Angle-resolved ultraviolet photoelectron spectroscopy and theoretical simulation of a well-ordered ultrathin film of tetratetracontane(n−C44H90)on Cu(100): Molecular orientation and intramolecular energy-band dispersion

Abstract

The electronic structure and molecular orientation of a tetratetracontane $(n\ensuremath{-}{\mathrm{C}}_{44}{\mathrm{H}}_{90};$ TTC) ultrathin film on a Cu(100) surface were studied by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) using synchrotron radiation. A well-oriented thin film of TTC was successfully prepared by vacuum evaporation in ultrahigh vacuum at room temperature. We observed a $(2\ifmmode\times\else\texttimes\fi{}1)$-like low-energy electron-diffraction (LEED) pattern for the deposited TTC film. This result indicates that the TTC molecules lie on the Cu(100) surface in two types of domains, rectangular to each other, in which the alkyl-chain axes are along the [110] and $[11\ifmmode\bar\else\textasciimacron\fi{}0]$ directions of the Cu(100) surface. The application of the dipole selection rules to the normal-emission ARUPS spectrum revealed that the C---C---C plane of TTC is parallel to the Cu(100) surface plane (flat-on orientation). The intramolecular energy-band dispersion of TTC was examined by changing the take-off angle of emitted electron along the [110] direction of the Cu(100) surface. The observed results support the conclusion about the direction of alkyl-chain axes by LEED observation. In order to analyze the molecular orientation more quantitatively, we also performed theoretical simulations of the angle-resolved photoemission spectra using the independent-atomic-center (IAC) approximation combined with ab initio molecular-orbital (MO) calculations for various molecular orientations. The simulated spectra for flat-on orientation are in excellent agreement with the observed spectra. These results once again verify the deduced molecular orientation, and also demonstrate the reliability of theoretical simulation with the IAC/MO approximation for compounds without a \ensuremath{\pi}-electron system. Furthermore, we observed a work function change of about -0.3 eV by adsorption of TTC. Such a decrease of the work function indicates the formation of a dipole layer at the interface, in contrast to the traditional picture of energy-level alignment assuming a common vacuum level at the organic/metal interface.