Transmission Electron Microscopy

Transcription

1 Transmission Electron Microscopy Wacek Swiech, CQ Chen, Jim Mabon, Honghui Zhou and Matt Bresin Frederick Seitz Materials Research Laboratory University of Illinois at Urbana-Champaign

2 Outline 1. Introduction to TEM 2. Basic Concepts 3. Basic TEM techniques Diffraction Bright Field & Dark Field TEM imaging High Resolution TEM Imaging 4. Scanning Transmission Electron Microscopy (STEM) 5. Aberration-corrected STEM / TEM & Applications 6. Microspectroscopy & spectromicroscopy X-ray Energy Dispersive Spectroscopy Electron Energy-Loss Spectroscopy 7. In-Situ TEM 8. Summary 2

3 Why Use Transmission Electron Microscopy? Optical Microscope Transmission Electron Microscope (TEM) High Resolution TEM Resolution 200 kev electrons l: Å < 500 nm A N is 0.95 with air up to 1.5 with oil Resolution limit: ~200 nm R = 0.66(C s λ 3 ) 1/4 Resolution limit: ~0.15 nm Sample thickness requirement: Thinner than 500 nm High quality image: <20 nm TEM is ideal for investigating thin foil, thin edge, and nanoparticles 3

4 e- x 10^3 Basic Concepts Electron-Sample Interaction 1. Transmitted electrons (beam) 2. Diffracted electrons (beams) (Elastically scattered) 3. Coherent beams 4. Incoherent beams 5. Inelastically scattered electrons 6. Characteristic X-rays Incident high-kv beam Specimen X-rays Incoherent beam (STEM) Diffracted beam Coherent beam Transmitted beam Inelastically scattered electrons magnetic prism TEM can acquire images, diffraction patterns, spectroscopy and chemically sensitive images at sub-nanometer to nanometer resolution. 4

5 Structure of a TEM Gun: LaB 6, FEG keV Gun and Illumination part TEM Sample Objective lens part View screen Mode selection and Magnification part 5

6 How does a TEM Obtain Image and Diffraction? Incident Electrons Sample < 500 nm Objective Lens Back Focal Plane Back Focal Plane Structural info First Image Plane First Image Plane u v Morphology 1 _ = u v f Object Image Conjugate planes View Screen 6

7 Electron Diffraction I SOLZ FOLZ d ZOLZ q Zero-order Laue Zone (ZOLZ) First-order Laue Zone (FOLZ). Bragg s Law 2 d sin q = nl Wavelength X-ray: about 1A Electrons: 100kV High-order Laue Zone (HOLZ) l is small, Ewald sphere (1/l) is almost flat Ewald Sphere 7

8 Electron Diffraction II Diffraction patterns from single grain or multiple grains 50 nm Single crystal Polycrystal Amorphous 8

9 Selected Area Electron Diffraction (SAED) Major Diffraction Techniques 1) Selected-Area Electron Diffraction 2) NanoArea Electron Diffraction 3) Convergent Beam Electron diffraction Example of SAED SAED aperture Selected-area aperture J.G. Wen, A. Ehiasrian, I. Petrov 9

10 Selected Area Electron Diffraction (SAED) 1. Illuminate a large area of the specimen with a parallel beam. 2. Insert an aperture in the first image plane to select an area of the image. 3. Focus the first imaging lens on the back focal plane Al matrix Al-Cu matrix L. Reimer and H. Kohl, Transmission Electron Microscopy, Physics of Image Formation,

11 Selected Area Electron Diffraction Advantages 1. Can observe a large area of specimen with a bright beam 2. Since the image plane is magnified, can easily select an area with an aperture A 50 mm aperture can select a 2 mm area 3. Useful to study thick films, bulk samples, and in-situ phase transformations Spherical Aberrations Disadvantages 1. Cannot select an area less than ~1 mm due to spherical aberrations and precision in aperture position 2. Difficult to record diffraction from individual nanoparticles or thin films Selected Area Aperture 11

