Epitaxial Thin Film Structures of Electroluminescent Materials
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NTRODUCTION p-sexiphenyl (6P), a model molecule for the polymer poly(para -phenylene) (PPP), is a very interesting material for studying basic physical properties of conjugated organic semiconductors. Additional interest in 6P originates in its huge potential for the application in optoelectronic devices [1-4], as it exhibits efficient blue electrolumines cence. Therefore, extensive studies have been performed on PPP and its oligomers, especially regarding anisotropic optical properties [5,6], texturing of thin films [7,8], and doping [9-13]. However, most of these studies (including sample preparation) were performed either in ambient atmosphere/high vacuum or solution. This often imposes additional uncertainties on the results as unwanted effects due to contamination or side-reactions with contaminants may occur. One very promising approach to find new intrinsic properties and interesting aftereffects is to study conjugated organic materials under ultrahigh vacuum (UHV) conditions [14-18]. Our experiments should serve to explore the potential of well-ordered or heteroepitaxial organic/inorganic structures for
future applications. We investigated in situ the quality of order at interfaces of 6P with Cu(100) and H-terminated Si(111) by means of infrared reflection absorption spectroscopy (IR-RAS) and low energy electron diffraction (LEED). Furthermore, the evolution of the infrared (IR) spectra of 6P upon n -doping by K was studied.
EXPERIMENTAL The infrared reflection absorption spectroscopy and low energy electron diffraction measurements were made in a custom ultrahigh vacuum setup at the Department of Chemistry, Division of Material Science, Nagoya University. For IR-RAS a FTIR spectrometer Mattson R/S-1, equipped with a mercury cadmium telluride detector, was used. Electron diffraction patterns were recorded with a low-current LEED optics from OCI Corp. p -sexiphenyl, purchased from Tokyo Chemical Industries Co. Ltd., was evaporated in situ from a pinhole -source onto polycrystalline Au, Cu(100) and Hterminated Si(111). The base pressure in t he UHV system was below 1×10-10 mbar. During the evaporation of 6P and K, from SAES S.p.A., the pressure remained below 5×10-9 mbar. The 6P thickness was monitored with a quartz microbalance. Atomic force microscopy (AFM) measurements were performed ex sit u with a Nanoscope Multimode TM (Nanoscope IIIa control system) in TappingMode TM.
RESULTS First, 6P was evaporated stepwise onto a Cu(100) single crystal, which was kept at 130°C. The cleanness of the Cu surface was confirmed by LEED. Fig.1 shows the IRRAS spectra series for increasing 6P thickness, up to 31 Å. The inset shows the chemical structure of 6P. The observed peaks positions correspond well with earlier published results [9,12,19,20], and can be assigned as follows: the peaks at 762 and 815 cm-1 are due to C-H out-of-plane vibrations. The peak at 1401 cm-1 corresponds to a C-C stretching mode perpendicular to the molecular axis. The 1482 cm-1 peak is assigned to a C-C stretching along the molecular axis. In
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