Transmission Electron Microscopy (TEM) By Austin Avery

Overview What is Transmission Electron Microscopy? History of TEM Theory of TEM The instrumentation How is TEM useful? Pros/Cons Summary

Transmission Electron Microscopy TEM is an instrumental technique that uses a tiny focused beam of electrons. These electrons interact with a very thin and tiny sample, usually only a couple atoms thick. After passing through the sample the electrons have changed course slightly and are detected by a photographic film or a CCD camera.

History of TEM In 1931 Max Knoll and Ernst Ruska developed the first TEM microscope, considered the first ‘electron microscopy’. The group was first interested in further developing the resolution of Cathode Ray Oscilloscopes. After WW2 Ruska was finally able to produce the first TEM with 100,000x resolution power.

TEM Theory The reason a TEM can have such a large resolution factor at such a small size is due to the de Broglie λ of electrons. The wavelength λ e = h/√[2m o E(1+(E/2m o c 2 ))] of electrons where h is planck’s constant, m o =9.11x kg, E is the energy of the accelerated electron, and c is speed of light.

Theory Because electrons have wave-particle duality properties, they can be produced at a specific energy and wavelength and then analyzed after being affected by a sample Max resolution d=λ/(2n sin α) Wavelegth λ Numerical Aperture

Instrumentation The TEM consists of an Electron gun made from a Tungsten filament or a Lanthanum hexaboride crystal The e - gun is charged with about kV before any electrons of reasonable energy will be released from the source. Then a series of electromagnetic and electrostatic lenses direct the electrons into a beam

Optics There are 3 sets of lens on a typical TEM Condenser lens: Electron beam formation Objector lens: focus of beam onto sample Projector lens: expands electron beam into analytical form onto analysis screen or CCD Magnification adjustments are made by varying the current through the quadrupole or hexapole lenses

Vacuum Components Vacuum system: ~10 -4 to Pa Very important to prevent arc between cathode and ground Mean free path of electrons Better beam focusing, less interaction with gas molecules

Sample Components Sample grids: Usually 3mm diameter mesh ring with 1-100μm size squares made of Cu, Mb, Au, or Pt Samples inserted into section with air locks to prevent large decreases in vacuum pressure Stages are designed to adjust the samples orientation when inside the TEM for more accurate readings

Electron Lens Components Electron lens: Focus parallel rays at a constant length TEM lenses are usually electromagnetic Made of Fe, Fe-Co, or Ni-Co because of their magnetic properties like magnetic saturation, hysteresis, and permeability

Aperture Components Filter electrons that can stray from the beam path and affect image quality Decrease beam intensity, helpful with beam sensitive samples Can be fixed or moveable, depending on the quality of the instrument or manufacturer Made of metallic substances thick enough to stop stray electrons but allow axial electrons through

Why use a TEM? A TEM is able to form images of sample molecules and atoms 10’s of thousands times smaller than any visible light microscopes Using the stage tuning a “tilt series” can be developed to resolve 3D images of samples

Scan Types and Uses Bright Field Diffraction Contrast Dark Field Image Crystallography-lattice defects Biological specimen-many Sub-atomic ratios

Pros/Cons Pros: Very high resolution Requires very little sample to test Quantitative and Qualitative Can be modified in many ways to account for different substances and requirements Cons: Tough sample prep. Hour consuming runs to get a few images Small field of view, may take several runs to find what is being studied Sample destruction, especially biological samples

Summary TEM is useful for small, nanoscale analytes TEM can create 3D images of samples TEM can be modified for different types of molecules and atoms TEM is not cheap TEM is GOOD! And that’s the way the cookie crumbles…

References Kirkland, E (1998). Advanced computing in Electron Microscopy. Springer. Hubbard, A (1995). The Handbook of surface imaging and visualization. CRC Press Joachim Frank, editor (2006). Frank, J. ed. Electron tomography: methods for three- dimensional visualization of structures in the cell. Springer