Crystallographic Orientation

Nanoparticles may have a random arrangement of the constituent atoms or molecules or the individual atomic or molecular units may be ordered into a regular periodic crystalline structure which may not necessarily be the same as that which is observed in a much larger system. If crystalline each nano-particle may be either a single crystal or itself composed of a number of different crystalline regions or grains of differing crystallographic orientations. Nanoparticles may also be quasi crystalline, the atoms being packed together in an icosahedral arrangement and showing non-crystalline symmetry characteristics. Such quasi crystals are generally only stable at the nanometre or at most the micrometer scale.

Nanoparticles may be present within another medium such as nanometre sized precipitates in a surrounding matrix material. These nanoprecipitaates will have a specific morphology and may possess certain crystallographic orientation relationships with the atomic arrangement of the matrix depending on the nature of the interface which may lead to coherency strains in the particle and the matrix. One such example is the case of self assembled semiconductor quantum dots which form due to lattice mismatch strain relative to the surrounding layers an whose geometry is determined by the details of the strain field. Another feature which may be of importance for the overall transport properties of the composite system is the connectivity of such nanometer sized regions or in the case of a nanoporous material nanopore connectivity.

In three dimensions we also have to consider collections of consolidated nanoparticles eg. A nanocrystalline solid consisting of nano metre sized crystalline grains each in a specific crystallographic orientation………



KELSALL R., NANOSCALE SCIENCE AND TECHNOLOGY, WILEY, 2005, P 3-4