Patterning porous matrices and planar substrates with quantum dots

Silica hydrogels and planar substrates were patterned with CdS nanoparticles using a photolithographic method based on the photo dissociation of thiols and cadmium-thiolate complexes. Silica hydrogels were prepared via a standard base-catalyzed route. The solvent was exchanged with an aqueous solution of CdSO4 and 2-mercaptoethanol, and the samples were then exposed to a focused ultraviolet beam. Planar substrates were patterned by illuminating a precursor solution spin coated on the substrates. CdS nanoparticles formed in the illuminated spots, and had a diameter below about 2 nm. The diameter of the spots illuminated by the UV beam could be varied from a few hundred to a few mu m, on both hydrogels and planar substrates. Samples were characterized with transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and optical absorption, photoluminescence and Raman spectroscopies. All these techniques confirmed the chemical identity of the CdS nanoparticles. To investigate the mechanism of nanoparticle formation, we took absorption spectra of the precursor solution as a function of irradiation time. In unirradiated solutions, we noticed a maximum at 250 nm, characteristic of Cd-thiolate complexes. The absorption at 250 nm decreased with increasing irradiation time. A new band appeared at 265 nm for exposures around 5 min, and that band shifted to 290 nm in samples exposed for 10 min. A yellow precipitate formed after about 30 min. XRD showed that the precipitate was cubic CdS, with a mean particle size of 1.4 nm. We attribute formation of CdS to the photodissociation of the thiols and of the Cd-thiolates. UV irradiation of these precursors yields a series of species that can react with Cd2+, such as RS center dot, S2- and H2S. Small CdS nanoparticles form in the initial stages of illumination, and present absorption bands in the 265-290 nm region. These CdS aggregates grow, coalesce and precipitate for longer irradiation times.