Toward Raman fingerprints of single dye molecules at atomically smooth Au(111)

The creation of a highly enhanced electromagnetic (EM) field underneath an STM-tip enables Raman spectroscopic studies of organic submonolayer adsorbates at atomically smooth single crystalline surfaces. To study the sensitivity of this technique, tip-enhanced resonance Raman (TERR) spectra of the dye Malachite Green Isothiocyanate (MGITC) on Au(111) in combination with the corresponding STM images of the probed surface region were analyzed. The detection limit for unambiguous identification of the dye and semi-quantitative determination of the surface coverage reaches ≤ 0.7 pmol/cm2, or approximately 5 molecules present in the enhanded-field region, which is confirmed by STM images. Because of well-defined adsorption sites at atomically smooth Au(111) surfaces, no variation in band positions or relative band intensities was observed at the single- or few-molecule detection level when employing TERR spectroscopy