Electron Energy-loss Spectrometry & Energy-Filtering TEM

Electron Energy-loss Spectrometry & Energy-Filtering TEM Ferdinand Hofer

Institute for Electron Microscopy Graz University of Technology & Graz Centre for Electron Microscopy

www.felmi-zfe.at April, 19th, 2011, Belgrade 1

Outline

• Electron energy-loss spectrometry (EELS) • Energy-filtering TEM (EFTEM) • High Hi h energy resolution l ti EELS • Experiments with low energy-losses • New Developments: STEM-EELS-EDX

April, 19th, 2011, Belgrade 2

Microscopes & New Methods

„Environmental“ Scanning electron microscope FEI Quanta 600

2. Installation worldwide

Development of methods and applications HR-SEM

57 employees, 11 microscopes et al., ….. ~ 30 cooperations with research groups / year ~ 100 cooperations with ith industry i d t AFM ti VEECO 3500

Zeiss Ultra

+ glovebox

From basic to applied research in materials & biological sciences

Analytical HR-TEM Tecnai F20 FEI „„Focused ion beam“ microscope p NANOLAB Nova 200 FEI

1. Installation in Austria April, 19th, 2011, Belgrade 3

History y of EELS & EFTEM Gatan Imaging Filter, 1991

better spectrometers Cs correctors monochromators

LEO EM911 with Omega filter, 1991 Zeiss EM902 with Henry-Castaing filter, 1984 Gatan Ser. EELS, 1982

2000

1986 Gatan PEELS 1986 Shuman, post-column energy filter 1981-1991 Krahl & Rose, corr. :-filter 1974 Rose & Plies, proposal of magnetic :-filter

Hillier & Baker, EELS for microanalysis, 1945 1962 Henry & Castaing: double magnetic prism & electrostatic mirror Boersch with monochromator 1949 Möllenstedt with a retarding grid 1942 Ruthemann, 1. EELS spectrum April, 19th, 2011, Belgrade 4

Electron Specimen p Interactions in TEM Incident high energy electrons 60 300 kV 60-300

Auger electrons X-rays Secondary electrons

Thin specimen 10-200 nm

Elastically scattered electrons Inelastically scattered electrons

TEM, HREM, ED

EELS, EFTEM


Measurement of the EELS-Spectrum p Electrons Eo Specimen Eo - 'E

Energy-filter

EELS-spectrum

I Imaging i by bof selecting l ti electrons Spectrum inelastic 'E electrons = Electron with energy-loss spectrum = (EELS) energy-filtering TEM (EFTEM)


EELS & Energy-filtering gy g TEM Recording g of 1. energy-filtered images

EDX detector

Philips CM20 + Gatan Imaging Filterimprovement, (1993) thickness maps, contrast FEI2. Tecnai F20 + HR-Gatan IMaging FilterLi-U (2002) Elemental distribution maps,

3. Distribution of chemical bonding

R Recording di off energy-filtered filt d iimages For recording of EEL-spectra

In-Column-Filters: Zeiss-SMT, JEOL April, 19th, 2011, Belgrade 7

Electron Energy-loss gy Spectrometry p y (EELS) ( ) process

energy loss [eV] phonons 0.02 - 0.10 EELS spectrum of a 20 nm thin~TiC specimen inter-/intra-band transitions 5 - 25 plasmons ~ 5 - 25 4735x inner-shell ionizations ~ 30 - 1000 (5000) background

information content TE [mrad] 5 - 15 "heat" p p properties, p , band g gap p 5 - 10 optical < 0.1 free electron density 1-5 element, chemical bonding

Li - U Zero-loss peak intensity

Core excitations

Ti L23

CK

Valence excitations

0

100

Edge fine structures 200

300

400

500

600

Energy Loss (eV) April, 19th, 2011, Belgrade 8

EELS & EFTEM Information Content Low-loss region ('E < 50 eV) • Specimen S i thickness thi k • Valence and conduction electron density • Complex dielectric function High-loss region ('E > 50 eV) • Elemental composition Li Li-U U • Chemical bonding and electronic structure • Coordination numbers • Interatomic I t t i distances di t Some further advantages: • High signal collection efficiency (approaching 100%) • High spatial resolution (at limit of TEM/STEM probe size) • High energy resolution (at limit of TEM of beam energy spread & instrumental instabilities) April, 19th, 2011, Belgrade 9

