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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AEM Analysis of NanoparticlesCharles Lyman Lehigh University, Bethlehem, PA
Based on presentations developed for Lehigh University semester courses and for the Lehigh Microscopy School
PASI - Electron Microscopy - Chile
First x-ray dot map
Duncumb and Cosslett (1956)
Dark-field images
Centered dark-field (tilted beam)
X-ray elemental images (x-ray maps)
Specimen thickness: 10 nm to 500 nm
Need counts, counts, counts
Auger elemental images
Special UHV instrument required
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
What elements are associated with each other?
Have I missed any elements?
Types of X-ray Mapping
Spectrum imaging
In the future:
PASI - Electron Microscopy - Chile
Dot Maps (since 1956)
density of x-ray dots photographed as beam scans (1 scan per element)
no intensity information
gray levels give intensity
can be made quantitative
Spectrum Images (since 1989)
no pre-set elements
PASI - Electron Microscopy - Chile
Single channel, single photograph
Time consuming
Qualitative only
Dim recording dot (100 sec frame)
WDS dot maps of Fe Ka in bulk specimen
Optimum recording dot (100 sec frame)
Optimum recording dot (300 sec frame)
PASI - Electron Microscopy - Chile
Stored in computer
Quantitative maps possible
20% dead time
EDS x-ray map of bulk specimen
Fe
Background
Si
Ca
K
Al
PASI - Electron Microscopy - Chile
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
Silicon drift detector (EDS)
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
WDS map (300 sec)
EDS map (900 sec)
Better peak-to-background but WDS not currently used for thin specimens
Low Fe phase missed
Map a non-existant element
Mobile species
Lock element in place with 10 nm of sputtered Cr
Fe map
Background map
Coat with 10 nm of Cr
PASI - Electron Microscopy - Chile
Too few counts per pixel
Drift of specimen during long map
From Williams and Carter, Transmission Electron Microscopy, Springer, 1996
1 nA in 20-50 nm
1 nA in 1-2 nm
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W-gun STEM
FEG STEM
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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
Under-sampling
Do not use this M to obtain a quality map
Most of pixel not sampled
PASI - Electron Microscopy - Chile
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
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
Where are individual elements?
Elemental associations
each pixel may have 10-100 counts
significant counts when add > 500 pixels together
PASI - Electron Microscopy - Chile
20nm
40 nm
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
Red-orange-yellow-white
Primary color images
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
Thin metal alloy with precipitates
Quantitative map using z-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
Spectrum-Imaging
Available for EELS
Multivariate Statistical Analysis
On the Lehigh CD
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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
Data rate = 10,000 cts/sec
DT = 40% dead time
Acquisition time = 10 minutes
Spectrum Image of Granite
Na, Ca, and Ti might not show up in global spectrum
Courtesy of David Rohde
EELS energy filters
PASI - Electron Microscopy - Chile
Bottom row: elements not known to be present
Hunt and Williams, Ultramicroscopy 38 (1991) 47-73
200 nm
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