Scanning tunneling microscopy and spectroscopy of rubrene on clean and graphene-covered metal surfaces

Rubrene (C₄₂H₂₈) was adsorbed with submonolayer coverage on Pt(111), Au(111), and graphene-covered Pt(111). Adsorption phases and vibronic properties of C₄₂H₂₈ consistently reflect the progressive reduction of the molecule-substrate hybridization. Separate C₄₂H₂₈ clusters are observed on Pt(111) as well as broad molecular resonances. On Au(111) and graphene-covered Pt(111) compact molecular islands with similar unit cells of the superstructure characterize the adsorption phase. The highest occupied molecular orbital of C₄₂H₂₈ on Au(111) exhibits weak vibronic progression while unoccupied molecular resonances appear with a broad line shape. In contrast, vibronic subbands are present for both frontier orbitals of C₄₂H₂₈ on graphene. They are due to different molecular vibrational quanta with distinct Huang-Rhys factors.