Hybrid gold/silica/nanocrystal-quantum-dot superstructures: synthesis and analysis of semiconductor-metal interactions.

We report on the synthesis and spectroscopic characterization of well-defined hybrid structures that consist of a gold core overcoated with a silica shell, followed by a dense monolayer of CdSe nanocrystal quantum dots (QDs). The dielectric silica spacer of a controlled thickness provides a simple means for tuning interactions between the QD emitters and the metal core. To illustrate this tunability, we demonstrate switching between QD emission quenching and enhancement by varying the silica shell thickness. Synthetic procedures developed here employ a final step of self-assembly of QDs onto the silica shell performed via simple titration of the QD solution with prefabricated core/shell Au/SiO2 particles. This approach allows us to perform an accurate quantitative analysis of the effect of the metal on the QD emission intensity. One important result of this analysis is that nonuniformity of nonradiative rates across the QD ensemble has a significant effect on both the magnitude and the shell-thickness dependence of the emission enhancement/quenching factors.