Ordered arrangement under appropriate conditions is a fundamental property of many systems, of which nanocrystals are only one example. A nanocrystal consists of an arrangement of atoms in a high degree of order, and when nanocrystals themselves take up a highly ordered arrangement a supracrystal is formed.
Similarity Between Nanoscale and Microscale Order
Nanocrystals can take up fcc (face-centered cubic) tetrahedral shapes which repeat to form more complex nanocrystals in decahedral or icosahedral arrangement, at nanometer level. At micrometer level, too, the supracrystals are similar in shape and properties to the nanometer scale crystal. The atoms are substituted by incompressible nanocrystals while the coating agents or alkyl chains take the place of the atomic bonds, keeping the nanocrystals together with a mechanical spring-like action. This has been shown in different areas.
- Crystal growth of supracrystals grown in solution at mesoscales are similar in shape to those grown at nanoscale
- Breathing modes of atoms are coherent at all scales from nanocrystals to supracrystals
- Again, longitudinal acoustic phonons travel at the speed of sound in supracrystals by the coherent movement of nanocrystals from the positions they occupy at equilibrium
As the supracrystal grows large, the mechanism behind its growth changes from a layered growth (supracrystal film) or heterogeneous growth, by slow evaporation of solvent from a colloidal nanocrystalline solution, to a homogeneous type of growth, or growth in solution where shaped supracrystals grow in regular octahedral, triangular, hexagonal or pentagonal stars. If the ordering is improved, the supracrystalline structure also becomes better. Procedures such as annealing at 350 °C, or phase transitions, can help control the supracrystal structure.
Small and large nanocrystals can be mixed, as long as each is uniform in its category, to acquire supracrystals. The large nanocrystals self-assemble in a well-defined pattern whose interstices are completed by small nanocrystals, producing binary structures of various kinds under specific conditions. The nanocrystals are typically stabilized by the use of ligands which are also an essential factor in determining the final structure, apart from the crystalline structure of the nanocrystals.
Several nanocrystals have been studied over the years. The opal is the classic example, being composed of silicate particles below a micrometer in size. Colorless opal is colorless, lacking order in the arrangement of silicate particles, while the same particles in order show natural size segregation and the consequent emergence of reflective properties. Thus the ordering of particles on a large scale causes intrinsic properties to emerge. This phenomenon can be used to create new materials for use in electronics or biomedical fields.
Controlling Supracrystal Structure
Colloidal nanocrystals can be synthesized under the control of various parameters to allow the required shape, size and composition to be obtained. This enables the self-assembly of 3D crystalline structures or supracrystals, made up of quasi-spherical nanocrystals with a narrow size distribution width, and which form fcc, bcc (body-centered cubic) and hcp (hexagonal close packing) crystal lattices.
The distance between nanoparticles and hence the crystal structure varies with factors such as;
- Vapor pressure
- Water adsorption on the interface of nanocrystals and the coating agent
- Coating agents which are typically alkyl chains
- Free coating agents or impurities and other organic molecules
Quasi-binary supracrystals can also be produced by ligand exchange during the process of supracrystal growth, or by a magnetic field, as by using nanocrystals which are of two different sizes and are both magnetic. In short, the function of nanocrystals in the supracrystal can be likened to that of atoms in nanocrystals, that is, to act as the building blocks of superlattice structures. The structures depend upon the mode in which the nanocrystals are organized.
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The selective formation of single-domain or polycrystalline nanocrystals is being studied using various techniques such as organometallic synthesis of either single or polycrystalline gold nanocrystals, which resulted in a mix of both types. This was followed by seeding of nanocrystals formed by this technique below a specific diameter to get a narrow size distribution width at the end.
To obtain supracrystals, nanocrystals of a single size with a coating agent dispersed in solvent are allowed to self-assemble in 3D superlattices or supracrystals, via slow evaporation. These nanocrystals are either single-domain, with large facets, or polycrystalline. While both self-assemble, the former are ordered both translationally and by orientation, causing increased stiffness, while the latter have long-range translational ordering. Similar segregation can be achieved by using other methods too.
Changing Supracrystal Properties
Mechanical properties of supracrystals, as studied by the atomic force microscope, also change based upon the strength of the interparticle interactions in supracrystal films which are made up purely of polycrystalline nanoparticles, but those which comprise single-domain nanocrystals show greater strength by one order of magnitude in Young’s modulus for elastic deformations. This varies between interfacial supracrystals which are formed at air-solvent interfaces as floating films, layer by layer, and those which are shaped supracrystals. Thus the mechanism of supracrystal growth also plays a major role in the mechanical properties.
Another strong influence is the use of a coating agent which can potentially impair the strength of the interactions between the atoms at the surface of the nanocrystal and the terminal groups of the coating agent, as well as the interactions between the alkyl chains of the coating agent. Another factor is the size of the nanocrystals which are the building blocks for the supracrystals. It is possible to change the Young’s modulus over three orders of magnitude by varying the coating agent, from one which is strongly bound to the interface with a non-solvent for the alkyl chains so that interdigitation is favored leading to homogeneous growth of the supracrystal to produce a large Young’s modulus, to a small one with a good solvent.
Another interesting area is in their electronic transport properties in thick supracrystals of microscale thickness. Those made up of 5 nm nanocrystals act differently from those which have 6, 7 or 8 nm thickness.