Variations in steady-state and time-resolved background luminescence from surface-enhanced resonance Raman scattering-active single Ag nanoaggregates.

We observed a background luminescence emission that was associated with surface-enhanced resonance Raman scattering (SERRS) of rhodamine 6G (R6G) molecules adsorbed on single Ag nanoaggregates and investigated the origin of the background luminescence. Thanks to the observation of single nanoaggregates, we clearly identified nanoaggregate-by-nanoaggregate variations in the steady-state and time-resolved background luminescence spectra of each nanoaggregate. From the variations in the steady-state spectra, two kinds of key properties were revealed. First, the background luminescence spectra were divided into four components: one fluorescence band corresponding to the monomers of R6G and three Lorentzian bands whose maxima were red-shifted from the fluorescence maximum of the monomer by several tens of nanometers. On the basis of the red-shifted luminescence maxima, and experimental and theoretical studies of background luminescence, we attributed the three background luminescences to fluorescence from aggregates (dimer and two kinds of higher-order aggregates) of R6G molecules on an Ag surface. Second, a positive correlation was observed between wavelengths of background luminescence maxima and wavelengths of plasmon resonance maxima. This positive correlation invoked the idea that the dipoles of both the background luminescence and the plasmon radiation are coupled with each other. From the key observations in the steady-state background luminescence spectra, we propose that two factors contribute to the variations in the steady-state background luminescence spectra; one is the aggregation (monomer, dimer, and two kinds of higher-order aggregates) of R6G molecules on an Ag surface, and the other is plasmon resonance maxima of single Ag nanoaggregates. Considering these two factors, we propose that the variations in the time-resolved background luminescence spectra are associated with deaggregation of R6G molecules (higher- to lower-order aggregates) and temporal shifts in the plasmon resonance maxima of single Ag nanoaggregates.