Colloidal crystal

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A colloidal crystal is an ordered array of colloidal particles and fine grained materials analogous to a standard crystal whose repeating subunits are atoms or molecules. [1] A natural example of this phenomenon can be found in the gem opal, where spheres of silica assume a close-packed locally periodic structure under moderate compression. [2] [3] Bulk properties of a colloidal crystal depend on composition, particle size, packing arrangement, and degree of regularity. Applications include photonics, materials processing, and the study of self-assembly and phase transitions.

Contents

A collection of small 2D colloidal crystals with grain boundaries between them. Spherical glass particles (10 mm diameter) in water. ColloidCrystal 40xBrightField GlassInWater.jpg
A collection of small 2D colloidal crystals with grain boundaries between them. Spherical glass particles (10 μm diameter) in water.
The connectivity of the crystals in the colloidal crystals above. Connections in white indicate that particle has six equally spaced neighbours and therefore forms part of a crystalline domain. ColloidCrystal 40xBrightField GlassInWater Connectivity.png
The connectivity of the crystals in the colloidal crystals above. Connections in white indicate that particle has six equally spaced neighbours and therefore forms part of a crystalline domain.
IUPAC definition

Assembly of colloid particles with a periodic structure that
conforms to symmetries familiar from molecular or atomic crystals.

Note: Colloidal crystals may be formed in a liquid medium or during
drying of particle suspension. [4]

Introduction

A colloidal crystal is a highly ordered array of particles which can be formed over a long range (to about a centimeter). Arrays such as this appear to be analogous to their atomic or molecular counterparts with proper scaling considerations. A good natural example of this phenomenon can be found in precious opal, where brilliant regions of pure spectral color result from close-packed domains of colloidal spheres of amorphous silicon dioxide, SiO2 (see above illustration). The spherical particles precipitate in highly siliceous pools and form highly ordered arrays after years of sedimentation and compression under hydrostatic and gravitational forces. The periodic arrays of spherical particles make similar arrays of interstitial voids, which act as a natural diffraction grating for light waves in photonic crystals, especially when the interstitial spacing is of the same order of magnitude as the incident lightwave. [5] [6]

Origins

The origins of colloidal crystals go back to the mechanical properties of bentonite sols, and the optical properties of Schiller layers in iron oxide sols. The properties are supposed to be due to the ordering of monodisperse inorganic particles. [7] Monodisperse colloids, capable of forming long-range ordered arrays, existing in nature. The discovery by W.M. Stanley of the crystalline forms of the tobacco and tomato viruses provided examples of this. Using X-ray diffraction methods, it was subsequently determined that when concentrated by centrifuging from dilute water suspensions, these virus particles often organized themselves into highly ordered arrays.

Rod-shaped particles in the tobacco mosaic virus could form a two-dimensional triangular lattice, while a body-centered cubic structure was formed from the almost spherical particles in the tomato Bushy Stunt Virus. [8] In 1957, a letter describing the discovery of "A Crystallizable Insect Virus" was published in the journal Nature . [9] Known as the Tipula Iridescent Virus, from both square and triangular arrays occurring on crystal faces, the authors deduced the face-centered cubic close-packing of virus particles. This type of ordered array has also been observed in cell suspensions, where the symmetry is well adapted to the mode of reproduction of the organism. [10] The limited content of genetic material places a restriction on the size of the protein to be coded by it. The use of a large number of the same proteins to build a protective shell is consistent with the limited length of RNA or DNA content. [11] [12]

It has been known for many years that, due to repulsive Coulombic interactions, electrically charged macromolecules in an aqueous environment can exhibit long-range crystal-like correlations with interparticle separation distances often being considerably greater than the individual particle diameter. In all of the cases in nature, the same iridescence is caused by the diffraction and constructive interference of visible lightwaves which falls under Bragg’s law.

