Luminescence of nanocrystalline ZnS: Pb2+

Nanocrystalline ZnS : Pb²⁺ is synthesised via a precipitation method. The influence of the size of the nanocrystals and the sulfide concentration used in the synthesis on the luminescence properties is investigated. Nanocrystalline ZnS : Pb²⁺ shows a white emission under UV excitation with a rather high quantum efficiency (∼5%). At least two luminescence centres are involved. One centre is identified as a Pb²⁺ ion located on a regular Zn²⁺ site and gives a red emission under 480 nm excitation. The luminescence properties of this emission are characteristic of ³P₀ → ¹S₀ (A-band) or charge transfer (D-band) transitions on Pb²⁺ ions. The other centres are not as well defined and give a broad green emission band under 380 nm excitation and also show luminescence properties typically observed for Pb²⁺. The green emission probably originates from a charge-transfer like D-band emission of Pb²⁺ in ZnS close to a defect (e.g. an S^2- vacancy or an O^2- ion on an S^2- site). A relation between the temperature quenching of the emissions and the band gap is observed and indicates that photoionisation occurs. The higher quenching temperature for the Pb²⁺ luminescence in smaller particles can be explained by widening of the band gap as a result of quantum size effects in the ZnS host.