sequence. The drain-source current versus bias voltage (I-V) curve exhibits a linear relationship for the WSe2/MoSe2/NbS2 system and its resistance is much lower than the MoSe2/WSe2/NbS2 MSJ. Detailed analysis reveals that the transport properties are governed by the electronic coupling between specific interlayer states. In WSe2/MoSe2/NbS2 configuration, large overlapping states are observed, which facilitate charge transfer and result in good ohmic contact. Our work may provide a theoretical guidance for the designing of next-generation ultrathin and flexible devices.

Cell imaging using two-photon excited CdS fluorescent quantum dots working within the biological window

Product: NanoDCAL
Date: 2019
Authors: Zhang N,Liu X,Wei Z,Liu H,Peng J,Zhou L,Li H,Fan H
Journal: Nanomaterials

In recent years, two-photon excited semiconductor quantum dots (QDs) have been the subject of intense investigation due to their long excitation wavelength which helps to achieve deeper penetration and higher image resolution in optical bioimaging. In this paper, water-soluble CdS QDs were synthesized using a hydrothermal method and applied to human liver hepatocellular carcinoma (HepG2) cells. The first-principles calculation suggested that the S-rich defected structure contributes to a narrower band gap compared to the pristine structure. The resulting fluorescence wavelength was significantly red shifted, which was attributed to the deep defect states emission. The large Stokes shifts (> 200 nm) of the QDs can eliminate the possible cross-talk between the excitation light and the emission light. Two-photon induced red fluorescence emission can avoid overlapping with the autofluorescence emission of biological samples. The uptake and cell viability measurements of the HepG2 cells showed a good biocompatibility and a low toxicity of CdS QDs. Two-photon excited scanning microscopy images revealed that the HepG2 cells incubated with CdS QDs emitted bright red upconversion fluorescence and the fluorescence brightness was 38.2 times of that of the control group. These results support CdS QDs as a good candidate for application in cellular imaging.

First principles research on the dynamic conductance and transient current of black phosphorus transistor

Product: NanoDCAL
Date: 2019
Authors: Wang B,Li J,Xu F,Jin H,Wan L,Yu Y,Wei Y
Journal: Journal of Physics D: Applied Physics

The validity of high frequency technique and time-domain measurement to nanoscale electronic devices provides an imperious demand to explore the ultrafast electron dynamics and nonlinear responses accompanied with material science in theory. In this work, we carried out a first principles calculation to research the dynamic response in both frequency and time domain of a nanoscale Cu/black phosphorus (Cu/BP) transistor. The system shows n-type transport behaviors due to the charge transfer from the Cu/BP contact to the central BP section, which is different from the p-type pristine BP transistor. By adjusting the gate voltage, on-off ratio of conductance can reach up to 10 3 which is expected to further increase with the length of the central BP section. The Cu/BP transistor always shows capacitive-like behaviors even at high frequency, and cut-off frequency is estimated up to 75 THz. Transient current evolution shows abundant quantum scattering behaviors, and two important time scales were analyzed. The tune-on time is comparable to the Fermi velocity of pristine BP, and is roughly independent of the magnitudes of bias voltages. The relaxation time is roughly hundreds of femtoseconds, which corresponds to the cut-off frequency up to a point and can be further reduced by dephasing effect. The rapid response of hundreds of femtoseconds indicates that the Cu/BP transistor maybe work as high frequency nanoscale electronic device.

Electronic structure and transport properties of bilayer graphene adsorbed by LiF2 super-halogen clusters and Li3O super-alkali clusters

Product: NanoDCAL
Date: 2019
Authors: Ma Y,Li D,Feng X,Zhang H,Liang C
Journal: Journal of Physics D: Applied Physics

The lack of a band gap limits the practical applications of graphene in electronic devices. Using first-principles calculations with a van der Waals correction, we investigated the electronic structure and transport properties of bilayer graphene (BLG) adsorbed by super-halogen clusters and super-alkali clusters. BLG sandwiched between a pair of LiF2 super-halogen and Li3O super-alkali molecules enables the energy band to open a gap of about 0.36 eV and 0.26 eV near the Fermi energy, making the system exhibit semiconducting properties, which are attributable to the strong dipole electric field between the LiF2 super-halogen and Li3O super-alkali molecules. We also found that Li3O and the adjacent layer of graphene provide an energy band at the bottom of the conduction band and that LiF2 and the adjacent layer of graphene provide an energy band at the top of the valence band. The spatial separation of the electrons and holes is highly suitable for obtaining highly efficient photoelectric conversion in photovoltaic cells. We used the density functional theory and the non-equilibrium Green's function to determine the transport properties of the pristine BLG and the molecule-adsorbing BLG. A negative differential resistance effect occurred and an asymmetrical IV curve was observed under positive and negative bias, which is due to the inherent electric field effect between the graphene layers in the molecule-adsorbed BLG.

Spin current generation by thermal gradient in graphene/h-BN/graphene lateral heterojunctions

Product: NanoDCAL
Date: 2019
Authors: Jiang P,Tao X,Kang L,Hao H,Song L,Lan J,Zheng X,Zhang L,Zeng Z
Journal: Journal of Physics D: Applied Physics

Electron transport driven by a temperature gradient in a transport junction constructed by connecting a zigzag hexagonal boron nitride (h-BN) nanoribbon between two graphene nanoribbons (Gr/BN/Gr) is studied by density functional calculations and non-equilibrium Green's function method. When the zigzag-edged graphene nanoribbons are in the ferromagnetic configuration, the BN barrier introduces a spin-dependent scattering and causes an 'X'-type spin-dependent transmission functions around the Fermi level at equilibrium. In a linear response approximation, this gives rise to a Seebeck thermopower with opposite signs for different spins. It drives electrons with different spins to flow in opposite directions under a finite temperature gradient. Calculations show that the charge current is zero while spin current is not, thus pure spin current is generated. These findings suggests the great importance of BN barrier in the generation of thermal spin current using graphene and the idea should be taken into consideration in the design of spintronic devices using two dimensional materials.

High intrinsic ZT in InP3 monolayer at room temperature

Product: NanoDCAL
Date: 2019
Authors: Zhang S,Niu X,Xie Y,Gong K,Shao H,Hu Y,Wang Y
Journal: Journal of Physics Condensed Matter

Two-dimensional thermoelectric (TE) materials which have the figure of merit ZT that is greater than 1.5 at room temperature would be highly desirable in energy conversion since the efficiency is competitive to conventional energy conversion techniques. Here, we report that the indium triphosphide (InP3) monolayer shows a large ZT of 1.92 at 300 K, based on the quantum calculations within the ballistic thermal transport region. A remarkably low and isotropic phononic thermal conductivity is found due to the flat lattice vibration modes, which takes a major responsibility for the impressively high ZT at room temperature. Moreover, a large ZT of 1.67 can still be achieved even under a 1% mechanical tension on the lattice. These results suggest that the InP3 monolayer is a promising candidate for low dimensional TE applications.

Large magnetoresistance and spin-polarized photocurrent in La2/3Sr1/3MnO3(Co)/quaterthiophene/La2/3Sr1/3MnO3 organic magnetic tunnel junctions

Product: NanoDCAL
Date: 2019
Authors: Han X,Mi W,Wang X
Journal: Journal of Materials Chemistry C

Organic magnetic tunnel junctions (OMTJs) have become one of the hot topics in spintronic devices due to their structural adjustability and long spin lifetime. However, the spin-dependent transport properties in OMTJs with different spatial spin-polarized interfaces and their manipulation by light are not yet clear. Here, the spin-dependent transport properties in La2/3Sr1/3MnO3/quaterthiophene/La2/3Sr1/3MnO3 (LSMO/T4/LSMO) and Co/T4/LSMO OMTJs with different spatial spin polarizations and their light modulation are investigated systematically by theoretical calculations. It is found that a large tunneling magnetoresistance (TMR) appears in the OMTJs with a large spatial spin-polarized spinterface, wherein TMR will be reduced in the OMTJs with spatial spin polarization inversed spinterfaces. Furthermore, perfect spin injection efficiency can be obtained in both OMTJs. Specifically, the magnetization alignments of the ferromagnetic electrodes in the Co/T4/LSMO OMTJ can control the switch of the spin channel. Moreover, the fully spin-polarized photocurrent and spin battery appear in parallel and antiparallel magnetization configurations of the LSMO/T4/LSMO OMTJ, respectively. The results contribute to understanding the spin transport properties and designing multifunctional organic spintronic devices.

BX1-BX2 (X1, X2 = P, As, Sb) lateral heterostructure: Novel and efficient two-dimensional photovoltaic materials with ultra-high carrier mobilities

Product: NanoDCAL
Date: 2019
Authors: Wang Q,Li J,Liang Y,Wang B,Nie Y
Journal: Journal of Materials Chemistry A

Identifying lateral heterostructures (LHSs) with suitable two-dimensional (2D) building blocks is still an urgent challenge in materials science and device physics. In this work, we propose a new series of novel LHSs based on the ideal members of 2D group-VA derivatives, namely BX1-BX2 (X1, X2 = P, As, Sb) with different connecting edges. Through evaluating their stabilities, electronic properties, and optically related electric behaviors by using first principles calculation methods, a battery of novel and excellent properties are demonstrated including large formation energies, moderate direct band gaps, ultrahigh carrier mobilities, and efficient optical absorptions. Moreover, the electronic devices based on these LHSs show good performance for photoelectric conversion in the visible light region, and more appreciable photocurrents, photon responsivities, external quantum efficiencies, energy conversion efficiencies, and fill factors are explored compared with some other reported 2D materials. We believe the new group of stable and novel BX1-BX2 LHSs can provide a new strategy for experimental design and application of future photovoltaic conversion devices and excitonic solar cells.

Spin-filter transport and magnetic properties in a binuclear Cu(ii) expanded porphyrin based molecular junction

Product: NanoDCAL
Date: 2019
Authors: Montenegro-Pohlhammer N,Urzúa-Leiva R,Páez-Hernández D,Cárdenas-Jirón G
Journal: Dalton Transactions

Although the magnetic and transport properties of molecular junction systems composed of metalled porphyrins or phthalocyanines have been broadly studied in recent years, to date no studies have been devoted to evaluate the aforementioned properties in junction systems featuring metalled expanded porphyrins as active elements. The present work reports a detailed theoretical study of the magnetic and electronic transport properties of the recently synthesized dinuclear Cu(ii)-naphthoisoamethyrin complex (PyCu2). This is the first work on performing these kinds of studies using a magnetically coupled metallic expanded porphyrin as a molecular kernel. DFT and wave function-based methods have been used to determine the nature of the magnetic interaction between the metallic centres, characterized by the exchange coupling constant J, showing that although this was found to be weakly antiferromagnetic, after an exhaustive analysis it turns out that the coupling has a ferromagnetic nature with a value of J = 14.2 cm-1. Once the magnetic ground state of PyCu2 was rigorously established, the spin resolved transport properties of the device composed of the expanded porphyrin attached to two gold nano-wires were studied by means of the combination of DFT and the nonequilibrium Green's function formalism, in order to explore PyCu2 prospects as a possible spintronic device.

Ballistic transport in bent-shaped carbon nanotubes

Product: NanoDCAL
Date: 2019
Authors: Wu Z,Xing Y,Ren W,Wang Y,Guo H
Journal: Carbon

We report theoretical investigations of ballistic quantum transport properties of smoothly bent semiconducting single-walled carbon nanotubes (SWCNTs). The SWCNT is doped into NxN and PxP forms where N and P stand for N-type and P-type doping, x takes N-type, P-type or intrinsic I-type. Our calculation is based on a state-of-the-art non-equilibrium Green's function approach combined with density functional theory. The smooth bent induces a small electron redistribution on the SWCNT arc, which leads to rich and major transport differences between the NxN and PxP tubes. Conductance G of NNN tubes does not change with the bending angle $beta$, while G of PPP tubes decreases with it. G of NPN and PNP tubes increases with $beta$, while that of the PNP tubes varies with it in an oscillatory manner. The bent induced transport phenomena can be well understood by analyzing the microscopic physics of the electronic density distribution and quantum interferences between scattering states which traverse different paths along the bent-shaped tubes. The predicted conductance versus bent angle is useful for estimating how a flexible system may behave when strained and/or bent.

Realizing robust half-metallic transport with chemically modified graphene nanoribbons

Product: NanoDCAL
Date: 2019
Authors: Song L,Jin S,Jiang P,Hao H,Zheng X,Zhang L
Journal: Carbon

Although many chemical modification schemes for achieving half-metallicity in zigzag-edged graphene nanoribbons (ZGNRs) have been proposed, practically, half-metallic transport is hardly observable with them due to the resulting negligible energy difference of the anti-ferromagnetic (AF) and ferromagnetic (F) configurations between the two edges. We propose a scheme to achieve robust half-metallic transport by such ZGNRs in which central carbon atoms are substituted by BN pairs. We build transport junctions by connecting the top edge of one ribbon with the bottom edge of another through a carbon tetragon or a carbon hexagon. For both connection styles, we consider two different relative orientations of the BN pairs in the two ribbons, namely, a “BN-BN” case with the BN pairs in the same direction and a “BN-NB” case with the BN pairs in the opposite directions. It is found that, for both connection styles, we can always get a BN configuration where the junction is always in a perfect half-metallic state, independent of the magnetic configurations. It is understood by the matching or mismatching of the spin polarity and spatial separation of the edge states of the two ribbons. This should be taken into consideration in the design of spintronic devices with graphene nanoribbons.

