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 pro perties of CdS nanoparticles with 13 - 120 Angstrom diameter depend on their particle size and the way of surface stabilization. The mean Cd -S distance of thiolcapped CdS nanoparticles (13-40 Angstrom) increase s with decreasing particle size with respect to CdS bulk. In contrast, mean interatomic distances in polyphosphate-stabilized particles (30- 120 Angstrom) are slightly contracted. This demonstrates that the cova lently bound thiol ligands induce the nanocluster lattice expansion. F rom the temperature dependence of the mean-square relative displacemen t (MSRD), a slight stiffening of the Cd-S bond can be observed with de creasing particle size. The static part of the MSRD, reflecting static disorder of the Cd-S bond, exhibits a maximum al cluster sizes of abo ut 30 Angstrom. For particles larger than 30 Angstrom diameter, the in crease of the static disorder follows the change of the surface to vol ume ratio. This ratio also determines the size dependence of the asymm etry of the interatomic Cd-S pair potential. Furthermore, in CdS bulk and CdS nanoparticles with the thermodynamically favored wurtzite stru cture this asymmetry is significantly enlarged in comparison to nanopa rticles with zinc blende structure.