Photoluminescence of small cadmium sulphide particles

Colloidal particles of CdS are produced by adding H2S to aqueous solutions of Cd(NO3)2(method 1) or rapid mixing of Na2S and Cd(NO3)2 solutions (method 2). Single crystals with cubic structure are obtained, their size being 50 and 20 Å, respectively. Type 1 particles are associated as dimers or trimers while type 2 particles form larger clusters of 2000–3000 Å size in aqueous solution.Luminescence studies, carried out mainly with type 1 sol, show the presence of a red emission (λmax≈ 700 nm) arising from sulphur vacancies and a very weak green fluorescence (λmax≈ 515 nm) due to free-carrier recombination. The red luminescence is extremely sensitive to the presence of acceptors such as methyl viologen (MV2+): 10–8 mol dm–3 MV2+ suffices to quench 50% of the emission. Kinetic analysis shows that only one MV2+ per CdS aggregate is required to quench completely the red luminescence. This effect is exploited to determine the aggregation number of CdS particles. MV2+ also induces a green emission (λmax≈ 530 nm, ϕ= 4 × 10–3) which is attributed to the formation of Cd vacancies.A green emission (λmax≈ 520 nm) is also obtained by substitutional doping of type 1 sol with chloride, and this arises from interstitial sulphur. Doping with Cu2+ produces the characteristic emission of this activator (λmax≈ 820 nm) only if particles are prepared under aerobic conditions or illuminated in the presence of oxygen. A lattice-defect model is evoked to explain this behaviour.