Dielectric Properties of Photo-Luminescent CdSe/CdS Mono-Shell and CdSe/CdS/ZnS Multi-Shell Nanocrystals Studied by TEM-EELS

Semiconductor nanocrystals (NCs) of CdSe/CdS (core/shell) mono-shell and CdSe/CdS/ZnS (core/shell/shell) multi-shell having bright and stable luminescence were investigated using high energy-resolution electron energy-loss spectroscopy (EELS) based on transmission electron microscopy to evaluate their dielectric properties in the visible light energy region. EELS spectra of the mono-shell and multi-shell NCs showed two-step structures attributed to interband transitions near band gaps of individual CdSe core and CdS shell. EELS simulations based on the classical dielectric theory revealed that the two-step structures were results of dielectric response from CdSe core and CdS shell. The intensity profiles of the two-step structures depend on a thickness of CdS shells and mono/multi-shell structures of NCs. Besides, the quantum confinement effect was observed in the spectra of mono- and multi-shell NCs, where onset energy of the CdSe core was larger than that of bulk CdSe. The EELS spectra of multi-shell NCs, however, did not present an evident onset due to a bandgap of ZnS outer-shell suggesting defects or amorphous state in ZnS outer-shell. Such dielectric properties of NCs obtained from EELS analysis could provide valuable information to evaluate the crystallinity of individual core and shells in NCs.