12 Nanoarea Electron Diffraction (NAED) Diffraction from a single Au rod 55 nm nanoprobe Technique 1. Focus the beam on the front focal plane of the objective lens and use a small condenser aperture to limit beam size 2. A parallel beam without selection errors A 10 mm aperture can form a ~ 50 nm probe size 3. Useful to investigate individual nanocrystals and superlattices A. B. Shah, S. Sivapalan, and C. Murphy Disadvantages 1. Very weak beam difficult to see and tilt the sample 2. More complex alignment than conventional TEM 3. High resolution images are difficult to obtain Need to switch between NAED and TEM modes) 12

13 Nanoarea Electron Diffraction 5 mm condenser aperture 30 nm M. Gao, J.M. Zuo, R.D. Twesten, I. Petrov, L.A. Nagahara & R. Zhang, Appl. Phys. Lett. 82, 2703 (2003) J.M. Zuo, I. Vartanyants, M. Gao, R. Zhang and L.A. Nagahara, Science, 300, 1419 (2003) This technique was developed by CMM 13

14 Aperture-Beam Nanoarea Electron Diffraction a) b) CeO S 011 S 020 S 002 C 111 C 220 C SrTiO 3 2 nm 5 n m 5 n m 20 mm condenser aperture 20 nm probe size v Condenser Aperture u Sample Conjugate planes This technique was developed by CMM 14

15 Applications of Electron Diffraction Tilting sample to obtain 3-D structure of a crystal Lattice parameter, space group, orientation relationship New materials discovered by TEM Quasi-crystal Carbon Nanotube 5-fold symmetry To identify new phases, TEM has advantages: 1) Small amount of materials 2) No need to be single phases 3) Determining composition by EDS or EELS 1, 2, 3, 4, 6-fold symmetry No 5-fold for a crystal Helical graphene sheet Disadvantage: needs experience More advanced electron diffraction techniques 15

16 Convergent Beam Electron Diffraction (CBED) Parallel beam Convergent-beam Sample Sample Back Focal Plane Large-angle bright-field CBED SAED CBED 1. Point and space group 2. Lattice parameter (3-D) strain field Bright-disk 3. Thickness 4. Defects Dark-disk Whole-pattern 16

17 Major Imaging Techniques Major Imaging Contrast Mechanisms: 1. Mass-thickness contrast 2. Diffraction contrast 3. Phase contrast 4. Z-contrast Mass-thickness contrast 1) Imaging techniques in TEM mode a) Bright-Field TEM (Diff. contrast) b) Dark-Field TEM (Diff. contrast) c) Weak-beam imaging hollow-cone dark-field imaging d) Lattice image (Phase) e) High-resolution Electron Microscopy (Phase) Simulation and interpretation 2) Imaging techniques in scanning transmission electron microscope (STEM) mode 1) Z-contrast imaging (Dark-Field) 2) Bright-Field STEM imaging 3) High-resolution Z-contrast imaging (Bright- & Dark-Field) 3) Spectrum imaging 1) Energy-Filtered TEM (TEM mode) 2) EELS mapping (STEM mode) 3) EDS mapping (STEM mode) 18

18 TEM Imaging Techniques Phase Contrast Image I. Diffraction Contrast Image: Contrast related to crystal orientation [111] Kikuchi Map [112] [001] [001] [110] Two-beam condition Transmitted beam Application: Morphology, defects, grain boundary, strain field, precipitates Diffracted beam 19

19 TEM Imaging Techniques II. Diffraction Contrast Image: Bright-field & Dark-field Imaging Sample Sample Objective Lens T D Aperture Back Focal Plane Diffraction Pattern Two-beam condition Bright-field Image First Image Plane Dark-field Image Bright-field Dark-field 20

20 TEM Imaging Techniques II. Diffraction Contrast Imaging At edge dislocation, strain from extra half plane of atoms causes atomic planes to bend. The angle between the incident beam and a few atomic planes becomes equal to the Bragg Angle Θ B. Dislocations & Stacking Faults Bright Field Image Specimen Principal Plane Near dislocations, electrons are strongly diffracted outside the objective aperture Objective Aperture R. F. Egerton, Physical Principles of Electron Microscopy, Diffracted Transmitted Beam Beam 21

21 Weak-beam Dark Field Imaging High-resolution dark-field imaging Near Bragg Condition g S g Taken by I. Petrov Exact Bragg condition Weak-beam means Large excitation error Planes do not satisfy Bragg diffraction Possible planes satisfy Bragg diffraction 1g 2g 3g Experimental weak-beam C.H. Lei Bright-field Weak-beam Dislocations can be imaged as 1.5 nm narrow lines 22

