Surface-enhanced Raman spectroscopy of the growth of ultra-thin organosilicon plasma polymers on nanoporous Ag/SiO2-bilayer films

A new method for the synthesis of thin bilayer films as surface-enhanced Raman spectroscopy (SERS) active substrates was developed which is based on the combination of plasma polymerization, plasma calcination and Ag-film deposition by means of physical vapor deposition. The surface morphology of prepared substrates was characterized by field emission scanning electron microscopy, atomic force microscopy and electrochemical impedance spectroscopy. These substrates lead to high surface enhancement factors proven by the spectroscopic analysis of adsorbed Trans-1,2 bis-(4-pyridyl) ethylene molecules. By this preparation technique, SERS-active films can be deposited on any substrate. The new SERS substrates were successfully applied to study the growth of ultra-thin hexamethyldisiloxane plasma polymer films. The Raman intensity of the CH-stretching vibration was studied as a function of the film thickness. The surface enhancement decreased sharply at about 20 nm. The resulting increase in the intensity of Raman peaks for thin adsorbed plasma polymer films was observed to be a combination of the electromagnetic enhancement mechanism and the high surface area increase of the rough Ag-surface.