Introduction to Transmission Electron Microscopy (Physical Sciences)

Transcription

1 Introduction to Transmission Electron Microscopy (Physical Sciences) Centre for Advanced Microscopy Program 9:30 10:45 Lecture 1 Basics of TEM 10:45 11:00 Morning tea 11:00 12:15 Lecture 2 Diffraction 12:15 12:30 Practice 1 Indexing diffraction patterns 12:30 13:00 Demonstration 1 Basics of TEM operation 13:00 14:00 Lunch 14:00 15:15 Lecture 3 - Imaging 15:15 15:30 Afternoon tea 15:30 16:00 Demonstration 2 Collection of diffraction patterns and images 16:00 16:30 EDX, STEM Centre for Advanced Microscopy 1

2 Transmission Electron Microscopy and Diffractometry of Materials. Brent Fultz and James Howe Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Transmission Electron Microscopy: Physics of Image Formation. L. Reimer H. Kohl Introduction to Conventional Transmission Electron Microscopy. Marc de Graef Scanning Transmission Electron Microscopy: Imaging and Analysis. S. J. Penycock and P. Nellist Essential software: Digital micrograph, ImageJ Other useful software: JEMS (available in CAM), Crystal Maker, CRISP Contact: Felipe.Kremer@anu.edu.au For initial sessions and training in the TEM, please contact: Felipe.Kremer@anu.edu.au or Frank.Brink@anu.edu.au Notes Centre for Advanced Microscopy 2

3 Part 1 The basics of TEM Centre for Advanced Microscopy e-learning Room Introduction Centre for Advanced Microscopy 3

4 What is a microscope? A. An instrument used for viewing very small objects, such as mineral samples or animal or plant cells, typically magnified several hundred times. Light microscope vs TEM Centre for Advanced Microscopy 4

5 What do we know about electrons? What do we know about electrons: Amber = elektron ( ) Negatively charged particles => We can apply an electric field to accelerate it! Electron Duality Behave as waves => Diffraction patterns when passing by crystals Behave as particles => Excitation of inner shell electrons => x-ray chemical analysis What do we know about electrons? Electron Duality Wave characteristics of the electron gives rise to coherent scattering (diffraction) Particle characteristics gives rise to characteristic X-rays Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Centre for Advanced Microscopy 5

6 What do we know about electrons? de Broglie wavelength Wave characteristics => Planck s constant Wavelength CLASSICAL RELATIVISTIC Why electrons? Resolution Light microscopy vs electron microscopy Rayleigh criterion Wavelength of the radiation Numerical aperture Collection angle Ex.: Green light (~500 nm) ~ 300 nm Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Centre for Advanced Microscopy 6

7 Why electrons? Resolution Light microscopy vs electron microscopy Rayleigh criterion Wavelength of the radiation Numerical aperture Collection angle Ex.: Green light (~500 nm) ~ 300 nm Wavelength as a function of the acceleration voltage (100 kv) = 3.7x10-3 nm (200 kv) = 2.5x10-3 nm (300 kv) = 2.0x10-3 nm Wavelength five orders of magnitude smaller than visible light!!! Why electrons? Resolution Increasing aperture size Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Centre for Advanced Microscopy 7

8 Resolution vs Magnification A B C d Mag = D/d A has same Mag than C B has same Res than C D What else can we see? Multitude of signals resulting from the electron matter interaction Incident high kv beam Backscattered electrons (BE) Secondary electrons (SE) Characteristic X-rays Auger electrons Visible light Absorbed electrons Electron-hole pairs specimen Bremsstrahlung X-rays Elastically scattered electrons Inelastically scattered electrons Direct beam Centre for Advanced Microscopy 8

9 What else can we see? Multitude of signals resulting from the electron matter interaction Incident high kv beam Backscattered electrons (BE) Secondary electrons (SE) Characteristic X-rays Auger electrons Visible light Absorbed electrons Electron-hole pairs specimen Bremsstrahlung X-rays Elastically scattered electrons Inelastically scattered electrons Direct beam Techniques Crystallographic information: Electron diffraction Selected area diffraction (SAD) Convergent beam electron diffraction (CBED) Imaging Dark field / Bright field imaging High resolution (transmission) electron microscopy (HRTEM or HREM) Chemical analysis: X-rays Energy dispersive X-ray spectroscopy (EDS or EDX) Electrons Electron energy loss spectroscopy (EELS) Centre for Advanced Microscopy 9

