For surface-sensitive spectroscopy, the samples were carried to our XPS-STM cluster in a UHV transfer vacuum suitcase attached to a load-lock, which allowed transferring them without breaking the UHV conditions. The XPS measurements were performed in a Phoibos 150 Specs spectrometer with a base pressure lower than 10 -10 torr. A monochromatised Al Kα small radiation source with a spot size of 1 mm 2 was used for the excitation of the electrons. The pass energy of the electron energy analyser was set at 20 eV. The take-off angle between the analysed photoelectrons and the substrate surface was 90°. The overlapping spectral features were resolved into individual components using the damped non-linear least squares method after subtraction of Shirley background using Gaussian-Lorentzian line shape. The surface atomic content was accomplished assuming a homogenous distribution of atoms and Scofield photoionization cross-section.