Structural, vibrational, elastic, electronic, and piezoelectric properties of binary γ-GeX and ternary γ-Ge2XX′ monolayers (X, X′= S, Se, and Te)

The recent synthesis of a new polymorph of two-dimensional (2D) germanium monochalcogenides, namely, γ-GeSe with a four-atomic-layer-thick hexagonal lattice, has received considerable attention due to its novel properties and potential applications. This exciting advancement paves the path for extensive experimental and theoretical investigations on the family of γ-MX crystals in which M and X are elements of group IV and VI, respectively. In this regard, herein we conduct first-principles-based calculations to explore the structural, vibrational, mechanical, electronic, and piezoelectric properties of γ-GeX and Janus γ-Ge2XX′ (X/X′: S, Se, and Te) monolayers. We performed a detailed analysis of the suggested systems' dynamical, thermal, and mechanical stability through phonon-band-dispersion calculations, ab initio molecular dynamics (AIMD) simulations, and elastic tensor analyses, respectively, and all six possible nanosheets are found to be stable. The computed Raman spectra of the monolayers reveal that, different from binary systems, the formation of Janus monolayers results in the appearance of additional Raman active modes. The mechanical response of the proposed crystals is examined by calculating in-plane stiffness (Y2D) and the Poisson's ratio (ν) within the elastic regime, and the obtained results ascertain their flexibility. It is found that similar to their binary counterparts, Janus monolayers are indirect-band-gap semiconductors, and their valence-band maxima show a Mexican hat dispersion along the high-symmetry points of the Brillouin zone. Additionally, it is demonstrated that the construction of Janus crystals enhances the piezoelectric coefficients of γ-GeX monolayers, both in the in-plane and out-of-plane directions. Our findings not only provide a comprehensive insight into physical and electronic properties of γ-GeX and γ-Ge2XX′ monolayers but also reveal their promising features for various nanoelectronic and nanoelectrochemical applications.