Nanostructured substrate with nanoparticles fabricated by femtosecond laser for surface-enhanced Raman scattering

A simple and fast method to fabricate nanostructured substrates with silver nanoparticles over a large area for surface-enhanced Raman scattering (SERS) is reported. The method involves two steps: 1) dip the substrate into a silver nitrate solution for a few minutes, remove the substrate from the solution, and then air dry and 2) process the silver nitrate coated substrate by femtosecond (fs) laser pulses in air. The second step can create silver nanoparticles distributed on the nanostructured surface of the substrate by the photoreduction of fs multiphoton effects. This study demonstrates that an enhancement factor (EF) greater than 5 10 5 , measured by 10 –6 M Rhodamine 6G solution, can be achieved. The proposed technique can be used to integrate the SERS capability into a microchip for biomedical and chemical analysis. Instruction Since the 1970s, the phenomena of surface-enhanced Raman scattering (SERS) by molecules adsorbed on a roughened metal surface have been widely investigated [1,2]. The enhancement factor (EF) can be as high as 10 14 –10 15 using Rhodamine 6G (R6G) which allows the technique to be sensitive enough to detect a single molecule on a rough noble metal (e.g., silver and gold) surface [3]. Different techniques to fabricate metallic nanoparticles on semiconductor substrates for SERS applications have been reported. For example, Qiu et al. fabricated silver-capped silicon nanowires using an electroless metal deposition method to achieve high SERS intensities [4]. Chan et al. demonstrated a porous silicon SERS substrate for small molecules by using a 6-step fabrication method [5]. Lin et al. developed a method to fabricate silicon wafer SERS substrates in the aqueous solution of silver nitrate machined by the fs laser [6]. Diebold et al. employed the fs laser pulse-train to fabricate a nanostructured silicon substrate followed by silver thermal evaporation for SERS detections [7]. In this paper, we report a simple method that can simultaneously fabricate nanostructures and generate silver nanoparticles by fs laser pulses over a large area of the silicon substrate. The SERS substrate was measured to have higher than 5 10 5 of EF using R6G as the analyte molecule at the excitation wavelength of 632.8 nm. This technique shows a possibility of integrating SERS capability to high order microdevice systems for chemical or biological detections. Experiment The SERS substrate is prepared in the following steps. First, a silicon wafer sample was cleaned ultrasonically in methanol and then dipped into a 1.0 M aqueous silver nitrate solution for 10 min at room temperature. Next, the sample was removed from the silver nitrate solution and air dried [8]. A thin layer of silver nitrate was formed on the sample surface. Finally, the silicon sample was processed by the fs laser micromachining system. The repetition rate, center wavelength and pulse width of the fs laser (Legend-F, Coherent, Inc.) are,