Controllable 2H/3R phase transition and conduction behavior change in MoSe2:Nb substitution by high pressure synthesis for promising thermoelectric conversion

Transition metal dichalcogenides (TMDs) are uniquely multifunctional materials with optical, electronic, and catalytic properties. Despite the advantages of low cost, low toxicity, and high abundance, the thermoelectric transport properties of MoSe2 were not extensively investigated. Meanwhile, MoSe2 bulk material with 3R phase was rarely reported compared to 2H phase. In this work, controllable phase transition from 2H to 3 R for MoSe2 bulk polycrystalline material was achieved with various Nb contents by a simple and feasible high-pressure method. The preferred orientation resulted in anisotropy of both electrical and thermal transport. The samples converted from n type for pristine sample to p type conduction after Nb doping. Meanwhile, the conduction type gradually changed from semiconductor to degenerated semiconductor. The electrical properties were distinctly improved by Nb doping systematically from the reduced bandgap and the enhanced carrier concentration and mobility. The lattice thermal conductivity was reduced by point defects and grain/phase boundaries generating from Nb doping. Maximum zT of 0.17 at 873 K was obtained for Nb0.04Mo0.96Se2, which is among the highest values for Te-free Mo dichalcogenides. The strategy of chemical doping and high-pressure synthesis provides an alternative route to achieve MoSe2 bulk materials with a controllable 2H/3R phase ratio for potential applications, which can be extended to other TMDs.