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