Preparation And Characterization Of Hfo2-based Rare Earth Doped Nanomateriais And Graphene Films

Low-dimensional nanomaterials have drawn continuous research attention because of their unique properties derived from their unique nanostructures, as well as their potential applications caused by the properties.In this dissertation, we prepared RE (rare earth) doped HfO2based nanomaterials, and studied the luminescent properties and the whith light emissions of the RE doped HfO2-based nanomaterials; at the same time, we prepared the large-area monolayer and bilayer graphene by chemical vapor deposition (CVD) process, and investigated the research issue of how to improve the size of graphene domains. The main points of this dissertation are listed as follows:(1) One-dimension uniaxially aligned HfO2nanotubes are prepared by radio frequency sputtering with electrospun polyvinylpyrolidone (PVP) nanofiber templates. The high temperature annealed monoclinic HfO2nanotubes have uniform intact structure with an average outer diameter of around200nm and a wall thickness of about25nm, which is orderly uniaxially aligned. The crystal quality of HfO2nanotubes are increased with increasing the annealing temperature. In addition, ZnO/HfO2:Eu core-shell nanocables are synthesized by the method of HfO2nanotubes, only ZnO nanowires substitute for the PVP nanofibers. The ZnO/HfO2:Eu nanocables show a uniform intact coaxial core-shell structure, the wall thickness of monoclinic HfO2shell is around50nm, the diameter of hexagonal ZnO core around120nm.(2) Photoluminescence (PL) properties of the RE (rare earth) doped HfO2(HfO2:Eu and HfO2:Tb) nanotubes and RE co-doped HfO2(HfO2:Eu&Tb) nanotubes have been studied in detail. The visible light emission peaks of the HfO2:Eu and HfO2:Tb nanotubes correspond to the5D0→7FJ (J=0-2) transitions of Eu3+and the5D4→7FJ (J=3-6) transitions of Tb3+, respectively. Comparing the PL spectrum of the annealed non-doped and RE doped HfO2films with nanotubes, we find the emission peaks (including the defect emissions and RE emissions) PL intensities of the HfO2:RE3+ nanotubes are higher by several orders of magnitude than that of the films. This enhancement in the PL can be ascribed to the high density of surface states of HfO2:RE nanotubes. According to this result, we investigated the PL mechanism of the RE doped HfO2nanotubes. Accordingly, the energy-transfer mechanism in the HfO2:RE nano-materials can be explained by the defect-related Auger transition model.(3) White-light emission of the HfO2:Eu&Tb nanotubes and ZnO/HfO2:Eu nanocables were studied. Through tuning the concentrations of the RE ions in the HfO2host, white-light emissions are achieved by tuning the proportion of the red, green, and blue three basic colors. The blue emission drive from the HfO2host defects, red and green emissions originate from the inner4f shell transitions of corresponding Eu3+ions and Tb3+ions, respectively. When the molar ratio of Eu/Tb/Hf=1.69/6.08/92.23, good white-light emission with commission International de I'Eclairage (CIE) coordinates (0.333,0.323) and color temperature (Tc)(5465K) are achieved in the HfO2:Eu&Tb nanotubes. Additionally, White-light band emission in ZnO/HfO2:Eu nanocables is designed, in which blue, green, and yellow emissions derive from ZnO defects and yellow and red emissions come from HfO2:Eu. The blue, green, and red emissions in the ZnO/HfO2:Eu nanocables are all greatly enhanced by the high desity of nanostructure defects, which is similar with the RE doped HfO2nanotubes. Therefore, the RE doped HfO2based nano-materials are suitable for the high efficient luminescent materials.(4) A systematic study on growth parameters in an atmospheric pressure chemical vapor deposition (APCVD) process show that the mean size of the graphene domains increases with increasing growth temperature and CH4partial pressure, while the density of domains decreases with increasing growth temperature and is independent of the CH4partial pressure. Our studies show the graphene domain density is highly dependent on the initial annealing temperature on Cu substrate, and is almost independent with the growth time. According to this analysis, an improved two-step synthetic process is developed to reduce domain density and achieve high quality full-surface coverage of monolayer graphene films. Electrical transport measurements demonstrated that the resulting graphene exhibits a high carrier mobility of up to3000cm2V-1s-1at room temperature.(5) We demonstrated a novel method to directly produce large-area AB stacking bilayer graphene by a low pressure CVD (LPCVD) on Cu substrates. The self-limiting effect of graphene growth on Cu foil can be broken through the introduction of a high H2/CH4ratio, which can saturate the growth of the graphene and make the Cu substate exposed on the upstream end. Bilayer graphene were epitaxially grown on downstream monolayer with the carbon fragments continuously supplied by the uncovered upstream Cu substrate. According to a systematic analysis of the dependence of the bilayer coverage and AB stacking ratio on the growth parameters, a rational design of a two-step CVD process was developed to achieve high coverage (>95%) bilayer graphene with a high AB stacking ratio exceeding90%. The bilayer graphene showed excellent high carrier mobility up to1700cm2/V·s at room temperature.(6) In order to reduce the influence of the domain interfaces on the quality of the graphene, we controlled the graphene nucleation density by a high H2/CH4ratio and annealing the Cu substrate in the Ar ambient, and firstly achieved graphene with millimeter-size domains on Cu substrates. The monolayer graphene domain size is～2mm, and bilayer graphene domain size is～0.2mm. This is a cost-effective, high-throughput method, and Cu foil is also cheap, so it's really suitable for the growth high quality large area graphene.