Because of the rarity and pathological properties, neither opal nor any of the organic viruses have been very popular in scientific laboratories. The number of experiments exploring the physics and chemistry of these “colloidal crystals” has emerged as a result of the simple methods which have evolved in 20 years for preparing synthetic monodisperse colloids, both polymer and mineral, and, through various mechanisms, implementing and preserving their long-range order formation.

Colloidal crystals are receiving increased attention, largely due to their mechanisms of ordering and self-assembly, cooperative motion, structures similar to those observed in condensed matter by both liquids and solids, and structural phase transitions. [13] [14] Phase equilibrium has been considered within the context of their physical similarities, with appropriate scaling, to elastic solids. Observations of the interparticle separation distance has shown a decrease on ordering. This led to a re-evaluation of Langmuir's beliefs about the existence of a long-range attractive component in the interparticle potential. [15]

Colloidal crystals have found application in optics as photonic crystals. Photonics is the science of generating, controlling, and detecting photons (packets of light), particularly in the visible and near Infrared, but also extending to the Ultraviolet, Infrared and far IR portions of the electromagnetic spectrum. The science of photonics includes the emission, transmission, amplification, detection, modulation, and switching of lightwaves over a broad range of frequencies and wavelengths. Photonic devices include electro-optic components such as lasers (Light Amplification by Stimulated Emission of Radiation) and optical fiber. Applications include telecommunications, information processing, illumination, spectroscopy, holography, medicine (surgery, vision correction, endoscopy), military (guided missile) technology, agriculture and robotics.

Polycrystalline colloidal structures have been identified as the basic elements of submicrometre colloidal materials science. [16] Molecular self-assembly has been observed in various biological systems and underlies the formation of a wide variety of complex biological structures. This includes an emerging class of mechanically superior biomaterials based on microstructure features and designs found in nature.

The principal mechanical characteristics and structures of biological ceramics, polymer composites, elastomers, and cellular materials are being re-evaluated, with an emphasis on bioinspired materials and structures. Traditional approaches focus on design methods of biological materials using conventional synthetic materials. [17] The uses have been identified in the synthesis of bioinspired materials through processes that are characteristic of biological systems in nature. This includes the nanoscale self-assembly of the components and the development of hierarchical structures. [18]

Bulk crystals

Aggregation

Aggregation in colloidal dispersions (or stable suspensions) has been characterized by the degree of interparticle attraction. [19] For attractions strong relative to the thermal energy (given by kT), Brownian motion produces irreversibly flocculated structures with growth rates limited by the rate of particle diffusion. This leads to a description using such parameters as the degree of branching, ramification or fractal dimensionality. A reversible growth model has been constructed by modifying the cluster-cluster aggregation model with a finite inter-particle attraction energy. [20] [21]

In systems where forces of attraction forces are buffered to some degree, a balance of forces leads to an equilibrium phase separation, that is particles coexist with equal chemical potential in two distinct structural phases. The role of the ordered phase as an elastic colloidal solid has been evidenced by the elastic (or reversible) deformation due to the force of gravity. This deformation can be quantified by the distortion of the lattice parameter, or inter-particle spacing. [22]

Viscoelasticity

Periodic ordered lattices behave as linear viscoelastic solids when subjected to small amplitude mechanical deformations. Okano's group experimentally correlated the shear modulus to the frequency of standing shear modes using mechanical resonance techniques in the ultrasonic range (40 to 70 kHz). [23] [24] In oscillatory experiments at lower frequencies (< 40 Hz), the fundamental mode of vibration as well as several higher frequency partial overtones (or harmonics) have been observed. Structurally, most systems exhibit a clear instability toward the formation of periodic domains of relatively short-range order Above a critical amplitude of oscillation, plastic deformation is the primary mode of structural rearrangement. [25]

Phase transitions

Equilibrium phase transitions (e.g. order/disorder), an equation of state, and the kinetics of colloidal crystallization have all been actively studied, leading to the development of several methods to control the self-assembly of the colloidal particles. [26] Examples include colloidal epitaxy and space-based reduced-gravity techniques, as well as the use of temperature gradients to define a density gradient. [27] This is somewhat counterintuitive as temperature does not play a role in determining the hard-sphere phase diagram. However, hard-sphere single crystals (size 3 mm) have been obtained from a sample in a concentration regime that would remain in the liquid state in the absence of a temperature gradient. [28]

