We report first-principles theoretical investigations of quantum transport in a monolayer WSe2 field effect transistor (FET). Due to strong spin-orbit interaction (SOI) and the atomic structure of the two-dimensional lattice, monolayer WSe2 has 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 I(τ, s) in the FET is labelled by both the valley index and spin index, realizing valleytronics and spintronics in the same device.

The resistance of copper grain boundaries (GBs) is calculated systematically through a full atomistic quantum approach. A set of twin GBs, including the coherent twin GB, is generated by density functional theory (DFT) total energy relaxation starting from the coincidence site lattice (CSL) model. The atomic structure of the GBs is used to construct two-probe transport junctions for quantum-transport analysis by carrying out DFTwithin the Green's function formalism. The specific resistivity calculated for the coherent twin GB is found to be quantitatively consistent with the available experimental and theoretical data. The specific resistivity and reflection coefficient of other more complex GBs are predicted. The interfacial energy density and specific resistivity are both found to inversely relate with the planar density of coincidence sites. Comparison of our calculated specific resistivities and reflection coefficients with the corresponding GB-averaged experimental quantities shines light on the microstructure of the samples.

The conversion of solar energy into hydrogen via water splitting process is one of the key sustainable technologies for future clean, storable, and renewable source of energy. Therefore, development of visible light-responsive and efficient photocatalyst material has been of immense interest, but with limited success. Here, we show that overall water splitting under visible-light irradiation can be achieved using a single photocatalyst material. Multiband InGaN/GaN nanowire heterostructures, decorated with rhodium (Rh)/chromium-oxide (Cr2O3) core-shell nanoparticles can lead to stable hydrogen production from pure (pH ∼ 7.0) water splitting under ultraviolet, blue and green-light irradiation (up to ∼560 nm), the longest wavelength ever reported. At ∼440-450 nm wavelengths, the internal quantum efficiency is estimated to be ∼13%, the highest value reported in the visible spectrum. The turnover number under visible light well exceeds 73 in 12 h. Detailed analysis further confirms the stable photocatalytic activity of the nanowire heterostructures. This work establishes the use of metal-nitrides as viable photocatalyst for solar-powered artificial photosynthesis for the production of hydrogen and other solar fuels.

We report theoretical analysis of thermal-spin and thermoelectric properties of noncollinear spin valves driven by a high-frequency ac voltage bias. The spin valve consists of two ferromagnetic contacts sandwiching a single-level or multilevel quantum dot (QD). A general formulation for the time-averaged thermal-spin and thermoelectric properties of spin valves is derived within the nonequilibrium Green’s function theory, which provides a starting point for further numerical calculations of these properties. Numerical results of a spin valve having a spin-degenerate single-level QD are given as an example. The ac bias induces various photon-assisted transmission peaks which can greatly enhance the Seebeck coefficients and the figures of merit, and offer a new possibility to tune both the spin-dependent and normal thermoelectric properties of the spin valve. Details of these properties and how they depend on the noncollinearity of the spin valve, magnetic polarization, ac frequency, ac bias, and other control parameters are reported. A particularly interesting result is the opposite dependency of the thermoelectric properties on the magnetic polarization and noncollinearity for contacts with or without spin accumulation.

A typical magnetic tunnel junction(MTJ) consists of a thin insulating layer(a tunnel barrier),sandwiched by two ferromagnetic electrode layers.When the relative magnetic configuration of both ferromagnetic electrode layers changes from parallel state to anti-parallel state with external magnetic field,the resistance of MTJ would become high from low,exhibiting tunnel magneto-resistance(TMR) due to spin-dependent electron tunnelling.Amorphous AlOx barrier MTJs were extensively studied and have been used in magneto-resistance random access memory(MRAM) and read heads of hard disk drives,since the discovery of room-temperature TMR in 1995.However,the spin electronic devices development of the next-generation high-speed,low-power-consumption and high-performance need much higher TMR ratio and a novel structure.In 2001,the first-principle calculation predicted that the TMR ratio of epitaxial Fe(001)/MgO(001)/Fe(001) MTJs would be over 1 000%,due to spin filter effect of MgO barrier for different symmetry spin polarized electron.In 2004 TMR ratios of about 200% were obtained in MTJs with a single-crystal MgO(001) barrier or a textured MgO(001) barrier.In 2008,the TMR ratio of 604% has been reported in pseudo-spin-valve MTJs with core structure of CoFeB/MgO/CoFeB.Recently,Quantum well(QW) resonances tunneling and spin-dependent Coulomb blockade magneto-resistance(CBMR) effect in MgO-barrier MTJs were proposed and demonstrated in theories and experiments.Magnetic sensors,MRAM,spin nano-oscillator and microwave detector based on MTJs have attracted attention of electron science and devices.In the paper we briefly introduced the investigation and development of magnetic tunnel junction material and its device applications.

