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Title: Molecular orientation-dependent ionization potential of organic thin films
Authors: Chen, W. 
Huang, H. 
Chen, S. 
Huang, Y.L.
Gao, X.Y. 
Wee, A.T.S. 
Issue Date: 25-Nov-2008
Citation: Chen, W., Huang, H., Chen, S., Huang, Y.L., Gao, X.Y., Wee, A.T.S. (2008-11-25). Molecular orientation-dependent ionization potential of organic thin films. Chemistry of Materials 20 (22) : 7017-7021. ScholarBank@NUS Repository.
Abstract: Orientation-dependent ionization potentials (IPs) of organic thin films have been investigated with the combination of synchrotron-based high-resolution photoemission spectroscopy and near-edge X-ray absorption fine structure measurements. Organic thin films of copper(II) phthalocyanine (CuPc) and its fully fluorinated counterpart of copper hexadecafluorophthalocyanine (Fi 16CuPc) with well-controlled orientation are used as model systems. Both molecules lie flat on the graphite surface and stand upright on Au(111) terminated by self-assembled monolayer of octane-1-thiol. The IP for the standing-up CuPc thin film (IP = 4.75 eV) is 0.40 eV lower than that of the lying-down film (IP = 5.15 eV). In contrast, the IP of the standing up F 16CuPc (EP = 6.50 eV) is 0.85 eV higher than that of the lying-down film (IP = 5.65 eV). This reversed orientation dependence in IP is explained by the opposite surface dipoles in the standing-up CuPc and F16CuPc molecular layers originating from the different intramolecular dipolar bonds exposed at the surfaces, i.e., a net upward pointing surface dipole in the standing-up CuPc thin film that lowers the IP, and a net downward pointing surface dipole in the standing-up F16CuPc thin film that increases the IP. © 2008 American Chemical Society.
Source Title: Chemistry of Materials
ISSN: 08974756
DOI: 10.1021/cm8016352
Appears in Collections:Staff Publications

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