||This article may be too technical for most readers to understand. (March 2013)|
The size of nanocrystals distinguishes them from larger crystals. For example, silicon nanocrystals can provide efficient light emission while bulk silicon does not and may be used for memory components.
When embedded in solids nanocrystals may exhibit much more complex melting behaviour than conventional solids and may form the basis of a special class of solids. They can behave as single-domain systems (a volume within the system having the same atomic or molecular arrangement throughout) that can help explain the behaviour of macroscopic samples of a similar material without the complicating presence of grain boundaries and other defects.
The traditional way to prepare nanocrystals of a new material involved choosing molecular precursors, surfactants, and solvents using optimized reaction conditions causing the atoms to self-assemble into monodisperse nanocrystals.
A newer, simpler strategy uses preformed nanocrystals as templates and chemical transformation to change the composition.
Solution-based mechanisms can chemically transform nanomaterials, allowing atoms to be easily and precisely incorporated, removed, or replaced from preformed templates. The approach uses oxidation, reduction, alloying, or atomic exchange reactions. In ionic nanocrystals, cation exchange can be driven by solvation energy differences between template and solvated ions. Ion solubilities can be controlled by adding selective coordinating species to the solution. In metal nanocrystals, atomic exchange reactions reflect reduction potential differences between the template metal and solvated metal ions. This galvanic replacement method involves a redox reaction. Placing a nanocrystal in a solution containing metal ions with a higher reduction potential oxidizes the templates' surface, dissolving its metal ions. The released electrons reduce the ions from the solution, which deposit at the template's surface.
Galvanic replacement also applies to ionic compounds. In oxide nanocrystals, a redox-couple reaction can occur between multivalent metallic ions. E.g., higher–oxidation state ions in manganese oxide nanocrystals have been replaced with solvated lower–oxidation state iron ions.
Atomic diffusion is a key parameter in such reactions. Chemical transformation tools provide complete composition control only within the atomic diffusion length. High nanocrystal surface-to-volume ratios expose the entire lattice to diffusion. The effective particle size range for these tools depends on the material, but can reach hundreds of nanometers.
- Cadmium telluride nanocrystals
- Magnetic nanoparticles
- Nanocrystal solar cell
- Nanocrystalline silicon
- Quantum dot
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