euglena viridis - “green in the middle, and before and behind white” antony van leeuwenhoek -...
TRANSCRIPT
Euglena viridis - “green in the middle, and before and behind white”
Antony van Leeuwenhoek - 1674
Resolution ½
Ernst Abbe1840 - 1905
0.61 λR.P. = ---------- N.A.
N.A. = n (sine α)n = index of refractionα = half angle of illumination
Refractive index is dependent on a ray of illuminationentering a medium of differing density causing the beam to bend
In the vacuum environment of an electron microscope the index of refraction is 1.0 and therefore N.A. depends solely on the half angle of illumination
0.61 λR.P. = ---------- N.A.
In light microscopy the N.A. of a lens and therefore resolutioncan be increased by a) increasingthe half angle of illumination,b) increasing the refractive indexof the lens by using Crown glassand c) decreasing the wavelength(λ) of illumination.
In electron microscopy the refractive index cannotexceed 1.0, the half angle is very small, and thus the only thing that can be adjusted is decreasingthe wavelength of illumination
Transmission Electron MicroscopyTransmission Electron Microscopy
Louis de BroglieLouis de Broglie19231923
Transmission Electron MicroscopyTransmission Electron Microscopy
h = Planck's constant (6.624 X 10-27 erg/second)
m = mass of an electron (9.11 X 10-28 gram = 1/1837 of a proton)
v = velocity of the electron
Transmission Electron MicroscopyTransmission Electron Microscopy
( 150 / V )1/2 Angstroms
Substituting 200 eV for V gives a of 0.87 Angstroms
For a beam of 100 KeV we get a wavelength of 0.0389
and a theoretical resolution of 0.0195 Angstroms!
But in actuality most TEMs will only have an actual
resolution 2.4 Angstroms at 100KeV
Transmission Electron MicroscopyTransmission Electron Microscopy
Ernst Ruska Ernst Ruska &&
Max KnollMax Knoll
19321932
Transmission Electron MicroscopyTransmission Electron Microscopy
Bill LaddBill Ladd19391939
Transmission Electron MicroscopyTransmission Electron Microscopy
James Hillier - RCA
EMB1940
Electron Sources
Thermionic Emitters
Field Emitters
Electron Sources
Work FunctionWork Function
Energy (or work) required to
withdraw an electron
completely from a metal
surface. This energy is a
measure of how tightly a
particular metal holds its electrons
Electron Sources
Thermionic Emittersutilize heat to overcomethe work function of a material.
Tungsten Filament (W)
Lanthanum HexaborideLaB6
Electron Sources
Tungsten emittersWire bent into a loop of variousdimensions.W (m.t. 3410 degrees C.)
Electron Sources
Increasing the filament current will increase the beam current but only to the point of saturation at which point an increase in the filament current will only shorten the life of the emitter
Electron Sources
Heat is applied by wayof separate resistancewire or ceramic mounts
Filament current is separate from heating current
Electron Sources
Similar in design to atungsten filament
LaB6 EmittersLaB6 Emitters
Electron Sources
Filament Current(Heating Current)Current running through the emitter
Beam CurrentCurrent generated by the emitter
Electron Sources
Filament Centering
Gun Horizontal
Gun Tilt
Electron Sources
Field EmitterField Emitter
Single oriented crystal of tungstenetched to a fine tip
The emission of electrons that are stripped from parent atoms by a high electric field
Electron Sources
A Field Emissiontip can be “cold” orthermally assisted tohelp overcome the work function but ultimately it is a highvoltage field of 3 KeVthat is needed
Electron Sources
Other Factors to consider?
Cost W= $15 LaB6 = $400 F.E. = $6000Lifetime 100 hr. 1000 hr 5-8,000 hr.