Superconductivity refers to a phase of matter in which charge carriers can be moved without dissipating energy. In this special phase, unlike a perfect metal conductor, any external magnetic field... Show moreSuperconductivity refers to a phase of matter in which charge carriers can be moved without dissipating energy. In this special phase, unlike a perfect metal conductor, any external magnetic field lines are expelled from the material. The phenomenon has been a focus of attention both in fundamental science research as well as technological application ever since it was first discovered in Leiden in the year of 1911. Recent fast progress in nano-engineering, fabrication and characterisation enable two-dimensional devices to be realised relatively easily in the lab via top-down or/and bottom-up methods. Van der Waals materials and thin films can be fabricated now with good control and reproducibility. This has not only paved the way for studying clean superconductivity in two dimensions.The advances in nanotechnology combined with the increasing understanding and exploration in solid state physics also allow more control over the superconducting properties of matter. This thesis contributes to the study of conventional phonon-mediated superconductors by exploring the possibility of manipulating (quasi-) two-dimensional (2D) superconductors' properties through the careful design of the devices. The investigations reported in this thesis include clean 2D superconductivity via a top-down fabrication method of exfoliating van der Waals superconductor crystals; understanding critical current magnetic oscillation in van der Waals heterostructure Josephson Junctions; increasing critical current density of thin film superconductor through controlled oxidation. And ambitiously, the increasing of critical temperature of a superconductor by manipulating the material with a superperiodic potential. Show less
Spectroscopic studies on fluorescent single molecules in organic condensed matter does not only provide information about the molecule itself, but also its near environment. By suppression of... Show moreSpectroscopic studies on fluorescent single molecules in organic condensed matter does not only provide information about the molecule itself, but also its near environment. By suppression of phonon-induced broadening of spectral lines through cooling to low temperatures, small changes in the spectral lines’ position can be observed in response to weak variations in local fields. These variations can for instance be caused by rearrangements of charges or minute changes in the crystal lattice around the molecule. Therefore, molecules are sensitive sensors to what happens at the nanoscale. This is exemplified by coupling to an external electric field, inducing a Stark shift of the molecule’s spectral lines, as shown in Chapter 4. Other dynamics, related to the crystal around the molecule, are resolved in the fluorescence of molecules on the surface of two-dimensional hexagonal boron nitride, shown in Chapter 5. In Chapter 2, 3 and 6, perylene molecules are studied in a new host crystal with the aim of detecting a ‘forbidden’ transition to the triplet state from the ground state, a transition required for building a single-molecule optical switch. Show less
In this Ph.D. thesis we study the interaction of low energy electrons with thin materials, namely layered materials (graphene, hexagonal boron nitride, molybdenum disulfide) and organic films. At... Show moreIn this Ph.D. thesis we study the interaction of low energy electrons with thin materials, namely layered materials (graphene, hexagonal boron nitride, molybdenum disulfide) and organic films. At these low energies the quantum mechanical wavelength of the electron wave function is in the order of a few Angstroms, thus comparable to the interlayer distance in layered materials. This leads to resonances in the electron reflection/transmission spectrum, comparable to the interference of light when it is reflected from a thin film. We use low energy electron microscopy (LEEM) and electron Volt transmission electron microscopy (eV-TEM) to determine the energy dependent electron mean free path (MFP) and identify resonant transmission/reflection states related to the unoccupied band structure. Furthermore, we use photoemission electron microscopy (PEEM) to image low energy electrons from a gold surface covered with a film of chiral organic molecules. We image the photoexcited electrons and compare the intensity of photoemission caused by different (circular) polarizations of light. Show less
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; Benschop, T.; Chen, X.; Krasovskii, E.E.; Dood, M.J.A. de; Tromp, R.M.; ... ; Molen, S.J. van der 2021
In twisted bilayer graphene (TBG) a moiré pattern forms that introduces a new length scale to the material. At the 'magic' twist angle of 1.1°, this causes a flat band to form, yielding emergent... Show moreIn twisted bilayer graphene (TBG) a moiré pattern forms that introduces a new length scale to the material. At the 'magic' twist angle of 1.1°, this causes a flat band to form, yielding emergent properties such as correlated insulator behavior and superconductivity [1-4]. In general, the moiré structure in TBG varies spatially, influencing the local electronic properties [5-9] and hence the outcome of macroscopic charge transport experiments. In particular, to understand the wide variety observed in the phase diagrams and critical temperatures, a more detailed understanding of the local moiré variation is needed [10]. Here, we study spatial and temporal variations of the moiré pattern in TBG using aberration-corrected Low Energy Electron Microscopy (AC-LEEM) [11,12]. The spatial variation we find is lower than reported previously. At 500°C, we observe thermal fluctuations of the moiré lattice, corresponding to collective atomic displacements of less than 70pm on a time scale of seconds [13], homogenizing the sample. Despite previous concerns, no untwisting of the layers is found, even at temperatures as high as 600°C [14,15]. From these observations, we conclude that thermal annealing can be used to decrease the local disorder in TBG samples. Finally, we report the existence of individual edge dislocations in the atomic and moiré lattice. These topological defects break translation symmetry and are anticipated to exhibit unique local electronic properties. Show less
In this work, we illustrate unconventional approaches towards the fabrication of edge functionalized graphene nanostructures and bidimensional architectures in polymeric and metallic supports, with... Show moreIn this work, we illustrate unconventional approaches towards the fabrication of edge functionalized graphene nanostructures and bidimensional architectures in polymeric and metallic supports, with an outlook towards molecular sensing devices. Particularly, starting from the most established knowledge on the chemistry of graphene, we selectively functionalize the edges of graphene either via electrochemistry, plasma chemistry and solution chemistry. In fact, the chemistry at the edges, particularly at the nanoscale, tailors the properties of graphene without perturbing the honeycomb lattice of carbon atoms, thus without compromising the intrinsic nature of graphene. Via unconventional tools such as microtomy and molecular break junctions, we finally realize chemically designed platforms such as transistors, nanogaps and nanoribbons to be further integrated into sensing devices, such as zero-depth nanopore. Remarkably, we demonstrate the possibility of achieving extremely precise graphene nanostructures while going beyond the highly complicated demands of conventional top-down fabrications. At the same time, we specifically address the chemistry at the edges of graphene moving beyond synthetic approaches. Selectively edge functionalized graphene becomes available also on large area films and tailored graphene nanostructures, looking for the integration of graphene in the next generation sensing devices. Show less
