Prediction of new phase 2D C2h group III monochalcogenides with direct bandgaps and highly anisotropic carrier mobilities

Abstract

Two-dimensional (2D) group III monochalcogenides (MX, M = Ga, In and X = S, Se, Te) are promising candidates for next-generation ultrathin optoelectronic devices due to their exotic properties. However, the lack of direct band gaps and the low hole mobilities in their conventional single-layer D_3h phase hinder their potential utility for various applications. In this work, new polymorphs of 2D MXs belonging to the space group C_2h are predicted through a global structural search based on artificial swarm intelligence and density functional theory calculations. We demonstrate that such monolayer polymorphs are thermodynamically and kinetically stable through phonon spectrum analysis and ab initio molecular dynamics simulations. Direct band gaps of 2.38 to 2.84 eV are revealed in all C_2h MX monolayers, a property that traditional 2D D_3h MXs do not possess. Calculations based on the Boltzmann Transport Equation method show that electron mobilities in C_2h monolayers are significantly higher in magnitude than those of the conventional D_3h phase. Anisotropic optical properties are predicted and high absorption coefficients covering the UV-visible spectra are also reported. All these features render the new C_2h MX monolayers promising candidates for potential applications in high-efficiency solar cells and anisotropic optoelectronic devices.