Coverage-Dependent Modulation of Charge Density at the Interface between Ag(001) and Ruthenium Phthalocyanine

When an organic film is deposited on a metal surface, charge layers are formed at the interface. These are an important feature of the interface electronic structure and play a crucial role as separation layers between electrodes and active layers in organic devices. Here, we report on a study of the interface between diruthenium phthalocyanine, (RuPc)₂, and the Ag(001) surface. The molecules form two different commensurable arrangements on the substrate, a low density one for a coverage well below the first monolayer and a high density one up to the completion of the monolayer. The focus of this study is on the interface states evolution with the molecular density on the metal surface and the charge distribution in the thin interfacial layer between molecules and substrate. From this investigation, conducted by low energy electron diffraction, scanning tunneling microscopy/spectroscopy, photoemission spectroscopy, and density functional theory, we have found that, even if individual molecules are characterized by a quite similar surface-to-molecule charge transfer pattern, the two molecular arrangements present different valence band structures and, more interestingly, different modulations of the interface charge. These charge modulations are governed by interfacial states energetically resonant with the molecular states, localized at the position of the molecules as well as by a reaction of the electronic cloud of the metal surface to the molecular adsorption due to a Pauli pushback effect. This complex, spatial charge modulation makes the (RuPc)₂/Ag(001) an interesting case of interaction intermediate between physisorption and chemisorption.