Electron Microscopes - Unit 1

Notes on Scanning and Transmission electron microscopes and the differences between them.

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Electron Microscopes'
A transmission electron microscopes (TEM) can magnify a
sample up to one million times. The sample must be cut
extremely thin. An electron beam is directed onto the sample to
be magnified and some of the electrons pass through and form a
magnified image of the specimen.
A scanning electron microscope (SEM) can magnify a sample
up to 100,000 times. A sharply focused electron beam
moves over the sample to create a magnified image of the
surface. Some electrons in the beam scatter off the sample
and are collected and counted by an electronic device. Each
scanned point on the sample corresponds to a pixel on a
television monitor; the more electrons the counting device
detects, the brighter the pixel on the monitor is. As the
electron beam scans over the entire sample, a complete
image is displayed on the monitor. SEMs are particularly
useful because they can produce
three-dimensional images of the surface of objects.
A SEM scans the surface of the sample bit by bit while a TEM which looks at a sample
all at once. The scanning transmission electron microscope (STEM) combines
elements of an SEM and a TEM and can resolve single atoms in a sample.
Electron beam passes through thin Electron beam scans over surface
sample. of sample.
Specially prepared thin samples or Sample can be any thickness and
particulate material are supported on is mounted on an aluminium stub.
TEM grids.
Specimen stage halfway down column. Specimen stage in the chamber at
the bottom of the column.
Image shown on fluorescent screen. Image shown on TV monitor.
Image is a two dimensional projection of Image is of the surface of the
the sample. sample.
Example of TEM image

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Thin section of E.coli bacteria
Example of SEM image
E.coli bacteria.…read more


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