Surface Molecular Structure of Self-Assembled Alkanethiols Evidenced by UPS and Photoemission with Synchrotron Radiation

Abstract

With the aim of investigating the configurational and conformational characteristics of monolayers of alkanethiols on gold by means of a photoemission experiment, we compare the valence data obtained from these layers using synchrotron radiation and ultraviolet photoemission spectroscopy (UPS). The incident energy used for synchrotron radiation has been chosen very close to the Cooper minimum of the 5d levels of gold, in order to limit as much as possible interference from the substrate. The recorded spectra differ markedly from one sample to another, notwithstanding their closely related chemical nature. These variations are compared consistently with semiquantitative simulations on model clusters and found thereby to reflect specifically the influence of the connectivity and of specific interactions on the electronic structure of long hydrocarbon chains.

Two-photon photoemission from ex-situ prepared butanethiol SAMs on Au (111)

“…Density functional calculations of individual (standing-up) alkanethiols in contact with a gold substrate derive HOMO orbitals of the alkyl chain at about 3 eV binding energy [22,25]. However, conventional (UPS) photoemission data on alkanethiol SAMs do not yield evidence for any adsorbate induced state in this energy regime [26,27]. Interestingly, a very similar spectral signature at 3 eV binding energy was also reported recently in a 2PPE study on tetra-tert-butyl-azobenzene (TBA) on Au (111) [28].…”

Comparison of Electronic Transport Measurements on Organic Molecules

“…The ultraviolet photoemission spectroscopy (UPS) data of alkane thiol monolayers on Au that have been reported do not show any evidence for molecular levels within~5 eV from the Au Fermi level, suggesting either a low cross-section for the HOMO or a deep-lying HOMO level. [50,51] It may be possible to decide between these possibilities by probing the LUMO level with complementary inverse photoemission spectroscopy (IPES) and two-photon photoemission (2PPE) data, though we are not aware of any published data. It is likely that, in addition to these, other types of experiments will be needed to unravel the mechanisms of current transport, even those operative through simple alkane chains.…”

Valence electron momentum spectroscopy of n-butane

“…Firstly, investigations on quasiparticle grounds of the electron binding energies of saturated hydrocarbons of varying size and complexity [6][7][8][9][10] have disclosed evidence for a mixing of the C 2s and C 2p ϩH 1s character of orbitals at the border of the inner and outer valence regions ͑14-22 eV͒. This phenomenon relates to long-range methylenic hyperconjugation in large zigzag planar n-alkane segments, 6,7 and leads to a strong dependence of the ionization spectrum to the conformation of chains.…”