Tuning the electronic properties of SAMs by embedded molecular dipoles

The present project aims at providing an atomic-level understanding of the electronic and structural properties of alkylthiolate based self-assembled monolayers that contain embedded (di)polar functional groups. Such systems are highly interesting both from a fundamental as well as from an application point of view, as the dipolar groups induce a potential discontinuity inside the monolayer electrostatically shifting the energy levels in the regions above and below the dipoles relative to each other. Such SAMs also allow tuning the substrate work-function independent of the docking chemistry and the identity of the SAM-ambient interface in a controlled manner. New insights are expected from combining a number of experimental surface-science techniques with quantum- mechanical and molecular-dynamics modelling. The former will be used in the group of Michael Zharnikov and include high-resolution X-ray and ultraviolet photoelectron spectroscopy, near-edge absorption fine structure spectroscopy, and Kelvin probe measurements. The simulations, which are to be performed in the group of Egbert Zojer, are ideally complementary to those experiments and include density-functional theory as well as classical force-field based calculations on metal slabs covered on one side by the organic adsorbate. To understand the relationship between the molecular structure and the SAM properties, we will systematically vary the length of the aliphatic chains, the positions of the embedded dipoles, as well as the chemical functionalities giving rise to the dipoles. The molecules will be provided by the group of David A. Allara, where also the infrared reflection absorption spectroscopy will be performed. By combining experiments and simulations we expect in-depths insights into the details of the electrostatic potential shifts between the parts of the SAM above and below the embedded dipoles and how these shifts are linked to the induced changes of the substrate work-function (where preliminary experiments and calculations indicate that this link is not straightforward).

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.