GISAXS studies of the synthesis and growth of CdS quantum dots from constituent atoms in SiO2 matrix

Grazing incidence small angle x-ray scattering (GISAXS) was used to study the synthesis and size evolution of CdS nanocrystals formed in SiO2 substrate by successive multi-energy implantation of constituent elements (three different ion doses) and subsequent thermal annealing. The 2D GISAXS patterns comprised of quasi-isotropic half-rings with an interference maximum related to spatial correlation between isolated CdS clusters. The analysis with the Guinier plot was compared with a more advanced model, described by the local mono-disperse approximation (LMA) weighted by the assumed size distribution (Gaussian), a cluster form factor (spherical) and a structure factor S of the assembly. The largest CdS nanoclusters (Rg = 3.2 nm) were found in a sample implanted with the highest dose and annealed at 1000oC, with an average intercluster distance of 15 nm. The same dose at 800oC yielded slightly smaller size and intercluster distance, indicating similarly successful CdS synthesis. Comparison of various doses at the same Ta showed that the higher dose favors the formation of bigger clusters, positioned further apart. The obtained values were compared with high-resolution TEM and optical absorption results. The analysis of GISAXS patterns proved a good uniformity of the implanted layer and yielded reliable estimates of nanocrystallite average size, shape and intercluster distance, as well as the size and distance distributions, both in plane and in the perpendicular direction.