Fabrication Of TiO₂Nanotube Array Heterojunctions And Property Studies In External Fields

The interactions of nanomaterials with external fields are expected to create newphysical and chemical properties, which provides a unique way for new materialdevelopment and device design. In this dissertation, highly-ordered TiO₂nanotubearrays （TNAs） with uniform size were fabricated. Three types of external fields （i.e.,light field, magnetic field, electric field） were introduced into the nanostructuresynthesis and property study, and we successfully designed and fabricated highperformance nano-structured devices based on TiO₂nanotube arrays, systematicallystudying the relationship among external field conditions, material structures andproperties, and its corresponding physical mechanisms were also discussed.TiO₂nanotube arrays and double-walled carbon nanotube （DWCNT） films weresynthetized by electrochemical anodization and chemical vapor deposition, respectively.A novel photodetector based DWCNT film/TNA heterojunctions has been designed andfabricated. This photodetector exhibited high photoresponse in a broad spectral regionfrom ultraviolet （UV） to visible, which effectively overcame the shortcomings oftraditional TiO₂thin film-based detector. In addition, we also investigated the effect ofchanges in the length of the TNAs on the opto-electric properties of the detector. Byoptimizing the length of the TNAs in the detector, not only higher photoresponse hasbeen achieved, but also the photosensitive region has been extended into near-infraredrange.We designed and fabricated DWCNT film/Cu₂O NP film/TNA heterojunctionphotodetector constructed by Cu₂O nanoparticle （NP） film synthetized by chemical bathdeposition process. The result demonstrated that a low dark current and enhancedvisible absortion were induced by Cu₂O NPs. Meanwhile, the energy bands of theDWCNT film/Cu₂O NP film/TNA heterojunctions were well matched, which favorablefor charge separations. Consequently, the photoresponse of DWCNT film/Cu₂O NPfilm/TNA heterojunction photodetector has been dramatically enhanced compared withDWCNT film/TNA heterojunction device.High magnetic fields were introduced in the annealing process of TiO₂nanotube arrays. We investigated the effect of high magnetic fields on the microstructure andvisible photoluminescence of TiO₂nanotube arrays. The result demonstrated the grainsize and visible light-emitting intensity were affected significantly by high magneticfields, but it was not effective for crystal orientation. The visible photoluminescenceproperties have been greatly enhanced by varying the intensity and direction of highmagnetic fields.We designed and fabricated resistive-switching memories based TiO₂nanotubearrays with two crystal structures （anatase and amorphous）, the resistive switchingbehavior of which was found to be different from that observed in traditional TiO₂film-based devices. The results demonstrate that the resistive switching inAg/TNA（anatase）/Ti device was induced by the formation and rupture of O-deficientTiO₂-xconducting filaments produced by oxygen vacancy migration, while the resistiveswitching in Ag/TNA（amorphous）/Ti device might be described as the dual effect ofoxygen vacancy migration and Ag conducting filament.