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Solution- and vapor-phase growth of ZnO nanostructures on various substrates and their device applications

Solution- and vapor-phase growth of ZnO nanostructures on various substrates and their device applications
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Preparing single crystalline materials on amorphous substrates has attracted much attention for the fabrication of transparent or flexible devices. A lack of heteroepitaxial relation between deposited materials and amorphous substrates and dissimilarity in their thermal expansion coefficient limit the growth of single crystalline materials on amorphous substrates. Semiconductor one-dimensional (1D) nanostructures by bottom-up method can provide a promising opportunity to prepare the single crystalline materials on various substrates due to nanometer-scale epitaxy. Although the remarkable progress in fabrication of 1D nanostructures has been achieved, only limited single crystalline substrates, such as silicon, sapphire, and compound semiconductor wafer, were widely adopted for growth of semiconducting 1D nanomaterials. This dissertation presents the growth of 1D semiconducting nanomaterials on various substrates by solution- and gas-phase growth methods and their device applications. The strategy to employ the amorphous substrates for the growth of 1D nanomaterials is (i) utilizing solution-phase growth method to decrease the growth temperature for use of amorphous substrates due to their low melting temperature, (ii) adopting a local heating method for obtaining high growth temperature locally without damage of amorphous substrates, (iii) employing lattice-matched graphene seed layer on the amorphous substrates for satisfying the heteroepitaxial relationship. High-quality ZnO nanostructures were obtained on various substrates using solution-phase growth method. Low growth temperature of solution-phase growth method enables the use of Si, glass, and graphene substrates. Various morphology of ZnO nanostructures were obtained by controlling the growth kinetics and surface energies of crystal planes. Diverse ZnO nanostructures such as nanorod, nanoflower, and nanodisk were grown by changing the growth conditions such as growth time, temperature, concentration of chemicals, and pH of solution. Position-controlled growth of ZnO nanostructures was successfully demonstrated by employing the polymer growth mask to induce the selective nucleation at the desired locations. Single-crystalline ZnO nanorod arrays were grown on the Si substrate. The position-controlled growth of ZnO nanorod and microrod array was achieved using the pre-patterned organic mask layers on the Si substrates. Due to the lack of heteroepitaxial relationship between ZnO and mask layers and nucleation sites on the organic resist layer, ZnO nuclei grows only on the patterned sites, where the ZnO seed layer was exposed to nutrient solution. In addition to position control, the diameter and length of ZnO rod arrays was controlled by adjusting the size of polymer mask layers and growth conditions such as growth time, temperature, concentration of chemicals, and pH of solution. Electron microscopy and X-ray diffraction (XRD) revealed that the selectively grown ZnO microrod arrays are highly crystalline. From the micro-photoluminescence (PL) measurement, ZnO microrod arrays are of excellent optical quality. ZnO nanorod arrays were directly grown on the few-layer-graphene (FLG) substrate without a seed layer. The density, dimension and morphology of the ZnO nanorods showed a clear dependence on the growth temperature, molar concentration and pH of the nutrient solution. More importantly, as determined by transmission electron microscopy (TEM), the ZnO nanostructures grown on FLG sheets were single-crystalline due to possible heteroepitaxial relationship between them. From the PL and cathodoluminescence (CL) spectra, strong near-band-edge emission was observed without any deep-level emission, indicating that the ZnO nanostructures grown on FLG substrates were of high optical quality. Controlled selective growth of ZnO nanoflower arrays on glass substrates was achieved by using wet chemical method and pre-patterned growth organic mask. With using the patterned substrates, the morphology of ZnO nanostructures were drastically from nanorod to nanoflowers. From the TEM investigation, the ZnO nanoflowers are defect-free and highly crystalline. Low-temperature PL and I?V measurements revealed that the metallic behavior of ZnO nanoflower. ZnO nanodisks were grown on the various substrates coated with ZnO nanoparticles. With using citrate ion, while the vertical growth rate along ZnO(0001) was reduced, the lateral growth rate of ZnO induced the formation of hexagonal disk morphology. Monochromatic CL images showed that luminescence was spatially localized near the boundary of the nanodisk, and spectral analysis in conjunction with intensity profile consistently ascribed the spatial localization of luminescence to whispering gallery mode-like-enhanced emission. High-quality ZnO nanostructures were successfully obtained on the various substrates