1. Institute of Modern Optics, School of Physics, Peking University, Beijing 100871, China 2. College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
Abstract：Metal nanowire arrays as surface-enhanced Raman scattering substrate, could bring high enhancement effect. According to the electromagnetic enhancement theory, the high enhancement factors come from the strong electromagnetic coupling between nanowires in the arrays. Gold nanowire arrays and gold nanobrushes were prepared by electro-deposition of gold into alumina template coated with gold film, then chemically etching alumina directly or rubbing gold film and chemically etching alumina. SEM showed the nanowires around 60 nm in diameter with length around 600 nm in the nanobrushes. In the gold nanobrushes, local nanowires collect in bundles after complete dissolution of alumina matrix due to the transverse tension during drying in the air. The two kinds of samples are different in structure. But optical transmission measurements of the two samples in the direction perpendicular to the substrate both showed plasma-enhanced absorption around 640-650 nm. The authors choose 4-mercaptopyridine (4-MP) as the probe molecules because 4-MP chemically adsorb on Au nanowires tightly by monolayer and align perpendicular to Au surface. SERS experiments were performend with a 632.8 nm laser. SERS signals come from the back scattering of the incident light propagating perpendicular to the Au substrates. And the Raman spectra were collected at more than 10 different points on each sample. A strong, locally inhomogeneous surface-enhanced Raman spectrum of 4-MP adsorbed on Au nanobrushes with enhancement factor as high as 1×106 was observed. At the same time, SERS of 4-MP from the gold nanowire arrays under the same experiment conditions showed the enhancement factor around 102. Considering the optical transmission measurements of the two samples, the enhancement factors both come from resonance field enhancement. According to the difference of the two kinds of samples, the higher enhancement factors of gold nanobrushes are related with the gold film. According to the localization theory of electromagnetic field, the field around the upper part of the nanowires of the nanobrushes is far stronger than that of the bottom part and on the film. So the gold film introduced higher enhancement factors in SERS. The Raman spectrum of 4-MP also showed that the CT mechanism affects the enhancement factors.
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