SEMICONDUCTOR CLUSTERS IN THE SOL-GEL PROCESS - QUANTIZED AGGREGATION, GELATION, AND CRYSTAL-GROWTH IN CONCENTRATED ZNO COLLOIDS

A new synthesis of ZnO wurtzite clusters (crystallite sizes 3-6 nm) is presented. This novel approach, employing ultrasound, allows one to produce relatively highly concentrated 0.1 M Q-ZnO colloids within a few minutes. These colloids remain in a dispersed state for weeks. They can be further concentrated into stable syruplike liquids (molarities of approximately 1 M). Under the extreme concentration conditions (10 M Q-ZnO colloids) employed, ZnO alcogels are formed, and self-induced crystal growth occurs in the alcogels (sizes ranging between 1 and 5 mm). Furthermore, the small crystallites can be cast on porous or nonporous supports and fired to obtain ZnO ceramic membranes and thin films. All ZnO materials investigated were colorless and did not opalesce, while they exhibited a bright luminescence when exposed to UV light. Progressive concentration from dilute sols to the compact crystal state produced structured luminescence excitation spectra with magic maxima ranging from 250 and 400 nm. In dilute suspensions, excitonic transitions in ''primary'' clusters can be observed. In concentrated suspensions, additional excitonic levels appear, attributed to the appearance of primary cluster aggregates. Gelation and crystal growth produce further excitonic levels. This process is explained as a growth of secondary cluster aggregates. One can destroy the crystals in an ultrasound field followed by dilution of the alcogels until the original spectroscopic properties from primary clusters are restored. An electronic correlation diagram and crystal growth mechanism, both based on aggregation, are proposed.