10 Instrumentation Basic requirements: - Intermediate voltages (200, 300, 400 kev) - Brilliant source: LaB 6, FEG, W hairpin Possible extras: - Scanning transmission electron microscope (STEM) - Energy dispersive X-ray detector (EDS) - Electron energy loss spectometer (EELS) - X-ray and EELS mapping software - Cold stage, heating stage, tensile stage, bias stage, etc. - SE & BSE detectors Essential extras: - Diffraction and image simulation software - Image processing software TEM scheme Gun assembly Condenser system Objective system Projector system Viewing/recording system Centre for Advanced Microscopy 10

11 TEM scheme Gun assembly Condenser system Illumination Objective system Image formation Projector system Magnification Viewing/recording system Jeol 2100F TEM instrument FEI CM300 Gun chamber High voltage cable Condenser aperture Objective aperture Selected area aperture Left side control panel Viewing chamber Goniometer Cold trap Right side control panel Centre for Advanced Microscopy 11

12 Right control panel Spot size Diffraction/Image Focus reset Magnification Beam shift Focus Filament knob Beam deflectors Left control panel CCD control Negative exposure control Tilt control Fine/coarse Brightness Centre for Advanced Microscopy 12

13 Electron guns Brightness Temporal coherence Spatial coherence Diameter d o Emission current i c Divergence semi-angle 0 Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Electron guns LaB 6 crystal Thermionic emission Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Diameter d o Emission current i c Divergence semi-angle 0 Centre for Advanced Microscopy 13

14 Electron guns Thermionic emission Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Diameter d o Emission current i c Divergence semi-angle 0 Electron guns Cold FEG (W tip) Reproduced: Transmission Electron Microscopy: Physics of Image Formation L. Reimer H. Kohl Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Centre for Advanced Microscopy 14

15 Electron guns Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Lenses Centre for Advanced Microscopy 15

16 Magnetic lenses Electro-magnetic lenses Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Magnetic lenses Lorentz force Cylindrical coordinates Convergent lens that produces a rotation to the image. Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Centre for Advanced Microscopy 16

17 Magnetic lenses Electron entering the lens field Electron spiralling in the field Conventional optical ray diagram A e - A A A A A Focus and magnification Remember: Electro-magnetic lens behave like convergent lenses d o d f d i o i Centre for Advanced Microscopy 17

18 Focus and magnification Remember: Electro-magnetic lens behave like convergent lenses d f Object Back focal plane (power spectrum of the object Fourier Transform) Aberrations Spherical aberration C s Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Centre for Advanced Microscopy 18

19 Aberrations Spherical aberration C s Chromatic aberration C c Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter #1 Condenser lens system Centre for Advanced Microscopy 19

20 Core functions: Probe size Convergence angle Brightness Condenser lens system Different modes of operation: Parallel illumination Focused beam Translating and tilting the beam Components: C1, C2, C3 lens Condenser aperture Condenser stigmator Focus Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Centre for Advanced Microscopy 20

21 C1 role (spot size) <1 Demagnification of the source Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter C2 role (brightness) CO or c/o Condenser-objective system Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Centre for Advanced Microscopy 21

22 C2 operation Most microscopes employ a condenser mini-lens system. (S)TEM Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter C2 operation Parallel beam achieved in both under- and over-focus. Which one should I chose to operate the microscope? Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Centre for Advanced Microscopy 22

23 C2 operation Parallel illumination should be achieved in over focus condition R R Convergence angle ( ) Spatial coherence At the microscope Misaligned aperture Aligned aperture Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Centre for Advanced Microscopy 23

24 Scanning coils Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter y Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter x Condenser astigmatism Perfect lenses Real lenses Small compensating field At the microscope Result Focusing the beam (C2) results in a elliptical view of the source. Astigmatism present. Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Centre for Advanced Microscopy 24

25 More on astigmatism The difference in the focus point between the two axis leads to the ellipsoidal shape of the electron beam. #2 Post-specimen lenses Centre for Advanced Microscopy 25

