Strong out-of-plane piezoelectricity and Rashba-type spin splitting in asymmetric structures: first-principles study for Janus γ-Sn2OX (X = S, Se, Te) monolayers

Abstract

In this paper, we propose a series of two-dimensional asymmetric Janus γ-Sn₂OX (X = S, Se, Te) monolayers and systematically calculate the electronic structure, piezoelectricity, and mobility of carriers by using the first-principles method. It is predicted that while γ-Sn₂OS and γ-Sn₂OSe monolayers are structurally stable, Janus γ-Sn₂OTe is found to be unstable with the negative frequencies appearing in its phonon spectrum. Both γ-Sn₂OS and γ-Sn₂OSe monolayers are semiconductors with indirect bandgaps. Particularly, γ-Sn₂OS has a camel's back-like structure in the top valence band and Rashba spin splitting with a Rashba energy of 3.50 meV is found in γ-Sn₂OS when the spin-orbit coupling is taken into account. Our calculated results reveal that the Janus γ-Sn₂OS and γ-Sn₂OSe monolayers are piezoelectric with out-of-plane piezoelectric coefficient d₃₁ predicted to be 0.18 and 0.26 pm V⁻¹, respectively. It is also indicated that γ-Sn₂OS and γ-Sn₂OSe exhibit highly directionally anisotropic carrier mobility and their electron mobilities are high enough for applications in nanoelectronic devices.