Optical and structural studies of phase separation in Zn _xCd _1-xSe/C core/shell nanocrystals

Zn _xCd _1-xSe/C core/shell nanocrystals with 31-39 nm diameter semiconductor cores and 11-25 nm-thick carbon shells were synthesized from solid state precursors in large scale amounts. Transmission electron microscopy (TEM) showed striations in the nanocrystals that are indicative of a composition modulation, and reveal a chemical phase separation and possible spinodal decomposition within the nanocrystals. The optical properties and carrier relaxation kinetics of the nanocrystals were examined with cathodoluminescence (CL) imaging and spectroscopy. As the excitation level is increased, carrier filling in two-dimensional phase-separated Cd-rich regions leads to a partial saturation of states before the onset of carrier filling in the higher bandgap homogenous Zn _0.47Cd _0.53Se regions. In order to model the state filling and calculate quasi-Fermi levels for electrons and holes, a random quantum well model was used to determine the electron and hole eigensates in the Cd-rich regions of the nanocrystals.