Novel auxetic semiconductors with high carrier mobility: first principles prediction of Janus Ge2XY (X/Y = S, Se, Te) monolayers

Recently, auxetic materials have attracted attention due to their unusual behavior and multifunctional applications. A negative Poisson's ratio has been found in some two-dimensional (2D) asymmetric layered materials. In this work, we predict a new class of 2D auxetic materials with the chemical formula Ge₂XY (X/Y = S, Se, Te) using ab initio calculations. We construct the crystal structure and evaluate the stability of Janus Ge₂XY monolayers under ambient conditions. Phonon dispersion spectra, cohesive energy calculations, and molecular dynamics simulations confirm the high structural stability of Ge₂XY. At the ground state, Ge₂XY monolayers are semiconductors with narrow band gaps ranging from 0.11 to 1.09 eV. We also calculate the mechanical properties, including elastic constants, Young's modulus, and Poisson's ratio. Importantly, the Ge₂XY monolayers represent ideal auxetic materials with a large negative Poisson's ratio. All three Ge₂XY systems possess Poisson's ratio values of around −0.2 along the x-axis. Moreover, Ge₂XY monolayers are predicted to have high electron mobility up to 10.92 × 10³ cm² V⁻¹ s⁻¹ (Ge₂STe). The combination of ideal auxetic behavior and tunable transport properties makes the Janus Ge₂XY structures promising materials for nanoelectronic and mechanical applications.