Biomedical Engineering Reference
In-Depth Information
defects within colloidal crystals is another prerequisite for some practical appli-
cations [ 118 ]. Linear defects, for instance, could be used as photonic waveguides
and point defects as microcavities [ 105 ]. However, this cannot be achieved through
conventional self-assembly methods alone, as the intentionally added defects will
substantially frustrate the crystal growth and locally induce disordering in colloidal
crystals [ 119 ]. Experimentally, a key challenging of controlling colloidal crystals
is lack of reliable methods to control colloidal crystallization of large and perfect
single crystals with predefined orientations, artificial defects, and patterns over fast
time scales. Obviously, the electrically controlled 2D colloidal crystallization is one
of the most promising technologies to acquire large and perfect 2D crystals based
on the principles outlined in Sect. 7.3.5 .
7.4.2
Fabrication of 3D Photonic Crystals
The technologies adopted to fabricate the 2D colloidal crystals can be extended
to fabricate 3D colloidal crystals with some modifications. Several other methods
have also been widely employed to assemble highly ordered 3D crystals with
large domain size, including sedimentation, repulsive electrostatic interactions, and
physical confinement. Among these, sedimentation in a gravitational field seems
to be the simplest approach for building 3D colloidal crystals [ 120 ]. A number of
parameters must be carefully controlled to grow colloidal crystals of high quality.
These parameters include the size, uniformity, and density of the colloids, as well
as the rate of sedimentation. The main disadvantages of this method are the poor
control over the structure and the thickness of the crystalline arrays, the long
preparation time, and the polycrystalline nature of the products. Highly charged
colloidal particles suspended in a solution can spontaneously self-organize into
ordered structures, driven by the minimization of electrostatic repulsive interactions
[ 121 - 125 ]. The colloidal crystals prepared using this method are typically non-
close-packed, because the repulsive electrostatic interactions keep the particles away
from each other. This method has very strict requirements regarding the experimen-
tal conditions such as the surface charge density, the colloidal concentration, and
the ionic strength. By leaving the colloidal suspension to a physical confinement,
it would self-assemble into long-range-ordered crystalline structures [ 126 - 128 ].
Colloidal crystals with domain sizes of square centimeters could be fabricated by
using a specially designed packing cell [ 129 - 131 ]. This method is relatively fast,
and it also provides tight control over the structures and the thickness of the 3D
colloidal crystals.
Search WWH ::




Custom Search