Scanning Tunneling Microscopy Reveals Surface Diffusion of Single Double-Decker Phthalocyanine Molecules at the Solution/Solid Interface

Abstract

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.

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

“…It should be remembered that C * is an equilibrium parameter, while the rates of formation and dissolution are not. The kinetics of adsorption, SAM formation, and SAM dissolution will depend strongly upon the adsorption energy of the solvent and its surface mobility . The free energy change associated with the formation of an ordered adlayer from molecules in solution was calculated from and Δ G f = – RT ln ( K eq ).…”

“…2 Further, the importance of solvent desorption in the initial tecton adsorption and the subsequent self-assembled monolayer (SAM) formation has recently been quantitatively demonstrated. 3 Solvent influence and control in self-assembly have been studied in various systems. 4−8 Depending on the delicate balance of tecton− solvent, tecton−tecton, tecton−surface, and solvent−substrate interactions, competitive deposition of solvent with adsorbate is possible.…”

“…A model for noncovalent self-assembly at the solution/solid interface has been proposed, in which a solution of tectons (or building blocks) are brought into contact with a solid support and an ordered adlayer spontaneously forms . Further, the importance of solvent desorption in the initial tecton adsorption and the subsequent self-assembled monolayer (SAM) formation has recently been quantitatively demonstrated . Solvent influence and control in self-assembly have been studied in various systems. − Depending on the delicate balance of tecton–solvent, tecton–tecton, tecton–surface, and solvent–substrate interactions, competitive deposition of solvent with adsorbate is possible .…”

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

“…A model for noncovalent self-assembly at the solution/solid interface has been proposed in which a solution of tectons (or building blocks) is brought into contact with a solid support, and an ordered adlayer spontaneously forms . In addition, the importance of solvent desorption in the initial tecton adsorption and the subsequent self-assembled monolayer (SAM) formation has recently been quantitatively demonstrated . Solvent influence and control in self-assembly have been studied in various systems. − Depending on the delicate balance of tecton–solvent, tecton–tecton, solvent–solvent, tecton–substrate, and substrate–solvent interactions, competitive deposition of solvent with adsorbate is possible .…”

Molecular Diffusion and Self-Assembly: Quantifying the Influence of Substrate hcp and fcc Atomic Stacking

“…5 In addition, SPM techniques combined with photoemission spectroscopies can provide detailed structural and chemical characterization of on-surface processes. 6−8 The temporal evolution of on-surface diffusion has been studied via a variety of techniques 9 with Arrhenius-based analysis of sequential SPM "images" providing the temperature dependent rates of on-surface processes from which activation barriers and thermodynamic quantities may be obtained under UHV 10 and liquid 11 conditions. Examples of such quantities include the energy barriers for molecular diffusion 12,13 and rotation.…”