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.