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

Rubrene (C(42)H(28)) was adsorbed with submonolayer coverage on Pt(111), Au(111), and graphene-covered Pt(111). Adsorption phases and vibronic properties of C(42)H(28) consistently reflect the progressive reduction of the molecule–substrate hybridization. Separate C(42)H(28) 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 C(42)H(28) 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 C(42)H(28) on graphene. They are due to different molecular vibrational quanta with distinct Huang–Rhys factors.