First-Principles Study on the 1T Phase of GaX (X=S, Se) Monolayers

Two-dimensional (2D) semiconductors gallium monochalcogenides GaX (X=S, Se), normally possessing the 2H structure, attract great research interest due to their potential applications as future electronical and optical devices. In this paper, we for the first time studied the structural properties and electronic structure of another possible phase of the GaX monolayers, namely the 1T phase by means of first-principles calculations. We found that the 1T phase of the GaX monolayers remain semiconducting with smaller band gaps compared to their 2H counterparts. In addition, we showed that the transition between the 1T and 2H phases for the GaX monolayers is significantly easier than for the MoS₂ monolayer. The 1T phase of the GaX monolayers exhibits a slightly higher energy than the 2H phase and the phonon spectrum confirms its dynamical stability; however the relative stability could be straightforwardly switched by introducing mechanical strains or p-type doping. This work opens up a new route for controlling the phase transition between the 1T and 2H phases of the GaX monolayers and will be benefit for their future applications in optoelectronic devices.