project duration: 15.05.2012-14.05.2015

Covalently-bonded self-assembled monolayers (SAMs) on metals have a wide variety of applications ranging from biology, via lithography, corrosion protection and sensing to organic electronic devices. When such SAMs are used for manipulating the electronic properties of surfaces, they usually contain polar chemical units. Typically, these units form the terminal groups of the SAMs, i.e., they are located at the SAM-ambient interface. This is far from ideal, as then changing the dipolar group also changes many SAM properties like its wetting properties or the growth of subsequently deposited layers. To avoid that, in the present project we studied the potential of SAM-forming molecules in which the polar units are “buried” within the molecular backbones. To understand the fundamental properties of such SAMs, we combined a variety of surface-science experiments (conducted primarily in the group of Michael Zharnikov at the Universität Heidelberg) with state of the art quantum-mechanical and molecular dynamics simulations (performed in the group of Egbert Zojer at Graz University of Technology). In the course of our studies, we were indeed able to realize aromatic SAMs with the desired properties which allowed changes of the work-function of a Au substrate by +/- 0.5 eV depending on the orientation of the embedded dipoles and compared to an apolar reference SAM. In these layers the intrinsic film properties could be rationalized at an atomistic level by means of the simulations. This paved the way for further experiments on mixed SAMs containing molecules with different dipole orientations for which a continuous tuning of induced work-function changes could be realized. On more fundamental grounds the above-mentioned study also showed that through a regular arrangement of embedded dipoles on surfaces one is able to locally shift the electrostatic reference energy within the adsorbates. This can be probed efficiently by x-ray photoemission spectroscopy in conjunction with the simulation of core-level shifts. This paved the way for proposing a novel concept for realizing materials with user-defined electronic properties that relies on collective electrostatic effects for realizing, for example, monolayer quantum-well and quantum-cascade structures. Finally, the peculiar charge transport properties through the above-described embedded-dipole SAMs also provided fundamental insight into the properties of molecular electronic devices.

1. A. Kovalchuk, T. Abu-Husein, D. Fracasso, D. A. Egger, E. Zojer, M. Zharnikov, A. Terfort, and R. C. Chiechi,* “Transition Voltages Respond to Synthetic Reorientation of Embedded Dipoles in Self-Assembled Monolayers”, Chemical Science, published on-line, DOI: 10.1039/C5SC03097H
2. Gernot J. Kraberger, David A. Egger, Egbert Zojer,* “Tuning the electronic structure of graphene through collective electrostatic effects”, Adv. Mater. Interfaces, 1500323 (2015). DOI: 10.1002/admi.201500323.
3. T. Abu-Husein, S. Schuster, D. A. Egger, M. Kind, T. Santowski, A. Wiesner, R. Chiechi, E. Zojer,* A. Terfort,* and M. Zharnikov,* “The Effects of Embedded Dipoles in Aromatic Self-Assembled Monolayers”, Adv. Funct. Mater. 25, 3943 (2015). DOI: 10.1002/adfm.201500899.
4. B. Kretz, D. A. Egger, and E. Zojer,* “A toolbox for controlling quantum states in organic monolayers”, Advanced Science, 1400016 (2015). DOI: 10.1002/advs.201400016.
5. V. Obersteiner, D. A. Egger,* G. Heimel, and E. Zojer,* ”Impact of Collective Electrostatic Effects on Charge Transport through Molecular Monolayers”, J. Phys. Chem. C 118, 22395 (2014); doi: 0.1021/jp5084955. Green OA