Phonon dispersion

Using a single colloidal crystal, phonon dispersion of the normal modes of vibration modes were investigated using photon correlation spectroscopy, or dynamic light scattering. This technique relies on the relaxation or decay of concentration (or density) fluctuations. These are often associated with longitudinal modes in the acoustic range. A distinctive increase in the sound wave velocity (and thus the elastic modulus) by a factor of 2.5 has been observed at the structural transition from colloidal liquid to colloidal solid, or point of ordering. [29] [30]

Kossel lines

Using a single body-centered cubic colloidal crystal, the occurrence of Kossel lines in diffraction patterns were used to monitor the initial nucleation and subsequent motion caused distortion of the crystal. Continuous or homogeneous deformations occurring beyond the elastic limit produce a 'flowing crystal', where the nucleation site density increases significantly with increasing particle concentration. [31] Lattice dynamics have been investigated for longitudinal as well as transverse modes. The same technique was used to evaluate the crystallization process near the edge of a glass tube. The former might be considered analogous to a homogeneous nucleation event—whereas the latter would clearly be considered a heterogeneous nucleation event, being catalyzed by the surface of the glass tube.

Growth rates

Small-angle laser light scattering has provided information about spatial density fluctuations or the shape of growing crystal grains. [31] [32] In addition, confocal laser scanning microscopy has been used to observe crystal growth near a glass surface. Electro-optic shear waves have been induced by an ac pulse, and monitored by reflection spectroscopy as well as light scattering. Kinetics of colloidal crystallization have been measured quantitatively, with nucleation rates being depending on the suspension concentration. [33] [34] [35] Similarly, crystal growth rates have been shown to decrease linearly with increasing reciprocal concentration.

Microgravity

Experiments performed in microgravity on the Space Shuttle Columbia suggest that the typical face-centered cubic structure may be induced by gravitational stresses. Crystals tend to exhibit the hcp structure alone (random stacking of hexagonally close-packed crystal planes), in contrast with a mixture of (rhcp) and face-centred cubic packing when allowed sufficient time to reach mechanical equilibrium under gravitational forces on Earth. [36] Glassy (disordered or amorphous) colloidal samples have become fully crystallized in microgravity in less than two weeks.

Thin films

Two-dimensional (thin film) semi-ordered lattices have been studied using an optical microscope, as well as those collected at electrode surfaces. Digital video microscopy has revealed the existence of an equilibrium hexatic phase as well as a strongly first-order liquid-to-hexatic and hexatic-to-solid phase transition. [37] These observations are in agreement with the explanation that melting might proceed via the unbinding of pairs of lattice dislocations.

Long-range order

Long-range order has been observed in thin films of colloidal liquids under oil—with the faceted edge of an emerging single crystal in alignment with the diffuse streaking pattern in the liquid phase. Structural defects have been directly observed in the ordered solid phase as well as at the interface of the solid and liquid phases. Mobile lattice defects have been observed via Bragg reflections, due to the modulation of the light waves in the strain field of the defect and its stored elastic strain energy. [16]

Mobile lattice defects

All of the experiments have led to at least one common conclusion: colloidal crystals may indeed mimic their atomic counterparts on appropriate scales of length (spatial) and time (temporal). Defects have been reported to flash by in the blink of an eye in thin films of colloidal crystals under oil using a simple optical microscope. But quantitatively measuring the rate of its propagation provides an entirely different challenge, which has been measured at somewhere near the speed of sound.

