Tuning structural and electronic properties of two-dimensional aluminum monochalcogenides: Prediction of Janus Al2XX′ (X/X′:O,S,Se,Te) monolayers

Abstract

The realization of ternary, single-layer transition metal dichalcogenides has suggested a promising strategy to develop two-dimensional (2D) materials with alternative features. In this study, we design and investigate Janus aluminum monochalcogenide monolayers, ${\mathrm{Al}}_{2}X{X}^{\ensuremath{'}}$ $(X/{X}^{\ensuremath{'}}=\phantom{\rule{0.28em}{0ex}}\mathrm{O},\phantom{\rule{0.28em}{0ex}}\mathrm{S},\phantom{\rule{0.28em}{0ex}}\mathrm{Se},\phantom{\rule{0.28em}{0ex}}\mathrm{Te})$ by using first-principles methods. Starting from binary constituents, the ternary structures are optimized without any constraint and ground-state configurations are obtained. The stability of these systems is tested by performing phonon spectra analysis and ab initio molecular dynamics simulations and all ${\mathrm{Al}}_{2}X{X}^{\ensuremath{'}}$ monolayers other than AlTeO are confirmed to be dynamically stable. Mechanical properties are examined by calculating Young's modulus and Poisson's ratio and subsequently compared with binary counterparts. Monolayers of ${\mathrm{Al}}_{2}X{X}^{\ensuremath{'}}$ have a brittle character but oxygenation makes them less stiff. The electronic structure is also analyzed and variation of the band gap with the type of chalcogen atoms is revealed. It is found that different from their binary counterparts, ${\mathrm{Al}}_{2}X\mathrm{O}$ monolayers are direct band-gap semiconductors. Additionally, modification of the electronic structure in the presence of biaxial compressive or tensile strain is investigated by taking into account possible indirect-direct band-gap transitions. Our results not only predict stable 2D ternary ${\mathrm{Al}}_{2}X{X}^{\ensuremath{'}}$ structures but also point out them as promising materials for optoelectronic applications.