Synthesis and Atomic-Scale Characterization of Oxide Thermoelectric Materials and Devices

Complex oxide thermoelectric (TE) materials are drawing increased interest for energy harvesting applications. Nb-doped SrTiO3 is seen as a promising material, because its power factor ( ) is comparable to the state-of-art TE bismuth telluride. The major challenge is the discovery of new high performance materials to enhance the thermoelectric figure of merit ZT = ( ). Recent results on other TE materials have shown that the thermal conductivity may be drastically reduced by reducing materials dimensions to the nano scale resulting in an increase in ZT. Furthermore, other studies have suggested that magnetic co-doping could facilitate enhancements in the ZT. Magnetic co-doping of Nb-doped SrTiO3 and Fe-doped SrTiO3 thin films was achieved by alternate ablation of vanadium or iron and Nb-doped SrTiO3 targets during pulsed laser ablation. The samples were then annealed to enhance crystallinity. A novel sample preparation method was developed to facilitate in-situ/ex-situ transmission electron microscopy analysis of sample morphology. The results of the transmission electron microscopy in-situ/ex-situ annealing characterization (TEM/STEM) suggest that the film start to crystallize at 400 ̊C. The use of oxygen is a must during the annealing process to minimize the point defects. EDS showed that the composition is function of temperature. Furthermore, the results suggest that the co-doping can be utilized to enhance the thermal transport characteristics. Fe-doped SrTiO3 is a promising thermoelectric material. The transmission electron microscopy STEM/TEM indicate that the films are highly crystal, with the capability to control the regions of the doping. This technique can be used to form Fe-doped two- dimensional electron gas–type structures that may be excellent TEs. Energy Dispersive x-Ray Spectroscopy (EDS) results indicate that Nb and Fe substitutes for Ti in the SrTiO3 lattice. x–Ray Photoelectron spectroscopy (XPS) results confirm the stoichiometry of the films. Physical properties measurements system (PPMS) results suggest that the transport properties need to be optimized for high carrier concentration, and better TE performance.