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PASI - Electron Microscopy - Chile
1Lyman - Mapping
Compositional
Mapping Charles Lyman Lehigh University,
Bethlehem, PA
Based on presentations developed for Lehigh University semester courses and for the Lehigh Microscopy School
PASI - Electron Microscopy - Chile
2Lyman - Mapping
X-ray Mapping is 50 Years Old
First x-ray dot map» Duncumb and Cosslett (1956)
3-D tomographic map» Kotula et al. (2006)
PASI - Electron Microscopy - Chile
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Types of Compositional Images in TEM/STEM
Dark-field images» Phase-specific DF images (any TEM)
– Centered dark-field (tilted beam)– Displaced aperture dark-field
» High-angle annular dark-field (HAADF) STEM images
X-ray elemental images (x-ray maps)» Specimen thickness: 10 nm to 500 nm» Need counts, counts, counts
– Make large: probe current, thickness, counting rate, time
Auger elemental images» Images of elements on the surfaces» Special UHV instrument required
EELS elemental images» Specimen thickness: < 30 nm
PASI - Electron Microscopy - Chile
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X-ray Mapping Compared with Other Mapping Methods
Mapping detection limits assumed to be about 0.1 x point detection limit
Friel and Lyman, Microsc. Microanal. 12 (2006) 2-25
PASI - Electron Microscopy - Chile
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X-ray Mapping
Important Questions» Where are specific elements located?» What elements are associated with each other?» Have I missed any elements?
Types of X-ray Mapping Qualitative
Which elements are present? Quantitative
How much of each element is present? Spectrum imaging
Entire spectrum is collected at each pixel In the future: “Every image an analysis, every analysis an image”
PASI - Electron Microscopy - Chile
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X-ray Map Acquisition
Dot Maps (since 1956)» density of x-ray dots photographed as
beam scans (1 scan per element)» no intensity information
Digital Images (starting about 1980)» gray levels give intensity» many element maps collected in 1 scan» can be made quantitative
Spectrum Images (since 1989)» store a spectrum at each pixel» no pre-set elements» “mine the data” off-line
Friel and Lyman, Microsc. Microanal. 12 (2006) 2-25
PASI - Electron Microscopy - Chile
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X-ray Dot Maps
Early X-ray Dot Maps Advantages
» Any x-ray detector» Rapid scanning provides survey
Disadvantages» Record CRT brightness is a variable» Single channel, single photograph» One element at a time» Time consuming» Qualitative only
SE image of flat-polished basalt
Dim recording dot (100 sec frame)
WDS dot maps of Fe Kin bulk specimen
Optimum recording dot (100 sec frame)
Optimum recording dot (300 sec frame)
PASI - Electron Microscopy - Chile
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Digital X-ray Maps
Modern X-ray Maps Advantages
» Up to 16 selected elements » Stored in computer» Photograph later» Dwell time per pixel» Background subtraction and
quantitation possible» Quantitative maps possible
Disadvantages» None
Collection parameters: 128x128 pixels 55 ms dwell time per pixel 20% dead time Total frame time = 15 min (900 sec)
EDS x-ray map of bulk specimenFe Background Si
Ca K Al
SE image of flat-polished basalt
PASI - Electron Microscopy - Chile
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Maximizing the Collected X-ray Counts
Maximize counts» Set pulse processor to a short
processing time for high count rate:
– 2,000 cps at 135 eV (long )– 10,000 cps at 160 eV (short )
» Use 50-60% dead time» More counts for same collection
(clock) time» Thin specimens rarely produce
high count rates Silicon drift detector (EDS)
» > 500,000 cps
Elemental detection» Collect > 8 counts/pixel to assure
element is present above background
Low count rate
Mid- count rate
High count rate
Low Fe counts
High Fe counts
0
1
5
11
8
59
Bulk specimen of basalt
PASI - Electron Microscopy - Chile
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WDS maps vs. EDS maps
FeFe Fe
WDS map (300 sec) EDS map (900 sec)
Better peak-to-background but WDS not currently used for thin specimens
Low Fe phase missed
PASI - Electron Microscopy - Chile
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X-ray Map Artifacts
Continuum image artifact» Collect a map for every element known in
specimen » Map a non-existant element
– null-element or continuum background map
Mobile species» Certain elements (e.g. Na, S) move under the
beam» Lock element in place with 10 nm of sputtered Cr
Fe map Background map
Coat with 10 nm of Cr
Friel and Lyman, Microsc. Microanal. 12 (2006) 2-25
PASI - Electron Microscopy - Chile
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Small Thin-Specimen Excitation Volume
From Williams and Carter, Transmission Electron Microscopy, Springer, 1996
1 nA in 20-50 nm 1 nA in 1-2 nm
Most serious problem for thin specimen map» Too few counts per pixel» Drift of specimen during long map
PASI - Electron Microscopy - Chile
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Maximum Map Magnification
Friel and Lyman, Microsc. Microanal. 12 (2006) 2-25
W-gun STEM
FEG STEM
For ~1 nA probe current
PASI - Electron Microscopy - Chile
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Oversampling & Undersampling
Field-emission STEM• Beam size ~ 2 nm
(~ 1nA)• R = x-ray spatial resolution
including beam size and beam spreading
• Let R = 2 nm = 1 pixel N = 128 pixels in a line L = 10 cm screen width
• M ≈ 400,000x
Over-sampling• M > 400,000x• M to 1,000,000x is OK
Under-sampling• M < 400,000x• M << 400,000x (survey)
Do not use this M to obtain a quality map
Most of pixel not sampled
M L
RN
PASI - Electron Microscopy - Chile
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Field-Emission STEM X-ray Maps
Pt-Rh catalyst sulfided with SO2
S. Choi, M.S. Thesis, Lehigh University (2001)
Map setup: probe size 2nm, probe current 0.5 nA, 128x128, 100 ms/pixel Original magnification = 500,000x
50 nm
ADF Image Pt map S map Background map
PASI - Electron Microscopy - Chile
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W-Gun Thin Specimen X-ray Maps
Map setup» 128x128 pixels» 2.6 sec/pixel» 12 hours» Original M ~ 10,000x
Images from Wong et al. quoted in Friel and Lyman, Microsc. Microanal. 12 (2006) 2-25
Freeze-dried section of rat parotid gland
PASI - Electron Microscopy - Chile
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Uses of Compositional Images
Location of elements and phases» Where are individual elements?» How does element concentration change (qualitatively)?
