期刊
ACS NANO
卷 8, 期 10, 页码 9954-9963出版社
AMER CHEMICAL SOC
DOI: 10.1021/nn5014534
关键词
2D crystallization; Marangoni flows; n-alkane; silicon; atomic force microscopy; scanning electron microscopy; X-ray diffraction
类别
资金
- FONDECYT [1100882, 1141105]
- U.S. National Science Foundation [DMR-0411748, DMR-0705974, DGE-1069091]
- graduate school of the German Research Foundation (DFG) [1276]
- Structure formation and transport in complex systems (Saarbruecken, Germany)
- CONICYT
- Binational German-Chilean Academic Exchange project (German Academic Exchange Service Project) [56206483/CONICYT, PCCI 044]
- U.S. DOE [DE-AC02-06CH11357]
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 similar to 0.15 mm/s. The critical vcr marks a transition between the evaporation regime, where the solvent's meniscus remains at the bulk's 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entrainment (Landau-Levich-Deryaguin) regime, where the solution is partially dragged by the substrate, covering the withdrawn substrate by a homogeneous film.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.
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