Groups: WA 2,4,5,7. History The electron microscope was first invented by a team of German...
-
Upload
edward-dark -
Category
Documents
-
view
218 -
download
3
Transcript of Groups: WA 2,4,5,7. History The electron microscope was first invented by a team of German...
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. 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
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 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 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: Biology Forensics Medicine Chemistry
Amazing resolution and 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 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