Non-spherical colloid based crystals

Crystalline thin-films from non-spherical colloids were produced using convective assembly techniques. Colloid shapes included dumbbell, hemisphere, disc, and sphero-cylinder shapes. [38] [39] Both purely crystalline and plastic crystal phases could be produced, depending on the aspect ratio of the colloidal particle. The low aspect ratio, such as bulge, eye-ball, and snowman-like non-spherical colloids, which spontaneously self-assembled to photonic crystal array with high uniformity. [40] The particles were crystallized both as 2D (i.e., monolayer) and 3D (i.e., multilayer) structures. [41] [42] [43] [44] [40] The observed lattice and particle orientations experimentally confirmed a body of theoretical work on the condensed phases of non-spherical objects. Assembly of crystals from non-spherical colloids can also be directed via the use of electrical fields. [38]

Applications

Photonics

Technologically, colloidal crystals have found application in the world of optics as photonic band gap (PBG) materials (or photonic crystals). Synthetic opals as well as inverse opal configurations are being formed either by natural sedimentation or applied forces, both achieving similar results: long-range ordered structures which provide a natural diffraction grating for lightwaves of wavelength comparable to the particle size. [45]

Novel PBG materials are being formed from opal-semiconductor-polymer composites, typically utilizing the ordered lattice to create an ordered array of holes (or pores) which is left behind after removal or decomposition of the original particles. Residual hollow honeycomb structures provide a relative index of refraction (ratio of matrix to air) sufficient for selective filters. Variable index liquids or liquid crystals injected into the network alter the ratio and band gap.

Such frequency-sensitive devices may be ideal for optical switching and frequency selective filters in the ultraviolet, visible, or infrared portions of the spectrum, as well as higher efficiency antennae at microwave and millimeter wave frequencies.

Self-assembly

Self-assembly is the most common term in use in the modern scientific community to describe the spontaneous aggregation of particles (atoms, molecules, colloids, micelles, etc.) without the influence of any external forces. [18] Large groups of such particles are known to assemble themselves into thermodynamically stable, structurally well-defined arrays, quite reminiscent of one of the 7 crystal systems found in metallurgy and mineralogy (e.g. face-centered cubic, body-centered cubic, etc.). The fundamental difference in equilibrium structure is in the spatial scale of the unit cell (or lattice parameter) in each particular case.

Molecular self-assembly is found widely in biological systems and provides the basis of a wide variety of complex biological structures. This includes an emerging class of mechanically superior biomaterials based on microstructural features and designs found in nature. Thus, self-assembly is also emerging as a new strategy in chemical synthesis and nanotechnology. [17] Molecular crystals, liquid crystals, colloids, micelles, emulsions, phase-separated polymers, thin films and self-assembled monolayers all represent examples of the types of highly ordered structures which are obtained using these techniques. The distinguishing feature of these methods is self-organization.

See also

Related Research Articles

<span class="mw-page-title-main">Colloid</span> Mixture of an insoluble substance microscopically dispersed throughout another substance

A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend the definition to include substances like aerosols and gels. The term colloidal suspension refers unambiguously to the overall mixture. A colloid has a dispersed phase and a continuous phase. The dispersed phase particles have a diameter of approximately 1 nanometre to 1 micrometre.

<span class="mw-page-title-main">Self-assembly</span> Process in which disordered components form an organized structure or pattern

Self-assembly is a process in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, local interactions among the components themselves, without external direction. When the constitutive components are molecules, the process is termed molecular self-assembly.

<span class="mw-page-title-main">Photonic crystal</span> Periodic optical nanostructure that affects the motion of photons

A photonic crystal is an optical nanostructure in which the refractive index changes periodically. This affects the propagation of light in the same way that the structure of natural crystals gives rise to X-ray diffraction and that the atomic lattices of semiconductors affect their conductivity of electrons. Photonic crystals occur in nature in the form of structural coloration and animal reflectors, and, as artificially produced, promise to be useful in a range of applications.

