SERS Surface Enhancement Factor

Raman spectrum of benzenethiol (BT). The Raman spectrum of BT was obtained as a reference (Figure 5.5). A concentrated solution of BT ([BT] = 15.39 M) was

characterized under the Raman conditions described in Table 5.1.From its spectrum it is possible to identify the in-plane bending (β) and stretching (ν) modes of BT, such as βCCC

and νCS (415, 619, 700, 1093 cm-1), βSH (919 cm-1), βCCC (1002 cm-1), βCH (1027, 1125,

1158, 1187 cm-1), and νCC (1584 cm-1). This information and the estimation of the

effective scattered volume provide all the necessary details to obtain the intensity and number of molecules of the non-SERS parameters of the enhancement factor

Figure 5.5. Raman spectrum of concentrated solution of benzenethiol when excited with a 632.8 nm laser.

SERS spectrum of BT adsorbed on Fischer’s patterns. The spectrum of the functionalized benzenethiol on the SERS platforms was obtained under the same

experimental conditions of the concentrated solution of BT. Figure 5.6 shows an example of one of the spectra obtained for each pattern. These patterns have been labeled

according their fabricated area dose exposure. From this group of spectra, it is possible to notice a change in intensity for the different patterns. The four most intense bands in the spectrum were chosen to quantify the SERS enhancement factor in these platforms. These bands correspond to: βCCC (999 cm-1), βCH (1024 cm-1), βCCC and νCS (1074 cm-1) and νCC

(1584 cm-1). The best SERS spectra are obtained for the platforms with the smaller triangle size and the largest interparticle distance (Figure 5.4).

Figure 5.6. SERS spectra of adsorbed benzenethiol on SERS platforms.

With such set of collected data, it is possible to determine the SERS enhancement factor of these structures. The SERS intensity is obtained from the spectrum itself, the metallic surface area is determined by the structural information collected from the SEM and AFM images. Finally the determination of the number of molecules probed can be calculated using the surface density parameter from the literature. Therefore, the results

obtained from these SERS platforms combined with the parameters from the normal Raman spectrum of a concentrated solution of BT, make possible the quantification of the SERS enhancement factor on these nanostructured platforms, as it is illustrated in Figure 5.6.a. From the graph it is possible to detect that for all the different arrays and using the different Raman signal, the estimated enhancement factor is of the order of 104-106. Now, it is also possible to detect that those arrays that were fabricated under a longer exposure of the electron beam, meaning a larger area dose that generates bigger features with smaller interparticle distance and with a red-shifter plasmon band, there is a decay of the enhancement factor. However, it seems there is a breaking point at 115 μC/cm2

, where the enhancement factor of all the arrays prepared by a larger area dose than that value,

present a sharper decaying slope.

Figure 5.7. SERS enhancement factor (a) and plasmon band (b) of the arrays fabricated by a different area dose.

A possible interpretation of this change in the decaying slope can be done by comparing the plasmon bands of such structures (Figure 5.6.b), and the correlation with the incident Raman beam, shown as a bold black line in Figure 5.6.b. As well as the region

corresponding to the Raman spectral region, illustrated as a gray area in Figure 5.6.b, where the Raman signal has been converted from wavenumbers to wavelengths and highlighted in Figure 5.6.b with dash lines. This plot shows that then enhancement is magnified when the plasmon band is localized between, or very close to, the incident beam and the Raman spectral region analyzed. A similar connection between the plasmon band and the enhancement factor has been given in the literature,12,17 strengthen the interpretation given here.

In addition, it is possible to notice that not all the SERS signals exhibit the same

enhancement factor, and from the four selected vibrational modes, the band that is closer to the incident frequency is the least enhanced. It is not surprising to find that the signals do not have the same enhancement factor, as each Raman tensor presents a different scattering cross section, which is also responsible of the intensity of the Raman band.

5.6. Conclusions

A complete analysis was given about the different parameters that need to be taken into consideration when quantifying the SERS enhancement factor. Using the concept and practical considerations described here it was possible to properly estimate the

enhancement factor of different arrays of gold nanotriangles.

A rigorous quantification of the enhancement factor of the fabricated structures was performed and analyzed. All the fabricated nanostructures present a large enhancement of the order of 104-106 obtainedin non-resonant conditions against benzenethiol,

comparison of the calculated enhancement factor and the plasmonic band of the arrays was given, establishing a correlation between both of them as well as with some of the experimental conditions, such as the Raman laser, or the scattering cross section of the Raman tensor of the probed molecule. All of these, factors that need to be taken into account when designing new experiments with these or other nanostructures.

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