A charge neutralizer filament was used to compensate for the charge build-up effect. The conditions used for survey scans were as follows: energy range = 0-1200 eV, pass energy = 160 eV, step size = 0.1 eV, and X-ray spot size = 2 mm in diameter. A binding energy range of 20-40 eV (depending on the examined peak) was used for high-resolution spectra with a pass energy of 20 eV. Argon (0.5 keV) was used as the sputtering source. The C 1s peak with a value of 285 eV was used for calibration in all spectra. Gaussian-Laurentius (GL) functions and a Shirley back-ground were used to fit the experimental XPS data.