Electron%20microscopy%20analysis%20of%20nm-sized%20particles%20and%20segregations

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Electron microscopy analysis of nm-sized
particles and segregations

• Frank Krumeich and Reinhard Nesper
• ETH Zurich, Laboratory of Inorganic Chemistry

• Electron microscopy valuable tools for the
  characterization of
  nanomaterials
• Scanning transmission electron microscopy
  STEM - HAADF-STEM - Combination with
  spectroscopic methods
• Comparison of methods

krumeich_at_inorg.chem.ethz.ch
www.microscopy.ethz.ch
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Electron Microscopy Methods for the
Characterization of Nanomaterials (Example
Vanadium Oxide Nanotubes)
SEM characterization of tubular morphology
EELS composition
V map
C map
Cross-sections of VOx nanotubes TEM and
elemental maps obtained by electron spectroscopic
imaging
TEM characterization of the wall structure
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STEM detectors
Scanning Transmission Electron Microscopy (STEM)
HAADFHigh Angle Annular Dark Field detector (T gt
3)
BFBright Field detector ADFAnnular Dark Field
detector (T 0.5 - 3)
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Scattering of Electrons at an Atom
Strong Coulomb interaction of an electron with
the nucleus ? scattering into high angles or
even backwards High angle annular dark field
detector (HAADF-STEM)? atomic-number (Z)
contrast
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HAADF-STEM of Small Metal Particles
50 nm
Au particles (bright contrast) on titania (Z
contrast)
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HAADF-STEM and EDXS Point Analyses
Pd/Pt particles on alumina
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HAADF-STEM and EDXS
WO3 segregations in the oxidation product of
Nb4W13O47 (Tox1000C)
Krumeich, Nesper, J. Solid State Chem. 179 (2006)
1658
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HAADF-STEM Elemental Distribution
HAADF-STEM of Nb4W13O49
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High-Resolution Electron Microscopy
HAADF-STEM
HRTEM
WO3 segregations in a bronze-type Nb-W oxide
2 nm
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Comparison HRTEM ? HAADF-STEM
HRTEM HAADF-STEM
basics interference of coherently scattered electron waves incoherent scattering
recording - time parallel 0.5 2 s serial 5-20 s (? problems)
cathode LaB6 (or FEG) FEG
resolution ca. 2 Å ca. 2 Å
obtainable information atomic positions (and elemental distribution) atomic positions and elemental distribution
image interpretation comparison with simulations Scherzer defocus atom columns dark direct atom columns always bright intensity Z2
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Analytical Electron Microscopy

• Qualitative and quantitative information about
  the composition EDXS, EELS
• Bonding, coordination, interatomic distances
  Fine structure in EELS (ELNES, EXEFS)
• Spatially resolved information about
  composition1. STEM EDXS and/or EELS2. ESI

Benefits

• Electron-matter interactions are mostly
  elastic ? high electron doses necessary
• Long measuring times ? high sample stability
  and absence of drift
• Ionization edges occur at different energies and
  are of different shape ? not all methods are
  equally suitable for all elements

Limitations
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Transmission Electron Microscope
Tecnai F30 Uacc 300kV, field emission cathode
(FEG) SuperTwin lens Cs 1.15 mm, point
resolution d lt 0.2 nm Equipment post-column
imaging filter, STEM, energy-dispersive X-ray
spectrometer Methods TEM, HRTEM, STEM, ED,
EDXS, EELS, ESI, EFTEM
Acknowledgements
EMEZ Electron Microscopy Center, ETH
Hönggerberg www.emez.ethz.ch
krumeich_at_inorg.chem.ethz.ch
www.microscopy.ethz.ch