Electronic and optical properties of supported C60 molecules studied by scanning tunneling microscopy

This thesis is primarily concerned with optical properties of adsorbed C60 molecules on metal and insulating substrates. These properties were studied on a single-molecule scale, using Scanning Tunneling Microscope-induced light emission spectroscopy (STM-LES). In the first part of the thesis (Chapter 2), we present the investigation of the structural and electronic properties of C60 islands on Au(111) and on ultrathin NaCl films. We further present a study of the structure of ultrathin sodium chloride films grown on Au(111), and the influence of the herringbone reconstruction on the interface state of a NaCl-covered Au(111) surface. In the second part of the thesis (Chapter 3 and 4), we show that applying a local technique such as STM induced optical spectroscopy enables the exploration of the optical characteristics of individual objects on the nanoscale. Contrary to conventional non-local techniques, the local character of this technique offers the unique possibility to select and probe individual atoms, molecules or clusters on surfaces. Here, we demonstrate the capability of STM-induced light emission spectroscopy (STM-LES) for the chemical recognition on a single-molecule scale. In Chapter 3, we present STM-induced light emission spectroscopy (STM-LES) of a clean Au(111) surface, Au(111) surface covered with islands of C60 molecules and of Au(111) covered with ultrathin NaCl films. We have performed an extensive investigation of the dependence of the gold plasmon spectra on tunneling current and voltage. We show the influence of nano-objects on the shape and intensity of the spectra. Furthermore, we investigated the influence of this experimental technique on morphological changes of surfaces and adsorbed layers. In Chapter 4, we present the first observation of energy resolved luminescence from selected C60 molecules excited by electrons tunnelling through a double barrier STM junction. A comparison with the luminescence spectra obtained by non-local spectroscopy from dispersed C60 molecules in rare gas matrices enables us to demonstrate the molecular origin of the detected light and to identify the observed spectral features. The present novel observation of both, fluorescence and phosphorescence constitutes a solid basis for the chemical identification of an individual C60 molecule.