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.
Scanning tunneling microscopy (STM) was used to observe and quantify single-molecule diffusion at the solution/solid interface and at the argon/solid interface. This work investigates the influence of the temperature, solvent, and STM tip on isolated molecular surface diffusion through analysis of the molecular trajectories in sequential STM images. The surface diffusion of Y[C6S-Pc]2 in phenyloctane was found to be thermally activated with almost no motion observed at 5 °C, whereas, above 30 °C molecular motion and/or adsorption/desorption are so rapid that it becomes difficult to track single molecules. The surface diffusion of molecules also depended on solvents; solvents with greater dipole moments (and presumably greater interaction with Au(111)) reduced diffusivity, while the absence of a solvent (i.e., argon/solid interface) increased diffusivity. At room temperature, the influence of the STM tip was quantified by varying the sample bias voltage, with the diffusion coefficient varying between 0.6 × 10–17 and 16 × 10–17 cm2/s. This is the first quantitative study of single-molecule (as opposed to vacancy) diffusion at the solution/solid interface. An important implication of this study is that even in the case of very strong adsorbate–substrate interactions, the STM tip can significantly mobilize surface molecules and thereby enhance the formation of self-assembled films. Moreover, because the tip-induced displacements are not unidirectional, one cannot diagnose tip-induced motion by analyzing the displacements at one set-point and scan rate. Particular care must be taken in any STM-based studies of self-assembly kinetics at the solution–solid interface.
The formation dynamics and stability of CoOEP at the solution/Au(111) interface are captured in situ using scanning tunneling microscopy (STM) in a dynamic solution flow cell at room temperature. The intermediate steps of self-assembly of CoOEP into an ordered monolayer were captured, and fractional coverage as a function of time was measured to extract characteristic parameters of the self-assembly process. Adlayer structure and formation under various solvents are compared to previous studies conducted on HOPG. The choice of substrate is found to have a dramatic influence on adlayer structure and stability. It was found that the CoOEP adlayer assembly on HOPG is an equilibrium process, and the monolayer can be readily formed within minutes of contact with solution (above a solvent-dependent threshold solution concentration); the dissolution of the formed adlayer is feasible, though the rate of dissolution is solvent-dependent. The assembly of an adlayer on Au(111) is kinetically driven, monolayer formation occurs within minutes, and dissolution is very slowonly minimal island dissolution was achieved after hours of pure solvent flow. Solvent incorporation of 1,2,4-trichlorobenzene (TCB) was observed to form a CoOEP pseudorectangular adlayer structure (REC) on both HOPG and Au(111), though a solvent-free pseudohexagonal structure (HEX) occurred at much higher concentrations of CoOEP on HOPG than on Au(111). This is likely due to the fact that CoOEP binds more strongly to Au(111) than HOPG, which promotes the REC to HEX transition on Au(111) at lower concentrations. Solvent incorporation of toluene (Tol) into a CoOEP adlayer on Au(111) was observed, but it did not incorporate into the adlayer on HOPG. There is a significant increase in the Arrhenius desorption rate factor (∼350) of toluene on HOPG relative to Au that is likely a driving factor for Tol coadsorption on Au. A very short-lived decane incorporated adlayer was also observed. The transformation on Au(111) from REC to HEX structure under 1 μM CoOEP in Tol occurred within ∼10 min, while under a solution of 470 μM CoOEP in TCB the transformation required ∼102 min. This variance is primarily due to the relative residence times of the solvent molecule on the Au(111) surface, where Tol has an estimated desorption rate 500 times greater than TCB. The unit cells of the CoOEP adlayer are also substrate-dependent. The commensurate TCB-incorporated REC structure on Au(111) contains two CoOEP but only one CoOEP on HOPG. Thus, the adlayer formation of CoOEP on Au(111) was more significantly affected by solvent than for adsorption on HOPG.
Self-Assembling systems that rely on non-covalent forces to drive ordering are the subject of intense study. Of particular interest are redox active systems like porphyrins and phthalocyanines that can generate self-assembled monolayers (SAMs) at the solution-solid interface. These offer the potential of creating highly ordered films from the simple process of solution dipping. In recent years, it has become clear that kinetics plays a significant role in determining the structure of these films. However, very little is known about the rates and mechanisms of the formation and stability of these interesting surface layers. In this talk we will present new data and models for the adsorption and desorption kinetics of non-covalent SAMs on gold surfaces. As might be suspected, the adsorption and desorption rates do not follow simple Langmuir behavior. Also, the potential for multilayer growth must be considered.
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