| AVS 59th Annual International Symposium and Exhibition | |
| Surface Science | Tuesday Sessions |
| Session SS-TuP |
| Session: | Surface Science Poster Session |
| Presenter: | J. Park, Seoul National University, Republic of Korea |
| Authors: | J. Park, Seoul National University, Republic of Korea T. Ueba, Osaka University, Japan L. Terawaki, Osaka University, Japan T. Yamada, Osaka University, Japan H. Kato, Osaka University, Japan T. Munakata, Osaka University, Japan |
| Correspondent: | Click to Email |
Electronic excitation at the interface between an organic molecular film and a substrate is of general importance for the area of organic electronics and light conversion processes. We demonstrate rubrene/HOPG as a model system for an organic film/substrate interface. Many efforts have been devoted to improve the performance of a rubrene thin film transistor after high carrier mobility was achieved for rubrene single crystals. The reasons for the poor efficiency of the thin films are attributed to the molecular geometry on the surface. To understand the mechanisms of charge transportation for organic molecular devices, it is the primary step to unravel the molecular electronic structures of both occupied and unoccupied states at interfaces between the film and the substrate.
We have performed two-photon photoemission (2PPE) spectroscopy for rubrene films formed on HOPG substrate. It is revealed a prominently enhanced unoccupied molecular peak, which is resonantly excited from the highest occupied molecular orbital (HOMO). Interestingly, the enhancement of the peak becomes less significant at the coverage higher than 1 monolayer, where the image potential state (IPS) peak on the substrate disappears. The resonance enhancement is moderate with s-polarization, by which the transition to IPS is completely suppressed. We ascribe that the excitation of the the level is mediated by the IPS on HOPG. Though the IPS wave function extends outside the molecules, it interacts with the unoccupied molecular orbital at the edges of molecular islands, causing the strong resonance enhancement of the unoccupied molecular level.
By clarifying the mechanism, the excitation process is expected to be useful to highly enhance the efficiency of organic molecular devices and light conversion processes. The energy of IPS is generally governed by the work function. It may be possible to tune the IPS level nearly resonant to an unoccupied level of organic films. This provides a way to tailor the electronic excitation efficiency.