Plasmonic silvered nanostructures on macroporous silicon decorated with graphene oxide for SERS-spectroscopy

A method for fabricating surface-enhanced Raman scattering (SERS)-active substrates by immersion deposition of silver on a macroporous silicon (macro-PS) template with pore diameters and depth ranging from 500-1000 nm is developed. The procedure for the formation of nanostructured silver films in the layers of macro-PS was optimized. Silver particles of dimensions in the nano- and submicron-scale were formed on the external surface of the macro-PS immersed in the water-ethanol solution of AgNO_3, while the inner pore walls were covered by smaller, 10-30 nm diameter, silver nanoparticles. Upon introducing the hydrofluoric acid to the reaction mixture, the size of nanoparticles grown on the pore walls increased up to 100-150 nm. Such nanostructures were found to yield SERS-signal intensities from CuTMpyP4 analyte molecules of the same order to those obtained from silvered mesoporous silicon reported previously. The tested storage stability for the silvered macro-PS-based samples reached up to 8 months. However, degradation of the SERS intensity under illumination by the laser beam during spectral measurements was observed. To improve the stability of the SERS-signal a hybrid structure involving graphene oxide deposited on the top of analyte molecules adsorbed on the Ag/macro-PS was formed. A systematic observation of the time evolution of the characteristic peak at 1365 cm^-1 showed that the addition of the oxidized graphene layer over the analyte results in ∼2 times slower decay of the Raman intensity, indicating that the graphene coating can be used to enhance the stability of the SERS-signal from the CuTMpyP4 molecules.