Fabrication And Properties Of Multilayer-coated Core-shell Structural Monodisperse Spheres And Close-packed Structure

Highly mono-dispersed metal or metal coated spheres assemble to build photonic crystals (PCs) with large dielectric contrast, which has been demonstrated to have complete photonic band gap (CPBG) even in the optical wavelengths. In this paper, the preparations and properties of metal or metal coated spheres and dielectric-metallic PCs have been studied. This is expected to help build a novel kind of metallic PCs with complete photonic band gap (CPBG) in optical and near infrared wavelengths.Firstly, the research progresses, hotspots and prospects of PCs are briefly reviewed. Then, the narrow size distributed and size controllable sub-micrometer silica spheres, silver coated silica spheres (SiO₂@Ag) and silver coated silica spheres with silica nano-layers (SiO₂@Ag@SiO₂) are prepared. The spherical, monodisperse and optical properties of the spheres used for PCs are investigated with TEM, SEM, XRD, UV-Vis, XPS and EDS.The sol-gel process to fabricate submicrometer SiO₂ spheres for PCs use is investigated, which shows that the quality of SiO₂ spheres is cross-affected by the composition parameters, pH value and TEOS concentration. A novel method to synthesize monodisperse, narrow size distributed and size tunable SiO₂ spheres is developed by controlling the number density of SiO₂ seeds, the reaction conditions and multi-step adding of TEOS.The silver layer formation processes on the surfaces of SiO₂ spheres is investigated. The layer thickness, density and uniformity are determined by the sensitization, composition and reaction conditions. It is confirmed that, silver nanoparticles are deposited on the surface of SiO₂ sphere by the rule of nucleation-growth mechanism, and each reaction step must be controlled effectively to obtain dense and uniform silver layer. Based on these investigations, a novel silver coating method is proposed, in which the nucleation and growth steps are separated and controlled independently during the silver layer formation. Using this method, dense and uniformsilver layer with high cover degree and large dielectric contrast is prepared.The nucleation-growth processes and kinetics of silver nanoparticles deposition are investigated, and a precise silver deposition procedure is given. The layer thickness can be adjusted by the quantity of SiO2@Ag spheres. Dense and uniform silver layers in thickness from 5 to 20nm are achieved, which are examined and proved by XRD, EDS > XPS and TEM. A novel wet chemistry method is presented to prepare silica coating on the surface of SiO2@Ag spheres. It is found that the surface modification process plays a key role on the formation of silica coating. With APS followed by PVP, a surface modification process is proposed to improve silica nanoparticles deposition, prohibit SiO2@Ag spheres aggregation, and prevent the silver particles from desquamation and dissolution. With this method, dense and uniform silica coating is prepared on the surfaces of SiO2@Ag spheres, and the related surface co-modification mechanism is proposed. Furthermore, spherical, monodisperse and narrow size distributed SiO2@Ag@SiO2 PCs units are obtained, which can be used to fabricate metallic photonic crystals.The pulling self-assembled process and mechanism are fully investigated. It is found that ordered spheres appear firstly on the top area of the meniscus formed on the interface between baseplate and liquid level by the force of surface tension. With the solvent evaporating, spheres are transported to the meniscus area continually, which lead to the increase of the spheres volume fraction. When the volume fraction reaches a certain value near 74% (the compact stacking filling fraction), phase transformation like phenomenon occurs and then the ordered structure forms. Subsequently, further rearrangement of the spheres and densification of the sample finally result in the formation of PCs during the following desiccation processes. Based on the above mechanism, silica PCs of - cm2 area with single crystal of 0.01mm2 areas are fabricated by controlling process parameters of temperature, humidity and spheres volume fraction.Finally The metal-dielectric spheres SiO2@Ag@SiO2 with 300 nm in diameter crystallize directly into close-packed structure by self-origination method, and directional photonic band gaps (PBGs) for the photonic crystalwere found in optical and near infrared wavelengths. The photonic band gaps could be affected by the refraction index of metal coated silica spheres. When the sphere's refraction index has been increased, so does the photonic band gap. The surface plasmon resonance bands of Ag nanoparticles, which exist in the spectrum under the wavelength 900nm, is only shown at the special angle at and their bands position never would be changed by the test angle in PC's spectrum.