![Electron Diffraction
• Au (111) ring [2.35 Å d-spacing]
With 200KV and L=65cm the (111)
ring should be at about 7.5mm from
the transmitted beam
Rd=lL
R=0.027A*650mm/2.35A](https://image.slidesharecdn.com/tem-1-240706043647-ba380071/85/Transmission-and-electron-microscope-Tem-ppt-38-320.jpg)
Transmission and electron microscope Tem ppt
- 2. The TEM system and components: •Vacuum system •Electron Gun •Electron Lenses •Sample Stage •More Electron Lenses •Viewing Screen •Camera Chamber
- 4. TEM Illumination control • Filament saturation • Filament centering • Spot size (Condenser Lens Current) • Condenser aperture
- 5. How to focus the TEM • Control objective lens current • Adjust astigmatism correction coils • Use large screen at low magnifications • Use small screen at high magnifications • Beware of lingering on an area too long
- 6. Focusing on a hole in a thin carbon film (a) underfocused objective lens (bright fringe); (b) at focus (no fringe); (c) overfocused objective lens (dark fringe). Note also the change in appearance of the carbon fine grain. Magnification ~750,000X.
- 7. Objective aperture located between upper and lower parts of polepiece, just under the specimen. The major function of the aperture is to help remove peripherally deflected electrons to enhance image contrast. In addition to the specimen and objective aperture, a chilled anticontaminator blade (see Figure 6.34) may also be inserted just above the specimen (or sometimes above and below the specimen) to prevent contaminants from condensing on specimen.
- 8. 8 Astigmatism Correction • Older stigmators were composed of pairs of magnetic slugs that could be mechanically rotated into position to compensate for astigmatism. • Newer microscopes use primarily electromagnetic stigmators since they are less expensive to build, easier to use, and somewhat more precise in their correction. • Electromagnetic stigmators may consist of eight tiny electromagnets encircling the lens field. • By varying the strength and polarity of various sets of magnets, one can control both amplitude and azimuth in order to generate a symmetrical magnetic field (Figure 6.35). • When stigmators become dirty, they will no longer effectively compensate for astigmatism and must be withdrawn from the microscope and cleaned.
- 9. 9 (A) Conceptual drawing of electromagnetic stigmator showing orientation of eight electromagnets around the lens axis. Strength and direction are controlled by adjusting appropriate combinations of magnets to generate a symmetrical field. The stigmator is located under the condenser and the objective lens polepieces.
- 10. Astigmatism Correction • Must correct condenser lens, objective lens and projector lens separately • Use both objective stigmator selectors – Use the first stigmator – Use the second one – Refocus – Repeat
- 11. Contrast Considerations • Resolving power is good , but… To see image features contrast is needed • How to increase contrast…? • (SEM contrast is derived from local topography and/or differential interactions of beam with sample)
- 12. Contrast Considerations • Mass-thickness Contrast (absorption) – Density x thickness – More dense or thicker areas look darker due to absorption of beam electrons. – Thickness fringes due to destructive interference as beam traverses the sample – Use stains to highlight specific areas • Uranium, manganese, osmium • Coat and/or shadow areas to generate contrast
- 13. Contrast Considerations • Other “deficiency” contrast mechanism – Electron scattering • Random – Amorphous materials change electron trajectories • Regular – Crystalline materials change trajectories uniformly
- 14. Contrast Considerations • Most of the beam – Goes right through the sample unperturbed – Other elastic interactions change the direction of electrons which can be selected for or eliminated from the image forming beam.
- 15. Brightness Considerations • Type of source (W, LaB6, FE) • Higher magnification means a strong Int-Proj lens, which means lower intensity • Hard to see on fluorescent screen • Ways to mitigate – use lower mags – converge beam with condenser lens – align beam as needed
- 16. Beam Energy Considerations • Higher voltages produces – shorter wavelength – better resolution – greater depth penetration of sample
- 17. Beam Energy Considerations • Lower voltages produces – greater contrast due to larger scattering angles for slow(er) moving electrons – less depth penetration – larger proportion of electrons involved in inelastic collision events
- 19. Magnification Resolution at Object (nm) 2,000 10.0 20,000 1.0 50,000 0.4 100,000 0.2 Most photographic emulsions used in electron microscopy can resolve image details of ~20µm, thus the resolution of object details will depend on the image magnification as shown in the table (resolution = 20µm/magnification):
- 20. 20 How to Obtain High Resolution • 2. Adjustments to the gun, such as the use of higher accelerating voltages, will result in higher resolution for the reasons already mentioned in the discussion on high contrast. • Chromatic aberration may be further lessened by using field emission guns since the energy spread of electrons generated from such guns is considerably narrower. (The energy spread for tungsten = 2 eV while field emission = 0.2–0.5 eV.) • In an electron microscope equipped with a conventional gun, a pointed tungsten filament will generate a more coherent, point source of electrons with better resolution capabilities.
