Symmetry-breaking induced large piezoelectricity in Janus tellurene materials.

Structural symmetry-breaking can lead to novel electronic and piezoelectric properties in two-dimensional (2D) materials. In this paper, we propose a 2D Janus tellurene (Te2Se) monolayer with asymmetric Se/Te surfaces and its derived multilayer structures. The band structure calculations show that the 2D Janus Te2Se monolayer is an indirect gap semiconductor, and the intrinsic mirror asymmetry combined with the spin-orbit coupling induces the Rashba spin splitting and the out-of-plane spin polarization. Moreover, the absence of both the inversion symmetry and out-of-plane mirror symmetry, together with flexible mechanical properties, results in large in-plane and out-of-plane piezoelectric coefficients that are valuable in 2D piezoelectric materials. Furthermore, the out-of-plane piezoelectric effects can exist in multilayer structures under different stacking sequences while the in-plane piezoelectric effect can only exist in some specific stacking patterns. The piezoelectric coefficients of the Janus Te2Se monolayer and multilayers exceed those of many Janus transition metal dichalcogenides and other well-known piezoelectric materials (e.g., α-quartz and wurtzite-AlN). The combination of the SOC-induced spin splitting and large piezoelectricity endows the Janus Te2Se structures with potential for applications in spintronics, flexible electronics and piezoelectric devices.