Résumé
An original sol-gel technique is used to prepare sodium borosilicate glasses doped by II-VI semiconductor nanocrystallites. A complete study of CdS and CdSe crystallites is presented, as well as preliminary results on CdTe crystallites. The 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. In the cases of CdSe and CdTe, selenious and telluric acids, respectively, added to the starting solution induce the formation of crystallites by a redox reaction during thermal degradation of the gel. This gel is then melted and quenched to provide the glass. The effects of further thermal annealing, at temperatures around and above the T
of the glass, are observed by optical absorption measurements. The latter are analyzed by using a theoretical calculation of excitonic energies and oscillator strengths, based on the envelope-function approximation. This model includes the confinement-induced complex mixing of valence-band states. Our results show that disorganized clusters with a rather narrow size-distribution controlled by initial conditions are present before the annealing. The latter quickly improves the crystalline quality. Meanwhile, the average size is slightly increased and the width of the size-distribution reduced down to an optimum, after a few hours (less than 10). For instance, CdS crystals of approximately 4 nm in diameter, with a half width at half maximum of the size distribution of approximately 0.23 nm are obtained after 6 hours of annealing.