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

Ein Rasterelektronenmikroskop (REM) tastet eine Probe mit einem Elektronenstrahl ab, um Informationen über die Topographie und Zusammensetzung der Probenoberfläche zu erhalten. 
REM verwendet eine Elektronenkanone, um einen negativ geladenen Elektronenstrahl zu erzeugen, der von einer positiv geladenen Anode nach unten angezogen wird. Elektromagnetische Linsen bündeln dann den Elektronenstrahl und verringern dessen Durchmesser.
Anschließend lenken Rasterspulen diesen fokussierten Elektronenstrahl entlang der x- und y-Achse ab und ermöglichen es ihm, einen rechteckigen Bereich der Probenoberfläche abzutasten.
Der hochenergetische Primärelektronenstrahl regt Elektronen in den Atomen einer Probe an, was zur Freisetzung von niederenergetischen Sekundärelektronen führt. Diese Sekundärelektronen werden vom Detektor ausgelesen und verwendet, um ein 3D-Bild der Probenoberfläche zu erzeugen. 
Die Elektronenstrahl-Proben-Wechselwirkung erzeugt auch charakteristische Röntgenstrahlung, die Informationen über verschiedene in der Probe vorhandene Elemente wie Eisen und Nickel liefert.

rasterelektronenmikroskopie

Scanning Electron Microscopy

Mit einem Rasterelektronenmikroskop (REM) werden die Oberflächenmerkmale einer Probe untersucht, indem ein Elektronenstrahl verwendet wird, der die Probenoberfläche zweidimensional abtastet. Typischerweise können Bereiche mit einer Breite zwischen etwa 1 Zentimeter und 5 Mikrometern abgebildet werden. Mit REM können Bakterien, Viren, Gewebe sowie größere Proben wie Insekten abgebildet werden. Herkömmliche REM-Aufnahmen ermöglichen eine 20- bis 30.000-fache Vergrößerung und eine räumliche Auflösung von 50 bis 100 Nanometern.

Fundamentale Prinzipien

Von der Elektronenkanone freigesetzte beschleunigte Elektronen haben eine hohe kinetische Energie (im Bereich von 5–30 keV). Wechselwirkungen zwischen Elektronen und Probe führen zu einer Abbremsung der Elektronen und einer Energiedissipation in Form unterschiedlicher Signale. Die Elektronen unterliegen zwei Arten der Streuung: elastisch und inelastisch. Inelastische Streuung führt zur Emission von Sekundärelektronen. Diese niederenergetischen Elektronen (~50 eV) sind die Elektronen der äußeren Hülle der Probenatome, die gerade genug Energie aufnehmen, um die Atomoberfläche zu verlassen. Durch die Streuung von Sekundärelektronen werden nur topografische Informationen bereitgestellt, da das Energieniveau der aus den inneren Bereichen der Probe austretenden Elektronen zu niedrig ist, um die Probenoberfläche zu verlassen.

Röntgenstrahlen werden auch durch inelastische Kollisionen der einfallenden Elektronen mit Elektronen in diskreten Orbitalen (Hüllen) von Atomen in der Probe erzeugt. Wenn die angeregten Elektronen in niedrigere Energiezustände zurückkehren, erzeugen sie Röntgenstrahlen einer festen Wellenlänge (bezogen auf den Unterschied in den Energieniveaus der Elektronen in verschiedenen Schalen für ein bestimmtes Element). Dadurch werden für jedes Element in einem durch den Elektronenstrahl angeregten Mineral charakteristische Röntgenstrahlen erzeugt.

Die elastische Streuung hingegen wird nicht durch verdrängte Elektronen aus den Atomen der Probe verursacht. Der Hauptelektronenstrahl wird nach der Wechselwirkung mit dem Kern zurückgestreut. Diese Elektronen ändern weder ihre Energie noch ihre Geschwindigkeit, sondern ändern aufgrund ihrer Wechselwirkung mit dem Kern ihre Richtung. Die Detektion dieser Elektronen liefert Informationen über die Zusammensetzung, und ihr unterschiedlicher Kontrast bei der Wechselwirkung mit Atomen unterschiedlichen Atomgewichts ermöglicht es dem Benutzer, Unterschiede in der Probenzusammensetzung zu erkennen. In biologischen Proben können damit eingebettete oder angelagerte Nanopartikel und Nanostrukturen mit höheren Atomgewichten wie Gold oder Eisen untersucht werden.

Rasterelektronenmikroskopie

Ein Rasterelektronenmikroskop (REM) wird verwendet, um die Oberflächenmerkmale einer Probe zu untersuchen, indem ein Elektronenstrahl verwendet wird, der die Probenoberfläche in einem ...

Education

Lehre

JoVE Core

Cell Biology

Zellbiologie

Unit 1

Einheit 1

Visualizing Cells, Tissues, and Molecules

Visualisierung von Zellen, Geweben und Molekülen

KEY TERMS AND CONCEPTS

KERNBEGRIFFE UND KONZEPTE

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

cell biology

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How SEM Reveals Surface Details

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