First-Principles Study on the Thermal Transport Properties of Monolayer 1T-Ag6X2 (X = S, Se, Te)

Cluster substitution is a novel strategy for optimizing the physical and chemical properties of functional materials, but it is not fully understood in related fields, especially in the fields of thermoelectricity and engineering of thermal management. The monolayer 1T-Ag6X2 (X = S, Se, Te) is obtained by substituting the transition metal atoms in traditional two-dimensional transition metal sulfides (TMDs) with octahedral cluster Ag-6. The phonon dispersion curves and ab initio molecular dynamics (AIMD) results show that monolayers 1T-Ag6S2 and 1T-Ag6Se2 have reliable stability. Therefore, the thermal transport performance of monolayers 1T-Ag6S2 and 1T-Ag6Se2 is studied systematically based on first principles. The results show that monolayers 1T-Ag6S2 and 1T-Ag6Se2 both have extremely low lattice thermal conductivity kappa(l), 0.27 and 0.30 W/mK at room temperature, respectively, which are much lower than those of two-dimensional 1T-MX2 and other two-dimensional materials. This is because, after the introduction of Ag-6 clusters, monolayers 1T-Ag6S2 and 1T-Ag6Se2 have more complex structures than traditional TMDs, with more tortuous phonon paths and greater phonon anharmonicity. It is also interesting to note that the contribution of optical and acoustic phonon modes to the total kappa(l) of monolayers 1T-Ag6S2 and 1T-Ag6Se2 is similar (usually, the kappa(l) of most two-dimensional materials is dominated by the acoustic component), which makes a reasonable explanation for the extremely low kappa(l) of these two materials. In this study, we further understand the important role of cluster substitution in regulating the thermal transport properties of two-dimensional materials and provide research ideas for designing new two-dimensional materials with low kappa(l).