Ultralow Thermal Conductivity and Very High Seebeck\nCoefficient in Two-Dimensional TeSe<sub>2</sub> Semiconductor

Emerging\nchalcogenide-based two-dimensional (2D) materials possess\nvarious unique yet fully explored properties and are thus considered\npromising candidates for next-generation optoelectronic and energy\nconversion applications. Here, TeSe<sub>2</sub> crystals with interesting\nthermoelectric features were synthesized using a simple solid-state\nreaction. High-resolution transmission electron microscopy reveals\nthat TeSe<sub>2</sub> stabilizes in a 2D atomic structure with helical\nchains, resembling 2D tellurene. The thermoelectric properties were\nmeasured from 2 to 390 K in a polycrystalline pellet, showing an ultralow\nthermal conductivity below 0.25 W m<sup>–1</sup> K<sup>–1</sup> and a very high positive Seebeck coefficient of up to 865 μV\nK<sup>–1</sup>. Particularly, the thermal conductivity shows\na hysteresis effect upon temperature cycling, which is tentatively\nexplained as cracks opening and partially closing. Optical measurements\nindicate that TeSe<sub>2</sub> is a semiconductor with two bandgaps\nat 1.43 and 1.65 eV. These results highlight that TeSe<sub>2</sub> is an intriguing 2D semiconductor with complex thermoelectric properties,\nwhich provides a platform to further study the interplay of emerging\n2D structure, thermal, and electronic properties.