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

Abstract

Recently, auxetic materials have attracted attention due to their unusual behavior and multifunctional applications. 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 Ge$_2$XY (X/Y = S, Se, Te) using {\it ab initio} calculations. We construct the crystal structure and evaluate the stability of Janus Ge$_2$XY monolayers under ambient conditions. Phonon dispersion spectra, cohesive energy calculations, and molecular dynamics simulations confirm the high structural stability of Ge$_2$XY. At the ground state, Ge$_2$XY 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 Ge$_2$XY monolayers exhibit ideal auxetic materials with a large negative Poisson's ratio. All three Ge$_2$XY systems possess Poisson's ratio values around $-0.2$ along the $x$-axis. Moreover, Ge$_2$XY monolayers are predicted to be high electron mobility up to $10.92\times 10^3$~cm$^2$V$^{-1}$s$^{-1}$ (Ge$_2$STe). The combination of ideal auxetic behavior and tunable transport properties makes the Janus Ge$_2$XY structures promising materials for nanoelectronic and mechanical applications.