Vapour Phase Transport Growth of One-Dimensional Zno Nanostructures and their Applications

Abstract

One-dimensional (1D) nanostructures have gained tremendous attention over the last decade due to their wide range of potential applications. Particularly, ZnO 1D nanostructures have been investigated with great interest due to their versatility in synthesis with potential applications in electronics, optics, optoelectronics, sensors, photocatalysts and nanogenerators. The thesis deals with the challenges and the answer to grow ZnO 1D nanostructure by vapor phase transport (VPT) continuously without any length limitation. The conventional VPT technique has been modified for the non-catalytic growth of ultralong ZnO 1D nanostructures and branched structures in large area with controllable aspect ratio. It has been shown that the aspect ratio can be controlled both by thermodynamically (temperature) and kinetically (vapour flux). The thesis also deals with the fabrication of carbon nanotube (CNT) -ZnO based multifunctional devices and the field emission performance of ZnO nanowires by employing various strategies. The entire thesis has been organised as follows: Chapter 1 deals with Introduction. In this chapter, importance of ultralong nanowires and significance of ultralong ZnO nanowires has been discussed. Various efforts to grow ultralong ZnO nanowire with their advantages and disadvantages have been summarised. Lastly the significance of forming ZnO nanowires based nano hybrid structures and importance of doping in ZnO nanowires and has also been discussed. Chapter 2 deals with experimental procedure and characterization. In this chapter, a single step VPT method for the growth of ultralong ZnO nanowires that incorporates local oxidation barrier for the source has been described. The synthesized nanowires were characterised by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman & photoluminescence. Chapter 3 deals with growth of ZnO nanowires, controlling the aspect ratio of ZnO nanowires, and role of other experimental aspects. In this chapter, a way to grow nanowires continuously without any apparent length limitation, a way to control the diameter of the nanowires kinetically without catalyst particle or seed layer and obtaining smaller diameter of the nanowires by non-catalytic growth as compared to that set by the thermodynamic limit has been discussed. Furthermore, the significance and importance of local oxidation barrier on source for protecting them from degradation, ensuring the continuous supply of vapour and enabling the thermodynamically and kinetically controlled growth of nanowires has been discussed. Lastly, the scheme for large area deposition and a method to use same source material for several depositions has been presented. Chapter 4 deals with multifunctional device based on CNT -ZnO Nanowire Hybrid Architectures same device can be used as a rectifier, a transistor and a photodetector. In this chapter, the fabrication of CNT arrays-ZnO nanowires based hybrid architectures that exhibit excellent high current Schottky like behavior with p-type conductivity of ZnO has been discussed. CNT-ZnO hybrid structures that can be used as high current p-type field effect transistors (FETs) and deliver currents of the order of milliamperes has been presented. Furthermore, the p-type nature of ZnO and possible mechanism for the rectifying characteristics of CNT-ZnO has been discussed. Lastly, the use of hybrid structures as ultraviolet detectors where the current on-off ratio and the response time can be controlled by the gate voltage has been presented and also an explanation for photoresponse behaviour has been provided. Chapter 5 deals with the substrate-assisted doping of ZnO nanowires grown by this technique. In this chapter, the non-catalytic growth of ZnO nanowires on multiwalled carbon nanotubes (MWCNTs) and soda lime glass (SLG) with controlled aspect ratio has been presented. The elemental mapping to confirm the presence and distribution of carbon and sodium in ZnO nanowires and the transport studies on both carbon and sodium doped ZnO has also been presented. Furthermore the stability of carbon doped ZnO has also been presented. Lastly, the advantage of growing ZnO nanowires on MWCNTs and overall advantage associated with this technique has been discussed. Chapter 6 deals with formation of ZnO nanowire branched structures. In this chapter, a possibility to grow ZnO nanowires on already grown ZnO nanowires has been demonstrated. The formation of branched structure during multiple growth of ZnO nanowire on ZnO nanowire has been presented and evolution of aspect ratio in these branched structures has been discussed. Furthermore, the advantage of using ZnO branched structures and also the ZnO nanoneedles on MWCNT mat for field emission has been presented. Chapter 7 summarizes all the findings of the thesis.