As the air-liquid interfacial assembly can yield well-ordered nanoparticle layers, this high control over the arrangement also facilitates the formation of more complex structures. For example, a layer-bylayer approach to stack nanoparticle films can result in multilayer films, Figure (f) and (g). Also, multi-layered structures can be obtained with desired structural properties in each layer. Similarly, mixing different nanoparticles in the colloidal phase results in a well-ordered distribution of the nanoparticles in the films. For example, mixing nanoparticles of two different sizes in different ratios leads to the formation of different binary lattices, Figure (h). Again, mixing nanoparticles of different shapes and materials can make more compositional and structural complexities possible. For a quick overview, Table summarizes the setups used for air-liquid interfacial self-assemblies in a selection of representative studies. (a) Ag nanoparticles self-assembled at the air-water interface using a Langmuir-Blodgett trough setup, the movable barrier can compress the film towards higher packing density. Reprinted with permission © 2007, Nature Publishing Group (b) Au nanoparticles self-assembled at a fixed airdiethylene glycol interface © 2020 CC-BY 4.0. (c) The presence of excess ligands at the interface inhibits wrinkling and folding of the nanoparticle film during compression. Reprinted with permission © 2017 Royal Society of Chemistry (i-vii: different stages of compression for different ligand concentrations) (scale bar: 40 µm). (d) Nie et al. showed that nanoparticles dispersed in polar solvents (e.g. ethanol) can also be trapped at the air-water interface by nebulization with electrospreading. Reprinted with permission © 2015, American Chemical Society (e) Network-like assemblies can be obtained by adding myristyl alcohol that forms polygonal domains. Reprinted with permission © 2011 John Wiley & Sons. (f) Multilayer Au nanoparticle film obtained at the air-liquid interface. Reprinted with permission © 2020 Nature Publishing Group (g) Multilayer 300 nm silica nanoparticle film obtained by layer-by-layer deposition of air-liquid self-assembled nanoparticle monolayers © 2016 Optica Publishing Group (h) Self-assembly of a mixture of 15-nm Fe3O4 and 6nm FePt nanocrystals at the air-liquid interface results in binary assemblies. Reprinted with permission © 2010, Nature Publishing Group. Closed packed binary lattice.