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

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 selfassembly 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.

Quantifying the Cooperative Process of Molecular Self-Assembly on Surfaces: A Case Study of Isophthalic Acids

“…Scanning probe microscopy and especially scanning tunneling microscopy (STM) offers the ability to perform experiments at the nanoscale in various solution environments, in different atmospheres, with the possibility to change and control temperature, pressure, and solution concentration . Thus, STM has not only become a primary tool for analyzing sub-molecular structural details of the formed self-assembled molecular networks (SAMNs) but also for following the time evolution of these systems, as well as for extracting quantitative thermodynamic parameters for the self-assembly process on surfaces. ,, …”

Influences on the Dynamics and Stability of Self-Assembly: Solvent, Substrate, and Concentration

“…Decane (Dec), 1-phenyloctane (PhO), toluene (Tol), and 1,2,4-trichlorobenzene (TCB) are the solvents considered (Figure ). The results on Au(111) reported here are compared to previous work on HOPG . A dense pseudohexagonal SAM structure is formed when solvents PhO and Dec are used.…”

“…14 Scanning tunneling microscopy (STM) is an ideal tool for studying SAMs because it provides submolecular resolution and can be used in a wide range of environments. STM can be performed on any conducting surface in UHV, 17 gaseous atmosphere, 18 and conducting 19 or nonconducting 20 solutions. Many of the STM studies of noncovalent self-assembly have focused on systems with tectons containing long alkyl chains or acidic moieties to enhance SAM stability and as a design parameter.…”

“…The results on Au (111) reported here are compared to previous work on HOPG. 20 A dense pseudohexagonal SAM structure is formed when solvents PhO and Dec are used. In the case of TCB and Tol, a combination of factors to be identified later results in the formation of solvent-incorporated pseudopolymorphs.…”

“…The lattice parameters of the observed adlayers under the various solvents on HOPG 20 are also presented in Table 1. Note that both HEX and REC structures on Au(111) contain two CoOEP molecules per unit cell, while on HOPG the HEX contains two and the REC just one CoOEP molecule per unit cell.…”