Synthesis and Self-Assembly of Au@SiO₂ Core−Shell Colloids

Gold nanoparticles have been coated with amorphous silica to form spherical colloids with a core−shell structure. The thickness of silica shells could be conveniently controlled in the range of tens to several hundred nanometers by changing the concentration of the sol−gel precursor or the coating time. These core−shell colloids could serve as the building blocks to fabricate photonic devices. In one demonstration, we showed that these core−shell particles could be assembled into long-range ordered lattices (or photonic crystals) over large areas that exhibited optical properties different from those crystallized from silica colloids. Transmission spectra of these crystals displayed both features that correspond to the Bragg diffraction of a periodic lattice and the plasmon resonance absorption of gold nanoparticles. Reflectance spectra taken from these crystals only showed peaks caused by Bragg diffraction. In another demonstration, these core−shell colloids were assembled into chains of different configurations by templating against well-defined channels patterned in thin films of photoresist. As suggested by previous studies, the plasmon coupling between gold cores makes these structures ideal candidates for nanoscale waveguides with lateral mode sizes well below the optical diffraction limit.