22 TEM Imaging Techniques II. Diffraction Contrast Image Two-beam condition for defects Howie-Whelan equation df 0 pi = f dz g exp{2pi(sz+g.r)} x g Dislocations Use g.b = 0 to determine Burgers vector b Stacking faults Phase = 2 p g R Each stacking fault changes phase 2 -p 3 Diffraction contrast images of typical defects Sample g Dislocations Dislocation loop Stacking faults 23

23 Lattice Beam Imaging Two-beam condition Many-beam condition [001] M. Marshall C.H. Lei 24

24 Lattice Imaging Delocalization effect from a Schottky-emission gun (S-FEG) From a LaB 6 Gun Field-Emission Gun Lattice image of film on substrates 25

25 High Resolution Transmission Electron Microscopy (HRTEM) 1 Df sch 2 Df sch f(x,y) = exp(isv t (x,y)) ~1 + i s V t (x,y) V t (x,y): projected potential Scherzer defocus 1 Df sch = (C s l) 2 Resolution limit 1 r sch = 0.66 C 4 s l Scherzer Defocus: Positive phase contrast black atoms 2 Scherzer Defocus: ("2nd Passband" defocus). Contrast Transfer Function is positive Negative phase contrast ("white atoms") Contrast transfer function J.G. Wen Simulation of images Software: Web-EMAPS (UIUC) MacTempas 26

26 e- x 10^3 Scanning Transmission Electron Microscopy Focused e-beam Dark-field Bright-field Probe size nm STEM x-rays Thickness <100 nm Incoherent Scattering i.e. Rutherford Coherent Scattering (i.e. Interference) Technique 1. Raster a converged probe across and collect the integrated signal on an annular detector (dark field) or a circular detector (bright field). 2. An incoherent image is chemically sensitive (Z-contrast) under certain collection angles 3. Annular dark field (ADF) STEM is directly interpretable and does not have contrast reversals or delocalization effects like HRTEM 4. STEM resolution is determined by the probe size, which is typically 0.2 to 0.5 nm for a modern FEG STEM. magnetic prism Inelastically scattered electrons 5. Since STEM images are collected serially, the resolution is typically limited by vibrations and stray fields 27

27 e- x 10^3 SEM vs STEM SEM Primary e-beam kev backscattered electrons STEM Primary e-beam kev x-rays secondary electrons <50 ev Auger electrons x-rays Probe size nm Thickness <100 nm Incoherent Scattering i.e. Rutherford Dark-field 1 mm Bright-field Coherent Scattering (i.e. Interference) STEM technique is similar to SEM, except the specimen is much thinner and we collect the transmitted electrons rather than the reflected magnetic electrons prism 28 Inelastically scattered electrons

28 TEM vs ADF-STEM Ge quantum dots on Si substrate Ir nanoparticles 1. STEM imaging gives better contrast 2. STEM images show Z- contrast TEM Z 10nm 5 nm q ADF-STEM Annular dark-field (ADF) detector I Z 2 Z-contrast image 5 nm Z-contrast imaging J.G. Wen L. Long 29

29 ADF STEM Applications Superlattice of LaMnO 3 -SrMnO 3 -SrTiO 3 Dopant atoms in Si A. B. Shah et al., 2008 P. M. Voyles et al.,

30 Bright Field STEM vs ADF STEM 40kX Si Ge Superlattice 1Mx 15Mx Ge x Si 1-x Si 40kX 1Mx 15Mx Ge x Si 1-x Si 31

31 1.3 mm ± 10 nm Spherical Aberration 2.4 m Since 1936, Scherzer proved that spherical and chromatic aberrations would ultimately limit the resolution of the electron microscope. The method to correct aberrations was well known, but experimental aberration correctors were not successful until ~1998 due to complexities in alignment and lack of computing power. 32

32 Spherical Aberration Correction C s + ( C s ) = 0 Before correction After correction 33

33 Spherical Aberration Corrector for TEM & STEM Hexapole C s Corrector Optical Axis Hexapole 1 Round Lens 1 Round Lens 2 N C s < 0 Uli Dahmen NCEM Max Haider CEOS, Germany Hexapole 2 N Harold Rose Univ. of Tech. Darmstadt e-beam 34

