Tip-enhanced Raman spectroscopy in ultra-high vacuum ; Spitzenverstärkte Raman-Spektroskopie im Ultrahochvakuum

Tip-enhanced Raman spectroscopy (TERS) is a promising new scanning probe technique for the investigation of the vibrational properties of molecules on surfaces. TERS possesses both, a very high sensitivity - down to the single molecule level - and a spatial resolution in the 10 nm range. In contrast to most TERS studies, we perform our experiments under ultra-high vacuum (UHV) conditions to ensure a high sample quality. Therefore, we have added a preparation chamber to the existing UHV-TERS set-up. Large parts of the work were devoted to improving the capabilities and the manageability of the UHV- TERS device. We have chosen the fullerene C60 on a Au(111) surface as sample system. From a large body of literature the vibrational as well as the adsoption properties of C60 are well understood. Compared to other molecules studied in UHV-TERS, C60 has a much smaller Raman cross-section. This allows to test if our experiment can be applied to a larger class of molecular adsorbates with comparably small Raman cross-sections. To separate the signals of C60 from those of foreign molecules, we have recorded time-resolved series of tip-enhanced Raman (TER) spectra. For the analysis we select series showing the typical vibrational features of C60 that were stable over several seconds to minutes. TER spectra of C60 show a remarkable dependence on the bias voltage applied between tip and sample, indicating a significant influence of the tip-sample separation on the signal strength, even on a sub-nanometer scale. Measurements on different morphologies of C60 on Au(111) exhibit shifts for certain Raman peaks that are discussed in the context of different adsorption sites and polymerisation in the C60 adlayer. Finally, we present experiments, in which scanning tunnelling microscopy images of C60 on Au(111) and TER spectra were acquired from the same spatial region. These results can act as the point of departure for future measurements with simultaneously recorded TERS maps and topographic images, that allow a correlation ...