Basics of HRTEM Lecture outline

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Basics of HRTEM Lecture outline

• Components of a HRTEM
• Electrons in the microscope
• Focusing in an EM
• Magnification in an EM
• Aberration
• Specimen considerations

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Components of a HRTEM

• Electron gun
• Electron acceleration
• Lenses
• Apertures
• Specimen stage
• Detection systems

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Electron sources for TEM

• Either thermionic or field effect emission
• Different tip materials
• Tungsten thermionic, lowest resolution, biggest
  energy spread
• LaB6 thermionic emission, good resolution, good
  brightness
• Carbon nanotube field emission, excellent
  resolution, poor brightness

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Tungsten tip
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LaB6 tip
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(a) Thermionic emission (b) FEG
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Acceleration system

• A series of discs that accelerate the electrons
  in stages
• From a high potential in 10 regular steps
• The final stage is at ground potential

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Lenses

• Two forms of electromagnets
• Based on octopoles and hexapoles
• Based on solid metal rings
• Constructed from an alloy of cobalt, iron and
  nickel
• Machined to a specific geometry
• A key concept is the bore
• This is the diameter of the hole that runs
  through the lens
• The necessity of the bore leads to the resolution
  limits of the microscopy
• The field in the bore can reach 3-4 Tesla
• By comparison, a field in a commercial magnet
  used in a scrap yard is approximately 0.12 T and
  the earths magnetic field at the surface is
  approximately 0.000 045 T

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Apertures

• The beam will have divergent components
• Apertures are used to filter these off
• The aperture can also be used to select just a
  small portion of the total beam
• This is of use in selected area diffraction (SAD
  or SAED)
• Apertures are constructed from small holes in
  metal sheets
• The holes are around x micrometres in diameter
• Apertures are moved in and out of the beam at
  various locations

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Specimen stage

• The sample is loaded on a small disc constructed
  of a mesh of metal (Cu, Ni, Au, etc) coated in
  conducting polymer and a thin film of amorphous
  carbon
• This is supported on the end of a long rod
• The rod is moved in the x, y and z direction
• Either using piezoelectric controls
• Or small gearing systems
• The specimen is supported in the beam, most often
  contained inside the objective lens

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Sample holders
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Detection system

• The direct detection system is a fluorescent
  screen
• Electrons striking the screen cause a light flash
• The intensity or brightness of the image relates
  directly to the intensity of brightness of the
  electrons striking the screen
• The flux of electrons is sufficiently high that
  the image is is constant
• A secondary detection system can be either
  photographic or digital

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Electrons in the microscope

• Electrons are accelerated through the potential
• The travel down the optic axis of the microscope
• Interacting with the fields from the lenses
• Interacting with atoms and electrons
• Samples are a combination of physical electron
  scatterers and potential interactions

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Diagram of all scattering interactions
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Scattering depends on
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Elastic vs inelastic scattering

• Inelastic scattering gives rise to the amplitude
  contrast
• This shows you the outlines of structures in the
  TEM
• It cannot reveal atomistic or crystallographic
  detail
• Elastic scattering gives rise to phase contrast
• This shows you the phase interaction of the
  electrons with the electric fields of the atoms
• It gives rise to the atomistic detail

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Equations governing electron motion

• Electrons move relativistically in the microscope
• I.e. close to the speed of light
• Their wavelength is given by

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Forces in a lens

• Electrons have a magnetic moment
• They experience a force in the magnetic field
  dictated by
• This obeys the right hand rule and so electrons
  spiral through the microscope

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In the lens

• The field lines in the lens are as shown in the
  image below
• This gives the focusing ability as if the lines
  were parallel there would be no focusing effect
• It also worsens the problems of image quality

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Focusing

• This is achieved by varying the strength of the
  lenses above and below the objective lens
• This has the effect of changing the length of the
  electron path
• This change of path changes the focus

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Over and under focus

• These conditions arise from the strength of the
  fields

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Magnification