"Nano-materials and Powder Diffraction"

 

EPDIC-10 Workshop

 

31 August 2006, Geneva, Switzerland

 

 

 

 

"Future Directions and Research Priorities:  . . . Greater emphasis needs to be placed on the fundamental understanding of materials rather than on applied science and product development.  Naturally, application of materials is the ultimate goal, but this needs to be built on a firm theoretical basis so that improvements can be made more efficiently and reliably.  Particular attention should be given to understanding a material's behavior from the atomic/nano-level via microstructure to macrostructure levels using advanced analytical techniques and computer modeling . . ."

 

[EuropeanWhite Book on Fundamental Research in Materials Science, http://www.mpg.de/doku/wb_materials/wb_materials_010_013.pdf

 

 

 

Different than for micro-to-macro materials, the major role in nano-crystals is played by surface phenomena which lead to most of the unique properties of nano-materials. A meaningful understanding and interpretation of those properties has to be based on reliable information about the underlying atomic structure. The basic and essentially only preeminent technique for determination of the atomic structure of matter is diffraction. The complex structure of nano-materials causes the standard methods of structural analysis to be of limited use so that non-standard and new approaches and tools for structural analysis are necessary. Iuliana Dragomir-Cernatescu & Robert L. Snyder will describe XRD techniques and instrumentation for specifically designed for nano-materials characterization.

 

Relative to standard crystalline materials a nano-crystal is not a single crystal:

 

(i), its structure is different in the bulk than at the surface, and

 

(ii), the grain size limits long-range interactions to the characteristic dimension(s) of the grain only.

 

In other words, each nano-crystal constitutes a unique object.

 

For the above reasons, the model of an ideal crystal lattice cannot approximate the structure of a nano-crystal.  In practice that means, that any structural analysis of such materials has to be based on “total scattering”, which comprises both coherent an incoherent components.  This topic will be covered by Simon Billinge.  Certainly, it is not always necessary to examine total scattering since useful information about the structure of nano-crystals can be obtained from the analysis of characteristic Bragg scattering also.  It should, however, be realized that such information is definitely incomplete since it refers only to coherently scattering part of the crystal.

 

No specific criterion for selection of the optimum type of radiation source for diffraction experiments on nano-materials can be recommended, since each one has its advantages and limitations.  The application of the less commonly used ones, electrons and neutrons will be presented by Stavros Nicolopoulus and Thomas E. Proffen, respectively. 

 

A particular role in investigating the structure of nano-materials is played by low angle scattering (Lutz Brügemann) and determination of the grain size distribution in the sample (Paolo Scardi). Those studies are very helpful to find the characteristic nano-dimensions of nano-materials.  As it turns out those techniques, when applied to nano-crystals, require a non-standard approach.  An alternative to powder diffractometry is diffraction on a single grain. Unfortunately, the radiation sources available today allow for such an analysis of grains of about 100 nm in size or larger only (Ian Robinson).  We may expect that a real breakthrough in characterization of nano-crystals will come with the X-ray laser.  Such an excellent research tool may be expected to become available in a few years from now (Jacek Krzywinski). We should start getting prepared for such a research opportunity today!

 

It is quite natural that, working in the field of experimental powder diffraction, we are well aware of the shortcomings imposed by diffraction methods, both by their technical limitations and by the available methods of elaboration of the experimental data.  Those limitations affect our ability to set novel tasks and envision new research horizons.  Therefore it is worthwhile to go beyond those limitations and embrace a different point of view, where “impossible is possible”. Numerical simulations bring about such opportunity, i.e. a virtual experiment without the technical limitations of experimental reality. These techniques have a great potential and their application to nano-materials is on the rise.  It can be a tremendous inspiration for us, experimenters, and a great opportunity for the modelers to verify their theoretical predictions. The potential of MD simulations will be presented by Izabela Szlufarska.

 

We believe that the above outlined program of the workshop will provide a good insight into the current state-of-the-art and the future of structural investigation of nano-crystals.