|M.Sc Student||Iris Amarilio-Burshtein|
|Subject||ZnO Nanowires - Structure and Properties Dependence on|
|Department||Department of Materials Science and Engineering||Supervisors||Professor Emeritus Lifshitz Yeshayahu|
|Ms. Tamir Shoshana|
|Full Thesis text - in Hebrew|
Miniaturization of basic device elements to nanometric dimensions is motivated by the demand for improved electronic, optical and sensing properties . Controlled growth of NSs in general and nanowires (NWs) in particular is difficult in currently available technologies/systems. A better understanding of the NWs growth mechanisms is required for their reproducible growth as well as for establishing the correlation between their size/morphology and their properties. This work focuses on growth of ZnO NWs exploring the parameters affecting their growth processes, structure and properties .
1D growth of NWs from the gas phase may occur in three different phase combinations: Vapor-Liquid-Solid (VLS), Vapor-Sold-Solid (VSS) and Vapor-Solid (VS). The first two require a catalyst for nucleation and growth of NWs while in the VS mechanism the NW growth is driven by the crystalline anisotropy and does not necessitate a catalyst. The present work attempts to better understand the nucleation and growth of ZnO (via VLS, VSS and VS) as an important study case for NW growth of oxides .
Laser ablation and thermal CVD were employed for fabrication of ZnO NSs. Zn vapor generated from solid ZnO was transported to heated substrates on which NSs grew. The effect of the following parameters was studied for both techniques: (1) gas composition (argon, oxygen), flow and pressure, (2) substrate type , (3) substrate temperature (800-9000C), (4) the effect of Au as a catalyst for NW nucleation and or growth, (5) process duration .
The nano-morphologies were characterized using: High Resolution Scanning Electron Microscopy (HRSEM) and High Resolution Transmission Electron Microscopy (HRTEM) both with an attached Energy Dispersive Spectroscope (EDS), X-ray Diffraction (XRD), Photo and Cathode-Luminescence (PL, CL ).
The main findings are: (1) Thermal CVD of ZnO:C yields NW growth in a narrow window of parameters and only on substrates deposited by a catalyst (catalyst-assisted nucleation, VS growth). Addition of oxygen suppresses nucleation and growth. (2) Laser ablation of ZnO yields mainly upstream growth which is affected more by the laser ablation shock waves than by the carrier gas. NWs nucleation is preceded by formation of ZnO bases. Diffusion through catalyst was excluded and a catalyst-assisted growth without such diffusion was proposed. 2D growth of NSs was caused by VSS and VS occurring simultaneously. Conditions for oriented growth of NWs were found. Oxygen addition to the laser ablation carrier gas leads to films comprised of large diameter rods. The effects of substrate temperature and concentration of growth species were elucidated .