- 21. 21 How to Obtain High Resolution • 3. Use apertures of appropriate size. – For most specimens, larger objective lens apertures should be used to minimize diffraction effects. – If contrast is too low due to the larger objective aperture, smaller apertures may be used but resolution will be diminished. – In addition, they must be kept clean since dirt will have a more pronounced effect on astigmatism. – Small condenser lens apertures will diminish spherical aberration, but this will be at the expense of overall illumination. – The illumination levels may be improved by altering the bias to effect greater gun emissions; however, this may thermally damage the specimen.
- 22. 22 How to Obtain High Resolution • It may take nearly an hour for the eyes to totally adapt to the low light levels, and this adaption will be lost if one must leave the microscope room. • Alignment must be well done and stigmation must be checked periodically during the viewing session. • The circuitry of the microscope should be stabilized by allowing the lens currents and high voltage to warm up for 1 to 2 hours before use. • Bent specimen grids should be avoided since they may place the specimen in an improper focal plane for optimum resolution. • In addition, they prevent accurate magnification determination and are more prone to drift since the support films are often detached.
- 23. 23 Magnification • Besides forming images with high resolution, the lenses of the electron microscope are able to further magnify these images. • Magnification refers to the degree of enlargement of the diameter of a final image compared to the original. • In practice, magnification equals a distance measured between two points on an image divided by the distance measured between these same two points on the original object, or
- 24. 24 Magnification • Consequently, if the image distance between two points measures 25.5 mm while the distance between these same two points on the object measures 5 mm, then the magnification is
- 25. 25 Magnification • As will be discussed later, there are at least three magnifying lenses in an electron microscope: the objective, intermediate, and projector lenses. • The final magnification is calculated as the product of the individual magnifying powers of all of the lenses in the system as shown in Equation 6.6.
- 26. Exposure Considerations • Low intensity situations lead to longer exposure times • Vibration will make edges blurry • High intensity situations lead to short exposure times (and concomitant error)
- 27. Sample Stability Considerations • High intensity or long exposure situations may cause sample to degrade (bonds break, polymer chain-scission, etc.) • Remember the contamination square in the SEM??? Same thing happens in the TEM- you will grow a nice carbon bump on samples as you look at them.
- 28. TEM Sample Prep for Materials
- 29. Imaging Modes in the TEM Bright Field Mode Dark Field Mode Diffraction Mode
- 30. Bright Field Imaging • Main portion of the transmitted beam is used to form the image – zero-order beam
- 31. Dark Field Imaging • If the transmitted beam is excluded from the image formation process – off-axis imaging – tilted beam imaging
- 33. 33 (top) Darkfield image obtained by tilting illumination system. (bottom) Same specimen viewed in standard bright field mode. Specimen consists of inorganic salt crystals.
- 34. Electron Diffraction • Elastic Scattering Events – Bragg diffraction • nl=2d sinq
- 35. Electron Diffraction • Four conditions in Back Focal Plane (BFP) of the objective lens: – No sample No reflections (only transmitted beam) – Amorphous Transmitted beam + random scattering – Polycrystal Transmitted beam + rings – Single crystal Transmitted beam + spots
- 36. Electron Diffraction Angle of incidence ~1/20 to even come close to satisfying the Bragg condition. Therefore only the lattice planes close to parallel to the beam are involved in diffraction.
- 37. Electron Diffraction • Think of TEM as a diffraction camera Transmitted Beam Diffracted Beam R L Rd=lL R is measured d is the unknown l is the electron wavelength L is the camera length (lL is the camera constant)
- 38. Electron Diffraction • Au (111) ring [2.35 Å d-spacing] With 200KV and L=65cm the (111) ring should be at about 7.5mm from the transmitted beam Rd=lL R=0.027A*650mm/2.35A
- 39. TEM Images Metal particles Polymer mix Electron Diffraction