Transmission electron microscope

Beam path in TEM with crystalline sample, shown in simplified form. The upper diffraction plane corresponds to the rear focal plane of the objective.

The transmission electron microscopy ( TEM stands for transmission electron microscope) is a mode for electron microscopes , which use a direct mapping of objects of electron beams allows. In the 1930s, the resolution of optical microscopes was first undercut by the pioneering work of Max Knoll and his then doctoral student Ernst Ruska . The latter was awarded the Nobel Prize in Physics in 1986 . The current limit of resolution is 0.045 nm.

functionality

A scientist at a transmission electron microscope

The electrons shine through the object, which must be sufficiently thin. Depending on the atomic number of the atoms that make up the object, the level of the accelerating voltage and the desired resolution, the sensible object thickness can range from a few nanometers to a few micrometers. The beam path runs in a vacuum so that the electrons are not deflected by air molecules. Typical acceleration voltages of TEM are 80 kV to 400 kV, with the range below 200 kV being used more for the investigation of biological materials (80 kV to 120 kV are usually used here), while material science tasks are more likely to be solved with 200 kV or higher voltages. The highest usable acceleration voltage is an essential performance characteristic of a TEM. However, the highest possible acceleration voltage is not always the most suitable for a particular study.

The higher the atomic number and the lower the acceleration voltage, the thinner the object has to be (see section “Sample preparation”). A thin object is also required for high-resolution images. The electrons supplied by the electron source are deflected by the condenser lens system in such a way that they evenly illuminate the object section to be observed and all of them strike the object approximately parallel to one another.

In the sample to be examined, the electrons change their direction of movement in the form of Rutherford scattering . In some cases they also lose kinetic energy ( inelastic scattering ). Elastically scattered electrons that leave the object at the same angle are focused in one point in the back focal plane of the objective lens.

With a diaphragm (lens diaphragm or contrast diaphragm), only the electrons that have not been scattered can pass in this plane. As atoms with a higher atomic number and thicker object areas scatter more, the resulting contrast is called mass thickness contrast . In the case of amorphous solids, this allows a very simple interpretation of the images obtained.

The contrast of crystalline materials follows more complicated laws and is called diffraction contrast . Since, under certain conditions, the image intensity shows strong variations with small local changes in the crystal structure (inclination, atomic distance), as they occur in the vicinity of crystal structural defects (various-dimensional defects) due to internal stresses in the lattice, the real structure of solids can be excellently shown investigate (see also Fig. dislocation lines).

The projective lens system throws the first intermediate image generated by the objective lens system, further enlarged, onto a detector. A fluorescent screen for direct observation, for example, which is usually coated with fluorescent zinc sulfide, can be used as such . If the image is to be recorded, photographic films or plates (image plates) or a CCD camera are used. CCD elements would be quickly destroyed by direct bombardment with the very high-energy beam electrons, which is why the electron intensity is first converted into light with a scintillator , which is then guided to the CCD chip via transfer optics (usually optical fiber bundles). The use of imaging plates has the advantage that the high-energy radiation does not damage them and the image can be recorded directly. Image plates are often used in electron diffraction.

By changing the projective lens system, the focal plane of the objective lens can also be shown enlarged instead of the intermediate image. This gives an electron diffraction pattern, which can be used to determine the crystal structure of the sample.

A transmission electron microscope can also be used to examine the surface morphology of objects that are too thick even for electrons to pass through directly. Instead of the original object, a surface impression is examined. A carbon footprint is very beneficial even for very rough surfaces .

Dislocation lines (dislocation contrast) of an alloy

Special procedure

In energy-filtered transmission electron microscopy (EFTEM), the kinetic energy of the electrons that has changed due to the passage of the object is used in order to be able to make chemical statements about the object, such as the distribution of the elements .

The high-resolution transmission electron microscopy (engl. High Resolution Transmission Electron Microscopy , HRTEM) allows the image of the atomic arrangement in crystalline properties and is based on the phase contrast , the coherence of the electron wave is utilized.

With the HAADF signal (HAADF stands for High Angle Annular Dark Field ) of the scanning transmission electron microscope , however, an incoherent high-resolution image can be achieved. Other special TEM methods are e.g. B. electron holography , differential phase contrast, Lorentz microscopy and high-voltage microscopy .

