Sub-wavelength size metallic silver particles display a strong absorption with a maximum near 480 nm (). This absorption is due to the electron oscillations induced by the incident light field, and is called the surface plasmon resonance [20
]. The AFM image of a typical SIF used for this study () shows silver particles ranging in size from 20 to 80 nm high.
Atomic force microscope image of a quartz slide coated with a silver island film.
We examined the emission spectra of [Ru (bpy)3]2+ in PVA on quartz (Q) and on SIFs (S). Quenching by oxygen occurs on both the quartz and SIF surfaces (). In an argon atmosphere the emission intensity of [Ru(bpy)3]2+ is about 7-fold larger on the SIF than on quartz (). Comparable increases in intensity were observed in an atmosphere of air or oxygen.
Calculation of the apparent increase in the radiative decay rate of [Ru(bpy)3]2+
Emission spectra and relative intensities of [Ru(bpy)3]2+ in PVA on quartz (Q) and silver island films (S). The horizontal lines show the emission intensities in argon, air or an oxygen atmosphere.
Changes in the rate of radiative decay can be detected by decrease in the lifetime [1
]. Hence, we measured the frequency-domain (FD) intensity decay of [Ru(bpy)3
in each atmosphere, and on quartz and on SIFs (). In each case there is a decreased lifetime on the SIFs. A decreased lifetime with an increased intensity () indicates an increase in the rate of radiative decay of [Ru(bpy)3
. Based on the intensity decays in , and ellipsometry for similar spin-coated samples [21
], we estimate the average film thickness to be in the range of 10 nm, but this estimate has considerable uncertainties.
Frequency-domain intensity decays of [Ru(bpy)3]2+ in PVA equilibrated with an argon, air or oxygen atmosphere. Top panels, on quartz. Bottom panels, on silver islands.
The FD intensity decays were analyzed in terms of the multi-exponential model (Eq. (1)
) and the results summarized in . Examination of these data reveal that the individual decay times, averages lifetime and amplitude-weighted lifetimes are shorter on SIFs than on quartz. Decreases in lifetime are observed in argon, air or oxygen. Increased intensities () and decreased lifetimes is strong evidence for an increase in the radiative decay rate.
Multi-exponential analysis of [Ru(bpy)3]2+ intensity decays in absence and presence of silver island films
It is of interest to use the intensity () and lifetime data () to estimate the increase in the radiative decay rate of [Ru(bpy)3]2+ on silver island films. This can be accomplished by using the definitions of the quantum yield (Q0) and lifetime (τ0) on the quartz surface
is the radiative decay rate and k
is the sum of the non-radiative decay rates. Using these expressions the radiative decay rate on quartz is given by
Now assume that proximity of [Ru(bpy)3]2+ to the SIFs results in an increase in the affected radiative decay rate to Γm. Assuming the non-radiative rates k are not changed due to the SIFs, the quantum yields and lifetimes near the metal (m) are given by
It is difficult to estimate the absolute quantum yields for [Ru(bpy)3]2+ in the thin PVA films. However, if ratios are used, the quantum yields can be replaced with the relative intensities (I0 and Im), assuming the SIFs have a minimal effect on the rate of excitation. Hence the ratios of Γm/Γ can be calculated using
The calculated values of Γm
are summarized in . This ratio was calculated using both the mean lifetime
and the amplitude-weighted lifetimes τ
. While it is not presently clear which type of weighted average lifetime should be used, the results are the same. The radiative decay rate of [Ru(bpy)3
increases about 20-fold when coated on silver island films. We stress that there are apparent values spatially averaged over these illuminated region of the sample and the thickness of the PVA film. Also, our calculation assumes no change in the rates of excitation or non-radiative decay. Even if the rate of excitation changed, this does not alter our conclusion that the emission intensities are higher and lifetime of [Ru(bpy)3
are smaller on SIFs than on quartz.