26 Z position corresponding to the eucentric plane (reference plane to which the calibrations are reproducible). Hence the sample should be kept at this height for best performance. Eucentric height Object plane (Eucentric plane is perpendicular to the optic axis). Sample does not move laterally when tilted. Best position for analysis. At this position if the objective is in focus, then the objective-lens current is at a standard value. Therefore it is possible to use the same lens-current value independent of sample. It is the object plane. Change strength of objective lens Change the height z of the sample 1) Adjust the eucentric current using standard focus. Eucentric height Object plane 2) Adjust the right of the sample until minimum contrast is achieved. Change strength of objective lens Change the height z of the sample standard focus Centre for Advanced Microscopy 26

27 Diffraction and image modes Objective aperture Selected area diffraction aperture (SAD) Image mode Image Objective lens Object Back focal plane Intermediate lens From diffraction to image mode: change in the strength of the intermediate lens Projector lens Centre for Advanced Microscopy 27

28 Objective aperture Selected area diffraction aperture (SAD) Diffraction mode Objective lens DP Object Back focal plane Intermediate lens Projector lens Diffraction mode Objective lens CM 300 L = 420 mm L = 880 mm Intermediate lens Projector lens Diffraction pattern L = 245 mm Centre for Advanced Microscopy 28

29 Diffraction mode Hitachi H7100 Objective lens Intermediate lens Projector lens Diffraction pattern Image mode Objective lens M = 5900x Intermediate lens Si SiO 2 Si Projector lens Images M = 115kx M = 195kx Centre for Advanced Microscopy 29

30 Bright and Dark field Bright Field Dirty Dark Field Centred Dark Field Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Bright field Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Centre for Advanced Microscopy 30

31 Dirt dark field Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Centred dark field Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Centre for Advanced Microscopy 31

32 Centred dark field Weak beam Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Aligning apertures Selected area aperture (sits in the image plane, hence it is aligned in real space) Aligned Misaligned Objective aperture (sits in the back focal plane, hence it is aligned in reciprocal space) Aligned Misaligned Even distribution of intensities Even distribution of intensities Centre for Advanced Microscopy 32

33 TEM is primarily a scattering technique Optics in TEM In materials with long range order this mean that diffraction patterns will be generated. The main steps in a TEM experiment are: Locate the region of interest Adjust the eucentric height Isolate the feature of interest with the SAD aperture. Switch to diffraction mode. Tilt the sample. Select which diffracted beams will contribute to the image formation. Back to image mode. Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Summary Electron scattering as a imaging tool Electron sources The TEM construction Basic TEM operation Lenses Pre-specimen Condenser system control of electron beam illumination on the sample Post-specimen Intermediate lenses Image / Diffraction modes Projector lenses Magnification Image / Diffraction mode Bright field imaging Dirt and Centred dark field modes Centre for Advanced Microscopy 33

34 Part 2 Diffraction Centre for Advanced Microscopy e-learning Room Scattering First Born approximation Ψ. Δ Δ 2. Ψ. Scattering center Scattering factor is the Fourier Transform of the scattering potential. Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Centre for Advanced Microscopy 34

35 Scattering Coherent forward scattering Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials In a crystal Ψ. Δ Ψ Δ. Distance between source and detection unknown and not actually relevant. Only relative intensities will be measured. Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Centre for Advanced Microscopy 35

36 When do we get diffraction spots? Lattice translations Ψ Δ. Laue condition Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials When do we get diffraction spots? I Coherent illumination Source λ 0 θ Plane θ B θ B θ B d See intensity only when angle θ is such that path difference, 2d sin θ = nλ Bragg s Law, θ B = Bragg angle Centre for Advanced Microscopy 36

37 k 0 k 0 2 k k 0 k k 0 = k = k 2 sin k Hence, k k Diffraction k = g (Laue condition) = B 2 sin or 2 sin 1 21 sin By comparison 2 sin and Shape factor and structure factor Crystal = lattice + basis Shape factor Structure factor Centre for Advanced Microscopy 37

38 Structure factor rules (extinction conditions) Example: Simple cubic One atom basis (0 0 0) Structure Factor Calculations F (hkl) = Σ i f i e 2πiK r i F (hkl) = f e 2πi(0) No extinction condition Extinction conditions Example: [001] ZA, Cubic unit cell P IP Primitive unit cell Centre for Advanced Microscopy 38