First principles modeling of pure black phosphorus devices under pressure

Product: NanoDCAL
Date: 2019
Authors: Rong X,Yu Z,Wu Z,Li J,Wang B,Wang Y
Journal: Beilstein Journal of Nanotechnology

Black phosphorus (BP) has a pressure-dependent bandgap width and shows the potential for applications as a low-dimensional pressure sensor. We built two kinds of pure BP devices with zigzag or armchair conformation, and explored their pressure-dependent conductance in detail by using first principles calculations. The zigzag BP devices and the armchair BP devices exhibit different conductance-pressure relationships. For the zigzag BP devices conductance is robust against stress when the out-of-plane pressure ratio is less than 15%, and then increases rapidly until the conductive channels are fully opened. For the armchair pure BP devices conductance decreases at first by six orders of magnitude under increasing pressure and then increases quickly with further increase of pressure until the devices enter the on-state. This shows that the pure zigzag BP devices are more suitable for the application as flexible electronic devices with almost constant conductance under small pressure, while armchair BP devices can serve as bidirectional pressure sensors. Real-space distributions of band alignments were explored to understand the different pressurerelated properties. We fitted a set of parameters based on the results from the empirical Wentzel-Kramers-Brillouin method, which provides an effortless approximation to quantitatively predict the pressure-related behaviors of large pure BP devices.

Spin-polarized quantum transport in Fe 4 N based current-perpendicular-to-plane spin valve

Product: NanoDCAL
Date: 2019
Authors: Feng Y,Cui Z,sheng Wei M,Wu B
Journal: Applied Surface Science

Fe 4 N has been confirmed to possess high spin polarization of 81.3% and low Gilbert damping constant of 0.021 ± 0.02 in the recent experiment. To explore the potential applications of Fe 4 N in spintronics devices, the current-perpendicular-to-plane spin valve employing Fe 4 N as electrode and Ag as spacer is simulated to study the spin polarized quantum transport by utilizing the first principles calculations combined with nonequilibrium Green's function. The project density of states (PDOS), transmission coefficient, spin-polarized current, magnetoresistance (MR) ratio and spin injection efficiency (SIE) as a function of bias voltage are studied. Our calculations reveal that spin down electron is the majority spin polarized electron and the absolute value of MR ratio of Fe 4 N/Ag/Fe 4 N at equilibrium reaches up to 174%, and it decreases with the bias increases. Besides, our results indicate that Fe 4 N/Ag/Fe 4 N device has stable SIE value of about 40% and stable MR ratios of about 150% when bias increases from 0 V to 0.1 V, and the device has a better performance within this voltage range.

Strain controlling transport properties of heterostructure composed of monolayer CrI3

Product: NanoDCAL
Date: 2019
Authors: Yang B,Zhang X,Yang H,Han X,Yan Y
Journal: Applied Physics Letters

The modulation of the magnetic state and spin orientation in two-dimensional (2D) intrinsic magnets is important for controlling the spin-dependent transport properties of 2D magnet-based heterostructures. In this work, using first-principles calculations, it is found that the Néel antiferromagnetic (AFM) state with in-plane spin and the ferromagnetic (FM) state with in-plane and out-of-plane spin can be achieved in monolayer CrI3 under appropriate in-plane strains. In particular, the conductance of the Graphite/monolayer-CrI3/Graphite van der Waals heterostructure increases with the increase in the tensile strain, and the rate of change in conductance reaches more than 1800% when the strain becomes larger than 20%, which is significantly larger than that of the van der Waals heterostructure with a nonmagnetic insulator as a barrier to the magnetic field. Interestingly, when the magnetic state in monolayer CrI3 is switched from the Néel AFM to FM state by strain, the anisotropy magnetoresistance (AMR) ratio of the Graphite/monolayer-CrI3/Graphite heterostructure changes from -34.8% to 70%. The changes of AMR and conductance with strain originate mainly from the variation of the bandgap of monolayer CrI3 and the average transmission channels of graphite. These findings enrich the method in tuning spin orientation and provide the route for controlling transport properties of the heterostructure by strain tuning spin orientation in 2D magnets.

Electronic transport of organic-inorganic hybrid perovskites from first-principles and machine learning

Product: NanoDCAL
Date: 2019
Authors: Li L,You Y,Hu S,Shi Y,Zhao G,Chen C,Wang Y,Stroppa A,Ren W
Journal: Applied Physics Letters

Using the data-mining machine learning technique and the non-equilibrium Green's function method in combination with density functional theory, we studied the electronic transport properties of the organic-inorganic hybrid perovskite MAPbI 3 . The band structures of MAPbI 3 from first-principles show that the ferroelectric and antiferroelectric dipole configurations have very little influence on the energy bandgap. Furthermore, we investigated the tunnel junctions made of MAPbI 3 and 48 different metal electrodes, with the same fixed lattice constant as MAPbI 3 . With the increase in the number of perovskite unit cells, the electron transmission coefficients are found to decrease exponentially in general. For data mining studies, several different methods are employed to develop models for predicting electron transport properties. In particular, the gradient boosting regression tree model was tested and found to be the most effective tool among all these algorithms for fast prediction of the electron transmission coefficients and performance ranking of all studied metal electrodes.

Total iron measurement in human serum with a smartphone

Product: NanoDCAL
Date: 2019
Authors: Serhan M,Sprowls M,Jackemeyer D,Long M,Perez ID,Maret W,Tao N,Forzani E
Journal: AIChE Annual Meeting, Conference Proceedings

Iron deficiency, a leading cause of anemia, is one of the globe's top nutritional disorders according to the World Health Organization. Hemochromatosis, on the other hand, is associated to excess iron, and is usually diagnosed late in the stages of irreversible organ damage. Since abnormally low or high blood iron levels are common worldwide and can be of serious detriment to human health, a ubiquitously available technique for measurement of blood iron could represent a substantial improvement in point-of-care medical technology for monitoring iron-related blood disorders and could potentially spark a trend toward proper early prevention of diseases and health maintenance throughout the life span. Here, we introduce a smartphone-based colorimetric detection system for iron measurement in human serum. The system is designed for point-of-care screening and iron monitoring, and was optimized to be low-cost while still allowing for accurate, rapid iron assessment. It employs a dry sensor strip with optimized chemistry in which iron ions are stripped from blood transport proteins, reduced from Fe(III) to Fe(II), and subsequently chelated with ferene, developing a visible color change for smartphone detection of total iron. We compare the common laboratory iron detection assay of human serum to that of our dry sensor strip. The prototype smartphone assay was sensitive to iron detection with a dynamic range of 50 - 300 µg/dL, sensitivity of 0.00047 a.u/µg/dL and coefficient of variation of 10.5% versus the standard lab approach with sensitivity of 0.00091 a.u/µg/dL and coefficients of variation of 2.2%. Further, a detection limit near 15 µg/dL provided by the smartphone system indicated the system's potential capability for detection of iron deficiencies.,Finally, drawn human venous blood sample processed for serum and measured for total iron were simultaneously sent to a commercial testing facility (LabCorp) and processed by the smartphone system, averaging errors of less than 3% around the true value of 231 µg/dL. In addition, spectrophotometric validation of the iron detection kinetics for the test conditions were investigated, rendering a more complete insight of the detection reaction. The new mobile-app based colorimetric assay agreed with the standard spectrophotometric method, and provides promising features of mobility and low-cost manufacturing for global healthcare settings.

Anomalous and Polarization-Sensitive Photoresponse of Td-WTe2 from Visible to Infrared Light

Product: NanoDCAL
Date: 2019
Authors: Zhou W,Chen J,Gao H,Hu T,Ruan S,Stroppa A,Ren W
Journal: Advanced Materials

Recently, an emergent layered material Td-WTe2 was explored for its novel electron–hole overlapping band structure and anisotropic inplane crystal structure. Here, the photoresponse of mechanically exfoliated WTe2 flakes is investigated. A large anomalous current decrease for visible (514.5 nm), and mid- and far-infrared (3.8 and 10.6 µm) laser irradiation is observed, which can be attributed to light-induced surface bandgap opening from the first-principles calculations. The photocurrent and responsivity can be as large as 40 µA and 250 A W−1 for a 3.8 µm laser at 77 K. Furthermore, the WTe2 anomalous photocurrent matches its in-plane crystal structure and exhibits light polarization dependence, maximal for linear laser polarization along the W atom chain a direction and minimal for the perpendicular b direction, with the anisotropic ratio of 4.9. Consistently, first-principles calculations confirm the angle-dependent bandgap opening of WTe2 under polarized light irradiation. The anomalous and polarization-sensitive photoresponses suggest that linearly polarized light can significantly tune the WTe2 surface electronic structure, providing a potential approach to detect polarized and broadband lights up to far infrared range.

Ferromagnetic, Ferroelectric, and Optical Modulated Multiple Resistance States in Multiferroic Tunnel Junctions

Product: NanoDCAL
Date: 2019
Authors: Yin L,Wang X,Mi W
Journal: ACS Applied Materials and Interfaces

In data storage devices, spin, ferroelectric, or optical indices have been utilized as information carriers, and the binate couplings among the three parameters are explored to increase the resistance states and resultant data-density. However, studies holding all of the three information indices are still blank, where the increasing number of information carriers from previous two to three provides opportunities for inducing novel phenomena and distinct resistance states. In this work, using the spin-electron-photon resolved theory, we demonstrate the feasibility of spin, ferroelectric, and optical interactions, which are further detected by a spin- and ferroelectric-modulated photovoltaic effect in La 2/3 Sr 1/3 MnO 3 /BiFeO 3 /Fe 4 N multiferroic tunnel junctions (MFTJs). Moreover, based on the spin- and ferroelectric-induced four resistance states in MFTJs, the special photovoltaic effect shall split each resistance state into light-on and light-off switching states, which finally lead to multiple resistance states. Besides, nearly 100% spin-polarized photocurrent and large tunneling magnetoresistance (electroresistance) are realized in these MFTJs. These results reveal that interacted spin, ferroelectric, and optical indices can simultaneously serve as information carriers in storage devices, which provide guidance for developing efficient data memories.

Photoexcited charge carrier behaviors in solar energy conversion systems from theoretical simulations

Product: NanoDCAL
Date: 2020
Authors: Wei W,Huang B,Dai Y
Journal: Wiley Interdisciplinary Reviews: Computational Molecular Science

Solar energy bears great potential in the substitution of conventional fossil fuels to enable a sustainable world. In order to harness and use solar energy, photocatalytic and photovoltaic systems made of semiconductor materials have been developed to convert sunlight into energy (electricity) based on the photoelectrochemical effects. Photoelectric events are related with the light–energy (electricity) conversion process, including photon absorption, photoexcited charge carrier separation and recombination, charge carrier transport, and photocurrent generation, as well as electron plasmonic resonance. We focus on the recent state-of-the-art simulation methods correspondingly mimicking these photogenerated charge carrier behaviors, taking electron–phonon, electron–exciton, and exciton–exciton interactions into account. Results can be obtained from the standard density function theory (DFT) for ground-state properties, many-body perturbation theory for band gap renormalization and optical absorption, time-domain DFT in combination with nonadiabatic molecular dynamics for photoexcitation dynamics, nonequilibrium Green's function and self-consistent theory for charge carrier transport properties, and classical Maxwell's equations in discrete dipole approximation for localized surface plasmon resonance absorption and near-field enhancement. In combination of the results from these simulation methods, a complete and consistent picture describing the fundamental photoelectric process in light–energy (electricity) conversion could be obtained. Simulation results offer guidelines for experimental efforts and provide new basic insights into the underlying mechanisms and the design principles for next-generation photocatalytic and photovoltaic devices of high solar light utilization efficiency. This article is categorized under: Structure and Mechanism > Computational Materials Science Electronic Structure Theory > Ab Initio Electronic Structure Methods Software > Simulation Methods.

Decorating a WSe2 monolayer with Au nanoparticles: A study combined first-principles calculation with material genome approach

Product: NanoDCAL
Date: 2020
Authors: Tsao HW,Kaun CC,Su YH
Journal: Surface and Coatings Technology

Using first-principles calculations, we investigate the effect of absorbing different sizes of gold nanoparticles on a WSe2 monolayer. We find that absorption is via van der Waals interaction, giving different stable distances and adsorption energies for various sizes of gold nanoparticles. The absorbed Au nanoparticles dope the WSe2 monolayer and induce flat bands around the Fermi level, affecting the conductance of the systems. In addition, these calculated results form a data set for further machine learning studies. Combined with artificial neural network and genetic algorithm, the absorption energies and electronic states of extended systems are predicted.

Tunable type-I/type-II transition in g-C3N4/graphyne heterostructure by BN-doping: A promising photocatalyst

Product: NanoDCAL
Date: 2020
Authors: Yun J,Zhang Y,Ren Y,Kang P,Yan J,Zhao W,Zhang Z,Guo H
Journal: Solar Energy Materials and Solar Cells

As a promising photocatalytic material, g-C3N4 has drawn tremendous research interest. However, the fast charge recombination and narrow range of solar absorption restrain its practical application. Herein, for the first time, based on extensive hybrid functional calculations, graphyne (Gyne), a new two dimensional material, is used to form a layered vdW-nanohybrid with g-C3N4 to enhance the photoelectrocatalytic activity of g-C3N4. A comprehensive theoretical study of interfacial properties of g-C3N4/Gyne heterostructure including the band structure, partial density of state, optical absorption, wave functions, charge density difference, band alignment and photocurrent density is acquired to provide deep insight into the photocatalytic performance. The calculated results show that g-C3N4/Gyne heterostructure exhibits tremendous photocatalytic performance as that of recently experimentally synthesized Gyne family based nanocomposite, g-C3N4/graphdiyne (g-C3N4/GDyne). The designed g-C3N4/Gyne heterostructure has a fourfold increase in photocurrent density (0.937 $mu$A/mm2) compared with that of g-C3N4 (0.233 $mu$A/mm2). More importantly, the photocatalytic performance of g-C3N4/Gyne can be further improved by doping 2BN-pairs into Gyne layer. Theoretical prediction indicates that g-C3N4/2BN-Gyne even realizes a sevenfold increase in photocurrent density (1.669 $mu$A/mm2) due to the type II band alignment, broadened light absorption range and much smaller effective mass, providing helpful physical mechanism information for further optimizing the optoelectronic properties of g-C3N4/GDyne and g-C3N4/Gyne. Our theoretical work provides stepping stone into the design of highly efficient g–C3N4–based photocatalysts and a fully coherent picture about the interfaces of g-C3N4/GDyne and g-C3N4/Gyne heterostructures can also be obtained.

