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Synergistically improved electronic and thermal transport properties in Nb-doped NbyMo1-ySe2-2xTe2x solid solutions due to alloy phonon scattering and increased valley degeneracy.

The layered 2H-MoSe2-based compounds have recently attracted considerable attention as novel thermoelectric (TE) materials for medium-to-high temperatures power generation. In this research, for the first time, dissolving Te in binary MoSe2 and thus forming MoSe2-2xTe2x solid solutions is shown to be very effective for reducing lattice thermal conductivity (κL) due to strong alloy scattering to phonons. MoSe1.2Te0.8 achieves the lowest κL of 2.87 Wm-1K-1 at room temperature among all solid solutions, an 85% decrease from that of pristine MoSe2 (18.69 Wm-1K-1). Band structure calculations and experiments have verified that Te alloying effectively increases the density-of-states effective mass of MoSe2-2xTe2x solid solutions via increased valley degeneracy, leading to enhanced Seebeck coefficient. Moreover, Nb doping modulates the density of holes of NbyMo1-ySe1.2Te0.8 samples to its optimum level and gives rise to the maximum power factor of 0.85 mWm-1K-2 at 823 K. On account of synergistic optimization of the electronic and thermal transport, Nb0.05Mo0.95Se1.2Te0.8 has achieved the highest ZT value of 0.34 at 823 K, representing a 70% enhancement as compared to the best result previously reported for MoSe2. This research documents that alloying combined with doping is an effective avenue to greatly enhance the TE performance of NbyMo1-ySe2-2xTe2x solid solutions.