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Electron microscopy

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Electron microscopy

  1. 1. ELECTRON MICROSCOPY Indrajit Bramhecha
  2. 2. Need Of EM • Resolution. • Depth of field • Microanalysis • Chemical analysis • Guidable Medium • Wave-particle duality Need Of Electrons Indrajit Bramhcha 2
  3. 3. Resolution where λ is the wavelength of the radiation, μ is the refractive index of the view medium, 𝝱 is the semi-angle of collection of the magnifying lens, 𝑚0 mass of electron, c light speed, e charge of electron, V applied electrical potential difference, h plank’s constant Wavelength of visible light is in range 390 to 700mm and can theoretically resolve up to 0.2𝝁m Wave length of electron is about 0.00370nm can theoretically resolve upto 0.012nm Abbe’s Equation R = 0.612ࣅ (Wave𝑙𝑒𝑛𝑔𝑡ℎ) μ sin 𝝱 𝑛𝑢𝑚𝑒𝑟𝑖𝑐𝑎𝑙 𝑎𝑝𝑒𝑟𝑡𝑢𝑟𝑒 De Broglie’s wave equation 𝝀 = ℎ 𝑚𝑣 = ℎ 2𝑚0 𝑒𝑉(1+ 𝑒𝑉 2𝑚0 𝑐2) ≈ 1.5 𝑉 1 2 𝑛𝑚 Indrajit Bramhcha 3
  4. 4. Types of EM • TEM • SEM • STEM • STM • AFM Indrajit Bramhcha 4
  5. 5. • Topography • Texture/surface of a sample • Morphology • Size, shape, order of particles • Composition • Elemental composition of sample • Crystalline Structure • Arrangement present within sample What can you see with an SEM?
  6. 6. Typical Application Of Sem In Polymer Analysis • Examine pull-out of fibers in composite materials upon fracture for evidence of poor wetting and bonding • Examine microcracking in surfaces, thin films, and coatings • Find pinholes in coatings • Examine grain shape and orientation, often important in extruded and rolled materials Indrajit Bramhcha 6
  7. 7. • Samples must be small enough to fit in sample chamber • Most modern microscopes can safely accommodate samples up to 15cm in height • Samples should be carefully prepared for high vacuum compatibility – excessively high outgassing and liquid evaporation is to be avoided • Samples must be electrically conductive • Polymer samples typically need to be sputter coated to make sample conductive • Ultra-thin metal coating • Usually gold or gold/palladium alloy • Coating helps to improve image resolution SEM Sample Preparation
  8. 8.  Once sample is properly prepared, it is placed inside the sample chamber  Once chamber is under vacuum, a high voltage is placed across a tungsten filament to generate a beam of high energy electrons (electron gun) and serves as the cathode  The position of the anode allows for the generated electrons to accelerate downward towards the sample  Condensing lenses “condense” the electrons into a beam and objective lenses focus the beam to a fine point on the sample Scanning Electron Microscopy
  9. 9. • Scanning coils move the focused beam across the sample in a raster scan pattern • Same principle used in televisions • Scan speed is controllable Scanning Electron Microscopy
  10. 10. SEM Sample Interactions
  11. 11. Indrajit Bramhcha Electron beam-sample interactions • The incident electron beam is scattered in the sample, both elastically and inelastically • This gives rise to various signals that we can detect (more on that on next slide) • Interaction volume increases with increasing acceleration voltage and decreases with increasing atomic number 11
  12. 12. Where does the signals come from? • Diameter of the interaction volume is larger than the electron spot  resolution is poorer than the size of the electron spot Indrajit Bramhcha 12
  13. 13. Indrajit Bramhcha 13
  14. 14. SE1  The secondary electrons that are generated by the incoming electron beam as they enter the surface  High resolution signal with a resolution which is only limited by the electron beam diameter SE2  The secondary electrons that are generated by the backscattered electrons that have returned to the surface after several inelastic scattering events  SE2 come from a surface area that is bigger than the spot from the incoming electrons  resolution is poorer than for SE1 exclusively Sample surface Incoming electrons SE2 Indrajit Bramhcha 14
  15. 15. Indrajit Bramhcha Factors that affect SE emission 3. Atomic number (Z) • More SE2 are created with increasing Z • The Z-dependence is more pronounced at lower beam energies 4. The local curvature of the surface (the most important factor) 15
  16. 16. Backscattered electrons (BSE)  A fraction of the incident electrons is retarded by the electro-magnetic field of the nucleus and if the scattering angle is greater than 180° the electron can escape from the surface  High energy electrons (elastic scattering)  Fewer BSE than SE  We differentiate between BSE1 and BSE2 Indrajit Bramhcha 16
  17. 17. BSE vs SE SE produces higher resolution images than BSE By placing the secondary electron detector inside the lens, mainly SE1 are detected Resolution of 1 – 2 nm is possible Indrajit Bramhcha 17
  18. 18. X-rays  Photons not electrons  Each element has a fingerprint X- ray signal  Poorer spatial resolution than BSE and SE  Relatively few X-ray signals are emitted and the detector is inefficient  relatively long signal collecting times are needed Indrajit Bramhcha 18
  19. 19. SEM Instruments • Electron Gun • Electron Column • Sample Chamber • Detectors • Analyzer (computer) Indrajit Bramhcha 19
  20. 20. Electron Gun • Thermionic gun • Field emission gun Emission Thermionic Field Emission W LaB6 FE Size (angstroms) 1 x 10 6 2 x 10 5 <1 x 10 2 Brightness (A/cm2.steradian) 104 – 10 5 105 – 10 6 107 – 10 9 Energy Spread (eV) 1 – 5 0.5 – 3.0 0.2 – 0.3 Operating Lifetime (hrs) >20 >100 >300 Vacuum (torr) 10-4 – 10 -5 10-6 – 10 -7 10-9 – 10 -10 Indrajit Bramhcha 20
