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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