Interfacial effects on leakage currents in Cu/$alpha$-cristobalite/Cu junctions

Product: NanoDCAL
Date: 2020
Authors: Lin KB,Su YH,Kaun CC
Journal: Scientific Reports

As the miniaturization trend of integrated circuit continues, the leakage currents flow through the dielectric films insulating the interconnects become a critical issue. However, quantum transport through the mainstream on-chip interfaces between interconnects and dielectrics has not been addressed from first principles yet. Here, using first-principles calculations based on density functional theory and nonequilibrium Green's function formalism, we investigate the interfacial-dependent leakage currents in the Cu/$alpha$-cristobalite/Cu junctions. Our results show that the oxygen-rich interfaces form the lowest-leakage-current junction under small bias voltages, followed by the silicon-rich and oxygen-poor ones. This feature is attributed to their transmission spectra, related to their density of states and charge distributions. However, the oxygen-poor interfacial junction may conversely have a better dielectric strength than others, as its transmission gap, from −2.8 to 3.5 eV, is more symmetry respect to the Fermi level than others.

Ab Initio and Theoretical Study on Electron Transport through Polyene Junctions in between Carbon Nanotube Leads of Various Cuts

Product: NanoDCAL
Date: 2020
Authors: Chen YR,Lin MK,Chan DH,Lin KB,Kaun CC
Journal: Scientific Reports

In this study we look into the interference effect in multi-thread molecular junctions in between carbon-nanotube (CNT) electrodes of assorted edges. From the tube end into the tube bulk of selected CNTs, we investigate surface Green's function and layer-by-layer local density of states (LDOS), and find that both the cross-cut and the angled-cut armchair CNTs exhibit 3-layer-cycled LDOS oscillations. Moreover, the angled-cut armchair CNTs, which possess a zigzag rim at the cut, exhibit not only the oscillations, but also edge state component that decays into the tube bulk. In the case of cross-cut zigzag CNTs, the LDOS shows no sign of oscillations, but prominent singularity feature due to edge states. With these cut CNTs as leads, we study the single-polyene and two-polyene molecular junctions via both ab initio and tight-binding model approaches. While the interference effect between transport channels is manifested through our results, we also differentiate the contributions towards transmission from the bulk states and the edge states, by understanding the difference in the Green's functions obtained from direct integration method and iterative method, separately.

Large spin Hall effect and tunneling magnetoresistance in iridium-based magnetic tunnel junctions

Product: NanoDCAL
Date: 2020
Authors: Zhou JQ,Zhou HY,Bournel A,Zhao WS
Journal: Science China: Physics, Mechanics and Astronomy

Magnetic tunnel junctions (MTJs) switched by spin-orbit torque (SOT) have attracted substantial interest owing to their advantages of ultrahigh speed and prolonged endurance. Both field-free magnetization switching and high tunneling magnetoresistance (TMR) are critical for the practical application of SOT magnetic random access memory (MRAM). In this work, we propose an MTJ structure based on an iridium (Ir) bottom layer. Ir metal is a desirable candidate for field-free SOT switching owing to its strong intrinsic spin Hall conductivity (SHC), which can be enhanced via doping. Herein, we study TMR in Ir-based MTJs with symmetric and asymmetric structures. Ir-based MTJs exhibit large TMR, which can be further enhanced by heavy metal symmetry owing to the resonant tunneling effect. Our comprehensive investigations illustrate that Ir-based MTJs are promising candidates for realizing SOT switching and high TMR.

Mesoscopic electronic transport in twisted bilayer graphene

Product: NanoDCAL
Date: 2020
Authors: Han Y,Zeng J,Ren Y,Dong X,Ren W,Qiao Z
Journal: Physical Review B

We numerically investigate the electronic transport properties between two mesoscopic graphene disks with a twist by employing the density functional theory coupled with the nonequilibrium Green's function technique. By attaching two graphene leads to upper and lower graphene layers separately, we explore systematically the dependence of electronic transport on the twist angle, Fermi energy, system size, layer stacking order, and twist axis. When choosing different twist axes for either AA-or AB-stacked bilayer graphene, we find that the dependence of conductance on twist angle displays qualitatively distinction, i.e., the systems with "top"axis exhibit finite conductance oscillating as a function of the twist angle, while the ones with "hollow"axis exhibit nearly vanishing conductance for different twist angles near the charge neutrality point. These findings suggest that the choice of twist axis can effectively tune the interlayer conductance, making it a crucial factor in designing of nanodevices with the twisted Van Der Waals multilayers.

In-Plane Dual-Gated Spin-Valve Device Based on the Zigzag Graphene Nanoribbon

Product: NanoDCAL
Date: 2020
Authors: Zhou M,Jin H,Xing Y
Journal: Physical Review Applied

Using the nonequilibrium Green's function combined with density-functional theory, we systematically study the transport properties of the zigzag graphene nanoribbon (ZGNR). When an external transverse electric field is applied, a ZGNR can exhibit half-metallic characteristics with its conductive channel localized at the edge positions. Accordingly, an in-plane dual-gated spin-valve device is proposed. Thanks to its unique design, the proposed device overcomes the bottleneck of current leakage and avoids contact issues. Remarkably, a 100% spin injection efficiency and a giant tunnel magnetoresistance of up to 107 are realized, which represent much better performance than that of traditional magnetic tunnel junctions. In addition, we also explore the effect of architecture reconstruction and impurity doping on spin-polarized transport. It is found that, in general, the tunneling process is hindered if disorder occurs at the conductive edge. By contrast, if we put the deformation or impurities away from the conductive edge, the transport properties may be barely degraded or even improved, depending on the type of disorder. It should be possible to apply these ubiquitous underlying principles to other materials, which could inspire the design of alternative spintronic devices with high performance.

Switching at less than 60 mv/decade with a “cold” metal as the injection source

Product: NanoDCAL
Date: 2020
Authors: Liu F
Journal: Physical Review Applied

Power dissipation is a great challenge for the continuous scaling down and performance improvement of CMOS technology, due to the thermionic-current switching limit of conventional MOSFETs. In this paper, we show that this problem can be overcome by using a "cold"metal as the injection source of a transistor; these metals are different from conventional metals and can filter out high-energy electrons to break the "Boltzmann tyranny."It is proved that the subthreshold swing of the thermionic current of a transistor using a "cold"-metal contact can be much smaller than 60 mV/decade at room temperature. Specifically, the two-dimensional transition-metal-dichalcogenide (TMD) "cold"metals NbX2 and TaX2 (X=S,Se,Te) are proposed as injection sources for FETs. Quantum transport simulations indicate that a promising switching efficiency and on-state current can be achieved using TMD "cold"-metal injection sources, which is beneficial for energy efficiency.

Significant tunneling magnetoresistance and excellent spin filtering effect in CrI3-based van der Waals magnetic tunnel junctions

Product: NanoDCAL
Date: 2020
Authors: Yan Z,Zhang R,Dong X,Qi S,Xu X
Journal: Physical Chemistry Chemical Physics

van der Waals (vdW) heterojunctions stacked by different two-dimensional (2D) layered materials not only exhibit the complementary effect of short plates, but also harbor novel physical phenomena. In particular, the emergence of 2D magnetic vdW materials has provided novel opportunities for the application of these materials in spintronics. However, to the best of our knowledge, to date, the spin-related transport mechanism in magnetic tunnel junctions (MTJs) based on these 2D vdW magnetic materials and the effect of pinning layers on their transport properties have not been elucidated by the non-equilibrium state theory. Herein, based on first-principles calculations, we report the spin-polarized quantum transport properties of sandwich-type vdW magnetic tunnel junctions (CrI3/h-BN/ntextperiodcenteredCrI3) comprising monolayer CrI3, a hexagonal boron nitride (h-BN) spacer layer, and n-layer CrI3 (n = 1, 2, 3, and 4). Considering the inter-layer antiferromagnetic coupling in n-layer CrI3, a few layers of CrI3 can be regarded as its own natural pinning layers. Especially, when n is equal to 3, an almost fully spin-polarized current and large tunnel magnetoresistance ratio (3600%) are obtained in the equilibrium state. Excitingly, due to different numbers of pinning layers in MTJs, the transport properties of these MTJs at positive bias voltages exhibit an interesting odd-even effect within a limited thickness of these pinning layers. Moreover, an almost perfect spin filtering effect and remarkable negative differential resistance (NDR) were observed in the MTJs where n was odd (n = 1 and 3). The observed non-equilibrium quantum transport phenomenon is explained by spin-dependent transmission coefficient at different bias voltages. Our results provide effective guidance for the experimental studies of the MTJs based on 2D magnetic vdW materials.

Reassessing destructive quantum interference in azulene-based devices

Product: NanoDCAL
Date: 2020
Authors: Saraiva-Souza A,Smeu M,Guo H
Journal: Physical Chemistry Chemical Physics

Quantum interference (QI) effects have recently attracted increased interest in electron transport studies of single molecular junctions. Although QI effects have been explained in a variety of molecular devices by different chemical rules, such as orbital-based prediction, the graphical scheme, and cross-conjugated states, recently, experimental and theoretical reports have claimed to have reached a better understanding of QI features. In particular, azulene molecule derivatives present an insightful case study where these simple rules of thumb can fail. Here, we explore the validity of graphical rules and the effects of closed loops in the azulene molecular structure. The electron transport behavior through an azulene core with different moieties (thiol, ethynyl-thiol, phenyl-thiol, and ethynyl-phenyl-thiol) was investigated with first-principles calculations combined with the non-equilibrium Green's function (NEGF) technique. The transmission spectra at zero bias show that the graphical rules are not sufficient to predict and explain the destructive QI effect in these azulene derivatives. Instead, closed-loop diagrams should be taken into account to properly describe the transport properties in those systems, but the presence of a closed-loop does not necessarily lead to the absence of destructive QI in the transmission spectrum. Our results indicate that the destructive QI effect is found when the azulene core is coupled at the 4,7Az-, 5,7Az- and 1,3Az-positions with ethynyl-phenyl-thiol moieties, while no obvious destructive QI effect is observed in the other azulene derivatives, either with the thiol, ethynyl-thiol or phenyl-thiol anchoring groups. We also demonstrated that the I-V curves depend more strongly on anchoring groups than the coupling position.

Thermal gradient driven spin current in BN co-doped ferromagnetic zigzag graphene nanoribbons

Product: NanoDCAL
Date: 2020
Authors: Song L,Jin S,Liu Y,Yuan L,Yang Z,Jiang P,Zheng X
Journal: Physica E: Low-Dimensional Systems and Nanostructures

Based on first principles calculations, we investigate thermal spin transport in a zigzag graphene nanoribbon (ZGNR) device with B and N atoms doped at the opposite edges. It is found that by applying a thermal gradient without a bias voltage across the BN co-doped ZGNR device, spin-up and spin-down charge currents flowing in opposite directions are induced, leading to spin current. Specifically, a giant Seebeck thermopower with opposite signs for the two spin channels is obtained in the linear response regime and the magnitude is dependent on the doping concentration. More importantly, by slightly tuning the chemical potential, pure spin current can be achieved. These findings suggest a feasible way for producing thermal spin current in ZGNRs and will be greatly instructive in the design of graphene based spintronic devices.

Highly spin polarized transport in $gamma$-zigzag graphyne nanoribbon junctions

Product: NanoDCAL
Date: 2020
Authors: Song L,Cao H,Chai X,Chen X,Ye Z,Zhou Y,Huang X,Zhu H,Zheng X
Journal: Physica E: Low-Dimensional Systems and Nanostructures

Using density functional calculations combined with non-equilibrium Green's functions, we investigate the BN co-doping effects on the electronic structure of zigzag $gamma$-graphyne nanoribbons and spin-dependent transport properties of related transport heterojunctions. The results show that, when properly selecting the doping sites of the B and N atoms at the two edges, half-metallicity is obtained due to the opposite energy shift in the two spin channels caused by the potential generated by the BN pairs. In transport junctions constructed by connecting two ribbons by only one edge of each ribbon to form a ‘Z'-shape structure, highly spin polarized transport is achievable, depending on the relative positions of the B and N dopants in the two ribbons. Our finding opens a new possibility of graphyne in spintronic device applications.

