Self-Assembly Dynamics and Stability through Concentration Control at the Solution/HOPG Interface
Kirill Gurdumov, Ursula Mazur, K. W. Hipps
Abstract
Understanding the formation kinetics and thermodynamics of self-assembled monolayers (SAMs) provides an insight into the delicate balance of intermolecular forces on the molecular scale. We herein investigate the growth, dynamics, and stability of a model noncovalent self-assembler, Co(II) octaethylporphyrin, at the solution–HOPG interface. Real-time imaging of the nucleation and growth of the self-assembled layer was captured and studied via scanning tunneling microscopy (STM) and further explored using computational methods. A custom STM solution flow cell was designed and implemented to allow for in situ monitoring of self-assembly at very low concentrations and with volatile solvents. Flow studies at low concentration provide an insight into early-stage formation kinetics and structure of the SAMs formed. It was found that the choice of organic solvent plays a dramatic role in the kinetics and structure of the SAM. These results, in turn, provide insight into the balance of the intermolecular forces driving the self-assembly. The role of the solvent was particularly strong in the case of 1,2,4-trichlorobenzene (TCB). Under TCB, a very stable rectangular structure is formed and stabilized by solvent incorporation. A transition to a solvent-free pseudo-hexagonal structure was only observed when the porphyrin was at near-solubility limit concentrations. Only the pseudo-hexagonal structure was observed in the porphyrin adlayer when toluene, decane, and 1-phenyloctane were used as solvents. Mixed solvent competition was tested and gave further insight into the role solvent plays in the thermodynamics and kinetics of self-assembly.