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Diffraction methods and electron microscopy

Diffraction methods and electron microscopy. Outline and Introduction to FYS4340 and FYS9340. FYS4340 and FYS9340. FYS4340 Theory based on ”Transmission electron microscopy” by D. B. Williams and C.B. Carter Part 1, 2 and standard imaging techniques (part 3) Practical training on the TEM

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Diffraction methods and electron microscopy

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  1. Diffraction methods and electron microscopy Outline and Introduction to FYS4340 and FYS9340

  2. FYS4340 and FYS9340 • FYS4340 • Theory based on ”Transmission electron microscopy” by D. B. Williams and C.B. Carter • Part 1, 2 and standard imaging techniques (part 3) • Practical training on the TEM • FYS9340 • Theory same as FYS4340 + additional papers related to TEM and diffraction. • Teaching training. • Perform practical demonstrations on the TEM for the master students.

  3. Basic TEM Electrons are deflected by both electrostatic and magnetic fields Force from an electrostatic field F= -e E Force from amagnetic field F= -e (v x B) Electron transparent samples Electron gun Sample position

  4. IntroductionEM and materials Electron microscopy are based on three possible set of techniqes Spectroscopy Imaging Chemistry and elecronic states (EDS and EELS). Spatial and energy resolution down to the atomic level and ~0.1 eV. Electrons With spatial resolution down to the atomic level (HREM and STEM) BSE X-rays (EDS) AE SE Diffraction E<Eo (EELS) From regions down to a few nm (CBED). Bragg diffracted electrons E=Eo

  5. Valence M 3d6 M 3p4 L 3d4 3s2 2p4 3p2 K Electron shell 2s2 2p2 1s2 L K Basic principles, electron probe Electron Auger electron or x-ray Characteristic x-ray emitted or Auger electron ejected after relaxation of inner state. Low energy photons (cathodoluminescence) when relaxation of outer stat. Secondary electron MENA3100

  6. Introduction EM and materials The interesting objects for EM is not the average structure or homogenous materials but local structure and inhomogeneities Defects Interfaces Precipitates Defects, interfaces and precipitates determines the properties of materials

  7. Resolution limitations of the VLM • 1839, George Airy: there should be a natural limit to the optical microscopes. • 1872, both Ernst Abbe and Hermann von Helmholtz: Light is limited by the size of the wavelength. Resolution of the eyes 0.1-0.2 mm Resolution of a good VLM ~300 nm

  8. Electron beam/cathode ray • 1857, The cathode-ray tube was invented • 1896, Olaf Kristian Birkeland experimenting with the effect of parallel magnetic fields on the electron beam of the cathode-ray tub concluded that cathode rays that are concentrated on a focal point by a magnet are as effective as parallel light rays that are concentrated by means of a lens.

  9. Electron optics • 1926, Hans Busch, ”Founder of the electron optics” published his theory on the trajectories of electrons in magnetic fields. • 1928, Graduate student Ruska worked on refining Busch’s work. • The energy of the electrons in the beam was not uniform resulting in fuzzy images. • Knoll and Ruska were able design and construct electron lenses and the first realization of an electron microscope.”

  10. Wave nature of electrons • 1897, J.J. Thomson • Concludes that electrons have particle nature. • 1924, Louis de Broglie • Hypothesis: Matter on the scale of subatomic particles possesses wave characteristics. The speed of low-mass subatomic particles, such as electrons, is related to wavelength . • 1927, Davisson and Germerand Thomson and Reid • Both demonstrated the wave nature of electrons by independently performing electron diffraction experiments λ=1.22/E1/2

  11. The first electron microscope • Knoll and Ruska, first TEM in 1931 • Idea and first images published in 1932 • By 1933 they had produced a TEM with two magnetic lenses which gave 12 000 times magnification. Electron Microscope Deutsches Museum, 1933 model Ernst Ruska: Nobel Prize in physics 1986

  12. The first commersial microscopes • 1939 Elmiskop by Siemens Company • 1941 microscope by Radio corporation of America (RCA) • First instrument with stigmators to correct for astigmatism. Resolution limit below 10 Å. Elmiskop I

  13. r2 r1 α Disk of least confusion Developments Realized that spherical aberration of the magnetic lenses limited the possible resolution to about 3 Å. • Spherical aberration coefficient r2 r1 α ds = 0.5MCsα3 M: magnification Cs :Spherical aberration coefficient α: angular aperture/ angular deviation from optical axis 2000FX: Cs= 2.3 mm 2010F: Cs= 0.5 nm

