Diffraction distortion in LEEM diffraction imaging makes it impossible to measure angles and distances accurately. In a LEEM this diffraction distortion changes when the sample measured is changed...Show moreDiffraction distortion in LEEM diffraction imaging makes it impossible to measure angles and distances accurately. In a LEEM this diffraction distortion changes when the sample measured is changed meaning it needs to be corrected each time. In this thesis the method for an in-place correction is explained and applied. The method successfully allows the measurement of angles and distances without requiring the measurement of too much data. However, it can still be improved upon so that less data is needed and a better correction is acquired.Show less
Crystalline pentacene layers can be damaged when exposed to low energy electrons. Using the Low Energy Electron Microscope this damage is researched. By analyzing the fading of the Low Energy...Show moreCrystalline pentacene layers can be damaged when exposed to low energy electrons. Using the Low Energy Electron Microscope this damage is researched. By analyzing the fading of the Low Energy Electron Diffraction (LEED) pattern of the layers the structural damage is quantified. Multiple techniques of analyzing the data are compared where a k-space filtering technique all-round gave the best results in dealing with the background. In one dataset, the height of the diffraction peaks decreases and their width broadens with increasing dose. In another dataset, the height decays exponentially and the width remains almost constant. In the data analyzed a threshold for damage to occur is found at an electron energy of around 6 eV. Damage in crystalline layers also simulated and compared to the experiments in LEEM.Show less
To better understand the LEEM spectra of 2D materials we explored usingtransfer matrices to model them. We applied transfer matrices to find ananalytical expression for the LEEM spectra of graphite...Show moreTo better understand the LEEM spectra of 2D materials we explored usingtransfer matrices to model them. We applied transfer matrices to find ananalytical expression for the LEEM spectra of graphite. We found that itresults in an approximate solution that correctly predicts the position ofthe minima and general shape of the curve in the 0-25 eV range. We alsoapplied transfer matrices to model the spectrum of few-layer grapheneon bulk hexagonal boron nitride. The modeling of graphene on hBN wasdone in a coherent, incoherent and a modified coherent case. We foundthat the model manages to show the 8 eV minima in the coherent case.But the general shape we found is less accurate than for graphite. Theother cases did not model the 8 eV dip of the spectra.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
We introduce a new technique called 3D-ARRES, with which the full free elec- tron eigenstates above the vacuum energy of van der Waals materials can be imaged by resolving the landing energy and in...Show moreWe introduce a new technique called 3D-ARRES, with which the full free elec- tron eigenstates above the vacuum energy of van der Waals materials can be imaged by resolving the landing energy and in-plane momentum of reflected electrons in LEEM. Compared to ARRES, 3D-ARRES measures the whole first Brillouin zone and not only along the high symmetry axes. This is done by inte- grating the LEED pattern on the camera at different incidence angles and land- ing energies and fitting the data using image analysis techniques. By comparing the three-dimensional (3D) band structures ARRES measurements on both bulk MoS 2 and h-BN, 3D-ARRES is shown to be effective.Show less
Amethod to measure local tilt angles of two-dimensional materials in LEEM is developed. To create these local tilt angles graphene is stamped on top of pillars which are 55 nm high and 1 μm in...Show moreAmethod to measure local tilt angles of two-dimensional materials in LEEM is developed. To create these local tilt angles graphene is stamped on top of pillars which are 55 nm high and 1 μm in diameter. The measurement method uses an aperture to select a spot on the sample and moved the sample to measure all over the sample. These spots are measured in diffraction space. By analyzing all diffraction images, a magnitude and an orientation of local tilt angles are obtained. The spatial resolution of this method is bound by the size of the aperture. In this experiment a spatial resolution of 338 nm is achieved. The angular resolution depends on the locating method of the diffraction spot. In this experiment an angular resolution of one degree is achieved.Show less
In this thesis we present and scrutinize a technique to reconstruct the surface profile of a sample using a low energy electron microscope (LEEM). This technique is added to the rich catalogue of...Show moreIn this thesis we present and scrutinize a technique to reconstruct the surface profile of a sample using a low energy electron microscope (LEEM). This technique is added to the rich catalogue of surface analysis techniques available in LEEM.We demonstrate that the surface profile of a sample can be deduced from the local tilt angles. This is done by studying the change in diffraction pattern. The procedure is then applied to two samples: a flake of MoTe2 that has a bump on it and a flake of MoTe2 that has collapsed into a trench. We show that we can correct for lensing effects that are introduced due to a non-flat surface. In these samples we determine the local tilt angle with an accuracy of 0.3°.Show less
In this study, the work function difference between the two terminations of SrTiO3 (STO) is investigated. The two terminations i.e. SrO and TiO2 are formed by annealing the STO sample at 1200°C for...Show moreIn this study, the work function difference between the two terminations of SrTiO3 (STO) is investigated. The two terminations i.e. SrO and TiO2 are formed by annealing the STO sample at 1200°C for 12 hours in air at ambient pressure. To collect more information about this perovskite oxide, we study this material in AFM and LEEM.We distinguish the two terminations, calculate the step heights between the unit cells and record LEEM Multi dark-field images showing different surface reconstructions. To measure the work function difference, three methods are used: Energy-filtered PEEM, Intensity-Voltage curves and the comparison between LEEM real-space data and simulations. All methods showed that the work function difference is lower than predicted by theory.Show less
