Unoccupied electronic structure and molecular orientation of rubrene; from evaporated films to single crystals

• Published in: Surface science 2016-07, Vol.649, p.7-13
• Main Authors: Ueba, T., Park, J., Terawaki, R., Watanabe, Y., Yamada, T., Munakata, T.
• Format: Article
• Language: English
• Subjects: Organic semiconductor; Photoelectron spectroscopy; Rubrene; Single crystal; Thin film; Unoccupied level
• ISSN: 0039-6028; 1879-2758
• DOI: 10.1016/j.susc.2016.01.015

Two-photon photoemission (2PPE) spectroscopy and ultraviolet photoemission spectroscopy (UPS) have been performed for rubrene single crystals and evaporated thin films on highly oriented pyrolytic graphite (HOPG). The changes in the 2PPE intensity from the single crystals by the polarization of the light and by the angle of the light incident plane against the crystalline axes indicate that the molecular arrangement on the surface is similar to that in the bulk crystal. On the other hand, in the case of evaporated films, the polarization dependence of 2PPE indicates that the tetracene backbone becomes standing upright as the thickness increases. In spite of the alignment of molecules, the broadened 2PPE spectral features for thick films suggest that the films are amorphous and molecules are in largely different environments. The film structures are confirmed by scanning tunneling microscopy (STM). The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) derived levels of the single crystal are shifted by +0.18 and −0.20eV, respectively, from those of the 0.8ML film. The shifts are attributed to the packing density of molecules. It is shown that the unoccupied electronic structure is more sensitively affected by the film structure than the occupied electronic structure. [Display omitted] •Change in electronic structures of rubrene from evaporated films to single crystal•Unoccupied levels are more sensitive to the film structure than occupied levels.•Correlation between electronic structures and film/crystal structures•Thick films are amorphous with standing tetracene backbone.