Near-field interactions between metal nanoparticle surface plasmons and molecular excitons in thin-films. Part II: Emission

We systematically study the effect of near-field plasmon-exciton interactions between metal nanoparticles (NPs) and molecular thin-films. In part I, we discussed the effect of these interactions on the absorption of the organic molecules, whereas in this part, we probe their impact on the emission of the organic molecules by time-resolved and time-averaged photoluminescence measurements. Through carefully designed experiments, we are able to exclusively probe the effect of the presence of metal NPs on the emission of organic molecules while eliminating their influence on molecular excitation. For this study, we utilize Ag NPs in a size regime where absorption dominates light scattering, as well as several organic materials that emit at different wavelengths throughout the ultraviolet-visible-near-infrared spectrum. By introducing a transparent, nonemissive spacer layer, we are able to accurately probe the range of plasmon-exciton interactions. We find a good qualitative agreement between the photoluminescence measurements and calculations based on an analytical model. In general, we observe a strong reduction of the emission quantum yield in the presence of the Ag NPs within a distance of <30 nm. However, for poor emitters in combination with embedding media with a high permittivity, we find an emission enhancement due to the Ag NPs. Finally, we compare the results obtained from both parts of this study to discuss the potential application of near-field plasmon-exciton interactions in thin-film opto-electronic devices and sensors.