Bob Ashley 6 21 2013 Overview Systems of the TEM Illumination Condenser lens system Specimen manipulation stage Imaging system Image formation Magnification Recording system Reading List ID: 188238
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Slide1
The Transmission Electron Microscope
Bob Ashley 6-
21
-2013Slide2
Overview
Systems of the TEM
Illumination
Condenser lens system
Specimen manipulation stage
Imaging system
Image formation
Magnification
Recording systemSlide3
Reading List
Practical Electron Microscopy for Biologists. G. A. Meek.Slide4
The Transmission Electron Microscope
Bizzola Electron Microscopy 1999Slide5
JEOL 2100 Lab6Slide6
Illumination System
Thermionic Emission Source
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 functionEnergy necessary to bring about electron emissionElectrons
would have no
order
Wehnelt Cap
Electrode that shapes and controls emission
Negative relative to filament
Anode
Positive respective to filament
WikipediaSlide7
Source Types
Tungsten
Wire filament
Lab6
Lanthanum hexaboride
crystal
Field Emission GunTungsten tipThermionic (Schotkky) and ColdSlide8
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 gunsSlide9
Coherent or Incoherent
Coherent
Waves have same wavelength and
phase, in the ideal sense would be perfectly coherent
Incoherent
Waves have modulating phase relationships and wavelengths
Temporal coherencyWavelength differentialSpatial coherencySize 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)Slide10
Comparing the Electron SourcesSlide11
Condenser Lens and ApertureSlide12
Condenser Lense
s
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.matter.org.uk
/
www.ammrf.org.au
/
C1 CrossoverSlide13
Left Control BoxSlide14
Specimen Chamber
Vacuum interlock system
www.ammrf.org.auSlide15
Specimen Holder and Stage
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 holderSlide16
Stage ControllerSlide17
Grid Types
Supports used must be strong yet electron transparent
Plastic (formvar)
Carbon
Holey carbon
Quantifoil
C-flatSlide18
Image Formation
Four fundamental processes
Scattering
Absorption
Gives rise to amplitude contrast
Contrast from absence of electrons
DiffractionUsed to enhance contrast in cryoEM but with loss of resolution
InterferenceGives rise to phase contrastHalo or fringe around objectSlide19
Specimen Beam Interactions: Scattering
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 electronAs 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 patternsSlide20
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 coherentInelasticAlmost always incoherentSlide21
Accelerating Voltages
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 damageSlide22
Mass Thickness
Typical thick sections are at 100nm while high resolution is limited to 10’s of nm.Slide23
Objective Lens and ApertureSlide24
Objective Lens
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 contrastAssociated stigmator to correct any astigmationSlide25
Objective Aperture
Bozzola 176Slide26
Right Controller Box Slide27
Intermediate and Projections Lenses
Similar in construction to objective lens
Major function is to assist in the magnification of the image from the objective lensSlide28
The Screen and CCDSlide29
Next Week:
CCDs and their function in the electron microscope