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The combination of scanning probe microscopy (topographic information) and Raman spectroscopy (chemical information) has proven to be an excellent tool for the investigation of adsorbates at atomically smooth metal surfaces [1,2]. Focussing the incident laser (632 nm) onto the apex of a gold STM-tip creates a giant electromagnetic field in the cavity between tip and substrate. Only the molecules situated in this enhanced field give rise to intense Raman scattering.
For this study, tip-enhanced resonance Raman (TERR) spectra of the triaryl dye Malachite Green Isothiocyanate (MGITC) adsorbed at a Au(111) single crystal substrate in (sub)monolayer quantities are analysed. Together with the corresponding STM images, the sensitivity of this technique as a probe for the detection and semi-quantitative determination of the dye coverage is highlighted.
The recorded TERR spectra decrease with decreasing surface coverage, but do not show any alteration in relative band intensities or band shifts. This indicates a comparable adsorption geometry at all surface concentrations and negligible interactions between the adsorbed molecules concerning the resulting resonantly enhanced Raman modes.
STM images recorded for surfaces prepared from varying adsorbate solution concentrations (4.5 x 10-5 to 1.2 x 10-8 M) show structures of approx. 1.5 nm diameter, which closely resembles the size of a MGITC molecule.
The maximum coverage is estimated to 1 x 1014 molecules/cm2 or 1260 molecules in the enhanced field region. Assuming a linear relation between coverage and spectral intensity (~1.3 x 105 cts/s per molecule), at present, the lower limit of detection lies at < 20 molecules, only about one order away from the single-molecule detection limit.
[1] B. Ren et al., Angew. Chem. Int. Ed. 2005, 36, 541.
[2] B. Pettinger et al., Phys. Rev. Lett. 2004, 92, 096101.
