Ultrasmall CdS nanoparticles were prepared by the size-selective photoetching technique. The smallest size
of the CdS nanoparticles was obtained by the irradiation with the monochromatic light having the wavelength
of 365 nm, while the absorbance of CdS nanoparticles monotonically decreased, for the whole wavelength,
to null if the irradiation was performed using a wavelength less than 365 nm. The surface modification of
CdS nanoparticles with thiophenol caused the red-shift of the absorption spectra due to the electric interaction
between the bound thiophenol molecules and the CdS core, the degree being enhanced with a decrease in the
particle size. 1H NMR signals of thiophenol bound on the surface of the smallest CdS nanoparticles prepared
by 365-nm irradiation were shifted upfield compared to those obtained for the larger thiophenol-modified
CdS nanoparticles prepared by the longer wavelength irradiation, and the signals appeared in positions similar
to those of chemically synthesized CdS cluster molecules. The observation with transmission electron
microscopy revealed that the smallest thiophenol-modified CdS nanoparticles had an edged pyramidal shape
with a size of about 1.7 nm in contrast to the larger CdS nanoparticles of almost spherical shape. The particle
size and chemical compositions of the smallest thiophenol-modified CdS nanoparticles roughly agreed with
those of Cd32S14(SC6H5)36路4DMF, which has been reported to be one of the largest chemically synthesized
CdS cluster molecules. The similarity between the emission and excitation properties indicated that the smallest
thiophenol-modified CdS nanoparticles had the energy structure almost equal to that of Cd32S14(SC6H5)36路4DMF.
It was found that the size-selective photoetching of CdS particles prepared in Nafion films could be achieved only for particles having the particle sizes smaller than about 12 nm in the diameter. The particles greater than this threshold size were photocorroded until complete dissolution was attained.
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