HOMO Level Pinning in Molecular Junctions: Joint Theoretical and Experimental Evidence

S. Rodriguez-Gonzalez, Z. Xie, O. Galangau, P. Selvanathan, L. Norel, C. Van Dyck, K. Costuas, C. D. Frisbie, S. Rigaut, J. Cornil

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27 Scopus citations

Abstract

A central issue in molecular electronics in order to build functional devices is to assess whether changes in the electronic structure of isolated compounds by chemical derivatization are retained once the molecules are inserted into molecular junctions. Recent theoretical studies have suggested that this is not always the case due to the occurrence of pinning effects making the alignment of the transporting levels insensitive to the changes in the electronic structure of the isolated systems. We explore here this phenomenon by investigating at both the experimental and theoretical levels the I/V characteristics of molecular junctions incorporating three different three-ring phenylene ethynylene derivatives designed to exhibit a significant variation of the HOMO level in the isolated state. At the theoretical level, our NEGF/DFT calculations performed on junctions including the three compounds show that, whereas the HOMO of the molecules varies by 0.61 eV in the isolated state, their alignment with respect to the Fermi level of the gold electrodes in the junction is very similar (within 0.1 eV). At the experimental level, the SAMs made of the three compounds have been contacted by a conducting AFM probe to measure their I/V characteristics. The alignment of the HOMO with respect to the Fermi level of the gold electrodes has been deduced by fitting the I/V curves, using a model based on a single-level description (Newns-Anderson model). The extracted values are found to be very similar for the three derivatives, in full consistency with the theoretical predictions, thus providing clear evidence for a HOMO level pinning effect. ©

Original languageEnglish (US)
Pages (from-to)2394-2403
Number of pages10
JournalJournal of Physical Chemistry Letters
Volume9
Issue number9
DOIs
StatePublished - May 3 2018

Bibliographical note

Funding Information:
The work of S.R.G. is supported by the Belgian National Fund for Scientific Research (F.R.S.-FNRS). We also acknowledge the Consortium des Equipements de Calcul Intensif (CECI) funded by the Belgian National Fund for Scientific Research (F.R.S.-FNRS) for providing the computational resources. J.C. is an FNRS research director. C.V.D. thanks the support of the National Institute for Nanotechnology (NINT), which is operated as a partnership between the National Research Council, Canada, the University of Alberta and the Government of Alberta. C.D.F. acknowledges financial support from the U.S. National Science Foundation (CHE-1708173). XPS and UPS were carried out in the Characterization Facility, University of Minnesota. This work was also supported by the Universite de Rennes 1 the CNRS, and the Agence Nationale de la Recherche (RuOxLux - ANR-12-BS07-0010-01)).

Funding Information:
The work of S.R.G. is 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 the Belgian National Fund for Scientific Research (F.R.S.-FNRS) for providing the computational resources. J.C. is an FNRS research director. C.V.D. thanks the support of the National Institute for Nanotechnology (NINT), which is operated as a partnership between the National Research Council, Canada, the University of Alberta, and the Government of Alberta. C.D.F. acknowledges financial support from the U.S. National Science Foundation (CHE-1708173). XPS and UPS were carried out in the Characterization Facility, University of Minnesota. This work was also supported by the Université de Rennes 1, the CNRS, and the Agence Nationale de la Recherche (RuOxLux - ANR-12-BS07-0010-01)).

Publisher Copyright:
Copyright © 2018 American Chemical Society.

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