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

يقوم المجهر الإلكتروني الماسح أو SEM بمسح عينة بشعاع إلكتروني لتوفير معلومات حول تضاريس وتكوين سطح العينة. 
يستخدم SEM مسدس إلكتروني لتوليد شعاع إلكتروني سالب الشحنة ينجذب إلى أسفل بواسطة أنود موجب الشحنة. ثم تركز العدسات الكهرومغناطيسية شعاع الإلكترون وتقلل قطره.
بعد ذلك ، تقوم ملفات المسح بتحويل شعاع الإلكترون المركز هذا على طول المحورين x و y وتسمح له بمسح منطقة مستطيلة من سطح العينة.
يثير شعاع الإلكترون الأولي عالي الطاقة الإلكترونات في ذرات العينة ، مما يؤدي إلى إطلاق إلكترونات ثانوية منخفضة الطاقة. تتم قراءة هذه الإلكترونات الثانوية بواسطة الكاشف واستخدامها لإنشاء صورة ثلاثية الأبعاد لسطح العينة. 
ينتج عن تفاعل عينة شعاع الإلكترون أيضا أشعة سينية مميزة ، والتي توفر معلومات حول عناصر مختلفة ، مثل الحديد والنيكل ، الموجودة في العينة.

المجهر الإلكتروني الماسح

Scanning Electron Microscopy

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يستخدم المجهر الإلكتروني الماسح (SEM) لدراسة الخصائص السطحية للعينة باستخدام شعاع إلكتروني يقوم بمسح سطح العينة بطريقة ثنائية الأبعاد. عادةً، يمكن تصوير المناطق التي يتراوح عرضها بين 1 سم إلى 5 ميكرومتر. يمكن استخدام SEM لتصوير البكتيريا والفيروسات والأنسجة بالإضافة إلى عينات أكبر مثل الحشرات. يعطي SEM التقليدي تكبيرًا يتراوح من 20X إلى 30000X ودقة مكانية تتراوح من 50 إلى 100 نانومتر.

المبادئ الأساسية

تتمتع الإلكترونات المتسارعة التي يطلقها مدفع الإلكترون بطاقة حركية عالية (تتراوح من 5 إلى 30 كيلو إلكترون فولت). تؤدي تفاعلات عينة الإلكترون إلى تباطؤ الإلكترونات وتبديد الطاقة على شكل إشارات مختلفة. تخضع الإلكترونات لنوعين من التشتت: مرن وغير مرن. يؤدي التشتت غير المرن إلى انبعاث الإلكترونات الثانوية. هذه الإلكترونات منخفضة الطاقة (~50 فولت) هي إلكترونات الغلاف الخارجي لذرات العينة التي تكتسب طاقة كافية لمغادرة سطح الذرة. يتم توفير المعلومات الطبوغرافية فقط عن طريق تشتت الإلكترونات الثانوية نظرًا لأن مستوى طاقة الإلكترونات الخارجة من المناطق الداخلية للعينة منخفض جدًا بحيث لا يتمكن من الخروج من سطح العينة.

تتولد الأشعة السينية أيضًا عن طريق الاصطدامات غير المرنة للإلكترونات الساقطة مع الإلكترونات الموجودة في المدارات المنفصلة (أغلفة) الخاصة بالذرات في العينة. عندما تعود الإلكترونات المثارة إلى حالات طاقة أقل، فإنها تنتج أشعة سينية ذات طول موجي ثابت (يرتبط بالاختلاف في مستويات طاقة الإلكترونات في الأغلفة المختلفة لعنصر معين). وهكذا، يتم إنتاج الأشعة السينية المميزة لكل عنصر في المعدن المثار بواسطة شعاع الإلكترون.

من ناحية أخرى، لا يحدث التشتت المرن بسبب الإلكترونات المزاحة من ذرات العينة. يتشتت شعاع الإلكترونات الرئيسي بعد تفاعله مع النواة. هذه الإلكترونات لا تغير طاقتها أو سرعتها ولكنها تغير اتجاهها بناء على تفاعلها مع النواة. يوفر اكتشاف هذه الإلكترونات معلومات تركيبية، كما أن تباينها المتفاوت عند التفاعل مع ذرات ذات أوزان ذرية مختلفة يسمح للمستخدم بتمييز الاختلافات في تكوين العينة. في العينات البيولوجية، يمكن استخدام هذا لدراسة الجسيمات النانوية و الهياكل النانوية المدمجة أو المرفقة ذات الأوزان الذرية الأثقل، مثل الذهب أو الحديد.
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يستخدم المجهر الإلكتروني الماسح (SEM) لدراسة السمات السطحية للعينة باستخدام شعاع إلكتروني يقوم بمسح سطح العينة في ...

