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

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Presentation on theme: "Euglena viridis - “green in the middle, and before and behind white” Antony van Leeuwenhoek - 1674."— Presentation transcript:

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

2 Resolution  ½

3 Ernst Abbe 1840 - 1905 0.61 λ R.P. = ---------- N.A. N.A. = n (sine α) n = index of refraction α = half angle of illumination

4 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

5 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

6 Transmission Electron Microscopy Louis de Broglie 1923

7 Transmission 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

8 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 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

9 Transmission Electron Microscopy Ernst Ruska & Max Knoll 1932

10 Transmission Electron Microscopy Bill Ladd 1939

11 Transmission Electron Microscopy James Hillier - RCA EMB 1940

12 Electron Sources Thermionic Emitters Field Emitters

13 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

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

15 Electron Sources Tungsten emitters Wire bent into a loop of various dimensions. W (m.t. 3410 degrees C.)

16 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

17 Electron Sources Heat is applied by way of separate resistance wire or ceramic mounts Filament current is separate from heating current

18 Electron Sources Similar in design to a tungsten filament

19 LaB6 Emitters


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

22 Electron Sources Filament Centering Gun Horizontal Gun Tilt

23 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

24 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

25 Electron Sources Other Factors to consider? Cost W= $15 LaB6 = $400 F.E. = $6000 Lifetime 100 hr. 1000 hr 5-8,000 hr.

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