More and square superlattices (Figure 2.6c).83, 95 For

More precise orientational control was investigated in the silver octahedra system. Silver octahedra can generate three
distinct types of 2D assemblies, which are hexagonal close-packed (HCP), open
hexagonal (OH), and square superlattices (Figure 2.6c).83, 95 For
Ag octahedra assembly, nanoscale surface chemistry is used to direct the
assembly. By systematically tuning the surface wettability of the Ag
nanoparticle with different hydrophobic/hydrophilic alkylthiols, a continuous assembly structural evolution at the oil-water interface was observed. Thoroughly investigation
on the effects of surface hydrophobicity proved that the particle-water and
particle-oil interactions directed the assembly process. With increasing
hydrophobicity of the alkylthiols, the
assembly structure evolution from HCP arrangement to OH arrays, a mixture of
planar and finally to standing octahedra clusters. This work successfully demonstrated
the concept of “one anisotropic particle, multiple superlattices”. The surface
chemistry control method was further extended to assembly Ag nanocubes into
three different assemblies.96 By functionalizing the Ag nanocubes with a binary mixture of
hydrophobic (thiol-terminated poly (ethylene glycol) (PEG)) and hydrophilic
thiol (hexadecanetiol) molecules, the
geometrical and spatial orientation can be controlled and directed to form three
different arrangements including a square close-packed (SCP) (planar), a linear
(tilting), and a standing configurations (Figure 2.6d). The mechanism of
formation of these superlattice structures
is similar to the reported particle-oil
and particle-water interactions in the earlier paper. In their previous work,
this interaction is directed by directly choosing ligands with different
hydrophobic and hydrophilic properties, however, in this work, this is achieved
by controlling the ratio of hydrophobic molecules in the binary ligands, and
further control the hydrophobicity of the whole particle system. Specifically,
SCP arrangement with a packing density of 100% was observed when thiol molecule
ratio is 0%, while when this ratio increased to 67%, the Ag nanocubes form a linear arrangement, when the ratio further
increased to 95%, a hexagonal configuration with standing nanocubes were
observed. Using the 67% ratio of thiol molecule binary ligands, this group
further demonstrated that the Ag nanocube configuration can be modulated by
controlling carrier solvent polarity without changing the surface functionality.97 By continuously decreasing the solvent polarity, the PEG
conformation can be tuned from swollen to coiled states, which then drive the
Ag nanoparticle to form linear, hexagonal, and square close-packed lattice at
the oil/water interface. It is worth to mention that all these three systems can fabricate nanoparticle assemblies
with wafer scale, the success of these works demonstrate the ability to tune the nanoparticle interfacial behavior at both the single nanoparticle level
and further control their macroscopic assemblies.

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