Fabrication, Properties And Self-assembly Of Building Blocks For 3D Photonic Crystals

Photonic crystals (PCs) have attracted much attention because of their ability to manipulate, confine, and control light. In this dissertation, three classes of monodisperse spheres, including SiO₂ spheres, SiO₂@CdS core-shell spheres and CdS hollow spheres, have been fabricated by chemical routes for using as building blocks for PCs. They have been self-assembled into three-dimensional (3D) face-centered-cubic (FCC) PCs. The optical properties of these self-assembled PCs have been investigated, and were compared with the theoretical calculations. Moreover, two methods of introducing well-defined defects in opal and CdS inverse opal PCs have been developed. Finally, monodisperse luminescent rare-earth doped spheres and luminescent rugby-like ZnO particles for photonic probes have been prepared, and their optical properties have been investigated. The significant results achieved in this dissertation are given as below:1. We have modified and optimized the traditional Stober method for preparation of monodisperse silica spheres. The obtained silica spheres are highly uniform with diameters ranging from 150-900nm and with relative standard deviation less than 5.0%. Opal PCs composed silica spheres have been fabricated by several self-assembly techniques, including gravity-sedimentation, centrifugation and vertical dip-coating methods, whose pseudo bandgaps have been controllably adjusted from blue to red optical regions by varying the diameters of the silica spheres from 190 nm to 310 nm. Moreover, well-defined micrometer-sized defects have been embedded in the interior of the opal PCs using a vertical dip-coating method.2. We have developed an ultrasound-assisted chemical bath depositon (CBD) method for the fabrication of monodisperse SiO₂@CdS core-shell spheres. Using this method, the thickness of the shell can be flexibly controlled from 10 nm to 60 nm by adjusting the reaction time, while substantially eliminating the unwanted separated CdS nanoparticles. The obtained SiO₂@CdS core-shell spheres were highly uniform with homogenous and dense CdS shells. It is believed that the ultrasonic irradiation plays a key role for the formation of homogenous and dense CdS shell onto the silica cores. Subsequently, monodisperse CdS hollow spheres were obtained by selectively dissolving the silica cores with a diluted HF aqueous solution. The obtained CdS hollow spheres are highly uniform, mechanically robust, and possess high refractive-index (n=2.45) and high air-filling ratio. Thesehollow spheres thus provide ideal building blocks for 3D semiconductor PCs. Both SiO2@CdS core-shells and CdS have been self-assembled into FCC structures by centrifugation, confirming the high quality of these building blocks for photonic applications.3. We have developed a simple way to fabricate inverse opal PCs. The method involves directly self-assembling of CdS hollow spheres into FCC PCs from solution and subsequently annealing at 400 °C to minimize the interstitial spaces between the spheres. With this method, the stable CdS inverse opals with high filling ratio and the size as large as 20umx20um have been obtained. It directly uses the high refractive-index semiconductor material as building blocks to self-assemble into FCC PCs, thus overcoming some of the problems associated with traditional templating methods, such as sophisticated filling process and destructive template-removal process.The micro-reflectance spectrum shows that the CdS inverse opal composed 400-nm hollow spheres possesses two pseudo bandgaps at 530 nm and 920 nm along [111] direction, which is consistent with the theoretical photonic bandgap calculations.4. We have developed a simple and straightforward method of precisely fabricating point defects in CdS inverse opal PCs with a variable pressure scanning electron microscope. Well-defined point defects, not only vacancies but also an individual impurity, were directly fabricated by electron-beam irradiation under a gas atmosphere. This method has proven extensively practicable for precisely processing many other materials, such as ZnO and Si. Judging from the various advantages, including high resolution (<200 nm), the versatility of the fabrication process and the convenient in-situ control of e-beam for both observation and defect fabrication, we believe that this method holds great promise for the development of 3D PBG-based devices.5. We have developed a versatile yet simple approach for the fabrication of monodisperse luminescent beads doped with rare-earth (RE). Using a negatively charged CdS porous shell surrounding silica cores as a host matrix, trivalent RE ions have been electrostatically adsorbed into the CdS/silica core-shell beads in the suspension. After annealing at 750 °C for 2 h, the RE-doped core-shell beads are encode with sharp and strong luminescence of RE3+. By varying the RE species, including Tb3+, Eu3+, Nd3+, Er3+ and (Tb3++Yb3+), a wide varity of characteristic luminescences of RE3+ from visible to near-infrared region havebeen readily encoded into the core-shell beads. These highly luminescent and uniform RE-doped beads thus provide a new class of spectrum-rich photonic probes or light source for photonic applications.6. Uniform ellipsoidal ZnO microparticles have been synthesized in aqueous solution by sonication at the temperature below 80 °C. The obtained ellipsoidal particles are highly uniform with a hexagonal cross-section. The morphologies of the ZnO particles have been tailored from rugby-like ellipsoidal to half-ellipsoidal by increasing the TEA concentration. Moreover, a significant enhancement of ultraviolet (UV) emission has been observed in ZnO by a thermal treatment at 200 °C. Based on the thermal desorption spectroscopy results, the origin of this enhancement effect was attributed to the reduction of non-irradiative centers and hydrogen passivation through desorption of adsorbed water and hydroxyl groups. Finally, we have developed a simple technique of directly writing sub-micrometer UV emission patterns in ZnO that were prepared using a wet-chemical method. The technique utilizes an electron beam in SEM to precisely control the local desorption to enhance the UV emission in the ZnO samples. With this technique, we have not only created optical nanotags on individual ZnO nanorods, but have also written sub-micrometer (-400 nm) UV-emission patterns on ZnO films, while keeping the surface morphology unchanged. This patterning technique is a straightforward and highly efficient method without the use of sophisticated lithographic processes, and has proven extensively applicable in various chemically-grown ZnO samples.