*6.5. Telluride*

Bulk tellurium (Te) is a new and high-quality thermoelectric material [174]. At the same time, due to its two-dimensional structure, telluride can be used as an effective means to further improve its thermoelectric properties [175]. In the bulk material, the tellurium atom is combined with a neighboring atom through a covalent bond and extends in a spiral shape. Adjacent spiral chains are then combined by van der Waals forces [176], so bulk tellurium belongs to a quasi-one-dimensional chain structure. However, the theory predicts that the structure of monolayer telluride is different from that of bulk tellurium. There are three possible crystal structures (α-Te, β-Te, γ-Te) [177]. Gao et al. [178,179], through first principles calculations, studied the thermal conductivity and thermoelectric properties of single-layer telluride with different structures, and they have been found to have low thermal conductivity and excellent thermoelectric properties. But only b-type telluride has been synthesized in experiments and the synthesis conditions are relatively harsh, so there is no experimental study on the physical properties of monolayer telluride [180,181]. Wang et al. developed a liquid-phase synthesis method [182], which can be used to prepare multi-layer telluride with the same bulk structure in large quantities. In further research, the research group and its collaborators used the Raman method to measure the room temperature suspended thermal conductivity of telluride with a thickness of 35 nm in the intra-chain direction (about 1.5 W/m·K) and compared with the bulk material (about 3 W/m·K, 300 K), there is a certain degree of reduction. They believe that the main reason for this is that the surface of the multi-layer telluride will interact with the phonons. Because the structure of this type of telluride is the same as that of the bulk material, its in-plane thermal conductivity will be different along the in-chain direction and the inter-chain direction.
