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

In this paper, for the newly proposed two-dimensional (2D) Janus MoAZ(3)H (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 MoAZ(3)H MLs have obvious valleytronic properties due to the broken inversion symmetry and strong spin-orbit coupling (SOC) effects, and the spin-valley coupling of MoAZ(3)H MLs can be enhanced from 173 to 283 meV by changing the A or Z element, comparable to the corresponding MoA(2)Z(4) MLs. The non-zero Berry curvatures with opposite signs at the K and K ' points can induce the valley Hall effect. Furthermore, Janus MoAZ(3)H MLs exhibit obvious in-plane and out-of-plane piezoelectric responses, and their independent d(11) and d(31) 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 MoA(2)Z(4) and MoSiGeN4 MLs, attributed to the unique Janus structure of the MoAZ(3)H MLs. More importantly, strain can tailor both the spin-valley coupling and piezoelectricity of the MoAZ(3)H MLs. Our findings not only enrich the excellent properties of the MoAZ(3)H MLs, but also indicate a direction for their application in valleytronics and energy conversion devices.