LB method is generic and can beapplied to both isotropic and anisotropic plasmonic building blocks, thestructure of the final nanoparticle array depends on the particle shape. Forexample, spherical nanoparticles normally formed close-packed arrays, and theinterparticle separation distance can be tailored in a controllable manner.
43, 66-70 Foranisotropic nanoparticles, each nanoparticle shape gave rise to acharacteristic packing type. Yang etal. reported that truncated Ag cubes showed face-to-face square like arrangement, while cuboctahedra Ag nanoparticle adopted a rhombohedral unit cell, Agoctahedral tended to assemble in a hexagonal lattice by using LB method.27 Further tuning the surfacecompression pressures resulted in areversible control of a series of phase transition of Ag cuboctahedra. From an ordered hexagonal assemblies to small islands and finally to the crystalline close-packed structure at low, intermediate, and high level of surfacepressure, respectively (Figure 2.4c-e). The robust and scalable LB assemblymethod can be also used to fabricate large scale of face-to-face ordered-packedAg nanocube arrays, in which the particle spacing can be tuned from around twotimes of particle diameter to 2-3 nm (Figure 2.4f-g).
71 Other anisotropic nanoparticlessuch as Au nanoprism were assembled intolarge-scale monolayers using LB method, however, the ordering of the obtainedassemblies need to be further improved.72, 732.2.5 Drying mediated air-water interfacialself-assemblySelf-assembly on air-water interfacial provides a flat and soft substrate for theformation of monolayers without “coffee ring’ that normally found on substratebased evaporation. However, without additional compression, it is hard toobtain close-packed monolayers over the macroscopicarea without microscopic defects or voids. An effective way to solvingthis problem is assembling monolayers ona water surface that has a slight upward convex curvature, as reported byAndres and co-workers.74 Withthis structure, the nanoparticles nucleated at the center of the apparatus andgrew outward to macroscopic size.
The obtained monolayer was found to beuniform over centimetre-scale withhexagonal close-packed structure. Such films can be transferred and patternedon a solid substrate using a PDMS pad.75This convex water surface was further used to fabricate 2D assemblies that stretchedacross micrometer scale holes by a drying mediated air-water interfacialapproach. This method provides a facile way towards large-scale, free-standingand close-packed 2D plasmonic assemblies.
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Normally, the fabrication involved threesteps: 1) Capping the plasmonic nanoparticles with thiol grouped molecule; 2) Droppingthe nanoparticle solution on top of a convex water that sits on a solid substrate with holes; 3) After the water fullyevaporated, the free-standing nanoparticle monolayer draped itself over the holey substrate (Figure 2.5a). Jeager andco-workers reported a free-standing Au nanoparticle membrane that with singleparticle thickness and flat and smooth surface by using this method (Figure2.5b).
36They found that DDT ligands not only prevent the particle from aggregation, but also provide tensile strength to make themembrane stable on the holey siliconnitride substrate without cracks or collapse. This method is robust and genericso it can be further adapted for othernanoparticles with different core materials, nanoparticles size, or even ligandtype.76Peng et al. modified this method and fabricated a free-standingplasmonic film on a naked TEM grid with micrometre-scaleholes.77The obtained film is robust and can be transferred from micro-grids to solidsubstrates with under a gas flow, whilemaintaining the local structure (Figure 2.5c).
