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

The nature of the interface between electrogenerated poly(3,4-ethylenedioxythiophene) and the Au substrate was studied. The adsorption of the relevant monomer was investigated and compared with that of other thiophene derivatives. Very thin films of the thiophene derivatives have been obtained through chemisorption processes from vapor and liquid phases, on Au polycrystalline substrates, Au nanoparticles, and a Au(111) single crystal and characterized with different techniques as XPS and Raman spectroscopies. Results show that the poly(3,4-ethylenedioxythiophene)/metal interface is far from being simply constituted by unreacted molecules in contact with the substrate.

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In molecular electronics, the interface between the organic semiconductors (either molecular or polymeric films) and metal electrodes is critical for the electronic level alignment and for the determination of charge injection barriers. Here is studied with valence band photoemission and near edge X-ray absorption fine structure (NEXAFS) spectroscopy the chemisorption of 3,4-ethylenedioxythiophene (EDOT) derivatives on Au(111) single crystal and polycrystalline surfaces.
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: 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.
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. NEXAFS indicates that the thiophene rings tend to adopt a planar configuration at the interface.

L. Pasquali et al., The Journal of Physical Chemistry C, 116, 15010-15018 (2012)