Structural Requirements for Surface-Induced Aromatic Stabilization
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Structural Requirements for Surface-Induced Aromatic Stabilization Takuya Hosokai1, Keiichirou Yonezawa2, Kengo Kato2, Rintaro Makino2, Jinpeng Yang2, Kaveenga Rasika Koswattage2, Alexander Gerlach3, Frank Schreiber3, Nobuo Ueno2, Satoshi Kera2 1 Department of Materials Science and Technology, Iwate University, 4-3-5 Ueda, Morioka, 0208551 Iwate, JAPAN 2 Department of Nanomaterial Science, Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, 2638522 Chiba, JAPAN 3 Institut für Angewandte Physik, Universität Tübingen, Auf der Morgenstelle 10, Tübingen 72076, GERMANY ABSTRACT Surface-induced aromatic stabilization (SIAS), a recently proposed mechanism leading to a formation of charge-transfer (CT) states at organic/metal (O/M) interfaces [G. Heimel, et al., Nat. Chem. 5, 187 (2013)], was investigated for an aromatic hydrocarbon, diindenoperylene (DIP), by means of synchrotron radiation-based ultraviolet photoelectron spectroscopy (UPS). By employing DIP and noble metal substrates (Ag and Cu), we confirmed the formation of CT states, indicating that an inclusion of a specific functional group with a hetero-atom within adsorbate molecules as suggested before is not necessarily required for the formation of CT states mediated by the SIAS. With a comparison of the mother and analogue molecules, perylene and PTCDA, we discuss the structural requirement for the realization of the SIAS. INTRODUCTION The electronic structure of O/M interfaces is crucial for Organic Electronics, in which charge carrier injection at the interface strongly influences the device performance [1-4]. One remaining issue in this research field is a formation of CT states at the O/M interfaces. While it is known that the CT state is caused by an electron-transfer from the metal substrate to the former lowest unoccupied molecular orbital (LUMO) of the adsorbate, it was not well understood until recently why the CT state forms unexpectedly in specific O/M systems [5-8]. Heimel et al. succeeded to explain the mechanism leading to the formation of CT states in terms of SIAS [9]; the stabilization of the -conjugation of the adsorbed molecules due to an electron donation from the substrate atoms to specific functional group (e.g. keto group or azo group) within the molecules lowers the LUMO level through the formation of a resonance state of the molecules, and subsequently induces the CT. In the literature [9] such a specific functional group with hetero-atoms is introduced as a key for the SIAS. However, it is not yet clear if the inclusion of hetero-atoms is prerequisite for realization of the SIAS. In this paper, we will show that even functional groups without hetero-atoms yield a SIAS. We studied the interface electronic structure of DIP (C32H16, Fig. 1) monolayers on Ag and Cu polycrystalline surfaces by means of synchrotron radiation (SR)-based UPS (hereafter referred to as just UPS). We found the formation of CT states for DIP on both the noble metal surfaces, and such an interface structure is very similar t
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