Decoupling, integer charge transfer, and surface reactions of organic molecules on 2D materials

Two-dimensional materials such as graphene, hexagonal boron nitride (h-BN) or blue phosphorene (BlueP) are attracting much attention in fundamental research and applications due to their unique physical properties such as, e.g., a layer-dependent band gap and an exceptionally high charge carrier mobility. The weak van der Waals interaction between the layers makes it possible to use these materials, among others, as interlayers to decouple organic semiconductors from metal surfaces. The decoupling is essential to preserve the intrinsic molecular properties such as the narrow optical absorption and emission which are normally altered because of the interaction with the metal. However, the question arises whether one monolayer is sufficient to effectively decouple organic molecules or not. In this thesis, this question is addressed by investigating the growth as well as the structural, optical and electronic properties of pentacene (Pen) monolayers adsorbed on h-BN grown on a Ni(111) surface. In addition, the influence of the molecular functionalization on the decoupling effect is investigated by comparing Pen to 6,13-pentacenequinone (P2O) monolayers on h-BN/Ni(111). Besides the decoupling effect, integer charge transfer (ICT) can occur for adsorbates on weakly-interacting substrates which can result in the formation of stable organic radicals (anions or cations) of former closed-shell molecules. The investigation of such systems, which have one unpaired electron in the highest occupied molecular orbital (also called open-shell systems) is of great interest for novel (opto)electronic devices, single molecular magnets as well as spintronic applications. In addition, the chemical properties of radicals differ from those of their neutral counterparts and open up new pathways for surface reactions. In this thesis, I discuss the reason for the ICT and present the impact on the structural, optical and electronic properties by comparing the adsorption of perylene-3,4,9,10-tetracarboxlic dianhyride (PTCDA) molecules on h-BN monolayers on Ni(111) with PTCDA on the BlueP-Au-network on Au(111). Furthermore, I demonstrate that the integer charge transfer is necessary for surface reactions such as the deoxygenation of PTCDA molecules and the graphitization on h-BN/Ni(111).