This polymer based air-water interfacial assembly is first used to assemble monolayered gold nanospheres (Figure 2.5e), in which the structural features can be adjusted by tunning the gold nanoparticle sizes.78 The assembly process begins with a ligand exchange process, in which citrate-reduced gold nanoparticle was obtained by spun down and redispersed in a mixture of THF and PS-SH. Then, the PS-capped gold nanoparticles were concentrated and finally dispersed in chloroform. The free-standing monolayered plasmonic nanosheets are fabricated by dropping the gold nanoparticle-chloroform solution onto the surface of Milli-Q water in a plastic Petri dish. Since the particles are capped by the hydrophobic ligands, the solution is quickly spread and solidify into a gold reflection on the surface of a water. The plastic Petri dish was covered to slow fluid flows and minimize solvent fluctuations. Finally, the monolayered particle film is generated at the air-water interface due to the rapid chloroform evaporation. The obtained thin particle sheets floating at the water surface can be as large as ?15 ?m in lateral dimensions with a thickness of ?13 nm.
After the successful assembly of nanosheets from gold nanosphere, this method was further modified and extended into a general approach by confining the assembly process on top of a water droplet as shown in Figure 2.5d.6 There are two key factors during the assembling process, which are the high concentration of the nanoparticles and the proper length of ligands. When the highly concentrated PS functionalized nanoparticles was spread onto the water surface, small patches of islands are firstly formed due to the rapid chloroform evaporation. As the slow water evaporation, these small islands are gradually forced to assemble into large-scale nanosheets due to the decreasing surface tension. The final lateral dimension of such kind of plasmonic sheets can reach up to ?3 mm with single particle thickness (Figure 2.5f). Moreover, the packing order of the nanosheets can be adjusted by tuning the length of the ligands, which is the result of the competition between van der Waals attraction forces and the steric hindrance of ligands. As the fabrication process is processed on holey grids, the freestanding nanosheets are directly generated without any need for further substrate removal and are transferable to any arbitrary substrates.
This assembly approach could be customized using various “meta-atoms” elements, such as [email protected] nanocube and nanobrick,6, 79, 80 bilayered Au nanosphere81 and Au rhombic dodecahedral and nanostar82 to form high-quality plasmonic nanosheets (Figure 2.5f-i). Both SEM and TEM characterizations revealed the assembled nanosheets are monolayered with different packing types. For example, the gold spheres, RDs and NStrs exhibited a hexagonally close-packed ordering, while NCs tend to pack side by side, isotropic NBs are aligned horizontally with random packing directions.