39 Structure factor rules (extinction conditions) Example: Body centred cubic Two lattice points per unit cell (0 0 0) and ( ½ ½ ½) Structure Factor Calculations F (hkl) = Σ i f i e 2πiK r i F (hkl) = f e 2πi(0) + f e2πi( ½ h + ½ k + ½ l) F (hkl) = f (1 + e πi( h + k + l) ) (N.B: e iθ = cos θ + i sin θ) F (hkl) = 0 when h + k + l is odd F (hkl) = 2f when h + k+ l is even Extinction conditions Example: [001] ZA, Cubic unit cell I I Body centred unit cell Centre for Advanced Microscopy 39

40 Structure factor rules (extinction conditions) Example: Body centred cubic Two lattice points per unit cell (0 0 0) and ( ½ ½ ½) Structure Factor Calculations F (hkl) = Σ i f i e 2πiK r i F (hkl) = f 1 e 2πi(0) + f 2 e2πi( ½ h + ½ k + ½ l) F (hkl) 0 when h + k + l is odd (N.B: e iθ = cos θ + i sin θ) F (hkl) = f 1 +f 2 when h + k+ l is even Extinction conditions Example: [001] ZA, Cubic unit cell I IP Body centred unit cell Centre for Advanced Microscopy 40

41 Extinction conditions Let s look at the shape factor Rectangular prism Lets focus in the x direction Centre for Advanced Microscopy 41

42 Let s focus in the x direction Geometric series Intensity is the square product of the amplitude Ψ Ψ Ψ Diffraction peaks broadened by the shape factor Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Let s look at the shape factor N x = 12 N y = 6 Diffraction peaks broadened by the shape factor Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Centre for Advanced Microscopy 42

43 Let s look at the shape factor Other shape factors Simple cubic (large crystal) Simple cubic (thin disc) Crystal real shape Diffraction intensities Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Ewald sphere The origin of the Ewald sphere lies at the base of the undiffracted beam k 0 Laue condition Diffraction occurs whenever the Ewald sphere touches a point in the reciprocal space. Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Centre for Advanced Microscopy 43

44 Ewald sphere Sample tilting tilting of the reciprocal space Effects of the Ewald sphere curvature 3D information through the higher order Laue zones Due to striking caused by the shape factor the diffraction spots appear dislocated Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Diffraction Crystalline sample d (d-spacing, Å) L (camera length, mm) 2θ r (reciprocal d-spacing, Å -1 ) tan2θ = r/l From Bragg equation 2θ = λ/d λ/d = r/l Centre for Advanced Microscopy 44

45 Diffraction Crystalline sample d (d-spacing, Å) L (camera length, mm) r (reciprocal d-spacing, Å -1 ) 2θ = r/l 2θ = λ/d d= L λ r -1 Camera constant (Å mm) Indexing a diffraction pattern 3.07 J. Wong-Leung, Dep. of Electronic Materials Eng. - ANU Centre for Advanced Microscopy 45

46 Indexing a diffraction pattern J. Wong-Leung, Dep. of Electronic Materials Eng. - ANU Indexing a diffraction pattern J. Wong-Leung, Dep. of Electronic Materials Eng. - ANU Centre for Advanced Microscopy 46

47 Indexing a diffraction pattern The diffraction pattern cannot be uniquely defined due to its symmetry. Once a set of reflections indexed the others must be indexed consistently. To find the zone-axis two approaches are possible since this vector is perpendicular to the ones in this plane #1 Dot product = 0 #2 Cross product. J. Wong-Leung, Dep. of Electronic Materials Eng. - ANU Powder diffraction (polycrystalline materials) In implanted Si 0.1 Ge 0.9 implanted region polycrystalline Unimplanted regions remain single crystal Ruixing (Andy) Feng - EME Centre for Advanced Microscopy 47

48 Powder diffraction (polycrystalline materials) Reproduced: Transmission Electron Microscopy: Physics of Image Formation L. Reimer H. Kohl Summary Diffraction is a result of coherent scattering Structure factor extinction conditions Shape factor size effects Bragg s Law and Laue conditions are equivalent statements Ewald sphere is a geometrical construct from Laue conditions which facilitates understanding of the diffraction in a TEM 3D information can be obtained from higher order Laue zones Polycrystalline samples lead to the formation of ring patterns Centre for Advanced Microscopy 48

