Ordered Layered Assemblies of Nanoparticles
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Ordered Layered Assemblies of Nanoparticles Nicholas A. Kotov Introduction The development of advanced materials from inorganic nanoparticles (NPs) is currently one of the most dynamic areas of science. NPs are attracting significant fundamental and commercial interest, with a wide range of applications including the next generation of optics, electronics, and sensors. In optical, electrical, and magnetic devices, NPs will be mostly used in the form of thin films. Currently, such films are typically made by the spincoating, spraying, or sometimes simple painting of nanoparticle–matrix mixtures. Layer-by-layer (LBL) assembly is one of the new methods of thin-film deposition, often realized as sequentially adsorbed (mono)layers of oppositely charged polyelectrolytes.1 LBL assembly has also been successfully applied to thin films of NPs and nanocolloids, that is, dispersed species with only one or two spatial dimensions in the nanoscale regime. A large number of studies on the assembly of semiconductor, metallic, magnetic, insulating, composite, and other materials have been undertaken.2–10 One of the major advantages of LBL assembly is its simplicity: the process requires neither sophisticated hardware nor high-purity components. This simplicity is combined with the high quality of the resulting coatings, whose thicknesses can be controlled at the nanometer level.11 LBL assembly is also quite universal: for virtually any aqueous dispersion of NPs, one can find an LBL pair with a steady film buildup.3 The lateral packing of the NPs in individual adsorption layers can also be controlled by different means, producing densely packed films.3,12,13 LBL coatings are also highly homogeneous, unlike the composite polymer/NP coatings obtained by other techniques,14 where phase separation may occur. Additionally, the universality of LBL affords the combination of NPs with other functional materials often used in LBL deposition such as
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dyes, proteins, and DNA, leading to a palette of multifunctional assemblies.15 In essence, LBL assembly can be a convenient method for processing NPs into thin films, both for advanced materials research and industrial production.
Layer-by-Layer Assembly of Nanoparticles and Nanocolloids LBL assembly is usually realized as the sequential (mono)layer adsorption of positively and negatively charged species, say A and B, by alternately dipping a substrate into solutions containing A and B. One solution is almost always a polyelectrolyte, while LBL partners of polyelectrolytes can be dispersions of NPs, clay sheets, proteins, dyes, DNA, viruses, or other species. Positively charged polyelectrolytes are readily adsorbed to the surfaces of glass, quartz, silica, most metals, and many other materials, due to the natural negative charge of oxides. Rinsing with a solvent between the adsorption steps— commonly water—removes the excess solution and leaves a thin (mono)layer of A (say, a polyelectrolyte) on the surface. Thereafter, the substrate is immersed into a solution/dispersion of oppositely char
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