Exploring The Thermoelectric Properties Of Transition Metal Chalcogenides: TcX2(X=S, Se, Te)

Transition metal chalcogenides (TMCs) as potential candidates for application in thermoelectric conversion field have been attracted a lot of attention. TcX₂(X=S, Se, Te) are also a member of the transition metal chalcogenide (TMC) family.

In the past, structural, mechanical, electronic and optical properties of TcX₂ have been investigated. But there is no research about the thermoelectric properties of these materials. TcS₂, TcSe₂, and TcTe₂ are isostructural crystalline forms and have triclinic structures with space group P-1. The unit cell of these materials contains four formula units (Z = 4) and corresponds to the distortion of a Cd(OH)₂ type cell, where the Tc atoms form Tc diamond shaped chains in the form of octahedral coordination and can be easily exfoliated in a monolayer form. These three compounds are all semiconductors.

Fig. 1. The band structure of TcS2 (a), TcSe2 (b) and TcTe2 (c) along the high symmetry k-points calculated by using LSDA+U. Image published with permission from Elsevier from https://doi.org/10.1016/j.jallcom.2018.06.119

By using first-principles density functional theory, deformation potential method and Boltzmann transport approach, the thermoelectric properties of TcX₂ (X=S, Se, Te) have been studied. The results show that TcX₂ (X=S, Se, Te) are semiconductors with the indirect band gap of 0.99 eV, 0.73eV, 0.10 eV, respectively. The conductivity has a larger value in the low-temperature region compared to the high-temperature region, and the conductivities for p-type doping are always greater than these for n-type doping for TcS₂ and TcTe₂ along all directions, for TcSe₂ along x- and z-direction, then increases as doping level raises.

Fig. 2. The Seebeck coefficients for TcS2(a), TcSe2(b) and TcTe2(c) along conveyor directions as a function of temperature under various doping levels. Image published with permission from Elsevier from https://doi.org/10.1016/j.jallcom.2018.06.119

The magnitude of electrical conductivity for TcX₂ (X=S, Se, Te) can reach 10⁸/Ωm, 10⁷/Ωm, 10¹¹/Ωm, respectively. For TcS₂, when the p-doping level is greater than 10²¹ cm^-3, the value of electronic thermal conductivity is always larger than 200W/mK. The maximum values of electronic thermal conductivity are 400W/mK, 600W/mK and 0.04W/mK along x-, y- and z-directions, respectively. For TcSe₂ the maximum value of conductivity is 8.5W/mK along x-direction, 18W/mK along y-direction and 0.03 W/mK along z-direction. For TcTe₂, the electronic thermal conductivity maintains a large value in all temperature value and doping levels.

Fig. 3. The power factor of TcS2(a), TcSe2(b) and TcTe2(c) along conveyor directions as a function of temperature under various doping levels. Image published with permission from Elsevier from https://doi.org/10.1016/j.jallcom.2018.06.119

TcS₂ and TcSe₂ have high S for both p-type or n-type doping, which is at the magnitude of 10^-4V/K, and the peaks of Seebeck coefficient shift up as the doping level is weakened. The Seebeck coefficients of TcTe₂ are always smaller than  V/K. The PF can be greatly enhanced by p-type doping for TcS₂ and TcTe₂ along all directions, for TcS₂ along the x-direction. Moreover, n-type doping is much better for other directions. These phenomena mean that TcX₂ has potential application in the thermoelectric field.

These findings are described in the article entitled Thermoelectric properties of TcX2(X=S, Se, Te), recently published in the Journal of Alloys and Compounds. This work was conducted by X. Jiang, L. Zhu, and K.L. Yao from the Huazhong University of Science and Technology.