Photoemission and X-ray Absorption Study of the Interface between 3,4-Ethylenedioxythiophene-Related Derivatives and Gold

Abstract

The chemisorption of 3,4-ethylenedioxythiophene (EDOT) derivatives on Au(111) single crystal and polycrystalline surfaces is studied with valence band photoemission and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Different EDOT derivatives are considered, namely EDOT itself, its dimer (bi-EDOT), 3′,4′-ethylenedioxy-2,2′:5′,2″-terthiophene (TET) and some of the relevant electro-generated polymers, being PEDOT at different stages of substrate coverage and the TET polymer. The interpretation of the valence band spectra is carried out by comparing with DFT calculations of the densities of states of different possible reaction products that can be formed at the interface. Simulation of the NEXAFS spectra of EDOT and thiophene and comparison with literature data on thiophene and oligothiophene films are used for the interpretation of the X-ray absorption spectra of bi-EDOT, TET on Au, and the TET polymer and PEDOT at different electro-polymerization times. Valence band features that are associated to EDOT products are identified together with new structures that are ascribed to reactive dissociation products at the interface. In particular the presence of unsubstituted thiophene/oligothiophene molecules is observed, together with possible alkyl chains. NEXAFS indicates that the thiophene rings tend to adopt a planar configuration at the interface.