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Comparison Between Scanning Electron and Transmission Electron Microscopes

BY: Muniba Safdar | Category: Biotech-Research | Submitted: 2010-10-23 10:11:11
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Article Summary: "Information about Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM); their differences, crystallographic information, composition and applications..."


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SCANNING ELECTRON MICROSCOPE:
SEMs are spotted after Reflecting Light Microscopes and produce similar information.

INVENTED: scanning electron microscope was designed by Zworykin et al in 1942.

MORPHOLOGY: the shape, size and arrangement of the particles (making up the object) are limited to a few nanometers. These particles are lying on the surface of the sample or have been exposed by grinding or chemical etching.

COMPOSITION: the sample is composed of elements and compounds and relative ratios, in areas ~1 micrometer in diameter.

Crystallographic Information: The arrangement of atoms in the specimen and their degree of order; only useful on single-crystal particles >20 micrometers.


DESIGN: it has two cathode-ray tubes that are synchronized with electron beam scanning the raster (parallel scanning line) pattern.

ELECTRON COLUMN
: two lenses, one aperture, electron gun, sample, various movable stages and detectors.

SAMPLE CHAMBER: the sample space is large. The sample allows the electrical connection and mechanical test apparatus.

TYPICAL VOLTAGE AND RESOLUTION: 1 to 50 kV, 30 angstrom or more depends on sample. SEM resolution has great depth of field.

DISPLAY: second cathode-ray tube provides a TV-like display. The display brightness and contrast is determined by detector output.

PHOTOGRAPHY: Captures the image digitally for analysis or by taking the photo of the cathode-ray tube display.


STANDARD DETECTOR: secondary electron (< 50 eV) uses scintillator. PMT (Photomultiplier tube) gives good topographical contrast.

SAMPLE FORM: big and thick sample are OK or we can use any clean solid.

SAMPLE PREPARATION: The first step for the sample preparation is to clean off dirt and grease. Then we have to check that insulators must be coated with conducting layer approximately 100 Ang thick. Then split off or evaporate metal or carbon.

MOST USEFUL FOR: scanning electron microscope is mostly used for looking at the surface or at the atomic composition, fracture, wear or corrosion surfaces, powders, polished, etched microstructures and other defects.

TRANSMISSION ELECTRON MICROSCOPE:
TEMs are spotted after Transmission Light Microscopes and will produce similar information.

INVENTED: transmission electron microscope was designed in 1933 by Ruska.

MORPHOLOGY: the shape, size and arrangement of the particles (making up the specimen) as well as their relationship to each other are limited on the scale of atomic diameter.

COMPOSITION: the sample is composed of elements and compounds and relative ratios, in areas a few nanometers in diameter.

CRYSTALLOGRAPHIC INFORMATION:
The arrangement of atoms in the specimen and their degree of order, detection of atomic-scale defects in areas a few nanometers in diameter.

DESIGN: it has one cathode-ray tube. There is no need of raster scan in TEM.

ELECTRON COLUMN: four lenses, two aperture, electron gun, sample and movable stage. Half the lenses and apertures are above the sample and half below the sample.

SAMPLE CHAMBER: sample space is large. The sample allows the electrical connection and mechanical test apparatus.

TYPICAL VOLTAGE AND RESOLUTION:
50 to 300 kV, even a million volts, 10 angstrom or more. Atomic planes are visible. The resolution of the TEM is better than SEM.

DISPLAY: inside the electron column there is a fluorescent screen at the bottom.

PHOTOGRAPHY: cartridge film is loaded inside the electron column under the fluorescent screen.

STANDARD DETECTOR: just the fluorescent screen and photographic film.

SAMPLE FORM: sample should be in foil or powder (< 1000 Ang) form, and should have thick or surface replicas.

SAMPLE PREPARATION: in case of TEM sample preparation usually takes a lot of work and may irreversibly change the material. To avoid this problem we have to use ion mill, focused ion beam electro-polishing, jet polishing, dimpling etc.

MOST USEFUL FOR:
TEM are used for the selected area electron diffraction, imaging of dislocations, tiny precipitates, grain boundaries and other defects.

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