Structure of nano-crystals: a key to understanding the unique properties of nano-materials
|Bogdan F. Palosz|
Polish Academy of Sciences, Institute of High Pressure Physics (UNIPRESS), Sokolowska 29/37, Warszawa 01-142, Poland
A key problem of "nanoscience" is understanding what is the origin of the difference in the properties between nano- and single crystals. In contrast to single- for nano-crystals one cannot disregard the fact that there is a large number of atoms located at the surface and, consequently, one has to accept that the crystallographic structure of a very small size particle may deviate from that in the bulk crystals. In a tentative model, a nanocrystal might be presented as a two-phase system formed by the grain core and the surface shell, both having characteristic dimensions.
There are no standard procedures established for performance of structural analysis of a nanocrystal. The problem is that its complex structure cannot be well represented by a unit cell. Only for this reason the methods which are used for elaboration of powder diffraction data based on Bragg equation should not be applied for examination of nanocrystalline materials. The values of lattice parameters calculated from individual Bragg reflection differ between each other, and because they vary with the diffraction vector Q, they are rather "apparent lattice parameters", alp's. Although Bragg equation is not fulfilled for very small crystals, it might be practical to base analysis of real structure of nanocrystals referring to Bragg type scattering and concentrate on interpretation of the deviations of diffraction images of nanocrystals from those corresponding to a perfect crystal lattice.
Examples of examination of distribution of strains among nano-grain core and surface shell has been examined at ambient conditions, but also under high pressures and high temperatures are given based on X-ray and neutron scattering experimental data. Advantages of the analysis referring to reciprocal and real spaces of nano-structures are discussed.
Presentation: invited oral at E-MRS Fall Meeting 2005, Symposium I, by Bogdan F. Palosz
See On-line Journal of E-MRS Fall Meeting 2005
Submitted: 2005-07-20 11:21 Revised: 2009-06-07 00:44