J. Rockenberger et al., EXAFS STUDIES ON THE SIZE DEPENDENCE OF STRUCTURAL AND DYNAMIC PROPERTIES OF CDS NANOPARTICLES, JOURNAL OF PHYSICAL CHEMISTRY B, 101(14), 1997, pp. 2691-2701
Size-dependent structural and dynamic properties of CdS nanocrystals w
ith 13-120 Angstrom diameter and of molecular crystals consisting of t
hree-dimensional superlattices of these nanoparticles have been determ
ined 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 na
noparticles with diameters from 13 to 40 Angstrom show an expansion of
the mean Cd-S distance whereas mean interatomic distances in polyphos
phate-stabilized particles with 30-120 Angstrom diameter are slightly
contracted with respect to CdS bulk. By measuring the EXAFS spectra be
tween 5 and 290 K, the total mean-square relative displacement could b
e separated into a static part which is independent of temperature and
into a dynamic temperature-dependent part. The temperature-independen
t mean-square relative displacement (or static disorder) of the Cd-S b
onds exhibits a maximum at cluster sizes of about 30 Angstrom separati
ng thermodynamically and kinetically controlled growth regimes. Vibrat
ional amplitudes of the Cd-S bonds are only very slightly damped with
decreasing particle size. Careful investigations of the asymmetry of t
he interatomic Cd-S pair potential indicate the possibility to disting
uish between CdS nanoparticles with zinc blende and wurtzite structure
in cases where alternative methods such as powder X-ray diffraction o
r transmission electron microscopy fail.