  21. 21. Electron guns  We want many electrons per time unit per area (high current density) and as small electron spot as possible  Traditional guns: thermionic electron gun (electrons are emitted when a solid is heated)  W-wire, LaB6-crystal Indrajit Bramhcha 21
  22. 22. Filament saturation Indrajit Bramhcha 22
  23. 23. Field emission electron source: High electric field at very sharp tip causes electrons to "tunnel" cool tip ——> smaller E in beam improved coherence many electrons from small tip ——> finer probe size, higher current densities (100X >)
  24. 24. Schottky field emission • A hot field emission gun has some advantages compared to cold field emitters. The major advantages are better beam current stability, less stringent vacuum requirements and the fact that there is no need for periodic emitter flashing (heating the cold filament for a short time each day) to restore the emission current. Indrajit Bramhcha 24
  25. 25. magnetic lens p q Magnetic lens (solenoid) Lens formula: 1/f = 1/p + 1/q M = q/pDemagnification: (Beam diameter) F = -e(v x B) f  Bo 2 f can be adjusted by changing Bo, i.e., changing the current through coil. Why suitable for high energy e Indrajit Bramhcha 25
  26. 26. Indrajit Bramhcha 26
  27. 27. SEM over look Indrajit Bramhcha 27
  28. 28. Specimen What comes from specimen? Backscattered electrons Secondary electrons Fluorescent X-rays high energy compositional contrast low energy topographic contrast composition - EDS Brightness of regions in image increases as atomic number increases (less penetration gives more backscattered electrons)
  29. 29. Backscattered electron detector - solid state detector electron energy up to 30-50 keV annular around incident beam repel secondary electrons with — biased mesh images are more sensitive to chemical composition (electron yield depends on atomic number) line of sight necessary
  30. 30. Back-Scattered Electron Detector BSE Micrograph Showing Crystalline Lamellae
  31. 31. Secondary electron detector - scintillation detector + bias mesh needed in front of detector to attract low energy electrons line of sight unnecessary
  32. 32. SEM Micrographs Crystalline Latex Particles Polymer Hydrogel Surface
  33. 33. SEM Micrographs SEM Images of PVEA Comb-like Terpolymers
  34. 34. • Morphology • Shape, size, order of particles in sample • Crystalline Structure • Arrangement of atoms in the sample • Imperfections in crystalline structure (defects) • Composition • Elemental composition of the sample What can we see with a TEM?
  35. 35. • Samples need to be extremely thin to be electron transparent so electron beam can penetrate • Ultramicrotomy is a method used for slicing samples • Slices need to be 50-100nm thick for effective TEM analysis with good resolution TEM Sample Preparation
  36. 36.  Instrument setup is similar to SEM  Instead of employing a raster scan across the sample surface, the electron beam is “transmitted” through the sample  Material density determines darkening of micrograph ◦ Darker areas on micrograph indicate a denser packing of atoms which correlates to less electrons reaching the fluorescent screen  Electrons which penetrate the sample are collected on a screen/detector and converted into an image Transmission Electron Microscopy
  37. 37. TEM Indrajit Bramhcha 37
  38. 38. TEM Micrographs Isotactic Polypropylene Particle
  39. 39. TEM Micrographs Large Ceria Nanoparticles Small Ceria Nanoparticles
  40. 40. TEM Micrographs Microcrystalline Cellulose
  41. 41. Indrajit Bramhcha 41
  42. 42. Pros • Easier sample preparation • Ability to image larger samples • Ability to view a larger sample area SEM Pros and Cons Cons  Maximum magnification is lower than TEM (500,000x)  Maximum image resolution is lower than TEM (0.5nm)  Sputter coating process may alter sample surface
  43. 43. Pros • Higher magnifications are possible (50,000,000x) • Resolution is higher (below 0.5Å) • Possible to image individual atoms TEM Pros and Cons Cons  Sample preparation  Sample structure may be altered during preparation process  Field of view is very narrow and may not be representative of the entire sample as a whole
  44. 44. Electrons Used Indrajit Bramhcha 44
  45. 45. STM Indrajit Bramhcha 45
  46. 46. STM Types Indrajit Bramhcha 46
  47. 47. STM principle Indrajit Bramhcha 47
  48. 48. AFM Indrajit Bramhcha 48
  49. 49. The Force that is experience by the tip is mainly due to Van der Waals force, typically 10-11 to 10-6 N at separation of ~ 1 Å. Can work with insulator Soft, flexible (low spring constant) Cantilever Tip Indrajit Bramhcha 49
  50. 50. More Scanning Probe techniques Lateral Force Microscopy (LFM): measures frictional forces between the probe tip and the sample surface Magnetic Force Microscopy (MFM): measures magnetic gradient and distribution above the sample surface; Electric Force Microscopy (EFM): measures electric field gradient and distribution above the sample surface; Scanning Thermal Microscopy (SThM): measures temperature distribution on the sample surface. Scanning Capacitance Microscopy (SCM): measures carrier (dopant) concentration profiles on semiconductor surfaces. Spin-resolved STM: atom-resolved magnetic microscopy. AFM MFM Indrajit Bramhcha 50
  51. 51. Indrajit Bramhcha 51
  52. 52. Thank You For Listening! Indrajit Bramhcha 52

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