단분산 SiO2 나노분말의 제조와 자기조립 광자결정의 광특성

Abstract

This study investigated the effects of particle growth mechanism, the variables of self-assembly, and defect healing, using SiO2 nanoparticles, to control the characteristics of self-assembled photonic crystal (PC). The particle growth mechanism was revealed to be nucleation and growth mechanism enabling the average size and size distribution of the SiO2 nanoparticles to be controlled. We were also able to precisely control the photonic bandgap of PC self-assembled via the evaporation method. We healed the defects and enhanced the optical properties of PC through the heat-treatment. It is important in particle fabrication to obtain a narrow size distribution and to control the average size of SiO2 nanoparticles. The well-known St?ber process was used to investigate the effect of process variables on the particle fabrication and growth mechanism of the particles. In all process, the concentrations of TEOS and NH3 were kept at 0.4 and 0.7 mol, respectively. Reaction temperature and H2O/EtOH mole ratio were varied over the range 40℃~70℃ and 0.1~0.5, respectively. The particles obtained at 60℃ showed the narrowest size distribution and were subsequently used in the self-assembly process. The formation and growth mechanisms were investigated by observing particle shape during the reaction. The reaction of TEOS was terminated within 26 minutes. After 2 minutes, spherical particles with smooth surface were observed through TEM observation. The nucleation proceeded continuously up to 12 minutes. Addition of particles with 310 nm average size did not affect the nucleation and growth behavior. The observation results were interpreted as being diffusion growth of monomer addition and hence were well matched with LaMer and Dinegar theory. During the seed addition experiment, monomodal distribution was obtained between 0.50 and 1.50 wt% and bimodal between 0.05 and 0.10 wt%. PC, which was fabricated by the particles of average size 197~360 nm, showed a photonic bandgap between 435 nm and 780 nm. A well defined photonic bandgap was obtained with particles of average size 318 and 360 nm. Thicker PC layers were fabricated on the slide glass, with a contact angle smaller than ITO glass. Varying the content of the particles in solution between 0.4 and 0.8 wt% produced better optical properties. Evaporation temperature showed no effect on optical properties for the slide glass. However, in ITO glass, the best optical properties were obtained between 80℃ and 90℃. Self-assembled PC features various defects which we tried to overcome through heat-treatment. When the self-assembled PC was heat-treated between 250℃ and 300℃, the optical properties were improved. Microstructure observation showed the healing of defects and the initiation of several cracks. The heat-treatment reduced the particle size and hence moved the photonic bandgap to the shorter wavelength region.