Observation of enhanced Raman scattering as a result of coupling between localized plasmons and surface polariton waves associated with metal-semiconductor nanocomposites
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We report the observation of enhanced Raman scattering as a result of the coupling between the localized surface plasmon (LSP) of metal nanoparticles (NP) and the surface plasmon polariton (SPP) associated with a thin metal surface. In the present study, we apply AuNPs composites for the SERS detection of the trinitrotoluene (TNT) explosive, as well as to other related nitroaromatic derivatives. The analysis of TNT attracts current interest due to homeland security issues. We describe the successful detection of 4-NBT, 4-ABT, and TNT by an AuNPs – TiO2 matrix associated with an Au coated p Si/SiO2 surface. The AuNPs matrix has been prepared by self-assembly of AuNPs – TiO2 composites and characterized by AFM and Raman measurements. AuNPs were prepared by capping highly monodisperse colloidal n-TiO2 particles with Au islands. Au capping was done by band-gap irradiation of colloidal TiO2 and HAuCl4 acid mixtures. The colloidal TiO2 nanoparticles were prepared by controlled hydrolysis of titanium tetra-isopropoxide with aqueous HClO4 solution (pH 1.5). The Au surfaces used in this research were microscopically smooth Au films deposited on semiconductor wafers cut from p-Si/SiO2 single crystals. The Au films were prepared by physical vapor deposition using standard techniques. The final composite hybrid wafers were characterized for thickness and composition by standard SEM (thickness: p-Si, 222 μm; SiO2, 171 μm; Au 8.4 μm). The smoothness of the Au surface was confirmed by optical microscopy (magnification 100 x), SEM, and AFM. Chemical etching of the smooth Au surface with full-strength aqua regia under well defined conditions also produced a matrix of AuNPs islands on the p-Si/SiO2 wafers. The etched surface galleries allowed the observation of enhanced Raman scattering with TNT and the other nitroaromatic derivatives.