Highly-anisotropic optical and electrical properties in layered SnSe

Anisotropic materials are of considerable interest because of their unique combination of polarization- or direction-dependent electrical, optical, and thermoelectric properties. Low-symmetry two-dimensional (2D) materials formed by van der Waals stacking of covalently bonded atomic layers are inherently anisotropic. Layered SnSe exhibits a low degree of lattice symmetry, with a distorted NaCl structure and an in-plane anisotropy. Here we report a systematic study of the in-plane anisotropic properties in layered SnSe, using angle-resolved Raman scattering, optical absorption, and electrical transport studies. The optical and electrical characterization was direction-dependent, and successfully identified the crystalline orientation in the layered SnSe. Furthermore, the dependence of Raman-intensity anisotropy on the SnSe flake thickness and the excitation wavelength were investigated by both experiments and theoretical calculations. Finally, the electrical transport studies demonstrated that few-layer SnSe field-effect transistors (FETs) have a large anisotropic ratio of carrier mobility (∼5.8) between the armchair and zigzag directions, which is a record high value reported for 2D anisotropic materials. The highly-anisotropic properties of layered SnSe indicate considerable promise for anisotropic optics, electronics, and optoelectronics.