Qualitative EELS Analysis y The simple case

Real world sample!!!! 165 eV = S L2,3 832 eV = La M5

532 eV = O K

227 eV = Mo M4,5 285 eV = C K 155 eV V=M Mo M3

779 eV = Co L3

= La-Co-Oxid

455 eV = Ti L2,3 532 eV =OK

= Mo-Sulfide with Ti-oxide inclusions April, 19th, 2011, Belgrade 10

Quantification Valid for very thin specimens, signal is integrated within integration window ' and collection angle E, useful for biological samples z e r o - lo s s peak

4735x

b a c k g ro u n d

(E ,' ) IAA(E, (E,') )

intensity

N

Ilow(E,') ' 0

50

' 200

300

400

e n e r g y - lo s s ( e V )

NA

I A E , ' E , ' V A E , ' I low l l loss

NA number of atoms per unit area Ilow(E,') intensity in low-loss region VA(E,') (E ') partial ionization cross cross-section section April, 19th, 2011, Belgrade 11

Quantitative EELS: Y-Ba-Cu-oxide YBa2Cu3O7, recorded at E0=200kV, E=7.6 mrad, D=1.5 mrad, t/O=0.20

Hofer & Kothleitner, Microsc.Microanal.Microstr. 7 (1996) 265 April, 19th, 2011, Belgrade 12

Chemical Bonding g and Band Structure ELNES = Electron Energy Loss Near Edge Structure a XANES = x-ray absorption near edge structure occupied

1s

bonding orbitals

unoccupied states

antibonding

„free“ electron states

B K edge

ELNES

EXELFS


Energy-filtering TEM ( (EFTEM) ) Electron spectroscopic p p imaging g g (ESI)


Zero-Loss Zero Loss Filtering Zero-loss filtering – removal of blurring effect of inelastic scattering U filt d bright Unfiltered b i ht field fi ld image i

Zero-loss filtered bright field image (0 ± 5) eV

200 nm

EFTEM zero loss imaging is essential for quantitative CBED F Hofer, P Warbichler; in “Transmission EELS in Materials Science”, eds. Disko & Ahn, Wiley (2004) April, 19th, 2011, Belgrade 15

Contrast Tuning by EFTEM Bright field image

Energy-filtered image @ (50 ± 2.5) eV

F Hofer, P Warbichler; in “Transmission EELS in Materials Science”, eds. Disko & Ahn, Wiley (2004) April, 19th, 2011, Belgrade 16

Energy-filtering TEM (EFTEM) Imaging of ionisation edges (elemental maps) 4735 4735x

zero-loss peak

background

core excitations

intensity

Ti L23 CK

valence excitations

0

100

200

300

400

500

600

Energy Loss (eV) EELS spectrum of a 20 nm thin TiC specimen April, 19th, 2011, Belgrade 17

Secondary Phases in Materials Ferritic-martensitic 10% Cr steel with W and Mo Type GX12CrMoWVNbN 10000 hours at 480oC

Fe M2,3 jump ratio image with rocking beam illumination

? Volume fraction of secondary phases

0.5 Pm TEM bright field image

Reduced diffraction features !

Hofer & Warbichler, Ultramicroscopy 63 (1996) 21 April, 19th, 2011, Belgrade 18

EFTEM & Steel Research Creep-resistant 10% Cr steel with W and Mo, 10000 hours at 480oC RGB-image: red = Mo, green = Cr, blue = V

Volume fraction of secondary

0.5 m phases = 7 vol%

0.5 µm

Fe2(Mo,W)

Particle size frequency curves measured d ffrom jump j ratio ti iimages

(Cr,Fe,Mo,W)23C6

VN

Hofer & Warbichler Warbichler, in: Transmission EELS in Materials Science, Springer (2004) April, 19th, 2011, Belgrade 19

EFTEM Concentration Maps p Quantitative phase distribution in a Ba-Nd-titanate ceramic

Nd2Ti2O7 0.01 r 0.04 BaNd titanate 0.31 r 0.07 0

0.3

Ba rich phase 500 nm

TEM image

Ba/Nd / atomic ratio map

-0.16 0 16

Ba/Nd

13 1.3

Reduction of diffraction effects & thickness variations F. Hofer et al., Ultramicroscopy 67 (1998) 63 April, 19th, 2011, Belgrade 20

Spatial Resolution EFTEM 2

d

'E 2 O 2 3 2 R ((C C E ) ((C S D ) (0.6 ) E0 E 2

Delocalisation of inelastic scattering

• Diffraction limit

• important for low energies 'E