Impurity states in Mo1−xMxSe2 compounds doped with group VB elements and their electronic and thermal transport properties†
The impurity states introduced by substitutional doping of Mo1−xMxSe2 with group VB (M = V, Nb and Ta), and their effect on the thermoelectric properties were systematically investigated via DFT calculations and experiments. We find that lightly doped Mo1−xMxSe2 (x = 0.01) exhibits a hopping conduction at low temperatures owing to the discrete impurity states near the valence band maximum (VBM) of the host. Increasing the doping level gives rise to an impurity band near the VBM of Mo1−xMxSe2, leading to the metallic characteristics. And, the impurity states increase the carrier effective mass due to the coincidence in energy of the topmost bands at Γ and K. Moreover, the substitution of Mo by M dramatically reduces the lattice thermal conductivity (κL) of Mo1−xMxSe2. As a result of the significantly improved electronic properties and reduced κL, M-doped Mo1−xMxSe2 samples exhibit remarkable enhancements in thermoelectric figure of merit ZT. The Ta-doped Mo0.95Ta0.05Se2 (x = 0.05) sample possesses the ZTmax of 0.12 and 0.14 measured perpendicular and parallel to the pressing direction, respectively. This work provides some insights towards understanding the effect of the impurity states on the thermoelectric properties of MoSe2 and other transition metal dichalcogenides (TMDs).