Part 1. Scanning tunneling mocroscopy study of I-POE monolayers on graphite superlattice

We investigated the physisorption of p-iodo-phenyl octadecyl ether molecules (I-POE) on graphite superlattice surfaces. We directly observed monolayers of I-POE adsorbed to either pristine or superlattice regions of a graphite surface using scanning tunneling microscopy (STM). The electron density of the superlattice was found to couple to the electron density of the I-POE monolayer in the STM images. The packing density of the monolayer and the orientations of I-POE lamella structures could be modified depending on the superlattice. The bias voltage-dependent STM images revealed the relative scales of the molecular orbital (MO) energy levels for I-POE and the density of states of the superlattice. Our observations suggest that the graphite superlattice can affect the STM images and the packing characteristics of an organic monolayer, unlike the relative inertness of flat pristine graphite surfaces. Care must be taken to avoid misinterpreting organic monolayer structures observed by STM on superlattice regions.
Part 2. Formation study of nanostructure formed by evaporating dipyrromethene-based solutions

Dichloromethane (DPM) liquid droplets containing a cobalt dipyrromethene complexes deposited on a graphite surface were found to form coffee ring, donut ring, molar ring, and volcano dot structures due to the redistribution of the solute during solvent evaporation. The process by which ring-like structures form from a liquid droplet depends on the characteristics of the physical dewetting process, the substrate, the solvent, and the solute dynamics, including the rate of evaporation of the liquid, the solubility, and the temperature gradient across the droplet.
We provide an example system in which the self-assembly of a nanostructure on a graphite surface driven by dipyrromethane molecules could be controlled by varying the solvent evaporation rate. We found that the shape and size of the DPM-based nanostructures depended on the drying temperature. The DPM derivatives, which can form neutral complexes with a variety of metal ions, have been used as flexible and versatile ligands in supramolecular chemistry applications. For example, bisdipyrromethene metal complexes featuring double helical or triangular structures have been reported. We interpreted the temperature dependence of the nanostructure shape and size as indicative of two formation mechanisms, the selection of which appears to be determined by the solvent evaporation rate.
At higher temperatures, the heat was readily transferred from the substrate to the pinned edge of the drop, thereby enhancing evaporation near the drop’s edges relative to the center. The CDT moved toward the drop’s edge and replaced fluid lost to evaporation. Increasing the substrate temperature increased the magnitude of the solute transfer toward the contact line. We showed that a higher substrate temperature enhanced the frequency of coffee ring formation.
The donut rings formed at room temperature were similar to the porphyrin-induced rings. No detectable fluorescence was observed from the insides of the several of the porphyrin rings. These results suggested that the centers of the rings were clear, indicating that the mechanism by which the toroid rings formed differed from the coffee stain mechanism. A pinhole mechanism is proposed, in which the formation of holes in the thinning liquid films is proposed to account for the ring formation.
The shape differences were attributed to the fact that the solvent evaporation rate controlled the self-assembly process that yielded the coffee stain and pinhole structures. The solvent evaporation rate may determine the ring formation mechanism.