34 Spherical Aberration Corrector for STEM Quadrupole-Octupole C 3 -C 5 corrector First sub-å image resolved in a STEM Ondrej Krivanek Nion Company P. E. Batson et al., Nature,

35 JEOL JEM2200FS with Probe UIUC The STEM can obtain 1 Å spatial resolution Multilayer wall Diffuser Piezo Stage Only if the instabilities of the room are controlled 37

36 Spherical Aberration Correction Image corrector Probe corrector JEOL 2010F C s = 1 mm Probe size 0.3 nm JEOL 2200FS with probe corrector Probe size 0.1 nm 38

37 Improved Resolution and Contrast with C s Corrector 63 pm Aberration correction combined with a high stability environment and high quality specimens allow for atomic resolution imaging over a large area. 4k x 4k image shown. 39

38 Sub-Å test using GaN film along [211] zone axis TEAM Microscope at LBNL 63 pm Annular dark field STEM image of hexagonal GaN [211] Fourier transform of the image; image Fourier components extend to below 50 pm. 40

39 New Applications Possible Only with Aberration Correction Ronchigram 1. Better Contrast for STEM Imaging with smaller probe 2. Reduced delocalization for HRTEM imaging 3. Sub-2Å resolution imaging at low voltage ( kv) for TEM and STEM X more probe current in the STEM for EELS and EDS spectroscopy 41

40 Analytical (Scanning) Transmission Electron Microscopy X-ray Energy Dispersive Spectroscopy Cathodoluminescence (CL: Visible light ) Bremsstrahlung X-rays Characteristic X-rays Incident High-kV Beam Back scattered electrons (BSE) Secondary electrons (SE) Auger Electrons Electron-Hole pairs Specimen Absorbed Electrons Elastically scattered electrons Direct Beam Inelastically scattered electrons Electron Energy Loss Spectroscopy 42

41 Characteristic Signals: EELS, EDS, XPS Signals AES XPS Incoming photon: h (EDS) Energy Loss Photons (XAS) (EELS) The emission of Auger electrons is an alternative to X-ray emission as an ionized atom returns to its ground state 43

42 Experimental XEDS, XPS, & EELS Copper L shell NiO: O K-shell and Ni L shell EDS L3 L2 EDS Ni L3 XPS EELS L2 EELS Energy (ev) Energy (ev) Energy resolution, Spatial resolution, Elements resolving 44

43 EELS: Electron Optical System Post-Column Filter (GIF) Compare: Electron Energy Analyzer 686/spectprism.jpg Williams and Carter 2009 V 0 = (V 1 R 1 + V 2 R 2 )/2R 0 Casaxps.com 45

44 EELS: Electron Optical System In-Column (Omega) Filter 4 Dispersive elements Integrated into column 46

45 Experimental EELS NiO Conduction/V alence bands X

46 STEM-EELS/EDS Spectrum Imaging: Spatially Resolved Analysis Spectral Imaging: 3D/4D data cube x-y image, z spectrum (EDS/EELS) EDS y e- x 10^3 x Image at DE1 Image at DE2 Image at DE3 Image at DEn DE Sub nanometer in Cs-corrected STEM! 48

47 e- STEM-EELS Spectrum Imaging: Line Scan STO ev e- x 10^ ev nm nm x 10^ bkgd Spectrum Bkgd Edge 1 2 nm nm ev 49

48 EF-TEM (Energy-Filtered TEM): Parallel Beam Use the slit to select electrons of a specific energy Allow only those electrons to fall on the screen or CCD Analogy to dark-field imaging Dark Field TEM EF-TEM Imaging Gatan Review of EFTEM Fundamentals 50

49 EF-TEM Imaging: Using Core Loss Pre-edge image Post-edge image Post-edge 1 Post-edge 2 Post-edge Jump Ratio Imaging Three-window method Three Window Mapping Jump ratio 5 0 n m 5 0 n m 51

50 EF-TEM Imaging: Using Zero-loss Spectrum imaging ZLP Low-loss Unstained/osmicated cebellar cortex Elastic electrons only Inelastic fog removed Contrast enhanced (Good for medium thick samples) CBED Pattern Gatan Review of EFTEM Fundamentals For Z < 12, the inelastic cross-section is larger than elastic cross-section 52