<span class="mw-page-title-main">Soft matter</span> Subfield of condensed matter physics

Soft matter or soft condensed matter is a subfield of condensed matter comprising a variety of physical systems that are deformed or structurally altered by thermal or mechanical stress of the magnitude of thermal fluctuations. These materials share an important common feature in that predominant physical behaviors occur at an energy scale comparable with room temperature thermal energy, and that entropy is considered the dominant factor. At these temperatures, quantum aspects are generally unimportant. Soft materials include liquids, colloids, polymers, foams, gels, granular materials, liquid crystals, flesh, and a number of biomaterials. When soft materials interact favorably with surfaces, they become squashed without an external compressive force. Pierre-Gilles de Gennes, who has been called the "founding father of soft matter," received the Nobel Prize in Physics in 1991 for discovering that methods developed for studying order phenomena in simple systems can be generalized to the more complex cases found in soft matter, in particular, to the behaviors of liquid crystals and polymers.

<span class="mw-page-title-main">Nanoparticle</span> Particle with size less than 100 nm

A nanoparticle or ultrafine particle is a particle of matter 1 to 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 100 nm in only two directions. At the lowest range, metal particles smaller than 1 nm are usually called atom clusters instead.

In materials science, the sol–gel process is a method for producing solid materials from small molecules. The method is used for the fabrication of metal oxides, especially the oxides of silicon (Si) and titanium (Ti). The process involves conversion of monomers into a colloidal solution (sol) that acts as the precursor for an integrated network of either discrete particles or network polymers. Typical precursors are metal alkoxides. Sol–gel process is used to produce ceramic nanoparticles.

<span class="mw-page-title-main">Nucleation</span> Initial step in the phase transition or molecular self-assembly of a substance

In thermodynamics, nucleation is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically defined to be the process that determines how long an observer has to wait before the new phase or self-organized structure appears. For example, if a volume of water is cooled below 0 °C, it will tend to freeze into ice, but volumes of water cooled only a few degrees below 0 °C often stay completely free of ice for long periods (supercooling). At these conditions, nucleation of ice is either slow or does not occur at all. However, at lower temperatures nucleation is fast, and ice crystals appear after little or no delay.

<span class="mw-page-title-main">Coffee ring effect</span> Capillary flow effect

In physics, a "coffee ring" is a pattern left by a puddle of particle-laden liquid after it evaporates. The phenomenon is named for the characteristic ring-like deposit along the perimeter of a spill of coffee. It is also commonly seen after spilling red wine. The mechanism behind the formation of these and similar rings is known as the coffee ring effect or in some instances, the coffee stain effect, or simply ring stain.

<span class="mw-page-title-main">Particle aggregation</span> Clumping of particles in suspension

Particle agglomeration refers to the formation of assemblages in a suspension and represents a mechanism leading to the functional destabilization of colloidal systems. During this process, particles dispersed in the liquid phase stick to each other, and spontaneously form irregular particle assemblages, flocs, or agglomerates. This phenomenon is also referred to as coagulation or flocculation and such a suspension is also called unstable. Particle agglomeration can be induced by adding salts or other chemicals referred to as coagulant or flocculant.

<span class="mw-page-title-main">Ceramic engineering</span> Science and technology of creating objects from inorganic, non-metallic materials

Ceramic engineering is the science and technology of creating objects from inorganic, non-metallic materials. This is done either by the action of heat, or at lower temperatures using precipitation reactions from high-purity chemical solutions. The term includes the purification of raw materials, the study and production of the chemical compounds concerned, their formation into components and the study of their structure, composition and properties.

<span class="mw-page-title-main">Lyotropic liquid crystal</span>

Lyotropic liquid crystals result when fat-loving and water-loving chemical compounds known as amphiphiles dissolve into a solution that behaves both like a liquid and a solid crystal. This liquid crystalline mesophase includes everyday mixtures like soap and water.

