High piezoelectricity Janus GaXI (X = S, Se, or Te): First-principles calculations

Abstract

Two-dimensional (2D) Janus materials have received considerable interest because of their robust piezoelectricity generated by breaking the central symmetry, which have potential applications in micro/nanomotor and flexible robot skins. However, in the previous 2D piezoelectric material research system, strain and electric polarization were limited to the base plane, greatly restricting its applications. Based on density functional theory (DFT), we have found the monolayer and multilayer Janus GaXI (X = S, Se, or Te) with high planar and vertical piezoelectricity. The maximum out-of-plane piezoelectric coefficient (d₃₃ = 19.96 p.m./V) of these materials is 7.8 and 2.6 times larger than those of the conventional three-dimensional (3D) piezoelectric materials α-quartz and AIN, respectively. The monolayer Janus GaTeI exhibits the largest in-plane piezoelectric coefficient (d₁₁ = 11.27 p.m./V). Additionally, it is worth noting that the multilayer Janus GaXI of six high-symmetry stacking sequences exhibits stronger out-of-plane piezoelectric polarization in the vertical direction than the monolayer. The structure of GaSI monolayer is similar to that of honeycomb graphene monolayer, and GaSI multilayer is also similar to graphene multilayer. In the Janus GaXI system, the sign of the electrostatic potential gradients, the direction of out-of-plane polarization and the sign of relaxed-ion d31 values are the same. Our study shows that monolayers and multilayer Janus GaXI have excellent piezoelectricity, and they have wildly potential applications in micro/nano-electronic devices.