Molecular materials have been a subject of interest in fundamental research and applications for decades, and have been studied as bulk crystals, (thin) films and as individual molecules, due to... Show moreMolecular materials have been a subject of interest in fundamental research and applications for decades, and have been studied as bulk crystals, (thin) films and as individual molecules, due to the large variety in their properties. This dissertation explores pentacene crystals near the two-dimensional limit, i.e. in films of one to a few monolayers, primarily using an experimental technique called Low Energy Electron Microscopy (LEEM). We utilize the interaction of low-energy electrons with the molecular layers to learn about the properties of the layers, and also employ low-energy electrons as an external stimulus to create controlled changes in the properties of the sample. We study the real-time dynamics of growth of pentacene layers on various substrates. We study beam irradiation damage to molecular layers due to exposure to low-energy electrons, with implications for technological processes such as electron-beam lithography. We study photoemission and secondary electron emission from the molecular layers and highlight the critical role of unoccupied electronic band structure above the vacuum level in these processes. Finally, we present the design of, as well as the first scientific measurements in, a cryogenic sample chamber for LEEM that enables novel experiments addressing questions in surface science and condensed matter physics. 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
