Size-dependent structure and dynamics of CdS nanoclusters

CdS nanoparticles with diameters from 14 -120 Angstrom and very narrow size distributions are studied by x-ray absorption fine structure spectroscopy between 5 and 296 K. Clear trends in the Cd-S distance as a function of size and surface stabilization of the nanoparticles could be observed. Particles with thiols as ligands show an expansion of the Cd-S distance with decreasing particle size whereas polyphosphate stabilized samples are slightly contracted with respect to CdS bulk. The static disorder of the nanoparticles is enlarged with respect to the bulk. In contrast, the dynamic behavior of the nanoparticles shows a slight stiffening of the Cd-S vibration with decreasing particle size. Measurements of the third moment of the pair distribution function indicate the possibility to distinguish between cubic and hexagonal structures in CdS nanoparticles.