Janus XM-GaS (M=Si, Ge, Sn; X=N, P) monolayers: Multifunctional properties for photocatalysis, piezoelectricity and second harmonic generation

Abstract Two dimensional (2D) Janus monolayers attract increasing attention since successful synthesis of MoSSe monolayer. In this work, we study the stability and multifunctional applications of Janus monolayers MX-GaS (M = Si, Ge; Sn X = N, P) using first-principles calculations. 2D MX-GaS monolayers are energetically, mechanically and dynamically stable. Among MX-GaS, single-layer SiP-GaS and GeP-GaS possess suitable band edges for photocatalytic water splitting and strong visible-light absorbance. The in-plane and out-of-plane second harmonic generation (SHG) coefficients of single-layer MX-GaS are comparable or beyond that of typical 2D SHG materials GaS, GaSe and MoS2. Single-layer SiN-GaS, GeN-GaS, SnN-GaS and SnP-GaS display in-plane piezoelectric coefficients, while single-layer SiN-GaS exhibits considerable out-of-plane piezoelectricity. This work will stimulate discoveries of novel properties and applications among 2D Janus Ⅴ-Ⅳ-Ⅲ-Ⅵ (Ⅴ = N, P; Ⅳ = Si, Ge, Sn; Ⅲ = Ga, In; Ⅵ = S, Se, Te) materials.