49 Practice Indexing diffraction patterns Centre for Advanced Microscopy e-learning Room Diffraction pattern analysis 2.18 J. Wong-Leung, Dep. of Electronic Materials Eng. - ANU Centre for Advanced Microscopy 49

50 Diffraction pattern analysis J. Wong-Leung, Dep. of Electronic Materials Eng. - ANU Diffraction pattern analysis 1.39 J. Wong-Leung, Dep. of Electronic Materials Eng. - ANU Centre for Advanced Microscopy 50

51 Part 3 Imaging Centre for Advanced Microscopy e-learning Room Contrast Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Centre for Advanced Microscopy 51

52 Contrast mechanisms in TEM Formed by incoherent scattering Visible without the objective aperture I Mass thickness contrast Contrast mechanisms in TEM Formed by coherent scattering Arises from crystals satisfying Bragg s condition Greatly enhanced inserting the objective aperture I Diffraction contrast Centre for Advanced Microscopy 52

53 Phase contrast-fresnel Fringes Reproduced: Transmission Electron Microscopy: Physics of Image Formation L. Reimer H. Kohl Intensity profile Phase contrast-fresnel Fringes Fresnel Fringes In focus: No dephasing added by the OL. Under focus: = 90º Over focus: = - 90º Centre for Advanced Microscopy 53

54 Shape factor Deviation vector How does the deviation vector affects the shape factor? Shape factor only depends on the deviation Diffraction contrast Deviation vector and the Ewald sphere The s vector points from the Ewald sphere toward the reciprocal points s > 0 if points up s < 0 if points down Centre for Advanced Microscopy 54

55 Kikuchi lines Kicuchi lines Roads through the reciprocal space helps to tilt the sample towards the desired zone-axis Forward peaked incoherent scattering Incoherent scattering plus coherent (diffraction) Kikuchi lines Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Kikuchi lines and the deviation Setting up a two beam condition Position of the Kikuchi line in relation to the diffraction spot changes with s Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Centre for Advanced Microscopy 55

56 Centred dark field vs Weak beam Tilting the sample means tilting the entire reciprocal space. Tilting the beam means tilting the Ewald sphere. Procedure: Tilt the sample achieve a good two beam condition (only the direct beam and +g are visible. As you tilt the sample the diffraction intensities change but the diffraction spots do not move! Now move the vector g to the centre in order to have this vector excited. Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials What is the effect of tilting the beam K 0 K 0 -g 0 +g -g 0 +g Centre for Advanced Microscopy 56

57 Weak beam Two beam Centred dark field Weak beam diffraction Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Phase-amplitude diagram Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Centre for Advanced Microscopy 57

58 Thickness fringes (Two-beam) 1 Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Thickness fringes Two beam condition: Total intensity divided between the direct beam the the diffracted one Extinction coefficient For s = 0, period of the thickness fringes is (dynamical) For s >> 0, shorter periods (kinetical) Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Centre for Advanced Microscopy 58

59 Bending contours Excites the pair +g and -g Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Dislocation strain fields Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Centre for Advanced Microscopy 59

60 High Resolution TEM High resolution imaging (phase contrast) Objectives: Know that High Resolution (HR) TEM microscopy is only possible for very thin crystals within the limits in which the Weak Phase Object Approximation is valid. Realize HR images can only be obtained if great care in tilting to a zone axis and aligning the aperture is taken. Learn that the Contrast Transfer Function can be used to describe the imaging characteristics of the microscope. Understand that contrast in HR images cannot be directly assessed without image simulation. Centre for Advanced Microscopy 60

61 HRTEM In high-resolution transmission electron microscopy (HRTEM or HREM) the phase of the diffracted electron wave is preserved and interferes constructively or destructively with the phase of the transmitted wave. This technique of phasecontrast imaging is used to form images of columns of atoms. Electrons that pass between atoms traverse the crystal with minimum change in Intensity. Electrons that pass through the columns of atoms undergo large changes in its intensity and its exit phase changes. The information preserved in phase of the wave interfering produces the phase contrast. A objective aperture large enough to allow for both the direct beam and the g vectors is a requirement HRTEM Sb 2 S 3 crystal High resolution is not looking at atoms. It is a result of phase contrast from the interference of electrons with columns of atoms. The resulting intensity 2D intensity distribution reflects the atomic arrangement in the sample and therefore rendering atomic resolution. Centre for Advanced Microscopy 61

