WHY X-RAY IS LIMITED FOR NANOCRYSTALS STRUCTURE ANALYSIS?

X-ray crystallography is well adapted to structure analysis of perfect single crystals larger than a few micrometers. X-ray crystal interactions are kinematical and thus the structure factors can be directly derived from the diffracted intensity data. For nanocrystal structural studies use of powder X-ray diffraction present severe limitations as the grain size is very small and there is a severe peak broadening ( see photo below ). Therefore it becomes very difficult to solve ab-initio unknown  nanostructures from  X-ray diffraction ( see  figure below ).

Transmission Electron Microscopy (TEM) is very well adapted to the imaging and the analysis of nanocrystals. By means of the electron micrograph of the studied specimen, it is possible to select (possibly in a defect free area) and probe a tiny  area, smaller than the nanocrystal size, in order to obtain an electron diffraction pattern.

ELECTRON DIFFRACTION, SOLUTION FOR NANOCRYSTALS STRUCTURE ANALYSIS?

Despite these interesting features, conventional  electron diffraction was rarely used in the past as a standard tool for crystal identification mainly because the electron interactions with matter are about 10,000 times stronger than that of X‑rays. As a result, the scattering is not kinematic but dynamical, so that the diffracted intensities are so much altered that they cannot be trusted and used for crystal structure determination, unless the crystal thickness is very thin or very demanding dynamical calculations are undertaken.

WHAT ABOUT PRECESSION ELECTRON DIFFRACTION?

Electron diffraction precession technique proposed by Vincent & Midgley [1] offers a solution to this problem by decreasing the dynamical behaviour of electron diffraction. This technique is equivalent to the Buerger precession technique used in X-ray diffraction, where the specimen is precessed with respect to the incident X-ray beam. In the electron precession technique, the electron beam is tilted and precessed along a conical surface, having a common axis with the TEM optical axis.

As a result of this precession movement:
- many more reflections in the reciprocal space are visible than conventional SAED  patterns
- reflection diffracted intensity  is much closer to the integrated intensity values
- resulting  precession diffraction pattern can be considered  much less dynamical and much closer to      kinematical  ( like X-Ray case ).

With precession electron diffraction kinematically forbidden reflections and multiple scatterings are greatly reduced, making space group identification easier. and also  reduces sensitivity of ED intensities to crystal thickness,  misorientation effects and Ewald  sphere curvature.

Using precession in a ED pattern  results to huge extension of  visible reflections at very high angles (diffraction  order up to 0.05 nm) and an important redistribution of electron  diffraction intensities due to the practical  elimination of dynamical diffraction/multiple diffraction contributions.

Several minerals, catalysts, and complex oxides have  been solved  ab initio from quasi‑kinematical  precession diffraction  intensities [ 2].

Download   HERE  short  PDF  presentation  about precession  and  SPINNING  STAR

HOW WE CAN REALIZE PRECESSION DIFFRACTION IN TEM?

NanoMEGAS  unique  precession  inteface  SPINNING STAR  can be adapted  to any TEM ( old or brand  new  from 100-400 kv ) . By precessing  incident beam at a constant angle around a zone axis in combination with a similar  precession (descan) of the ED pattern below the specimen, the equivalent mechanism of the precession  of the specimen is obtained.

SPINNING STAR  takes control  of  TEM coils in order that scanning and de-scanning of the beam are exactly compensated for any spot size and  any TEM ( even without STEM presence ) and in order that  ED  pattern remain stationary during precession.When  precession interface is switched on, is fully independent from working mode of the microscope. ;when precession is switched off, the system is fully disconnected  from  TEM

[1]  R.Vincent, P,Midgley, Ultramicroscopy ,  1996
[2]  Ultramicroscopy   Special issue  , proceedings Elcryst2005 vol 107, June –July 2007  , 2007