Preparation, Self-Organization And Properties Of SiO₂/Ag/SiO₂ Submicrometer Core-Shell Spheres For Photonic Crystals

Mono-dispersed metal or metal coated spheres are ideal 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.The preparations and properties of dielectric and metallic PCs units have been studied and some important conclusions and newly developed achievements have been obtained, which are expected to 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 submicrometer 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, FT-IR, XPS, BET and EDS. Finally, PCs of high qualities are self-organized with sedimentation, pulling and pressing methods and their optical properties are also studied.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 ofTEOS.The kinetics of the formation of SiO₂ spheres is studied, and the sol-gel and heat treatment procedures are optimized. The particle size is found to be as a function of the amount of TEOS, which is consistent with the experimental results. Thus, any size of SiO₂ spheres between 160nmand lum can be predicted and prepared, which is significant for the tunableness of the photonic band gap positions using different sizes of spheres.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 uniform silver layer with high coverdegree 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 Ag+ ions, which can be predicted by the experimental fitting equation. Dense and uniform silver layers in thickness from 5 to 40nm are achieved, which are examined and proved by XRD EDS XPS and TEM.The relationships between plasma resonance and particle interdistance of silver nanoparticles on the surfaces of SiO2@Ag spheres are investigated by UV-Vis and TEM. The absorption band is broadened and red shifted with the decrease in the particle interdistance, which can be explained by the dipole-dipole interaction between silver nanoparticles. Based on that, the relationship between the band changing and coverdegree of silver nanoparticles on the surface of SiO2 spheres is obtained. As a result, a novel method is proposed to estimate the coverdegree of silver nanoparticles on the surface of SiO₂ spheres.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...