Photoluminescence of small cadmium sulphide particles

Abstract

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 (λ_max≈ 700 nm) arising from sulphur vacancies and a very weak green fluorescence (λ_max≈ 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 (λ_max≈ 530 nm, ϕ= 4 × 10^–3) which is attributed to the formation of Cd vacancies.

A green emission (λ_max≈ 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 (λ_max≈ 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.