Photogalvanic effect in monolayer WSe2-MoS2 lateral heterojunction from first principles

Product: NanoDCAL
Date: 2020
Authors: Luo WM,Shao ZG,Qin XF,Yang M
Journal: Physica E: Low-Dimensional Systems and Nanostructures

Density functional theory calculations and non-equilibrium Green's function method are carried out to study the photovoltaic effect of monolayer WSe2-MoS2 lateral heterojunction under vertical irradiation. We report the photoresponse behavior under different polarized lights and photon energies combined with charge density difference, electronic structure, and the joint density states under illumination. According to the charge density difference, the electrons transfer from MoS2 to WSe2 before illumination, but here generates the photocurrent under the vertical illumination due to the broken spatial inversion asymmetry of the device. Remarkably, the photocurrent increases from zero then decreases, and finally a reverse breakdown occurs. The photocurrent increases from zero since the electrons will be excited from the valence band to the conduction band only if the photon energy is greater than the energy gap of heterojunction material. As the illumination energy increases, the concentration difference of the excited electrons first increases and then decreases, leading to the photocurrent first increases and then decreases. Finally, the current reverses and suddenly increases because the Zener breakdown occurs. Furthermore, the photoresponse of system for linearly polarized light is similar with that for elliptically polarized light. This work demonstrates the basic principle of photovoltaic effect of single-layer WSe2-MoS2 lateral heterojunction, which can be further applied for novel electronic and optoelectronic devices based on quantum confined 2D lateral heterostructures.

Transport and photoelectric properties of vertical black phosphorus heterojunctions

Product: NanoDCAL
Date: 2020
Authors: Sun C,Wang Y,Yang ZD,Shang Y,Zhang G,Hu Y
Journal: New Journal of Chemistry

The electronic structures and transport properties of the heterojunctions (X/BP) (X = BN, MoS2, or graphene) formed by vdW interactions of black phosphorus (BP) with insulative BN, semi-conductive MoS2, conductive graphene (G), and their corresponding intercalation composites X/Pt/BP and X/PtCl2/BP constructed by encapsulating Pt and PtCl2 into the heterojunction layers have been explored using density functional theory (DFT) and the nonequilibrium Green's function (NEGF). The linear photogalvanic effects of Pt and PtCl2 on the intercalation composites have been further studied by employing Keldysh nonequilibrium Green's function (KNEGF) methods. The BP and X layers are packed in a zigzag-zigzag (also an armchair-armchair) orientation. The band edges of BN/BP are mainly contributed by BP with a type-I alignment. The valence band of MoS2/BP is controlled by BP while the conduction band is dominated by MoS2, leading to a type-II alignment. Both BP and G contribute to the valence band and the conduction band; moreover, the Dirac-cone band ribbon of G and the direct band gap of BP are preserved in G/BP. For BN/BP-based systems, intercalating PtCl2 could largely improve the conductivity compared to inserting Pt. However, the case is exactly opposite in the G/BP-based species, where the conductivity of G/Pt/BP is much larger than those of G/PtCl2/BP and G/BP. While for MoS2/BP-based systems, Pt and PtCl2 exert similar effects on conductivity. Evident anisotropic transport property is found for BN/BP-based and G/BP-based systems, while no obvious anisotropic feature is observed for MoS2/BP-based systems. Under linear illumination, encapsulating PtCl2 shows a much stronger photoresponse than Pt. The strength of the photoresponse in PtCl2-doped systems can be tuned by the irradiation angle, the photon energy, and the type of X. All these fascinating properties can be interpreted from multiple factors such as band structures, Pt(PtCl2)-BP(X) interactions and BP-X interactions. These new 2D materials are especially attractive for electronic and optoelectronic devices. This journal is

Photon-assisted spin transport in blue phosphorene nanotubes

Product: NanoDCAL
Date: 2020
Authors: Li B,Zhu L,Wu C,Yao K,Chang CR
Journal: Nanotechnology

We have investigated photon-assisted spin injection into blue phosphorene nanotubes (PNTs) with ferromagnetic cobalt electrodes by nonequilibrium Green's function combined with light-matter interaction based on the first-order Born approximation. The results show the photo-induced spin current. The spin up and spin down photocurrents flow in opposite directions for zigzag blue nanotubes (ZPNTs) with anti-parallel magnetic configuration of the electrodes. By changing the structures of the blue phosphorene nanotube and the magnetization of the electrodes, multitudes of quantum spin transport properties are investigated, such as the nearly perfect photo-induced spin current and strong photo-polarization current signal. The results suggest that ZPNTs could serve as a potential material candidate for optical communication devices.

Largely enhanced thermoelectric effect and pure spin current in silicene-based devices under hydrogen modification

Product: NanoDCAL
Date: 2020
Authors: Qiao Q,Tan FX,Yang LY,Yang XF,Liu YS
Journal: Nanoscale

Based on the density functional theory and nonequilibrium Green's function methods, we launch a systematic study of the magnetic properties and thermoelectric effects in silicene-based devices constructed by using zigzag silicene nanoribbons (ZSiNRs). By modulating the adsorption site, it is found that the ground state of ZSiNRs varies from an antiferromagnetic state to a ferromagnetic state. Meanwhile, a spin-degenerate semiconductor evolves into a spin semiconductor. The spin and charge thermoelectric figure of merits have an almost equal value of about 60 in the narrow device, which originates from the spin-dependent conductance dips and high spin-filtering effects. Moreover, a thermally-driven pure spin current in the silicene-based devices is obtained in the absence of the gate voltage, and its magnitude is effectively enhanced as the device width increases. Our results suggest that the silicene-based devices have very good prospects for spin caloritronics.

Effectively modulating vertical tunneling transport by mechanically twisting bilayer graphene within the all-metallic architecture

Product: NanoDCAL
Date: 2020
Authors: Chen X,Wu T,Zhuang W
Journal: Nanoscale

Bilayer graphene possesses new degrees of freedom for modulating the electronic band structure, which makes it a tempting solution for overcoming the intrinsic absence of sizeable bandgaps in graphene and designing next-generation devices for post-silicon electronics. By twisting bilayer graphene, interlayer hybridized and twist angle-dependent van Hove singularities in the electronic band structure are generated and expected to facilitate the vertical tunneling transport between bilayer graphene. Herein, based on the ab initio quantum transport simulations, we designed a novel all-metallic vertical quantum transport architecture with the twisted bilayer graphene as the transport channel region and Au electrodes as the source/drain contacts to investigate the twist angle-dependent vertical transport properties. Enhancement in the ION/IOFF ratio by 2 orders of magnitude can be achieved by simply twisting the bilayer graphene. Compared to the traditional gate voltage modulation, which tunes the Fermi energy level alone, the current strategy shifts the Fermi energy level of the channel region away from the Dirac cone, moves the Fermi level and the van Hove singularities towards each other and promotes the vertical quantum transport due to the interlayer electronic hybridization. This dual modulation strategy of this novel mechanical gating device thus provides a potential new solution for designing novel vertical transistors.

Spin-polarized quantum transport in Si dangling bond wires

Product: NanoDCAL
Date: 2020
Authors: An Q,Hu C,Yu G,Guo H
Journal: Nanoscale

We report theoretical modeling of spin-dependent quantum transport properties of dangling bond wires (DBWs) on the Si(100)-2 × 1:H surface. A single spin-polarized dangling bond center (DBC) near the DBW may strongly affect transport as characterized by anti-resonances or dips in the transmission spectra. Such spin-dependent gating can be effective up to a distance of 1.5 nanometer between the DBW and the DBC. At the energies where anti-resonances occur, the scattering states of the system are found to be "attracted" to the DBC-rather than moving forward to the existing electrode. The variety of gating effects can be well organized by a physical picture, i.e. a strong hybridization or interaction between the spin-polarized DBW and DBC occurs with the same spin polarization (at DBW and DBC) and at similar energy. The sharp spin-resolved anti-resonance in the DBW gives rise to a spin-filtering effect up to 100% efficiency.

Edge induced band bending in van der Waals heterojunctions: A first principle study

Product: NanoDCAL
Date: 2020
Authors: Ou Y,Kang Z,Liao Q,Zhang Z,Zhang Y
Journal: Nano Research

The dangling bond free nature of two-dimensional (2D) material surface/interface makes van der Waals (vdW) heterostructure attractive for novel electronic and optoelectronic applications. But in practice, edge is unavoidable and could cause band bending at 2D material edge analog to surface/interface band bending in conventional three-dimensional (3D) materials. Here, we report a first principle simulation on edge band bending of free standing MoS2/WS2 vdW heterojunction. Due to the imbalance charges at edge, S terminated edge causes upward band bending while Mo/W terminated induces downward bending in undoped case. The edge band bending is comparable to band gap and could obviously harm electronic and optoelectronic properties. We also investigate the edge band bending of electrostatic doped heterojunction. N doping raises the edge band whereas p doping causes a decline of edge band. Heavy n doping even reverses the downward edge band bending at Mo/W terminated edge. In contrast, heavy p doping doesn't invert the upward bending to downward. Comparing with former experiments, the expected band gap narrowing introduced by interlayer potential gradient at edge is not observed in our free-standing structures and suggests substrate's important role in this imbalance charge induced phenomenon.[Figure not available: see fulltext.].

Metallic sandwiched-aerogel hybrids enabling flexible and stretchable intelligent sensor

Product: NanoDCAL
Date: 2020
Authors: Zhang H,Han W,Xu K,Zhang Y,Lu Y,Nie Z,Du Y,Zhu J,Huang W
Journal: Nano Letters

Flexible strain sensors have been widely investigated with their rapid development in human-machine interfaces, soft robots, and medical care monitoring. Here, we report a new in situ catalytic strategy toward the fabrication of metallic aerogel hybrids, which are composed of vanadium nitride (VN) nanosheets decorated with well-defined vertically aligned carbon nanotube arrays (VN/CNTs) for the first time. In this architecture, the two-dimensional VN nanosheets as the main bone structure are favorable for the flexible devices due to their excellent structural compatibility during the repetitive deforming process. In addition, the sandwiched aerogel hybrids form highly conductive 3D network, affording outstanding sensitivity for the strain-responsive behaviors. Further, the VN/CNTs-based flexible strain sensors are successfully fabricated, showing a high gauge factor of 386 within a small strain of 10%, fast response, and extraordinary durability. The monitoring of physical signals and an actual real-time human-machine controlling system based on the sensors are also presented.

Giant Conductance Enhancement of Intramolecular Circuits through Interchannel Gating

Product: NanoDCAL
Date: 2020
Authors: Chen H,Zheng H,Hu C,Cai K,Jiao Y,Zhang L,Jiang F,Roy I,Qiu Y,Shen D,Feng Y,Alsubaie FM,Guo H,Hong W,Stoddart JF
Journal: Matter

For neutral intramolecular circuits with two constitutionally identical branches, a maximum 4-fold increase in total conductance can be obtained according to constructive quantum interference (CQI). For charged intramolecular circuits, however, the strong electrostatic interactions entangle the quantum states of these two parallel pathways, thus introducing complicated transport behavior that warrants experimental investigation of the intramolecular circuit rules. Here, we report that a tetracationic cyclophane with parallel channels exhibits a 50-fold conductance enhancement compared with that of a single-channel control, an observation that supplements intramolecular circuit law in systems with strong Coulombic interactions. Flicker noise measurements and theoretical calculations show that strong electrostatic interactions between charged parallel channels—serving as the chemical gate to promote the effective conductance of each channel—and CQI boosts the total conductance of the two-channel circuit. The molecular design presented herein constitutes a proof-of-principle approach to charged intramolecular circuits that are desirable for quantum circuits and devices.

Strain-tunable photogalvanic effect in phosphorene

Product: NanoDCAL
Date: 2020
Authors: Wu JH,Zhai F,Lu JQ,Wu J,Feng X
Journal: Materials Today Communications

We investigate the strain manipulation of photocurrent induced by a linearly-polarized light in a phosphorene-based two-terminal system. A local out-of-plane gate voltage is applied to break the inversion symmetry of pristine phosphorene. The photocurrent under zero source-drain bias is calculated by the nonequilibrium Green's function formalism combined with density-functional theory. The photocurrent in the strain-free system is highly anisotropic and depends on the applied region of the gate voltage. Under a small tensile strain with strength ≈2%, the photoresponse for lights in the whole visible range is suppressed greatly. The photocurrent along the zigzag direction decay quickly with the strain strength. In contrast, the photocurrent along the armchair direction can be enhanced by a moderate strain strength ≈8% in some region of photon energy.

Layer-dependent band to band tunneling in WSe2/ReS2van der Waals heterojunction

Product: NanoDCAL
Date: 2020
Authors: Ou Y,Liu B,Kang Z,Liao Q,Zhang Z,Zhang Y
Journal: Journal of Physics D: Applied Physics

Van der Waals (vdW) heterostructures are promising for building tunneling field-effect transistors (TFETs), owing to an inherent narrow vdW gap between two stacked materials induced by the dangling bond free surface. However, the band to band tunneling (BTBT) of such a vdW heterostructure TFET strongly depends on the layer-dependent band structure variation at the interface. Here, we report a first principle simulation on the BTBT transition of the monolayer ReS2 based heterostructures with monolayer and bilayer WSe2. An obvious decrease of the turn-on gate voltage from 36 V to 12 V was achieved by adding a layer of WSe2 due to the band gap narrowing and momentum conservative $Gamma$-$Gamma$ tunneling. Under the gate voltage of 20 V with bias of 0.271 V, the upper limit of the BTBT saturate current density in bilayer WSe2 vdW heterojunction can reach 934 $mu$A $mu$m-1. These results show the bilayer WSe2 heterojunction could be an ideal candidate for lower power and high operating speed TFETs.

Stacking-Independent Ferromagnetism in Bilayer VI3 with Half-Metallic Characteristic

Product: NanoDCAL
Date: 2020
Authors: Long C,Wang T,Jin H,Wang H,Dai Y
Journal: Journal of Physical Chemistry Letters

Two-dimensional (2D) ferromagnetic (FM) materials have been identified as the foundation for next-generation electronic devices. However, there are still rare materials that show intrinsic ferromagnetism, which makes searching for new ferromagnetic materials a very important task. In this work, we systematically investigate the electronic and magnetic properties of bilayer VI3 based on density functional theory (DFT). Our results show that bilayer VI3 shows a half-metallic characteristic. Furthermore, we observe a robust ferromagnetic ground state in bilayer VI3, which is independent of the stacking configurations. A model including intralayer through-bond and interlayer super-superexchange (SSE) effects is employed to analyze the mechanism of the ferromagnetic coupling. On account of the half-metallic character, an application as a spin valve is then explored using quantum transport simulations, which show 100% magnetoresistance. Our results provide guidelines for the application of the bilayer VI3 for the next-generation spintronic devices.

