Exploring spatial and compositional control of conductive materials using atomic layer deposition [electronic resource]
Keywords: Chemical Engineering.

Consumer-driven demand for affordable, smaller and more sophisticated devices has driven advances in nanoscale engineering. Many of these products, whether they are consumer electronics, fuel cells or photovoltaics, are comprised of significant quantities of scarce, expensive and sometimes toxic materials. Manufacturing affordable and more environmentally benign products on decreasing length scales requires more economical use of scarce resources and the exploration of alternative and abundant materials. Platinum, as one example, is an outstanding catalyst used in automotive, fine chemical and fuel cell applications; however, it is extremely scarce and costly. Placing platinum only where needed and in small quantities to achieve desired performance may help economize its usage. Because nanoscale feature size and spacing influence catalytic behavior, being able to control these parameters may enable the careful engineering of more powerful platinum catalysts. Another scarce and expensive material whose usage follows rising consumer demand for electronics and photovoltaics is indium. Indium tin oxide is the industry standard transparent conducting oxide (TCO) for high-end electronics. The engineering of high-quality yet poorly-understood indium-free alternative metal oxides using earth-abundant and inexpensive materials may further the development of more powerful and advanced devices. In this dissertation, atomic layer deposition (ALD), a promising ultra-thin film growth technique, is used to selectively deposit small amounts of platinum and to alloy zinc tin oxide in order to address these concerns of scarcity and rising costs. ALD consists of self-limiting, gas-surface half-reactions separated by inert gas purges which allows for highly controlled growth of nanoscale materials. Because ALD is based on surface reactions, selectively blocking nucleation sites by passivating growth surfaces allows for the deposition of material only where desired. The spatial deposition of platinum using ALD is demonstrated in two studies. In the first, a water-soluble polymer, polymethacrylamide (PMAM) acts as a resist to ALD and allowed the deposition of very small features of platinum. In the second study, surface passivation by means of controlling defect site density in self-assembled monolayers (SAMs) also serves as a growth template to control the aerial density and size of platinum nanoparticles deposited via ALD. The highly tunable nature of the chemistry of each precursor half-reaction in ALD enables the careful alloying or doping of materials in any desired ratio. A range of compositions for the indium-free TCO alternative, zinc tin oxide (ZTO), is explored by varying the dosing cycle ratios of tin oxide (SnOx) and zinc oxide (ZnO) in each ALD super cycle. Because the effects of ZTO composition on conduction behavior and optical properties are not well understood, having nanoscale control over how the constituent metal oxides grow on one another is an effective approach to learning more about these properties. As a result, ALD is explored as a flexible system to allow for compositional sampling and variation of film lamination in ZTO. The characteristics of ZTO ALD and the structural and optical properties of these films were shown to vary significantly as a function of degree of lamination and composition. These findings suggest that having nanoscale control over these film characteristics via ALD can lead to further understanding of ZTO. The ALD of ZTO provides an excellent system for the nanoengineering of this material as an indium-free TCO or buffer layer in solar cells.