Studies Of Structures And Properties Of Core/Shell ZnO-based Quantum Dots

As the development of synthesis and characterization of nanostructures and micro/nano processing, nanotechnology has been attracted immense interests. When their sizes are in the range from1nm to100nm, materials exhibits many chemical and physical properties due to the quantum confinement effect and surface effect, which have potential application for semiconductor devices, magnetic materials, catalysis, photoelectrochemical electrodes and nanodevices. To realize practical applications, it is necessary to precisely tuning certain properties of nanoparticles and core/shell structuring is an effective way to achieve it. Extensive studies have proved that the electronic structure of nanoparticles can be continuously tuned via core/shell structure. In this thesis, we reported a surface-converting strategy for synthesize ZnO/ZnS core/shell nanoparticles and explore their application in diluted magnetic semiconductors, highly efficiency photoelectrochemical electrodes and nanogenerators. We exploited characterization techniques such as X-ray absorption spectroscopy, UV-vis absorption spectroscopy, photoluminescence spectroscopy and first-principles calculations to explain the mechanism and discover the path to further optimization.The main content in this dissertation is as follows:(1) Realizing ferromagnetic coupling in diluted magnetic semiconductor quantum dotsManipulating the ferromagnetic interactions in diluted magnetic semiconductor quantum dots (DMSQDs) is a central theme to the development of the next-generation spin-based information technologies, but remains a great challenge because of the intrinsic antiferromagnetic coupling between impurity ions therein. Here, we propose an effective approach capable of activating ferromagnetic exchange in ZnO-based DMSQDs, by virtue of a core/shell structure that engineers the energy level of the magnetic impurity3d levels relative to the band edge. This idea has been successfully applied to Zn0.96CO0.04O DMSQDs covered by a shell of ZnS or Ag2S. First-principles calculations further indicate that covering a ZnS shell around the Co-doped ZnO core drives a transition of antiferromagnetic-to-ferromagnetic interaction, which occurs within an effective depth of1.2nm underneath the surface in the core. This design opens up new possibility for effective manipulation of exchange interactions in doped oxide nanostructures for future spintronics applications.(2) ZnO@S-doped ZnO Core/Shell Nanocomposites for Highly Efficient Solar Water SplittingProhibiting the rapid holes recombination is a key issue for designing highly efficient photoelectrodes for solar water splitting. Here, we propose a strategy incorporating S atoms in the surface shell of ZnO nanoparticles to generate holes trapper for restraining the electron-hole recombination. The obtained ZnO@S-doped ZnO core/shell-like nanocomposite exhibits a high photocurrent density of1.08mA/cm2with conversion efficiency up to0.74%,8times larger than that of pristine ZnO nanoparticles. Comprehensively, the results from experimental and computational aspects reveal that the S dopants substituting the surface O sites narrow the band gap and introduce a mid-gap impurity band, contributing to the enhanced water splitting activity. This design provides some guides for future optimization of ZnO-based photoelectrodes.(3) A Novel Nanogenerator based on ZnO/ZnS core/shell electrets with Stabilized Quasi-permanent ChargeFlexible, efficient and robust ZnO-based nanogenerators with convenient integration process are particularly desirable for self-powered technology, which is however difficult to achieve simultaneously in traditional piezoelectric ZnO nanogenerators due to the complexity of the function unit for generating piezopotential. Here, we proposed a novel flexible electrostatic ZnO nanogenerator based on charged ZnO nanoscale electrets, in which extra charge is stabilized with a carefully devised type-â…¡core/shell structure. Via a previously reported surface-converting strategy, we successfully covered ZnS shells around ZnO nanoparticles, preventing UV-induced electron carriers form decay in the atmosphere; and then the charged electrets were assembled with a simple process into a nanogenerator, which is enabled to convert vibrational energy from the environment into electric power with a conversion efficiency of2.2%. Photoluminescence, X-ray absorption spectroscopy and first-principles studies revealed that in the charged ZnO/ZnS core/shell electrets, the injected electrons occupy the site of oxygen vacancies. This design opens up an alternative path for fabricating flexible ZnO-based nanogenerator for future nanotechnology application.