The Stöber process is a chemical process used to prepare silica particles of controllable and uniform size for applications in materials science. It was pioneering when it was reported by Werner Stöber and his team in 1968, and remains today the most widely used wet chemistry synthetic approach to silica nanoparticles. It is an example of a sol-gel process wherein a molecular precursor is first reacted with water in an alcoholic solution, the resulting molecules then joining together to build larger structures. The reaction produces silica particles with diameters ranging from 50 to 2000 nm, depending on conditions. The process has been actively researched since its discovery, including efforts to understand its kinetics and mechanism – a particle aggregation model was found to be a better fit for the experimental data than the initially hypothesized LaMer model. The newly acquired understanding has enabled researchers to exert a high degree of control over particle size and distribution and to fine-tune the physical properties of the resulting material in order to suit intended applications.

<span class="mw-page-title-main">Freeze-casting</span>

Freeze-casting, also frequently referred to as ice-templating, or freeze alignment, is a technique that exploits the highly anisotropic solidification behavior of a solvent in a well-dispersed slurry to controllably template a directionally porous ceramic. By subjecting an aqueous slurry to a directional temperature gradient, ice crystals will nucleate on one side of the slurry and grow along the temperature gradient. The ice crystals will redistribute the suspended ceramic particles as they grow within the slurry, effectively templating the ceramic.

<span class="mw-page-title-main">Self-assembly of nanoparticles</span>

Nanoparticles are classified as having at least one of its dimensions in the range of 1-100 nanometers (nm). The small size of nanoparticles allows them to have unique characteristics which may not be possible on the macro-scale. Self-assembly is the spontaneous organization of smaller subunits to form larger, well-organized patterns. For nanoparticles, this spontaneous assembly is a consequence of interactions between the particles aimed at achieving a thermodynamic equilibrium and reducing the system’s free energy. The thermodynamics definition of self-assembly was introduced by Professor Nicholas A. Kotov. He describes self-assembly as a process where components of the system acquire non-random spatial distribution with respect to each other and the boundaries of the system. This definition allows one to account for mass and energy fluxes taking place in the self-assembly processes.

Patchy particles are micron- or nanoscale colloidal particles that are anisotropically patterned, either by modification of the particle surface chemistry, through particle shape, or both. The particles have a repulsive core and highly interactive surfaces that allow for this assembly. The placement of these patches on the surface of a particle promotes bonding with patches on other particles. Patchy particles are used as a shorthand for modelling anisotropic colloids, proteins and water and for designing approaches to nanoparticle synthesis. Patchy particles range in valency from two or higher. Patchy particles of valency three or more experience liquid-liquid phase separation. Some phase diagrams of patchy particles do not follow the law of rectilinear diameters.

Nanosphere lithography (NSL) is an economical technique for generating single-layer hexagonally close packed or similar patterns of nanoscale features. Generally, NSL applies planar ordered arrays of nanometer-sized latex or silica spheres as lithography masks to fabricate nanoparticle arrays. NSL uses self-assembled monolayers of spheres as evaporation masks. These spheres can be deposited using multiple methods including Langmuir-Blodgett, dip coating, spin coating, solvent evaporation, force-assembly, and air-water interface. This method has been used to fabricate arrays of various nanopatterns, including gold nanodots with precisely controlled spacings.

<span class="mw-page-title-main">Quasi-crystals (supramolecular)</span> Supramolecular aggregates

Quasi-crystals are supramolecular aggregates exhibiting both crystalline (solid) properties as well as amorphous, liquid-like properties.

<span class="mw-page-title-main">Orlin D. Velev</span>

Orlin D. Velev is the INVISTA Professor in the Department of Chemical and Biomolecular Engineering at North Carolina State University. He is best known for his work in soft matter, colloid science, and nanoscience.

Judith M. Dawes is an Australian physicist who is Professor of Physics at Macquarie University. She studies the interactions of light at the nanoscale and the applications of lasers in sensing. She is a former president of the Australian Optical Society, and a Fellow of SPIE and Optica.

<span class="mw-page-title-main">Sphere packing in a cylinder</span> Three-dimensional packing problem

Sphere packing in a cylinder is a three-dimensional packing problem with the objective of packing a given number of identical spheres inside a cylinder of specified diameter and length. For cylinders with diameters on the same order of magnitude as the spheres, such packings result in what are called columnar structures.

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