Name: how sem reveals surface details
Uploaded: 2023-04-30T12:01:00.000Z
Duration: 1 min 7 s
Channel: JoVE

Un microscope électronique à balayage ou MEB scanne un échantillon à l’aide d’un faisceau d’électrons pour fournir des informations sur la topographie et la composition de la surface de l’échantillon. 
Le MEB utilise un canon à électrons pour générer un faisceau d’électrons chargé négativement attiré vers le bas par une anode chargée positivement. Les lentilles électromagnétiques concentrent alors le faisceau d’électrons et réduisent son diamètre.
Ensuite, les bobines de balayage dévient ce faisceau d’électrons focalisé le long des axes x et y et lui permettent de balayer une zone rectangulaire de la surface de l’échantillon.
Le faisceau d’électrons primaires de haute énergie excite les électrons dans les atomes d’un échantillon, ce qui entraîne la libération d’électrons secondaires de basse énergie. Ces électrons secondaires sont lus par le détecteur et utilisés pour générer une image 3D de la surface de l’échantillon. 
L’interaction faisceau-échantillon d’électrons produit également des rayons X caractéristiques, qui fournissent des informations sur différents éléments, tels que le fer et le nickel, présents dans l’échantillon.

la microscopie Électronique à balayage

Scanning Electron Microscopy

Un microscope électronique à balayage (MEB) est utilisé pour étudier les caractéristiques de la surface d'un échantillon à l'aide d'un faisceau électronique qui balaie la surface de l'échantillon de manière bidimensionnelle. En règle générale, des zones comprises entre environ 1 centimètre et 5 micromètres de largeur peuvent être imagées. Le MEB peut être utilisé pour visualiser des bactéries, des virus, des tissus ainsi que des échantillons plus grands comme des insectes. Le MEB conventionnel donne un grossissement allant de 20X à 30 000X et une résolution spatiale de 50 à 100 nanomètres.

Principes Fondamentaux

Les électrons accélérés libérés par le canon à électrons ont une énergie cinétique élevée (allant de 5 à 30 keV). Les interactions électron-échantillon conduisent à une décélération des électrons et à une dissipation de l’énergie sous forme de différents signaux. Les électrons subissent deux types de diffusion : élastique et inélastique. La diffusion inélastique provoque l'émission d'électrons secondaires. Ces électrons de faible énergie (~ 50 eV) sont les électrons de la couche externe des atomes de l'échantillon qui acquièrent juste assez d'énergie pour quitter la surface de l'atome. Seules les informations topographiques sont fournies par la diffusion des électrons secondaires puisque le niveau d'énergie des électrons sortant des régions internes de l'échantillon est trop faible pour sortir de la surface de l'échantillon.

Les rayons X sont également générés par des collisions inélastiques des électrons incidents avec des électrons dans des orbitales discrètes (coquilles) d'atomes dans l'échantillon. Lorsque les électrons excités reviennent à des états d’énergie inférieurs, ils produisent des rayons X d’une longueur d’onde fixe (liée à la différence des niveaux d’énergie des électrons dans différentes couches pour un élément donné). Ainsi, des rayons X caractéristiques sont produits pour chaque élément d’un minéral excité par le faisceau d’électrons.

La diffusion élastique, en revanche, n'est pas provoquée par des électrons délogés des atomes de l'échantillon. Le faisceau principal d'électrons est rétrodiffusé après interaction avec le noyau. Ces électrons ne changent pas d'énergie ou de vitesse mais changent de direction en fonction de leur interaction avec le noyau. La détection de ces électrons fournit des informations sur la composition, et leur contraste variable lors de l'interaction avec des atomes de poids atomiques différents permet à l'utilisateur de distinguer les différences dans la composition de l'échantillon. Dans les échantillons biologiques, cela peut être utilisé pour étudier des nanoparticules et des nanostructures incorporées ou attachées avec des poids atomiques plus lourds, comme l’or ou le fer.

La microscopie Électronique à Balayage

Un microscope électronique à balayage (MEB) est utilisé pour étudier les caractéristiques de surface d’un échantillon à l’aide d’un faisceau d’électrons qui balaye la surface de l’échantillon dans un ...

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Cell Biology

Biologie cellulaire

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Visualizing Cells, Tissues, and Molecules

Visualisation des cellules, des tissus et des molécules

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

how sem reveals surface details

A scanning electron microscope (SEM) reveals the surface details of a sample with an electron beam. The beam scans across the sample in a two-dimensional pattern. SEM can image areas from about 1 centimeter down to 5 micrometers wide.

SEM is useful for studying bacteria, viruses, tissues, and larger samples such as insects. Conventional SEM can produce magnification from 20X to 30,000X. It also offers spatial resolution of about 50 to 100 nanometers.

The process starts when the electron gun releases accelerated electrons. These electrons carry high kinetic energy, usually in the range of 5 to 30 keV. When they hit the sample, the electrons lose energy through electron-sample interactions.

These interactions produce different signals through elastic and inelastic scattering. Inelastic scattering creates secondary electrons, which are low-energy electrons of about 50 eV. They come from the outer shell electrons of sample atoms and can escape from the surface, so they mainly show topography, or surface shape.

Inelastic collisions can also generate X-rays. These X-rays come from electrons in discrete atomic shells that drop back to lower energy states. Because each element produces characteristic X-rays with a fixed wavelength, SEM can help identify elements in a mineral.

Elastic scattering gives a different result. In this case, the main beam of electrons is backscattered after interacting with the nucleus. These electrons keep the same energy and speed, but they change direction. Detecting backscattered electrons provides compositional information and can show contrast between atoms of different atomic weights.

This contrast is useful in biological samples as well. It can help researchers study embedded or attached nanoparticles and nanostructures with heavier atomic weights, such as gold or iron. That makes SEM valuable for both surface detail and composition.

How SEM Reveals Surface Details

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scanning electron microscopy

A scanning electron microscope (SEM) reveals the surface details of a sample with an electron beam. The beam scans across the sample in a two-dimensional pattern. SEM can image areas from about 1 centimeter down to 5 micrometers wide. SEM is useful for studying bacteria, viruses, tissues, and larger samples such as insects. Conventional SEM can produce magnification from 20X to 30,000X. It also offers spatial resolution of about 50 to 100 nanometers. The process starts when the electron gun...

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key terms and concepts

Learn how a scanning electron microscope scans samples, produces secondary electrons, X-rays, and backscattered signals, and reveals surface detail and composition.

How SEM Reveals Surface Details | JoVE K12

cell biology

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How SEM Reveals Surface Details

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