CdSe Nanoplatelets: Synthesis, heterostructure, ion exchange and optical properties

Abstract

CdSe nanoplatelets (NPLs) exhibit interesting opto-electronic properties. For example, they have narrow excitonic features and can therefore be used as monochromatic light emitters. Synthesis methods for CdSe NPLs emitting at 460 nm (460NPLs), 510 nm (510NPLs) and 550 nm (550NPLs) were reported by Dubertret et al. This thesis reports that these synthesis methods are reproducible and that the surface of the NPLs can be modified. The knowledge obtained from investigating surface modifications of quantum dots and nanorods could be used to modify the surface of the NPLs. This resulted in the formation of CdSe/CdS core/shell NPLs. A CdS/Cd0.5Zn0.5S/ZnS shell using SILAR with oleyl amine as a coordination ligand was also synthesized. Synthesis of multishell quantum dots results in particles with a high quantum yield (QY), but the QY of the multishell NPLs was reduced compared to bare NPLs. Furthermore, the thickness of the multishell NPLs was not homogeneous. The edge of the particle was thicker and an excess of cadmium and sulphur could be found at the edge. Zinc and selenium were homogeneously distributed in the multishell NPLs. A second modification reported in this thesis is the cadmium-to-mercury ion-exchange of CdSe NPLs, but this modification did not result in stable dispersions of NPLs. The exchange was only partially successful on CdSe QDs with a multishell. Furthermore, an adaptation to the synthesis of 510NPLs is reported. Instead of adding only cadmium acetate during the reaction, a mixture of cadmium- and sodium acetate was added. The cadmium acetate is added during the reaction to induce 2D growth, but addition of different acetate salts also resulted in the formation of CdSe NPLs. When cadmium acetate is added, a small amount of thicker or thinner NPLs are present in the sample. Addition of a mixture of sodium acetate and cadmium acetate resulted in the formation of 510NPLs only with a high QY of 60 %. The optical properties of CdSe NPLs are also investigated. Plausible energy transfer is observed between NPLs. The fluorescence intensity decay of the NPLs is multi-exponential. Photoluminescent lifetime measurements on NPLs in solvents with different refractive indices showed that none of the components of the multi-exponential decay are predominantly radiative. Trends obtained with these measurement could not be fitted by the models used in literature.