affect the gold volume fraction. While the first value has an impact on the peak magnitude, the latter affects its spectral position, so both are present as the absorption peak shifts from 580 to 610 nm of the C15 films in Figure 5.6.

One interesting thing in composite materials in general and for the presented films in particular is how the illumination-induced electromagnetic fields are distributed. This can be measured by SNOM. Figure 5.8 shows AFM and SNOM images for two of the studied linkers for films filtrated from 7.5 mL of solution. The images were taken with a commercial SNOM (Nanonics Imaging Ltd.) in illumination-reflection mode. The illumination was provided by a 409 nm laser through an aperture tip with an aperture size of 100 nm and the reflected light was collected in the far-field by a microscope and focused on an avalanche photodiode. Feedback was provided by the aperture tip through an AFM setup.

The images show that the field intensity distribution is more homogeneous in the C8 than in the C15 film. This seems to be due to the more homogeneous structure of the C8 film. To prepare theoretical predictions for the SNOM contrast for these kind of structures is quite challenging, as there are four contributions to the contrast from the sample alone: the dielectric properties of the three phases and the overall topology. Both images indicate that induced fields are largely confined to the linker-gold system. This means that the primary role of the voids is simply to reduce the overall polarization of the average film and hence its refractive index. Their influence on the resonance position will then be quite weak, which is exactly observed.

FIGURE 5.8 AFM (top) and SNOM (bottom) images for (a) the C8 cross-linker and (b) the C15 cross-linker for films filtrated from 7.5 mL of solution (the gray scale of the SNOM images is not linear, in order to improve their contrast).
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