What are Electron Microscopes?
- Electron Microscopes are scientific
instruments that use a beam of highly energetic electrons to examine
objects on a very fine scale. This examination can yield the following
information:
- Topography
- The surface features of an object or
"how it looks", its texture; direct relation between these
features and materials properties (hardness, reflectivity...etc.)
- Morphology
- The shape and size of the particles
making up the object; direct relation between these structures and
materials properties (ductility, strength, reactivity...etc.)
- Composition
- The elements and compounds that the
object is composed of and the relative amounts of them; direct
relationship between composition and materials properties (melting
point, reactivity, hardness...etc.)
- Crystallographic Information
- How the atoms are arranged in the
object; direct relation between these arrangements and materials
properties (conductivity, electrical properties, strength...etc.)
Where did Electron Microscopes Come From?
-
- Electron Microscopes were developed due
to the limitations of Light Microscopes which are limited by the
physics of light to 500x or 1000x magnification and a resolution of
0.2 micrometers. In the early 1930's this theoretical limit had been
reached and there was a scientific desire to see the fine details of
the interior structures of organic cells (nucleus,
mitochondria...etc.). This required 10,000x plus magnification which
was just not possible using Light Microscopes.
The Transmission Electron Microscope (TEM) was the first type of
Electron Microscope to be developed and is patterned exactly on the
Light Transmission Microscope except that a focused beam of electrons
is used instead of light to "see through" the specimen. It
was developed by Max Knoll and Ernst Ruska in Germany in 1931.
The first Scanning Electron Microscope (SEM) debuted in 1942 with the
first commercial instruments around 1965. Its late development was due
to the electronics involved in "scanning" the beam of
electrons across the sample. An excellent article was just published
in Scanning detailing the history of SEMs and I would encourage
those interested to read it.
How do Electron Microscopes Work?
-
- Electron Microscopes(EMs) function
exactly as their optical counterparts except that they use a focused
beam of electrons instead of light to "image" the specimen
and gain information as to its structure and composition.
The basic steps involved in all EMs:
- A stream of electrons is formed (by the
Electron Source) and accelerated toward the specimen using a positive
electrical potential
- This stream is confined and focused
using metal apertures and magnetic lenses into a thin, focused,
monochromatic beam.
- This beam is focused onto the sample
using a magnetic lens
- Interactions occur inside the irradiated
sample, affecting the electron beam
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