Novel valleytronic and piezoelectric properties coexisting in Janus MoAZ3H (A = Si, or Ge; Z = N, P, or As) monolayers

Abstract

In this paper, for the newly proposed two-dimensional (2D) Janus MoAZ₃H (A = Si, or Ge; Z = N, P, or As) monolayer (ML) materials, we theoretically explore the valleytronic and piezoelectric properties using first-principles calculations. The calculated results show that Janus MoAZ₃H MLs have obvious valleytronic properties due to the broken inversion symmetry and strong spin–orbit coupling (SOC) effects, and the spin-valley coupling of MoAZ₃H MLs can be enhanced from 173 to 283 meV by changing the A or Z element, comparable to the corresponding MoA₂Z₄ MLs. The non-zero Berry curvatures with opposite signs at the K and K′ points can induce the valley Hall effect. Furthermore, Janus MoAZ₃H MLs exhibit obvious in-plane and out-of-plane piezoelectric responses, and their independent d₁₁ and d₃₁ piezoelectric coefficients range from 1.62 to 8.60 pm V⁻¹ and from 0.058 to 0.325 pm V⁻¹, respectively. This piezoelectricity is much stronger than those of the corresponding MoA₂Z₄ and MoSiGeN₄ MLs, attributed to the unique Janus structure of the MoAZ₃H MLs. More importantly, strain can tailor both the spin-valley coupling and piezoelectricity of the MoAZ₃H MLs. Our findings not only enrich the excellent properties of the MoAZ₃H MLs, but also indicate a direction for their application in valleytronics and energy conversion devices.