Recent developments in the preparation and properties of nanometer-size spherical and platelet-shaped particles and composite particles

By using self-assembly molecules as a template, nanometer-sized plate-like metal oxide and semiconductor particles can be obtained by confined growth inside the lamellar bilayers of microemulsion systems. It was found that a strong chemical affinity between the metal salt and the polar head group of amphiphilic molecules and the anisotropic structure of microemulsion systems play a premier role in the anisotropic growth. Nanometer-sized composite particles (nano-composites) with a core-shell structure have been prepared by arrested precipitation of metal or semiconductor clusters in reverse micelles, followed by hydrolysis and condensation of organometallic precursors in the microemulsion matrices. Temporally discrete nucleation and growth at elevated temperature (70 degrees C) give the resulting particles a narrow size distribution and defined crystallinity. Both the size of the core particles and the thickness of the coating layers can be varied by controlling processing parameters such as the ratio of water to surfactant and the ratio of water to organometallic precursors. By controlling the pH conditions and aging temperatures, a transparent gel composing the nanometer-sized inorganic clusters has been obtained. Optical properties of nanometer-sized composite particles are reviewed. For silver metal clusters and nano-composites, the shift of the absorption peak at the surface-plasmon resonance frequency due to the classical limited mean-free path of the conduction electrons or quantum size effects has been observed. The enhanced third-order non-linear susceptibility of the silver nano-composites results from the local-field enhancement and size effects, which has been experimentally demonstrated by the optical phase-conjugation technique. (C) 1998 Elsevier Science S.A. All rights reserved.