Exploring the electronic band gap of Janus MoSeO and WSeO monolayers and their heterostructures

Abstract

Designing new two-dimensional (2D) materials based on the Janus structure has attracted great attention due to their novel properties induced by out-of-plane symmetry breaking. In this work, we have systematically investigated the structural and electronic properties of TMSeO (TM = Mo and W) monolayers, which can be obtained by oxygenation of the TMSe₂ counterpart. Phonon dispersion curves confirm good dynamical stability. MoSeO and WSeO pristine monolayers are Γ–K indirect gap semiconductors with energy gaps of 0.815(1.324) and 1.323(1.894) eV as determined using the PBE(HSE06) functional, respectively. Our calculations show that a single vacancy of each constituent atoms leads to magnetism-free 2D materials, although it induces a considerable energy-gap reduction. A small lattice constant mismatch between the MoSeO and WSeO monolayers allows the formation of heterostructures. Depending on the stacking, vertical heterostructures (VHSs) exhibit different electronic features, including being a semiconductor with a variable band gap, a semimetal, and metallic in nature. Meanwhile, in lateral heterostructures (LHSs) the semiconductor behavior remains, for which the band gap increases according to an increase in the WSeO block composition. This work provides a deeper insight into the TMSeO monolayers as well as their effective gap engineering based on the formation of defects and heterostructures. The obtained results may introduce new 2D materials, which have the potential for applications in high-performance optoelectronic nano devices.