Single-molecular diffusivity and long jumps of large organic molecules: CoPc on Ag(100)

Abstract

Energy dissipation and the transfer rate of adsorbed molecules do not only determine the rates of chemical reactions but are also a key factor that often dictates the growth of organic thin films. Here, we present a study of the surface dynamical motion of cobalt phthalocyanine (CoPc) on Ag(100) in reciprocal space based on the helium spin-echo technique in comparison with previous scanning tunnelling microscopy studies. It is found that the activation energy for lateral diffusion changes from 150 meV at 45–50 K to ≈100 meV at 250–350 K, and that the process goes from exclusively single jumps at low temperatures to predominantly long jumps at high temperatures. We thus illustrate that while the general diffusion mechanism remains similar, upon comparing the diffusion process over widely divergent time scales, indeed different jump distributions and a decrease of the effective diffusion barrier are found. Hence a precise molecular-level understanding of dynamical processes and thin film formation requires following the dynamics over the entire temperature scale relevant to the process. Furthermore, we determine the diffusion coefficient and the atomic-scale friction of CoPc and establish that the molecular motion on Ag(100) corresponds to a low friction scenario as a consequence of the additional molecular degrees of freedom.

Molecular motion of a nanoscopic moonlander via translations and rotations of triphenylphosphine on graphite

“…On the one hand such large diffusivities may be the consequence of an unusual low friction of PAHs on graphitic substrates 47 , 48 . On the other hand, for cobalt phthalocyanine, another planar molecule with about twice the mass compared to PPh 3 , a larger activation energy but a similar diffusivity with 9.6 × 10 −10 m 2 s −1 at 350 K was found on metal surfaces 49 . In the latter case it was noted that the additional molecular degrees such as rotations may in turn increase the diffusivity of large organic molecules.…”

“…In particular, for more complex molecules, we are only now starting to gain a better fundamental understanding of how the molecular degrees of freedom contribute to mass transport and the factors controlling thin film growth and heterogeneous catalysis 49 . While traditionally, the motion of simple molecules is often treated as point-like particles, molecular degrees of freedom can influence diffusion dynamics significantly, and rotations already play a role for simple molecules such as benzene on metal substrates 50 .…”

“…While traditionally, the motion of simple molecules is often treated as point-like particles, molecular degrees of freedom can influence diffusion dynamics significantly, and rotations already play a role for simple molecules such as benzene on metal substrates 50 . In larger molecules, rotations and conformational changes are intimately connected to translational motion guiding the molecule towards the lowest barrier of the energy landscape and thus, internal coordinates can accelerate the crossing of energy barriers 49 . Our study thus highlights the importance of exploring the full potential energy landscape and provides benchmark data as we seek to develop models that predict the rates of chemical reactions, covering dynamics at high temperatures and allowing a precise determination of diffusion coefficients and the pre-exponential factor.…”