N-Heteropolycycles on graphene: Supramolecular organization and charge transfer

This work aims at a more detailed understanding of non-covalent doping of graphene and Molybdenumdisulfide using aromatic N-heteropolycycles. Therefore a wide variety of molecules is first deposited via wet-deposition onto graphene, to then investigate the induced changes in Fermi energy of graphene with both Raman spectroscopy and Kelvin-Probe-Force Microscopy. The induced shifts in graphene are then correlated with both the molecular geometry and substituents, as well as their electronic properties to derive a structure-effect relation that can be used to guide the fine tuning of graphene’s electronic and surface properties. All methods in this work, form deposition to measurements are done under ambient conditions to mimic the conditions of real live applications and to avoid perfect lab conditions like ultra-high vacuum that are not viable for end-of-the line device production and use. This idea is also applied to the graphene that is used itself, as commercially available graphene was used for all experiments. With this framework in mind the first steps of this project were to establish a suitable sample preparation routine to achieve a reproducible and comparable graphene substrate for the subsequent measurements. Due to CVD-grown and wet-transferred graphene being contaminated with polymer residues and water this was a crucial step for reliable results in the end. Comparing a wide range of different molecules, it was also necessary and important to formulate a deposition procedure that can be applied to all molecules, although probably not optimized for the specific molecule it was optimized for the overall comparison of all molecules on even ground. The first results of the deposition of both TIPS-Tetraazapentacenes and Tetraazaperopyrenes show a strong dependence of the graphene doping on the acceptor strength (=LUMO energy) of the molecules and less on the overall molecular structure (i.e. substituents and substitutional patterns). With those established protocols and findings the relations of charge transfer interaction between molecules and graphene were further refined with additional acceptor molecules (Benzonapthyridines) and also demonstrated for the opposite interaction with donor-type Heterotriangulenes showing trends in-line with the theory. i In case of the Benzonaphthyridines and Heterotriangulenes the graphene modification was carried a step further by utilizing the molecule specific properties of possible protonation with common acids and oxidation via UV irradiation to introduce a second stage of charge transfer after initial deposition. This demonstrated that it is possible to further modify already assembled molecules on graphene (and as a consequence the Fermi level of graphene as well) without major damage to graphene itself, although in our cases the overall changes following the second modification were small, but could easily be increased with specific molecules engineered for those kinds of applications. To delve even further into the non-covalent nanomaterial doping all, the gained insights and experiences from graphene were then taken to Molybdenumdisulfide (MoS2) to see whether the rules of structure-effect-relations derived from graphene can be applied to a different class of nanomaterial and to see specific differences and whether they follow the theoretical expectations. With the self-assembly of molecules being guided by mainly π-π interactions on graphene, MoS2 shows that the acceptor strength is one of the driving factors not only for the electronic modification but also for the deposition itself