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By Nithin Ramadurai

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Contents Introduction Formation of the Super-lattice Types of colloidal crystals Methods of Synthesis Applications

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INTRODUCTION

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What are Colloidal crystals? Self-arranged mono-disperse, negatively charged colloidal particles in periodic crystal lattices. Most prevalent lattices : Face-Centered Cubic & Hexagonal Close Packed Lattice spacing is of the order of the wavelength of light.

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Driving Forces Brownian Motion of colloidal particles. Electrostatic Repulsion between the negatively charged colloidal particles. The Colloidal particles acquire negative surface charges in polar solvents, by a. Dissociation of ionizable groups b. Preferential adsorption of ions from suspension.

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Formation of the Super- lattice

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Self-Assembly process Spontaneous and reversible formation of ordered structures by non-covalent interactions. Ordered structure forms as the system approaches equilibrium, thus reducing its free energy. Thus the self-assembled crystal form is thermodynamically more stable than the dispersed form of the colloidal particles.

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Self-Organization process Involves the formation of 2 D or 3 D arrays onto the substrate. Smaller particles form 2 D arrays. Larger particles aggregate into 3 D arrays due to rapid increase in the Vander Waal forces with size.

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Types of Colloidal Crystals 1. Natural Colloidal Crystals 2. Synthetic Colloidal Crystals

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1) Natural Colloidal Crystals Viruses such as Tobacco Mosaic Virus, Bushy Stunt Virus and Tipula Iridescent Virus. Viruses are mono-disperse in construction. Fig 1. AFM of TMV crystal

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Opals They are fossilized colloidal crystals. Opals are made of silica spheres cemented together. The voids between the spheres is filled with strongly hydrated amorphous silica. The spheres and voids have different refractive indices.

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Fig 2: SEM of precious opal showing silica sphere structure Fig 3: Precious Opal

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Synthetic Colloidal Crystals Colloidal Crystals can also be synthetically prepared. These find applications as electronic and photonic devices. Numerous methods of preparation exist.

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Methods of synthesizing Colloidal Crystals 1. By Electro-deposition on patterned surfaces 2. By employing surface tensile forces and evaporation 3. By using pulses of compressed gas

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1) Colloidal crystallization by Electro deposition on patterned surfaces Aim : To form colloidal crystal of PMMA latex spheres. The negatively charged PMMA latex spheres were synthesized by surfactant-free starve-feed emulsion polymerization. Average diameter of latex spheres was around 580nm.

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The anode is patterned with grooves. The negatively charged PMMA spheres are deposited in the patterned grooves, on application of a potential. Random deposition occurred at low potentials(2.5 V/mm). Increase in potential results in migration of colloidal particles along the electrode surface forming HCP or FCC structures (depending on the groove width). The migration is due to electro-hydrodynamic flow near the surface.

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Fig 5: Surface relief patterns: (a) SEM image of 5 micron wide grooves with a height of 35nm; (b) AFM image of 500nm wide grooves with a height of 150nm designed to provide hexagonal packing for particles with diameter 580nm. 3

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Fig 6. SEM image of two-dimensional arrays of Colloidal crystal with hexagonal packing 3 Fig 7. SEM image of two-dimensional arrays of Colloidal crystal with square packing 3

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2) Preparation of free-standing colloidal crystal film Mono-dispersed silica particles were prepared by hydrolysis of Tetraethyl Orthosilicate (TEOS) in an alcoholic medium in presence of ammonia and water. Driving forces for crystallization: the surface-tensile forces, capillary forces, the phenomena of evaporation and the electrostatic interaction between the silica particles. The process occurs in a half-close environment formed by inverting a large beaker over a smaller one containing the silica suspension.

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Fig 8: Schematic of the formation procedure of free-standing silica colloidal crystal film at a water air interface. 5 (C) (D)

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Fig 9: Digital camera images of free-standing colloidal crystal film at the waterair interface. 5 Fig 10: SEM image of the free standing opal film: (a) the top-view, (b) the cross-sectional image, (c) and (d) are the magnified images of (b). 5

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3) Colloidal crystallization using pulses of compressed gas A variety of feed solutions can be employed in this method. The most commonly used gas is Air. Inert gases can be used as well. Driving force for crystallization: shear produced by the flow and hard-stopping motion caused by pulses of air.

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Fig 11: Schematic diagram of the air-pulse-driven system for the fabrication of uniform colloidal crystals 7

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Fig 12: (A) Photographs of a colloidal crystal formed in the capillary cell taken from different angles for (111) Bragg diffraction. (B) The cell is immersed in water with a prism on top to reduce the light reflection at the cell surface. 7

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The texture of the colloidal crystal obtained is dependent on the pressure of the air pulses. The texture improves with increasing pressure. Fig 13: TOM images of colloidal crystals processed at different air pressures. 7

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Of the three preparation methods discussed, the last one is the most versatile, in which various sample/feed solutions can be used to prepare the respective colloidal crystal. It can also be easily employed for large scale production. The skill of the operator does not affect the quality of the colloidal crystal produced. Good reproducibility of crystals.