Influence of Functional Diamino Organic Cations on the Stability, Electronic Structure, and Carrier Transport Properties of Three-Dimensional Hybrid Halide Perovskite

Product: NanoDCAL
Date: 2020
Authors: Zhang H,Li D,Liu B,Ma Y,Zhou W,Zhang D,Tang H,Yang A,Liang C
Journal: Journal of Physical Chemistry C

In recent years, organic-inorganic hybrid halide perovskites have attracted wide attention due to their excellent photoelectric properties. MAPbI3 and FAPbI3 have particularly shown great application prospects. However, due to the low thermal and chemical stability of these monoamino organic cation 3D perovskites, the durability of the devices in which they are used is affected. In this paper, an inorganic skeleton was introduced to functional diamino organic cations (PDA and DAB) to obtain more stable diamino 3D perovskites (PDAPbI4 and DABPbI4). For comparison with the 3D perovskites, two monoamino perovskites with the same molecular chain length ((PT)2PbI4 and (MBA)2PbI4) were also designed. The geometrical structures, thermodynamic stability, electronic properties, transport properties, and optical absorption properties of the four perovskites were calculated by the first-principles method. The results demonstrate that diamino organic cations can, to a certain extent, provide favorable channels for carrier migration, especially for carrier migration between Pb-I inorganic layers, and increase the coupling effect between Pb-I inorganic layers. In addition, the excellent optical absorption properties of diamino organic cation 3D perovskites in the solar irradiation range can contribute to the design and synthesis of more effective and stable organic-inorganic perovskite optoelectronic devices in the future.

Combined Impact of Denticity and Orientation on Molecular-Scale Charge Transport

Product: NanoDCAL
Date: 2020
Authors: Yasini P,Shepard S,Albrecht T,Smeu M,Borguet E
Journal: Journal of Physical Chemistry C

Reducing the dimensions of electronic devices to the nanoscale is an important objective with significant scientific and technical challenges. In molecule-based approaches, the orientation of the molecule and coordination to electrodes (denticity) can dramatically affect the electrical properties of the junction. Typically, higher conductance is associated with shorter transport distances and stronger molecule-electrode coupling; however, this is not always the case, as highlighted in this study. We focused on 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) molecules and have used the scanning tunneling microscopy break junction (STM-BJ) method to measure the electrical conductance of single molecules bridged between gold electrodes with different molecular orientations and with varying denticities. In conjunction with the experiments, density functional theory (DFT) and nonequilibrium Green's function (NEGF) calculations were performed to determine the conductance of four distinct molecular configurations. The calculated conductances show how different configurations and denticities influence the molecular orbital offsets with respect to the Fermi level and provide assignments for the experimental results. Surprisingly, lower denticity results in higher conductance, with the highest predicted molecular conductance being 0.6 G0, which is explained by the influence of molecule-electrode coupling on the energy of molecular orbitals relative to the Fermi level. These results highlight the importance of molecular geometry and binding configuration of the molecule to the electrode. Consequently, our findings have profound ramifications for applications in which orbital alignment is critical to the efficiency of charge transport, such as in dye sensitized solar cells, molecular switches, and sensors.

An Emerging All-Inorganic CsSnxPb1- xBr3(0 ≤ x ≤ 1) Perovskite Single Crystal: Insight on Structural Phase Transition and Electronic Properties

Product: NanoDCAL
Date: 2020
Authors: Xian Y,Zhang Y,Rahman NU,Yin H,Long Y,Liu P,Li W,Fan J
Journal: Journal of Physical Chemistry C

Owing to the high toxicity of Pb-based hybrid perovskites, a hypotoxic element, e.g., Sn, was employed to partially or completely replace Pb with the aim of gaining environmentally friendly lead-free photoelectronic device. Herein, for the first time we hydrothermally grow a series of all-inorganic CsSnxPb1-xBr3 (0 ≤ x ≤ 1) perovskite single crystals and carefully study their crystallographic structure and electronic structure that are subject to the varied x value. The phase transition and narrowed band gap from CsPbBr3 to CsSnBr3 are observed, which is similar to the evolution of MASnxPb1-xBr3 perovskites while varying the ratio of Sn/Pb. Likewise, the incremental concentration of Sn atoms in the lattice tend to cause the energy level splitting that likely derives from the orbital overlapping between Sn and/or Pb atoms, which thereby gives rise to the narrowed band gap. Equally important is that the incorporation of Sn atoms is demonstrated to be capable of tightening the electron orbital coupling between Sn and/or Pb atoms, which thereafter enables advantageous balancing of the orbital overlap in the crystal structure with balanced and highly ordered arrangement of [SnBr6]4- and/or [PbBr6]4- octahedra and thus allows us to improve carrier transport and to benefit the real ecofriendly application of photoelectronic devices.

Optical, Electronic, and Contact Properties of Janus-MoSO/MoS2Heterojunction

Product: NanoDCAL
Date: 2020
Authors: Wang T,Su M,Jin H,Li J,Wan L,Wei Y
Journal: Journal of Physical Chemistry C

Although oxidization is usually believed to have a negative effect on material performance, oxygen atoms may play a special role in oxidized two-dimensional (2D) transition-metal dichalcogenides (TMDs). In this work, we systematically study the optical, electronic, and contact properties of Janus-MoSO/MoS2 heterojunctions. Our results show that when a MoS2 monolayer becomes a Janus-MoSO monolayer, a vertical intrinsic electric field appears because of the discrepancy of electronegativity between S and O atoms. If this oxidation process takes place in a bilayer MoS2, an asymmetric heterojunction is formed. Janus-MoSO/MoS2 shows type-II band alignment with an extremely small exciton binding energy, which facilitates the photoinduced electron and hole separation. In addition, when the electrode Au contacts with the bilayer MoS2, a Schottky barrier is usually observed, whereas this barrier vanishes when Au contacts with Janus-MoSO/MoS2, thus improving the device performance. We believe that the proposed structure based on Janus-MoSO may significantly improve the performance of TMD materials, which thereby guide the designing of two-dimensional nanodevices.

Two-dimensional halogen-substituted graphdiyne: first-principles investigation of mechanical, electronic, optical, and photocatalytic properties

Product: NanoDCAL
Date: 2020
Authors: Feng Z,Li Y,Tang Y,Chen W,Li R,Ma Y,Dai X
Journal: Journal of Materials Science

Two-dimensional semiconductor materials with proper band gap can expand the optical absorption into visible and even infrared regions and have been proposed as the photocatalytic candidates for clean energy conversion and environmental pollution. Based on density functional theory, we investigate a new family of two-dimensional materials halogen-substituted graphdiyne (H-GDY, H=F, Cl, Br, and I). H-GDY is a new porous carbon-rich framework composed of 1,3,5-trihalogen benzene rings and butadiyne linkages. These H-GDY monolayers possess excellent mechanical, dynamic and thermal stabilities as demonstrated by elastic constant, cohesive energy, ab initio molecular dynamics simulation, and phonon dispersion. More significantly, these H-GDY monolayers are nonmagnetic semiconductors with wide-band-gap energy of 3.13, 2.82, 2.80, and 2.70 eV for F-GDY, Cl-GDY, Br-GDY, and I-GDY, and display good optical absorption in the visible region. Furthermore, all these H-GDY monolayers have suitable band edge for full water-splitting. Our theoretical investigation not only broaden GDY family, but also provides promising photocatalysts for water-splitting.

Tunneling magnetoresistance and light modulation in Fe4N(La2/3Sr1/3MnO3)/C60/Fe4N single molecule magnetic tunnel junctions

Product: NanoDCAL
Date: 2020
Authors: Han X,Mi W,Wang D
Journal: Journal of Materials Chemistry C

Due to the development of devices with rapid and high-density information storage capabilities, spin-polarized transport through a single molecule has attracted much attention. Here, the spin-dependent transport properties and light modulation of Fe4N/C60/Fe4N and La2/3Sr1/3MnO3/C60/Fe4N (LSMO/C60/Fe4N) single-molecule magnetic tunnel junctions were investigated systematically by first-principles quantum transport calculations. At equilibrium, a positive tunneling magnetoresistance (TMR) is found in the Fe4N/C60/Fe4N junction. A negative TMR appears in the LSMO/C60/Fe4N junction, which can become positive upon applying a bias voltage of 0.3 V. Moreover, the magnetization configuration and bias voltage can effectively tailor the spin injection of the Fe4N/C60/Fe4N junction, but cannot affect the LSMO/C60/Fe4N junction with a spin injection efficiency of 100%. Additionally, the LSMO/C60/Fe4N junction with different electrodes becomes a system lacking spatial reversal symmetry, so its photoresponse is much smaller than that of the Fe4N/C60/Fe4N junction. The fully spin-polarized photocurrent and spin battery can be obtained in both magnetic tunnel junctions by properly tailoring the photon energy and polarization angle. Interestingly, it is possible to switch the two poles of the spin battery by the magnetization configuration in the LSMO/C60/Fe4N junction. These results provide theoretical guidance for the design of light-modulated single-molecule spintronic devices.

Combinational modulations of NiSe2 nanodendrites by phase engineering and iron-doping towards an efficient oxygen evolution reaction

Product: NanoDCAL
Date: 2020
Authors: Zhou J,Yuan L,Wang J,Song L,You Y,Zhou R,Zhang J,Xu J
Journal: Journal of Materials Chemistry A

Despite the fulfilling progress in fabricating advanced nickel dichalcogenide catalysts towards an efficient oxygen evolution reaction (OER), previous efforts focused on the pyrite structure. In this work, combinational modulations of metallic NiSe2 nanodendrites by phase engineering and heteroatom doping are achieved to promote the OER. A series of marcasite NiSe2 (m-NiSe2) and Fe-doped marcasite NiSe2 (m-Ni1-xFexSe2) nanodendrites with various dopant contents have been controllably synthesized. On the one hand, phase engineering to synthesize m-NiSe2 nanodendrites offers a better intrinsic electronic conductivity than the pyrite phase. On the other hand, heteroatom Fe doping in m-NiSe2 nanodendrites further gains electronic benefits and simultaneously provides more electrochemical active sites owing to heteroatom displacement defects. Consequently, an optimized catalyst of m-Ni0.94Fe0.06Se2 nanodendrites with a moderate dopant content is developed, exhibiting significantly improved OER performance with a low overpotential of 279 mV at 10 mA cm-2, a small Tafel slope of 39 mV dec-1 and long operational stability for 35 h in 1.0 M KOH. In situ surface oxidation of the m-Ni0.94Fe0.06Se2 nanodendrites to form amorphous Fe-doped NiOOH/Ni(OH)2 shells during the OER process is demonstrated, which contributes to their superior activity and outstanding stability. This work provides valuable insights into the design of advanced OER electrocatalysts by means of combinational modulations of phase engineering and heteroatom incorporation.

The mechanism of magnetostructural transition in Heusler alloy Co2V1.5Ga0.5

Product: NanoDCAL
Date: 2020
Authors: Algethami OA,Zhang QQ,Tan JG,Wang XT,Liu ZH,Ma XQ
Journal: Journal of Magnetism and Magnetic Materials

Recently, magnetic field induced low magnetization tetragonal martensite to high magnetization cubic austenite transformation has been realized in Heusler alloy Co50V34Ga16 (Applied Physics Letters 112, 211903 (2018)). In this paper, first-principles calculations have been performed on Heusler alloy Co2V1.5Ga0.5 to reveal the mechanism for magnetostructural transition. It has been found that the alloy prefers to crystallize into ferromagnetic L21 type structure at austenite, with Co atoms tending to occupy the Wyckoff sites A (0, 0, 0) and C (0.5, 0.5, 0.5), V atoms occupying at site B (0.25, 0.25, 0.25), Ga and the extra V atoms entering D (0.75, 0.75, 0.75) site. Moments of Co and V atoms parallel to each other and a total formula moment of 2.95 $mu$B is achieved. A potential of tetragonal distortion from ferromagnetic cubic structure to non-magnetic tetragonal structure has been predicted from the view of energetically favorable state. The stability of tetragonal Co2V1.5Ga0.5 is further confirmed by the phonon spectrum. The peaks of dx2-y2 and dz2 states for Co and V(D) 3d states near the Fermi level for the cubic structure split at the tetragonal structure, implying the structural transition is mainly attributed to the band Jahn-Teller effect. The hybridization between the Co 3d states and the 3d states of V at D site plays an important role in the martensitic transformation. A volume contraction of 1.3% is obtained accompanying with the magnetostructural transition.

Rich topological nodal line bulk states together with drum-head-like surface states in NaAlGe with anti-PbFCl type structure

Product: NanoDCAL
Date: 2020
Authors: Wang X,Ding G,Cheng Z,Surucu G,Wang XL,Yang T
Journal: Journal of Advanced Research

The band topology in condensed matter has attracted widespread attention in recent years. Due to the band inversion, topological nodal line semimetals (TNLSs) have band crossing points (BCPs) around the Fermi level, forming a nodal line. In this work, by means of first-principles, we observe that the synthesized NaAlGe intermetallic compound with anti-PbFCl type structure is a TNLS with four NLs in the kz = 0 and kz = $pi$ planes. All these NLs in NaAlGe exist around the Fermi level, and what is more, these NLs do not overlap with other bands. The exotic drum-head-like surface states can be clearly observed, and therefore, the surface characteristics of NaAlGe may more easily be detected by experiments. Biaxial strain has been explored for this system, and our results show that rich TNL states can be induced. Furthermore, the spin-orbit coupling effect has little effect on the band structure of NaAlGe. It is hoped that this unique band structure can soon be examined by experimental work and that its novel topological elements can be fully explored for electronic devices.

