New design strategies for tuning electrode properties by SAMs

Chemically anchored self-assembled monolayers (SAMs) play a crucial role in the area of organic and molecular electronics. They are usually used to modify the electronic properties of interfaces or to act as the active elements of nano-scale devices. Of particular interest in this context are SAMs bonded to metal-electrodes, which, when substituted with polar units at their tails, modify charge-carrier injection. A complication in that context is that due to the dipole-dipole repulsion, SAM-formation in such layers becomes a challenge. To overcome that problem, we will design novel types of SAMs, where the layers are stabilized either by reducing dipole-dipole repulsion via distributing the dipolar units or by inducing hydrogen-bonds between the SAM-forming molecules. To realize the latter approach, we will study systems related to dye molecules like indigo or quinacridone. For such molecules bonded to metal surfaces, e.g., via thiolate anchors, we expect novel properties like work-function modifications of unprecedented magnitude as well as peculiar electronic characteristics arising from the intra-layer chemical and electrostatic coupling. To realize the full potential of the planned research, we will combine the ideally complementary expertise of six research groups: A. Terfort (Univ. Frankfurt) and co-workers will synthesize the required molecules either with distributed dipole moments or the H-bond network forming dyes. M. Zharnikov (Univ. Heidelberg) and his group will devise optimized strategies for growing such SAMs on metal substrates, characterize their structural electronic properties, and together with the group of R. Resel (TU Graz) will study the growth of organic semiconductors on modified electrodes. E. Zojer (TU Graz) and his co-workers will use atomistic modeling to identify the most promising molecular structures guiding the synthetic efforts and to explain experimental observations. K. Zojer (TU Graz) and her students will model, how SAM-modified electrodes are expected to change the charge carrier injection into organic thin-film transistors paying special attention to the role of inhomogenieties in the films electronic properties. Finally, the SAMs designed in this project will be applied in actual device-structures by B. Stadlober and her group (Joanneum Research). All above mentioned research efforts will be intimately linked in close feedback-loops guaranteed by several levels of information exchange. From the tight integration of so many different approaches we expect an unprecedented level of understanding of the electronic and structural properties of the studied systems. This has the potential to significantly impact the way one thinks about self-assembled monolayers and to promote their use in actual organic electronic devices. Moreover, the multi- disciplinary approach and the stimulating research environment will hugely broaden the scientific perspectives of all involved scientists, in particular the students.

The injection of charge carriers is a crucial process in virtually all electronic device. Controlling these injection processes is particularly difficult for organic semiconductors, where doping processes are much more complex than in their inorganic counterparts. In the current project, we, therefore, developed an alternative strategy for changing the injection efficiency. This approach is based on the adsorption of so-called polar self-assembled monolayers, where the particular advantage of our approach is that the dipoles are embedded into the molecular backbones of the assembled molecules. In this way, we were able to electrostatically change contact resistances by several orders of magnitude. Notably, we showed that our approach can also is compatible with flexible electronics and it can also be efficiently applied to more complex electronic circuits. Moreover, it is useful also beyond the field of organic electronics, as demonstrated by its application to MoS2-based devices. Of particular relevant for the project is that we did not only demonstrate a practically useful effect, but were also able to explain it at an atomistic level. To achieve that, it was necessary to draw from the complementary expertise of several groups in Germany and Austria that would cover the entire "value chain". Starting from chemical synthesis (A. Terfort and coworkers at the University of Frankfurt), it involved spectroscopic investigation of the interfaces by means of surface-science techniques (M. Zharnikov and coworkers at the University of Heidelberg), the quantum-mechanical simulation of the electronic properties of the monolayers (E. Zojer and coworkers at the Graz University of Technology), and the fabrication of electronic devices (B. Stadlober, Joanneum Research) and their study by means of simulation (K. Zojer, Graz University of Technology). These studies provided detailed insight into which types of molecules would be ideally suited for achieving the desired goals and which structural elements should be avoided. They also helped us to further develop the understanding of collective electrostatic effects occurring at virtually all types of interfaces and allowed us to show that these effects fundamentally impact the outcomes of x-ray photoelectron spectroscopy (XPS) experiments. Based on a combination of experimental and theoretical studies, we were able to show for a variety of interfaces that the data obtained by XPS experiments are not only determined by the chemical environment of the studied atoms (as is the common understanding in the scientific community), but are also crucially impacted by the local electrostatic potential that is significantly modified by the adsorbed polar monolayers. The results of our research have been published in numerous papers in high-ranking journals, they were presented as (invited) talks at international conferences, and also triggered the application for a follow-up project.