This document summarizes various methods for self-assembly of photonic crystals, including opals and inverse opals. It discusses how self-assembly provides an alternative to top-down fabrication for creating 3D periodic structures. Specifically, it describes how sedimentation, centrifugation, and physical confinement can be used to assemble colloidal spheres into crystalline structures. It also introduces methods like vertical deposition and floating assembly that rely on capillary forces and evaporation. The document concludes by presenting examples of using atomic layer deposition of TiO2 to infiltrate opal templates and coat ZnO nanorod arrays, creating novel 2D and 3D photonic crystal structures through self-assembly approaches.

1. Photonic Crystals : By Self Assembly
Presented By:
zaahir salam
NAST 733: Nanophotonics and Biophotonics
Submitted to:
Dr. P. Thangadurai ,Assistant Professor
Centre for Nano Science &Technology
Pondicherry University

2. What’s Self-assembly
 “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
components.”
• Spontaneous ordering of “building blocks” structure through
non-covalent interactions.
e.g: Formation of molecular crystals, Folding of globular proteins

3. 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
photonic crystals.
• 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
well-defined structures.

4. 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
3D-PC Opal
26% air
Inverse Opal
74% air for high dielectric
contrast
ALD

5. • 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
functionality.
• 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
major requirements.

6. • 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
µm.
• Spherical colloids have been prepared from various materials
that include organic polymers and inorganic ceramics.

7. • 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
PBG)

8. 2 µm
Optimized TiO2 Infiltration
433 nm TiO2 inverse opal

9. 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.

11. 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
growth.
spherical colloids with diameters >500 nm

12. • 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.

14. • 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.

15. 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.

16. 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
downward.

17. • 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
700 nm.
• 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.

22. • Self-assembly of particles can not only happen on the solid
substrates, but also on the surface of liquids.
“Floating self-assembly”
• 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).

25. ALD of TiO2 at 100ºC
433 nm opal infiltrated
with TiO2
433 nm TiO2 inverse opal
(111)
• 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.
300 nm
433 nm opal infiltrated
with 20 nm of TiO2
Cross-sections

26. Using ALD of TiO2 to create novel 2D Photonic
Crystals
X. D. Wang, E. Graugnard, J. S. King,
C. J. Summers, and Z. L. Wang

27. 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.

28. Thank You