P. Lefebvre et al., OPTICAL-PROPERTIES OF II-VI SEMICONDUCTOR NANOCRYSTALS PRODUCED BY SOL-GEL SYNTHESIS IN SODIUM BOROSILICATE GLASSES, Superlattices and microstructures, 15(4), 1994, pp. 447-451
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.