The use of graphene as a slide makes it easier to use the theoretical resolution of the TEM to image individual atoms.

Sample preparation

The main requirement for a TEM sample is that the area to be examined is around 10-100 nm thick; for some examinations and samples, thicknesses of a few 100 nm are sufficient.

Typical inorganic samples

To examine metals in the TEM, slices are first cut from the sample material and ground to a thickness of about 0.1 mm. In most cases, the metal can then be thinned by electrolytic polishing so that a small hole is formed in the center of the disc. At the edge of this hole the metal is very thin and can be irradiated with electrons.

Metals, for which electrolytic polishing does not give satisfactory results, as well as non-conductive or poorly conductive materials such as silicon , ceramics or minerals can be made transparent for electrons by ion thinning (also ion milling ). Since the removal rate of this process is in the range of a few micrometers per hour, the samples are first mechanically thinned. Commonly used here are so-called dimplers , with which a cavity is ground in the center of the sample disc, as well as the so-called " tripod method " , in which the sample material is manually formed into a wedge is sanded.

With the help of a focused ion beam system, samples can be obtained from a specific sample area. For this purpose, a lamella is cut out of the interesting area of ​​the sample with a gallium ion beam , transferred to a sample holder (“ lift-out ”) and thinned until it becomes electron-transparent. In another method, the sample is first mechanically thinned and then a transparent window is thinned into the edge of the sample (" H-bar ").

Nanoparticles , which are intrinsically electron- transparent , are applied in a suspension to an equally transparent carrier film (e.g. amorphous carbon , possibly with small holes in it). When the suspension dries off, the particles adhere to the film. During the investigation, either the carrier film and a particle are irradiated, or it is possible to find particles that adhere to the carrier film, but lie freely over a hole; in this case the carrier film does not interfere with the examination at all.

Metals can be examined in a similar way. For this purpose, the sample is coated with a thin carbon film, which is then removed by etching . The resulting replica not only reflects the topography of the sample, but with appropriate metallographic preparation, non-metallic inclusions and precipitates also adhere to it.

Biological samples

DNA plasmids in different conformations in the TEM image after BAC spreading (uranium negative contrasting) 60,000 × / 80 kV

Biological samples to be viewed in the TEM have to go through a series of preparations. It depends on the scientific question which method is used.

• Fixation - to make the sample more realistic. Use glutaraldehyde for cross-linking and hardening of proteins and osmium tetroxide , which lipids black colored and simultaneously fixed.
• Cryo fixation - the sample issnap frozenin liquid ethane at less than −135 ° C. The water does not crystallize, but forms vitrified (glass-like) ice. This method fixes the biological sample with the least amount of artifact. However, the contrast is very low.
• Dehydration - water is removed and gradually replaced with ethanol or acetone .
• Embedding - to be able to section tissue. Acrylic resins are mostly used for this.
• Sectioning - dividing the sample into thin slices ( ultra-thin sections ). These can be cut on an ultra- microtome with a diamond or glass blade.
• Immune gold staining - Occasionally, antibodies marked with gold particles are stained individual epitopes .
• Negative staining ( Negative Stain ) - Heavy atoms such as lead - or uranium -atoms scatter electrons more strongly than light atoms and thus enhance the contrast (mass thickness contrast). Here reagents such as uranyl acetate , osmium tetroxide , ruthenium tetroxide , tungstophosphoric acid or lead citrate are used.
• High-Pressure Freezing (HPF) - with this method, very small sample quantities are shock-frozen under high pressure. This method is particularly suitable for immune electron microscopy, as the surface structure of the sample is only minimally changed.

Commons : Transmission Electron Microscope  - Collection of Images, Videos and Audio Files
Wiktionary: Transmission electron microscope  - explanations of meanings, word origins, synonyms, translations

swell

1. ^ Information from the Nobel Foundation on the 1986 award to Ernst Ruska
2. Hidetaka Sawada, Naoki Shimura, Fumio Hosokawa, Naoya Shibata and Yuichi Ikuhara: Resolving 45-pm-separated Si-Si atomic columns with an aberration-corrected STEM. 2015, accessed November 16, 2016 .
3. Breakthrough thanks to graphs - ideal slide for TEM. In: Spectrum of Science. No. 1, 2009, pp. 21-22.