Surface-Functionalization-Dependent Optical Properties of II-VI Semiconductor Nanocrystals

We report a study of the surface-functionalization-dependent optical properties of II-VI zinc-blende semiconductor nanocrystals on the basis of ligand-exchange chemistry, isomaterial core/shell growth, optical spectroscopy, transmission electron microscopy, and X-ray powder diffraction. Our results show that the transition energy and extinction coefficient of the 2S(h3)/(2)1S(e) excitonic band of these nanocrystals can be strongly modified by their surface ligands as well as ligand associated surface atomic arrangement. The oleylarnine exchange of oleate-capped zinc-blende II-VI nanocrystals narrows the energy gap between their first and second excitonic absorption bands, and this narrowing effect is size-dependent. The oleylamine exchange results in the quenching, subsequent recovery, and even enhancing of the photoluminescence emission of these II-VI semiconductor nanocrystals. In addition, the results from our X-ray powder diffraction measurements and simulations completely rule out the possibility that oleatecapped zinc-blende CdSe nanocrystals can undergo zinc-blende-to-wurtzite crystal transformation upon ligand exchange with oleylamine. Moreover, our theoretical modeling results suggest that the surface-functionalization-dependent optical properties of these semiconductor nanocrystals can be caused by a thin type II isomaterial shell that is created by the negatively charged ligands (e.g., oleate and octadecyl phosphonate). Taking all these results together, we provide the unambiguous identification that II-VI semiconductor nanocrystals exhibit surface-functionalization-dependent excitonic absorption features.