Transition metal dichalcogenides (TMDs, MX2) are Van der Waals materials with properties such as the band structure depending on chemical structure and the number of layers. Low energy electron...Show moreTransition metal dichalcogenides (TMDs, MX2) are Van der Waals materials with properties such as the band structure depending on chemical structure and the number of layers. Low energy electron microscopy (LEEM) provides a manner of characterizing TMDs, by controlling the landing energies at which electrons reach the desired sample and measuring the reflection of the electrons. Besides real space imaging, our LEEM instrument is able to image reciprocal space, which is especially interesting to the characteristic hexagonal lattice of TMDs. This thesis will discuss a number of energy-resolution limiting factors. One of these factors is the energy distribution of electrons incident on the sample. It will discuss a method to correct for this energy dispersion and use it to decrease noise in electron reflectivity spectra. Also, fluctuations in electron current will be discussed and corrected for. Further, this thesis will examine oxidation of TMDs, as the rate of oxidation and impact on properties differs greatly depending on the specific composition of the TMD. It will discuss methods of exfoliating TMDs in vacuum, i.e. in situ, and show successful exfoliation in vacuum of around 10−6 mbar inside the LEEM. Also, the research will study exfoliation of Si/SiO2 substrates with gold evaporated to produce large flakes of few-layer TMDs. This yielded MoS2 few-layers flakes of up to 100 microns. The findings in this study regarding enhanced energy resolion in LEEM measurements of TMDs and improved techniques for TMD exfoliation will help the progress in understanding and characterization of TMD materials.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
