Electron Energy-loss Spectrometry. Energy-Filtering TEM

Transcription

1 Electron Energy-loss Spectrometry & Energy-Filtering TEM Ferdinand Hofer Institute for Electron Microscopy Graz University of Technology & Graz Centre for Electron Microscopy 1 Outline Electron energy-loss spectrometry (EELS) Energy-filtering TEM (EFTEM) High energy resolution EELS Experiments with low energy-losses New Developments: STEM-EELS-EDX 2

2 Microscopes & New Methods 2. Installation worldwide Development of methods and applications 57 employees, 11 microscopes et al.,.. ~ 30 cooperations with research groups / year ~ 100 cooperations AFM VEECO with 3500 industry + glovebox From basic to applied research in materials & biological sciences Environmental Scanning electron microscope FEI Quanta 600 HR-SEM Zeiss Ultra Analytical HR-TEM Tecnai F20 FEI Focused ion beam microscope NANOLAB Nova 200 FEI 1. Installation in Austria 3 History of EELS & EFTEM Gatan Imaging Filter, 1991 LEO EM911 with Omega filter, 1991 better spectrometers Cs correctors monochromators Zeiss EM902 with Henry-Castaing filter, 1984 Gatan Ser. EELS, 1982 Hillier & Baker, EELS for microanalysis, Gatan PEELS 1986 Shuman, post-column energy filter Krahl & Rose, corr. -filter 1974 Rose & Plies, proposal of magnetic -filter 1962 Henry & Castaing: double magnetic prism & electrostatic mirror Boersch with monochromator 1949 Möllenstedt with a retarding grid 1942 Ruthemann, 1. EELS spectrum 4

3 Electron Specimen Interactions in TEM Incident high energy electrons kv X-rays Auger electrons Secondary electrons Thin specimen nm Elastically scattered electrons Inelastically scattered electrons TEM, HREM, ED EELS, EFTEM 5 Measurement of the EELS-Spectrum Electrons E o Specimen E o - E Energy-filter EELS-spectrum Spectrum Imaging by of selecting inelastic electrons = electrons Electron with energy-loss E spectrum = (EELS) energy-filtering TEM (EFTEM) 6

4 EELS & Energy-filtering g TEM EDX detector Recording of 1. energy-filtered images Philips CM20 thickness + Gatan maps, Imaging contrast Filter improvement, (1993) FEI 2. Tecnai Elemental F20 + HR-Gatan distribution IMaging maps, Filter Li-U (2002) 3. Distribution of chemical bonding Recording of energy-filtered images For recording of EEL-spectra In-Column-Filters: Zeiss-SMT, JEOL 7 Electron Energy-loss Spectrometry (EELS) process energy loss [ev] E [mrad] information content phonons EELS spectrum of a 20 nm thin ~ TiC 0.02 specimen "heat" <usually not accessible> inter-/intra-band transitions optical properties, p band gap plasmons ~ 5-25 < 0.1 free electron density inner-shell ionizations 4735x ~ background (5000) 1-5 element, chemical bonding intensity Zero-loss peak Core excitations C K Ti L 23 Li - U Valence excitations Edge fine structures Energy Loss (ev) 8

5 EELS & EFTEM Information Content Low-loss region ( E < 50 ev) Specimen thickness Valence and conduction electron density Complex dielectric function High-loss region ( E > 50 ev) Elemental composition Li-U Chemical bonding and electronic structure Coordination numbers Interatomic t distances 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) 9 Qualitative EELS Analysis The simple case 532 ev = O K Real world sample!!!! 165 ev = S L 2,3 832 ev 227 ev = Mo M 4,5 = La M ev = C K 779 ev = Co L ev = Mo M ev = Ti L 2,3 532 ev = O K = La-Co-Oxid = Mo-Sulfide with Ti-oxide inclusions 10

6 Quantification Valid for very thin specimens, signal is integrated within integration window and collection angle, useful for biological samples zero-loss peak 4735x background N A (, ) I A A(, (, ) intensity I low (, ) N A I energy-loss (ev) low loss I A,,, A N A number of atoms per unit area I low (, ) intensity in low-loss region A (, ) ) partial ionization cross-sectionsection 11 Quantitative EELS: Y-Ba-Cu-oxide YBa 2 Cu 3 O 7, recorded at E 0 =200kV, =7.6 mrad, =1.5 mrad, t/ =0.20 Hofer & Kothleitner, Microsc.Microanal.Microstr. 7 (1996)

7 Chemical Bonding and Band Structure ELNES = Electron Energy Loss Near Edge Structure XANES = x-ray absorption near edge structure occupied unoccupied states 1s bonding orbitals antibonding free electron states ELNES B K edge EXELFS 13 Energy-filtering TEM (EFTEM) Electron spectroscopic p imaging g (ESI) 14

