Stacking domain boundaries occur in Van der Waals heterostacks whenever there is a twist angle or lattice mismatch between subsequent layers. Not only can these domain boundaries host topological... Show moreStacking domain boundaries occur in Van der Waals heterostacks whenever there is a twist angle or lattice mismatch between subsequent layers. Not only can these domain boundaries host topological edge states, imaging them has been instrumental to determine local variations in twisted bilayer graphene. Here, we analyse the mechanisms causing stacking domain boundary contrast in Bright Field Low-Energy Electron Microscopy (BF-LEEM) for both graphene on SiC, where domain boundaries are caused by strain and for twisted few layer graphene. We show that when domain boundaries are between the top two graphene layers, BF-LEEM contrast is observed due to amplitude contrast and corresponds well to calculations of the contrast based purely on the local stacking in the domain boundary. Conversely, for deeper-lying domain boundaries, amplitude contrast only provides a weak distinction between the inequivalent stackings in the domains themselves. However, for small domains phase contrast, where electrons from different parts of the unit cell interfere causes a very strong contrast. We derive a general rule-of-thumb of expected BF-LEEM contrast for domain boundaries in Van der Waals materials. Show less
Jong, T.A. de; Visser, L.; Jobst, J.; Tromp, R.M.; Molen, S.J. van der 2022
Terrace-sized, single-orientation graphene can be grown on top of a carbon buffer layer on silicon carbide by thermal decomposition. Despite its homogeneous appearance, a surprisingly large... Show moreTerrace-sized, single-orientation graphene can be grown on top of a carbon buffer layer on silicon carbide by thermal decomposition. Despite its homogeneous appearance, a surprisingly large variation in electron transport properties is observed.Here, we employ Aberration-Corrected Low-Energy Electron Microscopy (AC-LEEM) to study a possible cause of this variability. We characterize the morphology of stacking domains between the graphene and the buffer layer of high-quality samples. Similar to the case of twisted bilayer graphene, the lattice mismatch between the graphene layer and the buffer layer at the growth temperature causes a moiré pattern with domain boundaries between AB and BA stackings.We analyze this moiré pattern to characterize the relative strain and to count the number of edge dislocations. Furthermore, we show that epitaxial graphene on silicon carbide is close to a phase transition, causing intrinsic disorder in the form of co-existence of anisotropic stripe domains and isotropic trigonal domains. Using adaptive geometric phase analysis, we determine the precise relative strain variation caused by these domains. We observe that the step edges of the SiC substrate influence the orientation of the domains and we discuss which aspects of the growth process influence these effects by comparing samples from different sources. Show less
Transport experiments in twisted bilayer graphene haverevealed multiple superconducting domes separated by cor-related insulating states 1–5 . These properties are generallyassociated with strongly... Show moreTransport experiments in twisted bilayer graphene haverevealed multiple superconducting domes separated by cor-related insulating states 1–5 . These properties are generallyassociated with strongly correlated states in a flat mini-bandof the hexagonal moiré superlattice as was predicted by bandstructure calculations 6–8 . Evidence for the existence of a flatband comes from local tunnelling spectroscopy 9–13 and elec-tronic compressibility measurements 14 , which report two ormore sharp peaks in the density of states that may be asso-ciated with closely spaced Van Hove singularities. However,direct momentum-resolved measurements have proved to bechallenging 15 . Here, we combine different imaging techniquesand angle-resolved photoemission with simultaneous real- andmomentum-space resolution (nano-ARPES) to directly mapthe band dispersion in twisted bilayer graphene devices nearcharge neutrality. Our experiments reveal large areas with ahomogeneous twist angle that support a flat band with a spec-tral weight that is highly localized in momentum space. The flatband is separated from the dispersive Dirac bands, which showmultiple moiré hybridization gaps. These data establish thesalient features of the twisted bilayer graphene band structure. Show less
Jong, T.A. de; Kok, D.N.L.; Torren, A.J.H. van der; Schopmans, H.; Tromp, R.M.; Molen, S.J. van der; Jobst, J. 2019