Novel topological nodal lines and exotic drum-head-like surface states in synthesized CsCl-type binary alloy TiOs

Product: NanoDCAL
Date: 2020
Authors: Wang X,Ding G,Cheng Z,Surucu G,Wang XL,Yang T
Journal: Journal of Advanced Research

Very recently, searching for new topological nodal line semimetals (TNLSs) and drum-head-like (DHL) surface states has become a hot topic in the field of physical chemistry of materials. Via first principles, in this study, a synthesized CsCl type binary alloy, TiOs, was predicted to be a TNLS with three topological nodal lines (TNLs) centered at the X point in the kx/y/z = $pi$ plane, and these TNLs, which are protected by mirror, time reversal (T) and spatial inversion (P) symmetries, are perpendicular to one another. The exotic drum-head-like (DHL) surface states can be clearly observed inside and outside the crossing points (CPs) in the bulk system. The CPs, TNLs, and DHL surface states of TiOs are very robust under the influences of uniform strain, electron doping, and hole doping. Spin-orbit coupling (SOC)-induced gaps can be found in this TiOs system when the SOC is taken into consideration. When the SOC is involved, surface Dirac cones can be found in this system, indicating that the topological properties are still maintained. Similar to TiOs, ZrOs and HfOs alloys are TNLSs under the Perdew-Burke-Ernzerhof method. The CPs and the TNLs in both alloys disappear, however, under the Heyd-Scuseria-Ernzerhof method. It is hoped that the DHL surface property in TiOs can be detected by surface sensitive probes in the near future.

Mechanistic insight into the role of N-doped carbon matrix in electrospun binder-free Si@C composite anode for lithium-ion batteries

Product: NanoDCAL
Date: 2020
Authors: Shen K,Chen H,Hou X,Wang S,Qin H,Gao Y,Shen J
Journal: Ionics

To improve the long cyclic stability and rate capability of Si-based anode, we demonstrate a core-shell structural Si@NC composite decorates with N-doped carbon network using a low-cost, a simple process of electrospinning and low-temperature pyrolysis. Si@PVP/Urea fabric composite spun on the copper foil was directly carbonized and then was cut into wafers used as the electrode plates without extra conductive agent and binder. The enhanced rate capability and cyclic stability of special structural Si@NC is mainly ascribable to N-doped carbon matrix providing numerous active sites, which attract Li to those points in an efficient way, and the core-shell structures supply high mechanical strength for Si@NC composite. Importantly, almost 3-fold improvement in the capacity retention rate of the Si@NC has been observed at high current densities of 1.6 and 3.2 A g−1. Meanwhile, DFT calculations confirm that Li will be easily adsorbed by N-active sites in N-doped carbon model to strengthen chemical absorption ability, which could have more chance to grab the quickly moving Li in a brief period. It is significant for theoretical guidance of subsequent studies. The findings should make an important contribution providing a great possibility for the mass production and application to the field of lithium-ion battery.

Design and Simulation of Steep-Slope Silicon Cold Source FETs with Effective Carrier Distribution Model

Product: NanoDCAL
Date: 2020
Authors: Gan W,Prentki RJ,Liu F,Bu J,Luo K,Zhang Q,Zhu H,Wang W,Ye T,Yin H,Wu Z,Guo H
Journal: IEEE Transactions on Electron Devices

The cold source field-effect transistor (CSFET), enabled by novel source engineering, is a promising alternative to achieve sub-60 mV/dec steep-slope switching. For the first time, we develop an industry-standard TCAD approach for the CSFET with an effective cold carrier density of states (DOS) model which captures the underlying physics of DOS engineering, cold carrier injection, and thermalization in the device. The simulation scheme uses nonequilibrium Green's function (NEGF) simulation for calibration. The effects of source engineering, rethermalization, and channel tunneling are extensively investigated on a Si-based double-gate CSFET. Its merits are highlighted by comparison with a conventional MOSFET under various temperatures, thicknesses, and gate lengths, showing improved I _ scriptscriptstyle ON/I _ scriptscriptstyle OFF in ultrascaled MOSFET.

Improvement of valley splitting and valley injection efficiency for graphene/ferromagnet heterostructure

Product: NanoDCAL
Date: 2020
Authors: Xu L,Lu W,Hu C,Guo Q,Shang S,Xu X,Yu G,Yan Y,Wang L,Teng J
Journal: Chinese Physics B

The valley splitting has been realized in the graphene/Ni heterostructure with the splitting value of 14 meV, and the obtained valley injecting efficiency from the heterostructure into graphene was 6.18% [Phys. Rev. B 92 115404 (2015)]. In this paper, we report a way to improve the valley splitting and the valley injecting efficiency of the graphene/Ni heterostructure. By intercalating an Au monolayer between the graphene and the Ni, the split can be increased up to 50 meV. However, the valley injecting efficiency is not improved because the splitted valley area of graphene moves away from the Fermi level. Then, we mend the deviation by covering a monolayer of Cu on the graphene. As a result, the valley injecting efficiency of the Cu/graphene/Au/Ni heterostructure reaches 10%, which is more than 60% improvement compared to the simple graphene/Ni heterostructure. Then we theoretically design a valley-injection device based on the Cu/graphene/Au/Ni heterostructure and demonstrate that the valley injection can be easily switched solely by changing the magnetization direction of Ni, which can be used to generate and control the valley-polarized current.

Thermoelectric transport properties of magnetic carbon-based organic chains

Product: NanoDCAL
Date: 2020
Authors: Tan FX,Yang LY,Yang XF,Liu YS
Journal: Chemical Physics

We propose a perfect spin-caloritronics device based on a carbon-based organic chain. A spin-semiconducting property is achieved, which originates from the edge localized states. The appearance of spin-dependent transport gaps results in a large spin Seebeck coefficient. Moveover, the dimensionless spin thermoelectric figure of merit (FOM) at room temperature can be enhanced to as high as 35. Furthermore, the pure spin current or single-spin current can be produced at some chemical potentials under a temperature difference, and their transport directions can also be tuned by the chemical potential. Therefore, the carbon-based organic chain is well suited for designing the multifunctional spin-caloritronics devices.

The gas sensing performance of borophene/MoS2 heterostructure

Product: NanoDCAL
Date: 2020
Authors: Shen J,Yang Z,Wang Y,Xu LC,Liu R,Liu X
Journal: Applied Surface Science

Borophene is a new kind of two-dimensional (2D) nanomaterial that recently has received widespread attention. In present paper, using semiconducting MoS2 as substrate, we theoretically designed borophene/MoS2 heterostructure and investigated the adsorption behaviors of small gas molecules (CO, CO2, NO, NO2 and NH3) on borophene surface. Based on density functional theory, we discussed different adsorption configurations, adsorption energies, charge transfer and electronic band structures. The obtained results show that, except CO2, the rest four molecules adsorb on borophene through chemisorption. The interaction of CO2 and borophene is weak, thus can be viewed as a kind of physisorption. Furthermore, using non-equilibrium Green's function method, we calculated the transport properties of these systems. By analyzing the transport properties, we found that the heterostructure is very sensitive to NO molecule; present study suggests the heterostructure can be used as a 2D gas sensor.

Experimental observation of large tunneling anisotropic magnetoresistance in a magnetic tunnel junction without heavy metals

Product: NanoDCAL
Date: 2020
Authors: Quan Z,Zhang F,Yan Z,Liu H,Zhang W,Fang B,Zhou G,Zeng Z,Xu X
Journal: Applied Surface Science

Tunneling anisotropic magnetoresistance (TAMR) in the magnetic tunnel junctions (MTJs) driven by spin–orbit coupling (SOC) has attracted much attention for potential spintronic applications based on magnetic manipulation of electric transport. However, it has been believed that heavy metals are indispensable for the large TAMR. Here we experimentally show a TAMR of up to 46% in a La0.7Sr0.3MnO3 (LSMO) based MTJ without heavy metals. This value represents the largest TAMR for MTJs with half-metallic electrodes. We demonstrate that the TAMR ratio is enhanced by over one order of magnitude by tuning interface termination layer, achieving quasilocalized states at the interface Fermi level on the basis of magnetization orientation. These results are also supported by our first-principles density functional calculations. This finding illustrates a crucial role of interface tuning in the TAMR effect, opening a route for engineering the large anisotropic magnetoresistance by interface termination control and manipulating high spin polarization without using heavy metals.

Four distinct resistive states in van der Waals full magnetic 1T-VSe2/CrI3/1T-VSe2 tunnel junction

Product: NanoDCAL
Date: 2020
Authors: Li F,Yang B,Zhu Y,Han X,Yan Y
Journal: Applied Surface Science

Two-dimensional (2D) intrinsic magnets have been successfully utilized to make the multifunctional van der Waals (vdW) spintronic devices. In this work, we design a vdW magnetic tunnel junction (vdW MTJ) formed by a ferromagnetic (FM) monolayer CrI3 barrier sandwiched between two 2D FM 1T-VSe2 electrodes and investigate the magnetic anisotropy and the tunneling magnetoresistance (TMR) effect of this vdW MTJ by using first-principles calculations. It is found that different from the conventional MTJs, four different magnetic configurations can be achieved in the vdW MTJ based on 1T-VSe2/CrI3/1T-VSe2 heterostructure when the magnetic moments of top electrode are pinned to be [0 0 1] axis. Moreover, the conductance of vdW MTJ based on 1T-VSe2/CrI3/1T-VSe2 heterostructure is the highest (lowest) when the magnetic moments of barrier and bottom electrode are all along [0 0 1] ([001¯]) axis, and a highest TMR ratio of 178% can be obtained in this vdW MTJ. The large changes of tunneling conductance with different magnetic configurations originate mainly from the large variation of the effective majority- and minority-spin transmission channels of FM 1T-VSe2 for different magnetic configurations. Our results suggest that vdW MTJ based on 1T-VSe2/CrI3/1T-VSe2 heterostructure holds great potential in multi-states magnetic storage for spintronics.

First-principles study on strain-modulated negative differential resistance effect of in-plane device based on heterostructure tellurene

Product: NanoDCAL
Date: 2020
Authors: Hu J,Xiong W,Huang P,Wang Y,Cai C,Wang J
Journal: Applied Surface Science

We built an in-plane device using the semiconductor and metal phases of monolayer tellurene and investigated its transport properties by the Keldysh nonequilibrium Green's function method combined with density functional theory simulations. An obvious negative differential resistance (NDR) effect is found in the current-voltage curve of the in-plane device based on group-VI two-dimensional material for the first time, which can be explained by analyzing the transmission spectra and the electronic structure of the device. Further, the transport properties of the device under uniaxial and biaxial strains were also investigated. It is found that the local peak current of the device at small bias voltage exhibits a perfect exponential change with the x-axis strain, and the peak-to-valley ratio can be improved at most 602% by the strain. Our studies provide important support for the applications of tellurene in in-plane devices.

Ultrahigh tunneling magnetoresistance in van der Waals and lateral magnetic tunnel junctions formed by intrinsic ferromagnets Li0.5CrI3and CrI3

Product: NanoDCAL
Date: 2020
Authors: Li F,Yang B,Zhu Y,Han X,Yan Y
Journal: Applied Physics Letters

Two-dimensional (2D) intrinsic magnets have been used to construct magnetic tunnel junctions (MTJs) with a high tunneling magnetoresistance (TMR) ratio, including van der Waals (vdW) MTJs and lateral MTJs. In this work, we design vdW and lateral MTJs formed by a ferromagnetic (FM) CrI3 barrier and two half-metallic Li0.5CrI3 electrodes, respectively, and investigate the TMR effect of these MTJs using the non-equilibrium Green's function combined with density functional theory. Interestingly, it is found that due to the half-metallicity of the Li0.5CrI3 electrode, the total conductances of vdW and lateral MTJs for the parallel configuration (PC) of magnetizations of two electrodes are about 12 and 11 orders of magnitude larger than those for the antiparallel configuration (APC) of magnetizations of two electrodes, respectively. Consequently, the ultrahigh TMR ratios of up to 1.48 × 1014 and 2.86 × 1012 are achieved in the designed vdW and lateral MTJs, respectively. Remarkably, the TMR ratio of 1.48 × 1014 is the highest ratio in MTJs based on 2D materials. Moreover, due to the CrI3 barrier in vdW MTJs becoming FM half-metal, the majority-spin conductance of vdW MTJs for PC of magnetizations of two electrodes is about 2 orders of magnitude larger than that of lateral MTJs, and thus, the TMR ratio of vdW MTJs is about 2 orders of magnitude larger than that of lateral MTJs. Our results suggest that vdW and lateral MTJs formed by the FM CrI3 barrier and half-metallic Li0.5CrI3 electrodes hold great potential for applications in spintronic devices.

