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Issue 36, 2016
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Characterization of step-edge barrier crossing of para-sexiphenyl on the ZnO (10[1 with combining macron]0) surface

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Abstract

Mass transport processes of conjugated organic molecules (COMs) on inorganic surfaces are essential elements in thin film deposition for hybrid optoelectronic devices. Defects and in particular surface step-edges dictate the molecular nucleation and growth morphology, which itself determine many physical properties of the resulting hybrid interface. Here, we explore the detailed molecular kinetics and transport rates of a single physisorbed para-sexiphenyl (p-6P) molecule crossing a step-edge (a “hetero-Ehrlich–Schwoebel barrier”) on the inorganic ZnO (10[1 with combining macron]0) surface by a combination of all-atom molecular dynamics simulations and passage time theory. We determine temperature- and charge-dependent (free) energy landscapes, position-dependent diffusion coefficients, and ultimately the mean first passage time over the step-edges. We find two completely different step-edge crossing mechanisms, the occurrence and rates of which simultaneously depend on both electrostatic and thermal molecule-surface coupling. In weakly coupled systems, the molecule crosses the step relatively quickly (in nanoseconds) by log-roll mechanisms while for strongly coupled systems, it crosses relatively slowly (in microseconds) in a strictly perpendicular fashion. In the latter process, “internal friction” from intramolecular bending and torsional degrees of freedom contribute a significant corrugation to the overall crossing barrier. Furthermore, we show that crossing pathways can also change qualitatively with step-edge height. The great complexity in hetero-barrier crossing of COMs (in contrast to simple atoms) revealed in this study has implications on the interpretation and possible control of nucleation and growth mechanisms at surface defects in hybrid systems.

Graphical abstract: Characterization of step-edge barrier crossing of para-sexiphenyl on the ZnO (10 [[1 with combining macron]] 0) surface

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Publication details

The article was received on 28 Jul 2016, accepted on 10 Aug 2016 and first published on 12 Aug 2016


Article type: Paper
DOI: 10.1039/C6CP05251G
Citation: Phys. Chem. Chem. Phys., 2016,18, 25329-25341
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    Characterization of step-edge barrier crossing of para-sexiphenyl on the ZnO (10[1 with combining macron]0) surface

    K. Palczynski, P. Herrmann, G. Heimel and J. Dzubiella, Phys. Chem. Chem. Phys., 2016, 18, 25329
    DOI: 10.1039/C6CP05251G

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