Ultralow thermal conductivity and high thermoelectric performance of γ-GeSe: effects of dimensionality and thickness

Two-dimensional chalcogenide-based materials of group 14 elements are predicted as potential thermoelectric (TE) materials, though the figure of merit (ZT) obtained requires improvement to be commercially accessible. Herein, we have computationally modeled synthesized γ-GeSe and reduced-dimension 2D layers (monolayer, bilayer, trilayer, and quad-layer) and subjected them to first principles calculations to extract essential properties of TE. The ZT values obtained for the considered systems are found to be remarkably high (quad-layer: 2.8; trilayer: 3.1; bilayer: 3.8), at 900 K. The dimensionality reduction and reducing layers improved the ZT considerably compared to that of bulk γ-GeSe (0.8 at 900 K). The power factor decreases with decreasing layers, ultralow lattice thermal conductivities (k_L) are responsible for the high ZT. Ultralow k_L (>1 W m^-1 K^-1) was observed in 2D γ-GeSe at all temperature ranges, with the lowest k_L observed in the bilayer (0.15 W m^-1 K^-1) and trilayer (0.17 W m^-1 K^-1) at 900 K. The low k_L is also supported by the presence of high anharmonicity, high phonon scattering rates, low elastic constants, low group velocity, and low Debye temperature. (author)