This dissertation outlines a generally applicable electrochemistry-based approach towards the creation of template-directed monocrystalline nanowires. The project is presented as essentially two distinct components: nanoporous template development and control of the electrocrystallization of cadmium sulfide. A novel multilayer thin film adaptation of the familiar nanoporous anodic alumina template has been developed allowing the in situ removal of the electrically insulating alumina barrier layer at the pore bases. This barrier-free nanoporous template enables the electrodeposition of a wide variety of materials and ultimately serves as the thread interconnecting the two projects of template development and nanowire synthesis. The first half of this dissertation describes in detail the creation of the multilayer thin film precursor consisting of an Al anodization layer, a Ti diffusion barrier layer and a Pt active electrode underlayer on a Si-substrate. Three interlaced problems were solved towards the development of the multilayer template.
Michael Matthew Crouse
Firstly, Al-Pt or Al-Au thin film bilayers were found to create intermetallic compounds at the interface. Secondly, the Pt or Au present in the intermetallics deleteriously accelerated the OER side reaction during anodization of the Al thin film to form alumina. Thirdly, an accelerated OER prevented field-driven etching of the oxide barrier layer; thereby, not allowing the creation of the Pt active electrode at the pore bases. The solution to these interlaced problems was the deposition of a selectively removable Ti diffusion barrier layer between the Al and Pt layers. Following the successful development of this Si-based multilayer template, a “transparent” (25-27 % transmission) extension was developed by shrinking the Pt underlayer and using a float glass substrate. CdS was chosen as the prototypical material due the rich literature base and the ability to easily probe the microstructure using optical techniques. This electrocrystallization study is examined through the unifying concept of the electrochemical overpotential of the cathodic redox reaction. The overpotential components relating to: electrode kinetics, species transport within the pore, and crystallization are demonstrated to be separable. It is shown through the use of DC galvanostatic electrodeposition and the geometric confinement of the statistics of nucleation, that the crystalline quality of the nanowires can be controlled. The limiting case of monocrystalline nanowires can be achieved at low galvanostatic deposition rates where the system is allowed to choose its most energetically favorable nucleation overpotential. At the end of the work, an application of the project towards II-VI nanostructured solar cells is discussed with on-going and future work identified.
