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

Un microscopio elettronico a scansione o SEM scansiona un campione con un fascio di elettroni per fornire informazioni sulla topografia e la composizione della superficie del campione. 
Il SEM utilizza un cannone elettronico per generare un fascio di elettroni con carica negativa attratto verso il basso da un anodo caricato positivamente. Le lenti elettromagnetiche concentrano quindi il fascio di elettroni e ne riducono il diametro.
Successivamente, le bobine di scansione deviano questo fascio di elettroni focalizzato lungo gli assi x e y e gli consentono di scansionare un'area rettangolare della superficie del campione.
Il fascio di elettroni primari ad alta energia eccita gli elettroni negli atomi di un campione, portando al rilascio di elettroni secondari a bassa energia. Questi elettroni secondari vengono letti dal rivelatore e utilizzati per generare un'immagine 3D della superficie del campione. 
L'interazione fascio di elettroni-campione produce anche raggi X caratteristici, che forniscono informazioni su diversi elementi, come ferro e nichel, presenti nel campione.

microscopia elettronica a scansione

Scanning Electron Microscopy

Un microscopio elettronico a scansione (SEM) viene utilizzato per studiare le caratteristiche superficiali di un campione utilizzando un fascio di elettroni che scansiona la superficie del campione in modo bidimensionale. In genere, è possibile acquisire l'immagine di aree comprese tra circa 1 centimetro e 5 micrometri di larghezza. Il SEM può essere utilizzato per visualizzare batteri, virus, tessuti e campioni più grandi come gli insetti. Il SEM convenzionale offre un ingrandimento compreso tra 20X e 30.000X e una risoluzione spaziale compresa tra 50 e 100 nanometri.

Principi fondamentali

Gli elettroni accelerati rilasciati dal cannone elettronico hanno un'elevata energia cinetica (che varia da 5 a 30 keV). Le interazioni elettrone-campione portano alla decelerazione degli elettroni e alla dissipazione dell'energia sotto forma di segnali diversi. Gli elettroni subiscono due tipi di diffusione: elastica e anelastica. La diffusione anelastica provoca l'emissione di elettroni secondari. Questi elettroni a bassa energia (~50 eV) sono gli elettroni del guscio esterno degli atomi campione che acquisiscono energia appena sufficiente per lasciare la superficie dell'atomo. Solo l'informazione topografica è fornita dalla diffusione degli elettroni secondari poiché il livello di energia degli elettroni che escono dalle regioni interne del campione è troppo basso per uscire dalla superficie del campione.

I raggi X sono generati anche da collisioni anelastiche degli elettroni incidenti con gli elettroni presenti negli orbitali discreti (gusci) degli atomi nel campione. Quando gli elettroni eccitati ritornano a stati energetici inferiori, producono raggi X di una lunghezza d'onda fissa (correlata alla differenza nei livelli energetici degli elettroni nei diversi gusci per un dato elemento). Pertanto, vengono prodotti raggi X caratteristici per ciascun elemento in un minerale eccitato dal fascio di elettroni.

La diffusione elastica, d'altra parte, non è causata dalla rimozione degli elettroni dagli atomi del campione. Il fascio principale di elettroni viene retrodiffuso dopo l'interazione con il nucleo. Questi elettroni non cambiano la loro energia o velocità ma cambiano la loro direzione in base alla loro interazione con il nucleo. Il rilevamento di questi elettroni fornisce informazioni sulla composizione e il loro contrasto variabile in seguito all'interazione con atomi di diverso peso atomico consente all'utente di distinguere le differenze nella composizione del campione. Nei campioni biologici, questo può essere utilizzato per studiare nanoparticelle e nanostrutture incorporate o attaccate con pesi atomici più pesanti, come oro o ferro.

Microscopia elettronica a scansione

Un microscopio elettronico a scansione (SEM) viene utilizzato per studiare le caratteristiche superficiali di un campione utilizzando un fascio di elettroni che scansiona la superficie del campione in un ...

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Cell Biology

Biologia cellulare

Unit 1

Unità 1

Visualizing Cells, Tissues, and Molecules

Visualizzazione di cellule, tessuti e molecole

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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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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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