electron microscopy analysis of nm- sized particles and segregations frank krumeich and reinhard...
Post on 15-Jan-2016
224 views
TRANSCRIPT
Electron microscopy analysis of nm-sized particles and segregations
Frank Krumeich and Reinhard NesperETH Zurich, Laboratory of Inorganic Chemistry
[email protected] www.microscopy.ethz.ch
Electron microscopy: valuable tools for the characterization of nanomaterials
Scanning transmission electron microscopy STEM- HAADF-STEM- Combination with spectroscopic methods
Comparison of methods
Electron Microscopy Methods for the Characterization of Nanomaterials (Example: Vanadium Oxide Nanotubes)
SEM: characterization of tubular morphology
Cross-sections of VOx nanotubes: TEM and elemental maps obtained by electron spectroscopic imaging
V map C map
TEM: characterization of the wall structure
EELS: composition
STEM detectors
BFBright Field detector
ADFAnnular Dark Field detector (Θ = 0.5 - 3°)
Scanning Transmission Electron Microscopy (STEM)
HAADFHigh Angle Annular Dark Field detector (Θ > 3°)
2ed
d
σZ
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:
HAADF-STEM of Small Metal Particles
50 nm
10 nm
Au particles (bright contrast) on titania (Z contrast)
HAADF-STEM and EDXS: Point Analyses
Pd/Pt particles on alumina
PtPd
PtPt
Cu
Al
O
C
PtPd
Pt Pt
Cu
Al
O
C
HAADF-STEM and EDXS
WO 3 segregations in the oxidation product of Nb 4W 13O 47 (T ox=1000°C)
Krumeich, Nesper, J. Solid State Chem. 179 (2006) 1658
matrix
segregation
HAADF-STEM of Nb4W13O49
HAADF-STEM: Elemental Distribution
Single-crystal X-ray structure of Nb7W10O47
ca. 80% Nb 100% W
P21212 a=12.26, b=36.63, c=3.95 Å
(Krumeich, Wörle, Hussain, J. Solid State Chem. 149 (2000) 428)
High-Resolution Electron Microscopy
WO3 segregations in a bronze-type Nb-W oxide
HRTEM
2 nm
HAADF-STEM
Comparison: HRTEM ↔ HAADF-STEM
HRTEM HAADF-STEM
basicsinterference 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
• 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
• 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 EELS2. ESI
Benefits
Limitations
Transmission Electron Microscope
AcknowledgementsEMEZ: Electron Microscopy Center, ETH Hönggerberg
www.emez.ethz.ch
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
[email protected] www.microscopy.ethz.ch
Post-column filter