Experimental study of pristine and alkali metal doped picene layers: confirmation of the insulating phase in multilayer doped compounds

The electronic structure and the geometric arrange- ment of picene molecules adsorbed on Ag(111) were studied by means of photoemission and near-edge X-ray absorption _ne-structure spectroscopies (NEXAFS). While the valence band of a picene monolayer shows a clear metallic state that evolves with alkali metal doping, in the case of a picene multilayer there is no evidence of metallic states evolving with doping. Our data suggest that the bulk ultrahigh-vacuum _lms of Kxpicene are in an insulating phase, and we attribute this behavior to a strong electron_electron Coulomb interaction that is instead screened in the monolayer. The NEXAFS pro_les of di_erent picene layers show a coverage-dependent orientation, from _at molecules (monolayer) to an orientation of _40° of the molecular long axis with respect to the substrate surface (multilayer). The observed molecular orientations are in disagreement with the expected crystal structure of the bulk material and may explain the presence of insulating states in strongly correlated doped picene multilayers.