Presentation on theme: "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."— Presentation transcript:
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
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