1. Compositional Mapping
Charles Lyman Lehigh University, Bethlehem, PA
Based on presentations developed for Lehigh University semester courses and for the Lehigh Microscopy School

2. X-ray Mapping is 50 Years Old
First x-ray dot map
Duncumb and Cosslett (1956)
3-D tomographic map
Kotula et al. (2006)

3. 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

4. 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

5. X-ray Mapping Important Questions Types of X-ray Mapping
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”

6. 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

7. X-ray Dot Maps Early X-ray Dot Maps
WDS dot maps of Fe Ka in bulk specimen
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
Dim recording dot (100 sec frame)
Optimum recording dot (100 sec frame)
Optimum recording dot (300 sec frame)
SE image of flat-polished basalt

8. Digital X-ray Maps EDS x-ray map of bulk specimen 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 Fe
Background Si Ca K Al
Collection parameters:
128x128 pixels
55 ms dwell time per pixel
20% dead time
Total frame time = 15 min (900 sec)
SE image of flat-polished basalt

9. Maximizing the Collected X-ray Counts
Maximize counts
Set pulse processor to a short processing time t for high count rate:
2,000 cps at 135 eV (long t)
10,000 cps at 160 eV (short t)
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 Fe counts
Low count rate
High Fe counts 1 5
Mid- count rate 11
High count rate 8 59
Bulk specimen of basalt

10. WDS maps vs. EDS maps WDS map (300 sec) EDS map (900 sec) Fe
Low Fe phase missed
WDS map (300 sec)
EDS map (900 sec)
Better peak-to-background but WDS not currently used for thin specimens

11. X-ray Map Artifacts Fe map Background map 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
Coat with 10 nm of Cr
Friel and Lyman, Microsc. Microanal. 12 (2006) 2-25

12. Small Thin-Specimen Excitation Volume
Most serious problem for thin specimen map
Too few counts per pixel
Drift of specimen during long map
1 nA in nm
1 nA in 1-2 nm
From Williams and Carter, Transmission Electron Microscopy, Springer, 1996

13. Maximum Map Magnification
W-gun STEM
FEG STEM
For ~1 nA probe current
Friel and Lyman, Microsc. Microanal. 12 (2006) 2-25

14. 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

15. Field-Emission STEM X-ray Maps
Map setup: probe size 2nm, probe current 0.5 nA, 128x128, 100 ms/pixel
Original magnification = 500,000x
Pt-Rh catalyst sulfided with SO2
ADF Image
Pt map
S map
Background map
50 nm
S. Choi, M.S. Thesis, Lehigh University (2001)

16. W-Gun Thin Specimen X-ray Maps
Map setup
128x128 pixels
2.6 sec/pixel
12 hours
Original M ~ 10,000x
Freeze-dried section of rat parotid gland
Images from Wong et al. quoted in Friel and Lyman, Microsc. Microanal. 12 (2006) 2-25

17. 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 counts
significant counts when add > 500 pixels together

18. STEM-EDS Elemental Maps from Au-Ag Nanoparticles
STEM-ADF image
Ag map (Ag La)
Au map (Au La)
20nm
Courtesy of M. Watanabe

19. Profiles from Elemental Maps
STEM-ADF image
20nm
Courtesy of M. Watanabe

20. STEM-XEDS Analysis of Au-Pd/TiO2 Particles for Peroxide Synthesis
ADF Image
Au Map
Pd Map
40 nm
40 nm
O Map
Ti Map
RGB Image
Red = Ti
Green = Pd
Blue = Au
Courtesy C. Kiely, published in Enache et al., Science 311 (2006)

21. 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

22. High Resolution Quantitative Maps of Thin SpecimensNi
Thin metal alloy with precipitates
Quantitative map using z-factor analysis
Developed by M. Watanabe Al Mo
Specimen: Ni base alloy
Williams et al., High Resolution X-ray Mapping in the STEM, J. Electron Microsc 51 (suppl.) 2002, S113-S126

23. 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

24. 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 y
energy x
Specimen: polished granite
Courtesy of D. Rohde

25. Spectrum Image of Granite
Na, Ca, and Ti might not show up in global spectrum
Specimen: polished granite
Courtesy of David Rohde

26. Compositional Mapping in EELS
Sequential EELS mapping in STEM
EELS energy filters
From Williams and Carter, Transmission Electron Microscopy, Springer, 1996

27. Cu Co Be O Ti V Cr Fe EELS Spectrum Image
Top row: elements known to be present in beryllium-copper Cu Co Be O
200 nm Ti V Cr Fe
Bottom row: elements not known to be present
Hunt and Williams, Ultramicroscopy 38 (1991) 47-73

28. Summary X-ray Mapping EELS 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