Highly In-Plane Anisotropic 2D-ZrGeTe₄: A Promising Thermoelectric Material with a High Power Factor and Figure of Merit

Two-dimensional (2D) ternary transition metal chalcogenides (TMCs) are promising for thermoelectric (TE) applications because of their low structural symmetry, high in-plane anisotropy, and adjustable band gap. A high-performance TE material requires high electrical conductivity, a large power factor PF, and a sizable dimensionless figure of merit ZT. This study focuses on the TE properties, including electrical and phonon transport properties, of highly in-plane anisotropic 2D-ZrGeTe4 based on combined first-principles and semiclassical Boltzmann theory. First, our results prove that 2D-ZrGeTe4 is mechanically, dynamically, and thermodynamically stable as a free-standing monolayer, which can be attributed to the van der Waals nature of ZrGeTe4. Next, we show that ZrGeTe4 exhibits a tunable band gap, exhibiting an indirect-to-direct band gap transition from the 3D bulk to the 2D monolayer. The electron and hole mobilities are highly anisotropic and dominate in the x-direction. Furthermore, 2D-ZrGeTe4 exhibits a large PF and low lattice thermal conductivity, yielding a high theoretical ZT of 6 and 3 for n-type and p-type at 900 K, respectively. Owing to its rich anisotropy, band degeneracy capabilities, and extreme phonon anharmonicity, 2D-ZrGeTe4 could emerge as a promising candidate for advanced TE applications at high-temperature ranges.