EXAFS studies on the size dependence of structural and dynamic properties of CdS nanoparticles

Size-dependent structural and dynamic properties of CdS nanocrystals with 13-120 Angstrom diameter and of molecular crystals consisting of three-dimensional superlattices of these nanoparticles have been determined by extended X-ray absorption fine structure spectroscopy (EXAFS) at the Cd K edge at temperatures between 5 and 290 K. It is shown that these properties are governed by the surface-to-volume ratio and the way of surface stabilization of the nanoparticles. Thiol-capped CdS nanoparticles with diameters from 13 to 40 Angstrom show an expansion of the mean Cd-S distance whereas mean interatomic distances in polyphosphate-stabilized particles with 30-120 Angstrom diameter are slightly contracted with respect to CdS bulk. By measuring the EXAFS spectra between 5 and 290 K, the total mean-square relative displacement could be separated into a static part which is independent of temperature and into a dynamic temperature-dependent part. The temperature-independent mean-square relative displacement (or static disorder) of the Cd-S bonds exhibits a maximum at cluster sizes of about 30 Angstrom separating thermodynamically and kinetically controlled growth regimes. Vibrational amplitudes of the Cd-S bonds are only very slightly damped with decreasing particle size. Careful investigations of the asymmetry of the interatomic Cd-S pair potential indicate the possibility to distinguish between CdS nanoparticles with zinc blende and wurtzite structure in cases where alternative methods such as powder X-ray diffraction or transmission electron microscopy fail.