Synthesis and Self-Assembly of Metal-Coated Nanoparticles
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Synthesis and Self-Assembly of Metal-Coated Nanoparticles W. Park and T. Borsa Department of Electrical & Computer Engineering University of Colorado, Boulder, CO 80309-0425, U.S.A. ABSTRACT We report theoretical and experimental studies on the metal-dielectric photonic crystal structure, which is constructed by the self-assembly of metal-coated nanoparticles. The finitedifference time-domain (FDTD) simulations were carried out to predict the width and position of the 3D photonic bandgap. For fabrication, we first prepared gold-embedded silica nanoparticles containing extremely small gold nanoparticles. The core particles are then immersed in a goldcontaining solution in which the gold shells were formed by subsequent reduction. Formation of continuous shells was confirmed by scanning electron microscope (SEM) and the optical response due to the surface plasmons. We also performed self-assembly of both the goldembedded core particles and metal-coated particles. SEM micrographs showed the formation highly ordered structures. The optical reflectance spectra exhibited shifts of the Bragg reflection peak due to the change in refractive index produced by gold nanoparticles. INTRODUCTION Since Yablonovitch [1] and John [2] independently proposed the concept of photonic crystal (PC) in 1987, there has been extensive research on new optical phenomena observed in PC structures. One of the greatest excitements about PCs stems from the potential to achieve strong light localization and subsequent suppression/enhancement of spontaneous emission, which has long been believed an unalterable, intrinsic quantity. Realization of this hallmark phenomenon of cavity quantum electrodynamics requires an extremely high Q cavity with small mode volume. PCs can also be used to realize low-loss, sharp bend waveguides. Combined with high Q nanocavities, they will form the platform for highly integrated nanophotonic circuits. 3D PC structures can exhibit full 3D photonic bandgaps (PBGs) and therefore provide complete light control and confinement in, for example, 3D nanocavities and 3D photonic interconnects. This recognition naturally directs attention to the nanoparticle self-assembly technique which assembles highly mono-dispersed nanoparticles into fcc close-packed structure. Nanoparticle self-assembly provides a simple, fast and economical way to fabricate 3D PCs in which the fullest effects of photonic band formation are expected. Recent advances in this field enabled large-scale production of high quality artificial opals. Jiang et al. used a convective selfassembly process to produce high quality opals with well-controlled thickness [3]. An alternative approach was also developed in which forced sedimentation in a confinement cell was utilized for opal synthesis [4]. Theoretical studies showed that complete 3D PBGs are possible in inverse opal structures with background dielectric materials possessing refractive index greater than 2.8. Therefore, it has been extensively investigated to develop processes for infiltrating and inv
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