Date of Award
Doctor of Philosophy (PhD)
Sensors are devices which have been commonly used to measure the functional dependence and the variability of physical parameters like temperature, pressure, pH, voltage, current, concentration, and others. Among the numerous kinds of sensors in different areas, gas sensors have been widely used and investigated for gas monitoring. Gas sensors are of crucial importance for the detection of hazardous atmospheres, because toxic gases are frequently odorless, colorless, invisible, rapidly evaporating, and flammable, and would otherwise go unnoticed. Gas sensors have been used in a variety of applications among others for the detection of specific gas species and the detection of gas concentrations.
Compared to other materials systems used in gas sensor applications, Metal Oxide Semiconductor (MOS) sensors have attracted much attention for gas detection due to their low cost, simple design and ease of production, short response time, wide detection range, and resistance to harsh working environments. Among various semiconductor materials used in MOS gas sensors, ZnO is a well-known semiconducting metal oxide material used in gas sensor applications due to its good electrical property, wide band gap of 3.37 eV, ~60 meV exciton binding energy, low cost, and high mechanical stability. ZnO has been applied for MOS gas sensor applications due to its high electrochemical stability, non-toxicity, ease of doping, and low cost. In general, gas sensors based on ZnO tend to exhibit exceptional performance for ethanol detection with respect to high sensitivity, short response time, and fast recovery time.
In this dissertation, the sensing performance of novel innovative ZnO nanostructure gas sensor designs to ethanol vapor concentration detection were investigated and analyzed in terms of their sensing response, their response time, and recovery time. Currently, the shortcomings of state-of-the-art thin film ZnO gas sensors are lack of sufficient sensitivity, long response times, and long recovery times relying only on one reactive surface. My research is addressing these shortcomings by designing, fabricating and testing novel innovative 3-dimensional nanoscale ZnO sensor device architectures with increased surface-to-volume ratio using an integrated process approach combining hydrothermal growth of nanorods with Atomic Layer Deposition (ALD) wrap-around coatings. First and foremost, Aluminum doped ZnO (AZO) thin films were introduced to enhance the sensing performance of ZnO nanorod gas sensors by providing additional oxygen vacancies and extra electrons for the redox reactions using ALD technology. A roughly 100% improvement was achieved on the sensing response of ZnO nanorod gas sensors equipped with ALD AZO 3-D wrap-around coatings compared to conventional ZnO nanorod gas sensors. Secondly, the other key approach in this dissertation was to conceive a unique novel sensor architecture design to further improve the sensing performance of ZnO nanostructure gas sensors with an innovative increase of the surface-to-volume ratio. These novel nested ZnO nanorod/nanotube gas sensors exhibited a large improvement in their sensing response due to the increased surface-to-volume ratio with two additional reaction surfaces and extra reaction sites. The sensing response of ZnO gas sensors was improved up to a maximum of 150% with the novel nested coaxial nanorod/nanotube architecture compared to the sensing response of conventional ZnO nanorod gas sensors. The third approach was to investigate the sensing performance of ZnO nanotube sensors synthesized within porous templates by utilizing ALD and Al2O3 sacrificial layers. The sensing performance of these ZnO nanotube gas sensors was enhanced with increased surface-to-volume ratio by adding additional coaxial ZnO nanotubes. The enhancement can be further improved by adding additional coaxial ZnO nanotubes layers which provide each 2 additional reaction surfaces. Furthermore, ALD AZO coatings were introduced to further enhance the sensing performance of ZnO nanotube gas sensors synthesized in porous templates. With the combined benefits from approaches 1 and 2, the maximum gained enhancement reached up to 136% for the template replication case. The first two approaches established a bottom-up technology, which is subject to high variability from batch to batch hydrothermal growth. In contrast, nested ZnO nanotubes synthesized within porous templates enables a true top-down technology by using mask and photolithography patterning techniques from microelectronics to guarantee the reproducibility, which would render it ready for commercialization and to be transferred for industrial applications.
"Enhanced Sensing Performance of Novel Nanostructured ZnO Gas Sensors in Ethanol Vapor Concentration Detection Applications"
(2019). Doctor of Philosophy (PhD), Dissertation, Electrical/Computer Engineering, Old Dominion University, DOI: 10.25777/j9r7-vs61