We conduct a feasibility study of nitrogen-vacancy (NV) ensemble magnetometry of the two-dimensional ferromagnet Cr2Ge2Te6 (CGT). The studied sample consists of thin flakes of exfoliated CGT...Show moreWe conduct a feasibility study of nitrogen-vacancy (NV) ensemble magnetometry of the two-dimensional ferromagnet Cr2Ge2Te6 (CGT). The studied sample consists of thin flakes of exfoliated CGT stamped onto a diamond with shallow NV centers. First, we simulate the NV center response to the magnetic stray fields produced by a monolayer of CGT and conclude a good signal to noise ratio should be attainable in a shot noise limited picture. Subsequent room temperature photo luminescence experiments reveal two key challenges: optical dimming underneath the samples and inherent low contrast of the NV electron spin resonance (ESR) spectrum. We can explain the optical dimming by a near-surface nanophotonic effect. The low observed contrast can partially be accounted for by the ionization of the shallow NV centers to the neutral charge (NV0). Also, it is found that contrast can be largely regained by rigorous cleaning of the diamond surface.Show less
The coexistence of the semi-conducting 2H and quasi-metallic 1T’ monolayer MoS2 phases has great potential in, for example, low-resistance contacts. In this research MoS2 on gold samples have been...Show moreThe coexistence of the semi-conducting 2H and quasi-metallic 1T’ monolayer MoS2 phases has great potential in, for example, low-resistance contacts. In this research MoS2 on gold samples have been fabricated. Several different fabrication procedures have been tested including stamping, gold-mediated exfoliation and hydrogen plasma cleaning. Measurements identifying the 2H phase have been made using Atomic Force Microscopy and Low Energy Electron Microscopy. Although 1T’ could not be observed, insights are gained into the various fabrication processes.Show less
When Van derWaals materials are reduced to two-dimensional atomic crystals, their physical properties start to change. For most materials these properties and phenomena are still unresearched. Van...Show moreWhen Van derWaals materials are reduced to two-dimensional atomic crystals, their physical properties start to change. For most materials these properties and phenomena are still unresearched. Van der Waals materials can be created to be atomically thin, to layers of a single atom thick. One class of these materials are transition metal dichalcogenide materials (TMDs). There exist different crystalline structures of TMDs, which are called polytypes. Here, TaS2 flakes are fabricated in order to study charge density waves (CDWs). A protocol to fabricate large thin flakes is developed along with a simple and quick method to determine the layer thickness with the use of an optical microscope. The calculated thicknesses are then verified with Atomic Force Microscopy (AFM) measurements. Low Energy Electron Microscopy (LEEM) measurements are performed on thick flakes in order to research CDWs. Although confirmation of CWDs is absent, different domains of reflective electron intensity and various structures are observed. These features are compared with AFM measurements. The flake existing of different polytypes could be the cause for the observed contrasts. The reflection of electrons is observed until an energy of 150 eV.Show less
Majorana quasiparticles can arise at the ends of one dimensional superconducting quantum wires. As a true one-dimensional experimental realization of such a quantum wire, a carbon nanotube can be...Show moreMajorana quasiparticles can arise at the ends of one dimensional superconducting quantum wires. As a true one-dimensional experimental realization of such a quantum wire, a carbon nanotube can be employed. The one-dimensional carbon nanotube can inherit the superconducting properties by bringing it in close contact to a superconductor, such as van der Waals superconductor niobium diselenide. In this work, the design and fabrication of devices are discussed that contain a carbon nanotube connected to a few atomic layer thick niobium diselenide. Bi- or trilayers of niobium diselenide crystal flakes are obtained using mechanical exfoliation techniques in inert conditions. A stamping resolution of about 1 mm is obtained in positioning the flakes with respect to each other which is shown by Atomic Force Microscopy. Initial characterizations have shown an improvement of the conductance of more than two orders of magnitude. The stability diagrams, however, exhibit a complex behaviour which we were unable to explain in terms of a proximitized carbon nanotube. We have shown that it is possible to fabricate the complete hybrid device that is predicted to host Majorana Fermions in its non-trivial phase. This work was conducted at Regensburg University in the group of Professor Christoph Strunk.Show less
