Quantifying Image Charge Effects in Molecular Tunnel Junctions Based on Self-Assembled Monolayers of Substituted Oligophenylene Ethynylene Dithiols

Zuoti Xie, Valentin Diez Cabanes, Quyen Van Nguyen, Sandra Rodriguez-Gonzalez, Lucie Norel, Olivier Galangau, Stephane Rigaut, Jerome Cornil, C. Daniel Frisbie

Research output: Contribution to journalArticlepeer-review

Abstract

A number of factors contribute to orbital energy alignment with respect to the Fermi level in molecular tunnel junctions. Here, we report a combined experimental and theoretical effort to quantify the effect of metal image potentials on the highest occupied molecular orbital to Fermi level offset, ϵh, for molecular junctions based on self-assembled monolayers (SAMs) of oligophenylene ethynylene dithiols (OPX) on Au. Our experimental approach involves the use of both transport and photoelectron spectroscopy to extract the offsets, ϵhtrans and ϵhUPS, respectively. We take the difference in these quantities to be the image potential energy eVimage. In the theoretical approach, we use density functional theory (DFT) to calculate directly eVimage between positive charge on an OPX molecule and the negative image charge in the Au. Both approaches yield eVimage ∼-0.1 eV per metal contact, meaning that the total image potential energy is ∼-0.2 eV for an assembled junction with two Au contacts. Thus, we find that the total image potential energy is 25-30% of the total offset ϵh, which means that image charge effects are significant in OPX junctions. Our methods should be generally applicable to understanding image charge effects as a function of molecular size, for example, in a variety of SAM-based junctions.

Original languageEnglish (US)
Pages (from-to)56404-56412
Number of pages9
JournalACS Applied Materials and Interfaces
Volume13
Issue number47
DOIs
StatePublished - Dec 1 2021

Bibliographical note

Funding Information:
C.D.F. acknowledges financial support from the U.S. National Science Foundation (CHE-2003199). Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program (DMR-2011401). The work of S.R.G. has been supported by the Belgian National Fund for Scientific Research (F.R.S.-FNRS). We also acknowledge the Consortium des Équipements de Calcul Intensif (CÉCI) funded by F.R.S.-FNRSfor providing the computational resources. J.C. is an FNRS research director.

Publisher Copyright:
© 2021 American Chemical Society.

Keywords

  • Fermi level-HOMO offset
  • charge transport
  • image charge
  • molecular junction
  • photoelectron spectroscopy
  • single level model
  • tunneling

How much support was provided by MRSEC?

  • Shared

PubMed: MeSH publication types

  • Journal Article

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