UPS spectra were measured using
He I excitation (21.2 eV) and recorded with a constant pass energy of 1.5 eV in the ultrahigh vacuum (UHV) chamber of the XPS instrument. UPS binding energies were referenced to the Fermi edge of Au, which was sputter-deposited onto the sample ex-situ using a coater (Quorum Q150T) equipped with gold target. The gold-covered areas was a circle 2 mm in diameter.
The sample bias was set at value of \(-10\,\mathrm{V}\). Analyzer
was operated in a fixed analyzer transmission mode applying the
pass energy of 1.5 eV. The samples were cleaned by Ar ions bombardment using energy of 1 keV
and ion current density 1 nA/cm2. In order to determine the Fermi level position, the circular area with diameter of 2 mm was coated with 15 nm of gold. For this we used the coater (Quorum Q150T) equipped with gold target.