Due to their unique properties, nanostructured materials have developed into promising components for next-generation optoelectronic and energy converting devices. Their nanoscale dimensions and features allow for incredible interactions with phenomena occurring at similar length scales. Nanowires made of oxide materials are especially attractive for controlling the transport of phonons, photons, and charged species for added thermal and chemical stability. By controlling the nanowire dimensions, the propagation of these species can be tailored to be suitable in a variety of applications, even those operating at elevated temperatures.
One of these applications is thermoelectrics, where the conversion of heat to electricity occurs entirely in the solid-state. Though decades of research yielded little progress due to the limiting interdependencies of transport properties, the nanowire geometry allows for the decoupling of the otherwise tethered conduction of heat and charge. By merely existing in the size regime between the respective mean free paths of electrons and phonons, thermal conduction can be impeded by increased phonon boundary scattering while the electrons can proceed with mobility of a single crystal. However, further phonon scattering can occur with the addition of nanostructured features along the length of the nanowire. To accomplish this, InXO3(ZnO) n (X = Ga, Fe) polytypoid nanowires were synthesized using a facile, solid-state conversion scheme. Using aberration correction Z-contrast electron microscopy, detailed analysis of the resulting structure lead to the discovery of the formation of single atomic sheets of In existing within the ZnO matrix. Evidence is provided that these octahedrally coordinated In planes act as inversion domain boundaries and necessitate the existence of polarity-restoring "zig-zag" features between them. With experimental observations coupled with DFT calculations, both a structure model and formation mechanism are proposed. Single nanowire thermoelectric measurements on In-Ga-ZnO reveal simultaneous improvement in all the factors that contribute to the thermoelectric conversion efficiency. This is the first demonstration of enhancement of all three thermoelectric parameters in a nanowire system.
ZnO-based nanowires could also hold the key to nanosized optoelectronic interconnects and lasers. Because of its high optical gain, lasing is easily achieved in ZnO nanostructures via optical pumping. However, any actual device integration requires the input power to be electrical. The most common way to electrically pump a material is to create a homo-junction diode, meaning both n- and p-type conduction has to be present in a single material. Therefore, much work has been done to achieve p-type ZnO, with limited success and repeatability caused from compensation from intrinsically-occurring donor defects. A strategy avoiding such compensation was employed via in situ Li incorporation during the chemical vapor transport growth, followed by dopant activation annealing. X-ray diffraction and photoluminescence qualitatively confirmed the incorporation of Li into the ZnO nanowires using this method. Field effect transconductance exhibited p-type behavior after the annealing step. The sign of the Seebeck Coefficient confirmed that the majority carrier type was positive. However, the p-type behavior only lasted a matter of days, where the longevity may have a direct relation on the nanowire diameter. Unfortunately, the growth of these Li-doped ZnO nanowires was unable to be reproduced for a variety of possible reasons. However, the data collected may help make p-type conduction a reliable reality.
The large amount of surface in the nanowire geometry allows for exploration into interface effects on ionic transport for use as electrolytes in solid-oxide fuel cells. These energy producing devices currently only operate at high temperature, due to the relative energy needed for ionic conduction. Therefore, much effort has been directed towards designing material systems to conduct ions at lower temperature. One strategy is to utilize interfaces in nanostructured systems, which can exhibit strain and space charge effects, to provide pathways of lesser resistance for ionic migration. To explore the possibilities of these effects, core/shell CeO2/ZrO2 nanowires were studied. First, CeO2 were hydrothermally grown nanowires and dried into thin films. A ZrO2 coating of a few nanowires was added and calcined to promote crystallinity. TEM and STEM images show that there is no coherence between the two materials. However, the ionic conductivity increased by an order of magnitude upon addition of the ZrO2. Furthermore, the activation energy of the coated samples is decreased by 30%, indicating the interface is playing a role in changing the mechanism of ionic conduction. These results give hope to the eventual discovery of a nanostructured oxide material capable of ionic conduction at room temperature.
These studies on various oxide nanowire systems help provide experimental merit to the theoretical benefits and disadvantages of nanostructured materials. Thermoelectric material performance was enhanced due to the presence of nanostructured inclusions in converted ZnO-based polytypoid nanowires. P-type conduction was attained in Li-doped ZnO, but the larger surface to volume ratio of the nanowire geometry may indeed limit stability. Effects of the designed boundary between CeO2 and ZrO2 were seen in the conduction of oxygen because of the shear amount of interfacial density present in the nanowire thin films. These studies take advantage of the intrinsic properties of nanostructured materials and bring the implementation of nanowires to marketable applications closer to reality.