Surface Enhanced Raman Active Nanoparticles in Plasma Treated Silver surfaces
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Atomic Force Microscopy and other scanning probe techniques are today suitable to give a clear picture of the surface morphology both in microscopic and nanoscopic scales. For this reason in the last five years these techniques were employed to correlate surface morphology with a large variety of surface phenomena[6]. This has been also done for SERS active surfaces prepared in different ways. Among them here we mention electrochemical roughened metal surfaces[7] and coldly deposited silver particles produced by evaporation[8]. The aim of this work is twofold. First of all we introduce a new method to prepare SERS active samples through the use of plasma oxidation-reduction treatments of silver surfaces. We will test the SERS activity with a thin polystyrene film deposited on top of them correlating nanomorphology and SERS activity by using AFM technique. Secondly, we will show how it is possible to discriminate between chemical and electromagnetic SERS effect in polystyrene. This has been done by introducing an organic spacer between the thin film and the metal surface, chemisorbed in a self assembled way. EXPERIMENT SERS active surfaces were prepared by plasma oxidation and reduction cycles in a 75 W RF plasma machine (MARCH Plasmod Barrel type). Samples used consisted in a 0.2 mm thick silver foil 99.99% cut in lcm2 and square shaped. CO 2 and H2(5%)+N 2(95%) gases were used for oxidation and reduction respectively with a pressure inside the plasma chamber of about ltorr. It was found that CO 2 plasma provide a mild oxidation of the silver surfaces suitable to obtain different surface morphologies merely by changing the oxidation time[9]. In order to test the produced samples, nearly monodisperse (7500 amu) polystyrene were deposited on top by leaving a drop from a CHC13 solution 1% in weight. In this way we deposited a few thousand nanometers thick film. Alkanethiole molecules have been used as spacer in order to separate the polystyrene thin film from the rough silver surface. In particular the immersion (for about 15 min.) of each silver sample into pure dodecanethiole [CH 3(CH 2 )1 SH] and the subsequent rinsing into pure ethanol ensure a self assembling of one monolayer onto the silver surface as already reported in literature[l10]. Some samples with the self assembled film have been used to deposit the same amount of polystyrene as before. All Raman spectra were performed through the use of an Ar ion laser working at 514.5 nm and a Jobin Yvon U 1000 double monochromator equipped with two holographic gratings and connected to an Hamamatsu photomultiplier with a photon counting chain. Laser power has been always maintained at 10 mW on the sample and the estimated resolution was at around 5 cm'. This ensure a good signal to noise ratio with 0.5 sec. integration time. Atomic Force Microscopy (AFM) images were obtained in air using a Nanoscope IliA instrument in either contact or tapping mode. Etched silicon tips have been used in tapping mode while Si3N 4 ones have been used in contact mode. In particular, sil
