An exploration of substrates for surface-enhanced Raman scattering




Anema, Jason Robert

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Surface-enhanced Raman scattering (SERS) constitutes a spectroscopy of rapidly growing importance. To understand and control the enhancing surface is key for optimizing SERS. In this work, progressively more ordered substrates for SERS are investigated: gold clusters, scratched gold films, nanohole arrays in a copper film and nanoscale gratings in a gold film. Gold clusters constitute the most random SERS substrate: samples were prepared by alternating vapour deposition of gold and plasma-induced polymerization of styrene gas. The relationships between sample preparation parameters, gold-cluster morphology, and SERS intensity were elucidated. Using Wilson’s notation, vibrations were assigned to all bands between 250 and 1750 cm-1 in the ordinary Raman and SERS spectra of polystyrene. The orientation of polystyrene’s phenyl ring, relative to the gold surface, was determined. It has been suggested that reactions involving silver catalyze polystyrene degradation during SERS, but we found that silver is not necessary for the degradation to occur. Of intermediate order are scratched gold films. Polarization-dependent SERS was studied for scratched gold films placed in-situ and under electrochemical control. A quantitative method for evaluating the polarization dependence was developed. In addition, we established that polarization effects may be used to selectively remove solution-phase interference signals from the SERS spectrum of an adsorbed analyte. Nanohole arrays and nanoscale gratings, made by focused ion beam (FIB) milling, constitute the most ordered SERS substrates. SERS was observed for oxazine 720 molecules adsorbed on a copper film pierced by nanohole arrays. Each of the arrays had a different hole-periodicity, and the effect of this periodicity on SERS intensity was examined. Electric fieldstrength perpendicular to the surface of the metal was calculated for each array using the finite-difference time-domain (FDTD) method, and the array with the greatest calculated value matched the array that gave the greatest experimentally observed SERS intensity. Finally, SERS was observed for oxazine 720 molecules adsorbed on a gold film through which nanoscale gratings were milled. We varied the width of the grooves and the width of the metal lines between them, and we examined the resulting effect on SERS intensity.



Raman effect, SERS