Abstract
We have synthesized core/shell/shell (CSS) CdSe/ZnSe/ZnS quantum dots (QDs) and investigated their exciton dynamics using time correlated single photon counting (TCSPC). The unique synthetic method combines hot injection with successive injection of precursors in one pot. Transmission electron microscopy (TEM) shows that CSS QDs were 6 ± 2 nm in diameter. The elemental composition, determined by energy dispersive X-ray spectroscopy, was 3.3% cadmium, 8.6% selenium, 42.3% sulfur, and 45.8% zinc by mole. Photoluminescence spectroscopy (PL) showed that the PL quantum yield is increased from 0.9% for CdSe to 25% for CSS. Global fitting was used for the analysis of exciton dynamics for CdSe, CdSe/ZnS core/shell, and CSS QDs. The decays of the PL spectra for CdSe and CdSe/ZnS were fit with triple exponentials with lifetimes of 0.7, 8, and 30 ns and 0.7, 10, and 30 ns respectively, while the CSS spectrum was fit with a double exponential with lifetimes of 12 and 30 ns. We attribute the 0.7 ns component to nonradiative recombination through dangling bonds at the CdSe surface or at crystal lattice dislocations at the CdSe/ZnS interface. This study clearly demonstrates that the CSS approach can be used to substantially improve the optical properties of QDs desired for various applications.
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