High piezoelectric coefficients and rich phase transitions in ternary TlXY (X = S, Se; Y = Cl, Br, I) monolayers

Abstract

In this work, we investigated the structural, electronic, optical, and piezoelectric properties of TlXY (X = S, Se; Y = Cl, Br, I) monolayers using the first-principles calculations. Five types of TlXY monolayers are structurally stable. The band gap, which ranges from 0.371-2.196 eV, is direct for most TlXY and indirect for TlSeBr. With adequate flexibility, the in-plane and out-of-plane piezoelectric coefficients, d11 and d31, of TlXY can approach 77.05-90.55 pm/V and 0.70-1.08 pm/V, respectively, which is larger than those of reported 2D materials. Strain can cause a direct-to-indirect band gap transition or even a topological phase transition. Furthermore, TlSeBr and TlSI can undergo structural phase transitions, resulting in dramatic changes in electronic structure and optical absorption. Our results suggest 2D TlXY as a series of promising materials for optoelectronic and piezoelectric applications.