In this PhD thesis, the recombination of different atomic lattices in stacked 2D materials such as twisted bilayer graphene is studied. Using the different possibilities of Low-Energy Electron... Show moreIn this PhD thesis, the recombination of different atomic lattices in stacked 2D materials such as twisted bilayer graphene is studied. Using the different possibilities of Low-Energy Electron Microscopy (LEEM), the domain forming between the two atomic layers with small differences is studied. Superlattices in three such 2D material systems are studied. In twisted bilayer graphene, the small difference is caused by a twist of approximately one degree between the layers. In graphene on SiC, the difference is caused by the lattice mismatch between a buffer layer bound to the substrate and the next graphene layer. For both, we show that domains of different shapes and sizes occur and relate them to strain and lattice mismatch. The third system studied is tantalum disulfide. In this layered material, two different superlattices occur: a superlattice between atomic layers with different atomic arrangements in the layers, so-called polytypes, and the superlattices between the atomic lattice and the Charge Density Waves (CDW). CDWs cause a large temperature dependent resistivity change. The influence of a mixture of different polytypes on the precise CDW states is studied using LEEM spectroscopy and local Low-Energy Electron Diffraction. Show less
Jong, T.A. de; Chen, X.; Jobst, J.; Krasovskii, E.E.; Tromp, R.M.; Molen, S.J. van der 2022
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
Graphene nanoribbons (GNRs) are used as a current carrying substrate in investigation of current-induced forces in a low-temperature STM (chapter 2). We demonstrate induced migration of Co adatoms... Show moreGraphene nanoribbons (GNRs) are used as a current carrying substrate in investigation of current-induced forces in a low-temperature STM (chapter 2). We demonstrate induced migration of Co adatoms on GNRs and on Au(111) using voltage pulses from the STM tip and we argue that motion is due to thermal excitations rather than the wind force. In chapter 3 we show that voltage signal is induced in a graphene strip when a droplet of ionic liquid is moved across its surface. Here we show that even deionized water can induce voltage over charged graphene surface due to the polarizability of water molecules. In chapter 4 we present a method for fabrication of graphene nanoelectrodes which we further test electrically in a modified STM. For the first time we demonstrate that the gap between two graphene nanoelectrodes can be tuned with subnanometric precision Show less
In this thesis we study quantum transport phenomena on the nanometer scale, in two classes of materials: topological insulators with induced superconductivity and graphene superlattices. Both... Show moreIn this thesis we study quantum transport phenomena on the nanometer scale, in two classes of materials: topological insulators with induced superconductivity and graphene superlattices. Both topics are motivated by recent experimental developments: the first topic arose from the search for Majorana fermions in a quantum spin Hall insulator, the second topic arose from the search for massive Dirac fermions in the Kekulé band structure of graphene on a copper substrate. We focus on lattice models, solving them both numerically and analytically. 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
The thesis describes experimental steps towards reduction of friction on the macroscopic scale by scenarios of thermo- and superlubricity well-known on the nanoscale. The friction study involves... Show moreThe thesis describes experimental steps towards reduction of friction on the macroscopic scale by scenarios of thermo- and superlubricity well-known on the nanoscale. The friction study involves experiments on tailored Si nanopillar arrays, micropatterned Diamond-Like Carbon coating and high-quality graphene. Show less
This work covers two closely related topics: a theoretical study on the origins of friction and an experimental study on the growth of graphene. Both fundamental studies are focusing on the atomic... Show moreThis work covers two closely related topics: a theoretical study on the origins of friction and an experimental study on the growth of graphene. Both fundamental studies are focusing on the atomic processes involved. The study on friction treats the dissipation that takes places at one single friction contact. We show that the current explanations result in a discrepancy that we solve by evalutation of the mass involved: this mass is orders of magnitude smallar than assumed. The very small and dynamic mass at a friction contact forms an efficient channel of dissipation. This explanation allows us to understand and predict the friction behavior of surfaces at both the small and large scale. The study of graphene growth investigates the growth process of graphene at the atomic scale with a Scannning Tunneling Microscope in situ. We use our high- and, variable-temperature STM to determine the lowest nucleation temperature of graphene on Ir(111). Additionaly, individual steps that follow up each other during growth are clarified and presented. The graphene film closure is studied as well, which showed that graphene introduces internal strain in order to prevent local lattice defects. Our results are important for the improvement of the quality of graphene. Show less
