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

Un microscopio electrónico de barrido o SEM escanea una muestra con un haz de electrones para proporcionar información sobre la topografía y la composición de la superficie de la muestra. 
SEM utiliza un cañón de electrones para generar un haz de electrones cargado negativamente atraído hacia abajo por un ánodo cargado positivamente. A continuación, las lentes electromagnéticas concentran el haz de electrones y reducen su diámetro.
A continuación, las bobinas de escaneo desvían este haz de electrones enfocado a lo largo de los ejes x e y y le permiten escanear un área rectangular de la superficie de la muestra.
El haz de electrones primarios de alta energía excita electrones en los átomos de una muestra, lo que lleva a la liberación de electrones secundarios de baja energía. Estos electrones secundarios son leídos por el detector y utilizados para generar una imagen 3D de la superficie de la muestra. 
La interacción haz de electrones-muestra también produce rayos X característicos, que proporcionan información sobre diferentes elementos, como el hierro y el níquel, presentes en la muestra.

microscopía electrónica de barrido

Scanning Electron Microscopy

Se utiliza un microscopio electrónico de barrido (SEM) para estudiar las características de la superficie de una muestra mediante el uso de un haz de electrones que escanea la superficie de la muestra de manera bidimensional. Normalmente, se pueden obtener imágenes de áreas de entre ~1 centímetro y 5 micrómetros de ancho. SEM se puede utilizar para obtener imágenes de bacterias, virus, tejidos y muestras más grandes, como insectos. El SEM convencional ofrece un aumento que oscila entre 20X y 30.000X y una resolución espacial de 50 a 100 nanómetros.

Principios fundamentales

Los electrones acelerados liberados por el cañón de electrones tienen una alta energía cinética (que oscila entre 5 y 30 keV). Las interacciones electrón-muestra provocan una desaceleración de los electrones y una disipación de la energía en forma de diferentes señales. Los electrones sufren dos tipos de dispersión: elástica e inelástica. La dispersión inelástica provoca la emisión de electrones secundarios. Estos electrones de baja energía (~50 eV) son los electrones de la capa externa de los átomos de la muestra que adquieren la energía suficiente para abandonar la superficie del átomo. La dispersión de electrones secundarios solo proporciona información topográfica, ya que el nivel de energía de los electrones que salen de las regiones internas de la muestra es demasiado bajo para salir de la superficie de la muestra.

Los rayos X también se generan por colisiones inelásticas de los electrones incidentes con los electrones en orbitales discretos (capas) de los átomos de la muestra. A medida que los electrones excitados regresan a estados de energía más bajos, producen rayos X de una longitud de onda fija (relacionada con la diferencia en los niveles de energía de los electrones en diferentes capas para un elemento determinado). Así, se producen rayos X característicos para cada elemento de un mineral excitado por el haz de electrones.

La dispersión elástica, por otro lado, no es causada por electrones desalojados de los átomos de la muestra. El haz principal de electrones se retrodispersa después de la interacción con el núcleo. Estos electrones no cambian su energía ni su velocidad, pero cambian su dirección en función de su interacción con el núcleo. La detección de estos electrones proporciona información sobre la composición, y su contraste variable al interactuar con átomos de diferentes pesos atómicos permite al usuario distinguir diferencias en la composición de la muestra. En muestras biológicas, esto se puede utilizar para estudiar nanopartículas y nanoestructuras incrustadas o adheridas con pesos atómicos más pesados, como el oro o el hierro.

Microscopía electrónica de barrido

Un microscopio electrónico de barrido (SEM) se utiliza para estudiar las características de la superficie de una muestra mediante el uso de un haz de electrones que escanea la superficie de la muestra en un ...

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Cell Biology

Biología Celular

Unit 1

Unidad 1

Visualizing Cells, Tissues, and Molecules

Visualización de células, tejidos y moléculas

KEY TERMS AND CONCEPTS

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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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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.

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How SEM Reveals Surface Details

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