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

Um microscópio eletrônico de varredura ou SEM varre uma amostra com um feixe de elétrons para fornecer informações sobre a topografia e a composição da superfície da amostra. 
O SEM usa um canhão de elétrons para gerar um feixe de elétrons carregado negativamente atraído para baixo por um ânodo carregado positivamente. As lentes eletromagnéticas concentram o feixe de elétrons e reduzem seu diâmetro.
Em seguida, as bobinas de varredura desviam esse feixe de elétrons focalizado ao longo dos eixos x e y e permitem que ele escaneie uma área retangular da superfície da amostra.
O feixe de elétrons primário de alta energia excita elétrons nos átomos de uma amostra, levando à liberação de elétrons secundários de baixa energia. Esses elétrons secundários são lidos pelo detector e usados para gerar uma imagem 3D da superfície da amostra. 
A interação feixe de elétrons-amostra também produz raios-X característicos, que fornecem informações sobre diferentes elementos, como ferro e níquel, presentes na amostra.

microscopia eletrônica de varredura

Scanning Electron Microscopy

Um microscópio eletrônico de varredura (MEV) é usado para estudar as características da superfície de uma amostra usando um feixe de elétrons que varre a superfície da amostra de maneira bidimensional. Normalmente, áreas entre ~1 centímetro a 5 micrômetros de largura podem ser visualizadas. O MEV pode ser usado para criar imagens de bactérias, vírus, tecidos e também de amostras maiores, como insetos. O MEV convencional fornece uma ampliação que varia de 20X a 30.000X, e resolução espacial de 50 a 100 nanômetros. Princípios fundamentais Os elétrons acelerados liberados pelo canhão de elétrons possuem alta energia cinética (variando de 5 a 30 keV). As interações elétron-amostra levam à desaceleração dos elétrons e à dissipação da energia na forma de diferentes sinais. Os elétrons sofrem dois tipos de espalhamento: elástico e inelástico. O espalhamento inelástico causa a emissão de elétrons secundários. Esses elétrons de baixa energia (~50 eV) são os elétrons da camada externa dos átomos da amostra que adquirem energia suficiente para deixar a superfície do átomo. Apenas informações topográficas são fornecidas pelo espalhamento de elétrons secundários, uma vez que o nível de energia dos elétrons que saem das regiões internas da amostra é muito baixo para sair da superfície da amostra. Os raios X também são gerados por colisões inelásticas dos elétrons incidentes com elétrons em orbitais discretos (camadas) de átomos na amostra. À medida que os elétrons excitados retornam a estados de energia mais baixos, eles produzem raios X de comprimento de onda fixo (relacionado à diferença nos níveis de energia dos elétrons em diferentes camadas para um determinado elemento). Assim, raios X característicos são produzidos para cada elemento de um mineral excitado pelo feixe de elétrons. O espalhamento elástico, por outro lado, não é causado por elétrons desalojados dos átomos da amostra. O feixe principal de elétrons é retroespalhado após interação com o núcleo. Esses elétrons não mudam sua energia ou velocidade, mas mudam sua direção com base na interação com o núcleo. A detecção desses elétrons fornece informações de composição, e seu contraste variável após a interação com átomos de diferentes pesos atômicos permite ao usuário distinguir diferenças na composição da amostra. Em amostras biológicas, isso pode ser usado para estudar nanopartículas incorporadas, ou fixadas, e nanoestruturas com pesos atômicos mais pesados, como ouro ou ferro.

Microscopia Eletrônica de Varredura

Um microscópio eletrônico de varredura (SEM) é usado para estudar as características da superfície de uma amostra usando um feixe de elétrons que varre a superfície da amostra em um ...

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JoVE Core

Cell Biology

Biologia Celular

Unit 1

Unidade 1

Visualizing Cells, Tissues, and Molecules

Visualizando Células, Tecidos e Moléculas

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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

JoVE Core - Biologia Celular ilustra processos celulares complexos em vídeos de animação fáceis de entender e, ao mesmo tempo, abrangentes. Esta coleção foca nos componentes e funções essenciais das células, incluindo organelas e membranas, transporte celular e energia celular. É complementado por vídeos de Cientistas em ação que demonstram a aplicação da biologia celular em experimentos de pesquisa do mundo real.

How SEM Reveals Surface Details

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