Structural Fingerprinting of Nanocrystals in the Transmission Electron Microscope
P. Moeck, S. Rouvimov, S. Nicolopoulos
Portland State Unversity, US
Keywords: precession electron diffraction, TEM, crystallographic databases
Abstract: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) 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. Automation of the former two strategies shall lead to statistically significant results on ensembles of nanocrystals. The structural information that can be extracted from a HRTEM image of an approximately 5 to 10 nm thick nanocrystal is the projected reciprocal lattice geometry, the plane symmetry group, and 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. As 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 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 allow frequently for an unambiguous determination of the space group. Comparing kinematical electron diffraction simulations with experimental PEDs allows, therefore, for structure verifications.