Environmental Solid State Chemistry - Nanocystalline Materials - Geochemical Cycling
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Transmission Electron Micrographs of various nanoparticles
What is a transmission electron microscope (TEM)?
A TEM is a transmission microscope that uses a beam of electrons to examine objects at extremely high magnification. Transmission means that the beam passes through the sample and means that the sample must be very thin (less than 0.000001 m thick) so that the electrons can pass through the specimen. In addition, the system requires an ultra-high vacuum. Magnification can range from 50x (tpyical for an optical microscope) to more than 20,000,000x.
TEM is ideal for characterizing extremely small features. The animated GIF shown above is a compilation of TEM images of nanoparticles. Nanoparticles are so named because they have at least one dimension that is smaller than 100 or so nanometers. Light microscopy is not useful when such small particles are involved because the wavelength of visible light is on the order of 400 to 700 nanometers. With a good light microscope and a monochromatic light source, a microscopist could theoretically distinguish features of dimensions of approximately one-half the wavelength of light used (i.e., thousands of atoms across).
In order to increase the resolving power of a microscope, a smaller wavelength is required. The nonrelativistic wavelength of electrons typically used in modern TEM's ranges from 1 to 4 picometers. Now, the microscopist can distinguish features on the SUB-NANOMETER length scale (atomic resolution).
With TEM, we can characterize particle size, morphology, and microstructure. In addition, we can identify trace components (by using electron diffraction) and quantitatively determine composition at the nanometer length-scale (by using electron energy loss spectroscopy or energy dispersive x-ray spectroscopy).
Why use TEM in chemistry?
Many of the features that control chemical behavior and materials properties are extremely small, often on the nanometer length-scale. To characterize such features like dislocations, nano-texture, and nanoparticle morphology, electron microscopy is a technique of choice.
Where are some electron microscopes at the University of Minnesota?
At the Characterization Facility!Some Electron Microscopy Links
Microscopy Society of America
Ask-A-Microscopist
The National Center for Electron Microscopy
Microworld