Metallo-Dielectric Colloidal Films as SERS Substrate

Abstract

Along this chapter, we probe that the discrete dipole approximation models fairly well the optical response of periodic systems. Herein, we use it to model the reflectance and transmittance, at normal incidence, of colloidal films made of SiO 2 spheres. As the thickness increases from 1 to 12 layers, the photonic band gap shifts to the blue tending to the value corresponding to a 3D opal, 442 nm. A film with more than eight layers resembles the bulk properties of a 3D opal. Our results are compared to a real sample. Besides, we show that taking advantage of the wide and asymmetrical absorbance spectrum of an opal with Au NPs is possible to identify the contribution of each component in the overall spectrum, through a deconvolution analysis. Finally, we present the electric field intensity as the content of metal NP increases in a monolayer. We consider NPs one order of magnitude smaller than the silica spheres, and then, 6, 9, and 17 NPs are hosted in the void. Similar average electric field intensities, about 11 times the incident intensity, are obtained with Au and Ag NPs. But, the spots with these intensities cover a bigger area with Ag NPs than with Au NPs. and transmittance, at normal incidence, of colloidal films made of SiO 2 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.