Main parts of electron microscopes (EM)
Like in light microscopes in
Transmission Electron Microscopes (TEM) and
Scanning Electronen Microscopes (SEM) one can speak about a source (here an electron source, instead of a source of photons). In the column of the microscope one can find a number of diaphragms that limit dispersion from the electron beam, as well as lenses (here electro-magnetic coils) that affect the trajectory of the electrons:
- a condenser lens to concentrate the beam
- an objective lens for the focus
- stigmator lenses to obtain a nice round electron beam
- in SEM, coils to produce the xy scan of the electron beam onthe sample
- in TEM, a projection lens (beneath the objective lens) to project the image on a screen.
The sample that is being observed is held in a high-vacuum object chamber that can be reached from outside through an inslide chamber (number 4 in the pictures of the microscopes here below). In SEM this chamber can be found right below the column, but in TEM it is located about half way the column. These positions are related with the manner according to which an image is formed by means of electrons in TEM and SEM. Depending whether one wants to apply TEM or SEM techniques, the sample will have to be subjected to distinct preparative procedures (for
TEM or
SEM samples).
For further information on SEM, you can follow the links in the left menu, scroll down to next paragraph or open a summary in pdf or Word. Link to illustrated presentations on this site on IMAGING TECHNIQUES, i.e. light, video, confocal laser scanning and transmission electron microscopy.
Electrons and radiation in TEM and SEM
In TEM as well as SEM the sample is bombarded by a beam of electrons, so-called
primary electrons that originate from an electron gun. The principles of image formation,however, differ:
| Electrons in Transmission & Scanning Electron Microscopy |
| TEM | SEM |
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TEM:
In TEM most objects (e.g. cells) have to be cut in outermost thin sections (<100nm) to allow electrons to pass through and they need to be given additional contrast (be 'stained') by treatment with heavy metals that bind to specific structures in the sample. Electrons can be more or less pass through various region in the sample, depending on the local density of the material, but regions occupied by heavy metals will totally deflect electrons. Based on the density of transmitted electron, a projection image of the sample is raised by means of an intermediate fluorescing screen and a digital camera that produces gray-shades images. TEM are mainly used to show structures inside the object (see example of cell organelles here below). However, sometimes TEM are employed to visualize the contours of intact, but extremely small electron-translucent objects, like virusses or macromolecular aggregates. (More about the TEM technique elsewhere on this site).
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SEM:
In SEM the image is formed from secondary electrons that have been dislocated at the surface of the scanned sample by bombarding primary electrons from the electron gun. Those ejected electrons are captured by a detector and the information is converted into an electric signal, amplified and digitalized. The result is a topographical image of the surface of the object, e.g. the surface of a metal coating or lamellae of fish gills (see example here below). Besides secondary electrons, radiation (in particular X-rays and cathodoluminescence in typical samples) as well as back-scattered and so-called Auger electrons with an own energy level are produced upon interaction of atoms in the surface layer of the sample with the primary electron beam. These emission signals, which contain information among others on the element composition of the upper layer, can be received by selected detectors, as is the case in EDAX microscopes for example, and combined with the topographical image.
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| Transmission versus scanning electron microscopy |
| TEM | SEM |
 |  |
Transmission Electron Microscope (TEM)
1: Electronen cannon in the upper part of the column. 2 Electro-magnetic lenses to direct and focus the electron beam inside the column. 3: Vacuum pumps system. 4: Opening to insert a grid with samples into the high-vacuum chamber for observation. 5: Operation panels (left for alignment; right for magnification and focussing; arrows for positioning the object inside the chamber). 6: Screen for menu and image display. 7: Water supply to cool the instrument. |
Scanning Electron Microscope (SEM)
1: Electron gun in the upper part of the column (here a so-called field-emission source). 2 Electro-magnetic lenses to direct and focus the electron beam inside the column. 3: Vacuum pumps system. 4: Opening to insert the object into the high-vacuum observation chamber in conventional SEM mode. 5: Operation panel with focus, alignment and magnification tools and a joystick for positioning of the sample. 6: Screen for menu and image display. 7: Cryo-unit to prepare (break, coat and sublimate) frozen material before insertion in the observation chamber in Cryo-SEM mode. 8: Electronics stored in cupboards under the desk. 9: Technicians Mieke Wolters-Arts and Geert-Jan Janssen discussing a view. |
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A. Example of application of Transmission Electron Microscopy (TEM)
Organelles in a pollen grain of tobacco (Nicotiana tabacum; AF = Actin filaments; G = Golgi apparatus; Mi = Mitochondrion; Mt = Microtubule).
(Zoom; Imaging: A.M. Wolters-Arts, Radboud University Nijmegen). | B. Example of application of Scanning Electron Microscopy (SEM)
Overview of gills of a fish, the mudskipper (Periophthalmus argentilineatus).
(Zoom; Imaging: G. Kruitwagen and H.P.M. Geurts, Radboud University Nijmegen). |