8 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) 15 Contrast Tuning by EFTEM Bright field image Energy-filtered (50 ± 2.5) ev F Hofer, P Warbichler; in Transmission EELS in Materials Science, eds. Disko & Ahn, Wiley (2004) 16

9 Energy-filtering TEM (EFTEM) Imaging of ionisation edges (elemental maps) zero-loss peak 4735x background core excitations Ti L 23 nsity inte C K valence excitations Energy Loss (ev) EELS spectrum of a 20 nm thin TiC specimen 17 Secondary Phases in Materials Ferritic-martensitic 10% Cr steel with W and Mo Type GX12CrMoWVNbN Fe M 2,3 jump ratio image with hours at 480 o C rocking beam illumination? Volume fraction of secondary phases 0.5 m TEM bright field image Hofer & Warbichler, Ultramicroscopy 63 (1996) 21 Reduced diffraction features! 18

10 EFTEM & Steel Research Creep-resistant 10% Cr steel with W and Mo, hours at 480 o C RGB-image: red = Mo, green = Cr, blue = V Particle size frequency curves measuredfrom jump ratio images 0.5 µm Fe 2 (Mo,W) (Cr,Fe,Mo,W) 23 C 6 VN Volume fraction of secondary 0.5 m phases = 7 vol% Hofer & Warbichler, in: Transmission EELS in Materials Science, Springer (2004) 19 EFTEM Concentration Maps Quantitative phase distribution in a Ba-Nd-titanate ceramic Nd 2 Ti 2 O BaNd titanate Ba rich phase 500 nm TEM image Ba/Nd atomic ratio map Ba/Nd Reduction of diffraction effects & thickness variations F. Hofer et al., Ultramicroscopy 67 (1998) 63 20

11 Spatial Resolution EFTEM 2 2 E d R (C 2 (C E ) ) C S ( ) 0 2 Delocalisation of inelastic scattering important for low energies E<100eV small angles for STEM & EFTEM Chromatic aberrations of objective lens important for EFTEM increases with width of energy selecting slit Diffraction limit Spherical aberrations of objective lens for STEM & EFTEM Statistical noise and electronic instabilities High tension, specimen drift, PSF of detector,... Radiation damage!!! 21 Spatial Resolution of EFTEM & must be optimised for edge type and element, Kothleitner & Hofer, Micron (1998) for 200 kv TEM with Cs = 0,47 mm, Cc = 1 mm, = 20 ev, E = 500 ev Only with FEG (Schottky Emitter) 0.7 nm 0.3 nm Krivanek et al., J.Microsc. 180 (1995) Egerton & Crozier, Micron 28 (1997) 22

12 EELS With High Energy Resolution (HR-EELS) 23 How to Achieve High Energy Resolution? 1. Electron sources with better energy resolution LaB 6 ~ (inevat FWHM of zero loss peak) undersaturated ~ 0.7 FEG Schottky emitter ~ 0.6 Cold FEG ~ 0.35 without E < 0.3eV only with monochromators with e.g. Boersch, TH Berlin (1962) 2. Deconvolution of EELS-spectra (recorded at e.g. 0.7 ev?) using maximum entropy methods (Gloter, Colliex et al., Ultramicroscopy 2003) 24

13 Graz System (FEI/Gatan) Ultrascan 2k / DigiScan (Gatan) 02/2005 (DS: 1 st installation) Monochromator (FEI) 02/2003 (2 nd installation) Improved HT tank (FEI) 06/2002 (300kV tank) High resolution imaging filter (Gatan): 10/2001 TECNAI F20 (FEI) FEG-TEM with Super Twin lens: 12/2000 TEM resolution: info limit: STEM resolution: energy resolution: 0.24 nm 0.10 nm 0.20 nm 0.18 ev financial support: 25 The Monochromator gun lens FEG monochr. Monochromator aperture Wien-filter Beam Accelerator Magnetic field Electric field Selection slit Condenser lens 1 Slit Condenser lens 2 Monochromated Monochromator e-beam ON Dispersed e-beam Objective Specimen 26

14 The Shape Hitachi HF-300 (300kV, CFEG) + Gatan GIF2002 (FWHM 0.28 ev) Fermi tail FWHM +27% Cold FEG Monochromator Schottky FEG +62% Tunnelingt ail FEI Tecnai F20 (200kV, Schottky) + Gatan HR-GIF + monochromator (FWHM 0.22 ev) K.Kimoto et al. Micron 36 (2005) 185 Important t for VEELS, band gaps 27 High Energy Resolution EELS HR-EELS of Co L 2,3 of CoO recorded with a monochromator TEM LaB 6 ~ 065eV 0.65 FEG ~ 0.55 ev ~ 0.20 ev Calculated with crystalfield multiplet theory (FMF de Groot) Mitterbauer, Kothleitner et al., Ultramicroscopy (2005) 28

15 EELS & EFTEM Spectrum Imaging 29 EELS Spectrum Imaging g (SI) STEM-EELS spectrum imaging Jeanguillaume & Colliex, Ultramicroscopy (1989) EFTEM SI Sequential recording of energy-filtered images STEM-EELS SI E 30

