Date of Award

3-21-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Applied Science

First Advisor

Jingbiao Cui

Abstract

As a transparent, wide bandgap semiconductor, ZnO offers an expansive range of potential uses in various technological arenas such as electronics, optoelectronics, photonics, sensors, and energy conversion. However, a current obstacle to the realization of ZnO based electronics and optoelectronics is the lack of a reliable and reproducible method for fabricating high quality p-type ZnO. In addition, there remains a difficulty in tuning the various properties of ZnO materials, especially nanostructures, via low cost and low temperature deposition techniques. In this work, some of these deficiencies have been addressed. Undoped and Ag-doped ZnO nanowires, as well as highly uniform and dense ZnO films, were obtained by an inexpensive, low temperature, electrochemical technique in aqueous solution. The effects of electrochemical growth conditions and Ag-doping on the structural, optical, and electrical properties of the ZnO nanowires were investigated in detail. Ag-doping was found to induce significant changes in the various physical properties of the ZnO nanowires. Importantly, a range of experimental and theoretical results indicate Ag is doped into the ZnO nanowire structure and leads to p-type properties of the nanowires. The room temperature photoluminescence (PL) of the nanowires illustrates bandgap reduction, while intense emissions from a free electron to neutral acceptor were induced in the low temperature PL upon Ag-doping. The electrical properties of the Ag-doped nanowires were probed with photoelectrochemical cell measurements, providing further evidence for their p-type nature. The mechanism of Ag-doping in the nanowires was explored with cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. Interestingly, the presence of Ag+ in the growth process catalyzes and enhances the electrochemistry, shifting the ZnO growth conditions to an O-rich environment. These conditions enable a more efficient Ag- and p-type doping process. The XPS and DFT results combine to show that Ag substitution for Zn is the dominant dopant coordination in the nanowires. The interaction between Ag 4d and O 2p orbital levels establishes an impurity band in ZnO, which shifts the Fermi level toward the valence band and induces p-type conductivity. This is the first report of p-type doping of ZnO nanowires via a low temperature electrodeposition technique. These results indicate that p-type ZnO can be obtained by an economical, low temperature process, opening up possibilities for low-cost, advanced optoelectronic devices based on ZnO nanowires.

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