Structural fingerprinting of nanocrystals in the transmission electron microscope

Three novel strategies for the structurally identification of nanocrystals in a Transmission Electron Microscope (TEM) are presented. Either a single High-Resolution Transmission Electron Microscopy (HRTEM) image or a single Precession Electron Diffractogram (PED) may be utilized. The structural identification information is in both cases collected from individual nanocrystals. PEDs from fine-grained crystal powders may also be used. Automation of the former two strategies leads to statistically significant results for ensembles of nanocrystals and has recently been achieved for the case of PEDs. "Crystal orientations & structures" maps can now be obtained from PEDs quickly and reliably at spatial resolutions that are superior to those of the competing electron backscattering Kikuchi diffraction technique of scanning electron microscopy.The structural information that can be extracted from a HRTEM image of an approximately 5 to 10 nm thin nanocrystal is the projected reciprocal lattice geometry and symmetry as well as a few structure factor amplitudes and phase angles. Except for the structure factor phase angles, the "same kind" of structural information can be extracted from a PED. (Note that while HRTEM images reveal the 2D space group symmetry of the projected electrostatic potential to the spatial resolution of the microscope, only its 2D point group symmetry can be obtained from a PED. Note also that PEDs are neither limited by electron optical aberrations of the objective lens nor by the spatial resolution of the TEM.) Since precession electron diffraction avoids crystal orientations that result in the simultaneous excitation of more than one strong diffracted beam, quasi-kinematic reflection intensities are obtained for nanocrystals with thicknesses of up to approximately 50 nm. Simultaneously present reflections in higher order Laue zones and systematic absences in both the higher and the zero order Laue zones facilitate the determination of 3D space group information.Searching for structural information that is extracted by our novel strategies in crystallographic databases and matching it with high figures of merit to that of candidate structures allows for highly discriminatory identifications of nanocrystals, even without additional chemical information as obtainable in analytical TEMs. As an alternative to the commercial databases, one may use open-access databases, which provide together some 150,000 crystal structure data sets (that include atomic coordinates).