Laterally Grown ZnO Nanowires for Sensing Applications
Authors:   , Matthews Campbell 
Published: November 2021

<p>Zinc oxide nanowires are a semiconducting material that has many uses in electronic applications. In particular, ZnO nanowires have been used in field effect transistors and applied as sensors for the detection of gases, biomolecules, UV light and as pressure sensors. ZnO nanowires can be fabricated using many different methods, but most require the use of high tempertures and have extensive setup costs. Hydrothermal growth, however, provides a cheap and low temperture method for growing ZnO nanowires. Much work has been done on the synthesis and charcetristaion of ZnO nanowires grown using hydrothermal growth, in partiuclar for photovoltaic applications. Little work has been done on the performace of hydrothermally grown ZnO nanowires in field effect transtors.  This thesis looks at applying hydrothermally grown ZnO nanowires as field effect transistors (FET). The FETs are characterised and developed with the intention of using them in senseing applications. The nanowire FET structure is optimised for sensing by developing a method that constrains the nanowires to exclusively lateral growth. A Ti capping layer is fabricated on top of a ZnO seed layer. The ZnO seed layer is then etched with dilute acid so that the Ti layer overhangs the ZnO. This acts as a physical barrier to vertical wire growth from the ZnO seed layer. The maximum deviation of the nanowires from the horizontal can be controlled by etching for different times.  Two device types are fabricated using different size nanowires. One uses large nanowires, or nanorods (diameter 400 nm), while the second device type uses a hybrid structure of large nanorods with much thinner nanowires (diameter 20 nm) growing off them. Both device types are characterised as FETs in dry conditions and also when immersed in a number of different liquids. Two different gating setups are also used with the Si/SiO₂ substrate used as a backgate and a Ag/AgCl electrode inserted into liquid as a topgate.  The large nanorods only show field dependence when wet due to the large capacitance of the elctric double layer and enhanced band bending. The wet nanorods can achieve on/off ratios of 10³. In contrast, the thinner nanowires show field dependence both when dry and when wet. On/off ratios of more than 10⁴ are achieved. In general the nanowires have superior on off ratios and smaller off current due to their larger surface to volume ratio.  Attempts are made to functionalise the nanowires with aptamers so that they can be used as a biosensor. The functionalisation procedure is documented, however the overall procedure proves to be unsuccessful due to the instability and dissolution of the nanowires in tris buffer. The rate of decay in buffer solution is investigated.  Both device types are also tested as gas sensors for humidity and ethanol detection. The nanorods show no apparent detection, while the nanowires show some response to ethanol. Further development of the experimental setup is necessary to better characterise the devices.  Finally future work on these nanowires is discussed and possible improvements proposed for future development as biosensors and gas sensors.</p>