Controllable Growth And Optimal Optoelectronic Properties Of Ain And In₂O₃Nanomaterials

Controllable growth of semiconductor nanomaterials is the basis of high-speed, high-frequency, micromation nano-optoelectronic device. Mastering the techniques of controllable growth of semiconductor nanomaterials will help us seek high performance of these nanomaterials and show wider applications in many aspects.As a III-V semiconductor material, AIN possesses many attractive features, for example, high energy gap of6.2eV, high thermal stability and negative electron affinity, which have the broad applications such as in cold cathode field emission. Revently, the structure defects of AIN one dimmensional (ID) nanomaterials have led to the disappointing field emission property. Thus, further improvement in structal defects is an important research subject. As an indirect-band gap semiconductor material, In2O3possesses many attractive optoelectronic features which lead to important applications in the field of conductive thin film and gas sensor. However, due to weak photocatalytic activity of the surfaces on traditional structures, the researches in water splitting property mainly focused on the composite materials of doping or heterostructure, whereas its potentiality remained latent. Deep exploration on intrinsic material will overcome these shortcomings so that in-depth study will be imperative.In this study, we use the CVD equipment to conduct the growth of AIN and In2O3 nanostructural materials. By observing the growth process and controlling the experimental conditions, we achieve the controllable growth of AIN and In2O3respectively, and enhanced field emission and photocatalytic properties were reached finally. The main contents and innovations in the dissertation are listed as fellows:1. Under VLS mechanism, by analyzing the experimental principle, we made an first attempt to add a mass flowmeter at the end of the traditional CVD equipment to "monitor" the reaction process and thus fabricated ID hierarchical AIN nanostructure which has a thin nanowire on top and a nanocolumn at the bottom. The diameter ratio of the nanowire and nanocolumn is as high as1/10. By observing the growth process and analyzing the growth mechanism, we reveal that the ID structural growth is based on the "dynamic equilibrium" principle between absorbing and evaporation of the catalytic drop. We reach the controllable growth of this hierarchical AIN nanostructure under guidance of the principle. The turn-on field of2.7V/μm and the threshold field of about7.1V/μm for the multi-level AIN nanostructure are much lower than those from many other AIN nanostructures and our experiments clearly show that this hierarchical AIN nanostructure will be an excellent field emission material.2. So far, theoretical studies on the photocatalytic property of In2O3are very little and little work only focused on the bulk material and doping. In this study, we firstly performed the theoretic calculation on different crystal face of In2O3bcc structure. By calculating densities of states (DOSs) of the bulk and slabs along the{100} and{111} directions of In2O3and comparing the results from both the bulk materials and{111} surface, we found that the cleaving of the{100} surface creates a new valence sub-band which is just below but very close to the Fermi level. During light irradiation, the excited holes from valence sub-band are concentrated on the{100} facets and the excited electrons are in bulk, whereas the excited holes and excited electrons on the{111} are in bulk. This leads to the excellent photocatalytic water splitting property of{100} surface on In2O3. Afterwards, we performed geometry optimization on the{111} and{100} slabs with adsorbed H2O molecules, the results show that the subband is still stable even the H2O molecules exist. 3. By observing the growth of In2O3octahedron under different temperature, we proposed a growth mechanism of In2O3truncated octahedrons. Because of the different surface energy of the{111} and{100} facets, they have different growth rates. By changing the flow and temperature, we "cut" the crystal facets of In2O3successfully and fabricated uniform truncated In2O3nanocrystals on a large-scale on a silicon substrate for the first time. The high photocurrent of1.4mA/cm2and long-time oxygen evolution without attenuation indicate that the In2O3truncated octahedrons are promising for photocatalytic oxygen evolution, and our results provide evidence that crystal cutting plays an important role in photofunctionization of materials.