Solvothermal synthesis of n-type Bi(SeTe) nanoplates for high-performance thermoelectric thin films on flexible substrates.

To improve thermoelectric performance of materials, the utilization of low-dimensional materials with a multi-alloy system is a promising approach. We report on the enhanced thermoelectric properties of n-type Bi(SeTe) nanoplates using solvothermal synthesis by tuning the composition of selenium (Se). Variation of the Se composition within nanoplates is demonstrated using X-ray diffraction and electron probe microanalysis. The calculated lattice parameters closely followed Vegard's law. However, when the Se composition was extremely high, an impurity phase was observed. At a reduced Se composition, regular-hexagonal-shaped nanoplates with a size of approximately 500 nm were produced. When the Se composition was increased, the shape distribution became random with sizes more than 5 μm. To measure the thermoelectric properties, nanoplate thin films (NPTs) were formed on a flexible substrate using drop-casting, followed by thermal annealing. The resulting NPTs sufficiently adhered to the substrate during the bending condition. The electrical conductivity of the NPTs increased with an increase in the Se composition, but it rapidly decreased at an extremely high Se composition because of the presence of the impurity phase. As a result, the Bi(SeTe) NPTs exhibited the highest power factor of 4.1 μW/(cm∙K) at a Se composition of x = 0.75. Therefore, it was demonstrated that the thermoelectric performance of Bi(SeTe) nanoplates can be improved by tuning the Se composition.