Spontaneous Formation of Nanopatterns in Velocity-Dependent Dip-Coated Organic Films: From Dragonflies to Stripes

• DOI: 10.1021/nn5014534
• Publication Date: 2014-09-04T21:47:44Z
• Authors: 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
• Keywords: Dewetting; Dip-coating; Mesoscopic physics

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solvent's meniscus remains at bulk's entrainment (Landau-Levich-Deryaguin) is partially dragged substrate, covering withdrawn homogeneous film.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