Raman Scattering Improves When Substrates Are Adorned with Silver and Gold
Gold may glitter, but silver really shines when it comes to substrates for surface-enhanced Raman scattering. SERS can augment a Raman signal by 14 to 15 orders of magnitude, which makes the technique invaluable for research. Repeatable SERS results often involve a substrate, and some of the best enhancement results to date have come from silver films deposited over nanostructures.
Researchers have developed a multilayer gold surface-enhanced Raman scattering (SERS) substrate by depositing a gold film over nanostructures (GFON) and topping it with thin silver-island films and with another gold layer (inset). The result is a greater enhancement of the Raman signal, demonstrated by the two spectra. Courtesy of Honggang Li, University of Maryland Baltimore County.
Now a team from the University of Maryland Baltimore County, under the direction of Brian M. Cullum, has developed a multilayer gold substrate that works just about as well as silver and is more stable, with a shelf life measured in months instead of days.
The group deposited a 100-nm-thick gold film over 430-nm silica spheres that were dispersed in a monolayer across a glass slide. They deposited silver oxide islands about 10 Å thick, an amount designed to conserve silver without sacrificing enhancement.
Team member Honggang Li noted that the material forced some of the configuration. “Ideally, we would prefer a thin film of silver oxide, but due to self-adhesion, the silver tends to form islands.”
The researchers topped everything off with another gold layer, then measured the resulting Raman signal enhancements using a custom setup comprising a 10-mW CW HeNe laser from JDS Uniphase, a spectrometer made by Acton with a resolution of 5 cm–1 and a CCD made by Roper Scientific. They found that adding the silver island and second gold layer resulted in a fifteenfold increase in the SERS signal as compared with that of the single gold film over nanostructures.
The group is investigating the use of other dielectric materials as replacements for silver, which might produce even greater SERS improvements while being cheaper and simpler to fabricate. Even without that, Li noted that the substrates could be useful in field applications, which require reproducibility and long-term storage.
Applied Spectroscopy, December 2006, pp. 1377-1385.
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