Liquid-air interface self-assembly: A facile method to fabricate long-range nanoparticle monolayers

A liquid-air interfacial assembly approach is a technique to fabricate long-range well-ordered nanostructures in either two or three dimensions. In this work, magnetic nanoparticles were synthesized by co-reduction of iron acetylacetonate (Fe(acac)3) and platinum acetylacetonate (Pt(acac)2) in benzyl ether. A few drops of the as-made nanoparticle suspension with varying concentrations were spread on the surface of diethylene glycol, ethylene glycol, and deionized water. After solvent evaporation, a substrate was placed under the liquid surface and then gently lifted up to collect nanoparticle layers. Although the nanoparticles are irregular in shape, they could well self-assemble on the diethylene glycol surface, forming an extended monolayer with small defects. By contrast, multilayer domains were frequently obtained when deionized water and ethylene glycol were used as the liquid subphases. An appropriate nanoparticle concentration was also a crucial factor in this approach. The lower concentration could not form an extended monolayer while the higher concentration led to multilayer assemblies. The order of the nanoparticle layers also strongly depended on the uniformity of the nanoparticles.