Collective electrostatics determining photoexcitations and charge transport in self-assembled monolayers

A. Egger1,2, V. Obersteiner1, T. Taucher1, I. Hehn1, A. Kovalchuk3, R. Chiechi3, T. Abu-Hussein4, A. Terfort4, S. Schuster5, M. Zharnikov5 and E. Zojer1

1.Institute of Solid State Physics, NAWI Graz, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria

2.Department of Materials and Interfaces, Weizmann Institute of Science, Rehovoth 76100, Israel

3.Stratingh Institute for Chemistry & Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

4.Institut für Anorganische und Analytische Chemie, Universität Frankfurt, Max-von-Laue-Straße 7, 60438 Frankfurt, Germany

5.Angewandte Physikalische Chemie, Universität Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany

It has been extensively discussed during the past decade that collective (often also termed cooperative) electrostatic effects arising as a consequence of the regular arrangements of dipoles dominate the electronic properties of self-assembled monolayers.[1,2]  They are, for example, responsible for a decoupling of interfacial level alignment from SAM-induced  work-function modifications.[3] Recently, it has also been suggested that they can be used to design adsorbate systems with unprecedented properties like interfacial quantum cascades or quantum wells.[4] The present contribution aims at highlighting that such effects also play a crucial role for charge transport and photoionization processes involving SAMs. We show that collective electrostatics results in fundamentally different current-voltage characteristics for SAMs compared to isolated molecules, rendering, for example, the direct comparison between calculations employing periodic boundary conditions and single molecule experiments problematic. For example, collective electrostatics can be responsible for a switching of the transport polarity [5] or can significantly impact the measured transition voltage [6]. It is omnipresent in SAMs-based junctions, as there are always dipole moments present due to the polar docking groups and bonding-induced charge rearrangements [7], but polar groups can also be incorporated next to the docking sites [5,8] or within the backbones [6]. Finally, it will also be described, how collective electrostatic effects shift core level energies (on top of the well-known chemical shifts), an effect often overlooked in literature [9]. This renders x-ray photoelectron spectroscopy as useful tool for probing the electrostatic properties of nanostructures, making it, for example, capable to probe the homogeneity of mixed self-assembled monolayers on a molecular length scale.[10]

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[6] A. Kovalchuk, T. Abu-Husein, D. Fracasso, D. A. Egger,E. Zojer, M. Zharnikov, A. Terfort, and R. C. Chiechi, Chem. Science 7, 781 (2016).

[7] V. Obersteiner, D. A. Egger, and Egbert Zojer, J. Phys. Chem. C 119, 21198-21208 (2015).

[8] V. Obersteiner, D. A. Egger, G. Heimel, and E. Zojer, J. Phys. Chem. C 118, 22395 (2014).

[9] T. C. Taucher, I. Hehn, O. T. Hofmann, M. Zharnikov, and E. Zojer, J. Phys. Chem. C 120, 3428 (2016).

[10] I. Hehn, S. Schuster, T. Wächter, T. Abu-Husein, A. Terfort, M. Zharnikov, and E. Zojer, submitted.