Bands alignment between organic layers of Alq3, Gaq3, Erq3 and graphene on 6H-SiC(0001)

This paper concerns physicochemical properties of three interfaces composed of organic layers of Alq3, Gaq3 or Erq3 vapor deposited on graphene/6H-SiC(0 0 0 1) surfaces. A graphene termination of 6H-SiC(0 0 0 1), about one monolayer thick, was produced in situ by thermal annealing of the substrate. The formation of the final interfaces and their characterization were also performed in situ under ultrahigh vacuum conditions. X-ray/UV photoelectron spectroscopies (XPS/UPS) were employed in this study. XPS results showed that the organic layers did not react chemically with the graphene/6H-SiC(0 0 0 1) surfaces at room temperature. In turn, UPS measurements showed that the adsorption of organic layers decreased the position of the vacuum level of the initial surface and one covered with the organic layers which initiated the surface dipoles. The vacuum level was lowered by 0.15 eV, 0.4 eV, and 0.9 eV for Alq3, Gaq3, Erq3 layers, respectively. The positions of highest occupied molecular orbitals (HOMOs) were also found. They were located at 1.55 eV, 1.75 eV, and 2.0 eV below the Fermi level, respectively, for Alq3, Gaq3, and Erq3 layers. The band energy diagrams for the three interfaces were constructed.