Self-Assembly Dynamics and Stability through Concentration Control at the Solution/HOPG Interface

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 1phenyloctane 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.