Institute of Solid State Physics
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SS22 WS22 SS23 WS23 SS24 WS24 Guidelines for Master Students
Embedded Dipole SAMs as a Versatile Tool for Interface Engineering Applying covalently-bonded self-assembled monolayers (SAMs) is a promising strategy for tuning interface properties in the area of organic electronics. This is particularly true when employing polar molecules, whose ordered arrangement allows tuning the electronic structure of interfaces exploiting collective electrostatic effects.1 A common approach to realize such polar molecules is the use of polar tail group substituents. This, however, suffers from the distinct disadvantage that tail-group substituents also change other surface properties and, thus, might adversely affect, for example, the growth of subsequent organic semiconductor layers. To avoid that, we decided to introduce SAM-forming molecules with polar entities embedded within the aromatic2 or aliphatic3 molecular backbones. This allows decoupling the tuning of electronic properties from modifying the surface characteristics of the SAMs. Employing such embedded dipole SAMs allows generating a variety of fundamental insights into how polar layers modify interface properties:1 One can, for example, show that core-level spectroscopy does not only provide information on the chemical environment of certain atoms, but - as a consequence of electrostatic core-level shifts - can also be used as a tool for probing the local electrostatic situation at a surface.4 Under certain circumstances the locality of these electrostatic shifts then even allows the determination of the homogeneity of mixed SAMs.5 Through tuning the interfacial level alignment, embedded dipoles can also modify the transition-voltage in monolayer transport experiments.6 When employing embedded dipole SAMs in electronic devices, like thin-film transistors, one can change contact resistances by several orders of magnitude by modifying electrode work-functions.7 This is achieved without changing the growth of the organic semiconductor layer and suitably oriented embedded dipoles even allow the realization of n-type transistors on flexible substrates employing Au electrodes.7 Notably, the beneficial effect of embedded dipole SAMs is not restricted to organic semiconductors but is observed also for MoS2-based transistors8 and recently also the possibility of including a series of distributed dipoles has been explored.9 Finally, it should be noted that the inclusion of embedded dipole linkers also has considerable potential for tuning the electronic properties of other materials classes, like, for example, metal-organic frameworks.10,11 |
