Structural Study of II-VI Core-Shell Quantum Dots by Synchrotron Anomalous Powder Diffraction and Small Angle X-ray Scattering

Research Division, National Synchrotron Radiation Research Center
○Hwo-Shuenn Sheu Wei-Ju Shih Wei-Tsung Chuang Ying-Huang Lai U-Ser Jeng

The need of nano size materials with bright and stable fluorescence for various applications covering from electro-optics to biology is extensively increasing. The main strategy to increase photoluminescence quantum yield and stability of nanocrystals is to grow a passivating shell on the cores surface. The passivating shell is not only protecting the core from reducing surface degradation but also play a role of charge transition. As opposed to type-I quantum dots (QDs), type-II core-shell QDs have both valence and conduction bands in the core lower than those in the shell materials. Upon electronic excitation, one carrier is predominantly confined to the core, while the other is located at the shell. The rational design and synthesis of the II-VI nanocrystals type-II quantum dots, such as CdS, CdSe, CdTe, ZnS, ZnSe, and ZnTe are reported by P. T. Chou et al. In this study, we use synchrotron PXRD and SAXS to elucidate the multiphase structure of the QDs of CdSe, CdTe, and CdTe/CdSe (core-shell). Moreover the anomalous effects around Se, and Zn K-edge are also applied to differentiate the alloy phase from the core-shell multiphase. The well overlapped anomalous SAXS profile for the CdTe/CdSe showed possible two phase structure, with a thin layer of CdSe shell caped on the CdTe core. The mean particle size of CdTe/CdSe is 7.4 nm with 18% polydispersity. The CdSe/ZnTe core-shell structure and other II-VI core shell QDs were also reported.