Organic spintronics refers to control spin dependent transport through organic materials. In the last two decades, extraordinary development has been achieved for organic-spintronics. A series of theoretical and experimental studies have been done to reveal the mechanisms of spin dependent transport properties. The theoretical analysis is based on the non-equilibrium Green’s function formalism provides a mathematical framework for solving the transmission coefficients in the Landauer formula from atomistic first principles without any phenomenological parameters. In this article, we provide a brief theoretical review on organic spintronics devices and device physics therein.

In theory, graphene should exhibit very high carrier mobility, however its measured mobility is usually much lower. This discrepancy between theory and empirical observations is believed to be due to disordered scattering. We present a systematic first principles study of diffusive impurity scattering in short channel graphene. We achieve good agreement with experimental data and show that mobility in graphene is strongly influenced by impurity scattering. In general, the results show that diffusive scattering can play a key role in determining the carrier transport properties of short channel devices. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4748326]

We report an extensive theoretical investigation of impurity doping and alloying effects to spin-polarized quantum tunneling in Fe/MgO/Fe magnetic tunneling junctions. We find that with proper impurity atoms, an optimal device characteristic regime can be established where the junction resistance R-J is nonlinearly quenched while the tunnel magnetoresistance ratio (TMR) is only linearly reduced. As a consequence, the spin transfer torque is significantly increased by a factor similar to the R-J reduction. We search for this optimal device regime using first-principles calculations by doping MgO with different impurity atoms. We predict that Zn has the desired property due to a well-balanced effect of producing gap states that mediate charge transport, thus quenching R-J, and producing relatively weak interchannel coupling to alleviate the detrimental diffusive scattering effect on the TMR.

Magnetic tunnel junctions (MTJs) with the core structure of CoFeB/MgAlOx/CoFeB were fabricated using magnetron sputtering technique. The MgAlOx tunnel barrier was obtained by plasma oxidation of an Mg/Al bilayer in an Ar+O2 atmosphere. Series of MTJs were fabricated with different Mg layer thicknesses (tMg), and Al layer thickness was fixed at 1.3 nm. The annealing effect on the tunneling magnetoresistance (TMR) ratio was investigated, and TMR ratio of 65% at room temperature (RT) was shown when it was annealed at 375 °C with the tMg=0.5 nm. The temperature dependence of conductance can be fit by the magnon-assisted tunneling model by adding spin independent tunneling contribution for the samples investigated here, and the spin independent conductance GSI varies with tMg, possibly due to less oxidation for thicker Mg layer.

Recently, preliminary experimental results of solar-to-hydrogen generation by wafer level InGaN nanowires were reported [Z. Mi et al. Nano Lett., 2011, 11 (6), pp 2353-2357]. In the present paper we report a theoretical investigation on the dissociation process of water molecules on wurtzite GaN (100) surface (M-Plane) using the density functional theory (DFT). We calculated the structure and energetic of the water adsorption, reaction barrier energies and pathway for water dissociation. The results suggest that the adsorption of H2O is more favorable near Gallium atoms than near Nitrogen atoms and we determined the likely binding sites of water molecules on GaN (100) surface. We also analyzed a model for hydrogen evolution reaction on GaN (100) that involves three steps, where a water molecule first dissociates into a hydrogen atom plus the OH group, followed by the dissociation of the hydroxyl group, and finally the two hydrogen atoms recombine to form molecular hydrogen. For these reactions, the atomic positions and the reaction barriers were determined.

Using ab initio methods, we have calculated the characteristic inverse length scale for spin polarized tunneling in Langmuir-Blodgett (LB) films contacted by Fe electrodes. As a function of energy, the scale is found to vary significantly around its mean value, and the variation is much more drastic in parallel configuration than that of anti-parallel configuration. This unusual behavior can be understood from the point of view of interface resonance, which shows very sharp features in the k-space resolved transmission probability.

The structural andmagnetic properties of ferromagnetic nanotubes fabricated by a low cost electrodeposition method are investigated. The fabrication of various elemental ferromagnetic materials are described, such as Fe, Co, and Ni, and ferromagnetic alloys, such as NiFe, CoPt, CoFeB, and CoCrPt nanotube arrays, in aluminum oxide templates and polycarbonate membranes with different diameters, wall thicknesses, and lengths. The structural, magnetic, and magnetization reversal properties of these nanotubes are investigated as a function of the geometrical parameters. The angular dependence of the coercivity indicates a transition from the curling to the coherent mode for the ferromagnetic nanotubes. The results show that nanotube fabrication allows the outer and inner diameter, length, and thickness of the nanotubes to be tuned systematically. The magnetization processes of ferromagnetic nanotubes are influenced by the wall thickness.

We report first-principles analysis of equilibrium and nonequilibrium quantum transport in a Langmuir-Blodgett (LB) film contacted by Fe electrodes. For several LB molecules with different lengths, transport is in the tunneling regime. We found that spin-polarized transport is closely related to a magnetic proximity effect that causes the LB film to develop small but finite magnetic character that determines spin-polarized current and produces unusual tunnel magnetoresistance (TMR) ratio versus bias voltage. This unusual TMR was observed experimentally.