Surface-enhanced Raman scattering from vapor-deposited copper, silver, and gold. Excitation profiles and temperature dependence

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Abstract
Surface-enhanced Raman spectra (SERS) of pyridine adsorbed on copper, silver, and gold films vapor deposited on low temperature substrates are reported. Similar spectra were also obtained on a sputter-cleaned silver single crystal. Excitation spectra (450–750 nm) for all three metals revealed an overall increase in SERS activity at longer wavelengths, the relative increase being greater for copper and gold than silver. A broad excitation maximum near 2 eV was observed for the 1006 cm−1 pyridine SERS signal on silver. A lesser-defined maximum was revealed for copper in the same general vicinity (1.7–2.0 eV) while a broad onset extending below 1.7 eV was observed for gold. Temperature studies (15–300 K) indicate that the observed SERS originate from molecular pyridine chemisorbed to the metal surfaces. In the cases of copper and gold, SERS were also observed from samples maintained at room temperature in vacuum. The intensities of the SERS signals were proportional to incident (cw) laser power at low power densities (<1 MW/cm2).