Transcript Electron Microscopy! - University of Northern Colorado

ELECTRON MICROSCOPY
Groups: WA 2,4,5,7
History
 The electron microscope was first
invented by a team of German
engineers headed by Max Knoll
and physicist Ernst Ruska in 1932
 They used Louis de Broglie’s
theory of electron waves
developed in 1924
 If you increase a particle’s
momentum, its wavelength will
decrease, allowing for higher
resolution.
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Having higher resolution means
having a higher degree of detail
visible in a photographic image.
History
Velocity
 Need to know mass of
electron, its charge and
electric potential
 80 kV electrons have a
velocity of 150,000 km/s
(1.5 x 10^8 m/s)
 Wave particle duality
concept of quantum
physics asserts that all
matter exhibits both wave
and particle like properties
Diffraction pattern of
Electron Waves
Overview
 Electron microscopy (EM) is a
technique that uses an
electron microscope that
sends a beam of electrons
instead of light (photons) to
create an image of the
specimen
 A series of electromagnetic
lenses and apertures are used
to reduce the diameter of the
beam
 Electrons are controlled by
changing the current through
the lenses
Mechanics
Thermionic Guns
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These are the most commonly found electron guns.
Heats a filament
Gives energy to electrons in atomic orbitals
Allows the electron to cross potential energy barrier
Mechanics
Field Emission Guns
 An electrostatic field is
produced
 Reduces the potential
energy barrier of an
electron
 Allows electrons with
enough energy to cross
barrier
 These guns often give a
brighter picture, but
require very good
vacuums.
Mechanics
Electromagnetic Lens
 The thick black bands represent the
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iron casing
The blue rings represent a wire that
coils around to create a solenoid
The red lines represent the magnetic
field lines
The blue lines represent electron
beam pathway
The field focuses the electrons to a
focal point – the stronger the field,
the shorter the focal path.
Electrons adopt a helical trajectory.
Scattered Detection
 Electrons interact with
specimen and secondary
electrons are produced
 When the secondary
electrons are accelerated:
 create energy to produce a
flash
 Flash detected by the
Everhart-Thornley Detector
 Detector sends the info to a
computer screen.
Types
Transmission Electron Microscopes
• Electrons travel through condenser
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lenses, specimen, objective lens,
then projection lens before placing
an optical image on a fluorescent
plate
Beam speed is between 40 and 400
kiloelectron volts
Works like a projector
Specimen limited to 100 nm
thickness
Cannot view surface
Types
Scanning Electron
Microscopes
 Beam speeds between 50 and
30,000 volts
 Beam interact with surface
and reactions are recorded by
sensors
 Interacts by include
producing heat, producing
low energy electrons, high
leveled backscattered
electrons, light and/or x-ray
emissions
 Rotate the specimen in X,Y
and Z directions
A comparison between light microscopy and two
types of electron microscopy
Optical v. Electron
Light Microscope
Electron Microscope
Advantages
 The electron microscope can be
beneficial to certain studies:
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Biology
Forensics
Medicine
Chemistry
 Amazing resolution and
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magnification power (2 million
times)
Chemical composition of
specimen
2D and 3D (SEM) images
Able to visualize structures that
are impossible to see with other
equipment
Higher depth of field
Limitations
 Preservation methods
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must be taken, on the
object such as plating,
dehydration, or
freezing.
Must be a small sample
Sample also must be in
vacuum
Radiation
Very expensive to buy
and maintain
Black and White Images