XPS measurements were performed with a vacuum-cluster-tool (ULVAC-PHI VersaProbe II) at a base pressure inside the analysis chamber below 5x10 -9 mbar. The setup is equipped with a monochromatic X-ray source (Al Kα = 1486.6 eV) set at 1.69 mA and 15 kV with a spot size of 200 µm. The lateral resolution of the machine is, however, dependent on the spot size of the analyzer and is expected to be one order of magnitude lower. The machine is periodically calibrated for having the Fermi edge of a clean silver foil at 0 eV.