Microscopic friction and diffusion of surface adsorbed molecules measured in real time by neutron scattering

In general our knowledge about friction and diffusion mechanisms on microscopic length scales is still very vague. Nevertheless it has been shown in recent years that microscopic processes determine the macroscopic friction behaviour between surfaces. If we can gain more detailed insight into the microscopic processes, we would be able to find improved solutions for a multitude of scientific and technical questions, such as reduction of friction in bearings, reduction of wear, better control and understanding of liquids in confinements, or we could even dream up motorisation solutions for nano-motors. We have recently started a research programme on the diffusion of carbon based model systems, which is already changing our picture of diffusion at the nanometre scale [1,2]. An adequate description of the diffusion of adsorbed molecules requires experimental and theoretical techniques that, one the one hand, cover the time and spatial dimensions of molecular diffusion, and which, one the other hand, are able to differentiate between different diffusion processes, such as rotations, translations of vibrations. Scanning probe microscopy has seen an impressive development in recent years and was able to deliver friction measurements on molecular length scales [3]. Scanning techniques are in general, however, limited to low temperature measurements, due to their limited frame repetition time. Neutron spectroscopy can deliver the complimentary information needed at temperatures of technological relevance, covering time scales of 10^-12–10^-6 s, on molecular length scales [4]. Substantial improvements in spectrometer performance allow us to investigate diffusion of molecules on surfaces in intricate detail. The interpretation is underpinned by extensive molecular dynamics (MD) simulations, which can now be performed on rather small computing clusters - even for large systems [1,2].

[1] H. Hedgeland, P. Fouquet et al., Nature Phys. 5, 561 (2009).
[2] P. Fouquet et al., Carbon 47, 2627 (2009).
[3] M. Dienwiebel et al., Phys. Rev. Lett. 92, 126101 (2004).
[4] P. Fouquet et al., Z. Phys. Chem. 224, 61 (2010).