Novel auxetic semiconductors with high carrier mobility: first principles prediction of Janus GeXY (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 GeXY (X/Y = S, Se, Te) using calculations. We construct the crystal structure and evaluate the stability of Janus GeXY monolayers under ambient conditions. Phonon dispersion spectra, cohesive energy calculations, and molecular dynamics simulations confirm the high structural stability of GeXY. At the ground state, GeXY 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 GeXY monolayers represent ideal auxetic materials with a large negative Poisson's ratio. All three GeXY systems possess Poisson's ratio values of around -0.2 along the -axis. Moreover, GeXY monolayers are predicted to have high electron mobility up to 10.92 × 10 cm V s (GeSTe). The combination of ideal auxetic behavior and tunable transport properties makes the Janus GeXY structures promising materials for nanoelectronic and mechanical applications.