Vibrational spectroscopy with a scanning tunnelling microscope: The role of the tip

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Inelastic electron tunnelling spectroscopy is an important technique, which enables the investigation of vibrational or magnetic excitations at the atomic scale. Vibrational spectroscopy adds chemical sensitivity to the capabilities of a scanning tunnelling microscope and thus represents an important means in, e.g., surface chemistry. It further enables the analysis of magnetic excitations such as single-atom spin flips. Such investigations are crucial to the development of magnetic data storage devices at the ultimate size. However, very often the excitation cross sections in inelastic electron tunnelling are weak. Further, in vibrational spectroscopy out of the manifold of possible vibration modes only a few modes or even none are usually observed in the spectra. Selection rules have been developed to rationalize these observations in terms of the electronic structure of the adsorbate and substrate and the symmetry of the vibrational modes. The role of the tip, however, has scarcely been discussed – both experimentally and theoretically. The main target of this project is filling this gap by systematically investigating prototypical single-atom and single-molecule vibrational spectra with controllably modified tips.