Photonic crystals by self assembly

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Photonic Crystals by Self Assembly

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Photonic crystals by self assembly

  1. 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. 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. 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. 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. 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. 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. 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. 8. 2 µm Optimized TiO2 Infiltration 433 nm TiO2 inverse opal
  9. 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.
  10. 10. 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
  11. 11. • 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.
  12. 12. • 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.
  13. 13. 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.
  14. 14. 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.
  15. 15. • 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.
  16. 16. • 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).
  17. 17. 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
  18. 18. 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
  19. 19. 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.
  20. 20. Thank You

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