2D diffusion of poly-aromatic hydrocarbons (PAH) on graphite : a novel perspective on the origin of friction in graphitic systems

Orateur : Irene CALVO-ALMAZAN (Cavendish Laboratory, University of Cambridge, J. J. Thompson Avenue, Cambridge, CB3 0HE, UK)

The nature of friction attracts multiple interests, in one hand due to its high economical impact [1], and in the other hand, because it sheds light on the fundamental processes which govern the diffusive behavior of atomic and molecular species at surfaces [2]. In particular, poly-aromatic hydrocarbons (PAH) such as benzene (C6H6), naphthalene (C10H8) or pyrene (C16H10) adsorbed on exfoliated graphite serve as model systems to understand the microscopic origin of friction [2]. In this work, we discuss how high resolution neutron spectroscopy presents a fundamental insight in the study of molecular diffusion and friction dissipative processes on surfaces. Time-of-flight and neutron spin-echo spectroscopy cover the typical time/length window (pico-seconds and sub-nanometer length scale) [2] where atomic diffusion takes place. We have systematically studied the effect of coverage, temperature and molecular size on the diffusive behavior of polyaromatic hydrocarbons (PAH) adsorbed on exfoliated graphite. The comparison of the diffusion of benzene (C6H6) and pyrene (C16H10) reveals how the interaction between molecules contributes to the dissipation of their kinetic energy. We find that the friction parameter is strongly dependent on coverage and molecular size. Finally, we propose a description of the molecule/molecule interaction through a model of rough hard disks which exchange linear and angular momentum during binary collisions [3].

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