Impact of Film Structure on Ionization Energy of Titanyl-Phthalocyanine in Thin Films
- PDF / 13,219,286 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 32 Downloads / 201 Views
Impact of Film Structure on Ionization Energy of Titanyl-Phthalocyanine in Thin Films Takuya Hosokai1, Keiichirou Yonezawa2, Rintaro Makino2, Noritsugu Kanayama2, Tomoyuki Koganezawa3, Koji K. Okudaira2, Satoshi Kera2, Nobuo Ueno2 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, 26385522 Chiba, JAPAN 3 Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo-cho, 6795198 Hyogo, JAPAN ABSTRACT Structure-related ionization energy (IE) of vacuum-deposited titanyl-phthalocyanine (OTiPc) thin films was investigated by using in situ ultraviolet photoelectron spectroscopy (UPS) and X-ray diffractometry. Distinct molecular orientations (i.e. lying-flat and standing-up orientation) in different polymorphous (i.e. monoclinic β-phase and triclinic α-phase) were observed on a surface of polycrystalline (poly-) Au and octadecyltrichlorosilane-self assembled monolayer (OTS-SAM). For the two structures IE of the highest occupied molecular orbital (HOMO) of OTiPc thin films altered significantly by 0.55 eV. The different IE was attributed to surface dipole potential and strong intermolecular interaction. INTRODUCTION A relationship between electronic and film structures is a central issue in discussing electronic and charge transport properties of organic thin films. IE of frontier MOs of organic thin films, which is an important parameter for the energy level alignment at interfaces in various organic devices, is known to be influenced by molecular orientation and molecular packing structure [1-3]. In particular, a surface dipole potential, which originates from local dipole moment existing at an end of polar group (such as >Cδ- - Hδ+ or >Cδ+ - Fδ-) of molecules and at sides of π-conjugated molecular plane, has recently been proposed to explain the orientationdependent IE of organic thin films [3]. In fact, this concept, which is somewhat similar to the surface plane dependent work function of metal single crystals [4], has been successively adopted for various molecular thin films, such as (perfluorinated-) pentacene, (perfluorinated-) copper Pc, diindenoperylene, (alkyl-) oligothiophene, and even π-conjugated polymers [3,5-8]. The general understanding of the structure-related IE of organic thin films is reaching a sufficient level. The remaining issue to be considered in IE is interplay of the local-bond related surface dipole and molecular permanent dipole as well as contribution of structure-dependent intermolecular interaction. So far the orientation-dependent IE has been investigated only for non-polar molecules, as aforementioned molecules. For such molecules the permanent dipole was not necessarily considered in discussing their IE, although the permanent dipole could be induced by an intermolecular or a molecule-substrate interaction (in a film and a solid interface system)[9,10]. On the other hand, polar organic molecules
Data Loading...
