Metal-enhanced luminescence in colloidal solutions of CdSe and metal nanoparticles: investigation of density dependence and optical band overlap

The photoluminescence (PL) of semiconductor nanoparticles (SNP) is strongly modified when the semiconductor is in the proximity of a metal surface or a metal nanoparticle (MNP). The effect may be due to two different phenomena which are (a) (Forster) resonant energy transfer ((F)RET) between the semiconductor and the metal and (b) the enhanced electric field around metallic structures that arises from surface plasmon oscillations. Here we present experimental evidence for enhancement and quenching of the PL of dilute SNP colloidal solutions depending on the amount of admixed MNP and the position of the MNP plasmon band with respect to the excitation wavelength and the optical bands in the SNP. The average distance between an MNP and its next neighbor MNP is varied between ∼0.1 and 2 μm by varying the MNP concentration, whereas that between MNP and SNP as well as between SNP and SNP is kept at about 0.1 μm. A model function based on the rate equations of the system is developed that yields a satisfactory description of the measured data by considering solely FRET between the particle species. The derived function is an extension of the Stern–Volmer equation, as it not only accounts for the energy transfer from the fluorescent SNPs to the MNPs, but also for the transfer of excitation from MNPs to SNPs and between MNPs. This theory provides a deeper insight into the mechanisms of metal-enhanced fluorescence and fluorescence quenching phenomena.