SELF-ASSEMBLY DYNAMICS AND ENERGETICS AT THE SOLUTION/SOLID INTERFACE: CONCENTRATION AND SOLVENT CONTROL

Kirill Gurdumov

doi:10.7273/000004363

A comprehensive investigation on the formation dynamics and energetics of adlayer formation at the at the solution/solid interface has been performed using scanning tunneling microscopy (STM) with a custom solution flow cell design that offers new insight into the self-assembly process. Understanding the formation kinetics and thermodynamics of self-assembled monolayers (SAM) provides insight into the delicate balance of intermolecular forces on the molecular scale. We herein investigate the growth, dynamics, and stability of a model non-covalent self-assembler -- Co(II) octaethylporphyrin at the solution/solution interface on the HOPG and Au(111) surfaces. Real-time imaging of the nucleation and growth of the self-assembled layer was captured and studied by in-situ 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 insight into early-stage kinetics and structural formation of a SAM. 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) on both HOPG and Au(111). 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 extremely high concentrations of porphyrin were present in solution. Similarly, in the case of CoOEP adsorbed on Au(111) under toluene, a solvent-incorporated rectangular structure was observed that, like the TCB case, transitioned into a pseudo-hexagonal structure, but this transition occurred at much lower concentrations of porphyrin. Toluene co-adsorption was not observed on HOPG. When deposited from decane, a short-lived pseudo-rectangular structure was observed on Au(111) but not on HOPG. Only the pseudo-hexagonal structure was observed in the porphyrin adlayer when 1-phenyloctane were used as a solvent. On HOPG, mixed solvent competition was tested and gave further insight into the thermodynamic and kinetic roles that solvents play in self-assembly.

SELF-ASSEMBLY DYNAMICS AND ENERGETICS AT THE SOLUTION/SOLID INTERFACE: CONCENTRATION AND SOLVENT CONTROL

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