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  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
 R.Vincent, P,Midgley, Ultramicroscopy , 1996
 Ultramicroscopy Special issue , proceedings Elcryst2005 vol 107, June –July 2007 , 2007
STEP 1 with spinning star obtain precession patterns from one or several ZA of the same
STEP 2 measure precession intensities from one or several ZA patterns using film, CCD or image
plates ; for high accuracy use electron diffractometer Pleiades
STEP 3 merge several ED zone axis intensities assuming known space group symmetry ;
solve structure using direct method software ( SIR2008, MICE , SHELX , FULPROF etc..)
STEP 4 most probable nanostructure solution will show up with list of all atomic positions