Growth and characterisation of Au Colloid catalysed zinc oxide nanowires.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis concerns new and in-depth research into the use of eclipse pulsed laser deposition (EPLD) to grow ZnO and ZnMgO nanowires and the use of chemically synthesised gold colloids and atomic clusters with precisely controlled dimensions as catalysts to control the dimensions of the resulting nanowires. The nanowires grown by these methods were characterised using a range of electrical and optical techniques. ZnO nanowires grown by collaborators using different growth methods were also characterised and compared to those produced by EPLD. A new method was developed for applying the nanoscale quantities of catalyst metal required for vapour-liquid-solid (VLS) nanowire growth. Chemically synthesised gold colloids were tethered to the surface of the sapphire substrate without aggregation using a silanising ligand, 3-aminopropyl trimethoxysilane (APTMS). ZnO and ZnMgO nanowires were grown on these gold colloid tethered substrates using EPLD. The operating parameters of the EPLD system were optimised to produce well- ordered arrays of nanowires. These nanowires had a tapered morphology and their tip diameters could be reproducibly controlled across a range of 5-40 nm by selecting the diameters of the gold catalyst colloid nanoparticles. Atomically precise, non-metallic gold clusters were used to investigate whether a lower limit exists on the size of Au nanoparticle catalysts for VLS ZnO nanowire growth. Chemically synthesised clusters of 101 and 9 Au atoms were separately applied to sapphire substrates using the APTMS ligand tethering method and used to catalyse ZnO nanowire growth by EPLD. These nanowires had tip diameters of less than 1.5 nm (the measurement limit of the scanning electron microscope used), indicating that non-metallic Au clusters are able to catalyse VLS nanowire growth in ZnO. A method was developed for fabricating permanent ohmic and Schottky contacts onto individual nanowires using electron beam lithography (EBL). The nanowires were found to be very resistive, but highly sensitive to UV illumination, with more than three orders of magnitude increase in current measured after 30 s of 365 nm UV illumination while being held at a constant voltage of 1V. This photocurrent was highly persistent and decayed to pre-illumination levels over approximately 5000 s in atmosphere. By illuminating contacted nanowires in vacuum and in atmosphere, it was shown that the photocurrent in ZnO nanowires is generated by two parallel mechanisms: the formation of electron-hole pairs across the bandgap and the hole-induced desorption of oxygen adsorbed on the crystal surface. Photogenerated electron hole pairs decayed exponentially with time both in atmosphere and in vacuum, but the desorbed surface oxygencould not be recovered in vacuum, leading to a permanent increase in conductivity while the nanowire remained under vacuum conditions. Photoluminescence (PL) and x-ray diffraction spectroscopy (XRD) were used to assess the crystallinity, the dominant donor impurities and preferential growth direction of the ZnO nanowires grown by EPLD. The dominant excitonic PL peak was found to be I9, indicating that indium was the most significant impurity present. PL line widths were very narrow, indicating excellent crystal quality. No detectable PL emission in the visible defect band region was observed, with at least five orders of magnitude increase in emission intensity between the excitonic UV emission and any defect band emission. The complete absence of defect band emission is unusual even in ZnO nanowires and is a further indicator of high crystal quality. The preferred growth direction of the nanowires was confirmed by XRD to be in line with the ZnO <002> plane. An ultra-thin sputtered film of palladium was used for the first time as a catalyst to grow ZnO nanowires. Despite having a significantly higher melting point than the EPLD ZnO growth temperature, a size dependent melting point reduction effect is proposed that enables the palladium film to dewet into liquid droplets that catalyse the VLS nanowire growth.