Monodispersed silica spheres with diameters of 100-500 nm were prepared by hydrolysis of tetraethyl orthosilicate (TEOS) in the presence of water and ammonia in ethanol medium. Silica colloidal sphere sizes were controlled by changing ammonium concentration at fixed TEOS and water concentrations. These silica spheres settle out from their alcoholic dispersion to form closely packed three-dimensional (3-D) particle arrays-i.e., photonic crystals that exhibit vivid optical diffraction due to a high degree of ordering of the particles. The lattice constant of the opal photonic crystal structure, as determined by optical diffraction, was found to be in a good agreement with that observed by SEM measurements. Highly purified silica nanospheres could also self-assemble to form 3-D crystalline colloidal arrays (CCAs) in water, which give narrow and continuously tunable diffraction peaks in the visible range. By grafting hydrocarbon chains on silica surfaces using the silane coupling agent octadecyltrimethoxysilane, hydrophobic silica nanospheres were obtained, and they readily self-assemble at the air-water interface to form closely packed two-dimensional (2-D) particle arrays. Such 2-D particle arrays could be collected onto solid substrates to form monolayer or multilayer thin films with controlled film thickness according to the particle size.