INFLUENCE OF CD CONTENT AND SE DOPING ON THE FORMATION OF CDSE NANOCRYSTALS IN SILICA XEROGELS - A SAXS STUDY

Small-angle X-ray scattering (SAXS) experiments were carried out to ch aracterize the structure of the composite formed by CdSe nanocrystals embedded in a porous silica matrix (silica xerogels containing Cd with formamide addition and ultrasound treatment). SAXS results from sampl es before Se diffusion indicate the presence of heterogeneities with a bimodal size distribution which was associated to the existence of me sopores (pores of several hundred A) immersed in a nanoporous matrix ( characteristic pore radii of 20-30 Angstrom). The diffusion of Se indu ces the nucleation and growth of CdSe nanocrystals. The average size o f the nanocrystals increases with Cd content. Higher Se doses promote the formation of larger nanocrystals (radius of gyration of similar to 30 to 50 Angstrom). Anomalous scattering results confirm the existenc e of Se aggregation associated with CdSe nanocrystal formation and sug gest that only partial segregation of Cd and Se occurs.