Surface diffusion of aromatics through van der Waals landscapes and beyond
1London Centre for Nanotechnology, University College, London, WC1H 0AH, UK
2Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge, CB3 0HE, UK
3Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
4Institute of Experimental Physics, University of Wrocław, 50-204 Wrocław, Poland
5Shulich Faculty of Chemistry, Technion, Haifa 32000, Israel
6Institut Laue-Langevin, CS 20156, 38042 Grenoble Cedex 9, France
7Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, New Jersey 08854, USA
8Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK
9Department of Chemistry, University of Reading, Whiteknights, RG6 6AD, UK
Beginning with the simple aromatic system of benzene molecules adsorbed on a graphite surface , we utilise a combination of helium spin-echo spectroscopy (HeSE) and density functional theory (DFT) to explore the surface motion of a number of cyclic molecules with conjugated bonding [2-4]. We establish accurately the surface energy landscapes that could template the molecules' self-assembly and give insight into the dominant modes of motion along the diffusion pathways.
The weakly physisorbed system of benzene/Cu(001) shows bonding dominated by the van der Waals interactions that have previously been difficult to estimate with density functional calculations. Here we couple experimental surface dynamical data with molecular dynamics simulations to allow comparison with van der Waals DFT. Additionally, we gain an increased understanding of the effect of the molecular modes of motion on the density of states at the transition state and subsequent molecular friction and surface dynamics.
As we develop our understanding of aromatic systems, of particular interest are the series of five-membered rings: cyclopentadienyl (C5H5), pyrrole and thiophene where, by substituting one of the ring atoms, we influence charge transfer  and the effect of the contribution of the molecules' vibrational states to the resultant motion . The dynamic friction in these systems is discussed in terms of the rate of energy transfer between the adsorbed molecules and the substrate and we consider the mechanisms associated with the molecular degrees of freedom that could lead to the high values seen in experiment .
Our most recent measurements extend to aromatic polycyclic and macrocyclic compounds. These larger molecules raise new questions for the refinement of the established models of diffusion familiar within the surface dynamics and neutron scattering communities.
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