Electronic Transport Inhibiting of Carbon Nanotubes by 5f Elements

Product: NanoDCAL
Date: 2020
Authors: Wang J,Gong K,Lu F,Xie W,Zhu Y,Wang Z
Journal: Advanced Theory and Simulations

Based on the combination of the non-equilibrium Green's function and density functional theory, a theoretical method for studying the transport behavior of high angular momentum 5f electrons is developed and the transport properties of the structure for actinide atoms embedded in carbon nanotubes (An@CNTs, An = Ac, Th, Pa and U) is reported. Results show that An@CNTs have lower transmission coefficients than that of CNTs. Furthermore, electrical bias to the U@(4, 4)/(5, 5) CNTs induces an additional transition spectral peak, which demonstrates that the U@(4, 4)/(5, 5) CNTs has a lower resistance. Therefore, 5f electrons of actinide atoms inhibit the electronic transport of CNTs. These findings may provide fresh insight into the transport properties of systems having higher angular momentum electrons.

Symmetry-Dependent Transport Properties of $gamma$-Graphyne-based Molecular Magnetic Tunnel Junctions

Product: NanoDCAL
Date: 2020
Authors: Yang Y,Zhou M,Xing Y
Journal: Acta Physico-Chimica Sinica

Electronic, thermoelectric, transport and optical properties of MoSe2/BAs van der Waals heterostructures

Product: NanoDCAL
Date: 2021
Authors: Li Y,Feng Z,Sun Q,Ma Y,Tang Y,Dai X
Journal: Results in Physics

The density functional theory (DFT) calculations were performed to systematically study the geometrical, electronic, thermoelectric, transport and optical properties of MoSe2/BAs van der Waals heterostructures (vdWHs). The different MoSe2 and BAs stacking configurations effect hardly on the band structure. The MoSe2/BAs vdWHs possesses excellent dynamical, thermal and mechanical stability, with a direct bandgap of 1.04 eV and type-I band alignment. The Seebeck coefficient and ZT suggest the possibility of MoSe2/BAs vdWHs for thermoelectric applications. The in-plane strains and external electric field can modulate the band structure to achieve the transition from type-I to type-II band alignment, and the direct band gap feature remains preserved. Under the in-plane strains and external electric field, one can find a remarkably high optical absorption coefficient ($sim$105 cm−1) in the visible-ultraviolet region and the red shift in the optical absorption spectrum. Its high energy conversion efficiency of 20.08% making the heterostructure extremely potential in solar energy harvesting of low-dimensional excitonic solar cells. These properties of MoSe2/BAs vdWHs show their promising applications in optoelectronic, nanoelectronic and thermoelectronic fields.

Phonon spectrum and electronic structures of WTe2: A first-principles calculation

Product: NanoDCAL
Date: 2021
Authors: Xu Z,Luo B,Siu ZB,Chen Y,Huang J,Li Y,Sun C,Chen T,Jalil MB
Journal: Physics Letters, Section A: General, Atomic and Solid State Physics

We investigate the electronic structures of intrinsic and doped WTe2 by first-principles calculations based on the density functional theory. We first calculate the phonon dispersion and phonon density of states of WTe2. We found that all the phonon modes have positive energies, and that the optical phonon branches have high eigenvalues, which indicates that the structure of WTe2 is thermodynamically stable. In the range of phonon energy from 0 to 10 meV, the acoustic and optical branches overlap, and thus there is no gap between them. The grouping of acoustic phonons and optical branches has implications for the thermal transport properties of WTe2. We next analyzed the electronic band structures of intrinsic and doped WTe2. We show that doping with other chalcogens in the same group of elements as Te reduces the energy band gap but leaves the overall band structure relatively unchanged. However, doping with elements from other groups, such as C and H, greatly modifies the electronic band structure, especially near the Fermi level. In fact, doping with such elements can elicit a transition of WTe2 from a gapped to a gapless phase.

High sensitivity and anisotropic broadband photoresponse of Td-WTe2

Product: NanoDCAL
Date: 2021
Authors: Xu Z,Luo B,Chen Y,Li X,Chen Z,Yuan Q,Xiao X
Journal: Physics Letters, Section A: General, Atomic and Solid State Physics

The photogalvanic effect (PGE) enables the generation of photocurrent and also offer a high polarization sensitivity in a broadband range, showing potential applications in the low-power two dimensional (2D) optoelectronics, however the photocurrent of PGE is generally small. Here, we investigated the PGE for the 2D Td-WTe2 monolayer by employing the quantum transport simulations, and proposed the physical mechanism to effectively enhance the photocurrent of PGE at small bias voltage. The photocurrent of PGE can be generated in the 2D Td-WTe2 monolayer when the linearly polarized light of vertical illumination was applied. In the whole visible and near-infrared range we find the biggish photocurrent which reach up to saturate for the most photon energies under a small bias. The photocurrent of junction exhibits a cosine dependence with respect to the polarization angle. The magnitude of the largest photocurrent can be evidently enhanced about 1×104 times for a photon energy of 2.4 eV than the one around 0 eV under the bias of 0.2 V in the zigzag direction, but 7×102 times at 0.9 V in the armchair direction. Moreover, a higher polarization sensitivity can be obtained. In addition, a strong anisotropy of photocurrent can be displayed between the zigzag and armchair Td-WTe2, and that the photocurrent of zigzag direction is almost 3 times larger than that one of the armchair direction. These results show that in the visible and near-infrared range the 2D Td-WTe2 monolayer play a potential candidate for the optoelectronics in future.

Surface states and related quantum interference in ab initio electron transport

Product: NanoDCAL
Date: 2021
Authors: Li D,Bertelsen JL,Papior N,Smogunov A,Brandbyge M
Journal: Physical Review Research

Shockley surface states (SS) have attracted much attention due to their role in various physical phenomena occurring at surfaces. It is also clear from experiments that they can play an important role in electron transport. However, accurate incorporation of surface states in abinitio quantum transport simulations remains still an unresolved problem. Here we go beyond the state-of-the-art nonequilibrium Green's function formalism through the evaluation of the self-energy in real-space, enabling electron transport without using artificial periodic in-plane conditions. We demonstrate the method on three representative examples based on Au(111): a clean surface, a metallic nanocontact, and a single-molecule junction. We show that SS can contribute more than 30% of the electron transport near the Fermi energy. A significant and robust transmission drop is observed at the SS band edge due to quantum interference in both metallic and molecular junctions, in good agreement with experimental measurements. The origin of this interference phenomenon is attributed to the coupling between bulk and SS transport channels and it is reproduced and understood by tight-binding model. Furthermore, our method predicts much better quantized conductance for metallic nanocontacts.

Intersecting topological nodal ring and nodal wall states in superhard superconductor FeB4

Product: NanoDCAL
Date: 2021
Authors: Zhou F,Liu Y,Wang J,Kuang M,Yang T,Chen H,Wang X,Cheng Z
Journal: Physical Review Materials

Novel materials with both topological nontrivial states and superconductivity have attracted considerable attention in recent years. Single-crystal FeB4 was recently synthesized and demonstrated to exhibit superconductivity at temperatures lower than 2.9 K, and its nanoindentation hardness was measured to be 65 GPa. In this study, based on first-principles calculations and the low-energy ktextperiodcenteredp effective Hamiltonian, we found that this Pnnm-type superhard FeB4 superconductor hosts topological behaviors with intersecting nodal rings (INRs) in the kx=0 and kz=0 planes and nodal wall states in the ky=$pi$ and kz=$pi$ planes. The observed surface drum-head-like (D-H-L) states on the [100] and [001] surfaces confirmed the presence of INR states in this system. According to our investigation results, FeB4, with its superconductivity, superior mechanical behaviors, one-dimensional and two-dimensional topological elements, and D-H-L surface states, is an existing single-phase target material that can be used to realize the topological superconducting state in the near future.

Structure, phase stability, half-metallicity, and fully spin-polarized Weyl states in compound Na V2 O4: An example for topological spintronic material

Product: NanoDCAL
Date: 2021
Authors: He T,Zhang X,Yang T,Liu Y,Dai X,Liu G
Journal: Physical Review Materials

Here we systematically investigate the structure, phase stability, half-metallicity, and topological electronic structure for a topological spintronic material NaV2O4. The material has a tetragonal structure with excellent dynamical and thermal stabilities. It shows a half-metallic ground state, where only the spin-up bands are present near the Fermi level. These bands are demonstrated to form a nodal line with the double degeneracy on the kz=0 plane. The nodal line is robust against spin-orbit coupling, under the protection of the mirror symmetry. The nodal line band structure is very clean, thus the drumhead surface states can be clearly identified. Remarkably, the nodal line and drumhead surface states have the 100% spin polarization, which are highly desirable for spintronics applications. In addition, by shifting the magnetic field in-plane, we find that the nodal line can transform into a single pair of Weyl nodes. The nodal-line and Weyl-node fermions in the bulk, as well as the drumhead fermions on the surface are all fully spin-polarized, which may generate interesting physical properties and promising applications.

Gate tunable self-powered few-layer black phosphorus broadband photodetector

Product: NanoDCAL
Date: 2021
Authors: Guo X,Zhang L,Chen J,Zheng X,Zhang L
Journal: Physical chemistry chemical physics : PCCP

Utilizing the unique gate induced giant stark effect in few-layer black phosphorus (BP), we theoretically propose a broadband photodetector device based on pure few-layer BP using atomic first-principles calculations. By applying a vertical gate voltage in the few-layer BP, the intrinsic inversion symmetry of the system can be broken. We found that the photocurrent can be generated via the photogalvanic (or photovoltaic) effect (PGE) without the need for an external bias voltage, which means the gated few-layer BP photodetector is self-powered and the dark current can be greatly suppressed. Most importantly, due to the giant stark effect of the gated few-layer BP, the photodetection range can be well controlled and further enlarged from the mid-infrared range (MIR) to the far-infrared range (FIR). Furthermore, the few-layer BP based photodetector device also presents high polarization sensitivity with extinction ratios up to 104 and a large anisotropic photoresponse. Our numerical findings pave a feasible way for the few-layer BP's novel application in self-powered and well-controlled broadband photodetectors.

The spin-dependent transport properties of defected zigzag graphene nanoribbons with graphene nanobubbles

Product: NanoDCAL
Date: 2021
Authors: Ni Y,Li J,Tao W,Ding H,Li RX
Journal: Physical Chemistry Chemical Physics

Zigzag-edged graphene nanoribbons (ZGNRs) have important applications in spintronics and spin caloritronics. While in the preparation of a ZGNR, defects like the graphene nanobubbles often appear, which may affect the physical properties of the ZGNR. In this paper, we studied the transport properties of a defected ZGNR with a graphene nanobubble by performing first-principles quantum transport calculations. The results show that when the nanobubble is intact and locates at the centre, the spin polarization and magnetoresistance tend to drop off in the low bias voltage cases, compared to the ideal ZGNR. While when the nanobubble is split and locates at the edge, all the transport properties are significantly affected and altered, such as the spin polarization, the giant magnetoresistance effect and the spin Seebeck effect. Meanwhile, some new results are obtained from the device, including the negative differential resistance effect and the pure thermal-induced spin-current.

Photogalvanic effect in chromium-doped monolayer MoS2 from first principles

Product: NanoDCAL
Date: 2021
Authors: Liu PP,Shao ZG,Luo WM,Yang M
Journal: Physica E: Low-Dimensional Systems and Nanostructures

Non-equilibrium green's function fromalism combined with density functional theory is used to investigate the photogalvanic effect of chromium (Cr)-doped monolayer molybdenum disulfide (MoS2) from first principles. Since the Cr-doped monolayer MoS2 belongs to the C2v point group which is non-inversion symmetric, the photocurrent can be generated at zero bias when irradiated by linearly or circularly polarized light. Then, the photoresponse for the circular photogalvanic effect (CPGE) and linear photogalvanic effec (LPGE) at different photon energies are investigated. It is found that the photoresponse of the CPGE is one order of magnitude larger than the LPGE and there are peaks around 1.5 eV, 1.9 eV and 2.0 eV for the CPGE. Especially, the photocurrent reaches the maximum value at the photon energy of 1.5 eV (red light) for the CPGE. These results can be explained by the electron transition between valence bands and conduction bands, which is proportional to density of states according to the Fermi's golden rule. Furthermore, we investigate the dependence of the photocurrent on polarization angle $theta$ and $phi$ for the LPGE and CPGE. Since the Cr-doped monolayer MoS2 can generate a large photocurrent in the visible light range, it can be used as an excellent photogalvanic material. This work indicates that Cr-doped monolayer MoS2 can provide some theoretical references for the research and design of novel electronic or optoelectronic devices.

Two-dimensional centrosymmetrical antiferromagnets for spin photogalvanic devices

Product: NanoDCAL
Date: 2021
Authors: Jiang P,Tao X,Hao H,Liu Y,Zheng X,Zeng Z
Journal: npj Quantum Information

Spin-dependent photogalvanic effect (PGE) in low-dimensional magnetic systems has recently attracted intensive attention. Based on first-principle transport calculations and symmetry analyses, we propose a robust scheme to generate pure spin current by PGE in centrosymmetric materials with spin polarization antisymmetry. As a demonstration, the idea is successfully applied to a photoelectric device constructed with a zigzag graphene nanoribbon (ZGNR), which has intrinsic antiferromagnetic coupling between the two edges and spin degenerate band structure. It suggests that spin splitting is not a prerequisite for pure spin current generation. More interestingly, by further introducing external transverse electric fields to the two leads to lift the spin degeneracy, the device may behave multifunctionally, capable of producing fully spin-polarized current or pure spin current, depending on whether the fields in the two leads are parallel or antiparallel. Very importantly, our scheme of pure spin current generation with PGE is not limited to ZGNR and can be extended to other two-dimensional (2D) centrosymmetric magnetic materials with spin polarization antisymmetry, suggesting a promising category of 2D platforms for PGE-based pure spin current generation.

