Synthesis and characterization of zinc-blende CdSe-based core/shell nanocrystals and their luminescence in water

Abstract

Zinc-blende CdSe cores are used as a substrate for the synthesis of core/shell nanocrystals, such as CdSe/ ZnS, CdSe/ZnSe/ZnS, and CdSe/CdS/ZnS. Only two monolayers of shell coverage for each material suffice to enhance the photoluminescence (PL) quantum efficiency and achieve similar to 50% PL efficiency in water from all core/shell nanocrystals after ligand exchange with 3-mercaptopropionic acid. Powder X-ray diffraction (XRD) patterns and high-resolution transmission electron microscopy images confirm the coherent epitaxial growth of the zinc-blende shell for core/shell nanocrystals. The PL spectra obtained at 5 K illustrate the effects of the shell composition on deep-trap emission, which manifests the role of hole-trapping surface defects. The spectral shift in both the first absorption maximum and PL band varies with the shell composition following the simple band-offset picture. The shell-to-shell variation of the. spectral shift and changes in XRD patterns suggests that the contraction of CdSe lattice occurs with the concomitant redshift in the PL band, most notably with the ZnS shell. Water-soluble nanocrystals show longer PL lifetimes than organic-soluble ones. The zinc-blende structure is considered a viable alternate replacing the wurtzite structure for the uniform growth of shells and the isotropic incorporation of capping ligands.