Transmission Electron Microscopy is a key technique to characterize nanoparticles andnanostructures, and to study their interactions with cells and tissues. At CNI, we apply advanced transmission electron microscopy techniques across theareas of materials science, nanotoxicology and biology, and we strive todevelop novel methods and to extend the range of applications. Specifically, weare developing electron diffraction methods based on the precession of theelectron beam. The precession electron diffraction unit mounted on our Transmission Electron Microscope (Zeiss Libra 120 Plus), enables us to use the TEM as a single crystal diffractometer for structure solution of micro- andnanocrystals. Precession electron diffraction is a way of collecting diffractionpatterns in which the electron beam processes on a cone with a vertex fixed onthe specimen plane. During the precession movement the pattern is tilted offthe zone axis, with few reflections excited at the same time, and the dynamicalinteractions between the diffracted beams are strongly reduced. The resultingpattern is quasi-kinematical ("x-ray like"), and the diffractionsintensities extracted from these patterns enable crystal structure solution(Fig. 1).
Precession electron diffraction combined with a data collection of atilt series of diffraction pattern (one every 1°) can give a full coverage ofthe reciprocal space, which can be reconstructed in 3D tomography (Fig. 2). Intensitydata extracted from this data collection are also suitable for structuresolution, since they are recorded in random crystal orientations with few spotsexcited at the same time. The precession unit installed in our laboratoryrepresents a unique facility in Italy, and will be devoted to structuresolution of new crystalline organic compounds and pharmaceuticals.Veryrecently we acquired an ASTAR unit for orientation and phase mapping in the TEM.
With this system it is possible to acquire a set of diffraction patterns(normally between 10000 and 40000) while the beam is scanning an area ofseveral mm2. The patterns are recordedusing a dedicated fast CCD camera (collection rates 100 to 10 frame persecond), which makes the entire data collection lasting only few minutes. Thethousands of recorded patterns are compared via software with a set of patternsgenerated according to the crystal structures present in the sample. For eachpattern a cross correlation routine determines to which crystal phase thepattern belongs and to which crystal orientation it corresponds. In this way itis possible to have at the same time a phase map and an crystal orientation mapof the sample with a resolution that in our microscope is between 5 and 10 nm(Fig. 3). We are actively seeking industrial and academicpartners to extend the use of this facility to a range of materials, and wecollaborate with Nanomegas Company, the manufacturer of the Digistar device forprecession electron diffraction and ASTAR unit from phase and orientationmapping, for further technical developments of this promising technique.