Langhui Wan

Electronics and optoelectronics of lateral heterostructures within monolayer indium monochalcogenides

Lateral heterostructures have attracted a great deal of attention due to their advanced properties, which may open up unforeseen opportunities in materials science and device physics. Here, we demonstrate a novel type of lateral heterostructure within monolayer indium monochalcogenides. The thermal stability of the structure is obtained based on the ab initio molecular dynamics calculations. Our results reveal that the proposed lateral heterostructures have direct bandgaps, tunable electronic properties, and type-II band alignment. In addition, the predicted carrier mobilities exceed 103 cm2 (V s)−1, which are 1–2 orders of magnitude higher compared to those of transition metal chalcogenide (TMD) materials. For the first time, the photoresponse and photovoltaic performance of such lateral heterostructures are evaluated based on the first-principles calculations. Upon illumination, the photoinduced current is generated throughout the heterojunction, with an external quantum efficiency up to 7.1%. These results make indium monochalcogenide lateral heterostructures promising candidates for next-generation of electronic and optoelectronic devices.

Definition of current density in the presence of a non-local potential

In the presence of a non-local potential arising from electron-electron interaction, the conventional definition of current density J(c) = (e/2m)([(p-eA)ψ](*)ψ-ψ(*)[(p-eA)ψ]) cannot satisfy the condition of current conservation, i.e., [Formula: see text] in the steady state. In order to solve this problem, we give a new definition of current density including the contribution due to the non-local potential. We show that the current calculated based on the new definition of current density conserves the current and is the same as that obtained from the Landauer-Büttiker formula. Examples are given to demonstrate our results.

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

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.

NONLINEAR THERMOELECTRIC TRANSPORT THROUGH A DOUBLE BARRIER STRUCTURE

We present a theoretical analysis of thermoelectric transport in the nonlinear regime. The thermopower and thermoconductance at finite temperature gradient are calculated numerically for a double barrier structure using Landauer Buttiker like formula. The thermopower is found to oscillate with the chemical potential. Thermopower can either be negative or positive which is well correlated with the behavior of the electric conductance. The thermal conductance is positive definite showing that the heat energy is always transferred from hot end to cold end. As the chemical potential is varied, nonlinear thermal conductance exists plateau-like features.

THE INFLUENCE OF THE COUPLING STRENGTH ON THE ELECTRON TRANSPORT THROUGH THE BENZENE-1,4-DITHIOLATE MOLECULAR JUNCTION

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.

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

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 Hartree-Fock 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.