Transmission Electron Microscopy

Transcription

1 Transmission Electron Microscopy Wacek Swiech, Honghui Zhou, Jim Mabon, Changqiang (CQ) Chen 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. Spectroscopy X-ray Energy Dispersive Spectroscopy Electron Energy-Loss Spectroscopy 7. In-Situ TEM 8. Summary

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: ~100 nm R = 0.66(C s λ 3 ) 1/4 Resolution limit: ~0.25 nm Sample thickness requirement: Thinner than 500 nm High quality image: <20 nm TEM is ideal for investigating thin foil, thin edge, and nanoparticles

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.

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

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

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

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

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

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, 2008.

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

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 Disadvantages A. B. Shah, S. Sivapalan, and C. Murphy 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)

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

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 v Condenser Aperture u Sample Conjugate planes 5 n m 20 mm condenser aperture 20 nm probe size This technique was developed by CMM

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

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

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

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 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 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 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 electrons magnetic prism 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 q 10nm 5 nm ADF-STEM Annular dark-field (ADF) detector I Z 2 Z-contrast imaging Z-contrast image J.G. Wen 5 nm L. Long

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., 2002

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

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 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, 2002.

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

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

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.

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.

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

40 Analytical (Scanning) Transmission Electron Microscopy 40 X-ray Energy Dispersive Spectroscopy Characteristic X-rays Bremsstrahlung X-rays Incident High-kV Beam Specimen Secondary electrons (SE) Back scattered electrons (BSE) Auger Electrons Elastically scattered electrons Forward Scattered electrons Inelastically scattered electrons Electron Energy Loss Spectroscopy

41 EDS and EELS in STEM Mode with Aberration Correction 41 ~ 1-10 nm Further refined probe Down to ~ nm Carries more current Spatially Resolved Analysis Sub nanometer in aberration-corrected STEM!

42 Some relevant transition events explored in EELS and EDS X-ray Energy Loss David Muller 2006: EDS: named by the initial core-hole state, e.g., Kα: 2p 1s EELS: named by the initial state, e.g., L 3 : 2p 3/2 [3S 1/2, 3d 3/2, 3d 5/2 ]

43 SEM X-ray Energy Dispersive Spectroscopy in STEM STEM Masashi Watanabe: X-ray Energy-Dispersive Spectrometry in Scanning Transmission Electron Microscopes in Scanning Transmission Electron Microscopy Imaging and Analysis reproduced from Williams and Carter: Transmission electron microscopy (2009) To High enhance spatial the resolution detection (~ nm sensitivity compared with ~ μm in SEM) refined incident probe Tilt the sample for a more efficient collection significantly reduced interaction volume (directly related to the specimen thickness) Use a higher probe current Use an aberration corrected STEM Degraded analytical sensitivity significantly reduced interaction volume (directly related to the specimen thickness) More collection probe limited current by can microscope be applied column in a detector similar probe configuration dimension (0.3 sr out of 4 π sr)

44 X-ray Energy Dispersive Spectroscopy Applications TEM mode spot, area STEM mode spot, line-scan and 2-D mapping Spatial resolution ~ 1 nm Line scan 2-D mapping HAADF voids Area Mapping Au Fe Ti Mo A B Ti Al A Cr Nb B Si Al Ga Au Ti Ti 0.85 Nb 0.15 metal ion etch A. Ehiasrian, I. Petrov Mo L. Wang

45 e- x 10^3 Electron Energy Loss Spectroscopy Electron Spectrometer 17/1686/spectprism.jpg Post-column Gatan Imaging Filter (GIF) In-column JEOL Omega-type Energy filter Gatan Imaging Filter (GIF)

46 Electron Energy Loss Spectrum ZLP Low-loss Ti-L Core-loss O-K ~ 50 ev Mn-L x100 La-M Zero-loss (ZLP) Energy Loss (ev) Low-loss spectrum (< 50 ev) due to interactions with weakly bound outer-shell electrons Plasmon peak (oscillations of weakly bound electrons) Inter- and Intra-Band transitions Core-loss spectrum (> 50 ev) due to interactions with tightly bound inner-shell electrons Inner shell ionization

47 Fe 2 O 3 Spectrum Comparison between EELS and EDS O-K bkgd Fe-L 3,2 Spectrum Bkgd Edge Energy Loss (ev) ev Fe-L ev Energy (KeV)

48 Correlations of Core-loss EELS Features With Electronic Structure Empty States Conduction/valence bands Core-shell energy levels O-2p O-1s Neighboring atoms Ni-3d Ni-2p Ni-M 3,2 O-K Ni-L 3,2 Gatan: Review of EELS Fundamentals

