Name: how tem reveals tiny structures
Uploaded: 2023-04-30T12:01:08.000Z
Duration: 1 min 15 s
Channel: JoVE

Un microscope électronique à transmission ou TEM est utilisé pour étudier la structure interne et la composition d'un échantillon.
L’instrument est constitué d’un canon à électrons qui émet un faisceau d’électrons. Le faisceau est ensuite focalisé sur l’échantillon à l’aide de deux ou plusieurs lentilles de condensateur électromagnétiques.
Les électrons transmis à travers l’échantillon sont collectés et focalisés par des lentilles objectives pour former l’image intermédiaire. Cette image est encore agrandie par des objectifs intermédiaires et de projecteur supplémentaires.
L’image finale peut être capturée à l’aide de dispositifs d’imagerie sensibles aux électrons, tels qu’un écran phosphorescent ou une caméra CCD.
La MET forme une image bidimensionnelle en noir et blanc d’un échantillon. Les zones plus sombres représentent des régions denses qui transmettent peu ou pas d’électrons. En revanche, les zones plus claires indiquent les régions à travers lesquelles le plus d’électrons sont transmis.
La MET est également utilisée pour analyser la composition des échantillons. L’interaction faisceau d’électrons-échantillon génère des rayons X caractéristiques qui peuvent être utilisés pour identifier et quantifier les différents éléments présents dans un échantillon.

la microscopie Électronique à transmission

Transmission Electron Microscopy

En 1931, le physicien Ernst Ruska, s'appuyant sur l'idée selon laquelle les champs magnétiques peuvent diriger un faisceau d'électrons tout comme les lentilles peuvent diriger un faisceau de lumière dans un microscope optique, a développé le premier prototype du microscope électronique. Ce développement a conduit au développement du domaine de la microscopie électronique. Dans le microscope électronique à transmission (MET), les électrons sont produits par un élément de tungstène chaud et accélérés par une différence de potentiel dans un canon à électrons, ce qui leur donne jusqu'à 400 keV d'énergie cinétique. Après avoir quitté le canon à électrons, le faisceau d'électrons est focalisé par des lentilles électromagnétiques (un système de lentilles condensateurs) et transmis à travers un échantillon à visualiser. L'image de l'échantillon est reconstruite à partir du faisceau d'électrons transmis. L'image agrandie peut être visualisée soit directement sur un écran fluorescent, soit indirectement en l'envoyant, par exemple, vers un appareil photo numérique ou un écran d'ordinateur.

L'ensemble de l'installation comprenant le canon à électrons, les lentilles, l'échantillon et l'écran fluorescent est enfermé dans une chambre à vide pour éviter la perte d'énergie du faisceau. Les modèles modernes de TEM à haute résolution peuvent avoir un pouvoir de résolution supérieur à 0,5 Å et des grossissements supérieurs à 50 millions de fois. A titre de comparaison, le meilleur pouvoir de résolution obtenu en microscopie optique est actuellement d'environ 97 nm.

Une limite du MET est que les échantillons doivent avoir une épaisseur d’environ 100 nm et que les échantillons biologiques nécessitent une préparation spéciale impliquant une « fixation » chimique pour les stabiliser en vue d’un découpage ultra fin. Pour surmonter ces limitations, plusieurs avancées dans les techniques MET, telles que la cryo-MET, ont été réalisées pour éliminer les artefacts et permettre l'imagerie directe des échantillons sans les étapes dommageables pour l'échantillon de fixation et de déshydratation de l'échantillon.

Ce texte est adapté de <a href="https://openstax.org/books/university-physics-volume-3/pages/6-6-wave-particle-duality?query=transmission%20electron&target=%7B%22type%22%3A%22search%22%2C%22index%22%3A0%7D#fs-id1163711954948">Openstax, University Physics Volume 3, Chapter 6 Photons and Matter Waves, Section 6.6: Wave-Particle Duality.</a>

La microscopie Électronique à Transmission

En 1931, le physicien Ernst Ruska s’appuyant sur l’idée que les champs magnétiques peuvent diriger un faisceau d’électrons tout comme les lentilles peuvent diriger un faisceau de lumière dans un ...

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how tem reveals tiny structures

Transmission electron microscopy uses electrons to reveal very small structures inside a specimen. The technique grew from the first electron microscope prototype developed in 1931 by physicist Ernst Ruska. He built on the idea that magnetic fields can direct an electron beam, just as lenses direct light in an optical microscope.

In a transmission electron microscope, a hot tungsten element produces electrons. An electron gun then accelerates them with a potential difference, giving the beam up to 400 keV of kinetic energy. Electromagnetic lenses, arranged as condensing lenses, focus the beam before it passes through the sample.

The image is formed from the electrons that move through the specimen. That reconstructed image can be viewed on a fluorescent screen or sent to a digital camera or computer monitor. The full system, including the electron gun, lenses, specimen, and screen, sits inside a vacuum chamber to reduce energy loss from the beam.

Modern high-resolution TEM models can resolve details smaller than 0.5 Å and reach magnifications above 50 million times. By comparison, the best resolving power of light microscopy is about 97 nm. TEM does have limits, because samples must be about 100 nm thick, and biological samples need chemical fixing before ultrathin slicing.

To reduce these limits, researchers have developed newer TEM methods such as cryo-TEM. These approaches help remove artifacts and can allow direct imaging without the sample-damaging steps of fixation and dehydration.

How TEM Reveals Tiny Structures

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Optical microscopy uses light and lenses to make tiny biological samples visible. Antonie van Leeuwenhoek built the first compound optical microscope in the 17th century to view blood cells, bacteria, and yeast cells. In 1830, Joseph Jackson Lister developed an essentially modern light microscope. During the 20th century, microscopes gained better magnification and resolution. A sample is placed on a glass slide and clipped onto the stage, which is the flat platform of the microscope. The...

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transmission electron microscopy

Transmission electron microscopy uses electrons to reveal very small structures inside a specimen. The technique grew from the first electron microscope prototype developed in 1931 by physicist Ernst Ruska. He built on the idea that magnetic fields can direct an electron beam, just as lenses direct light in an optical microscope. In a transmission electron microscope, a hot tungsten element produces electrons. An electron gun then accelerates them with a potential difference, giving the beam...

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key terms and concepts

Learn how transmission electron microscopy uses electrons, magnetic lenses, and a vacuum to image ultra-thin samples at extreme magnification.

How TEM Reveals Tiny Structures | JoVE K12

cell biology

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How TEM Reveals Tiny Structures

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