Detection and characterization of submonolayer adsorbates at smooth metal surfaces by Tip Enhanced Raman Spectroscopy

We employ Tip Enhanced Raman Spectroscopy (TERS) to obtain Raman Spectra from a (sub)monolayer of adsorbates on a single crystalline metal surface, which cannot be obtained from Normal Raman Scattering (NRS) because of the very low cross section. This problem of usually very low band intensities for NRS is solved in a simple and effective way: the excitation laser beam is focussed onto a gold STM tip which is in tunneling contact with the sample surface and thus in close vicinity to the adsorbed molecules. Due to the excitation of surface plasmons localized in the vicinity of the tip apex, the tip will emit a very strong electromagnetic field. Only the molecules situated in the enhanced field region (approx. 20-50 nm radius) underneath the tip apex give rise to intense characteristic Raman bands. To demonstrate the ability of this new technique, we have studied a variety of adsorbate species at a Au(111) substrate: large, resonant molecules such as the dye Malachite Green Isothiocyanate (MGITC) are examined as easily as smaller, nonresonant molecules like SCNor CN-. Also biologically relevant species, such as the DNA bases adenine, cytosine, guanine and thymine, are detected and identified without difficulty even at submonolayer concentrations. With regard to future developments, strategies for the application of TERS in electrochemical and UHV systems, which permit the control of the environment, will be discussed