Gap-mode enhanced Raman scattering of organic monolayers on flat Au(111) surface

A metal-molecular junction is recognized as a fundamental issue of molecular-based nano-devices. Charge transport through organic layers is strongly affected by chemical and physical properties of metal-molecular interfaces. In order to investigate molecular structures adsorbed on metal surfaces, surface-enhanced Raman scattering (SERS) is widely utilized as a powerful spectroscopic tool. Because of the electromagnetic origin of SERS, however, conventional SERS spectroscopy is applicable only for ``rough'' metal surfaces. Since various adsorption sites are exposed on rough metal surfaces, it is difficult to obtain intrinsic information of metal-molecular junctions. Here, we provide a simple method of enhanced Raman spectroscopy for organic monolayers on ``flat'' metal surfaces based on gap-mode plasmon excitation. The gap-mode enhanced Raman spectra measured on ``flat'' Au(111) single crystal facets were compared with conventional SERS spectra, and adsorption site dependence of molecular structures was discussed.