Investigation of ZnO nanorods as chemical and biological sensors

Zinc oxide (ZnO) is a metal-oxide semiconductor with a direct wide band gap and high exciton binding energy at room temperature. It has been applied to many applications such as solar cells, light emitting diodes, nano-generators and chemical sensors. In this thesis, a solution phase synthesis method has been used to produce ZnO nanorods on conductive substrates at low cost. By controlling the growth conditions, ZnO nanorods with an aspect ratio of 51 in a single step was achieved. The morphology, crystallisation, optical properties, band structures and carrier lifetime were analysed. Chemical sensors such as gas sensors play a very important role in our life and in industry. ZnO nanorods have been investigated as a sensing material to detect harmful gases such as NO2 and NH3. However, such a traditional sensing platform only works at high operating temperatures, which limits its application to portable devices or wireless devices. In order to decrease the working temperature, a Schottky diode was assembled by evaporating a gold layer on the top of the ZnO nanorods. Gas sensing results showed that this diode had good responses to NH3 gas at room temperature. It also displayed high selectivity to NH3 over acetone, CO2, CO and ethanol. Long-term tests demonstrated good stability over 7-weeks. ZnO nanorods are also a suitable sensing material for light-addressable potentiometric sensors (LAPS) that are designed to detect pH, redox ions and characterise cells and tissue. In contrast to the traditional electrolyte-insulator-semiconductor (EIS) structures, ZnO nanorods without an insulator were applied in LAPS and showed a spatial resolution of 3 μm. LAPS based on ZnO nanorods were used as a disposable biosensor for the detection of α-chymotrypsin. The effect of the ZnO morphology on the spatial resolution was also investigated.