Structural fingerprinting in the transmission electron microscope: overview and opportunities to implement enhanced strategies for nanocrystal identification

Abstract

This paper illustrates the prospective need for structural fingerprinting methods for nanocrystals. A review of the existing fingerprinting methods for crystal structures by means of transmission electron microscopy (TEM) which work for a single setting of the specimen goniometer is given. Suggestions are made on how some of these methods could be enhanced when nanocrystals and novel instrumentation are involved, i.e. when either the kinematic or quasi-kinematic scattering approximations are sufficiently well satisfied. A novel strategy for latticefringe fingerprinting of nanocrystals from Fourier transforms of high-resolution phase contrast transmission electron microscopy (HRTEM) images is briefly outlined. Nanocrystal structure specific limitations to the application of this strategy are discussed.

Precession electron diffraction and its utility for structural fingerprinting in the transmission electron microscope

“…For various reasons, structural fingerprinting of ensembles of nanocrystals is not feasible by means of standard laboratory-based powder X-ray diffractometry (XRD) [1]. Crystalline vanadium-oxide nanotubes [2], for example, show only one strong Bragg peak when a Cu-Ka X-ray tube is used for the recording of a powder X-ray diffractogram.…”

“…This followed from both the commercial availability of precession electron diffraction add-ons to existing TEMs* and the realization that the structures of nanocrystals can be both solved and refined on the basis of PED data from individual nanocrystals. Any technique that facilitates the solving and refining of nanocrystals can also be used for structural fingerprinting [1,5].…”

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

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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.

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“…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]. …”

“…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].…”