Size dependence of structural and dynamic properties of CdS-nanoparticles

Temperature-dependent EXAFS (Extended X-ray Absorption Fine Structure) measurements between 5 and 290 K show that structural and dynamic properties of CdS nanoparticles with 13-120 Å diameter depend on their particle size and the way of surface stabilization. The mean Cd-S distance of thiolcapped CdS nanoparticles (13-40 Å) increases with decreasing particle size with respect to CdS bulk. In contrast, mean interatomic distances in polyphosphate-stabilized particles (30-120 Å) are slightly contracted. This demonstrates that the covalently bound thiol ligands induce the nanocluster lattice expansion. From the temperature dependence of the mean-square relative displacement (MSRD), a slight stiffening of the Cd-S bond can be observed with decreasing particle size. The static part of the MSRD, reflecting static disorder of the Cd-S bond, exhibits a maximum at cluster sizes of about 30 Å. For particles larger than 30 Å diameter, the increase of the static disorder follows the change of the surface to volume ratio. This ratio also determines the size dependence of the asymmetry of the interatomic Cd-S pair potential. Furthermore, in CdS bulk and CdS nanoparticles with the thermodynamically favored wurtzite structure this asymmetry is significantly enlarged in comparison to nanoparticles with zincblende structure.