Raman signals from molecules on metal nanostructures can be enhanced by orders of magnitude due to electromagnetic (EM) effect where surface plasmon resonance takes place in nanogaps in the metal nanostructures. On the other hand, chemical (CM) effect due to metal-molecule interaction on the metal nanostructures can contribute the signal enhancement and modify shape of Raman spectra (i.e., relative peak-intensities and vibrational energies). Here, we investigated surface enhanced Raman scattering (SERS) of molecules (4-aminothiophenol) adsorbed on metal nanostructures with different nanogap widths to resolve the EM and CM effects on the Raman signals. We demonstrated that the SERS intensity increased with the decrease in the nanogap width. The signal enhancement was mostly due to the EM effect. On the other hand, we found that the spectrum shape was independent on the nanogap width. The interaction of adsorbed molecule with the metal was constant with irrespective of the nanogap widths, which explained the constant contribution of CM effect and similar spectrum shape for the SERS signal of the samples with the different nanogap widths.
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