euglena viridis - “green in the middle, and before and behind white” antony van leeuwenhoek -...

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.