Elemental associations» How are elements combined?
Particle and precipitate sizing» classification by chemistry and size
Quantitative analysis using stored maps» combine pixels within a phase» each pixel may have 10-100 counts» significant counts when add > 500 pixels together
PASI - Electron Microscopy - Chile
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STEM-EDS Elemental Maps from Au-Ag Nanoparticles
20nm
STEM-ADF image Ag map (Ag L) Au map (Au L)
Courtesy of M. Watanabe
PASI - Electron Microscopy - Chile
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Profiles from Elemental Maps
20nm
STEM-ADF image
Courtesy of M. Watanabe
PASI - Electron Microscopy - Chile
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40 nm
STEM-XEDS Analysis of Au-Pd/TiO2 Particles for Peroxide Synthesis
ADF Image Au Map Pd Map
O Map Ti Map RGB Image Red = Ti
Green = Pd
Blue = Au
Courtesy C. Kiely, published in Enache et al., Science 311 (2006) 362-365
40 nm
PASI - Electron Microscopy - Chile
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Color in X-ray Maps
Thermal color scale (look up table)» Red-orange-yellow-white» Indicates intensity in quantitative maps
Primary color images» red=Si; green=Al; blue=Mg» yellow = red+green
(yellow shows location of Si+Al)
From Goldstein et al., Scanning Electron Microcopy and X-ray Microanalysis, Springer, 2003
From Newbury et al., Advanced Scanning Electron Microcopy and X-ray Microanalysis, Plenum, 1986
PASI - Electron Microscopy - Chile
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High Resolution Quantitative Maps of Thin Specimens
Thin metal alloy with precipitates
Quantitative map using-factor analysis
» Developed by M. Watanabe
Williams et al., High Resolution X-ray Mapping in the STEM, J. Electron Microsc 51 (suppl.) 2002, S113-S126
Ni
Al Mo
Specimen: Ni base alloy
PASI - Electron Microscopy - Chile
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Recent Ways to Find Element Associations
Spectrum-Imaging» Available from most EDS companies» Available for EELS
Multivariate Statistical Analysis » Next lecture
LISPIX» Powerful image processing program by D. Bright (NIST)» Color overlays, scatter diagrams, mining spectrum-image data
cubes» On the Lehigh CD
PASI - Electron Microscopy - Chile
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Spectrum Imaging: A Spectrum at Every Pixel
Collect a spectrum at each pixel» Best way to analyze unknowns
Collect ‘x-y-energy’ data cube» 256x196 pixels x1024 channels x32bit spectra
(for spectrum image of granite)
Use good EDS mapping practice» Specimen: bulk, flat polished» Vo = 15 kV» Ip = 2.9 nA» M = 600x» Dwell time = 0.13 µs per pixel» Data rate = 10,000 cts/sec» DT = 40% dead time» Acquisition time = 10 minutes
x
y
energy
Courtesy of D. Rohde
Specimen: polished granite
PASI - Electron Microscopy - Chile
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Spectrum Image of Granite
Na, Ca, and Ti might not show up in global spectrum
Courtesy of David Rohde Specimen: polished granite
PASI - Electron Microscopy - Chile
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Compositional Mapping in EELS
Sequential EELS mapping in STEM EELS energy filters
From Williams and Carter, Transmission Electron Microscopy, Springer, 1996
PASI - Electron Microscopy - Chile
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EELS Spectrum Image
Cu Co Be O
Ti V Cr Fe
200 nm
Top row: elements known to be present in beryllium-copper
Bottom row: elements not known to be presentHunt and Williams, Ultramicroscopy 38 (1991) 47-73
PASI - Electron Microscopy - Chile
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Summary
X-ray Mapping» Thickness not critical» Match pixel size to x-ray excitation volume» Collect as many counts as possible» Always map for an element that is not present
(background map)
EELS Mapping» Higher spatial resolution than x-ray mapping (since
beam spreading is not an issue)» Specimen must be very thin