4.2 Effect of metal NPs' load on the SERS EF

Opals with inclusions of Au NPs offer several of the following features: [16, 17, 32]:


#### Figure 8.

Electric field intensity of (a) 6, (b) 9, and (c) 17 Au NPs at the intersite of three SiO2 spheres. The lower panels (d), (e), and (f) are the same systems but with Ag NPs.

λopal and λAu are the central wavelengths; and σopal and σAu are the respective FWHM. The corresponding fitting values are presented in Table 2. The addition of the two Gaussians give rise to a distribution (blue soft line) that fits the experimental distribution, see panels of Figure 7. The red and green lines are the Gaussian curves attributed to the contribution of Au and silica arrays in the composite, respectively. SiO2 film spectrum is very well adjusted to a Gaussian with a peak around 568 nm and a FWHM of 26 nm (see Figure 7). Meanwhile, when relatively low, medium, or high Au NPs concentration is incorporated in the opal, an asymmetrical and wide absorbance spectrum is noticed. At low concentration (composite 1), apparently still dominates the band assigned to the SiO2 array and the outcome is a distribution with a slightly negative asymmetry. At medium (composite 2) and high (composite 3) concentration, it seems that the Au NP signal starts to predominate

UV–Vis absorbance spectrum of the films made of SiO2 spheres before and after infiltration with low (composite 1), medium (composite 2), and high (composite 3) degree of Au NP loading. Blue non–soft line is the experimental measurement; red and blue lines are the Gaussian curves associated to the Au and silica

Film λopal σopal λAu σAu

Composite 1 567 nm 44 nm 527 nm 8 nm Composite 2 570 nm 48 nm 530 nm 20 nm Composite 3 578 nm 39 nm 532 nm 23 nm

arrays, respectively; blue soft line is the sum of blue and green lines.

SiO2 568 nm 26 nm

Gaussian fitting parameters for absorbance bands.

and the result is a distribution with a positive asymmetry.

Figure 7.

Nanorods and Nanocomposites

Table 2.

102

The thickness of the colloidal film and load of Au NPs are factors that have been exploited to succeed in the fabrication of an SERS substrate with high qualities. With inverse opal templates, nanoparticle film thickness has proved to have little effect on SERS signal intensity for substrates thicker than a monolayer. On the other hand, opals with Au NPs show a larger enhancement when the film has less than 10 silica layers compared to the thicker ones, and a possible explanation for this is a contribution of multiple scattering within the opal.

system, we limited ourselves to the section defined by three silica spheres with N NPs in the void among them. The NPs considered here are one order of magnitude (diameter of 20 nm) smaller than the silica spheres. Therefore, it is possible to host 6, 9, and 17 NPs in the void. Same configuration was used for Au and Ag NPs, finding similar electric field intensities for both of them, about 11 times the incident intensity. But, the spots with these intensities cover a bigger area with Ag NPs than with Au NPs. This indicates that the SERS EF of a specific concentration of mole-

The authors thankfully acknowledge the computer resources, technical exper-

Special thanks are given to Dr. Enrique Sánchez, from the Institute of Physics

at the Benemerita Universidad Autonoma de Puebla, for his comments and facilities for the synthesis and characterization of the materials presented in

tise, and support provided by the Laboratorio Nacional de Supercomputo del Sureste de Mexico, CONACyT network of national laboratories. A.L. González thanks the financial support of Benemerita Universidad Autonoma de Puebla through the VIEP projects GORA-EXC17-G and 100504244-VIEP2018. A. Santos thanks CONACYT by the grant number 258670. M. Toledo Solano thanks support from Mexican National Council for Science and Technology (CONACyT) through

cules would be larger using Ag NPs than Au NPs.

Metallo-Dielectric Colloidal Films as SERS Substrate DOI: http://dx.doi.org/10.5772/intechopen.90313

The authors declare no conflict of interest.

DDA discrete dipole approximation

SEM scanning electron microscopy

HCP hexagonal compact packing

SERS surface enhanced Raman scattering

Acknowledgements

Grant A1-S-38743.

Thanks

Figures 6 and 7.

Abbreviations

NPs nanoparticles PBG photonic band gap

FCC face centered cubic

EF enhancement factor

R reflectance T transmittance EM electromagnetic

105

Conflict of interest

Limiting ourselves to the contribution to the SERS enhancement due to the top layer of the composite film, we study the effect of NP concentration. For illustrating low, medium, and high content of metal NPs in the surface layer of the thin film, 6, 9, and 17 metal NPs in the interspace among three dielectric spheres have been considered. An estimation of the single molecule SERS EF can be done through the SERS EF ≈E<sup>4</sup> approximation [38], with E being the intensity of the local field. Usually, the size of the metal NPs is one order of magnitude smaller than the size of the dielectric spheres; therefore, we have considered metal NP diameter of 20 nm and SiO2 dielectric sphere of 200 nm. The spatial configuration of E values for Au and Ag NPs is presented in Figure 8. To decrease computational cost, only the area limited by the triangular zone was considered as the target in DDA calculation. The incident EM plane wave is parallel to the plane and travels from left to right, and its wavelength is of 633 nm (red laser). Notice that the SiO2 spheres apparently are not touching one each other, and the reason is that the plane that is presented is the one that crosses the array as depicted in the side view drawn. In general, the average electric field intensity is of the same order of magnitude for all cases, reaching values 10–12 times the incident electric field, the field being more intense in the spots among NPs, and in the region between the SiO2 spheres surface and the Au NPs close to it.

### 5. Conclusions

Along this chapter, we have proved that DDA is a good option to study the optical response of periodic systems. Herein, we use it to model the reflectance and transmittance, at normal incidence, of colloidal films made of SiO2 spheres with a specific diameter of 200 nm. We show that as the thickness increases from 1 to 12 layers, the center of the photonic band gap shifts to the blue tending to the value of 444 nm. This value is very close to the one corresponding to a 3D opal with HCP structure, 442 nm. Analyzing the trend of the position of the BG, a film with more than eight layers resembles that of a 3D artificial opal. We also compared our results with a 3D artificial opal made with silica spheres of the same size as the one studied by us. We found a good qualitative agreement, and the differences are attributed mainly to the presence of defects in the sample.

Artificial opals with Au NPs are of interest due to their many applications, and one of them is as SERS substrate. A main issue is how to be sure the NPs are being incorporated in the sample. Usually, reflectance spectrum does not show an evident contribution due to the presence of the plasmonic NPs. However, we present here that the absorbance spectrum is wider and asymmetrical compared to the opal without Au NPs. Furthermore, with a deconvolution analysis, it is possible to identify the contribution due to the Au NPs and the silica spheres in the overall spectrum.

For SERS applications, the calculation of electric field intensity gives an idea of the enhancement factor that can be reached by the substrate. At the end of the chapter, we present the spatial distribution of electric field intensity as the amount of metal NPs increases in a monolayer. Taking advantage of the periodicity of the

Metallo-Dielectric Colloidal Films as SERS Substrate DOI: http://dx.doi.org/10.5772/intechopen.90313

system, we limited ourselves to the section defined by three silica spheres with N NPs in the void among them. The NPs considered here are one order of magnitude (diameter of 20 nm) smaller than the silica spheres. Therefore, it is possible to host 6, 9, and 17 NPs in the void. Same configuration was used for Au and Ag NPs, finding similar electric field intensities for both of them, about 11 times the incident intensity. But, the spots with these intensities cover a bigger area with Ag NPs than with Au NPs. This indicates that the SERS EF of a specific concentration of molecules would be larger using Ag NPs than Au NPs.
