Nanoscale 3D Chemical Mapping by Spectroscopic Electron Tomography
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Nanoscale 3D Chemical Mapping by Spectroscopic Electron Tomography Günter Möbus1,2, Ron C. Doole2, Beverley J. Inkson1,2 Dept of Engineering Materials, University of Sheffield, Sheffield S1 3JD, UK Dept of Materials, University of Oxford, Oxford OX1 3PH, UK ABSTRACT Electron Tomography is shown to be applicable to problems of materials science if a contrast mechanism is used which provides a projection relationship for crystals not depending on lattice plane orientation. Energy filtered TEM (EFTEM) in its mode of electron spectroscopic imaging (ESI) and STEM-EDX-Mapping are, subject to limitations, suitable image formation techniques. The spectroscopic operation not only allows to overcome Bragg scattering artefacts, but offers the possibility of recording 4-dimensional data (volume and energy) of a region of interest, otherwise only known from NMR and XAS/XANES tomography at larger length-scales and from field-ion microscopy (atom probe) under restrictive conditions. INTRODUCTION Electron tomography is a successful discipline in the biomedical research area, where cell organelles, viruses, and macromolecular complexes can now be routinely reconstructed in 3D with close to 1 nm resolution [1,2]. Within the multiple choice of image-valued TEM-disciplines, presented in table I, biomedical carbon-based objects are imaged by either exploiting the weak-phase object (WPO) approximation within brightfield (BF) phase contrast TEM, or using absorption contrast for thicker specimens [3]. The 3D reconstruction is then simply achieved by the filtered backprojection algorithm [2]. Thicker, especially crystalline materials, and heavier atomic numbers are not treatable this way. The projected image depends strongly on (i) bend contours, (ii) dislocation contrast, (iii) grain-to-grain contrast, or most generally: (iv) variation of greyscale upon tilt-series even for a perfect single-crystal particle due to lattice plane orientation (zone-axis deviations, excitation error), rather than projected mass-thickness. Residual phase-contrast also introduces strong focus dependency (Figure 1). Table I. Tomographic suitability of common TEM contrast mechanisms. -----------------------------------------------------------------------------------------------------------Techniques BF-Absorption WPO, HREM EELS/ESI EDX-Mapping HAADF-STEM
Properties/Limitations low resolution, amorphous objects only only very thin, light atoms, binary tomography? thickness up to 50…300nm, residual Bragg contrast extreme exposure times, bad SNR, geometry restriction better resolution than EELS/EDX-mapping
Spectroscopic no no / Z-contrast? yes yes no / Z-contrast
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Figure 1: Two BF-TEM image pairs of an Al2O3 nanocrystal: (a,b) two focus values, (c,d) two tilt values, 5 degrees apart. Intensity fluctuations complicating tomography. Chemical mapping [4,5] techniques based on EELS-imaging and EDX-mapping have been proven to be applicable for spectroscopic tomography [6-9] for crystalline materials of heavier atomic number, also comprising the case whe
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