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

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 D3h phase hinder their potential utility for various applications. In this work, new polymorphs of 2D MXs belonging to the space group C2h 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 C2h MX monolayers, a property that traditional 2D D3h MXs do not possess. Calculations based on the Boltzmann Transport Equation method show that electron mobilities in C2h monolayers are significantly higher in magnitude than those of the conventional D3h phase. Anisotropic optical properties are predicted and high absorption coefficients covering the UV-visible spectra are also reported. All these features render the new C2h MX monolayers promising candidates for potential applications in high-efficiency solar cells and anisotropic optoelectronic devices. This journal is