Growth, Structure and Structural Phase Transitions in Two-dimensional Atomic and Supermolecular-Assembled Layers

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 LTPyB molecules on the Cu(111) surface.

 

In the present work, we have discovered a new oxygen-induced structure, denoted as “ringed $(\sqrt{3} \times \sqrt{3}) R30^{\circ}$. This phase exhibits a domain wall roughening transition at intermediate temperature and a second-order two-dimensional phase transition at higher temperature. The one-dimensional roughening temperature, at which the domain wall energy goes to zero, is determined to be $759 \pm 7.1K$. The dynamical scaling exponent is also determined to be $2.29 \pm 0.08$ , 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 $T_c = 768.3 \pm 0.2K$ . We also determine the static critical exponents and the dynamical critical exponent to be $\alpha = 0.64 \pm 0.06$, $v = 0.63 \pm 0.11$, and $z = 2.01 \pm 0.14$ respectively. For graphene on Ru(0001), 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, 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. Finally, we explored the growth and self-assembly of LTPyB supremolecular networks on the Cu(111) surface. We find the structure of the two-dimensional network to be $(10 \times 10) R30^{\circ}$. With increasing temperature, the ordered network undergoes a reversible transformation to a dilute phase. In this work, an electron beam induced effect is observed on the ordering-disordering process. We find that steps are impermeable to molecular motion.