LB method is generic and can be applied

LB method is generic and can be
applied to both isotropic and anisotropic plasmonic building blocks, the
structure of the final nanoparticle array depends on the particle shape. For
example, spherical nanoparticles normally formed close-packed arrays, and the
interparticle separation distance can be tailored in a controllable manner.43, 66-70 For
anisotropic nanoparticles, each nanoparticle shape gave rise to a
characteristic packing type.  Yang et
al. reported that truncated Ag cubes showed face-to-face square like arrangement, while cuboctahedra Ag nanoparticle adopted a rhombohedral unit cell, Ag
octahedral tended to assemble in a hexagonal lattice by using LB method.27 Further tuning the surface
compression pressures resulted in a
reversible 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 surface
pressure, respectively (Figure 2.4c-e). The robust and scalable LB assembly
method can be also used to fabricate large scale of face-to-face ordered-packed
Ag nanocube arrays, in which the particle spacing can be tuned from around two
times of particle diameter to 2-3 nm (Figure 2.4f-g).71 Other anisotropic nanoparticles
such as Au nanoprism were assembled into
large-scale monolayers using LB method, however, the ordering of the obtained
assemblies need to be further improved.72, 73

2.2.5 Drying mediated air-water interfacial
self-assembly

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Self-assembly on air-water interfacial provides a flat and soft substrate for the
formation of monolayers without “coffee ring’ that normally found on substrate
based evaporation. However, without additional compression, it is hard to
obtain close-packed monolayers over the macroscopic
area without microscopic defects or voids. An effective way to solving
this problem is assembling monolayers on
a water surface that has a slight upward convex curvature, as reported by
Andres and co-workers.74 With
this structure, the nanoparticles nucleated at the center of the apparatus and
grew outward to macroscopic size. The obtained monolayer was found to be
uniform over centimetre-scale with
hexagonal close-packed structure. Such films can be transferred and patterned
on a solid substrate using a PDMS pad.75
This convex water surface was further used to fabricate 2D assemblies that stretched
across micrometer scale holes by a drying mediated air-water interfacial
approach. This method provides a facile way towards large-scale, free-standing
and close-packed 2D plasmonic assemblies. Normally, the fabrication involved three
steps: 1) Capping the plasmonic nanoparticles with thiol grouped molecule; 2) Dropping
the nanoparticle solution on top of a convex water that sits on a solid substrate with holes; 3) After the water fully
evaporated, the free-standing nanoparticle monolayer draped itself over the holey substrate (Figure 2.5a). Jeager and
co-workers reported a free-standing Au nanoparticle membrane that with single
particle thickness and flat and smooth surface by using this method (Figure
2.5b).36
They found that DDT ligands not only prevent the particle from aggregation, but also provide tensile strength to make the
membrane stable on the holey silicon
nitride substrate without cracks or collapse. This method is robust and generic
so it can be further adapted for other
nanoparticles with different core materials, nanoparticles size, or even ligand
type.76
Peng et al. modified this method and fabricated a free-standing
plasmonic film on a naked TEM grid with micrometre-scale
holes.77
The obtained film is robust and can be transferred from micro-grids to solid
substrates with under a gas flow, while
maintaining the local structure (Figure 2.5c).

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