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Near-infrared SERS Measurements on Metal Coated Porous Silicon Photonic Crystals

Surface-enhanced Raman scattering (SERS) spectroscopy is a highly sensitive technique, widely used for identification and structural characterization of biological materials and chemical substances, which overcomes the sensitivity drawback of conventional Raman spectroscopy. In the past decade, it has been shown that cheap, easy-to fabricate, efficient and reproducible SERS-active substrates can be produced using metallic nanoparticles grown on the porous silicon surface. Porous silicon is a nanostructured material obtained by anodization of crystalline silicon in hydrofluoric acid solution. Typically, the excitation wavelengths used in SERS measurements are in the visible range, while the aim of this study is the structural optimisation of SERS-active porous silicon substrate for Raman near-infrared (NIR) excitation. This wavelength range is more appropriate for analysing biological molecules. To obtain SERS effect in NIR spectral range, photonic crystals with quenched silicon substrate band gap photoluminescence and efficient reflectance in NIR wavelength region had to be produced. Photonic crystals were made using computer-controlled current source whose programmed current density determines the porosity-depth profile and consequently the refractive index of periodic dielectric structure. Nanostructured metal coating was synthesized by immersion of porous silicon photonic crystal into metal salt aqueous solution. The structural properties of prepared NIR SERS-active substrates were investigated with SEM microscopy, the optical properties with UV-VIS-NIR reflection spectroscopy and the vibrational properties with ATR spectroscopy. SERS measurements were performed with FT-Raman spectrometer. The results have shown high SERS-sensitivity with the concentration detection limit of 10-6 M of typical probe molecule Rhodamine 6G by using the laser excitation at 1064 nm.

porous silicon, SERS, photonic crystal, near-infrared

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