OPTICAL-PROPERTIES OF II-VI SEMICONDUCTOR NANOCRYSTALS PRODUCED BY SOL-GEL SYNTHESIS IN SODIUM BOROSILICATE GLASSES

Size quantization effects on excitonic optical transitions are studied by optical absorption experiments on sodium borosilicate glasses dope d with CdS, CdSe or CdTe nanocrystals. The original sol-gel synthesis procedure involves the preparation of complex cadmium-based molecules in an aqueous solution to which other inorganic precursors are added, in order to start the gelation of the vitreous matrix. The formation o f semiconductor crystallites is achieved by a redox reaction, during t he thermal degradation of the gel, which is then melted and quenched t o provide the glass. The effects of thermal annealing, at temperatures around and above the T-g of the glass, are investigated by spectrosco pic measurements. The latter are analysed by using a calculation of ex citonic energies and oscillator strengths, based on the envelope-funct ion approximation, including the confinement-induced mixing of valence -band states. Our studies show that disorganised clusters with a rathe r narrow size-distribution, controlled by initial conditions, are pres ent before the annealing, which rapidly improves the crystalline quali ty. Meanwhile, the width of the size-distribution function is reduced down to an optimum, after a few hours of treatment.