Lecture 3 Brief Overview of Traditional Microscopes
• Optical Microscope;
• Scanning Electron Microscope (SEM);
• Transmission Electron Microscope (TEM);
• Comparison with scanning probe microscope (SPM)
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Scanning Electron Microscope (SEM), Lecture notes of Physical Chemistry
In scanning electron microscopy (SEM) an electron beam is focused into a small probe and is rastered across the surface of a specimen. • Several interactions ...
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Lecture 3 Brief Overview of Traditional Microscopes
- Optical Microscope;
- Scanning Electron Microscope (SEM);
- Transmission Electron Microscope (TEM);
- Comparison with scanning probe microscope (SPM)
General philosophy
Human beings use two kinds of means to gauge
objectives:
Seeing --- through eyes (light) ↔ optical microscope
↔ electronic microscope (higher resolution due to short
wavelength);
Touching --- through hands (probe) ↔ SPM.
Converting wavelength (nm) to electron energy (eV): 1240.7/λ(nm), for example, a 500 nm light corresponds to 1240.7/500 = 2.48 eV
Development of microscope takes us back to 400 years ago
Robert Hooke’s "Micrographia" (1665)
A hair knot
Red blood cell
Flu virus
Advantages and Disadvantages of Optical Microscope
Advantages:
- Direct imaging with no need of sample pre-treatment, the only
microscopy for real color imaging.
- Fast, and adaptable to all kinds of sample systems, from gas, to liquid,
and to solid sample systems, in any shapes or geometries.
- Easy to be integrated with digital camera systems for data storage and
analysis.
Disadvantages:
- Low resolution, usually down to only sub-micron or a few hundreds of
nanometers, mainly due to the light diffraction limit.
Electronic Microscope for higher resolution
- Resolution limit of optical microscopes is due to the light diffraction; roughly optical
resolution can be estimated as wavelength λ/2NA (NA is the numerical aperture of
lens, usually ~ 1.0): for white light, average wavelength is around 500 nm, the best
resolution is thus a few hundreds nm.
- Decreasing the wavelength is the way to improve the resolution, though nobody would deal with UV light.
- Electron wave is a unique medium that can be used in imaging. By accelerating
the electrons into high energy beam (via high voltage), the wavelength thus
created is far shorter than white light. For example, for an electron beam produced
from a 20 kV gun, the wavelength is only 1240.7/20,000 (eV) = 0.06 nm = 0.6 Å, corresponding to a resolution limit of λ/2 = 0.3 Å --- theoretically, it can be used to
image a species as small as 0.3 Å. Most atoms are in size of 2-3 Å.
History of Electronic Microscope
The Nobel Prize in Physics 1986
Ernst Ruska
Born: 25 December 1906, Heidelberg, Germany
Died: 27 May 1988, West Berlin, Germany
Affiliation at the time of the award: Fritz-Haber-Institut der Max-Planck-
Gesellschaft, Berlin, Federal Republic of Germany
"for his fundamental work in electron
optics, and for the design of the first
electron microscope"
What is SEM?
- In scanning electron microscopy (SEM) an electron beam is focused into
a small probe and is rastered across the surface of a specimen.
- Several interactions with the sample that result in the emission of
electrons or photons occur as the electrons penetrate the surface.
- These emitted particles can be collected with the appropriate detector to
yield valuable information about the material.
- The most immediate result of observation in the scanning electron
microscope is that it displays the shape of the sample.
- The resolution is determined by beam diameter.
Detailed working diagram of SEM
Basic components of SEM
Electron optics:
1. condenser lens --- focusing the electron beam to the objective lens.
2. objective lens --- responsible for the size of electron beam impinging on sample surface
3. electromagnetic coils --- responsible for driving the raster scanning by deflecting the
electron beam.
- Sample and sample holder:
1. size --- centimeters.
2. rotation --- sample can be rotated freely at three dimensions, x, y, z, to achieve imaging
at all directions.
3. conductivity --- required for sample to be measured. For non-conducting samples, like
biological specimens, metallic coating is required.
- Transducers (detectors):
1. scintillation device --- doped glass or plastic target that emits a cascade of visible
photons when struck by electrons.
2. semiconductor transducers --- when struck by electrons, electron-hole pairs are
generated, thus increasing the conductivity.
Brief schemes showing how SEM works
Brief schemes showing how SEM works