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

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

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.

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

• 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.

• 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.

• 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)

2 µm

Optimized TiO2 Infiltration

433 nm TiO2 inverse opal

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

growth.

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

downward.

• 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.

• 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).

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

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

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.

Thank You