Detecting structural variances of Co3O4 catalysts by controlling beam-induced sample alterations in the vacuum of a transmission electron microscope

Abstract

This article summarizes core aspects of beam-sample interactions in research that aims at exploiting the ability to detect single atoms at atomic resolution by mid-voltage transmission electron microscopy. Investigating the atomic structure of catalytic Co3O4 nanocrystals underscores how indispensable it is to rigorously control electron dose rates and total doses to understand native material properties on this scale. We apply in-line holography with variable dose rates to achieve this goal. Genuine object structures can be maintained if dose rates below ~100 e/Å2s are used and the contrast required for detection of single atoms is generated by capturing large image series. Threshold doses for the detection of single atoms are estimated. An increase of electron dose rates and total doses to common values for high resolution imaging of solids stimulates object excitations that restructure surfaces, interfaces, and defects and cause grain reorientation or growth. We observe a variety of previously unknown atom configurations in surface proximity of the Co3O4 spinel structure. These are hidden behind broadened diffraction patterns in reciprocal space but become visible in real space by solving the phase problem. An exposure of the Co3O4 spinel structure to water vapor or other gases induces drastic structure alterations that can be captured in this manner.

Comment on, “On the influence of the electron dose-rate on the HRTEM image contrast”, by Juri Barthel, Markus Lentzen, Andreas Thust, ULTRAM12246 (2016), http://dx.doi.org/10.1016/j.ultramic.2016.11.016

“…It was concluded that the image contrast is independent of the electron dose-rate and that strong electron beam-induced atom displacements from their equilibrium positions are absent that would significantly exceed isotropic Debye-Waller factors B = 8 π /3 < u 2 > at room temperature [1] . These findings stand in marked opposition to our recent reports that the electron dose-rate indeed has a marked influence on the HRTEM image contrast [2][3][4][5] . We welcome the opportunity to address the underlying facts and assumptions and find it appropriate to address this apparent discrepancy by demonstrating that the findings in [1] are in fact contained in the results of [2][3][4][5] , which apply to HRTEM images and to imaging in a Scanning Transmission Electron Microscopy (STEM).…”

“…We expanded the common acquisition practice of single images at a later point of time to include dose-rates as low as 1 e/ Å 2 s, an illuminated sample area that can be more than 100 times smaller than reported in Ref. [1] but remains free of fringes from the presence of apertures, the ability to dramatically alter beam currents by 4-5 orders of magnitude in a fraction of a second without an introduction of uncontrolled lens aberrations, and the ability to correct for residual lens aberrations < 0.5 Å after the acquisition of low-dose-rate image series [2][3][4][5]11] . In fact, our reconstructed electron exit wave functions or in-line holograms exhibit an unmatched sub-Å ngstrom resolution and superior signal-to-noise ratios (SNRs) even if low-doserates < 10 e/ Å 2 s are employed that are much too small to capture interpretable information in single high resolution images because of their poor SNRs.…”

“…9 ]. However, they can also exceed 5 × 10 7 e/ Å 2 s for the acquisition of atomic resolution high angle annular dark field images [2] . Thus, electron microscopy studies employ dose rates that are altered by 7-8 orders of magnitude.…”

“…However, it is unexplored in which imaging conditions and on what time scales the Born-Oppenheimer approximation remains applicable. The concept was recently questioned for investigations that enable sub-Å ngstrom resolution with single atom sensitivity in real space images [2][3][4][5] . One expects limitations because a detection of single atoms demands a dramatic increase of beam current densities to produce enough elastic Coulomb scattering events of the probing electrons with the nuclei of the investigated material to make atoms visible.…”

Visualizing Redox Chemistry in Oxide Surfaces at Atomic-Resolution

“…A systematic variation of the electron illumination demonstrates that low dose-rates are needed for alleviating beam-induced alterations in the oxide catalysts. Correspondingly, the HRTEM image signal reaches levels at the detection limit but can beneficially be recovered using low dose-rates in conjunction with in-line holography in order to maintain atomic-resolution and -sensitivity of the oxide surface structures (Figure 1) [2][3][4][5][6]. Second, we employ this concept of dose-fractionating electron detection to uncover redox properties of vanadium oxide supported on anatase titanium dioxide nanoparticles (VOx/TiO2) (Figure 2) [6].…”