SummaryThinning out MoS2 crystals to atomically thin layers results in the transition from an indirect to a direct bandgap material. This makes single layer MoS2 an exciting new material for electronic devices. In MoS2 devices it has been observed that the choice of materials, in particular for contact and gate, is crucial for their performance. This makes it very important to study the interaction between ultrathin MoS2 layers and materials employed in electronic devices in order to optimize their performance. In this work we used NC-AFM in combination with quantitative KPFM to study the influence of the substrate material and the processing on single layer MoS2 during device fabrication. We find a strong influence of contaminations caused by the processing on the surface potential of MoS2. It is shown that the charge transfer from the substrate is able to change the work function of MoS2 by about 40 meV. Our findings suggest two things. First, the necessity to properly clean devices after processing as contaminations have a great impact on the surface potential. Second, that by choosing appropriate materials the work function can be modified to reduce contact resistance.

In this work we study the effect of thermal processing of exfoliated graphene on mica with respect to changes in graphene morphology and surface potential. Mild annealing to temperatures of about 200°C leads to the removal of small amounts of intercalated water at graphene edges. By heating to 600°C the areas without intercalated water are substantially increased enabling a quantification of the charge transfer properties of the water layer by locally resolved Kelvin probe force microscopy data. A complete removal on a global scale cannot be achieved because mica begins to decompose at temperatures above 600°C. By correlating Kelvin probe force microscopy and Raman spectroscopy maps we find a transition from p-type to n-type doping of graphene during thermal processing which is driven by the dehydration of the mica substrate and an accumulation of defects in the graphene sheet.

The controlled creation of defects in silicon carbide represents a major challenge. A wellknown and efficient tool for defect creation in dielectric materials is the irradiation with swift (E kin Z500 keV/amu) heavy ions, which deposit a significant amount of their kinetic energy into the electronic system. However, in the case of silicon carbide, a significant defect creation by individual ions could hitherto not be achieved. Here we present experimental evidence that silicon carbide surfaces can be modified by individual swift heavy ions with an energy well below the proposed threshold if the irradiation takes place under oblique angles. Depending on the angle of incidence, these grooves can span several hundreds of nanometres. We show that our experimental data are fully compatible with the assumption that each ion induces the sublimation of silicon atoms along its trajectory, resulting in narrow graphitic grooves in the silicon carbide matrix.

We show that the work function of exfoliated single layer graphene can be modified by irradiation with swift (E kin = 92 MeV) heavy ions under glancing angles of incidence. Upon ion impact individual surface tracks are created in graphene on SiC. Due to the very localized energy deposition characteristic for ions in this energy range, the surface area which is structurally altered is limited to ≈ 0.01 µm 2 per track. Kelvin probe force microscopy reveals that those surface tracks consist of electronically modified material and that a few tracks suffice to shift the surface potential of the whole single layer flake by ≈ 400 meV. Thus, the irradiation turns the initially n-doped graphene into p-doped graphene with a hole density of 8.5 × 10 12 holes/cm 2 . This doping effect persists even after heating the irradiated samples to 500 • C. Therefore, this charge transfer is not due to adsorbates but must instead be attributed to implanted atoms. The method presented here opens up a new way to efficiently manipulate the charge carrier concentration of graphene.

We present atomic force microscopy and scanning Kelvin probe data obtained under ultra-high vacuum conditions from graphene exfoliated on crystalline SrTiO(3) substrates. The contact potential difference shows a monotonic increase with the number of graphene layers until after five layers of saturation is reached. By identifying the saturation value with the work function of graphite we determine the work function of single and bilayer graphene to be Φ(SLG) = 4.409 ± 0.039 eV and Φ(BLG) = 4.516 ± 0.035 eV, respectively. In agreement with the higher work function of single-layer graphene with respect to free-standing graphene, our measurements indicate an accumulation of charge carriers corresponding to a doping of the exfoliated graphene layer with electrons.