Modulation of electronic and magnetic properties of nanostructures on a monolayer of MoS${}_2$/Au(111)

Fundamental electronic and magnetic properties of single atoms and molecules depend crucially on their immediate environment. Understanding its exact influence allows for the manipulation and thus tailoring of said properties. In this thesis, we investigate four different nanostructures on a monolayer of MoS${}_2$/Au(111) by means of scanning tunnelling microscopy (STM) and atomic force microscopy (AFM). First, single sulfur vacancies in the MoS${}_2$ layer were investigated. Here, the missing sulfur atoms are imaged with a three-fold symmetric shape in the STM topographies. Several defect related in-gap states were observed by scanning tunnelling spectroscopy (STS), most prominently a sharp resonance at zero bias. We identified it as a Kondo resonance by temperature and magnetic field dependent measurements. For defects located in regions of the MoS${}_2$, where the underlying Au(111) was missing, the Kondo resonance was absent, probably due to a different charge state of the defect. We then investigated the magnetic properties of single iron atoms on the MoS${}_2$ monolayer. We found that the exact position of the atoms in the moiré superstructure, which forms due to the lattice missmatch of the MoS${}_2$ and the underlying Au(111), has a crucial impact on those properties. Atoms on the moiré minimum exhibit an inelastic excitation, which is a signature of a spin-flip excitation and atoms on the moiré maximum show a Kondo resonance, which is a sign of increased spin screening by the substrate electrons. The transition between those two observed features was continuous along the moiré structure. We explain these changes by a local variation of the density of states along the moiré modulation. An impact of the moiré was also observed on single manganese atoms: they exhibit a Kondo resonance in the weak coupling regime, with the amplitude varying along the moiré. We were able to manipulate single manganese atoms into dimers and found three distinct regimes of interatomic exchange coupling, depending on their atom separation. We focused on the dimers with the closest spacing of one atomic lattice site. They show a direct exchange coupling, which leads to a singlet ground state. The excitation energy from the ground state to the excited triplet state also varies along the moiré structure. In a model calculation we establish that the hybridisation of manganese d-levels with substrate electrons leads to a renormalisation of the singlet-triplet excitation energy. A comparison of experimental values and theoretical prediction suggests that several conduction channels are involved in the renormalisation. Finally, the high-dipole molecule Ethyl-DADQ was investigated. Pre-characterisations on a bare Au(111) substrate show that the molecular dipole is preserved and that the molecules cause a local lowering of the work function of the substrate. The investigations on MoS${}_2$ allow for a better energy resolution, which helps to resolve several vibronic sidebands of the positive ion resonance. Their intensities vary depending on the electrostatic environment of the molecule, suggesting a partial damping of some vibrational modes.