Electronic structure and molecular orientation of well-ordered polyethylene oligomer (n-C44H90) on Cu(100) and Au(111) surfaces studied by UV photoemission and low energy electron diffraction

The electronic structure and molecular orientation of tetratetracontane (n- C44H90) films on Cu(100) and Au(111) surfaces were investigated by angle-re solved UV photoemission spectroscopy (ARUPS) and low energy electron diffra ction (LEED). The observed ARUPS spectra showed the drastic take-off angle dependence due to intramolecular band dispersion. A 2x1-like LEED pattern w as observed for both substrates. From these results and theoretical simulat ion of ARUPS spectra based on independent-atomic center (IAC) approximation , we found that the C-C-C plane of the adsorbed TTC molecule is parallel to the substrate surface and its molecular axis is along a [110] direction fo r both substrates. We also measured the work function change by adsorption of TTC. The observed values were c.a. -0.3eV and -0.7eV for Cu(100) and Au( 111) systems, respectively. Such decrease of the work function indicates th e existence of a dipole layer at the interfaces in contrast to the traditio nal picture of energy level alignment at organic/metal interface assuming a common vacuum level at the interface. The dipole formation in such physiso rbed systems can be explained by the polarization of the TTC molecule due t o an image force. (C) 1999 Elsevier Science B.V. All rights reserved.