JEOL JEM-4000FX TEM
Basic Science
Why Use TEM?
Like other electron microscopes, the transmission electron microscope uses a beam of electrons as a "light source" to take high resolution images of samples. In transmission electron microscopy, the electron beam is actually passed through a specimen, hence "transmission" electron microscopy. The TEM works like a projector, and the images recorded are essentially "shadows" of the internal electron density of the specimen. The more electron-dense regions appear darker in the image. In order for the TEM to provide clear images, however, the specimen must be very thin, with a thickness of less than 100 nm. To prepare specimens, microscopists often use the ultramicrotome to cut thin sections of material or the focused ion beam to mill the sample to the correct thickness. (See also Leica Ultracut UCT.)
Because the specimen is so thin and small (it must fit on a 3 mm x 3 mm copper grid), any data obtained from TEM analysis is not necessarily representative of the entire specimen. Also, TEM images are notorious for containing many artifacts, making the interpretation of the images difficult.
The TEM is capable of imaging transmitted, diffracted, scanning transmitted, secondary, and backscattered electrons, as well as X-rays. Backscattered electrons, however, are not collected from typical TEM specimens because they are not thick enough to generate a backscattered electron signal. Under ideal conditions, point to point resolution in transmission mode is 0.2 nm with a lattice resolution of 0.14 nm at 400 kV accelerating voltage. At that operating voltage, the best secondary electron resolution is 1-2 nm.
Structural investigations can be made with this instrument using transmitted and diffracted electrons. The analyst uses bright field images of specially thinned specimens to select areas for crystallographic study using diffracted electrons. The ability to tilt a specimen in two directions permits identification of crystalline orientations.
