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 MicroscopeElectron 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