Synthesis And PL Investigations Of ZnO-based Three-dimensional Ordered Nanostructures

Research on ZnO has generated great interests in recent years for its promising versatile applications in optoelectronics, transparent conducting materials, sensors and biomedical sciences, especially on short wavelength light-emitting, UV lasing, due to its wide direct gap of 3.37eV, large exciton binding energy of 60 meV at room temperature. However, one important problem should be overcome before ZnO could potentially make inroads into the world of optoelectronics devices: the growth of p-type-conductivity ZnO crystals. Inspite of many decades of investigations, some of the basic properties of ZnO still remain unclear. For example, the nature of the residual n-type conductivity in undoped ZnO films, whether being due to impurities of some native defect or defects, is still under some degree of debate. Interpretations of PL and Raman spectra are still a subject of debates due to the complexity of ZnO microstructure. Therefore, the fabrication of ZnO nanostructures to investigate their RRS and PL spectra is very important.There is an increasing interest in three-dimensional (3D) ordered nanostructures due to their growing applications in separations, sensors, catalysisand bioscience, especially in photonic crystals (PCs). Photonic crystals, characterized by a periodically refractive index varying on lengths of the order of the light wavelength, can result in a photonic band-gap that blocks certain frequencies of photons. With controllable defects, photonic crystals have the ability to control the flow of light, which has gained a huge interest for possible applications: such as efficient collimators and all-optical microchips. Photonic crystals also provide an environment that strongly modifies the vacuum fluctuations and has novel influences on optical behaviors of emitting species embedded in photonic crystals. The study of this effect requires efficient emission in the medium and a photonic band gap (PBG) that overlaps the emission spectrum. PC composite materials are currently attracting much attention for their possible applications, and the physics associated with the interaction between their framework and electromagnetic radiationTemplate methods using colloidal crystals such as silica and polystyrene (PS) provide a simple and effective route for fabricating 3D ordered materials. In the last few years, various types of ordered composite material and ordered porous material have been synthesized using SiO₂ opal and PS opal. Although the fabrication of 3D ordered materials has been widely explored, there is still dearth of the systematical investigation of optical properties of these materials.Disorder is always present in artificial PC structures and causes considerable random scattering which strongly influences the optical spectra. However, current research is mainly focused on fabrication and optical characterization of stop-bands and there are only a few experimental reports concerning the effect of random scatteringThe aim of this work is to prepare ZnO-based three-dimensional ordered nanostructures by template methods using colloidal crystals and to investigate the effects of the ordered nanostructures on optical performance in these systems. The combination of a PCs structure and efficient emission of ZnO promises potential applications in in optoelectronics. Our works are as follows:1. We successfully synthesized ZnO-based three-dimensional ordered composite materials using SiO2-opal template and polystyrene (PS)-opal template.2. The UV PL peak of ZnO films with ordered nanostructures exhibit more significant red-shift and broadening than that of compact ZnO nanocrystal film with increasing excitation power, which is due to the stronger local heating effect in ordered ZnO nanostructures. A simple model based on laser heating effects is used to analyze UV PL of ZnO films, which agrees well with the experiment data. It was found that the electron-phonon coupling strengths determined by the ratio of second- to firstorder Raman scattering cross sections from the resonant Raman spectra agree with that determined by laser heating effects from PL spectra, which provides further evidence that our analyses are feasible.3. We have investigated resonant Raman scattering (RRS) and photoluminescence of ZnO inverse opal prepared by electrodeposition. The intensities of both 577 cm-1 Raman scattering and green emission get weaker after aging or UV laser irradiation. The RRS and green emission intensities are highly correlated for different samples, regardless of the means of sample treatment. According to the deposition-temperature and annealing-temperature dependences, we propose that the origins of the two peaks are related to surface hydroxide.4. We investigated the optical properties of 3D-ordered ZnO nanostructure using temperature-dependent and power-dependent photoluminescence (PL) spectra. Compared with compact ZnO film also prepared by electrodeposition, the 3D-ordered structure exhibits a marked increase of ultraviolet luminescence intensity and thermal activation energy. It was also proved that the PL red-shift with increasing excitation power and PL intensity-reduction at high excitation power is due to the laser heating effect in ZnO nanostructure. Furthermore, our results show that the optical properties of ZnO samples at high temperatures are very different from that at room temperature. At high temperature, ZnO samples exhibit larger thermal activation energies and lower exciton-LO-phonon coupling strengths.5. A remarkably enhanced yellow-green photoluminescence (PL) was observed from ZnO nanocrystals infiltrated into SiO₂ opal photonic crystals. It was clearly visible to the naked eye under the excitation of an Xe lamp and had substantially improved thermal stability over pure ZnO nanocrystals. The PL spectrum shape of a ZnO-SiO₂ composite opal can be modified by annealing an SiO₂ opal or choosing an SiO₂ opal with different lattice parameters. The enhancement of PL intensity is interpreted based on the dependence of the PL intensity on the size of SiO₂ microspheres as well as the anisotropy of the photoluminescence excitation (PLE) spectra.6. We systematically investigated the photoluminescence (PL) and transmittancecharacteristics of ZnO-SiO₂ opals with varied positions of the stop-band and film thicknesses. An improved ultraviolet (UV) luminescence was observed from ZnO-SiO₂ composites over pure ZnO nanocrystals under 325nm He-Cd laser excitation at room temperature. The UV PL of ZnO nanocrystals in SiO₂ opals with stopbands center of 410nm is sensitive to the thickness of opal films, and the UV PL intensity increases with the film thickness increasing. The PL spectra of ZnO nanocrystals in SiO₂ opals with stop-bands center of 570nm show a suppression of the weak visible band. The experimental results are discussed based on the scattering and/or absorbance in opal crystals. 7. The photoluminescence (PL) properties of ZnO films fabricated by electrodeposition were investigated by using annealing treatment at various temperatures and in different atmospheres. The PL spectra are composed of a dominant UV emission and a weak green emission at room temperature. Our experimental reveals that the optimum annealing condition for UV emission exists at 400 8C in N2 atmosphere. A correlation between the UV PL intensity and Raman scattering intensity is first observed below 500 8C both in N2 and 02 atmosphere under resonant excitation. We suggest the 575 cmK1 Raman peak is strongly affected by a non-radiative center, and the intense UV emission of ZnO annealed at 400 8C in N2 atmosphere is due to the reduction of this center. Two different green emission bands are observed, which show different dependences of the PL intensity on the annealing temperature.