16 Semiconductor Cross-section EFTEM-SI ev, 3eV steps, ev, 5 ev steps Ti L 2,3 NK OK Specimen preparation: Ion milling (Gatan) Acquisition with 200 kv TEM & GIF Tridium by M. Kundmann, Gatan Spatial drift correction with SDSD script by B. Schaffer (Ultramicroscopy 2004) 31 Valence EELS / Low-loss EELS Physical properties at nanometre resolution! Incident electron Dielectric properties Band gaps Bulk plasmons Surface plasmons & modes Intraband & interband transitions Atom cores Problems & limitations Polarization wave Valence electrons Delocalisation of inelastic electron scattering erenkov radiation for semiconductors Removal of the zero-loss peak 32

17 EFTEM SI of low-loss EELS with a very small energy-selecting slit width ( ev) with HR-GIF Real = deformed data cube Corrected = ideal data cube Correction for Non-isochromaticity Spatial drift Energy drift B.Schaffer, G.Kothleitner, W.Grogger, Ultramicroscopy 106 (2006) 1129 Available on Digital Micrograph Scripting Homepage: Plasmon position maps = physical property mapping e.g. Si-B-C-N ceramic, W.Sigle, Ultramicroscopy 96 (2003) 565 GaN-AlN-multilayer, B.Schaffer, Ultramicroscopy 106 (2006) 1129 Band gap mapping e.g. ZnO-TiO 2 -multilayer, G.Kothleitner, in: MSA-Proceedings (2007), Florida 33 Plasmon Position Map by EFTEM SI InAs quantum structures in InP matrix ~0.2 ev Cooperation with W. Neumann & H. Kirmse, Humboldt Universität Berlin 34

18 Low-loss Mapping by EFTEM-SI Au-nanorod with 0.3 ev slit (HR-GIF), step width 0.1 ev, 55 images total acquisition time = 5 min 512 x 512 pixel Raw data, unprocessed! carbon 0.7 ev 1.0 ev Bernhard Schaffer on Sunday, May 11, 2008, 22: ev 4.5 ev 100 nm 35 Monochromated EFTEM SI of Au (low-loss) Narrow slit (0.3 ev) combined with monochromator gives an energy resolution of ~0.4 ev. This allows EFTEM imaging close to the zero-loss peak: e.g. showing surface plasmon modes of Au nanoparticles. Image size: Energy range: Slit width: Energy steps: Energy resolution: Acq. time: 512 x 512 px -1 to 4.5 ev 0.3 ev 0.1 ev ~400 mev 17 min ev ev ev 36

19 Project ASTEM = Austrian Scanning Transmission Electron Microscope Scanning TEM mit kv Titan High-brightness gun (XFEG) Ultrafast scanning Super-EDX (ChemiSTEM) Cs-probe corrector Remote control microscope Specs: 0.07 nm resolution 0.40 na beam current / 1A probe 0.18 ev energy resolution Supported by 37 ASTEM Titan cubed from March 31 st April 5 th, 2011 Austrian Scanning Transmission Electron Microscope 38

20 ASTEM Inner Structure of Materials Atom columns in silicon crystal viewed in [110] orientation STEM HAADF image without Cs corrector STEM HAADF image with Cs corrector nm Advantage: better spatial resolution with higher SNR, shorter acquisition times 39 ASTEM Elemental Mapping Phase boundary in perowskite SrTiO 3 /BaTiO 3 Elemental mapping at the atomic scale using higher energy-losses viewed in [001] O Sr Atom columns Ti G.A. Botton et al., Ultramicroscopy 110 (2010) 925 Ba Sr Ti 40

21 EDX vs. EELS FEI Titan G2 (200 kv) FEI ChemiSTEM 10 ms dwell time/pixel both for EDX as well as EELS Two separate experiments (not simultaneous) Background-corrected, integrated intensities for the edges No post-data-acquisition processing other than colorization of the elemental signals Sampling: Å/pixel for EDX 0.32 Å/pixel for EELS Speed: 100, spectra/s (EDX) 1,000 spectra/s (EELS) Courtesy Bert Freitag (FEI) 41 Summary EELS elemental analysis of Li - U at nanometre resolution EFTEM for studying interfaces, secondary phases, defects in solids, biomaterials & devices, nanoparticles High energy-resolution EELS with monochromators (< 0.2 ev) Monochromated EFTEM spectrum imaging (0.5 ev, 1 nm) for elemental mapping, band gap and plasmon imaging Elemental mapping at atomic resolution (EDX & EELS) Bright Future: Atomic resolution analysis with Cs correctored microscopes, structure & local physical properties, e.g. in-situ microscopy, TEM-STM-AFM, tomography. DM Scripting Database: D. Mitchell & B. Schaffer, Ultramicroscopy (2005) 42