We investigate the presence of $(trans-10b,10c-dimethyl-10b,10c-dihydropropane)$ (DHP) photochromic molecular switches on 2D gold nanoparticle networks after molecular place exchange. We study the...Show moreWe investigate the presence of $(trans-10b,10c-dimethyl-10b,10c-dihydropropane)$ (DHP) photochromic molecular switches on 2D gold nanoparticle networks after molecular place exchange. We study the networks using spectoscopy, as the presence of the molecular switches on the network changes the surface plasmon resonance absorption frequency. We observed a redshift of $20\ nm$ and an additional peak broadening of $2\ nm$ (FWHM) of the absorption peak of the network compared to before molecular place exchange. From this we conclude that the molecules are present on the network after molecular place exchange.Show less
Van der Waals materials such as graphene are layered materials that can be created in single atom thickness. In most cases there is more than one way to stack subsequent layers, often leading to...Show moreVan der Waals materials such as graphene are layered materials that can be created in single atom thickness. In most cases there is more than one way to stack subsequent layers, often leading to domains of different stackings. In this work stacking domains in few layer stacks of graphene and \mos are studied using Low Energy Electron Microscopy. From dark field LEEM measurements on few layer epitaxial graphene on SiC it is concluded that two different types of domains exists: domains created from nucleation during growth and triangular stress domains induced from lattice mismatch with the underlying SiC. A detailed comparison between epitaxial and quasi-freestanding graphene is made. As part of this comparison deintercalation of the latter to reform the former is performed. The hydrogen diffusion out of the material occurring here is studied and linked to defects in the material and aforementioned stacking domains. For MoS2 no domains were analysed within the scope of this project, but a comparison of flatness for different substrates is made and using muLEED measurements a method to determine the number of layers is demonstrated and the two different orientations of the 2H polytype are experimentally identified.Show less
In medical imaging, model observers are used to define a new method of task based image quality assessment. In this thesis a novel search algorithm is presented that detects possible lesions in a...Show moreIn medical imaging, model observers are used to define a new method of task based image quality assessment. In this thesis a novel search algorithm is presented that detects possible lesions in a digital anthropomorphic 2D and 3D lung phantoms and defines the detectability of the candidate lesions using a non-prewhitening matched filter with an eye filter (NPWE) model observer. Sets of phantom images were simulated for a range of noise levels and two types of noise (Gaussian white noise and CT-like noise). The candidate lesions were classified as true positives and false positives. A proof of concept study showed promising results in the detectability trends the search algorithm described. The trends showed that with increasing noise levels the detectability of true positives decreased. When comparing the detectability indexes of the true positives and false positives, the differences between them became smaller for increasing noise levels. In the future, the algorithm can be applied to the analysis of real CT scans of a lung phantom containing lesions, and used to obtain Free Response Operating Characterisic (FROC) curves.Show less
In this thesis we present two distinct interferometric set-ups for displacement measuring interferometry in cryogenic Low Energy Electron Microscopy (LEEM), specifically for use in ESCHER, the LEEM...Show moreIn this thesis we present two distinct interferometric set-ups for displacement measuring interferometry in cryogenic Low Energy Electron Microscopy (LEEM), specifically for use in ESCHER, the LEEM at Leiden University. The set-ups are based on: 1. circular polarization and 2. 3x3 fiber couplers to obtain a quadrature measurement. Whilst the 3x3 coupler based set-up is limited by thermal drift and other environmental effects, the circular polarization based method displays a 3\sigma resolution of 1.75 nm, and we provide a framework for further increasing this resolution, including a set of alternate set-ups with slight added complexity but a great potential gain in resolution. Keywords: Displacement, Interferometry, Fiber Couplers, Polarization, LEEM, cryogenic, DMI, Quadrature, Fiber.Show less
For this project we investigated the effect of different spacing between nanoparticles in arrays on molecular exchange. To study this dependence we synthesized 10nm gold nanoparticles capped by...Show moreFor this project we investigated the effect of different spacing between nanoparticles in arrays on molecular exchange. To study this dependence we synthesized 10nm gold nanoparticles capped by different alkanethiols. By letting these nanoparticles self-assemble into nanoparticle arrays and immersing these arrays in a solution containing conjugated molecules, we formed networks of gold nanoparticles interconnected by these conjugated molecules. By measuring the electrical and optical properties of these networks, as well as imaging them with a SEM, we were able to characterize the spacing of nanoparticles dependence of molecular exchange. We conclude that the molecular exchange is not dependent on the spacing between nanoparticles. This might be due to the network quality.Show less
