Boosting Thermoelectric Performance of 2D Transition-Metal Dichalcogenides by Complex Cluster Substitution: The Role of Octahedral Au₆Clusters

The concept of element substitution was introduced with the discovery of classic semiconductors in the early 1930s. While it has been demonstrated as an effective strategy to tune the physical properties of related materials over many decades, it is physically limited to the atomic size mismatch between the dopant and the host. From another perspective, if a complex cluster can be chemically introduced into a system with a similar structure, it can be regarded as the equivalent cluster version of substitution. Complex atomic configurations usually offer more tortuous phonon paths and stronger phonon anharmonicity; however, the phenomenon of complex cluster substitution is generally less studied compared with the traditional element substitution. In this work, we take the first step using density functional theory (DFT) calculations to learn the electrical and thermal transport properties of a 1T phase transition-metal dichalcogenide (TMD) monolayer incorporated with octahedral Au6 clusters, i.e., T-Au6S2. It is found that complex cluster substitution leads to a higher phonon scattering frequency and ultralow lattice thermal conductivity (0.167 and 0.171 W/mK at 700 K along the x axis and y axis). Besides, the introduction of Au6 clusters can effectively optimize the electronic structures, balance the relationship between the Seebeck coefficient and the electrical conductivity, and thus improve the power factor. Consequently, T-Au6S2 exhibits a high thermoelectric figure of merit ZT of 3.75 (3.79) at 700 K along the x axis (y axis). Our work demonstrates that complex cluster substitution is a promising route to improve the TE conversion efficiency for low-dimensional semiconductors.