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 confin ed growth inside the lamellar bilayers of microemulsion systems. It wa s found that a strong chemical affinity between the metal salt and the polar head group of amphiphilic molecules and the anisotropic structu re of microemulsion systems play a premier role in the anisotropic gro wth. 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 ma trices. Temporally discrete nucleation and growth at elevated temperat ure (70 degrees C) give the resulting particles a narrow size distribu tion and defined crystallinity. Both the size of the core particles an d the thickness of the coating layers can be varied by controlling pro cessing parameters such as the ratio of water to surfactant and the ra tio of water to organometallic precursors. By controlling the pH condi tions and aging temperatures, a transparent gel composing the nanomete r-sized inorganic clusters has been obtained. Optical properties of na nometer-sized composite particles are reviewed. For silver metal clust ers and nano-composites, the shift of the absorption peak at the surfa ce-plasmon resonance frequency due to the classical limited mean-free path of the conduction electrons or quantum size effects has been obse rved. The enhanced third-order non-linear susceptibility of the silver nano-composites results from the local-field enhancement and size eff ects, which has been experimentally demonstrated by the optical phase- conjugation technique. (C) 1998 Elsevier Science S.A. All rights reser ved.