Photonic Crystals : By Self Assembly
NAST 733: Nanophotonics and Biophotonics
Dr. P. Thangadurai ,Assistant Professor
Centre for Nano Science &Technology
“Self-assembly is the autonomous organization of components
into patterns or structures without human intervention.”
Includes “processes that involve pre-existing components
(separate or distinct parts of a disordered structure), are
reversible, and can be controlled by proper design of the
• Spontaneous ordering of “building blocks” structure through
e.g: Formation of molecular crystals, Folding of globular proteins
Need of Self Assembly?
• The technical challenges and barriers to extending
conventional microlithography from 2D to 3D patterning have
made it possible to consider alternative approaches to the
fabrication of long-range ordered lattices to be used as
• In addition to the holographic and two-photon techniques
cannot help in overcoming these barriers.
• Self-assembly represents another alternative route that has
been extensively explored to generate 3D periodic lattices with
3D Photonic Crystals: Opals & Inverse Opals
For 3D PC’s: “top-down” approaches are difficult.
“Bottom-up” approach: self-assembly
Most common 3D photonic crystal is the opal. Close-packed silica
spheres in air
Opal is used as a template to create an inverse opal. Close-packed
air spheres in a dielectric material
74% air for high dielectric
• The final structure is usually determined by the characteristics
of the building blocks since the information that defines the
self-assembly process has already been coded in the building
blocks in the form of topology‚ shape‚ or surface
• Final structure is often at a thermodynamic equilibrium state‚
such a process has the tendency to reject defects and thus lead
to the formation of structures having greater order than could
be reached in non-self-assembling systems.
• More importantly‚ the inherently parallel nature of self-
assembly makes it a promising candidate for large-scale
production where low-cost and high throughput represent two
• To fabricate PhCs with a complete band gap we can use
Colloidal Crystals as templates.
• Colloids are usually referred to as small particles with at least
one characteristic dimension in the range of a few nm to one
• Spherical colloids have been prepared from various materials
that include organic polymers and inorganic ceramics.
• Normally a material with a high refractive index is infiltrated
into the voids of CC and then the spheres are removed to
obtain an inverse opal structure. In this step, not only is the
refractive index contrast increased, but also the connectivity
and topology of dielectric medium are improved.
Sintered Opal Infiltrated Opal Inverted Opal
Self Assembly ALD Etch
Infiltration of opal with high index materials
ZnS:Mn n~2.5 @ 425 nm (directional PBG)
TiO2 (rutile) navg~ 3.08 @ 425 nm (omni-directional
Sedimentation and Centrifugation
• Sedimentation (Hunter, 1993) and centrifuge (Holland et al.,
1999; Yan et al., 2000) seem to be the simplest methods to
obtain crystalline arrangement of microbeads.
Sedimentation under Gravity
Although it looks simple, this process involves a coupling of several complex
processes including gravitational settling, translational diffusion and
crystallization. Three parameters, namely the size and density of colloidal spheres
and the rate of sedimentation, should be controlled carefully to allow the crystal
spherical colloids with diameters >500 nm
• Repulsive electrostatic interactions between highly charged
spherical colloids have been widely exploited to organize these
colloids into hexagonal close packing or face-centered cubic
crystals with thicknesses up to hundreds of layers.
• However, careful control over particle sedimentation velocity can allow
one to obtain a single crystalline phase.
• To control the sedimentation velocity, two methods were applied.
One is to use a proper solvent. Although both silica and polymer
spheres can disperse in water, water is sometimes not a suitable solvent for
sedimentation. For small particles, ethanol, which has a lower density and
viscosity than water, has been found to be a good solvent. With large
particles, a mixture of water and ethyl glycol or ethanol is a good choice
(Blanco et al., 2000; Velev et al., 2000; Stachowiak et al., 2005).
The other method is to use extra forces. The surface charge of the
colloidal spheres can respond to a macroscopical electric field, thus the
velocity of the sedimentation can be controlled by using an electric field
parallel to gravity direction (Holgado et al., 1999; Rogach et al., 2000).
• Large quantity of defects and the large possibility of forming a mixture of
colloidal crystal phase reduce its application in fabrication of PhCs.
Self-assembly under physical confinement
In this method, colloidal spheres were assembled into a highly ordered structure in a
specially designed packing cell under continuous sonication. Only under sonication was
each colloidal sphere placed at the lattice site represented as a thermodynamic minimum.
In this method the number of the layers was controllable because it is solely determined
by the distance between the two substrates and the diameter of the spheres.
Vertical deposition method
• In this method, a substrate such as a flat glass or silicon wafer
is placed vertically in a colloidal suspension (Jiang et al.,
1999a; Jiang et al., 1999b).
• The withdrawal of substrate or the evaporation of solvent
causes the meniscus to wipe off the substrate surface vertically
• Under the combinational influence of convection flow and capillary force, colloidal
particles accumulate to and organize at the edge of meniscus.
• This method works well for silica and latex particles of diameter below 500 and
• However, it has two limitations:
first, the long time of evaporation and
second, more crucially, deposition is limited to smaller colloidal spheres that
sediment slower than the solvent evaporates.
• Self-assembly of particles can not only happen on the solid
substrates, but also on the surface of liquids.
• The surface of mercury and gallium were used as substrates to
allow the growth of CCs (Griesebock et al., 2002)
• Polymer microspheres (PS) were found also self-organizing at
the surface of water (Im et al., 2002; Im and Park, 2002; Zeng
et al., 2002; Im et al., 2003). Electrohydrodynamic deposition
method was employed by Trau et al. (1996; 1997) to fabricate
CCs of both silica spheres (0.9 μm in diameter) and PS spheres
(2 μm in diameter).
ALD of TiO2 at 100ºC
433 nm opal infiltrated
433 nm TiO2 inverse opal
• TiO2 infiltration at 100ºC produces very smooth and conformal surface coatings
with rms roughness ~2Å.
• Heat treatment (400C, 2 hrs.) of infiltrated opal converts it to anatase TiO2,
increasing the refractive index from 2.35 to 2.65, with only a 2Å increase in the
rms surface roughness.
433 nm opal infiltrated
with 20 nm of TiO2
Using ALD of TiO2 to create novel 2D Photonic
X. D. Wang, E. Graugnard, J. S. King,
C. J. Summers, and Z. L. Wang
TiO2 Coated ZnO Arrays
Aligned ZnO nano-rods in a hexagonal
matrix on a sapphire substrate.
Aligned ZnO nano-rods coated with 100
nm of TiO2 at 100°C.