A multifunctional Mg₂Si monolayer with negative Poisson's ratio and ultrahigh thermoelectric performance at room temperature

Flexible thermoelectric materials exhibit robust fracture resistance, rendering them highly promising for applications in wearable thermoelectric devices. Here, we design an environmentally friendly two-dimensional (2D) magnesium-silicon monolayer with negative Poisson's ratio and high thermoelectric conversion efficiency by CALYPSO package and first-principles calculations. The calculations indicate that the stable 2D Mg2Si monolayer is a semiconductor with in-plane negative Poisson's ratio of -0.364. The thermoelectric figure of merit (ZT) values of the p-type doped Mg2Si monolayer in the x direction are 2.51 at room temperature and 3.92 at 500 K, exceeding the threshold value of 2.0 for conventional thermoelectric materials. The remarkable thermoelectric performance of the Mg2Si monolayer is attributed to the presence of resonant bonds between magnesium and silicon atoms, as well as the strong coupling between acoustic and optical phonon modes, leading to pronounced phonon anharmonicity. In addition, the four-phonon scattering process also effectively reduces the lattice thermal conductivity. And since a portion of phonon mean free paths are lower than the Ioffe-Regel limit, a two-channel model is considered for calculating the lattice thermal conductivity. These findings offer valuable insights into comprehending the diverse structural, thermoelectric, and mechanical properties of the Mg2Si monolayer, and provide an avenue for the exploration and development of wearable thermoelectric devices.