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Application of Colloidal Crystals 1. As Electronic and Optical materials 2. As Chemical Sensors 3. As SERS substrates 4. Colloidal crystallization used as a model process of general crystallization

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1) Colloidal crystals as electronic and optical materials a) Quantum dots They are nano-particulate semiconductors which are essentially colloidal crystals. They have properties between those of bulk semiconductors and those of discrete molecules. Quantum dot technology is used in computing (solid- state quantum computation), biological analysis (replacement for organic dyes), photovoltaic cells, light emitting devices (QD-LED), etc.

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b) Colloidal crystals are used in the creation of precisely tunable Fabry-Perot interferometers. c) Photonic crystals Are essentially colloidal crystals. Composed of periodic dielectric nano-structures that affect propagation of electromagnetic waves. Selective propagation of certain wavelengths, based on the photonic band gap (PGB). d) As photovoltaic devices.

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2) Colloidal Crystals as chemical sensors- Environmental application Colloidal crystals are used in the colorimetric determination of pollutants such of Volatile Organic Compounds (VOCs). Fig 14: Color change of the colloidal crystal-based chemical sensor due to the introduction of acetone 9

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3) Colloidal crystal films used as SERS substrates Unique optical properties. Interesting structural properties such as 3 D periodicity and large surface areas, making them desirable as template materials. Gold-coated 3 D ordered colloidal crystal films can be used as SERS substrates.

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4) Colloidal crystallization used as a model for the process of general crystallization Unlike regular crystallization, here, transformations involve much larger time scale and length scale. A confocal microscope can be used to record the process of crystallization. Fig 15: Confocal microscope images of crystallization 2

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Conclusions The self-assembly and self-organization process of the colloidal particles is attributed to the inter-particle forces and externally created fluxes, which is dependant on the method of synthesis. Of the three methods of synthesis of colloidal crystals, the method employing compressed gas pulses is the most versatile.

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Artificially engineered colloidal crystals are widely used as electronic & photonic devices, as chemical sensors for environmental applications, as SERS templates and substrates. The process of colloidal crystallization is used as a model to study the process of general crystallization. Scope for future study could involve developing industrial-based production methods of colloidal crystal films.

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References 1. Paul Hiemenz and Raj Rajagopalan. PRINCIPLES OF COLLOIDS AND SURFACE CHEMISTRY- 3 RD EDITION, Chapter 13 pg- 579 & 580. 2. Ahuja, P. & Sharma, P. (2006). COLLOIDAL CRYSTALS AND SUPERLATTICES. PHILICA.COM Article number 68. 3. Lewis, Patrick C., Kumacheva, Eugenia, Allard, Mathieu and Sargent, Edward H.(2005). COLLOIDAL CRYSTALLIZATION ACCOMPLISHED BY ELECTRODEPOSITION ON PATTERNED SUBSTRATES, Journal of Dispersion Science and Technology,26:3,259 265. 4. Nina V. Dziomkina and G. Julius Vancso (2005). COLLOIDAL CRYSTAL ASSEMBLY ON TOPOLOGICALLY PATTERNED TEMPLATES, Soft Matter, 2005, 1, 265279 (The Royal Society of Chemistry 2005). 5. Wenjiang Li, Tao Fu, Sailing He (2006). PREPARATION OF FREE-STANDING SILICA 3D COLLOIDAL CRYSTAL FILM AT WATERAIR INTERFACE, Materials Science and Engineering A 441 (2006) 239244.

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6. Tsutomu Sawada, Toshimitsu Kanai, Akiko Toyotama (2006). COLLOIDAL CRYSTAL AND METHOD AND DEVICE FOR MANUFACTURING COLLOIDAL CRYSTAL GEL. European Patent Application EP 1 647 843 A1. 7. Toshimitsu Kanai, Tsutomu Sawada, Akiko Toyotama and Kenji Kitamura (2005). AIR- PULSE-DRIVEN FABRICATION OF PHOTONIC CRYSTAL FILMS OF COLLOIDS WITH HIGH SPECTRAL QUALITY. Adv. Funct. Mater. 2005, 15, No. 1, January. 8. F. Meseguer (2005). COLLOIDAL CRYSTALS AS PHOTONIC CRYSTALS. Colloids and Surfaces A: Physicochem. Eng. Aspects 270271 (2005) 17. 9. Tatsuro Endo, Yasuko Yanagida, Takeshi Hatsuzawa (2007). COLORIMETRIC DETECTION OF VOLATILE ORGANIC COMPOUNDS USING A COLLOIDAL CRYSTAL-BASED CHEMICAL SENSOR FOR ENVIRONMENTAL APPLICATIONS. Sensors and Actuators B 125 (2007) 589595. 10. Daniel M. Kuncicky, Brian G. Prevo and Orlin D. Velev (2006). Controlled assembly of SERS substrates templated by colloidal crystal films. J. Mater. Chem., 2006, 16, 1207 1211 (The Royal Society of Chemistry 2006).

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