Thermal and thermoelectric transport in nanostructures for energy applications

Issues concerning the control of thermal energy assume an ever increasing role in modern electronic technologies. In an effort to address this challenge, the rapid progress in nanotechnology has introduced new ways to control heat flow at microscopic length scales. Heat is additionally an ubiquitous energy resource and may be directly converted into clean and renewable electricity using thermoelectric materials. While the available thermoelectric conversion efficiency remains modest, nanostructures also provide means for improving the thermoelectric performance of bulk materials, concurrently promoting various desirable material attributes such as transparency and flexibility. This thesis elucidates nanoscale thermal phenomena concerning heat carrying phonons per se and as part of thermoelectric energy conversion. The main goals of the work are to explore the interplay of structural disorder and order for controlling nanoscale thermal transport in thin film multilayer and nanowire systems, and to generate new routes for fabricating economical and ecological large-area thermoelectric structures for sensing and energy harvesting applications. The thermal transport studies presented in this thesis show that thermal conductivity of amorphous nanostructures may be moderately tuned by interfaces regardless of the absence of crystalline order. In crystalline multilayers, however, interfaces may be rationally designed for an enormous reduction in thermal conductivity enabled by the wave interference of coherent thermal phonons. Coherent phonons may also contribute to thermal conductivity suppression observed in semiconductor core-shell nanowires, making them promising constituents for thermoelectric systems. Generally, manifestations of phonon coherence pave way for novel thermal design through phononic band structure engineering. The work also presents advanced nanofabrication routes for large-area thermoelectric nanocomposites. Particularly, the thermoelectric properties of zinc oxide thin films are transferred to a three-dimensional polymeric template by atomic layer deposition, allowing for a two-fold power output in reference to planar structures. In addition, scalable solution-based methods are used for facile fabrication of organic thermoelectric graphene nanocomposites. Finally, the work demonstrates new types of thermoelectric application prototypes with intriguing properties enabled by nanostructuring, including fully inkjet printed flexible thermoelectric circuits, the first planar fully transparent thermoelectric p-n modules, and a novel distributed thermocouple architecture of a transparent and flexible touch panel. The results not only provide new fundamental insight into phononic processes, but also enable new technological solutions for energy harvesting. Thus, the work has potentially profound implications on the emerging fields of nanophononic thermal engineering as well as transparent and flexible thermoelectrics.