Growth and Thermoelectric Properties of SnSe2 Single Crystals

Abstract

The development of renewable energy is yielding very promising results. However, fossil fuels will continue to be the world's primary source of energy for the near future. During energy conversion, they both pollute the environment and produce a large amount of waste heat. This scenario not only raises urgent concerns but also a promising future for the development of thermoelectric materials that can convert waste heat directly into electricity. The practical applications of thermoelectric materials are still very limited. However, recent advances in discovering new materials and developing new fabrication processes to increase thermoelectric performance are significant. Layered materials possess strong covalent bonds within the layers and weak van der Waals interactions between them. These materials have a naturally low thermal conductivity due to their anisotropic crystal and electronic structure, making them as promising for thermoelectric materials. Layered tin diselenide (SnSe2), a non-toxic and abundant component of the Sn-Se family, has recently attracted the interest of researchers for a variety of thermoelectric applications. In this study, we proceed to fabricate SnSe2 single-crystal to study thermoelectric properties using temperature gradient technique. Through careful control of sample growth parameter, especially the maximum temperature and cooling rate, we were able to obtain large-size and high-quality samples. The resulting undoped SnSe2 single crystal exhibits properties of n-type semiconductors with a carrier concentration about 10^18 cm^-3 at room temperature. Because of the layered structure, SnSe2 single crystals show anisotropy between the crystal directions in thermoelectric properties. The thermoelectric performance of the undoped sample is poor; however, due to the low lattice thermal conductivity, the ZT value along the out-of-plane direction is markedly higher than along the in-plane direction in the whole temperature range from 300 - 673 K. The maximum out-of-plane ZT value, 0.15, was obtained at 673 K, with a low total thermal conductivity of 0.43 W m^-1 K^-1. Furthermore, electronic doping, according to ARPES studies, has the potential to improve the power factor of SnSe2 single crystal. Among the various dopants studied, bromine (Br) and then chlorine (Cl) are effective electron donors for increasing the material's carrier concentration. Br doping raises carrier concentrations to 7×10^19 cm^-3 at 300 K while having little impact on carrier mobility or lattice thermal conductivity. As a result, the thermoelectric performance of the SnSe2 single crystal improves significantly in both directions, especially along the out-of-plane direction. Maintaining low thermal conductivity while increasing the power factor advanced the ZT out of plane value increase to 0.54 at 673 K. Chlorine, another halogen element, has also been studied for increasing the carrier concentration of single crystalline SnSe2. Chlorine has been shown to be efficient at raising carrier concentrations to as much as 10^20 cm^-3. Interestingly, some of the Cl doped samples had significantly higher conductivity than the Br doped samples at the same carrier concentration, while the differences in Seebeck coefficient values are negligible. These results indicates that the ZT value would rise much more than in Br doped SnSe2.