Page 166 - Science at the nanoscale

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Formation and Self-Assembly at the Nanoscale
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biosensing devices, etc. The self-assembly process is one of the
most general strategies available for the formation of regular
nanostructures.
Self-assembly of Nano-sized Components
7.3.3
In principle, components of any size can self-assemble just like
molecules when the thermodynamic conditions are met. A com-
mon example is the precious opal, which is basically a self-
assembly of quartz particles ∼150 to 300 nm in diameter. These
spherical particles adopt a close-packed hexagonal order in crys-
The size distributions and packing order of
talline phase.
these particles determine the colour and quality of the precious
opal. Light waves scattered from the planes of particles produce
interference patterns, thus giving opal its beautiful and iridescent
appearance.
Researchers have long demonstrated that regularly sized sil-
ica particles can form well-ordered self-assembled structures.
Scanning Electron Microscopy (SEM) images show the hexagonal
packing order observed for ∼800 nm silicate spheres (Fig. 7.9(a))
and a reasonably long-range order obtained in large area
(Fig. 7.9(b)). This two-dimensional ordered arrangement is often
prepared by spin-casting or solvent evaporation from a solution
containing the micron-sized silicate spheres.
Capillary interaction is found to be the main driving force for
self-assembly of micron- and nano-sized particles. In the presence
of floating or submerged particles, the originally flat liquid surface
Figure 7.9. Scanning Electron Microscopic (SEM) images showing ch07
close-packed assembly of micron-sized silicate nanoparticles. (from
author’s lab).