The optimization of thin film p-CuO/n-ZnO heterostructures for use in selective gas detection Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/02870z915

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  • Since bulk p-CuO/n-ZnO heterocontacts were first proposed for gas detection, rapid development has taken place in improving the overall functionality of these structures. While bulk heterocontacts have been shown to exhibit desirable sensitivity and selectivity characteristics, these devices suffer from innate diffusion and ZnO/CuO connectivity drawbacks that limit their effective use. To address these issues, thin film p-CuO/n-ZnO heterostructures have been fabricated via wet chemical (sol-gel) processes so as to examine their potential use in reducing environments. Individual ZnO and CuO sol-gel processes have been developed with the goal of optimizing thin film porosity, crystallinity, and preferred orientation for enhanced gas sensing capability. Particular attention was given to the effects of solution chemistry and pyrolysis temperature on desired thin film properties. For ZnO, control over film microstructure was attained through fabrication modes based on the solvents 2-methoxyethanol (MOE) and dimethylformamide (DMF). Monoethanolamine (MEA) was employed as a chelating ligand in specific solutions. Optimum preferred orientation for DMF-based ZnO films was seen to exist at a solution chemistry of 5% water and a 1:1 molar ratio of Zn to MEA. An increase in the drying temperature yielded a monotonic decrease in the electrical resistivity of these films. For the MOE-based process, a lowering of the pyrolysis temperature led to an increase in ZnO film porosity. CuO thin films were deposited through a solution route based on isopropanol. Scanning electron microscopy (SEM) revealed the CuO films to possess a level of porosity much higher than that seen in the ZnO films. Thin film p-CuO/n-ZnO heterostructures were fabricated in two configurations; ZnO on CuO (ZnO/CuO) and CuO on ZnO (CuO/ZnO). The results of current-voltage (I-V) tests showed the CuO/ZnO structures to display enhanced sensing characteristics to 4000 ppm hydrogen when compared to the ZnO/CuO structures. This finding was attributed to the inherently high porosity of the top CuO layer which in turn allowed for improved gas diffusion to the heterostructure interface. The phase equilibrium between CuO and ZnO exhibits limited solubility. As such, a novel microstructure formed by combining CuO and ZnO precursors has been explored with the expectation that the films will phase separate. For the deposited films, a variance in both the annealing temperature and time was found to yield a microstructure comprised of individual ZnO and CuO grains. The co-existence of these two structures was confirmed through Energy Dispersive Spectroscopy (EDS). It is expected that the high level of connectivity between the ZnO and CuO along with negligible barriers to gas diffusion will lead to superior sensing characteristics.
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