Date of Award

2010

Thesis Type

PhD

Document Type

Thesis

Divisions

Faculty of Science

Department

Department of Physics

Institution

Universiti Malaya

Abstract

Research concerning one-dimensional nanomaterials has grown tremendously ever since carbon nanotubes were discovered by Iijima (1991), silicon nanowires were grown by Lieber's group (1998), and ZnO nanowires and nanobelts were found by Wang's group from Georgia Tech (Pan et al., 2001). Such researches on one-dimensional structures have been one of the most important branches of science during recent decades. We can now observe the impact of these studies on the technology of one-dimensional materials and also the advanced prospect of developing new technologies. Based on this motivation, this thesis attempts to investigate some unknown properties of ZnO nanostructures as one of the most attractive materials in this decade. The aim of this work is to study the morphology, structure, and optical properties of undoped and Mg-, Al-, In-, S-, and Sn-doped ZnO nanostructures by means of various characterization tools. These include the field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX) analysis, field emission Auger electron spectroscopy (AES), transmission electron microscopy (TEM) and select area electron diffraction (SAED), photoluminescence spectroscopy, and Raman spectroscopy. In addition, field emission study is carried out on Mg-doped ZnO nanowires. Morphological and optical studies on the undoped ZnO nanowires grown on Si(100) and Si(111) substrates in a conventional and modified thermal evaporation set-up were carried out. The results showed that a better crystalline quality of ZnO nanowires can be achieved if it was grown in the modified set-up. In addition, size effects on optical properties of nanowires were investigated. Mg-doped ZnO nanowires were also grown in the conventional and modified thermal evaporation set-up. In the first method, ZnxMg1-xO nanowires were grown in the conventional thermal evaporation set-up using sintered ZnxMg1-xO powder. In the second step, ZnxMg1-xO nanowires were grown in the modified thermal evaporation set-up. Furthermore, effect of gold catalyst on growth process of ZnxMg1-xO nanowires was studied. In this step, band-gap engineering of ZnO nanowires was possible due to Mgdoping. It was observed that gold catalyst played a significant role in the growth process of ZnxMg1-xO nanowires. In addition the optical properties of ZnxMg1-xO nanowires, field emission characteristic of ZnxMg1-xO nanowires that were grown in the modified set-up were investigated. The other element that was used as doping material in ZnO nanowires was Al. Ultra thin film AlN was used as a source material of Al-doped ZnO nanowires. Characterizations indicated that Al has diffused from AlN thin film into ZnO nanowires. Further study on the self-catalytic role of In during growth of ZnO nanowires is also described in this thesis. EDX and AES measurements indicated that the grown nanowires were heterostructure of ZnO and ZnInO. Analyses of the early growth process have revealed that indium may play a self-catalytic role. Therefore, the vapor-liquid-solid (VLS) mechanism was likely to be responsible for growth of ZnO/ZnInO nanowires. The final work report in this thesis is on the comparative studies between the optical properties of anionic and cationic doping in ZnO nanobelts. Sulfur as a cation and tin as anion can be doped into ZnO nanostructures. The results have shown significant different in optical properties for the two types of nanobelts. The PL spectrum of the S-doped ZnO nanobelts showed a broad visible emission with no detectable ultraviolet (UV) peak, while the PL spectrum for the Sn-doped sample showed two emission bands: UV emission band and green emission bands with a weaker peak. A weak peak in the UV region appeared after annealing the S-doped ZnO nanobelts in air ambient. Further, bonding vibrational modes of S-doped ZnO nanobelts were studied by Raman spectroscopy. It was shown in this study that there was a significant difference in the behavior of S-doped ZnO nanobelts in comparison with undoped ZnO nanobelts.

Initial

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Additional Information

Thesis (PhD) - Faculty of Science, Universiti Malaya, 2010.

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