Abstract
The defect engineering of TiO2 and ZnO has attracted immense interest in recent years since it was reported to be an efficient tool to adjust and specify the properties of these materials. For instance, it was demonstrated that the appearance of oxygen vacancies in TiO2 after thermal treatment in oxygen-deficient atmosphere has a positive impact on many applications such as fuel cell, photocatalyst, photosensors. It was determined that the condition of treatment crucially influences the formation of defects. Various instruments were used to analyze the properties of metal oxide semiconductors after introduction of defects. However, most of studies available in the literature were concentrated on the application of the structures, and systematic approach to investigate the behavior of defects has been often overlooked or only slightly covered. Simply to say, without basic knowledge and solid background about the defect engineering of TiO2 and ZnO those attempts can be referred to “trial-and-error pathway”. The main purpose of this research is to discuss and investigate the introduction of defects in TiO2 and ZnO. The obtained results would allow to predict the perspective properties of these materials and accurately tune some of their useful and promising applications following a certain pattern and road map. In the first part, TiO2 thin film prepared by atomic layer deposition (ALD) was subjected to annealing in hydrogen at 350-500 oC. Hydrogenation resulted in appearance of disordered states and oxygen vacancies which induced the increased surface roughness. Furthermore, it was revealed that oxygen vacancies were mostly formed on the surface, and their concentration and distribution uniformity increased with increasing temperature. The most uniform distribution of oxygen vacancies with the highest surface density was obtained after annealing at 500 oC, which is consistent with the lowest electron-hole recombination rate and highest photoactivity of the modified TiO2. Although hydrogenation of TiO2 is considered as a simple and effective approach to improve the photoefficiency, inability to precisely control the thickness of disordered phase and lack of good comformality over different morphologies limits the wide application of this technique. In order to solve this problem, in the second part it is shown a simple one-step and low temperature process to deposit an amorphous titanium dioxide overlayer by ALD on the crystalline TiO2 film. This composite structure was beneficial for improved photocatalytic properties since the as-deposited amorphous layer showed unique properties such as local disorder and presence of oxygen vacancies which are similar to disordered shell created by hydrogenation. Furthermore, it was demonstrated that by changing the cycle number of ALD process the thicknesses of disordered shell and crystalline substrate could be precisely controlled with an accuracy of atomic scale. It allows to tune the photoefficiency of TiO2 composite. In the third part, ZnO prepared by ALD and annealed in hydrogen at 10 bar and 350-450 oC was investigated in terms of defect distribution and allocation. Hydrogenation induced simultaneous formation of oxygen and zinc vacancies whose concentrations were closely related to the temperature of treatment. Spatial distributions of these defects were analyzed by photoluminescence confocal mapping which revealed that their localized appearances were linked to each other. It was also demonstrated that nanomechanical mapping of elastic modulus distribution could be used to assess the allocation of accumulated defects on the topmost surface of ZnO with a depth resolution of only several atomic layers. The correlation between the surface morphology and the accumulated defects was established. The fourth part is dedicated to ZnO nanorods grown on ITO and annealed in hydrogen at 10 bar and 350 oC. Hydrogenation resulted in narrowed band gap and enhanced photoelectrochemical efficiency. The origin of such improvement is discussed in terms of newly generated surface oxygen and zinc vacancies whose formation is attributed to initial filling of native defects with hydrogen atoms and subsequent breaking of Zn-O bonds. Hence, the effect of dominant role of only one type of defect on the improved performance of disordered ZnO should be reconsidered. In the last part of this work, low-temperature hydrogen plasma treatment with different lengths of time was used to fabricate a p-type surface conductive ZnO film with controlled concentration of holes. The distribution and concentration of charge carriers on the topmost surface of the film was assessed with a depth resolution of several atomic layers that allowed to eliminate any influence from the bulk. A theoretical framework was constructed to provide a rationale of the p-type surface conductivity and justify its relation to the treatment time.