Euglena viridis - “green in the middle, and before and behind white” Antony van Leeuwenhoek - 1674.

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Presentation transcript:

Euglena viridis - “green in the middle, and before and behind white” Antony van Leeuwenhoek

Resolution  ½

Ernst Abbe λ R.P. = N.A. N.A. = n (sine α) n = index of refraction α = half angle of illumination

Refractive index is dependent on a ray of illumination entering 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 λ R.P. = N.A. In light microscopy the N.A. of a lens and therefore resolution can be increased by a) increasing the half angle of illumination, b) increasing the refractive index of the lens by using Crown glass and c) decreasing the wavelength ( λ ) of illumination. In electron microscopy the refractive index cannot exceed 1.0, the half angle is very small, and thus the only thing that can be adjusted is decreasing the wavelength of illumination

Transmission Electron Microscopy Louis de Broglie 1923

Transmission Electron Microscopy h = Planck's constant (6.624 X erg/second) m = mass of an electron (9.11 X gram = 1/1837 of a proton) v = velocity of the electron

Transmission 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 and a theoretical resolution of Angstroms! But in actuality most TEMs will only have an actual resolution 2.4 Angstroms at 100KeV

Transmission Electron Microscopy Ernst Ruska & Max Knoll 1932

Transmission Electron Microscopy Bill Ladd 1939

Transmission Electron Microscopy James Hillier - RCA EMB 1940

Electron Sources Thermionic Emitters Field Emitters

Electron Sources Work 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 Emitters utilize heat to overcome the work function of a material. Tungsten Filament (W) Lanthanum Hexaboride LaB6

Electron Sources Tungsten emitters Wire bent into a loop of various dimensions. W (m.t 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 way of separate resistance wire or ceramic mounts Filament current is separate from heating current

Electron Sources Similar in design to a tungsten filament

LaB6 Emitters

Electron Sources Filament Current (Heating Current) Current running through the emitter Beam Current Current generated by the emitter

Electron Sources Filament Centering Gun Horizontal Gun Tilt

Electron Sources Field Emitter Single oriented crystal of tungsten etched to a fine tip The emission of electrons that are stripped from parent atoms by a high electric field

Electron Sources A Field Emission tip can be “cold” or thermally assisted to help overcome the work function but ultimately it is a high voltage field of 3 KeV that is needed

Electron Sources Other Factors to consider? Cost W= $15 LaB6 = $400 F.E. = $6000 Lifetime 100 hr hr 5-8,000 hr.