Effect of silver-nanoparticle aggregation on surface-enhanced Raman scattering from benzoic acid

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Abstract
Silver nanoparticles were fabricated using the gas-evaporation method, and were deposited directly onto various substrates for different measurement purposes. Surface images obtained by transmission electron microscope and atomic force microscope show that the films fabricated consist of Ag nanoparticles rather than a smooth Ag layer or Ag islands. A series of Ag-nanoparticle films fabricated at different evaporation rates but with mass thickness fixed at 100 Å was used to study the surface-enhanced Raman scattering (SERS) from benzoic acid. The major differences among the films fabricated are the degrees of aggregation of Ag nanoparticles, while their mean grain sizes fall in the same length scale. Every film studied exhibits a surface-plasma resonance peak at ∼3800 Å with a long tail extending to the visible region, and the extinction of aggregated films is much stronger than that of dispersed films. In emphasizing the electromagnetic effect, the attenuated total-reflection method was employed in obtaining the SERS spectrum, where the excitations of surface plasmons and the enhanced Raman intensities were measured simultaneously. Densely aggregated films give stronger enhancements than dispersed films do, and a factor difference as large as 15 on the enhanced Raman intensities was observed between the most densely and the most loosely aggregated films studied. The enhancements are electromagnetic in nature, and we attribute them to the excitations of surface plasmons.