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Polarity-Controlled Epitaxial Growth of ZnO Crystal Arrays in Solution for Optoelectronic Devices

Polarity-Controlled Epitaxial Growth of ZnO Crystal Arrays in Solution for Optoelectronic Devices
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Diverse micro- and nanostructured materials have been extensively studied and have proven to have extraordinary performances far beyond those of their bulk counterparts. However, hierarchical organization of the elements into three-dimensional (3D) structures is needed to achieve their full benefits. Up to now, with the goal of producing tailored 3D structures, various methods have been developed. Despite recent advances of these approaches, however, the large-scale production of complex, high resolution, and fully 3D structures is still challenging. In addition, most of the approaches developed so far are limited to either polymeric structures or their replicas consisting of polycrystalline inorganic materials. In this respect, a solution phase synthesis for 3D fabrication is a promising alternative considering its potential capability for commercial-scale production at a low cost. Particularly, if the conditions are set to enable heterogeneous solidification to occur at a low level of supersaturation, epitaxial growth of 3D crystal array in solution is also available, as suitable for constructing single crystalline 3D structures. In this dissertation, we first demonstrated the artificially-regulation of vertical and in–plane crystallographic orientations of ZnO crystals by appropriate patterning of growth masks and seed layer control in low temperature. This enabled the formation of three types of ZnO nanocrystal morphologies (i.e., ZnO nanoflowers, columnar joints and hexagonal pillars). These morphology–controlled ZnO nanocrystals exhibited clear differences in light propagation characteristics, which could be ascribed to strong light guiding in the 1D nanostructures. Second, we established control of the preferential growth direction of ZnO crystals by polarity-selective crystallization during the low-temperature solution-phase synthesis. This feature was further combined with multistage epitaxial growth of the nanocrystal constituents of hexaplates and hexagonal rods, producing a 3D, single crystalline semiconductor with excellent luminescent characteristics. Third, we have investigated the surface polarity-dependent luminescent properties of the ZnO hexagonal rods. We have also shown that the surface defects associated with the near-band edge emission quenching could be eliminated by appropriate thermal annealing. Finally, to further extend the applicability of our approach to functional device applications, we established the heteroepitaxial growth of GaN layers on 3D ZnO crystal templates, and demonstrated the pixel-type light emitting diode array. In addition, vertical aligned ZnO crystal array and graphene hybrid structures have been employed to develop a number of electronic optoelectronic devices, such as ultraviolet detector, vertical transistor and pressure/touch sensors. These results illustrate the expendability of this work to many other materials and related device applications.
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