  14. v - Δv v Chromatic aberration Disk of least confusion Chromatic aberration coefficient: dc = Ccα ((ΔU/U)2+ (2ΔI/I)2 + (ΔE/E)2)0.5 Cc: Chromatic aberration coefficient α: angular divergence of the beam U: acceleration voltage I: Current in the windings of the objective lens E: Energy of the electrons 2000FX: Cc= 2.2 mm 2010F: Cc= 1.0 mm Thermally emitted electrons: ΔE/E=kT/eU Force from amagnetic field: F= -e (v x B)

  15. Developments ~ 1950 EM suffered from problems like: Vibration of the column, stray magnetic fields, movement of specimen stage, contamination. Lots of improvements early 1950’s. Still far from resolving crystal lattices and making direct atomic observations.

  16. Observations of dislocations and lattice images • 1956 independent observations of dislocations by: Hirsch, Horne and Wheland and Bollmann -Started the use of TEM in metallurgy. • 1956 Menter observed lattice images from materials with large lattice spacings. • 1965 Komoda demonstrated lattice resolution of 0.18 nm. • Until the end of the 1960’s it was mainly used to test resolution of microscopes.

  17. Menter, 1956

  18. Use of high resolution electron microscopy (HREM) in crystallography • 1971/72 Cowley and Iijima • Observation of two-dimensional lattice images of complex oxides • 1971 Hashimoto, Kumao, Hino, Yotsumoto and Ono • Observation of heavy single atoms, Th-atoms

  19. 1970’s • Early 1970’s: Development of energy dispersive x-ray (EDX) analyzers started the field of analytical EM. • Development of dedicated HREM • Electron energy loss spectrometers and scanning transmission attachments were attached on analytical TEMs. • Small probes making convergent beam electron diffraction (CBED) possible.

  20. 1980’s • Development of combined high resolution and analytical microscopes. • An important feature in the development was the use of increased acceleration voltage of the microscopes. • Development of Cs corrected microscopes • Probe and image • Improved energy spread of electron beam • More user friendly Cold FEG • Monocromator Last few years

  21. Electron beam instruments • Transmission Electron microscope (TEM) • Electron energies usually in the range of 80 – 400 keV. High voltage microscopes (HVEM) in the range of 600 keV – 3 MeV. • Scanning electron microscope (SEM) early 1960’s • dedicated Scanning TEM (STEM) in 1968. • Electron Microprobe (EMP) first realization in 1949. • Auger Scanning Electron Microscopy (ASEM) 1925, 1967 • Scanning Tunneling Microscope (STM) developed 1979-1981 Because electrons interact strongly with matter, elastic and inelastic scattering give rise to many different signals which can be used for analysis.

  22. Electron waves • Show both particle and wave properties • Electrons can be accelerated to provide sufficient short wave length for atomic resolution. • Due to high acceleration voltages in the TEM relativistic effects has to be taken into account. Charge e Restmass mo Wave ψ Wave length λ λ = h/p= h/mv de Broglie (1925) λ = h/(2emoU)1/2 U: pot. diff. λ = h/(2emoU)1/2 * 1/(1+eU/2moc2)1/2

  23. The Transmission Electron Microscope Relations between acceleration voltage, wavevector, wavelength, mass and velocity

  24. 3,8 Å 1,1 nm Simplified ray diagram Parallel incoming electron beam Si Sample Objective lense Diffraction plane (back focal plane) Objective aperture Selected area aperture Image plane MENA3100 V08

  25. JEOL 2000FX • Wehneltcylinder • Filament • Anode • Electrongun 1. and 2. beam deflectors • and 2. condenser lens • Condenseraperture • Condenser lens stigmator coils • Condenser lens 1. and 2. beam deflector • Condensermini-lens • Objective lens pole piece • Objectiveaperture • Objective lens pole piece • Objective lens stigmators • Image shift coils • Objectivemini-lens coils (low mag) • 2. Image shift coils • 1., 2.and 3. Intermediate lens • Projector lens beam deflectors • Projector lens • Screen Electron gun Illumination system Mini-lens screws Specimen Intermediate lens shifting screws Projector lens shifting screws

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