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

Tomas P. Corrales, Mengjun Bai, Valeria Del Campo, Pia Homm, Piero Ferrari, Armand Diama, Christian Wagner, Haskell Taub, Klaus Knorr, Moshe Deutsch, Maria Jose Retamal, Ulrich G. Volkmann, Patrick Huber

Research output: Contribution to journalArticlepeer-review

30 Scopus citations

Abstract

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entrainment (Landau-Levich-Deryaguin) regime, where the solution is partially dragged by the substrate, covering the withdrawn substrate by a homogeneous film.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.

Original languageEnglish
Pages (from-to)9954-9963
Number of pages10
JournalACS Nano
Volume8
Issue number10
DOIs
StatePublished - 28 Oct 2014

Bibliographical note

Publisher Copyright:
© 2014 American Chemical Society.

Keywords

  • 2D crystallization
  • Marangoni flows
  • X-ray diffraction
  • atomic force microscopy
  • n -alkane
  • scanning electron microscopy
  • silicon

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