Transcript COLLOIDAL cRYSTALS

By
Nithin Ramadurai
Contents
 Introduction
 Formation of the Super-lattice
 Types of colloidal crystals
 Methods of Synthesis
 Applications
INTRODUCTION
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.
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.
b.
Dissociation of ionizable groups
Preferential adsorption of ions from suspension.
Formation of the Superlattice
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.
Self-Organization process
 Involves the formation of 2D or 3D arrays onto the
substrate.
 Smaller particles form 2D arrays.
 Larger particles aggregate into 3D arrays due to rapid
increase in the Vander Waal forces with size.
Types of Colloidal
Crystals
1.
Natural Colloidal Crystals
2. Synthetic Colloidal Crystals
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
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.
Fig 2: SEM of precious opal showing
silica sphere structure
Fig 3: Precious Opal
Synthetic Colloidal Crystals
 Colloidal Crystals can also be synthetically prepared.
 These find applications as electronic and photonic
devices.
 Numerous methods of preparation exist.
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
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.
 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.
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
Fig 6. SEM image of two-dimensional arrays of
Colloidal crystal with hexagonal packing3
Fig 7. SEM image of two-dimensional arrays of
Colloidal crystal with square packing3
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.
(D)
(C)
Fig 8: Schematic of the formation procedure of free-standing silica colloidal crystal film at a water–
air interface. 5
Fig 9: Digital camera images of free-standing
colloidal crystal film at the water–air 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
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.
Fig 11: Schematic diagram of the air-pulse-driven system for the fabrication of uniform colloidal crystals 7
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
 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
 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.
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
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 (solidstate quantum computation), biological analysis
(replacement for organic dyes), photovoltaic cells,
light emitting devices (QD-LED), etc.
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.
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
3) Colloidal crystal films used as
SERS substrates
 Unique optical properties.
 Interesting structural properties such as 3D periodicity
and large surface areas, making them desirable as template
materials.
 Gold-coated 3D ordered colloidal crystal films can be used
as SERS substrates.
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
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.
 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.
References
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PATTERNED SUBSTRATES”, Journal of Dispersion Science and Technology,26:3,259 — 265.
4.
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