THESIS
2016
xi, 81 pages : illustrations ; 30 cm
Abstract
The growth, structure and structural phase transitions of two-dimensional layers on crystalline metallic substrates have been investigated using low energy electron microscopy (LEEM) and micro-low energy electron diffraction (μLEED). The complementary capabilities of LEEM and μLEED for studying real space and reciprocal space with high spatial and temporal resolution have provided unique insight about (1) one-dimensional roughening and two-dimensional critical phenomena in a newly discovered oxide layer at the Cu(111) surface; (2) defects, incommensurability and polymorphism in monolayer and bilayer graphene on the Ru(0001) surface; and (3) ordering and growth of supramolecular-assembly of TPyB molecules on the Cu(111) surface.
The oxidation of the Cu(111) surface has been well studied...[
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The growth, structure and structural phase transitions of two-dimensional layers on crystalline metallic substrates have been investigated using low energy electron microscopy (LEEM) and micro-low energy electron diffraction (μLEED). The complementary capabilities of LEEM and μLEED for studying real space and reciprocal space with high spatial and temporal resolution have provided unique insight about (1) one-dimensional roughening and two-dimensional critical phenomena in a newly discovered oxide layer at the Cu(111) surface; (2) defects, incommensurability and polymorphism in monolayer and bilayer graphene on the Ru(0001) surface; and (3) ordering and growth of supramolecular-assembly of TPyB molecules on the Cu(111) surface.
The oxidation of the Cu(111) surface has been well studied for many years and several different complex oxide structures have been identified. In the present work, we have discovered a new oxygen-induced structural phase, denoted as the “ringed (√3 × √3)?30
o” structure. This phase exhibits a domain wall roughening transition at intermediate temperature and a second-order two-dimensional phase transition at higher temperature. The domain wall energy is determined quantitatively from domain wall fluctuations using capillary wave analysis. It decreases with increasing temperature and the one-dimensional roughening temperature, at which the domain wall energy goes to zero, is determined to be 759.5 ± 7.1?. The dynamical scaling exponent of domain wall fluctuations is also determined to be 2.29 ± 0.09, which indicates that domain wall motion is mediated by a two-dimensional evaporation-condensation mechanism. Observations of critical fluctuations that occur above the roughening temperature reveal the divergence of the heat capacity at the critical temperature ?
? = 768.2 ± 0.3?. Static critical exponents that characterize the divergences of the heat capacity and correlation length are determined to be ? = 0.72 ± 0.08 and ? = 0.50 ± 0.04, respectively, and the dynamical critical exponent is determined to be ? = 1.92 ± 0.18.
Our investigations of graphene on the Ru(0001) surface were aimed at mitigating the proliferation of small-angle rotational domains and small angle grain boundaries that were observed previously both by other researchers during growth by chemical vapor deposition (CVD). We introduce a combined segregation/CVD growth method that is able to produce large domains with uniform orientation over several micron length scale. Using scanning μLEED imaging with 250nm spatial resolution to probe graphene lattice rotations and lattice constant, we discovered that the graphene lattice is never perfectly aligned with the substrate high symmetry directions. Furthermore, very clear evidence is obtained of incommensurability and polymorphism, in particular, the occurrence of 25-on-23 and 12-on-11 periodic structures. These results contradict an authoritative structure determination of a 25-on-23 periodicity based on surface x-ray diffraction measurements. The flexibility of graphene on metals is attributed to the coupling of in-plane strain and out-of-plane buckling. Therefore, our observations may shed light on controversies over lateral periodicity and corrugation in g/Ru(0001).
Finally, we explored the growth and self-assembly of TPyB supremolecular networks on the Cu(111) surface. We find the structure of the two-dimensional network to be (9.5 × 9.5)?30
? . With increasing temperature, the ordered network undergoes a reversible transformation to a dilute phase. In the course of this work, an electron beam induced effect is observed on the ordering-disordering process. Interestingly, the effect of the electron beam on the molecular layer even extends outside the edge of the electron beam. However, this action at a distance is terminated at steps at the edges of terraces that are illuminated by the electron beam. This indicates that significant lateral mass transport facilitates equilibration over large lateral length scales on a flat terrace, and more importantly that steps are impermeable to molecular motion.
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