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 Kotula et al. (2006)

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

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

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”

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

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

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

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

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

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

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 20-50 nm 1 nA in 1-2 nm From Williams and Carter, Transmission Electron Microscopy, Springer, 1996

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

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

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)

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

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

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

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

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) 362-365

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

High Resolution Quantitative Maps of Thin Specimens Ni 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

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

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

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

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

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

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