Narrowing the size distribution of polydisperse CdS nanocrystals whose average diameter was 42 Å and standard deviation was 19 Å was achieved utilizing size selective photocorrosion with sequential irradiation with monochromatic light whose wavelength was changed step by step from 490 to 430 nm in air-saturated sodium hexametaphosphate solution. With decreasing the wavelength of irradiated light, the first exciton peak was gradually developed in the absorption spectrum of the resulting CdS colloid, and the nearly monodisperse CdS nanocrystals of 22 Å were finally obtained, which were thought to be the smallest particles that were present in the original CdS colloid. Analyses of the amount of sulfate ions produced by photocorrosion of Q-CdS colloids revealed that the number of Q-CdS particles in the colloid decreased with promotion of photocorrosion, suggesting that during the course of photocorrosion photocorroded CdS particles were agglomerated to give larger particles which were further photocorroded. The molar absorption coefficient of CdS particles at the first exciton peak was found to be independent of the particle size.

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)36·4DMF, 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)36·4DMF.

Positively charged CdS nanoparticles having diameter of 3.0 ± 0.2 nm were prepared by the chemical modification of their surfaces with thiocholine. Chains of size-quantized CdS nanoparticles were prepared by using the electrostatic interaction between positively charged nanoparticle surfaces and the phosphate groups of DNA molecules. The observation by transmission electron microscopy revealed that the CdS nanoparticles were arranged in a quasi one dimension with dense packing. The line width of a nanoparticle array was equal to the diameter of CdS nanoparticles that was ca. 3.0 nm. The average distance between the centers of the adjacent nanoparticles was estimated to be 3.5 nm, which was almost equal to the length of 10 base pairs in DNA double strands.

Two-dimensionally organized CdS nanoparticle films were prepared with the use of the Langmuir−Blodgett (LB) technique. 2-Aminoethanethiol-modified CdS nanoparticles were spread on a water subphase which contained glutaraldehyde as a cross-linking agent for binding the surface-modified CdS nanoparticles with each other, and the cross-linking reaction was undertaken under several different compressions of the CdS nanoparticle layer spread on the water subphase. Surface pressure−area isotherms of the CdS monoparticulate layer taken after the cross-linking were largely different depending on the area used for the cross-linking. By transfer of the CdS monoparticulate film prepared at the air−water interface to the 2-aminoethanethiol-modified gold substrate using the LB technique, CdS monoparticulate films containing different surface concentrations were prepared on the gold substrate. It was easily done to cumulate the monoparticulate film on the gold substrate using the same transfer technique. The prepared CdS nanoparticles films showed n-type photosensitivities which were enhanced by increasing the number of cumulation of monoparticulate films.

Size-quantized ZnS thin films were prepared by sequential underpotential deposition of S and Zn on Au{111} substrates. Observations of the high-resolution electron microscope and the energy-dispersive X-ray spectrometer revealed that ZnS films were epitaxially deposited on the Au{111} facet with a stoichiometric composition. The scanning tunneling microscope images showed that the surface of the prepared films was flat in an atomic level and no large protrusion was contained, while etch pits with the height of the monatomic step of Au{111}, which were formed in the UPD of the first S layer on the naked Au surface, were seen through ZnS films with independence of the number of ZnS layers deposited. The ZnS film-deposited Au electrodes showed anodic photocurrents in an aqueous solution containing triethanolamine as a sacrificial electron donor. The energy gap determined from the action spectra decreased with an increase of the film thickness up to ca. 15.5 ZnS layers at which the bulk band gap was obtained.