As shown in Fig FT IR spectra of

Fig. 3. (a) Luminescence enhancement factor as a function of spacer thickness for silver nanoparticle layers shown in Fig. 2(a) and (d). (b) Absorption spectrum of the reference sample for photoluminescence measurements. (c) Photoluminescence spectra. The red curve is for the sample with the layer of large, closely packed silver nanoparticles [Fig. 2(a)] covered with a 30-nm-thick SOG layer. Black curve is photoluminescence spectrum of the reference sample. (For interpretation of the references to color in this PF562271 figure legend, the reader is referred to the web version of this article.)Figure optionsDownload full-size imageDownload as PowerPoint slide
To verify that no other effect was responsible for the long-range photoluminescence enhancement, we analyzed the data for signs of, for example, the microcavity effect and mirror-reflection effect at the silver nanoparticle layer interface.
Fig. 3(b) shows the absorption spectrum of the photoluminescence reference sample. The absorption was 54% at 375 nm, which was the excitation wavelength for photoluminescence measurements. This means that the absorption of the emission layer could still be approximately doubled at the excitation wavelength. For the sample with the silver nanoparticle layer, the excitation light could reflect at the surface of this layer, but this reflection is not possible in the reference sample. Such reflected light would again excite the emissive layer. However, the absorption of the emissive layer cannot exceed 100%. If the absorption is enhanced to 100%, the enhancement factor would be less than two-fold, which is not to sufficient to explain the enhancement factor observed in the sample of Fig. 3(a) (8.5-fold at 30 nm spacer thickness).