Bob Ashley 6-21-2013

• Systems of the TEM
• Illumination
• Condenser lens system
• Specimen manipulation stage
• Imaging system
• Image formation
• Magnification
• Recording system

Practical Electron
Microscopy for
Biologists. G. A. Meek.

The Transmission Electron
Microscope
Bizzola Electron Microscopy 1999

JEOL 2100 Lab6

• Electron source or gun
• Provides first coherent crossover of electron beam
• High voltage leading to filament- heated metal wire (cathode)
• Tungsten or Lab6 (lanthanum hexaboride)
• Work function
• Energy necessary to bring about electron emission
• Electrons would have no order
• Wehnelt Cap
• Electrode that shapes and controls emission
• Negative relative to filament
• Anode
• Positive respective to filament
Wikipedia

• Tungsten
• Wire filament
• Lab6
• Lanthanum hexaboride crystal
• Field Emission Gun
• Tungsten tip
• Thermionic (Schotkky) and Cold

Field Emission
Source
Allows for emission of electrons from fine tip via charge differential of tip and anodes.
• Brighter
• More coherent source
• Atomic diameter point source is future of FE guns

• Coherent
• Waves have same wavelength and phase, in the ideal sense would be perfectly coherent
• Incoherent
• Waves have modulating phase relationships and wavelengths
• Temporal coherency
• Wavelength differential
• Spatial coherency
• Size of source
Temporal coherence length= product of velocity and planks constant divided by the energy spread
Spatial coherence length= wavelength divide by 2 times the alpha (angle formed from source at at the specimen)

Condenser Lens and Aperture

Condenser Lenses
• C1 is spot size
• Determines size of beam on specimen
• C2 is beam brightness knob
• Varies and magnifies C1
• Aperture is a physical aperture in range of sizes
• Reduces spherical aberration
• Associated stigmator to correct astigmatism www.ammrf.org.au/ www.matter.org.uk/

Specimen Chamber
• Vacuum interlock system www.ammrf.org.au

• Holds sample in place on top of copper grid
• Moved with stage in x,y,z
• Tilting holder
• Side or top entry
• Room temperature
• Cryo holder

• Supports used must be strong yet electron transparent
• Plastic (formvar)
• Carbon
• Holey carbon
• Quantifoil
• C-flat

• Four fundamental processes
• Scattering
• Absorption
• Gives rise to amplitude contrast
• Contrast from absence of electrons
• Diffraction
• Used to enhance contrast in cryoEM but with loss of resolution
• Interference
• Gives rise to phase contrast
• Halo or fringe around object

• Elastic (Rutherford scattering)
• Electron collides with or passes close to a nucleus of atom, no loss of energy of initiating electron
• changes direction without losing velocity or energy
• Inelastic
• Electron collides with cloud electrons, measurable loss of energy of initiating electron
• As sample thickness increases more electrons are backscattered
• Assumed to occur only once in TEM (either the e- scatters or it doesn’t)
• Sometimes referred to as the direct transmitted beam if no scattering occurred
• Can either forward or back scatter
• Scatter measured in spatial deviation manifests as contrast, scatter of angular deviation manifests as diffraction patterns

Scattering and Coherence
• Coherence
• Electrons remain in phase with one another after passing through sample
• Incoherent electrons are those that have no consistent phase relationship upon passing through sample
• Elastic
• Usually coherent
• Inelastic
• Almost always incoherent

• How does this all relate in choice of your electron accelerating voltage or kV?
• The faster the speed of the electrons the better resolution obtained
• But at the sacrifice of contrast
• Slower electron speeds have more opportunity for inelastic scattering, inelastic scattering produces energy
(heat) therefore lower kV has more specimen damage

Mass Thickness
Typical thick sections are at
100nm while high resolution is limited to 10’s of nm.

Objective Lens and Aperture

• Most important lens
• Forms initial image further magnified by other lenses
• Responsible for focus
• Blocking of more peripherally deflected electrons with Objective aperture
• The larger the aperture used the more phase contrast
• Important for cryo EM and higher resolution
• The smaller the aperture the more aperture contrast
• Associated stigmator to correct any astigmation

Bozzola 176

Intermediate and Projections
Lenses
Similar in construction to objective lens
Major function is to assist in the magnification of the image from the objective lens

Next Week:
CCDs and their function in the electron microscope