The coexistence of high piezoelectricity and superior optical absorption in Janus Bi 2 X 2 Y (X = Te, Se; Y = Te, Se, S) monolayers.

Bismuth chalcogenide and its derivatives have been attracting attention in various fields as semiconductors or topological insulators. Inspired by the high piezoelectric properties of Janus Bi 2 TeSeS monolayer and the excellent optical absorption properties of the Bi 2 X 3 (X = Te, Se, S) monolayers, we theoretically predicted four new-type two-dimensional (2D) monolayers Janus Bi 2 X 2 Y (X = Te, Se; Y = Te, Se, S) using the first principles combined with density functional theory (DFT). The thermal, dynamic, and mechanical stabilities of Janus Bi 2 X 2 Y monolayers were confirmed based on ab initio molecular dynamics (AIMD) simulations, phonon dispersion, and elastic constants calculations. Their elastic properties, band structures, piezoelectric, and optical properties were systematically investigated. It was found that Janus Bi 2 X 2 Y monolayers have a typical Mexican hat-shaped valence band edge structure and, therefore, have a ring-shaped flat band edge, which results in their indirect band gaps. The results show that Janus Bi 2 X 2 Y monolayers are semiconductors with moderate band gaps (0.62-0.98 eV at the HSE + SOC level). After considering the electron-phonon renormalization (EPR), the band gaps are reduced by less than 5% at 0 K under the zero-point renormalization (ZPR) and further reduced by approximately 10% at 300 K. Besides, Janus Bi 2 X 2 Y monolayers also exhibit excellent optical absorption properties in the blue-UV light region, with the peak values at the order of 8 × 10 5 cm -1 . Particularly, the Janus Bi 2 Te 2 S monolayer was found to exhibit a piezoelectric strain coefficient d 11 of up to 20.30 pm V -1 , which is higher than that of most of the 2D materials. Our results indicate that Janus Bi 2 X 2 Y monolayers could be promising candidates in solar cells, optical absorption, and optoelectronic devices; especially, a Janus Bi 2 Te 2 S monolayer can also be an excellent piezoelectric material with great prospects in the fields of mechanical and electrical energy conversion.