On jeudi 3 avril 2008, May, Frank wrote: > The mathematical description of "crystals" is valid for the bulk of the > volume. However, the description suffers from a physical "termination of > series" effect at the surface of the crystal. For very large crystals, the > amount of surface is much greater relative to the total volume of the > crystal. However, when the crystalline material approaches what we now > refer to as the "nano" state (perhaps less than 40 Angstroms - 8 to 10 unit > cells), the amount of surface becomes significant. The chemical > composition - oxidation state of surface atoms - is intuitively different > at the surface of a material than within the bulk. How does one deal with > that situation? The effects are present in the powder pattern; how does > one model them?
One probably cannot deal efficiently with that using Rietveld refinement, as if you obtain a powder of such nano-particles, there will probably be a size distribution which will have a much greater influence on the powder pattern than the small lack of electronic density on the borders. Of course if you have a significant departure from the bulk structure (near the surface or not), you may still be able to model it, but rather using a PDF analysis (talking the background into account). I guess the study of CdSe nanoparticles is one good example of this : http://dx.doi.org/10.1103/PhysRevB.76.115413 A probably better (but more complex) way would use coherent diffraction on a single nano-particle. See the recent article using electron diffraction: http://www.nature.com/nmat/journal/v7/n4/abs/nmat2132.html Vincent -- Vincent Favre-Nicolin http://vincefn.net Université Joseph Fourier http://www.ujf-grenoble.fr CEA/ Institut Nanosciences & Cryogénie http://inac.cea.fr ObjCryst & Fox http://objcryst.sourceforge.net
