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

A scanning electron microscope or SEM scans a sample with an electron beam to provide information about the topography and composition of the sample surface.&#160;&#160;
SEM uses an electron gun to generate a negatively charged electron beam attracted downward by a positively charged anode. Electromagnetic lenses then concentrate the electron beam and reduce its diameter.
Next, scanning coils deflect this focused electron beam along the x- and y-axis and allow it to scan a rectangular area of the sample surface.
The high-energy primary electron beam excites electrons in the atoms of a sample, leading to the release of low-energy secondary electrons. These secondary electrons are read by the detector and used to generate a 3D image of the sample surface.&#160;
The electron beam-sample interaction also produces characteristic X-rays, which provide information about different elements, such as iron and nickel, present in the sample.

scanning-elektronenmicroscopie

Scanning Electron Microscopy

Een scanning-elektronenmicroscoop (SEM) wordt gebruikt om de oppervlaktekenmerken van een monster te bestuderen door gebruik te maken van een elektronenbundel die het monsteroppervlak op een tweedimensionale manier scant. Normaal gesproken kunnen gebieden met een breedte tussen ~1 centimeter en 5 micrometer in beeld worden gebracht. SEM kan worden gebruikt om bacteriën, virussen, weefsels en grotere monsters zoals insecten in beeld te brengen. Conventionele SEM geeft een vergroting variërend van 20x tot 30.000x en een ruimtelijke resolutie van 50 tot 100 nanometer.

Fundamentele principes

Versnelde elektronen die vrijkomen door het elektronenkanon hebben een hoge kinetische energie (variërend van 5-30 keV). Elektron-monsterinteracties leiden tot vertraging van de elektronen en dissipatie van de energie in de vorm van verschillende signalen. De elektronen ondergaan twee soorten verstrooiing: elastisch en inelastisch. Inelastische verstrooiing veroorzaakt de emissie van secundaire elektronen. Deze laagenergetische elektronen (~50 eV) zijn de buitenste schilelektronen van de monsteratomen die net genoeg energie verwerven om het atoomoppervlak te verlaten. Alleen topografische informatie wordt verschaft door de verstrooiing van secundaire elektronen, aangezien het energieniveau van de elektronen die de interne gebieden van het monster verlaten te laag is om het monsteroppervlak te verlaten.

Röntgenstralen worden ook gegenereerd door inelastische botsingen van de invallende elektronen met elektronen in discrete orbitalen (schillen) van atomen in het monster. Terwijl de aangeslagen elektronen terugkeren naar lagere energietoestanden, leveren ze röntgenstralen op met een vaste golflengte (gerelateerd aan het verschil in energieniveaus van elektronen in verschillende schillen voor een bepaald element). Zo worden karakteristieke röntgenstralen geproduceerd voor elk element in een mineraal dat wordt geëxciteerd door de elektronenbundel.

Elastische verstrooiing wordt daarentegen niet veroorzaakt door losgeraakte elektronen uit de monsteratomen. De hoofdbundel elektronen wordt terugverstrooid na interactie met de kern. Deze elektronen veranderen hun energie of snelheid niet, maar veranderen hun richting op basis van hun interactie met de kern. Detectie van deze elektronen levert informatie over de samenstelling op, en hun variërende contrast bij interactie met atomen met verschillende atoomgewichten stelt de gebruiker in staat verschillen in de monstersamenstelling te onderscheiden. In biologische monsters kan dit worden gebruikt om ingebedde of vastgehechte nanodeeltjes en nanostructuren met zwaardere atoomgewichten, zoals goud of ijzer, te bestuderen.

Scanning-elektronenmicroscopie

A scanning electron microscope (SEM) is used to study the surface features of a sample by using an electron beam that scans the sample surface in a ...

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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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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

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How SEM Reveals Surface Details

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