Spontaneous formation of nanopatterns in velocity-dependent dip-coated organic films : from dragonflies to stripes

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Autor/in:
Verlag/Körperschaft:
Hamburg University of Technology
Erscheinungsjahr:
2014
Medientyp:
Text
Schlagworte:
  • 2D crystallization
  • atomic force microscopy
  • Marangoni flows
  • n -alkane
  • scanning electron microscopy
  • silicon
  • X-ray diffraction
  • 540: Chemie
Beschreibung:
  • We present an experimental study of the micro- and mesoscopic structure of thin films of medium length n-alkane molecules on the native oxide layer of a silicon surface, prepared by dip-coating in a n-C32H66/n-heptane solution. Electron micrographs reveal two distinct adsorption morphologies depending on the substrate withdrawal speed v. For small v, dragonfly-shaped molecular islands are observed. For a large v, stripes parallel to the withdrawal direction are observed. These have lengths of a few hundred micrometers and a few micrometer lateral separation. For a constant v, the stripes quality and separation increase with the solution concentration. Grazing incidence X-ray diffraction and atomic force microscopy show that both patterns are 4.2 nm thick monolayers of fully extended, surface-normal-aligned alkane molecules. With increasing v, the surface coverage first decreases then increases for v > vcr ∼ 0.15 mm/s. The critical vcr marks a transition between the evaporation regime, where the solvents meniscus remains at the bulks surface, and the entrainment (Landau-Levich-Deryaguin) regime, where the solution is partially dragged by the substrate, covering the withdrawn substrate by a homogeneous film. The dragonflies are single crystals with habits determined by dendritic growth in prominent 2D crystalline directions of randomly seeded nuclei assumed to be quasi-hexagonal. The stripes strong crystalline texture and the well-defined separation are due to an anisotropic 2D crystallization in narrow liquid fingers, which result from a Marangoni flow driven hydrodynamic instability in the evaporating dip-coated films, akin to the tears of wine phenomenology.
Beziehungen:
DOI 10.1021/nn5014534
Quellsystem:
TUHH Open Research

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