using vapor-phase growth method. Local microheater arrays were fabricated on glass substrates for obtaining high growth temperature locally without damage on the glass substrates. Graphene seed layers were prepared on the amorphous SiO2 substrates for providing single crystalline seed layers. Diverse morphologies of ZnO nanostructures such as nanorods, nanotubes, and nanowalls were achieved by changing growth conditions. Selective nucleation and growth of ZnO nanostructures were presented by introducing pre-patterned substrates, local heating method, and artificial defects on the substrates. ZnO nanorods were selectively grown on glass substrates using microheating method combined with catalyst-free metal-organic chemical vapor deposition (MOCVD) method. For the use of glass substrates, a microheating method using a series of microheaters was developed, which provided well-controlled local heating without increasing the surface temperature of the glass substrates. Accordingly, ZnO nanorod arrays were selectively grown at specific positions on glass substrates by using local microheating. As determined by TEM, the ZnO nanorods exhibited high quality single crystallinity. Additionally, strong excitonic emission was observed for the ZnO nanorods on glass substrates. ZnO nanorods were grown vertically on graphene layers using a catalyst-free metal-organic vapor phase epitaxy (MOVPE) method. The growth behavior of ZnO nanostructures on graphene layers was different from that of SiO2 surface, presumably due to the heteroepitaxial relationship between ZnO and graphene. The origin of ZnO nucleation on FLG sheets was discussed in terms of defect mediated crystal nucleation and growth. The morphology, aspect ratio, and number density of the ZnO nanostructures on FLG sheets depended strongly on the growth parameters including temperature, buffer layer, and flow rate of DEZn. Furthermore, interesting growth behavior, the formation of aligned ZnO nanoneedles in rows and vertically aligned nanowalls, was observed, presumably resulting from enhanced nucleation at graphene step edges. Micro-Raman spectroscopy revealed the direct growth of ZnO nanostructures on graphene layers and no evolution of structural defects during the growth. High-resolution TEM inspection of the interface between ZnO and FLG sheets displayed the heteroepitaxial growth of ZnO nanostructures on graphene layers. PL spectra of the ZnO nanostructures on graphene layers included a free exciton PL peak with no carbon-related defect peak, suggesting that high-quality ZnO nanostructures were grown on the graphene layers without deterioration of the graphene layers during MOVPE. From CL spectra and monochromatic CL image, free-excitonic emission was originated not from the surface of graphene but from the ZnO nanostructures. Position-controlled AlN/ZnO coaxial nanotube heterostructures were grown on Si substrates using a catalyst-free MOVPE method. ZnO nanotubular structure was formed by preferential growth of nanowalls along the edge of patterned mask layers. In order to control the spatial arrangement and morphology of ZnO nanowalls, SiO2 growth mask were prepared by conventional lithography and etching technique. The coaxial AlN/ZnO nanotube heterostructure arrays were fabricated by coating as-grown ZnO nanotubes with an AlN thin film using a low-pressure MOVPE system. HR-TEM analysis revealed the abrupt interface between the single-crystalline ZnO and polycrystalline AlN coating layer. Position- and morphology-controlled growth of ZnO nanowalls were demonstrated by the local oxygen plasma treatment on graphene layers without using any growth mask or metal catalyst. The nucleation sites of ZnO nanowalls were easily controlled by the area exposed to the oxygen-plasma treatment because ZnO nucleation and growth were enhanced at the O2 plasma treated surface of graphene and depressed at the chemically inert graphene surface. XRD and PL measurement revealed that high quality ZnO nanowalls formed on graphene layers. Three dimensional hybrid nanomaterials of ZnO nanostructures on graphene films were successfully synthesized. Large-area graphene films were prepared on Ni metal thin films on Si substrates using chemical vapor deposition (CVD) method. The morphology and dimensions of the ZnO nanostructures were highly dependent on the growth temperature. Photoluminescence indicated the high optical quality of ZnO nanowalls were grown on graphene films. UV absorption spectra of ZnO nanowalls on CVD-graphene exhibited good optical transparency in the visible range of light. Field emission device applications based on position-controlled ZnO nanostructure arrays and photocatalysis application of ZnO nanostructures on graphene films were presented. Field emission characteristics of the position-controlled ZnO nanostructure arrays were investigated and micro-display arrays based on the position- and morphology-controlled ZnO nanostructure arrays were proposed. Under frontside and back side illumination, the photocatalytic activity of ZnO nanowalls/graphene films transferred onto the quartz was investigated.
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