51 ELLS Near Edge Fine Structure (ELNES): White Lines Williams and Carter 2009 White lines in transition metals Changes in the Cu L 2,3 edge with oxidation 53

52 Graphitic Tribological Layers in M-on-M Hip Replacements Science, 334, 1687, 2011 HREM image and EELS spectrum showing the graphitic nature of a thin surface layer 54

53 EDS Quantification Cliff-Lorimer equation for thin film EDS EELS (Ratio) Absolute quantification (atoms / nm²) Ik: sum of counts in edge k, IL: total intensity (including zero loss), σk: partial cross-section for ionization. D integration window, collection angle Williams and Carter

54 e- e- x 10^3 e- x 10^3 STEM-EELS Measurement of Local Thickness T=1.89 E P Thin specimen h h ne 2 P 2p 2p 0 m Log-ratio method t l ln( I I ) p o T t = thickness λ P = Plasmon MFP, I o = zero-loss peak ev ev I T = total spectrum Thick specimen E p 2E p ev Relative thickness: Reliable, Fast and online, High spatial resolution 56

55 Atomic ratio Quantitative Composition Analysis: Combining EDS-EELS Combine STEM-EDS & STEM-EELS Na/Te(Pb x S 1-x ) Relative thickness (l) 57

56 Comparison of EELS and EDS EELS (primary) Atomic composition Chemical bonding Electronic properties Surface properties. Spatial resolution: nm EDS (Secondary) Atomic composition only Spatial resolution: 1 nm 10 nm Energy resolution: ~ 1 ev Energy resolution: ~ 130 ev (Mn K α ) Relatively difficult to use The interpretation sometimes involves theoretical calculation High collection efficiency (close to 100 %) Sensitive to lighter elements Signal weak in high loss region Easy to use The interpretation is straightforward Low collection efficiency (1-3%) (tilting helps to some degree) Sensitive to heavier elements Low yield for light elements 58

57 Comparison of EELS and EDS: Primary vs Secondary EELS: Events causing ionization EDS: Events after Ionization EDS: low x-ray yield for lower Z EDS: Low energy x-ray below detection limit EDS: higher energy favorable (>1KV most favorable) EELS: Lower loss favorable (<1KV most favorable) One Ionization Event (~EELS) Several Relaxation Events (~EDS/AES) E loss > E c >E EDS 59

58 Resolution and Sensitivity (Detection Limit) Beam diameter & spreading Bulk Thin Ultra-thin E0 energy, t thickness, Z Typical MMF: 0.1 1%, goes inversely with R Thin foil: better R, worse MMF FEG higher current/ energy CS correction improves both 0.01% 0.01% 0.01% 3x10 6 atoms 300 atoms 1 atom Williams and Carter 2009 XEDS-TEM: approaching atomic-level detection and sub-nm spatial resolution MMF (~0.01%) correspond to ~ 10 7, 300, and <1 atom 60

59 Ag-Au Core-Shell : Elemental (EDS) Mapping Science 337, 954 (2012) 61

60 List of TEMs and Functions at UIUC 1. JEOL JEM 2010 LaB6 TEM TEM, low dose, NBD, HRTEM, in-situ experiments JEOL 2010 LaB6 JEOL 2100 Cryo 2. JEOL JEM 2100 LaB6 (Cryo-TEM) TEM, Low dose, special cryo-shielding 3. Hitachi H-9500 TEM (LaB6, 300 kv) Environmental TEM (in-situ heating and gas reaction) Gatan K2 camera for fast recording at 400 frames per second or 1600 frames per second using partial frame. Dynamic TEM (expected) 3. JEOL JEM 2010 F Analytical (S)TEM TEM, BF, DF, NAED, CBED, EDS, STEM, EELS, EFTEM, Z-contrast imaging Hitachi H-9500 JEOL 2200FS 4. JEOL JEM 2200 FS C s corrected (S)TEM Ultra high resolution Z-contrast STEM, BF STEM, NBD, CBED, EDS, EELS, EFTEM 6. Hitachi H-8600 TEM (W) Biological samples, staff assisted only Heating stage (up to 1000 C) Cooling stage (with liquid N 2 ) JEOL 2010F 62

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