2 nd Lecture
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Scanning electron microscope
 The Key Components of a Scanning Electron Microscope
Low-voltage electron microscope
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Comparision of light microscope and electron microscope:
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Principles of Microscopy:
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Wavelength
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Resolution
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Resolving Power (RP)
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Reflection
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Transmission
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Absorption
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Refraction
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Condenser
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As the electron beam copy the object, it interacts with the surface of the
object, dislodging secondary electrons from the surface of the
specimen in unique patterns. A secondary electron detector
attracts those scattered electrons and, depending on the number of
electrons that reach the detector, registers different levels of
brightness on a monitor. Additional sensors detect backscattered
electrons (electrons that reflect off the specimen's surface) and Xray (emitted from beneath the specimen's surface). Dot by dot, row
by row, an image of the original object is scanned onto a monitor for
viewing (hence the "scanning" part of the machine's name).
Generally, the image resolution of an SEM is poorer than that of a TEM.
However, because the SEM image relies on surface processes
rather than transmission, it is able to image bulk samples up to many
centimeters in size and (depending on instrument design and
settings) can produce images that are good representations of the
three-dimensional shape. Another advantage of SEM is its variety
called environmental scanning electron microscope (ESEM) can
produce images of sufficient quality and resolution with the samples
being wet or contained in low vacuum or gas. This greatly facilitates
imaging biological samples that are unstable in the high vacuum of
conventional EM.
The low-voltage electron microscope (LVEM) is a combination of SEM,
TEM and STEM in one instrument, which operates at relatively low
electron accelerating voltage of 5 kV.
Low voltage reduces the specimen damage by the incident electrons
and increases image contrast that is especially important for
biological specimens.
Electron gun: they produce the steady stream of electrons necessary
for SEMs to operate.
Lenses: they aren't made of glass. Instead, the lenses are made of
magents capable of bending the path of electrons. By doing so, the
lenses focus and control the electron beam, ensuring that the
electrons end up precisely where they need to go.
Sample chamber: The sample chamber of an SEM is where
researchers place the specimen that they are examining. Because
the specimen must be kept extremely still for the microscope to
produce clear images. The sample chambers of an SEM do more
than keep a specimen still. They also manipulate the specimen,
placing it at different angles and moving it so that researchers don't
have to constantly remount the object to take different images.
Detectors: To detect the electron beam interacting with the sample
object, Everhart-Thornley detectors register secondary electrons,
which are electrons dislodged from the outer surface of a specimen.
These detectors are capable of producing the most detailed images
of an object's surface. Other detectors, such as backscattered
electron detectors and X-ray detectors, can tell researchers about
the composition of a substance.
Vacuum chamber: SEMs require a vacuum to operate. Without a
vacuum, the electron beam generated by the electron gun would
encounter constant interference from air particles in the atmosphere.
Wavelength: Length of a light ray; represented by the Greek letter
lambda () Equal to the distance between two adjacent crests or
troughs of a wave. The shorter the wavelength used, the greater the
resolution that can be attained
Resolution: Refers to the ability to see two items as separate and
discrete units
Resolving Power (RP) of a lens: is a numerical measure of the
resolution that can be obtained with that lens. The smaller the
distance between objects that can be distinguished, the greater the
resolving power of the lens.
Reflection: If the light strikes an object and bounces back (giving the
object color).
Transmission: The passage of light through an object
Absorption: The light rays neither pass through nor bounce off an
object but are taken up by the object
Refraction: The bending of light as it passes from one medium to
another of different density.
Condenser:
lens system located under the microscope stage that
focuses light onto the specimen