Dresden 2011 – wissenschaftliches Programm
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O: Fachverband Oberflächenphysik
O 40: Metal substrates: Adsorption of organic / bio molecules V
O 40.5: Vortrag
Mittwoch, 16. März 2011, 12:15–12:30, PHY C213
Stabilization by rotational entropy - A general attribute of ’large’ adsorbates? — •Thomas Waldmann1, Harry E. Hoster1,2, and R. Jürgen Behm1 — 1Ulm University, Institute of Surface Chemistry and Catalysis, 89069 Ulm, Germany — 2Current address: Technische Universität München, Centre for Electromobility, 50 Nanyang Drive, Singapore 637553
Large molecules have in general considerable moments of inertia. At sufficiently high temperatures and not too high coverages this can lead to a stabilization of the adlayer by rotational entropy, as shown, e. g., in a recent thermal desorption (TPD) study [1] of the bisterpyridine derivative 2,4’-BTP on graphite (HOPG). In this presentation we used 2,4’-BTP on Ag(111) as a model system, where this behavior was directly observed by scanning tunneling microscopy (STM) at 300 K. Domains of freely rotating 2,4’-BTP molecules are in a dynamic equilibrium with static disordered 2D islands (2,4’-BTP)n (n>1) and individual molecules (n=1) stabilized by lateral (N...H) and vertical (N...Ag) interactions [2]. Evaluating a large number of molecules in STM images, we determined the mean energy difference between the rotating and the non-rotating adsorbates in the 2D islands. The stabilization by rotational entropy is compared with that of other typical small (O2, N2, acetylene) and large (benzene, PTCDA, 3,3’-BTP, HB-DC, MPc) adsorbates at 300 K.
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T. Waldmann et al., ChemPhysChem 11, 1513 (2010).