Site-selective growth of nanowires for sensor-applications

Abstract

Tin(IV)oxide, germanium and tungsten(VI)oxide are semiconducting materials with resistance values depending on adsorbed molecules altering the space charge layer in proximity to the surface. Previous studies have shown the potential of these nanostructured materials in chemiresistor-based sensors. Especially nanowires stand out due to their excellent crystallinity and high surface area to volume ratio. They can be used in sensors as single-nanowire devices or as a network between electrodes. Nanowires can outperform other morphologies due to the few grain boundaries and the high surface-to-volume ratio. It is of advantage to integrate nanowires in-situ on the desired area, because it can be tedious and time-consuming to integrate them into electronic devices afterwards. Therefore, process parameters for the site-selective growth of tin(IV)oxide and germanium nanowires on micromembranes by use of low-pressure chemical vapour deposition (CVD) have been optimised. The deposition temperature and the precursor flux are crucial parameters to obtain the desired nanowire density and quality. Furthermore, the morphology of tungsten(VI)oxide nanostructures that were obtained by use of aerosol-assisted chemical vapour deposition has been investigated in order to obtain a reliable procedure for the growth in a cold wall CVD reactor. The efficient growth of nanowires requires a pre-treatment including the light induced ligand exchange in W(CO)6. The gained knowledge on larger substrates has been used to achieve the site-selective deposition of tungsten oxide nanowires on micromembranes. The desired site-selective deposition of all three materials on one chip containing four micromembranes has been demonstrated. This is the first process, in which different materials are site-selectively deposited on a single chip. Additionally, experiments about the surface modification of nanowires with metal organic frameworks have been conducted and a setup has been constructed to allow liquid phase epitaxy of metal organic frameworks.