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

In this paper, we propose a series of two-dimensional asymmetric Janus gamma-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 gamma-Sn2OS and gamma-Sn2OSe monolayers are structurally stable, Janus gamma-Sn2OTe is found to be unstable with the negative frequencies appearing in its phonon spectrum. Both gamma-Sn2OS and gamma-Sn2OSe monolayers are semiconductors with indirect bandgaps. Particularly, gamma-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 gamma-Sn2OS when the spin-orbit coupling is taken into account. Our calculated results reveal that the Janus gamma-Sn2OS and gamma-Sn2OSe monolayers are piezoelectric with out-of-plane piezoelectric coefficient d(31) predicted to be 0.18 and 0.26 pm V-1, respectively. It is also indicated that gamma-Sn2OS and gamma-Sn2OSe exhibit highly directionally anisotropic carrier mobility and their electron mobilities are high enough for applications in nanoelectronic devices.

Automated summary

In this paper, a series of two-dimensional asymmetric Janus γ-Sn2OX (X = S, Se, Te) monolayers are proposed and the electronic structure, piezoelectricity, and mobility of carriers are systematically calculated using the first-principles method. It is predicted that γ-Sn2OS and γ-Sn2OSe monolayers are structurally stable, while Janus γ-Sn2OTe monolayer is found to be unstable. Both γ-Sn2OS and γ-Sn2OSe monolayers are semiconductors with indirect bandgaps. Moreover, these monolayers exhibit piezoelectric properties and highly directionally anisotropic carrier mobility, making them suitable for nanoelectronic devices.