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PASI - Electron Microscopy - Chile 1 Lyman - Mapping Compositio nal Mapping Charles Lyman Lehigh University, Bethlehem, PA Based on presentations developed for Lehigh University semester courses and for the Lehigh Microscopy School

Compositional Mapping

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Compositional Mapping. Charles Lyman Lehigh University, Bethlehem, PA. Based on presentations developed for Lehigh University semester courses and for the Lehigh Microscopy School. X-ray Mapping is 50 Years Old. First x-ray dot map Duncumb and Cosslett (1956) 3-D tomographic map - PowerPoint PPT Presentation

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Page 1: Compositional  Mapping

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

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X-ray Mapping is 50 Years Old

First x-ray dot map» Duncumb and Cosslett (1956)

3-D tomographic map» Kotula et al. (2006)

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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

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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

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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”

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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

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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)

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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Profiles from Elemental Maps

20nm

STEM-ADF image

Courtesy of M. Watanabe

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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

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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

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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

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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

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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

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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

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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

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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

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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