Electronic and optical properties of Janus-like hexagonal monolayer materials of group IV-VI

We report $ab\phantom{\rule{4pt}{0ex}}initio$ calculations of the electronic and optical response properties of Janus-like hexagonal monolayers materials of group IV-VI. By combining phonons spectra calculations and ab initio molecular dynamics simulations, we verify that some two-dimensional (2D) group IV-VI hexagonal materials are dynamically unstable. Our results of electronic band structure based on both density functional theory (DFT) with Heyd-Scuseria-Ernzerhof hybrid functional (HSE) and ${\mathrm{G}}₀{\mathrm{W}}₀$ formalisms indicate that the dynamically stable materials exhibit indirect band gaps. The calculations clearly reveal that the hexagonal crystal structure of the group IV-VI monolayers plays a crucial role not only in determining the width of the band gap but also on the electron-hole screening effect, and consequently the strength of the exciton binding energy. Despite their strong binding, exciton binding energies in these nanomaterials are found to be in agreement with a 2D Mott-Wannier model. We also show that most of the stable 2D materials exhibit strong optical absorbance in the visible range, being promising materials for ultra-thin-film photovoltaic applications. Thus, our systematic analysis on electronic and optical properties of hexagonal monolayer materials of group IV-VI will complement forthcoming experimental measurements and can pave the way for technological applications of those materials.