Evolution and universality of two-stage Kondo effect in single manganese phthalocyanine molecule transistors

Product: NanoDCAL
Date: 2021
Authors: Guo X,Zhu Q,Zhou L,Yu W,Lu W,Liang W
Journal: Nature Communications

The Kondo effect offers an important paradigm to understand strongly correlated many-body physics. Although under intensive study, some of the important properties of the Kondo effect, in systems where both itinerant coupling and localized coupling play significant roles, are still elusive. Here we report the evolution and universality of the two-stage Kondo effect, the simplest form where both couplings are important using single molecule transistor devices incorporating Manganese phthalocyanine molecules. The Kondo temperature T* of the two-stage Kondo effect evolves linearly against effective interaction of involved two spins. Observed Kondo resonance shows universal quadratic dependence with all adjustable parameters: temperature, magnetic field and biased voltages. The difference in nonequilibrium conductance of two-stage Kondo effect to spin 1/2 Kondo effect is also identified. Messages learned in this study fill in directive experimental evidence of the evolution of two-stage Kondo resonance near a quantum phase transition point, and help in understanding sophisticated molecular electron spectroscopy in a strong correlation regime.

Tunable conductance and spin filtering in twisted bilayer copper phthalocyanine molecular devices

Product: NanoDCAL
Date: 2021
Authors: Liu JH,Luo K,Huang K,Sun B,Zhang S,Wu ZH
Journal: Nanoscale Advances

We investigate theoretically the quantum transport properties of a twisted bilayer copper phthalocyanine (CuPc) molecular device, in which the bottom-layer CuPc molecule is connected to V-shaped zigzag-edged graphene nanoribbon electrodes. Based on a non-equilibrium Green's function approach in combination with density-functional theory, we find that the twist angle effectively modulates the electron interaction between the bilayer CuPc molecules. HOMO (highest occupied molecular orbital)-LUMO (lowest unoccupied molecular orbital) gap, spin filtering efficiency (SFE) and spin-dependent conductance of the bilayer CuPc molecular device could be modulated by changing the twist angle. The conductance reaches its maximum when the twist angle$theta$is 0° while the largest SFE is achieved when$theta$= 60°. The twist angle-induced exotic transport phenomena can be well explained by analyzing the transmission spectra, molecular energy level spectra and scattering states of the twisted bilayer CuPc molecular device. The tunable conductance, HOMO-LUMO gap and spin filteringversustwist angle are helpful for predicting how a two-molecule system may behave with twist angle.

Pure spin current generation with photogalvanic effect in graphene interconnect junctions

Product: NanoDCAL
Date: 2021
Authors: Zhou YH,Yu S,Li Y,Luo X,Zheng X,Zhang L
Journal: Nanophotonics

We investigate the photovoltaic behaviors of magnetic graphene interconnect junctions, which are constructed by zigzag graphene nanoribbons (ZGNRs), with the aim to produce pure spin current by photogalvanic effect (PGE). Two kinds of interconnect junctions are designed by connecting two 6-ZGNR with a carbon hexagon (C6) and a carbon tetragon (C4), respectively. It is found that zero charge current is produced under irradiation of light in both structures due to the presence of spatial inversion symmetry. Nevertheless, behind the zero charge current, net pure spin current is produced in the structure with a C6, but not in the structure with a C4. This difference originates from their different edge state distribution and different spatial inversion symmetry of the spin density. However, interestingly, local edge pure spin current can be obtained in both structures. More importantly, the pure spin current generation is independent of the photon energy, polarization type or polarization angle, suggesting a robust way of generating pure spin current with PGE and new possibility of graphene's applications in spintronics.

Novel two-dimensional layered mosi2z4 (Z = p, as): New promising optoelectronic materials

Product: NanoDCAL
Date: 2021
Authors: Yao H,Zhang C,Wang Q,Li J,Yu Y,Xu F,Wang B,Wei Y
Journal: Nanomaterials

Very recently, two new two-dimensional (2D) layered semi-conducting materials MoSi2N4 and WSi2N4 were successfully synthesized in experiments, and a large family of these two 2D materials, namely MA2Z4, was also predicted theoretically (Science, 369, 670 (2020)). Motivated by this exciting family, in this work, we systematically investigate the mechanical, electronic and optical properties of monolayer and bilayer MoSi2P4 and MoSi2As4 by using the first-principles calculation method. Numerical results indicate that both monolayer and bilayer MoSi2Z4 (Z = P, As) present good structural stability, isotropic mechanical parameters, moderate bandgap, favorable carrier mobilities, remarkable optical absorption, superior photon responsivity and external quantum efficiency. Especially, due to the wave-functions of band edges dominated by d orbital of the middle-layer Mo atoms are screened effectively, the bandgap and optical absorption hardly depend on the number of layers, providing an added convenience in the experimental fabrication of few-layer MoSi2Z4-based electronic and optoelectronic devices. We also build a monolayer MoSi2Z4-based 2D optoelectronic device, and quantitatively evaluate the photocurrent as a function of energy and polarization angle of the incident light. Our investigation verifies the excellent performance of a few-layer MoSi2Z4 and expands their potential application in nanoscale electronic and optoelectronic devices.

Highly anisotropic gas sensing of atom-thin borophene: a first-principles study

Product: NanoDCAL
Date: 2021
Authors: Li J,Chen X,Yang Z,Liu X,Zhang X
Journal: Journal of Materials Chemistry C

Two-dimensional (2D) materials have promising applications in ultra-sensitive gas molecule detection owing to their thinness. Among the atom-thin 2D crystals, graphene with a nearly isotropic 2D structure has been widely used in gas sensors. Here, we propose a gas sensor device structure with strongly anisotropic atom-thin $beta$12borophene on top of an MoS2substrate for detecting inorganic small molecules in the environment. The electronic properties are systematically investigated to understand the interaction between the gas molecules and $beta$12borophene. Nonequilibrium Green's function is used to study the transport properties of $beta$12borophene on the MoS2substrate for gas sensing. It is found that the gas sensing performance of $beta$12borophene is highly anisotropic, especially for NH3that has the largest anisotropic gas sensing ratio of 17.43. Such a highly anisotropic gas sensing ratio could be utilized to realize the detection of specific gas molecules with strong tolerance to errors due to device and environmental fluctuations. Our results suggest the promising application of highly anisotropic atom-thin 2D crystals for error tolerance gas sensors.

Quasi three-dimensional lead iodide perovskite using pyridine-2,5-diamine and 4,4′-bipyridine with tunable electronic structure, carrier transport, optical absorption properties

Product: NanoDCAL
Date: 2021
Authors: Zhou W,Tang H,Li D,Zhang D,Zhang H,Yang A,Liang C
Journal: Journal of Alloys and Compounds

The introduction of functional diamino organic cations between inorganic Pb–I octahedrons to form a new quasi three-dimensional (3D) diamino perovskite is a strategy for expanding the family of perovskite materials, obtain excellent photovoltaic materials, and solve the problems of low thermal and chemical stability of traditional perovskites. In this work, the electronic structure, carrier transport and optical absorption properties of two new perovskites with pyridine-2,5-diamine (PDA) and 4,4′-bipyridine (BPD) as the organic cations are investigated using first-principles calculations. It is found that diamino pyridine and bipyridine have a strong effect on the perovskite band structure. The results show that, unlike for the traditional perovskite where the organic cations do not contribute to the conduction band minimum (CBM) and valence band maximum (VBM), the C–C pp$pi$∗ anti-bonding orbital and the 2p orbital of N in the pyridine ring of PDA and BDP contribute to the CBMs of the two perovskites. Interestingly, the NH of two pyridine rings in 4,4′-bipyridine (BPD) is similar to diamine, and the bound state provided by the C–C pp$sigma$∗ orbital appears in the band gap of (BPD)PbI4, strongly affecting its optical properties. The results show that diamino pyridine and bipyridine effectively adjust the band structure of the organic-inorganic perovskite, and also affect the carrier transport and optical absorption characteristics. In particular, bipyridine perovskite displays better and more balanced electron and hole transport characteristics and greater optical absorption, which is helpful for the design and fabrication of more efficient and stable organic-inorganic perovskite photovoltaic devices.

Long-Lasting Orientation of Symmetric-Top Molecules Excited by Two-Color Femtosecond Pulses

Product: NanoDCAL
Date: 2021
Authors: Xu L,Tutunnikov I,Prior Y,Averbukh IS
Journal: Frontiers in Physics

Impulsive orientation of symmetric-top molecules excited by two-color femtosecond pulses is considered. In addition to the well-known transient orientation appearing immediately after the pulse and then reemerging periodically due to quantum revivals, we report the phenomenon of field-free long-lasting orientation. Long-lasting means that the time averaged orientation remains non-zero until destroyed by other physical effects, e.g., intermolecular collisions. The effect is caused by the combined action of the field-polarizability and field-hyperpolarizability interactions. The dependence of degree of long-lasting orientation on temperature and pulse parameters is considered. The effect can be measured by means of second (or higher-order) harmonic generation, and may be used to control the deflection of molecules traveling through inhomogeneous electrostatic fields.

First-Principles Investigation on Electrochemical Performance of Na-Doped LiNi1/3Co1/3Mn1/3O2

Product: NanoDCAL
Date: 2021
Authors: Gao Y,Shen K,Liu P,Liu L,Chi F,Hou X,Yang W
Journal: Frontiers in Physics

The cathode material LiNi1/3Co1/3Mn1/3O2 for lithium-ion battery has a better electrochemical property than LiCoO2. In order to improve its electrochemical performance, Na-doped LiNi1/3Co1/3Mn1/3O2 is one of the effective modifications. In this article, based on the density functional theory of the first-principles, the conductivity and the potential energy of the Na-doped LiNi1/3Co1/3Mn1/3O2 are calculated with Materials Studio and Nanodcal, respectively. The calculation results of the band gap, partial density of states, formation energy of intercalation of Li+, electron density difference, and potential energy of electrons show that the new cathode material Li1−xNax Ni1/3Co1/3Mn1/3O2 has a better conductivity when the Na-doping amount is x = 0.05 mol. The 3D and 2D potential maps of Li1−xNaxNi1/3Co1/3Mn1/3O2 can be obtained from Nanodcal. The maps demonstrate that Na-doping can reduce the potential well and increase the removal rate of lithium-ion. The theoretical calculation results match well with experimental results. Our method and analysis can provide some theoretical proposals for the electrochemical performance study of doping. This method can also be applied to the performance study of new optoelectronic devices.

Gate controllable optical spin current generation in zigzag graphene nanoribbon

Product: NanoDCAL
Date: 2021
Authors: Zhang L,Chen J,Zhang L,Xu F,Xiao L,Jia S
Journal: Carbon

Considering the demand for long spin communication distance in spintronics, graphene presents micrometer spin relaxation length at room temperature, making it one of the most promising two dimensional spintronic materials. However, achieving efficient spin injection (including pure spin current and spin polarized current) by reducing the spin dependent scattering between graphene and other materials like contact is still a core challenge. Here, we propose a novel approach to generate spin current in zigzag graphene nanoribbon (ZGNR) via photogalvanic (or photovoltaic) effect (PGE) from atomic first principle calculations. By designing ZGNR based device with spatial inversion symmetry, we find that the PGE induced pure spin current can be hiddenly generated without accompanying charge current. Furthermore, through applying a dual gate in the system, the generated pure spin current can be controlled when dual gate voltages have the opposite signs. Interestingly, when the signs of dual gate voltages are the same, the pure spin current can turn into the fully spin polarized current. More importantly, the generated spin current via PGE is independent of photon polarization and incident angles. Our investigations demonstrate ZGNR's great potential application in noninvasive spin injection of the graphene based spintronic device.

Thickness dependence of spin torque effect in Fe/MgO/Fe magnetic tunnel junction: Implementation of divide-and-conquer with first-principles calculation

Product: NanoDCAL
Date: 2021
Authors: Huang BH,Chao CC,Tang YH
Journal: AIP Advances

In this study, we develop a divide-and-conquer (DC) method under the framework of first-principles calculation to prevent directly solving Hamiltonian of a large device with time-consuming self-consistent process. The DC implementation combined with JunPy package reveals the oscillatory decay of layer-resolved spin torques away from the MgO/Fe interface, and suggests a very thin Fe layer thickness below 2 nm to preserve the efficient current-driven magnetization switch. This newly developed JunPy-DC calculation may efficiently resolve current self-consistent difficulties in noncollinear spin torque effects for novel spintronic applications with complex magnetic heterostructures.

Correlated energy-level alignment effects determine substituent-tuned single-molecule conductance

Product: NanoDCAL
Date: 2021
Authors: Ivie JA,Bamberger ND,Parida KN,Shepard S,Dyer D,Saraiva-Souza A,Himmelhuber R,Mcgrath DV,Smeu M,Monti OL
Journal: ACS Applied Materials and Interfaces

The rational design of single-molecule electrical components requires a deep and predictive understanding of structure-function relationships. Here, we explore the relationship between chemical substituents and the conductance of metal-single-molecule-metal junctions, using functionalized oligophenylenevinylenes as a model system. Using a combination of mechanically controlled break-junction experiments and various levels of theory including non-equilibrium Green's functions, we demonstrate that the connection between gas-phase molecular electronic structure and in-junction molecular conductance is complicated by the involvement of multiple mutually correlated and opposing effects that contribute to energy-level alignment in the junction. We propose that these opposing correlations represent powerful new "design principles"because their physical origins make them broadly applicable, and they are capable of predicting the direction and relative magnitude of observed conductance trends. In particular, we show that they are consistent with the observed conductance variability not just within our own experimental results but also within disparate molecular series reported in the literature and, crucially, with the trend in variability across these molecular series, which previous simple models fail to explain. The design principles introduced here can therefore aid in both screening and suggesting novel design strategies for maximizing conductance tunability in single-molecule systems.

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