Complex Surface Diffusion Mechanisms of Cobalt Phthalocyanine Molecules on Ag(100)

Abstract

We used time-lapsed scanning tunneling microscopy between 43 and 50 K and density functional theory (DFT) to explore the basic surface diffusion steps of cobalt phthalocyanine (CoPc) molecules on the Ag(100) surface. We show that the CoPc molecules translate and rotate on the surface in the same temperature range. Both processes are associated with similar activation energies; however, the translation is more frequently observed. Our DFT calculations provide the activation energies for the translation of the CoPc molecule between the nearest hollow sites and the rotation at both the hollow and the bridge sites. The activation energies are only consistent with the experimental findings, if the surface diffusion mechanism involves a combined translational and rotational molecular motion. Additionally, two channels of motion are identified: the first provides only a channel for translation, while the second provides a channel for both the translation and the rotation. The existence of the two channels explains a higher rate for the translation determined in experiment.

Adsorption and Reaction of PbPc on Hydrogenated Epitaxial Graphene

“…For instance, for a H coverage of Θ H = 3.3 × 10 –3 ML, τ ≈ 100 s is found; i.e., the diffusivity should be at maximum 10 –15 cm 2 /s if we assume a similar PbPc density and that each PbPc can release one H atom. However, such diffusivity values for phthalocyanine molecules are even in a worst case scenario by orders of magnitude too low. , For instance, for CuPc on Ag(100) diffusivity values of 10 –3 –10 –10 cm 2 /s are reported if extrapolated to 300 K. , We will see below that the reaction in fact can take place spontaneously if the PbPc is in the vicinity of the H atom; therefore, the PbPc gas density seems to be significantly lower and supports our coverage calibration from above by using the SEM contrast. Qualitatively, the decay for the higher initial H coverage is faster; i.e., single PbPc molecules can release more than one H center.…”

Non-Brownian Interfacial Diffusion: Flying, Hopping, and Crawling

“…In the absence of knowledge to the contrary, surface diffusion has traditionally been described in an analogous way, i.e., as two-dimensional (2D) Brownian motion. This is a plausible scenario for strongly bound adsorbates on metal surfaces in contact with gases or in a vacuum, , which move from one well-defined surface site to an equivalent adjacent site without desorbing, executing 2D random walks, where individual steps are activated processes. For sufficiently long trajectories this motion is Fickian and Gaussian.…”

Mass Transport in Surface Diffusion of van der Waals Bonded Systems: Boosted by Rotations?

“…For example, in the diffusion of pyrrole (C 4 H 4 NH), zero-point motion in internal coordinates of the molecule gives rise to enhanced diffusion barriers . In larger molecules still, studies using scanning probe microscopy , and helium spin–echo spectroscopy have shown that there is an intimate connection between rotational transitions of a molecule and its translational motion. Rotation affects translation through coupling where quasi-static rotation steers the molecule toward the lowest barrier of the energy landscape, or where the barrier is reached by a series of frustrated rotations resulting in a favorable geometry on approach to the transition state .…”