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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]

Janus group III monochalcogenide structures, which are predicted to have many promising applications in optoelectronics and photocatalytic water splitting, have been of particular interest recently. In this study, electronic properties and optical characteristics of the Janus Ga2SeTe monolayer under a biaxial strain εb and electric field E were considered using the density functional theory. Our calculations demonstrated that the Janus Ga2SeTe monolayer is dynamically and thermally stable and exhibits a semiconducting characteristic with a moderate direct band gap at equilibrium. The band gap of Janus Ga2SeTe monolayer at equilibrium, which is calculated by PBE method and corrected by HSE06 hybrid functional, is smaller than that of both GaSe and GaTe monolayers. Mulliken population analysis shows that there has been a redistribution of charge during the formation of the Janus structure, especially there is a large difference in charge between the two Ga layers in Janus Ga2SeTe monolayer. The biaxial strain has greatly altered the electronic structure of the Janus Ga2SeTe monolayer and direct–indirect band gap transitions were found at appropriate strain εb. While the effect of the E on electronic properties and especially optical properties is weak, the optical absorbance of the Janus Ga2SeTe monolayer can be enhanced by strain engineering, up to 14.42 × 104 cm−1 at εb=−7% in the near-ultraviolet region. The optical absorbance of the Janus Ga2SeTe monolayer is activated in the visible light region that is following its calculated band gap value. This work not only systematically presents the electronic and optical properties of the Janus Ga2SeTe monolayer in the presence of strain engineering and electric field but can also motivate experimental studies for applications in nanoelectromechanical and optoelectronic devices. [ABSTRACT FROM AUTHOR]