High band degeneracy and weak chemical bonds leading to enhanced thermoelectric transport properties in 2H–MoTe2

• Published in: Journal of solid state chemistry 2021-08, Vol.300, p.122227, Article 122227
• Main Authors: Zhang, Cheng, Li, Zhi, Zhang, Min, Li, Ziwei, Sang, Hao, Xie, Sen, Wang, Zhaohui, Xie, Hongyao, Luo, Jiangfan, Wang, Wei, Ge, Haoran, Liu, Yong, Yan, Yongao, Liu, Wei, Tang, Xinfeng
• Format: Article
• Language: English
• Subjects: 2H–Mo1–xNbxTe2; Band degeneracy; Electronic-thermal transport properties; Hopping conduction; Impurity band; Weak bonds
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/j.jssc.2021.122227

2H-MoX2 (X ​= ​S, Se and Te) based compounds are promising medium-to-high thermoelectric materials, with distinct advantages of excellent thermal stability and superior mechanical flexibility. As illustrated by our band structure calculations and electronic-thermal transport measurements, the p-type 2H–MoTe2 achieves the best thermoelectric properties among the binary 2H-MoX2 analogues, arising from the high band degeneracy and the weak Mo–Te chemical bonds. Multiple valence bands at the Γ, K Z, and H points converge in energy, and the impurity band induced by the doping of Nb contributes to the valence band convergence, leading to a large carrier effective mass of ~4.1 m0 and thus the excellent power factors in 2H–Mo1–xNbxTe2 with x ​> ​0.05. A hopping electrical conduction is observed in the 2H–Mo1–xNbxTe2 with a slight amount of Nb, which could be ascribed to the formed discrete impurity levels in the vicinity of valence band edge. In addition, the doping of Nb enhances the defect scattering of phonons and thus markedly reduces the lattice thermal conductivity of 2H–Mo1–xNbxTe2. Finally, 2H–Mo1–xNbxTe2 with x ​= ​0.07 obtains the largest ZT value of 0.20 and 0.14 ​at 823 ​K, when measured perpendicular and parallel to the pressing directions, respectively. The high band degeneracy as well as the weak chemical bonds are the two well-known strategies for effectively optimizing the electrical and thermal transport properties of thermoelectric (TE) materials. This work discovers that the aforementioned two strategies are well applied in the binary 2H–MoTe2, leading to their superior TE performances among binary MoX2 (X ​= ​S, Se and Te) compounds. Theoretically calculations of the electronic band structure and the systematical experimental studies reveal the convergence of multiple valence bands at the Fermi level in the Nb doped Mo1−xNbxTe2 compounds. Thus, significantly improved electrical properties are obtained in the highly Nb doped samples. Furthermore, the weak Mo–Te bonds and the substitutional defects of Nb on Mo sites result in obviously strengthened phonon scattering, and thus the much reduced lattice thermal conductivity. Finally, the optimal TE figure of merit ZT are acquired in 2H–MoTe2 among all binary 2H-MoX2 analogues. [Display omitted] •Systematical investigations of thermoelectric properties of 2H–Mo1−xNbxTe2via theoretical calculations and experiments.•High band degeneracy leading to a large carrier effective mass and power factor of 2H–