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

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.