Predicted novel Janus γ -Ge2 XY ( X/Y=  S, Se, Te) monolayers with Mexican-hat dispersions and high carrier mobilities.

This work is motivated by the recent fabrication of a new four-atom-thick hexagonal polymorph from group IV monochalcogenide, so-called γ -GeSe (Lee et al 2021 Nano Lett. 21 4305). In this paper, we propose and examine the structural characteristics, electronic properties, and carrier mobility of monolayers Janus γ -Ge2 XY ( X / Y = S, Se, or Te) based on comprehensive first-principles calculations. Monolayers γ -Ge2 XY are confirmed to be structurally stable. Our calculations reveal that γ -Ge2 XY monolayers are indirect semiconductors with Mexican-hat-like dispersions in the top valence band. While the effect of the electric field on the energy band dispersions of γ -Ge2 XY monolayers is weak, the energy band dispersions are changed drastically in the presence of strain, especially compressive strain. Interestingly, a structural phase transition from semiconductor to metal is observed in γ -Ge2 XY under compressive strain. γ -Ge2STe and γ -Ge2SeTe possess high electron mobility with values of 3.22 × 10 3 and 8.33 × 10 3 cm2 V−1 s−1, respectively. Our findings not only explore the fundamental physical properties of γ -Ge2 XY but also open up new opportunities in the design of high-performance electronic nanodevices based on layered nanomaterials with Mexican-hat-like dispersions. [ABSTRACT FROM AUTHOR]