Ag@CdS quantum dots were synthesized by a colloidal method. CdS nanoparticles stabilized by L-cysteine were doped with varying silver concentrations. Ag@CdS nanoparticles were characterized by UV-visible spectroscopy, steady-state and time-resolved fluorescence, X-ray diffraction, Fourier transform infrared resonance (FTIR), and transmission electron microscopy. The effect of dopant concentration and effect of aging of Ag@CdS colloids were studied. At low silver content, emission was found to enhance, while at high silver content, emission quenching was observed after aging. Ag-CdS nanocolloids exhibited multiexponential decay with tau2 being a long-lived component. X-ray diffraction spectra of as prepared samples indicated that silver occupies an interstitial position, while for annealed samples, Ag+ ions get oxidized to Ag2+ and substituted for Cd2+ in the CdS lattice. FTIR studies have shown that cysteine acts as a monodentate ligand for cadmium and silver for as-prepared sample, whereas it acts as a tridentate ligand for annealed samples toward silver. Transmission electron microscopy of as-prepared samples showed spherical-shaped composites with a uniform layer of capping molecules.

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A simple approach is discussed to synthesise CdS nanoparticles using various pure alcohols and their mixtures with water as a reaction medium. The study highlights that the solvent plays an effective role not only in controlling the particle size but also the structure. The size and phase of CdS nanoparticles were controlled by thermal reaction between cadmium precursor and thiourea in alcohol and their mixtures with water. Use of pure alcohols for the synthesis produces cubic structure as revealed from X-ray diffraction (XRD), while mixture of water with alcohol gives rise to hexagonal phase. XRD showed the presence of CdS nanoparticles with size 10-13 nm in an alcohol-water mixture whereas use of alcohols alone showed size in the range 1-3 nm. The composition and structural features were also studied by Fourier transform infrared spectroscopy, scanning electron microscopy and transmission electron microscopy. Thiourea, in the presence of pure alcohol seems to be responsible for reduction in particle size as compared to that in the presence of alcohol/water mixture. The possible mechanism for the structural phase transition is discussed on the basis of change in surface energy and solvent-solvent interaction.

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