Colloidal Nanoparticle-Layer Formation Through Dip-Coating: Effect of Solvents and Substrate Withdrawing Speed

Abstract

The effect of solvents of colloidal solution and substrate withdrawing speed during dip-coating on the nanoparticle-layer formation of sterically stabilized iron oxide ͑␥-Fe 2 O 3 ͒ nanoparticles on Si substrates is investigated. From the solutions with 10 13 -10 14 mL −1 particle concentrations dispersed in hexane, octane, decane, and tetradecane, the multilayer is formed from hexane while the uniform monolayer is formed from octane, decane, and tetradecane. Because the particle layer is formed by direct adsorption and convective flux of particles from solution to substrate, the multilayer formation from hexane is due to the highest interaction energy between particle and substrate and the highest convective flux to the meniscus region of the substrate caused by the highest evaporation rate. Lowering the substrate withdrawing speed increases the particle coverage from ϳ60 to ϳ80% at 0.001 mm/s as locally forming the multilayer regions, which is explained by increased convective flux at reduced speed. These results verify that the colloidal particle delivery to the substrate is determined by factors such as particle-substrate interactions, substrate withdrawing speed, and the evaporation rate of solvent.The chemically synthesized colloidal nanoparticles, having a highly controllable size and shape with excellent uniformity and having the possibility to form two-and three-dimensional ordered self-assembly upon drying solvent by the van der Waals interaction between particles, have been highlighted as one of the most promising building blocks for nanodevice applications with their unique electrical, optical, and magnetic properties. 1-6 In particular, the formation of an ordered, self-assembled layer on the solid substrate is highly required to fabricate solid-state devices such as electroluminescence devices, 7,8 sensors, 9 photodetectors, 10 photonic crystals, 11 electrical memory devices, 12-14 and magnetic storage devices. 15,16 Among the various approaches to form the ordered layer, such as the Langmuir-Blodgett method, 17-19 spin-coating, 7,20 and dipcoating, 21-27 the dip-coating process has been known to readily form either a self-limited monolayer, thanks to the stabilization of particle by surfactant, 27 or a multilayer by alternatively adsorbing glue layers and particles. 15,21 The particle-layer formation during dipcoating has been explained to be driven either by the "convective assembly" in meniscus 22-24 or by the "direct adsorption" from the solution. 21,26,27 In "convective assembly," the particles with a size from several tens of nanometers to micrometers move toward liquid meniscus driven by incoming solution flux resulting from the solvent evaporation and eventually pinned at the edge of meniscus. As the substrate is pulled out with a controlled withdrawing speed, the particles at the meniscus condense to form an ordered layer by lateral capillary force. 28,29 Because the convective assembly is kinetically controlled, the parameters such as evaporation rate of solvent, substrate withdrawing spee...

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