(6cb) Bottom-up Design of Nanostructured Thermoelectric Materials from Solution Phase Synthesized Nanowires, Nanocrystals and Heterostructures

Thermoelectric (TE) devices convert heat flow into electrical current or vice versa, which have many applications including waste heat recovery and solid state cooling. Recently, significant progress has been made in the development of nanostructured TE materials with high figure of merit (ZT), which is a measure of the performance of a thermoelectric material. By reducing the grain size of a polycrystalline thermoelectric material, or by incorporating nanoinclusions into a host thermoelectric material, the thermal conductivity can be greatly reduced due to the phonon scattering effect, which is the main reason for most of the experimentally observed ZT enhancements. To achieve the desired nanostructures, a top-down approach involving high temperature (above 1000 ºC) melt synthesis followed by ball milling and thermal sintering is commonly used, which is a tedious and energy intensive process. Meanwhile, a bottom-up approach based on solution phase synthesized nano building blocks could also be utilized to obtain similar nanostructures, which is, however, advantageous to the top-down approach due to the low-energy, low-cost and scalable features of the solution synthesis. Moreover, with the synthetic tools developed in the past decades, nano-sized building blocks with tunable size, composition and morphology are possible, which provide a versatile platform for the design of novel TE materials. Here, we show the application of bottom-up approach towards nanostructure TE materials in different systems including nanocrystalline Ag₂Te from Ag₂Te nanowires, PbTe-Ag₂Te nanocomposites from PbTe-Ag₂Te dumbbell heterostructures, and AuAg-PbSe superlattice from AuAg alloy nanoparticles and PbSe nanocrystals, etc. We demonstrate thermoelectric properties of the bottom-up fabricated nanomaterials are tunable by controlling the synthesis condition and post-synthesis treatment of the nano building blocks. We also show that the binary heterogeneous structures allow us to modulate the charge carrier transport through electronic band engineering, which may provide another mean to enhance ZT in addition to the phonon scattering effect.