FIGURE 5.5 Reflectance and transmittance for the C8 and C15 linker films filtrated from 4 mL of solution.

different to that in bulk gold. The dielectric properties of the films produced by the solution method (scheme (b) in Figure 5.1) were determined using reflectance ellipsometry in the wavelength range 300 to 800 nm. In the following discussion we will concentrate on the results from the continuous regime, i.e., films that completely covered the substrate. The only samples that could not be measured were the thinnest C2 films, which were too scattering and hence did not have a sufficient signal-to-noise ratio for measurements. The general appearance of the thinnest, island-like films was a dullish gray, the continuous films looked shiny golden, and the thickest films were starting to look dullish gold.

The complex refractive index for the three linker lengths studied can be seen in Figure 5.6, along with the result from our double EMA model, which will be explained in Section 5.5. The angular brackets in the graph are used to highlight the average or effective nature of the refractive index, as opposed to properties of a homogeneous, single-phase material.

The main feature of the spectra is a single damped oscillator structure in the observed region, which is blueshifted as the linker length increases. This behavior is distinctly different to the bulk properties of gold, as can be seen by comparing the results in the first three parts of Figure 5.6 to the fourth panel, which shows the bulk response from Ref. [36]. The single oscillator is basically the surface plasmon resonance for the individual gold particles, shifted by the different dielectric medium (the linker molecules) and by the interaction between the gold particles. It is well known that particle aggregation causes a redshift in the absorption spectrum compared to that from isolated or dilute metallic particles.37 The blueshift is caused by the increasing particle separation due to an increase in the linker length, as this increases the average particle-particle distance. Although there is no contact between gold particles because of intervening linker molecules, interaction takes place between these particles even for longer C15 linkers.

Figure 5.6 also shows that the continuous films made from a given linker behave very similarly for different thicknesses, a strong indication that the material can be treated as a homogeneous effective medium in each case. It also indicates that surface plasmon effects across the whole outer surface of the film are not an issue here, even though the films can become conductive as a whole through tunneling by the molecules.24'25 If such plasmon effects were present, the equivalent optical layer used in our analysis would have (n), (k) values, which would depend on the film thickness.38 Only the C15 linker films show some deviations in (n) and (k) for the different thicknesses and these, having a lower conductivity due to longer linker chain lengths, are least likely to support macroscopic surface plasmons. The observed shift might be due to ongoing structural changes for the C15 linker, as might be expected

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