Phase-dependent electronic properties of monolayer and multilayer anthracene films on graphite [0001] surfaces

The evolution of the electronic properties of thin anthracene films on highly oriented pyrolytic graphite [0001] substrates as a function of temperature has been investigated using ultraviolet photoelectron spectroscopy. The change in the valence line shape with increasing substrate temperature has been univocally associated with the occurrence of different molecular orientations and structural phases (e.g., "flat-lying" mono- and multilayer films and a multilayer phase with a "standing-up" orientation). These thin-film phases are characterized by ionization energies varying up to 0.9 eV; the surface dipole and possibly to a minor extent polarization energy contributions are shown to be related to the specific molecular packing/molecular orientation and to the interplay between the molecule-molecule vs molecule-substrate interactions. Furthermore, for the monolayer system, a vibrational fine structure of the highest occupied molecular orbital (HOMO) is clearly revealed, thus allowing a detailed study of the HOMO hole-vibration coupling and the temperature-dependent broadening. Showing a large number of recently discussed contributions to the valence line shape, anthracene thin films emerge as "benchmark" systems to study the behavior of holes relevant for the charge transport in organic electronic devices.