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CeNS Colloquium

Place: Kleiner Physik-Hörsaal, Geschwister-Scholl-Platz
Date: 12.11.10, Time: 15:30 h

Entropically-stablised Molecular Rhombus Tilings

Prof. Peter Beton
School of Physics & Astronomy, The University of Nottingham


The deposition of the molecule terphenyl tetracarboxylic acid (TPTC) on graphite from a solution in nonanoic acid leads to the formation of a random network which can be mapped onto a rhombus tiling. Images of the molecular networks are acquired using a scanning tunneling microscope and can be analysed using a lifting dimension which transforms the rhombus tiling to a three dimensional interface and the statistics of molecular placement can be determined from an analysis of the roughness of this virtual surface. We find that the TPTC network is consistent with an entropically-stabilised arrangement consistent with equilibrium spatial statistics. We also observe topological defects which can propagate through the molecular network giving rise to local rearrangements of molecules which allows the overall system to transfer between different configurations. However these processes are very slow and the molecular networks may be considered as dynamically-arrested raising important questions related to their growth, since the equilibrium configuration cannot evolve through temporal fluctuations. To address this question we have developed a numerical model for growth based on a lattice gas model and find that direct growth to a structure with equilibrium statistics is possible for parameters which are close to a phase boundary. In addition we show that tiling correlations may be used to distinguish equilibrium and kinetic origins of disorder. At a microscopic level the random tiling arises from a degeneracy between two alternative hydrogen bonding configurations and we have also investigated both theoretically and experimentally the influence on tiling statistics of the breaking of this degeneracy. Experimentally this can be achieved through small changes in molecular structure, a change of solvent or through the co-deposition of an additional molecule which stabilises one of the possible bonding geometries of a TPTC dimer. We also describe the transformation of the random phase to a more ordered phase using the tip of a scanning tunnelling microscope and the re-growth of the random network with modified statistics.

Collaborators: M.O. Blunta, J.M. Russella, A. Stannarda, J.P. Garrahana, M. del Carmen Giminez-Lopezb, X. Linb, M. Schröderb, N.R. Champnessb
a, School of Physics & Astronomy, University of Nottingham, UK.
b, School of Chemistry, University of Nottingham, Nottingham, UK.