Abstract: Two-dimensional (2D) post-transition metal chalcogenides (PTMCs) have attracted attention due to their suitable bandgaps and lower exciton binding energies, making them more appropriate for electronic, optical, and water-splitting devices than graphene and monolayer transition metal dichalcogenides. Of the predicted 2D PTMCs, GaSe has been reliably synthesized and experimentally characterized. Despite this fact, quantities such as lattice parameters and band character vary significantly depending on which density functional theory (DFT) functional is used. Although many-body perturbation theory (GW approximation) has been used to correct the electronic structure and obtain the excited state properties of 2D GaSe, and solving the Bethe–Salpeter equation (BSE) has been used to find the optical gap, we find that the results depend strongly on the starting wavefunction. In an attempt to correct these discrepancies, we employed the many-body Diffusion Monte Carlo (DMC) method to calculate the ground and excited state properties of GaSe because DMC has a weaker dependence on the trial wavefunction. We benchmark these results with available experimental data, DFT [local-density approximation, Perdew-Burke-Ernzerhof (PBE), strongly constrained and appropriately normed (SCAN) meta-GGA, and hybrid (HSE06) functionals] and GW-BSE (using PBE and SCAN wavefunctions) results. Our findings confirm that monolayer GaSe is an indirect gap semiconductor (Γ-M) with a quasiparticle electronic gap in close agreement with experiment and low exciton binding energy. We also benchmark the optimal lattice parameter, cohesive energy, and ground state charge density with DMC and various DFT methods. We aim to present a terminal theoretical benchmark for pristine monolayer GaSe, which will aid in the further study of 2D PTMCs using DMC methods.

Abstract: Recently, two-dimensional (2D) group-III nitride semiconductors such as h-BN, h-AlN, h-GaN, and h-InN have attracted attention because of their exceptional electronic, optical, and thermoelectric properties. It has also been demonstrated, theoretically and experimentally, that properties of 2D materials can be controlled by alloying. In this study, we performed density functional theory (DFT) calculations to investigate 2D B1–xAlxN, Al1–xGaxN, and Ga1–xInxN alloyed structures. We also calculated the thermoelectric properties of these structures using Boltzmann transport theory based on DFT and the optical properties using the GW method and the Bethe–Salpeter equation. We find that by changing the alloying concentration, the band gap and exciton binding energies of each structure can be tuned accordingly, and for certain concentrations, a high thermoelectric performance is reported with strong dependence on the effective mass of the given alloyed monolayer. In addition, the contribution of each e–h pair is explained by investigating the e–h coupling strength projected on the electronic band structure, and we find that the exciton binding energy decreases with increase in sequential alloying concentration. With the ability to control such properties by alloying 2D group-III nitrides, we believe that this work will play a crucial role for experimentalists and manufacturers focusing on next-generation electronic, optoelectronic, and thermoelectric devices.

Prof. Zhibo Zhang was recently appointed to the Editorial Board of a

high-impact journal—Remote Sensing of Environment.

Remote Sensing of Environment is a highly-selective journal that

serves the Earth observation community with the publication of results

on the theory, science, applications, and technology of remote sensing

studies. Its impact factor was 9.085 in 2019.

Prof. Zhang is the leader of the Aerosol, Cloud, Radiation-Observation,

and Simulation (ACROS) group.

Abstract: We investigate Janus and alloy structures of PtX_nY_2−n (X, Y = S, Se, Te; 0≤n≤2) materials on the basis of first-principles plane-wave simulations. Using cluster-expansion theory to study alloys of PtX_nY_2−n monolayers at various concentrations, for half coverage (n=1), our results indicate that Janus-type structures are not energetically the most favorable for PtXY monolayers; however, they are dynamically and thermally stable. To distinguish Janus PtXY structures, we report the Raman-active modes and compared them with those of bare PtX₂ monolayers. The electronic band gaps calculated with use of hybrid functionals are on par with available experimental data. Spin-orbit coupling significantly modifies the electronic band structure of PtXY monolayers. Because of the electronegativity differences of different chalcogen atoms on each surface of Janus PtXY structures, the arising dipole moment significantly modifies the band alignments on both surfaces. We find that hydrogen-evolution and oxygen-evolution reactions occur on different surfaces and that applied strain enhances the catalytic activity. We also investigate the monovacancy and stacking effects on the electronic properties of PtX₂ and PtXY structures. Our results indicate that due to their intrinsic dipole moments and band gaps, Janus PtXY monolayers are perfect candidates for water-splitting reactions.