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
TaS2 is a Van der Waals material which can exist in more than on crystal structure(polytypes). Different of these crystal structure can exist at once on a sample of TaS2 creating different domains...Show moreTaS2 is a Van der Waals material which can exist in more than on crystal structure(polytypes). Different of these crystal structure can exist at once on a sample of TaS2 creating different domains of crystal structures. In this work a Low Energy Electron Microscope is used to perform reflectivity measurements using Low Energy Electron Diffraction on different domains. The measured reflectivity of the domains is than compared to simulated reflectivity of different known polytypes of TaS2. A method on how to perform LEED reflectivity calculations is explored and used to compare to two LEED measurements on TaS2 and the crystal structures of the observed domains is analyzed.Show less
Multiple external tuning parameters affecting the different Charge Density Wave (CDW) phase transitions in few-layer 1T-TaS2 have been reported in literature. The formation of CDWs still lacks a...Show moreMultiple external tuning parameters affecting the different Charge Density Wave (CDW) phase transitions in few-layer 1T-TaS2 have been reported in literature. The formation of CDWs still lacks a predictive theory, so understanding may be gained from experiments on the effects favoring or suppressing CDW transitions. In this project we performed electronic transport measurements on ~ 100nm thin 1T-TaS2 at 1.55K to 290K. Flakes of 1T-TaS2 were exfoliated and stamped on top of Au contacts to form a Hall bar. The Charge Density Wave transition is observed between T = 100K and 160K with a relaxation time in the range of hours. Consequently, the transition can be suppressed by fast cooling and we find a range of metastable states, that are supposedly coexisting spatially separated domains of NC- and C-CDW. Under application of a magnetic field, the change of sign of the Hall voltage at the transition is confirmed. Both CDW phases show positive magnetoresistance along the channel with similar shape, whilst the effect is more pronounced in the C-CDW phase. The magnetoresistance effect is attributed to Weak Anti Localization, which would also explain a decrease in resistivity seen in the supercooled NC-CDW phase at low temperature. The slow transition and the magnitude of resistivity increase, that is about two times lower than reported in literature, may be caused by defects in the crystal hindering long-range CDW order.Show less
Lower emission current was implemented in an ESCHER LEEM based on a specs P90 FE-LEEM. The electrons that are used for the imaging are provided by a cold field emitter, meaning that a voltage is...Show moreLower emission current was implemented in an ESCHER LEEM based on a specs P90 FE-LEEM. The electrons that are used for the imaging are provided by a cold field emitter, meaning that a voltage is used to extract the electrons from an metallic emitter tip. The relationship between the extractor voltage and the emission current is inherently inconsistent, and for low emission current, high noise on our measurement of the emission current prevents the use of a feedback loop as is customary for higher emission currents(above 0.6 µA). Instead a model was made that fits the relationship between emission current and extractor voltage on a day to day basis, using high emission current data points from a range where a feedback loop can be used. With this the relationship between emission current and extractor voltage can be extrapolated for emission currents below 0.6 µA, to find the right extractor voltage for a desired emission current. Then the electron detector of the LEEM, which is normally used as a camera for imaging techniques, was used to verify the model using image intensity as a measure for the emission current. Using the developed model to set extractor voltage from emission current setpoint, emission currents of down to I_emis ≈ 2.5·10^(−13) µA were observed. The obtained emission current data was in good agreement with CFE theory for many orders of magnitude, however at I_emis ≈ 0.007 µA, a negative curvature was observed that is not in agreement with CFE theory. This makes the model less precise. At I_emis ≈ 1.23·10^(−7) µA, it deviated by an order of magnitude. At I_emis ≈ 2.5·10^(−13) µA the model deviated from the emission current by a factor of approximately 5 orders of magnitude. It is not yet clear whether this negative curvature is an artifact of our measurement or a deviation form CFE theory, however it is suspected to be an artifact of our measurement caused by either the emission profile of our electron emitter changing or lensing of the extractor plate. Multiple other options exist. Until the cause of this negative curvature is found, we can not assume the model to be a precise indication of the emission current. Despite this a working model was provided that allowed us to use significantly lower emission currents. Using lower emission current we were able to image a pentacene layer grown on hBN flakes exfoliated on silicon with significantly less charging compared to using high currents.Show less
The electronic properties of graphene on silicon carbide have opened up a wide variety of technological applications. During the growth of this graphene, thermal contraction causes strain between...Show moreThe electronic properties of graphene on silicon carbide have opened up a wide variety of technological applications. During the growth of this graphene, thermal contraction causes strain between the graphene layers. This strain induces domains of different crystallographic stacking which influence the electronic properties. The shape and geometry of those domains are given by the local strain and can be seen on images obtained with low-energy electron microscopy. In this research, we extract strain maps from these images to gain more understanding about what drives the formation of these stacking domains. We find that the substrate step edges affect the shape of the domains and therefore provide new parameters to be used for strain engineering.Show less
Electromigration in bismuth is studied as a way to create bismuth(111) bilayers. Temperature-dependent electromigration measurements have been performed and a model incorporating Joule heating is...Show moreElectromigration in bismuth is studied as a way to create bismuth(111) bilayers. Temperature-dependent electromigration measurements have been performed and a model incorporating Joule heating is used to describe those. An activation energy for diffusion between 100 and 180meV is found. Furthermore, in-situ electromigration experiments have been performed in a scanning electron microscope. This allowed us to link events in the conductance traces of bismuth constrictions during electromigration to visual features. Specifically, remerging of the bismuth electrodes was found to cause increases in conduction.Show less