49 Electron Energy Loss Spectrum Applications Thickness measurement Low-Loss Measuring the collective excitations, e.g., the combined response of the valence electrons and the electron magnetic field. Specimen thickness Optical and electronic properties Polarization response (complex dielectric function) Band structure, e.g., band gap Gatan: Review of EELS Fundamentals Core-Loss Elemental composition: the core-level binding energy is unique Near edge fine structure (ELNES): transition is only possible to local unoccupied states information about electronic structure (i.e., bonding) Extended fine structures (EXELFS): atomspecific radical distribution of near neighbors

50 EELS Near Edge Structure (ELNES) Chemical Bonding π* - Sp 2 bonded C-C Sp 2 bonding C-H bonding C-C Sp 2 bonding C-H* bonding C-C Sp 3 bonding C-H bonding C-C Sp 3 bonding V J Keast, J. Microscopy, 203, 135 (2001)

51 EELS Near Edge Structure (ELNES) Oxidation State of Transition Metal Oxide CaMnO 3 LaMnO 3 Energy Loss (ev) H. Zhou LaMnO 3 M. Varela, et.al., PHYSICAL REVIEW B 79, (2009) CaMnO 3 Chemical Shift L 3 /L 2 ratio (White Line Ratio) O-K edge near edge fine structure (ELNES) (owing to the hybridization between O-2p and the transitional metal 3 d)

52 Counts Counts Counts STEM EELS Spectrum Imaging VO V-L 2 3, Spectrum Image ev VO 1+x V-L 3, ev 6000 NiO Ni-L 3, ev Survey Image Spectrum Image Extracted Spectrum V-L mapping Ni-L mapping Each image pixel carries one EELS spectrum spatial and spectroscopy information are stored together H. Zhou

53 Atomic-column Resolution Spectrum Imaging Atomic-column resolution Spectrum Imaging Ti-L O-K Mn-L La-M Scan Direction DE Electron Energy Loss Substrate Side Ti L O K Mn L La M LaMnO 3 SrTiO 3

54 Atomic-column Resolution Spectrum Imaging La 0.7 Sr 0.3 MnO 3 SrTiO 3 Superlattice La Ti Mn RGB Image D. A. Muller et al., Science, 2008.

55 Comparison of EELS and EDS EELS Atomic composition Chemical bonding Electronic properties Surface properties. Spatial resolution: nm EDS 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

56 Energy Filtered Transmission Electron Microscopy Forming images with electrons of selected energy loss 1. Unfiltered image or diffraction pattern (DP) is formed 2. Image or DP is transformed into the spectrum 3. Part of the spectrum is selected by energy-selecting slit 4. Selected part of spectrum is transformed back into an energy-filtered image or DP Gatan: Review of EFTEM Fundamentals

57 Energy Filtered Transmission Electron Microscopy Spectrum imaging Spectrum imaging ZLP imaging Plasmon imaging ZLP Edges Edge imaging Low-loss Ti O Mn La x Image at DE1 Image at DE2 Image at DE3 Image at DEn y DE e- x 10^3 EFTEM Spectrum Imaging: Energy filtered broad beam technique Fills data cube taking one image at each energy at a time Less time STEM Spectrum Imaging Focused probe method Fills data cube by taking one spectrum at each location at a time Less does

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

59 EFTEM Plasmon Peak imaging EFTEM Plasmon Peak imaging Spectrum imaging Spectrum image (20 images) Al mapping image W mapping image A B Al W 30 nm A B J.G. Wen

60 EFTEM Edge Peak imaging Pre-edge image Post-edge image Image Ti Three-window Jump Ratio Imaging method Ti EELS spectrum Si Three Window Mapping Jump ratio J.G. Wen

61 EFTEM Edge Peak imaging Gatan: Review of EFTEM Fundamentals

62 List of TEMs and Functions at UIUC 1. JEOL JEM 2010 LaB6 TEM TEM, low dose, NBD, HRTEM, in-situ experiments 2. JEOL JEM 2100 LaB6 (Cryo-TEM) TEM, Low dose, special cryo-shielding 3. JEOL JEM 2010 F Analytical (S)TEM TEM, BF, DF, NAED, CBED, EDS, STEM, EELS, EFTEM 4. JEOL JEM 2200 FS C s corrected (S)TEM Ultra high resolution Z-contrast STEM, BF STEM, NBD, CBED, EDS, EELS, EFTEM 5. Hitachi H-9500 TEM In-situ heating and gas reaction, HREM, NAED, 300 kv, LaB6 electron source Gatan K2 camera for fast recording at 400 frames per second or 1600 frames per second using partial frame. 6. Hitachi H-600 TEM, 100 kv For biological applications, staff-assisted only. JEOL 2010 LaB6 JEOL 2200FS Heating stage (up to 1000 C) Cooling stage (with liquid N 2 ) JEOL 2100 Cryo Hitachi H-9500 JEOL 2010F

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