Theoretical study of piezoelectric and light absorption properties, and carrier mobilities of Janus TiPX (X = F, Cl, and Br) monolayers.

Janus TiPX (X = F, Cl, and Br) monolayers were systematically investigated through first-principles calculations. The Janus TiPX monolayers exhibit mechanical and dynamic stability. Two monolayers are indirect bandgap semiconductors, except the TiPBr monolayer, which has the features of a quasi-direct bandgap semiconductor. Biaxial strain can modify the band gap of single layers. The Janus TiPX monolayers have remarkable flexibility and piezoelectric properties. In particular, the TiPF monolayer shows high horizontal (44.18 pm V) and vertical piezoelectric coefficients (-3.59 pm V). These values exceed those of conventional bulk materials, like GaN (3.1 pm V) and α-quartz (2.3 pm V). All of the monolayers have absorption coefficients of 10 cm for visible and ultraviolet (UV) light, which are one order of magnitude greater than that of MoSSe. Furthermore, TiPX monolayers have high carrier mobility. Janus TiPX monolayers represent a class of two-dimensional (2D) materials with exceptional properties and multifunctionality, holding significant promise for various applications in piezoelectric sensors, solar cells, and nano-electronic devices.