High-resolution electron microscopy and microanalysis of cds and cdte quantum dots in glass matrices

CdS and CdTe quantum dots (QDs) precipitated from borosilicate glass matrices, exhibiting quantum confinement effects, have been characterized by optical spectroscopy and high-spatial-resolution electron microscopy. The particles appear to have formed initially by homogeneous nucleation and were randomly distributed, but the morphology of the particles and therefore growth processes differ. The CdTe particles were spherical single crystals, often with small facets on {111} planes, or at most bicrystals. The measured average diameter of these particles was about 14 nm. They had a zinc-blende cubic structure with lattice constant corresponding to bulk CdTe crystals. For CdS the particles were relatively large polycrystals, composed of smaller single crystals, whose size was about 3·3 nm. The structure of these nanocrystals was wurzite-type hexagonal, with a lattice corresponding to CdS. Particle-size effects on absorption peak shifts were observed for both CdTe and CdS samples. Microscopically observed QD sizes were compared with predictions of theoretical models based on optical absorption features of quantum confined structures.