Molecular motion at surfaces determines the kinetics of processes such as heterogeneous catalysis and thin-film growth, with the diffusivity being controlled by excitation across a translational barrier. Lineshape broadening upon inelastic scattering from surfaces as illustrated in Figure 1a can be used to determine the characteristics of molecular diffusion on pico- to nanosecond timescales with low-energy neutral beams being the gentlest probing technique . Using this technique‚ we have studied the diffusion of simple molecules on 2D materials [1,2] but only recently has the approach  been extended to larger and more complex molecules [3,4]

Specifically, using neutron spectroscopy, we follow the nanoscopic motion of triphenylphosphine (P(C₆H₅)₃ or PPh₃ as illustrated in Figure 1b on graphite. This "nanoscopic moonlander" follows a motion similar to that of a molecular motor: As illustrated in the video, PPh₃ rolls over the surface with a fast motion of its "legs" and  and a slower lateral motion.

Beyond the naïve view

In the traditional view of atomistic surface diffusion, motion is realised by a random walk of the molecule which needs to cross a barrier between two neighbouring potential wells. The molecule, often described as a point-like particle, moves by jumping from one favourable adsorption site to one of the nearest adjacent sites  as illustrated in the inset on the right-hand side of Figure 1b, following the energetically most favourable route across the potential energy surface (PES). However, the current study shows that this simple picture of a point-like particle is now longer applicable to large molecules.

When considering larger and more complex molecules such as PPh₃, the molecular degrees of freedom contribute to the motion. As illustrated in Figure 1b, PPh₃ exhibits a pyramidal tripod-like structure, with the central P-atom on top, surrounded by 3 hexagonal phenyl groups. The molecule bonds to graphite with its phenyl groups pointing towards the surface in analogy to the "legs" of a nanoscopic moonlander.

The importance of internal molecular motion

As can be seen from the video below,  PPh₃ moves over the surface with an almost negligible activation energy for rotations and motion of the phenyl groups and a comparably small activation energy for translation. While internal molecular motions dominate at low temperatures the molecule follows an additional translational jump-motion across the surface at higher temperatures [3].

Figure 2: Video illustrating the motion of a single triphenylphosphine molecule over graphite in a top view, as extracted from a molecular dynamics simulation at a temperature of 300 K.

 The unique behaviour of PPh₃ is due to its three-point binding with the surface, which increases the adsorption energy compared to "flat" molecules of similar size. However, the effective diffusion barrier remains small as its "legs" in form of the phenyl group make it easier to cross the diffusion barrier [3].  

Measurements across the entire temperature range

It illustrates that the molecular degrees of freedom in larger molecules are intimately connected with its diffusivity [3,4]. The latter is not only true for PPh₃ with its tripod-like structure but also for large complex molecules with a planar structure such as Cobaltphtalocyanine (CoPc, as illustrated in Figure 1a  where molecular rotations mean that the effective diffusion barrier becomes smaller at higher temperatures [4]. Hence,  only upon exploring the full potential energy landscape  benchmark data can be provided as we seek to develop models that predict the rates of chemical reactions. Both studies further highlight the importance of considering a wide temperature range to capture the complete dynamics of molecular motion on surfaces - as only at high and also technologically relevant temperatures the full complexity of the molecular motion emerges [3,4].

[1] A. Tamtögl, E. Bahn, M. Sacchi, J. Zhu, D. J. Ward, A. P. Jardine, S. J. Jenkins, P. Fouquet, J. Ellis and W. Allison, Nat. commun., 12, 3120 (2021).
[2] A. Tamtögl, M. Sacchi, I. Calvo-Almazán, M. Zbiri, M. M. Koza, W. E. Ernst, P. Fouquet, Carbon 126, 23 (2018).
[3] Tamtögl, M. Sacchi, V. Schwab, M. M. Koza, P. Fouquet. Comm. Chem., 7, 78 (2024)
[4] Sabik, J. Ellis, H. Hedgeland, D.J. Ward, A.P. Jardine, W. Allison, G. Antczak, .A. P. Jardine, W. Allison, G. Antczak, A. Tamtögl, Front. in Chem. 12, 1355350 (2024).