Page 133 - Optofluidics Fundamentals, Devices, and Applications

Optofluidic Colloidal Photonic Crystals 113

very high repulsive strengths assemble into bcc structures at low con-
centrations (in equilibrium). According to recent reports, binary mix-
tures of oppositely charged small and large particles exhibit more
complex crystal structures, which are generated by the complex
action of both attractive and repulsive electrostatic interactions.
Usually, it is very difficult to fabricate uniform colloidal crystals
on a large scale. Cracks, defects, and grain boundaries are formed dur-
ing the crystallization process. The polydispersity of the particle size,
the roughness of the surface, the low stability of suspension, and the
presence of impurities may be partial reasons for this. Moreover, an
inhomogeneous capillary force between the particles during the evap-
oration of the solvent is the main factor leading to the formation of
poor colloidal crystals with cracks or uneven film thicknesses.
However, crystallization in a capillary helps to overcome this problem.
As mentioned earlier, evaporation occurs at one end of the capillary,
and thus nucleation for crystallization also begins at that end due to
the increased concentration. In addition, crystal growth is achieved by
the supply of particles from the bulk through the convective flow
induced by evaporation. In this way, high-quality colloidal crystals
gradually grow inside the capillary. The prepared crystal has a low
crack density. Moreover, the thickness of the crystal can be kept uniform
because the solid wall of the capillary confines the crystallization.
Kim et al. fabricated colloidal crystals using a soft elastomer mold
[15]. Soft lithography involves a family of techniques for replicating
structures using elastomeric stamps and molds. By using the soft-
lithography technique, the researchers could make various soft
replica molds with grooves, patterns, and microchannels. After bond-
ing a soft mold on a substrate, a colloidal suspension was injected
and crystallized by the evaporation of the solvent at one end. In this
case, soft molds with various patterns acted as replica matrices for
the colloidal crystals. A scheme of the evaporation-induced crystalli-
zation presented in Fig. 6-2a and b shows the replicated colloidal
crystal patterns fabricated using the soft molds. In addition, porous
hierarchical structures could be prepared using the same soft-litho-
graphic approach [16,17]. Yang et al. prepared colloidal crystals using
PS particles, and then their interstices were filled with a titania
precursor. Finally, a network of porous inverse opal structures was
created after calcination and removal of the polymeric particles.
Also, Moon et al. and Kamp et al. prepared colloidal crystals in a
circular glass capillary [18,19]. In these experiments, the silica sus-
pension introduced in the capillary was evaporated at one of its ends,
and cylindrical colloidal crystals could be obtained after several
hours. In addition, inverse-opal-type porous crystals could be
prepared by infiltration of the polymer through the interstices and
subsequent particle removal. The colloidal crystals in the cylindrical
capillary always have the (111) plane of the fcc structure at their
surface. Figure 6-2c and d show SEM images of colloidal crystals