Band structure of transition metal oxides plays a critical role in many applications such as photo-catalysis, photovoltaics, and electroluminescent devices. In this work we report findings that the band structure of MoO3 can be significantly altered by a distortion in the octahedral coordination structure. We discovered that, in addition to epitaxial type of structural strain, chemical force such as hydrogen inclusion can also cause extended lattice distortion. The lattice distortion in hydrogenated MoO3 led to a significant reduction of the energy gap, overshadowing the Moss-Burstein effect of band filling. Charge doping simulations revealed that filling of conduction band drives the lattice distortion. This suggests that any charge transfer or n-type electron doping could lead to lattice distortion and consequentially a reduction in energy gap.

Based on first-principles density functional theory calculations, we systemically study the properties of two-dimensional buckled single-layer bismuth (b-bismuthene). The structure, stability, and electronic properties are mainly discussed by PBE + SOC method and the hybrid functional HSE06 method is used to further revise the band gap. The optimized b-bismuthene is determined to be dynamically and thermally stable with an indirect band gap. In particular, there is a peculiar Rashba spin-splitting emerging in the valence band maximum (VBM) states. Interestingly, the Rashba energy could be effectively modulated by in-layer biaxial strain. By applying in-layer biaxial strain, one can find that b-bismuthene has indirect–direct band gap and semiconductor–semimetal transitions. Moreover, we also study the electronic structure of bilayer b-bismuthene that is sensitively dependent on the interlayer distance. We demonstrate that the electric field (E-field) leads to a breaking of the Rashba-type splitting near the VBM of single b-bismuthene. More importantly, there is a synergistic effect when both strain and electric field are applied at the same time. The E-field induced band splitting character could be modified by the strain strength. Thus, this study indicates that b-bismuthene may be a potential material in both electronic and spintronic devices.

We predict, through first-principles calculations, a different class of Janus two-dimensional (2D) materials $X{\mathrm{Sn}}_{2}Y$ ($X, Y=\mathrm{P}$, As, Sb, and Bi; $X\ne Y$). It has found that these 2D monolayers have intrinsic polarization effect owing to presence of mirror-symmetry breaking. $\mathrm{Sb}{\mathrm{Sn}}_2\mathrm{Bi}, \mathrm{As}{\mathrm{Sn}}_2\mathrm{Sb}$, and $\mathrm{P}{\mathrm{Sn}}_2\mathrm{As}$ have a semiconducting feature, however $\mathrm{As}{\mathrm{Sn}}_2\mathrm{Bi}, \mathrm{P}{\mathrm{Sn}}_2\mathrm{Bi}$, and $\mathrm{P}{\mathrm{Sn}}_2\mathrm{Sb}$ have a metallic feature with inverted band structure. When the spin-orbit coupling (SOC) is considered, the double Rashba effects are found in $\mathrm{Sb}{\mathrm{Sn}}_2\mathrm{Bi}$. Band inversion coupling with SOC leads to unexpected spin-valley splitting characteristics in $\mathrm{As}{\mathrm{Sn}}_2\mathrm{Bi}, \mathrm{P}{\mathrm{Sn}}_2\mathrm{Bi}$, and $\mathrm{P}{\mathrm{Sn}}_2\mathrm{Sb}$, showing a circle-type Berry curvature. The electronic and spin properties of Janus $X{\mathrm{Sn}}_{2}Y$ monolayers are tunable via applying external strain and electric field, resulting in Rashba-type spin-splitting, $p_z/p_{xy}$ band inversion, and a Dirac cone. In particular, the sombrero band dispersion and Van Hove singularity are revealed in $\mathrm{P}{\mathrm{Sn}}_2\mathrm{As}$ by exerting tensile strain. We therefore investigate the Lifshitz transition and inverse Edelstein effect (also referred as spin-charge conversion) of the strained $\mathrm{P}{\mathrm{Sn}}_2\mathrm{As}$. On the basis of the k·p model in the clean limit, the Drude coefficient for spin-charge conversion is estimated to be about 0.086 $e$/Å as tensile strain of +3%. Finally, the synergic effect of tensile strain and electric field is considered to manipulate the spin-charge conversion. Our results provide a class of 2D materials to investigate the Rashba effect, Lifshitz transition, and spin-charge conversion.

We study the equilibrium geometry and electronic structure of alloyed and doped arsenene sheets based on the density functional theory calculations. AsN, AsP and SbAs alloys possess indirect band gap and BiAs is direct band gap. Although AsP, SbAs and BiAs alloyed arsenene sheets maintain the semiconducting character of pure arsenene, they have indirect-direct and semiconducting-metallic transitions by applying biaxial strain. We find that B- and N-doped arsenene render p-type semiconducting character, while C- and O-doped arsenene are metallic character. Especially, the C-doped arsenene is spin-polarization asymmetric and can be tuned into the bipolar spin-gapless semiconductor by the external electric field. Moreover, the doping concentration can effectively affect the magnetism of the C-doped system. Finally, we briefly study the chemical molecule adsorbed arsenene. Our results may be valuable for alloyed and doped arsenene sheets applications in mechanical sensors and spintronic devices in the future.

The impact of halogenation, in particular Cl and F, on the work functions of indium tin oxide (ITO) surfaces was studied using density functional theory calculations. We found that a strong surface dipole layer induced by the halogen, rather than a change in the electrochemical potential (that is, Fermi level) of the ITO, led to a dramatic increase in the work function. However, the work function for F-coated ITO was lower than that of Cl-coated ITO. This result contradicts the well-known fact that F is much more electronegative than Cl. Detailed computations reveal that both electronegativity and atomic size collectively contribute to the extraordinarily high work function of Cl-ITO. Additionally, the work function increases linearly with increasing surface halogen coverage for both systems, which was consistent with experimental data.

As the isoelectronic counterpart of phosphorene, monolayer group IV–VI binary MX (M = Ge, Sn; X = Se, S) compounds have drawn considerable attention in recent years.