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

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entrainment (Landau-Levich-Deryaguin) regime, where the solution is partially dragged by the substrate, covering the withdrawn substrate by a homogeneous film.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

Subjects

2D crystallization

atomic force microscopy

Marangoni flows

n -alkane

scanning electron microscopy

silicon

X-ray diffraction

DDC Class

540: Chemie

Funding Organisations

U.S. National Science Foundation

Deutsche Forschungsgemeinschaft (DFG)

More Funding Information

Support by FONDECYT Grant No. 1100882 and 1141105, U.S. National Science Foundation Grant Nos. DMR-0411748, DMR-0705974, and DGE-1069091, and graduate school 1276 of the German Research Foundation (DFG), “Structure formation and transport in complex systems” (Saarbruecken, Germany), is gratefully acknowledged, as are scholarships to M.J.R. by CONICYT and to Pa.H., M.J.R., and U.V. by the Binational German–Chilean Academic Exchange project (German Academic Exchange Service Project No. 56206483/CONICYT Project No. PCCI 044). The Advanced Photon Source at Argonne National Laboratory is supported by the U.S. DOE Contract No. DE-AC02-06CH11357.

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