Transcript Slide 1

Electron microscopy analysis of nmsized 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
[email protected]
www.microscopy.ethz.ch
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
Scanning Transmission Electron Microscopy (STEM)
STEM detectors
HAADF
High Angle Annular Dark
Field detector (Θ > 3°)
BF
Bright Field detector
ADF
Annular Dark Field detector
(Θ = 0.5 - 3°)
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:
dσ e
2
Z
d
HAADF-STEM of Small Metal Particles
50 nm
10 nm
Au particles (bright contrast) on titania (Z contrast)
HAADF-STEM and EDXS: Point Analyses
Al
C
O
Pt
Pd
Cu
Pt
Pt
Al
C
O
Pd/Pt particles on alumina
Pt
Pd
Cu
Pt Pt
HAADF-STEM and EDXS
matrix
segregation
WO3 segregations in the oxidation product of Nb4W13O47 (Tox=1000°C)
Krumeich, Nesper, J. Solid State Chem. 179 (2006) 1658
HAADF-STEM: Elemental Distribution
ca. 80% Nb
100% W
Single-crystal X-ray structure
of Nb7W10O47
P21212 a=12.26, b=36.63, c=3.95 Å
(Krumeich, Wörle, Hussain, J. Solid
State Chem. 149 (2000) 428)
HAADF-STEM of Nb4W13O49
High-Resolution Electron Microscopy
HRTEM
WO3 segregations in a
bronze-type Nb-W oxide
HAADF-STEM
2 nm
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
Analytical Electron Microscopy
Benefits
• Qualitative and quantitative information about the
composition: EDXS, EELS
• Bonding, coordination, interatomic distances:
Fine structure in EELS (ELNES, EXEFS)
• Spatially resolved information about composition:
1. STEM + EDXS and/or EELS
2. ESI
Limitations • 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
Transmission Electron Microscope
Tecnai F30
Uacc= 300kV, field emission cathode (FEG)
SuperTwin lens: Cs = 1.15 mm, point resolution
d < 0.2 nm
Equipment: post-column imaging filter, STEM,
energy-dispersive X-ray spectrometer
Methods: TEM, HRTEM, STEM, ED, EDXS,
EELS, ESI, EFTEM
Post-column filter
Acknowledgements
EMEZ: Electron Microscopy Center, ETH Hönggerberg
www.emez.ethz.ch
[email protected]
www.microscopy.ethz.ch