We determined the spatial resolution of the new very low energy transmission electron microscopy technique called eV-TEM [1] and used it to image gold nanoparticles deposited on graphene in order...Show moreWe determined the spatial resolution of the new very low energy transmission electron microscopy technique called eV-TEM [1] and used it to image gold nanoparticles deposited on graphene in order to determine whether it is possible to image for example DNA and proteins with low energy electrons. By transferring graphene to a flat grid with small circular holes in it we created new samples with flatter, less wrinkled graphene that make performing eV-TEM easurements on graphene easier and increase their quality. We improved the alignment of the imaging system of the microscope and determined the resolution of eV-TEM using the new samples to image graphene. We found a method to deposit 10nm gold nanoparticles on graphene suitable for eV-TEM measurements and a method to deposit ferritin on graphene that we should be able to image as well. We conclude that the spatial resolution of the current set-up of eV-TEM is 10nm and that it is possible to image gold nanoparticles deposited on graphene with eV-TEM.Show less
In the search of small and functional molecular devices, we have created and investigated 20 different self-assembled multilayers (SAMTs) each consisting of two self-assembled monolayers (SAM) on...Show moreIn the search of small and functional molecular devices, we have created and investigated 20 different self-assembled multilayers (SAMTs) each consisting of two self-assembled monolayers (SAM) on top of each other. We compare the molecular combinations to their inverse combinations, investigate their length dependence on the conductance and look for interesting features. Each SAM consists of one type of molecule (there are five different ruthenium complex molecules in total) and each bilayer is grown on an indium tin oxide substrate. Then current-voltage (I-V) curves are taken using conductive atomic force microscopy (C-AFM) to characterize the samples after the relative humidity (RH) of the setup has been brought to ≈ 5% and ≈ 50% respectively and the rectification ratio (RR) for each molecular combination is calculated. Double layers consisting of the same Ru-complexes and combinations of different molecules are measured. We find that while the shape of the I-V curves and the conductance of the combinations are different compared to those of their inverse combination, the rectification ratio (RR) (and in particular the direction of rectification) is not. The highest RR found is RRhumid = 10^(-2.0±0.6) at 1.0 V for the combination with bottom layer 1-Ru-N and top layer 2-Ru-N (decoupled). Furthermore, we see that while most bilayers have current values that agree to a conductance that decreases exponentially with the length, some do not. Lastly, we observe hysteresis in three samples. Our findings in this exploratory experiment contribute to the knowledge of charge transport in junctions consisting of layers of complex molecules.Show less
In this work, we investigate methods to determine the average length for cotunneling in self-assembled gold nanoparticle arrays spaced with alkanethiols. Cotunneling currents can be as low as 10 fA...Show moreIn this work, we investigate methods to determine the average length for cotunneling in self-assembled gold nanoparticle arrays spaced with alkanethiols. Cotunneling currents can be as low as 10 fA, requiring the development of robust measurement techniques to lower the noise floor. We present a comparison of different methods for finding the effective cotunneling length N. Preliminary findings on a variety of arrays indicate the onset of the cotunneling regime, but that there is no distinction in N between choice of alkanethiol in the array.Show less
Combining an applied thermal potential with Conductive Atomic Force Microscopy (C-AFM) enables the measurement of thermoelectric properties of Self-Assembled Monolayers (SAM). Such measurements can...Show moreCombining an applied thermal potential with Conductive Atomic Force Microscopy (C-AFM) enables the measurement of thermoelectric properties of Self-Assembled Monolayers (SAM). Such measurements can be used to show the presence of destructive quantum interference in molecules. To study the feasibility of measuring thermoelectric potentials over SAMs with C-AFM, we have simulated the temperature distribution around the tip in a typical C-AFM setup with finite element simulations in Comsol. These show that a sufficiently large temperature difference can develop across the molecular layer to measure the thermoelectric properties.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
Carbon Nanomembranes (CNMs) are two-dimensional materials made by cross-linking (interconnecting) the molecules of a Self Assembled Monolayer (SAM). CNMs can be chemically tailored on one or both...Show moreCarbon Nanomembranes (CNMs) are two-dimensional materials made by cross-linking (interconnecting) the molecules of a Self Assembled Monolayer (SAM). CNMs can be chemically tailored on one or both sides leading to a variety of possible properties (e.g. mechanical, conductive, chemical, etc.). Once these properties are understood and controlled, CNMs might be a candidate for (among other things) functional devices based on molecules. In this thesis, it is described how to create laterally patterned CNMs by cross-linking aromatic based self assembled monolayers. Cross-linking is done by in situ low-energy electron irradiation with low energy electron microscopy (LEEM). This novel method allows for new types of experiments giving a brand new perspective. We show that a new structure (possibly CNM) was created after electron irradiation of SAM followed by annealing, furthermore, we have learned that our SAMs show an electron irradiation induced effect. This effect is most accurately described as a front shaped like the irradiated area, slowly shrinking in size until its disappearance. This effect was observed right after irradiation if certain conditions on the illumination time and electron energy were met. This phenomenon was imaged live and down to micrometer scale. These measurements have brought us to the conclusion that this front is not shrinking with a constant speed but is accelerating just before its disappearance. Furthermore, our measurements hint for a connection between the parameters illumination time & electron energy and the visibility and speed of a front.Show less
