Surface enhanced Raman scattering (SERS) spectroscopy is well-known as an efficient method of molecules detection. Rough metallic surfaces or closely packed metallic nanoparticles arrays are the main attributes used to achieve Raman signal enhancement [1]. The signal enhancement is observed when the studied molecule is found next to the metal surface that is characterized by strong electromagnetic field appearing under the external excitation. This effect is known as Localized Surface Plasmon Resonance (LSPR).
The promising use of a guide mode resonator (GMR) for additional amplification of SERS-signal has been shown in recent reports [2]. The GMR is regarded as a diffraction grating with subwavelength dimensions on a dielectric template. The excitation wave tends to travel along the surface interacting with the material and followed by transmitting or reflecting processes. The rise in SERS-activity of GMR/metallic nanostructures is observed due to strong coupling between these two structures and can be controlled by tailoring the height and periodicity of the loaded dielectric grating.
Nanostructured silicon is considered to be an efficient substrate for SERS-active sensors development. Porous silicon (PS) is one of the silicon nanostructures types, that is widely used as a template for deposition of varying metallic forms such as silver and gold nanoparticles, dendrites, quasi-continuous films [3], etc. Furthermore, it is reported that PS can take part in Raman signal enhancement while being measured [4], which raises interest in its use as a substrate for SERS-active sensors fabrication.
In our work we propose a new approach of amplification of the already strong local field that appears between silver nanoparticles located at several nanometers from each other, using porous silicon periodic structures as the GMR for SERS-application. According to this approach the nanostructure of GMR/porous silicon/ silver nanoparticles has been fabricated in several steps. At first, the periodic gratings have been formed on a silicon wafer by direct laser interference patterning using Femto-lab workstation equipped with Yb:KGW femtosecond laser Pharos. The wavelengths of 515 nm and 40 kHz frequency were set in the second harmonic oscillator. The laser power and values of pulses were ranging from 16 mW to 160 mW and from 1000 pulses to 25000, respectively for searching the surface of the silicon grating close to the flat one in order to avoid additional light scattering by irregularities. Then, the fabricated GMR has been porosified by electrochemical etching in aqueous-based electrolyte including 10% hydrofluoric acid at current density 140 mA/cm2. And finally, the silver nanoparticles have been deposited on GMR/porous silicon by immersion deposition from 3 mM silver salt solution for an hour.
The morphology and optical properties of the nanostructures produced have been studied. The application of the structure to SERS-spectroscopy have been investigated, using Rhodamine 6G as a target molecule and laser wavelength 532 nm. It has been established that GMR/PS/silver nanoparticles system exhibits higher SERS-activity in comparison with PS/silver nanoparticles structure.
The experimental results are supported by the modelling results enabling us to offer way of production of SERS-active porous silicon based substrates. We demonstrate that the most effective sensor with properties individually selected for the used laser wavelength in the SERS-measurements may be produced by varying the pitch of GMR, porosity of silicon and shape of nanoparticles.