Principle : The transmission electron microscopy (tem or tem in english for transmission electron microscopy is a microscopy technique based on the principle of diffraction of electrons and can achieve a magnification of x5, 000 000.
it consists of placing a sample that is sufficiently thin under an electron beam used in coherent beam, and to view the hologram obtained is the diffraction pattern in the focal plane of the objective, that is, to use another lens to get the figure fourier transform of the diffraction pattern that is observable by the impact of electrons on a fluorescent screen or to record it on a plate photo.
the limit of resolution depends on the wavelength of de broglie of electrons, so their voltage acceleration, it would therefore be of the order of magnitude of the picomètre in an ideal case. but because of the strong aberrations, it is not in reality that of a few ångstroms.
The transmission electron microscope has two main modes of function depending on whether you get a picture or a snapshot of diffraction :
- image mode : the electron beam passes through the sample. depending on the thickness, the density or the chemical nature of the latter, the electrons are more or less absorbed. by placing the detector in the image plane, one can, for transparency, to observe an image of the irradiated area. it is this principle that is used in biology to observe cells or cups thin bodies.
- mode diffraction mode : this mode uses the behavior wave of the electron wave (de broglie) (this is modeled by the quantum physics). when they encounter the material organized (crystals), so they will be diffracted, that is to say deflected in certain directions depending on the organization of atoms. the beam is diffracted into several smaller beams, and these recombine to form the image, thanks to magnetic lenses (electromagnets which deflect the electrons).