Reliably modeling the electronic structure of self-assembled monolayers

Self-assembled monolayers of organic molecules covalently bonded to (noble) metals have been in the focus of multi-disciplinary research for a number of years. Their electronic properties are of particular interest for the use in molecular and organic electronics. Moreover their experimentally well characterized and highly reproducible properties render them ideal test-systems for benchmarking theoretical approaches. In the course of past research efforts a lot has been learned about the fundamental aspects that link the chemical structure of a SAM to its structural and electronic characteristics. However, when trying to predict SAM-properties on a more quantitative level, two main problems arise: In a large number of experimental studies, especially when applying SAMs in devices, one is dealing with non-perfectly ordered layers; moreover, the usually applied simulation method, namely density functional theory (DFT) in the framework of the local density or generalized gradient approximation, suffers from several intrinsic shortcomings. The goal of the present project is to assess and, if possible, correct these two aspects: Using molecular dynamics simulations, we will investigate the impact of temperature on the structural parameters of various SAMs and, more importantly, test the consequences of an incomplete coverage of the metal surface. Moreover, we will study the consequences of imperfections of the metal substrates on the dynamic SAM properties. From the obtained arrangements of the molecules, we will carefully extract test structures for performing electronic-structure calculations, thus, correlating structural imperfections with the resulting electronic properties of the SAM-covered metal. In parallel, we will work on strategies for addressing the main shortcomings of "conventional" DFT. This includes the application of newly developed correction schemes to account for van der Waals interactions. We will also test approaches to correct for the self-interaction error, excessively benchmark hybrid functionals for the description of metal/organic interfaces and apply post-DFT methods. To account for the well-known band-gap problem of DFT and for the polarization of the metal induced by long-range correlation effects, we will also evaluate computationally far less costly "effective" approaches. The envisioned research will involve intensive collaborations with international as well as national partners experienced either in the modeling or the experimental investigation of metal/organic interfaces. These will be crucial for a successful completion of the envisioned research program that would be too ambitious to be pursued only in a single group. Additionally, access to high-quality experimental data will allow a thorough validation of the obtained results.

Organic semiconductors play an increasingly important role in everyday life, mostly because they nowadays are the key elements of virtually all high-quality displays in handheld applications. Also the share of TVs based on organic light-emitting devices is constantly increasing. Organic semiconductors also have a significant potential for applications in large-area solar cells and flexible electronics just to name a few. An element crucially determining the functionality of all these devices is the interface between the electrode material and the organic semiconductor layer, as this is, where efficiency-limiting processes like charge-carrier injection or extraction occur. Beyond those already market-relevant aspects, organic-inorganic hybrid interfaces also represent key building blocks for a large variety of nanoscopic devices, where in many cases the interface provides the actual device functionality. Bearing that in mind, the current project focusses on developing a strategy that allows a truly quantitative prediction of the electronic properties of such interfaces, to use that strategy for obtaining structure-to-property-relationships of unprecedented quality, and eventually also to develop novel approaches for the computational design of materials. To achieve that, several methodological challenges had to be overcome addressing issues related to structural disorder at interfaces and deficiencies of the commonly applied methods used for simulating their electronic properties. With the necessary tools in hand, it was then possible to address a number of problems encountered in experiments explaining, e.g., the impact of structural changes on the electronic structure of the interface, the explanation of geometric and optical properties of complex multi-component interfaces, or the clarification of the role played by hitherto overlooked reactions crucially changing the structure of adsorbed molecules. A central outcome of the project is also that we were able to show that so-called collective electrostatic effects that arise from the superposition of the electric fields generated by regular assemblies of polar elements at the interface can fundamentally change important quantities, like the ballistic transport of charge through interfaces or the binding-energies of the electrons. Finally, based on this insight, we developed the idea of exploiting those inevitable effects to control the electronic properties of interfaces at will leading to the novel concept of electrostatic materials design.