Fabrication and Characterization of ZnO/CuO Core-Shell Nanowire Arrays

Research into nanomaterials has become more and more popular because of their unique properties compared to bulk materials. Amongst various functional materials, zinc oxide (ZnO), with a direct electron energy band gap of 3.34 eV at room temperature, is an important optoelectronic material with an intrinsically n-type semiconducting property. However, to form a p-type ZnO semiconductor is still a challenge. Copper oxide (CuO), compared to ZnO, has a much smaller band gap, 1.2 eV, and shows an intrinsically p-type semiconducting property. It has been suggested that when CuO is alloyed with ZnO properly, a p-n semiconductor heterojunction can be formed to be utilized in solar cell and gas sensor applications. In this thesis, ZnO/CuO core-shell nanowire arrays have been successfully fabricated by a simple three-step process. ZnO nanowire arrays were first grown by the hydrothermal method using ZnO seeded substrates. Copper then was deposited on as-grown ZnO nanowire arrays by a DC sputtering method. Thermal oxidation of copper nanofilm was utilized to enable the formation of ZnO/CuO core-shell nanowire arrays. The Cu nanofilm thermal oxidation behavior on the three-dimensional (3D) ZnO nanowire arrays was systematically studied by introducing different oxygen flows and different pressures. It has been suggested that increasing oxygen flow rate might increase local partial oxygen pressure, thereby increasing the degree of oxidation throughout each single ZnO/Cu core-shell nanowire. Higher pressure might favor the formation of Zn2SiO4 at the interface of ZnO and silicon substrates. ZnO/CuO core-shell nanowire arrays have exhibited better absorption efficiency in visible region as compared to the pure ZnO nanowire arrays, which suggests that ZnO/CuO core-shell nanowire arrays have strong potential as nanoscale building blocks in solar cells and light emission devices. In this thesis layout, the first chapter gives general concepts and background on ZnO and CuO nanowires. Chapters 2 and 3 will provide the experimental methodologies and some important parameters to control. Chapter 4 focuses on the results and discussion on the characterization, growth mechanism, and Cu nanofilm oxidation behavior on 3D ZnO nanowire arrays. Chapter 5 concludes this thesis work and provides suggestions for the future work