Pressure-Induced Anisotropic to Isotropic Thermal Transport and Promising Thermoelectric Performance in Layered InSe

Decoupling electron and phonon transport is beneficial to improve the thermoelectric figure of merit ZT. As a result of the structural anisotropy and compressibility, layered materials become an excellent platform for the simultaneous optimization of the entangled thermoelectric parameters. Here, we comprehensively investigate the effect of hydrostatic pressure on the electronic and thermal transport properties of layered γ-InSe by performing first-principles calculations along with the Boltzmann transport theory. With the increase of pressure, the lattice thermal conductivity of γ-InSe along the cross-plane direction increases, whereas it experiences a decrease along the in-plane direction. The ratio of thermal conductivity along the in-plane direction to that along the cross-plane direction decreases from 6.95 at 0 GPa to 1.26 at 8 GPa, showing a transition from anisotropic to isotropic thermal transport. Such abnormal pressure-dependent thermal conductivity stems from the competition between the enhanced phonon group velocity and the suppressed phonon lifetime. Furthermore, it is found that the highest ZT can reach 2.7 and 1.2 for p-type and n-type at 0 GPa and 800 K along the cross-plane direction, which can be explained by the combined effect of lower thermal conductivity and larger electrical conductivity contributed by the Se-pz orbital. The decoupling of electron and phonon transport can be realized along the in-plane direction by applying external pressure; the pressure-enhanced ZT results from the increase of the Seebeck coefficient and the decrease of thermal conductivity. These findings pave the way for robust tuning of thermoelectric properties in layered materials by applying external pressure.