MXenes are a large family of layered transition metal carbide/nitride materials that possess a number of desired properties such as flexible chemical composition, high mechanical strength, and excellent structural stability. Although MXene based semiconductors have attracted considerable recent research attention in the search of novel 2D electronic materials, accurate understanding of their electronic properties has not been established. In this work, we carry out fully converged GW quasiparticle calculations for M₂CO₂ (M = Hf, Zr, and Ti) MXene based 2D semiconductors and alloys using newly developed accelerated GW methods. The quasiparticle band gaps of single-layer Hf₂CO₂, Zr₂CO₂, and Ti₂CO₂ are predicted to be 2.45, 2.13, and 1.15 eV, respectively. The narrow band gap of Ti₂CO₂ is attributed to the low energy of Ti 3d as compared with the Hf and Zr d states. Considering their chemical similarity, it is expected that Hf_2−2xTi_2xCO₂ semiconductors can be synthesized without difficulties. We show that the quasiparticle band gap of Hf_2−2xTi_2xCO₂ (0 ≤ x ≤ 1) semiconductor alloy can be continuously tuned from 2.45 to 1.15 eV, offering a unique 2D semiconductor with a moderate and tunable gap for future electronics applications.

We present a systematic and comparative study of the structural and electronic properties of Cu-based ternary and quaternary semiconductors using first-principles electronic structure approaches. The important role that Cu $d$ electrons play in determining their properties is illustrated by comparing results calculated with different exchange–correlation energy functionals. We show that systematic improvement of the calculated anion displacement can be achieved by using the Heyd-Scuseria-Ernzerhof (HSE06) functional compared with the Perdew-Burke-Ernzerhof (PBE) functional. Quasiparticle band structures are then calculated within the $G^0$ $W^0$ approximation using the crystal structures optimized within the HSE06 functional and starting from the PBE+$U$ mean-field solution. Both the calculated quasiparticle band gaps and their systematic variation with chemical constituents agree well with experiments. We also predict that the quasiparticle band gap of the prototypical semiconductor Cu${}_2$ZnSnS${}_4$ in the kesterite phase is 1.65 eV and that the gap of the stannite phase is 1.40 eV. These results are consistent with available experimental values, which vary from 1.45 to 1.6 eV.

Although the GW approximation is recognized as one of the most accurate theories for predicting materials excited states properties, scaling up conventional GW calculations for large systems remains a major challenge. We present a powerful and simple-to-implement method that can drastically accelerate fully converged GW calculations for large systems, enabling fast and accurate quasiparticle calculations for complex materials systems. We demonstrate the performance of this new method by presenting the results for ZnO and MgO supercells. A speed-up factor of nearly two orders of magnitude is achieved for a system containing 256 atoms (1024 valence electrons) with a negligibly small numerical error of ±0.03 eV. Finally, we discuss the application of our method to the GW calculations for 2D materials.

The quasiparticle band gap is one of the most important materials properties for photovoltaic applications. Often the band gap of a photovoltaic material is determined (and can be controlled) by various factors, complicating predictive materials optimization. An in-depth understanding of how these factors affect the size of the gap will provide valuable guidance for new materials discovery. Here we report a comprehensive investigation on the band gap formation mechanism in organic-inorganic hybrid perovskites by decoupling various contributing factors which ultimately determine their electronic structure and quasiparticle band gap. Major factors, namely, quasiparticle self-energy, spin-orbit coupling, and structural distortions due to the presence of organic molecules, and their influences on the quasiparticle band structure of organic-inorganic hybrid perovskites are illustrated. We find that although methylammonium cations do not contribute directly to the electronic states near band edges, they play an important role in defining the band gap by introducing structural distortions and controlling the overall lattice constants. The spin-orbit coupling effects drastically reduce the electron and hole effective masses in these systems, which is beneficial for high carrier mobilities and small exciton binding energies.

The relative phase stability of VO${}_2$ is one of the most fundamental issues concerning the metal-insulator transition in this material but has been so far largely unexplored theoretically. We investigate the relative stability of various phases of VO${}_2$ using different levels of energy functionals within density functional theory (DFT). It is found that straightforward applications of several popular energy functionals, including the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional, result in a wrong prediction for the ground state of VO${}_2$. In particular, although the HSE and DFT+$U$ methods are able to produce a band gap in the M${}_1$ phase, they strongly favor the formation of local magnetic moments, a result that clearly disagrees with experiments. We also examine the effect of the occupation and the redistribution of the $d$ derived $t_{2g}$ (i.e., $d_{xz}$, $d_{yz}$, and $d_{x^2-y^2}$) orbitals of V atoms on the calculated relative phase stability of VO${}_2$. We find that a small change in $d$ occupation can result in a drastically different theoretical prediction. With the introduction of an orbital-dependent potential, a complete separation between the $d_{x^2-y^2}$ derived valence band and $d_{xz}$ and $d_{yz}$ derived conduction bands in the M${}_1$ phase is achieved, resulting in a slight redistribution of the $d$ occupation and a more faithful account of the polarization of the $t_{2g}$ orbitals. This slight rearrangement of the $d$ occupation also leads to a relative phase stability of VO${}_2$ (including structural and magnetic phases) that agrees well with experiment.

A layered solid boron hydride structure (B${}_2$H${}_2$) consisting of a hexagonal boron network and bridge hydrogen which has a gravimetric capacity of $8\mathrm{wt}\mathrm{\%}$ hydrogen is predicted. The structural, electronic, and dynamical properties of the proposed structure are investigated using first-principles electronic structure methods. The absence of soft phonon modes confirms the dynamical stability of the proposed structure. Charging the structure significantly softens hydrogen related phonon modes. Boron modes, in contrast, are either hardened or not significantly affected by electron doping. Furthermore, self-doping the structure considerably reduces the energy barrier against hydrogen release. These results suggest that electrochemical charging or self-doping mechanisms may facilitate hydrogen release while the underlying boron network remains intact for subsequent rehydrogenation.