Structural Fingerprinting of Nanocrystals in the Transmission Electron Microscope Supported by Open-Access Crystallographic Databases

Abstract

It is well known that nanocrystals cannot be fingerprinted structurally from powder X-ray diffraction patterns [1][2][3]. Three strategies for the structurally identification of nanocrystals in a Transmission Electron Microscope (TEM) have, therefore, been developed. Either a single High-Resolution Transmission Electron Microscopy (HRTEM) image or a single Precession Electron Diffractogram (PED) can be employed. The structural identification information is in both cases collected from an individual nanocrystal. PED from fine-grained crystal powders may also be utilized.The structural information that can be extracted from a HRTEM image of an approximately 5 nm thick nanocrystal is the projected reciprocal lattice geometry, the plane symmetry group, and a few structure factor amplitudes and phase angles. While the structure factor amplitudes suffer from dynamical diffraction effects and are in addition modified by the (not precisely known) contrasttransfer function of the objective lens, the structure factor phase angles are remarkably stable against dynamical diffraction effects and slight crystal misorientations [3,4].Except for the structure factor phase angles, the same "kind" of structural information can be extracted from a PED pattern. Instead of the plane symmetry of the HRTEM images, the 2D point symmetry of the PED is used and no structure factor phases are accessible. The information that can be utilized for structural fingerprinting is in the PED case, however, not limited to the directly interpretable resolution of the TEM. Since precession electron diffraction avoids crystal orientations that result in the simultaneous excitation of more than one strong diffracted beam (as much as this is possible), quasi-kinematic reflection intensities are obtained for nanocrystals with thicknesses up to approximately 25 nm. Simultaneously present reflections in higher order Laue zones and systematic absences in both the higher and the zero order Laue zones of a few projections allow frequently for an unambiguous determination of the space group [1][2][3].