62 Contrast transfer function (CTF) It is the function that describes how the microscope transfers information,,, 2sin Aperture function Envelope function It is the function that describes how the microscope transfers information CTF Contrast transfer function (CTF) Aperture function Spherical aberration Resolution limit Vector of the reciprocal space Defocus Ideal T(u) Centre for Advanced Microscopy 62

63 Contrast Transfer Function Contrast inversion J. Wong-Leung, Dep. of Electronic Materials Eng. - ANU Centre for Advanced Microscopy 63

64 Through focus and thickness series J. Wong-Leung, Dep. of Electronic Materials Eng. - ANU Effect of OA at Scherzer defocus J. Wong-Leung, Dep. of Electronic Materials Eng. - ANU Centre for Advanced Microscopy 64

65 Resolution J. Wong-Leung, Dep. of Electronic Materials Eng. - ANU Amplitude vs Phase contrast J. Wong-Leung, Dep. of Electronic Materials Eng. - ANU Centre for Advanced Microscopy 65

66 FFT and CTF f = 0 f = nm f = 500 nm Astigmatism Centre for Advanced Microscopy 66

67 Summary The optimum focus condition for HR is not at optical focus but in an underfocus condition known as the Scherzer (de)focus. The contrast in HR images is strongly dependent on defocus (just a click or two on the TEM panel!). Atoms are neither black or white in HR images. Atoms in a thin crystal modify the phase of the electron wave in ways that may fortuitously result in a structure image where atomic columns have darker contrast. Through-focus and through-thickness series can be used to monitor changes in phase contrast Summary Bright field (BF) and dark field (DF) imaging are less demanding than high resolution (HR) microscopy. BF and DF microscopy are based on excluding scattered electrons. HR microscopy is based on including as many beams as feasible and examining the amplitude variation due to phase shifting. HR images can sometimes be interpreted in terms of atomic scale potentials. All imaging modes require pre-alignment of the specimen in diffraction mode. Centre for Advanced Microscopy 67

68 Part 4 EDS and STEM Centre for Advanced Microscopy E-learning room Energy dispersive X-ray spectroscopy Particle characteristics gives rise to characteristic X-rays Energy levels are characteristic of each element hence allowing for chemical information to be obtained. Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Centre for Advanced Microscopy 68

69 Energy dispersive X-ray spectroscopy In EDS spectrum, the x-ray peaks from different elements have intensities that depend on: 1) The path and energy of the high-energy electron passing through the sample; 2) The ionization cross-sections of the elements; 3) The fluorescence yields; 4) The probabilities that the emitted x-rays are seen by the detector; Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Reproduced from: Brent Fultz James Howe Transmission Electron Microscopy and Diffractometry of Materials Energy dispersive X-ray spectroscopy 15 kv, Bulk 200 kv, 100 nm thick film C Pb Centre for Advanced Microscopy 69

70 Thin film limit K AB (Cliff-Lorimer factor) is a constant for a specific energy-detector configuration and is independent of sample thickness and composition. Accurate quantitative measurements require the knowledge of the following parameters: 1) Probe diameter; complicated calculation (or measuring directly with camera) Δ 1) Current; Faraday cup 2) Convergence angle; Using diffraction Scanning TEM Centre for Advanced Microscopy 70

71 Scanning Transmission Electron Microscopy Reciprocity Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Scanning Transmission Electron Microscopy Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science. David B. Williams and C. Barry Carter Δ Reproduced: Transmission Electron Microscopy: Physics of Image Formation L. Reimer H. Kohl. Centre for Advanced Microscopy 71

72 Scanning Transmission Electron Microscopy Reproduced from: Transmission Electron Microscopy: A Textbook for Materials Science David B. Williams and C. Barry Carter Ronchigram Centre for Advanced Microscopy 72

73 What is the right aperture? Ronchigram no C s Centre for Advanced Microscopy 73

74 Ronchigram no C s Ronchigram no C s Centre for Advanced Microscopy 74

75 Ronchigram C s > 0 STEM + EDS = Chemical maps Centre for Advanced Microscopy 75