Effects of metallic ions on the photoluminescence properties of CdS quantum dots/poly(amidoamine) dendrimer nano-composites

CdS quantum dots(QDs) were prepared using poly(amidoamine) (PAMAM) dendrimer template, and effects of such metallic ions as Cd²⁺, Zn²⁺, Pb²⁺, Cu²⁺, Mn²⁺ on the photoluminescence (PL) properties of CdS QDs were studied using spectrophotometric titration. The results show that Cd²⁺ and Zn ²⁺ ions enhance the PL intensity of CdS QDs, while Pb²⁺, Cu²⁺ and Mn²⁺ ions quench it. These are attributed to the surface modification of metallic ions on CdS QDs. Excess Cd²⁺ ions can reduce the non-recombination centres, increase the passivation of PAMAM dendrimers on the surface of CdS QDs, and form a Schottky-like barrier which enhances the PL efficiency greatly. Excess Zn²⁺ ions can also increase the PL intensity markedly in the same way as Cd²⁺ ions because of their similar lattice parameters. However, a PbS or CuS shell on CdS QDs emerges due to the substitution of Cd²⁺, which leads to the decrease of band-edge emitting of CdS QDs. And the quenching effect of Pb ²⁺ is stronger than Cu²⁺. The coordination between Cd ²⁺ and dendrimers can be damaged by Mn²⁺ ions, which results in weaker passivation of the QDs surface detects. Since Mn²⁺ ions themselves are new quenching centres, the quenching effect is mild in spite of a lower-energy Schottky-like barrier formed by Mn²⁺ ions.