Year

2013

Degree Name

Master of Engineering - Research

Department

Institute for Superconducting and Electronic Materials

Abstract

Nanoscience and nanotechnology has become one of the most active research areas for decades; new approaches for nanoscale fabrication are of fundamental importance in the advancement of science and technology, these developments in methodology stimulated numerous research activities. Thermal evaporation, laser ablation, chemical vapor deposition and other synthetic routes have enabled the synthesis of inorganic nanoscale materials with various morphologies. These nanomaterials form the building blocks for a range of uses such as nanoscale devices, energy storage, sensors and nanomedicine.

Physical evaporation synthetic route plays a key role in inorganic nanomaterial growth. In this approach, materials are first evaporated and then condensed to form solid materials. Thermal evaporation is one of the widely used methods; it is a simple and an effective way for the synthesis of nanostructures during the last decade. This method is usually processed in a tube furnace, in which the source materials are vaporized at an elevated temperature and the vapour phase(s) condense(s) under certain conditions. Since tube furnace is used as heating element, it is not applicable for the source materials with low volatility or high melting point (> 1800 ℃), and if the temperature gradient is low (approximately 20 °C/cm) even in a multi-zone tube furnace, which is the driving force for atom movement in most cases. A laser ablation method was developed by using a pulsed laser as secondary heating element; the target materials were ablated by the laser beam, creating a dense, hot vapor of plasma species. In the case of growing nanowires - the hot vapor condenses into nanometer diameter catalytic cluster as the species cool through collisions with the buffer gas. The clusters define the size and direct the growth of the crystalline nanowires by a vapor-liquid-solid (VLS) mechanism. This method ablate target materials with high melting point into gas phase, but the temperature gradient for the nanomaterial growth mainly depends on the tube furnace, it has been successfully applied on the nanowires growth of C, Si and Ge.

The objective of this work was to study the nanocrystal growth of ZnO with high crystallinity and clean surface by a new physical evaporation method. This method based on the utilization of high temperature yield by focused light beam and the large temperature gradient formed in the growth chamber. The starting material can be pure material or a mixture with catalyst materials in a crucible free condition. The work in this thesis is directed to the growth study and characterization of nanosized ZnO. ZnO nancrystal and nanostructure with various morphologies have been successfully grown by this new method. A systematic and detailed growth study has been carried out, relating to the influence of growth conditions on the morphology and phase structure of the synthesized product(s). The physical properties of selected samples with typical morphologies were studied. The growth mechanisms of ZnO nanocrystals by this method have also been investigated. This new method has shown some unique features and could be adapted for growing nanocrystals and nanostructures of other materials, particularly for the materials that can absorb infrared easily.

In this thesis, after a literature review on the ZnO nanomaterials growth, a systematic study on growth impact factors will be reported. The experiments demonstrate that pure phase ZnO nanocrystals with diversify morphologies could be obtained on Si substrate by using light heating evaporation method.

Targets of ZnO containing 2% of MnO can be used to produce single morphology ZnO nano-tetrapods with arms of ≤ 50 nm diameter on the inner wall of the chamber . ZnO nanostructure with different morphologies were grown on the different area of the horizontally positioned Si substrate, this is likely due to the effects of the local “turbulence” or flow speed and pressure of the vapor phase and carrier gas. The result has shown great potential for the large scale production of ZnO tetrapods by light evaporation method.

The morphology of the nanostructures was found to be quite sensitive to local substrate environment and the substrate’s temperature. Therefore various nanosized morphologies have been grown from the same target by changing these two growth conditions. Gas flow rate was also found to affect the morphology of the samples, our study revealed that the flow rate of 100 sccm is a moderate value used to strike a balance between the oxygen-vapor concentration and resident time in the present method.

The study on the effect of ZnO-carbon ratio on the growth of nanocrystals indicated that ZnO nanostructures with diverse morphologies can be grown by using this new method. The growth mechanism can be described as Vapour-Solid, tip-growth and vapourliquid- solid (VLS) with the increasing of the carbon content.

Raman spectra has been studied on the nanowires and nanostructures evaporated from ZnO:C = 200:10 and 200:200 targets. The results indicate that both samples showing peak shift correspond to E2 high − E2 low and E2 high compared to bulk material. We propose that there is tensile stress in these samples. The larger stress in the nanostructures evaporated from the Zn:C=200:200 target is likely attributed to the more oxygen defects in it since the target contained much more C, which will react with oxygen during the growth.

The UV–VIS optical absorption spectra of these two samples did not show any clear difference. They have a strong excitonic absorption peak located at 371 nm, which can be attributed to the large exciton binding energy and the good optical quality of the nanocrystals, our result agrees well with the previous report on ZnO nanowires.

Overall, the work presented in the thesis is mainly based on the ZnO nanocrystals growth and characteriazation. These results are important for nanosized the ZnO fabrication, as well as providing a new physical approach for nanocrystal growth.

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