For many complex materials systems, low-energy electron microscopy (LEEM) offers detailed insights into morphology and crystallography by naturally combining real-space and reciprocal-space... Show moreFor many complex materials systems, low-energy electron microscopy (LEEM) offers detailed insights into morphology and crystallography by naturally combining real-space and reciprocal-space information. Its unique strength, however, is that all measurements can easily be performed energy-dependently. Consequently, one should treat LEEM measurements as multi-dimensional, spectroscopic datasets rather than as images to fully harvest this potential. Here we describe a measurement and data analysis approach to obtain such quantitative spectroscopic LEEM datasets with high lateral resolution. The employed detector correction and adjustment techniques enable measurement of true reflectivity values over four orders of magnitudes of intensity. Moreover, we show a drift correction algorithm, tailored for LEEM datasets with inverting contrast, that yields sub-pixel accuracy without special computational demands. Finally, we apply dimension reduction techniques to summarize the key spectroscopic features of datasets with hundreds of images into two single images that can easily be presented and interpreted intuitively. We use cluster analysis to automatically identify different materials within the field of view and to calculate average spectra per material. We demonstrate these methods by analyzing bright-field and dark-field datasets of few-layer graphene grown on silicon carbide and provide a high-performance Python implementation. Show less
Electron microscopy has become an extremely important techniquein a wide variety of elds. The resolving power is vastly superiorto light microscopes and electron microscopy has proven tobe valuable... Show moreElectron microscopy has become an extremely important techniquein a wide variety of elds. The resolving power is vastly superiorto light microscopes and electron microscopy has proven tobe valuable in elds ranging from archaeology and geology to biology andcondensed-matter physics.A major disadvantage is that the electron energy used in conventional ElectronMicroscopy (EM) ranges from 10’s to 100’s of keV. Such energetic electronscan signicantly damage the specimen. This is especially relevant in thestudy of biological samples and organic materials in general. Major eorts arebeing made to avoid this radiation damage from interfering with the studyof such materials. There are several approaches to minimize damage in EM.These include developing better detectors such that lower electron doses aresucient to form an image, and lowering the electron energies to several keV.In this dissertation I present the development of, and measurements with, atransmission electron microscope that uses electron energies ve orders ofmagnitude lower than in conventional Transmission Electron Microscopes(TEMs). The energies we use are in the order of a few eV. Hence, we call ourtechnique ’eV-TEM’. Show less
Torren, A.J.H. van der; Molen, S.J. van der; Aarts, J. 2016
By combining low-energy electron microscopy (LEEM) with pulsed laser deposition (PLD), we have created a unique set-up to study the first stages of growth of complex metal oxides. We... Show more By combining low-energy electron microscopy (LEEM) with pulsed laser deposition (PLD), we have created a unique set-up to study the first stages of growth of complex metal oxides. We demonstrate this by investigating the growth of SrTiO3 (STO) and LaAlO3 (LAO) on STO in real-time. We follow growth by monitoring the intensity and the full-width-half-maximum (FWHM) of the specular diffracted beam at various energies. For layer-by-layer growth, we find the anticipated intensity peaks at the completion of each layer, and an oscillatory FWHM with the maximum at half-layer coverage. In the LAO on STO case, for optimal growth conditions and a LAO thickness above the critical thickness of 4 unit cells the interface between the band insulators shows conductivity. We obtain an electronic fingerprint of the growing material, by measuring the intensity of the specular beam as a function of energy at regular intervals during growth. Extending this fingerprint with the intensity dependence on the momentum parallel to the surface allows us to extract the band dispersion of unoccupied electron states of the sample surface. Significant differences in the unoccupied band structure develop between samples which are conducting and non-conducting. Show less
Low Energy Electron Microscopy (LEEM) is a microscopy technique typically used to study surface processes. The sample is illuminated with a parallel beam of electrons under normal incidence and the... Show moreLow Energy Electron Microscopy (LEEM) is a microscopy technique typically used to study surface processes. The sample is illuminated with a parallel beam of electrons under normal incidence and the reflected electrons are projected onto a pixelated detector, where an image is formed. In the first part of this thesis, we use LEEM to study the behavior of submonolayers of gold on Si(111). After a thorough analysis of the Si-Au system, we describe the behavior of these (sub)monolayers when exposed to alkanethiols. In the second part of this thesis we move away from pure surface physics and introduce two new applications for LEEM. The first of these, Low-Energy Electron Potentiometry (LEEP), can be used to visualize electrical conductance. We show that for layered two-dimensional materials we can obtain a higher resolution in LEEP experiments. Finally, in chapter 6, we present a new method to measure the dispersion relation of unoccupied states in two-dimensional layered materials. Show less