In this thesis, the formation of hexagonal boron nitride (h-BN) __nanomesh__ structures and of graphene on Rhodium (111) is studied experimentally. The structures of h-BN and graphene are extremely... Show moreIn this thesis, the formation of hexagonal boron nitride (h-BN) __nanomesh__ structures and of graphene on Rhodium (111) is studied experimentally. The structures of h-BN and graphene are extremely similar: both of them are single atomic layers with a honeycomb lattice, and the lattice constants are nearly identical. Both materials introduce novel properties and have the potential for a variety of applications. In this thesis, the layers were grown by chemical vapor deposition (CVD) on Rh(111). During growth, the formation processes were tracked by scanning tunneling microscopy (STM). This was performed in situ, namely during deposition at the elevated temperatures, required for the growth. In this way, we have obtained detailed knowledge of the formation mechanisms. In this thesis, basic surface science principles are employed to explain the observed, special growth behavior. Our understanding of the mechanisms at play has enabled us to compose new, improved deposition recipes that result in higher quality nanomesh and graphene layers. This knowledge is not only valuable for these specific systems, but it also deepens our general insights into deposition and growth of atomically thin layers. Show less
This dissertation is about transport and electronic properties of two types of electronic states occuring at the edges, which are protected by symmetry between positive and negative energies. One... Show moreThis dissertation is about transport and electronic properties of two types of electronic states occuring at the edges, which are protected by symmetry between positive and negative energies. One type of these states is shown to occur universally in graphene. It is also described how another type of edge states, Majorana fermions, can be used for topological quantum computation. Show less
The central topic in this thesis is the effect of topological defects in two distinct types of condensed matter systems. The first type consists of graphene and topological insulators. By... Show moreThe central topic in this thesis is the effect of topological defects in two distinct types of condensed matter systems. The first type consists of graphene and topological insulators. By studying the long-range effect of lattice defects (dislocations and disclinations) we find that the graphene electrons mimic fundamental Dirac electrons in spaces with curvature and torsion. We show that these long-range effects influence interferometric transport measurements: (i) Emphasizing the importance of electron dephasing in graphene; (ii) Enabling a characterization of neutral Majorana states, which are important for quantum computation applications, and conjectured to exist in topological insulators. Considering also the microscopic structure of graphene dislocations, we interpret local tunneling experiments on graphite grain boundaries. The second type of systems we study are the high temperature cuprate superconductors, where the strongly interacting electrons lead to coexisting symmetry breaking orders in the pseudogap phase. We observe and describe the interplay of nematic (orientational) and stripe (translational) orderings in local tunneling experiments, with stripe dislocations playing the key role. We also describe the observed phonon anomaly in cuprates through the effect of metallic stripes. Show less
In this thesis we consider several effects of a Dirac spectrum in photonic crystals on the scattering and propagation of light. We calculate the effect of a Dirac point (a conical singularity in... Show moreIn this thesis we consider several effects of a Dirac spectrum in photonic crystals on the scattering and propagation of light. We calculate the effect of a Dirac point (a conical singularity in the band structure) on the transmission of radiation through a photonic crystal. We find that the transmission at the Dirac point is inversely proportional to the longitudinal dimension of the crystal. Further we propose a method to detect the pseudospin-1/2 Berry phase produced by the Dirac-type band structure of a triangular-lattice photonic crystal. In addition we show that the half-integer spin and the associated Berry phase remain observable in the presence of disorder in the crystal: the destructive interference caused by the Berry phase suppresses the reflected intensity at an angle which is related to the angle of incidence by time-reversal symmetry causing extinction of coherent backscattering. We test all our predictions numerically by solving the full Maxwell equations. We also demonstrate that the Goos-Hanchen effect, which has been observed for the first time in optics and has applications in particular in photonic crystals, is of high importance for graphene. We show that the Goos-Hanchen effect in an n-doped channel with p-doped boundaries doubles the degeneracy of the lowest propagating mode, introducing a two-fold degeneracy on top of the usual spin and valley degeneracies. This can be observed as a stepwise increase by 8e^2/h of the conductance with increasing channel width. Show less
The theoretical foundation for the work reported here is provided by Landauer's scattering theory of electron transport. The three main ingredients of a scattering problem are (1) a set of... Show moreThe theoretical foundation for the work reported here is provided by Landauer's scattering theory of electron transport. The three main ingredients of a scattering problem are (1) a set of reservoirs that emit and absorb particles, (2) the particles themselves, that propagate as waves between the reservoirs and (3) a scatterer that obstructs free propagation. In this thesis two classes of problems are considered. The first class results when the physical quantities characterizing the reservoirs or the scatterer are not constant in time. The second class results when wave propagation is described by the Dirac equation rather than the Schroedinger equation, as is the case in a 2D form of carbon, called graphene. Show less