Contents Introduction Formation of the Super-lattice Types of
colloidal crystals Methods of Synthesis Applications
Slide 3
INTRODUCTION
Slide 4
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.
Slide 5
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.
Slide 6
Formation of the Super- lattice
Slide 7
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.
Slide 8
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.
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
Slide 11
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.
Slide 12
Fig 2: SEM of precious opal showing silica sphere structure Fig
3: Precious Opal
Slide 13
Synthetic Colloidal Crystals Colloidal Crystals can also be
synthetically prepared. These find applications as electronic and
photonic devices. Numerous methods of preparation exist.
Slide 14
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
Slide 15
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.
Slide 16
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.
Slide 17
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
Slide 18
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
Slide 19
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.
Slide 20
Fig 8: Schematic of the formation procedure of free-standing
silica colloidal crystal film at a water air interface. 5 (C)
(D)
Slide 21
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
Slide 22
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.
Slide 23
Fig 11: Schematic diagram of the air-pulse-driven system for
the fabrication of uniform colloidal crystals 7
Slide 24
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
Slide 25
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
Slide 26
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.
Slide 27
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
Slide 28
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.
Slide 29
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.
Slide 30
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
Slide 31
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.
Slide 32
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
Slide 33
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.
Slide 34
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.
Slide 35
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.
Slide 36
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).