Nanowires : from Microchip Integrated Synthesis to Application Examples
Both the high demand in the downscaling of electronic devices and the novel properties of nanowires (NWs) as a result of their high surface-to-volume (S/V) ratio and their quantum mechanical properties have driven NWs to be among the favorite research areas in recent years and are expected to remain one of the hot topics in the next decade. Thus the synthesis of NWs which is under intensive study is not only aimed at basic research but also for a wide spectrum of device fabrication and applications.The aim of this research is to explore simple synthesis techniques of metal oxide (semiconductor) and metal-metal oxide hybrid nanostructures with proper contact lines on different substrates and investigate NWs for various device applications. Three novel synthesis routes were developed and utilized in this thesis: Thin Film Fracture (TFF),Flame Transport Synthesis (FTS) and Thermal Annealing Method (TAM). As compared to many other synthesis techniques in the literature, the here used techniques are cost effective, can be easily scaled up, offer a less complex synthesis processes and offer easy integration of NWs into lithographically patterned chips. The three synthesis techniques employed in synthesizing both 1D semiconductor and semiconductor-metal hybrid structures spanning from device fabrication to results will be presented. Both FTS and TFF approaches are implemented for the fabrication of the semiconductor (mainly ZnO) nano-microstructures based UV photodetectors. Comparison of the UV photodetectors performances built from interpenetrating ZnO nano-microstructures fabricated by burner-FTS (B-FTS) and crucible-FTS (C-FTS) techniques will also be presented. Fastest response/recovery time constant (~32 ms) and an on/off ratio ~4.5103 at 2.4 V under 365 nm UV light irradiation for the B-FTS based photodetectors is reported. The growth mechanism, properties and measurements as sensor devices will be presented in detail. Besides that the TFF based ZnO NW specimens showed a promising result as oxygen gas sensors. 1D semiconductor-metal hybrid structures are fabricated using TFF and TAM techniques and offer additional functionalities. A controlled shrinking of Ti NWs (TFF) by electrochemical oxidation, TiO2-Ti hybrid NWs with a promising potential as NWFET have been demonstrated. In addition to the interesting FET characteristics, the hybrid NWFET showed a drastic and fast response in the drain current when it is exposed to different partial pressures of oxygen gas. One of the most important feature of these NWs is that they all exhibit granular structure. This characteristic makes them suitable especially for sensoric applications. The other novel approach for the chip integrated synthesis of iron oxide nanostructures with a variety of unique characteristics is TAM. A thermally initiated diffusion, oxidation and reshaping of Fe thin film in atmospheric air condition driven for the synthesis of single crystalline iron oxide NWs. Detailed investigations on the NWs proved that some specimens are able to form metal-metal oxide hybrid nanostructure with an iron-rich core region, the detailed characteristics are described as well.
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