Enhanced out-of-plane piezoelectricity in two-dimensional TlP by surface hydrogenation

In this article, we identify a new ground state (TlPH2) of surface-hydrogenated TlP from a possible crystalline configuration. The crystalline structure of TlPH2 is based on the previously proposed TlP with further hydrogenation. It is predicted to be more favorable than the previously proposed TlP and is confirmed to be dynamically stable. The calculated band structures demonstrate that TlPH2 is a direct bandgap semiconductor with a gap of 0.801 and 1.402 eV based on the DFT-PBE and HSE06 methods, respectively. It is a potential piezoelectric material with coefficients of −24.73 and −0.79 pm/V for d11 and d31, respectively. These coefficients are comparable to those of two-dimensional (2D) group III–V buckled honeycomb monolayers and more remarkable than those of the surface-modified penta-graphene and group-III phosphides. In particular, the combination of direct bandgap semiconducting behavior and robust piezoelectric response positions TlPH2 as a multifunctional material for nanoscale applications, such as sensors, actuators, electric field generators, and energy harvesters.