Discovery of Novel Two-Dimensional Photovoltaic Materials Accelerated by Machine Learning.

Searching for novel, high-performance, two-dimensional photovoltaic (2DPV) materials is an important pursuit for solar cell applications. In this work, an efficient method based on the machine learning algorithm combined with high-throughput screening is developed. Twenty-six 2DPV candidates are successfully ruled out from 187093 experimentally identified inorganic crystal structures, whose conversion efficiencies are predicted by density functional theory calculations. Our results indicate that Sb2Se2Te, Sb2Te3, and Bi2Se3 exhibit conversion efficiencies that are much higher than those of others, which make them promising 2DPV candidates for further applications. The superior photovoltaic performance is then analyzed, and the hidden structure-related relationships with photovoltaic properties are established, thus providing important information for the further examination of 2DPV materials. Given the rapid development of the database of materials, this approach not only provides an efficient way of searching for novel 2DPV materials but also can be applied to exploration of a broad range of functional materials.

Data-Driven Systematic Search of Promising Photocatalysts for Water Splitting under Visible Light.

Searching for highly efficient, environmentally friendly, and noble metal free photocatalysts is an important topic in the photocatalytic field. The combination of data-driven high-throughput screening and density functional theory (DFT) might be one of the most effective ways. To this end, we have carried out a systematic search of the Materials Project Database. A high-throughput screening method is employed, and six criteria are selected as the indicators. The screening reduces the list of candidates from 83989 to 22 structures that show promise as photocatalysts. The electronic and optical properties are then determined by DFT calculations. Our results indicate that ZnSe, Ga2Se3, and Na2Zn2O3 show suitable direct band gaps, efficient optical absorption, and appropriate band edge positions, which are suitable for photocatalytic water splitting in the visible light region. We believe that this work not only proposes novel photocatalysts but also provides an efficient way of searching for advanced photocatalysts for water splitting.

Photoinduced valley-polarized current of layered MoS2 by electric tuning.

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 (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 (K') index. For bilayer MoS2, the spin-up (down) current can be induced at both K and K' 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.

Modulation of Electronic Structure of Armchair MoS2 Nanoribbon.

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 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, while 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 MoS2 based practical nanoelectronic device and photo¬voltaic device.

Shot noise of spin current and spin transfer torque.

We report the theoretical investigation of the shot noise of the spin current (S(σ)) and the spin transfer torque (S(τ)) for non-collinear spin polarized transport in a spin-valve device which consists of a normal scattering region connected by two ferromagnetic electrodes (MNM system). Our theory was developed using the non-equilibrium Green's function method, and general nonlinear S(σ) - V and S(τ) - V relations were derived as a function of the angle θ between the magnetizations of two leads. We have applied our theory to a quantum dot system with a resonant level coupled with two ferromagnetic electrodes. It was found that, for the MNM system, the auto-correlation of the spin current is enough to characterize the fluctuation of the spin current. For a system with three ferromagnetic layers, however, both auto-correlation and cross-correlation of the spin current are needed to characterize the noise of the spin current. For a quantum dot with a resonant level, the derivative of spin torque with respect to bias voltage is proportional to sinθ when the system is far away from resonance. When the system is near resonance, the spin transfer torque becomes a non-sinusoidal function of θ. The derivative of the noise of the spin transfer torque with respect to the bias voltage Nτ behaves differently when the system is near or far away from resonance. Specifically, the differential shot noise of the spin transfer torque Nτ is a concave function of θ near resonance while it becomes a convex function of θ far away from resonance. For certain bias voltages, the period Nτ(θ) becomes π instead of 2π. For small θ, it was found that the differential shot noise of the spin transfer torque is very sensitive to the bias voltage and the other system parameters.

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 Green's 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 Green's 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.

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.