Education

JoVE Core

Cell Biology

بيولوجيا الخلية

Unit 1

الوحدة الأولى

Visualizing Cells, Tissues, and Molecules

تصوير الخلايا والأنسجة والجزيئات

KEY TERMS AND CONCEPTS

المصطلحات والمفاهيم الأساسية

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

imaging biological samples with optical microscopy

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How Light Microscopes Bring Samples Into View

Optical microscopy uses light and lenses to make tiny biological samples visible. Antonie van Leeuwenhoek built the first compound optical microscope in the 17th century to view blood cells, bacteria, and yeast cells. In 1830, Joseph Jackson Lister developed an essentially modern light microscope. During the 20th century, microscopes gained better magnification and resolution. A sample is placed on a glass slide and clipped onto the stage, which is the flat platform of the microscope. The...

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Microscope Contrast for Live, Unstained Cells

Phase-contrast and differential interference contrast microscopy help scientists see live cells without stains. Both methods boost contrast so thin, transparent structures become easier to study under the microscope. Phase-contrast microscopes use interference between light that passes straight through a cell and light that is refracted by cell parts. The microscope uses an annular stop in the condenser to make a hollow cone of light. The objective lens also contains a phase plate with a phase...

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Microscope Slide Prep and Staining

Microscope slide preparation includes wet mounts, fixation, and staining. These steps help make specimens easier to see under a light microscope. They also help preserve cells and highlight important structures. A wet mount is the simplest slide preparation. The specimen is placed in a drop of liquid on the slide. A liquid sample can be added directly with a dropper, and a solid sample, such as a skin scraping, can be placed on the slide before the liquid is added. The liquid is sometimes...

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Fluorescent Antibody Staining in Cells

Fluorescent antibody staining uses a fluorescence microscope to make cells and microbes visible in bright colors against a dark background. The microscope works with fluorochromes, which are fluorescent dyes that absorb energy from a light source and then give off visible light. Some fluorochromes are natural, such as chlorophylls. Others are added to the specimen to improve contrast. Common examples of fluorochromes include Texas red and FITC. The method can also use nucleic acid dyes such as...

immunocytochemistry and immunohistochemistry

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Antibody Staining in Cells and Tissues

Immunocytochemistry (ICC) and immunohistochemistry (IHC) use antibodies to detect specific proteins or antigens in samples. ICC focuses on individual cells, such as blood cells and stem cells. IHC is used for tissue samples. These methods were first published by Albert Coons in 1941, when they were used to detect pneumococcal antigen in mouse tissue sections from mice infected with Pneumococcus. Both techniques can produce either a colored signal or a fluorescent signal. Enzyme-conjugated...

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Confocal Microscopy for Sharp Cell Images

Confocal microscopy is an advanced imaging method used to capture sharp fluorescence images. Its main advantage is that it blocks out-of-focus light from the sample with pinholes. That helps produce high-resolution images with strong contrast. Unlike a standard optical microscope, a confocal microscope uses a focused laser beam to scan the sample surface at different z-planes. This makes it especially useful for thick specimens, such as biofilms. These samples can often be examined alive and...

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Protein movement in living cells can be measured with fluorescence-based methods such as FRAP, FRET, and PET. These techniques help scientists track how proteins move, interact, and change distance inside the cell. Fluorescent recovery after photobleaching, or FRAP, measures protein movement rates within the cell. In this method, a small region is exposed to a strong laser beam. The beam permanently bleaches fluorophore-tagged proteins in that area. Over time, bleached proteins diffuse away...

total internal reflection fluorescence microscopy

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Surface-Only Imaging with TIRF Microscopy

Total internal reflection fluorescence microscopy, or TIRF, is a microscope method used to view fluorophores near a solid surface with a higher refractive index, such as a glass coverslip. It is useful when scientists want to study signals close to the cell surface. TIRF works when light from a medium with a lower refractive index, such as air, hits the glass coverslip at a critical angle. Instead of entering the glass, the light undergoes total internal reflection. This reflection creates an...

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Atomic Force Microscopy for Surface Mapping

Atomic force microscopy is used to map the surface details of materials. It can examine ceramics, glass, polymers, and biological samples. The method gives far more resolution than an optical imaging system and produces three-dimensional surface profiles instead of flat 2D images. The probe is the key part of an AFM setup. It includes the cantilever and tip assembly, and it is also the part that is replaced most often. Constant contact with samples wears down the tip, so probe choice depends...

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Electron microscopy uses electrons instead of visible light to make images with much higher detail than a light microscope. Visible light limits the resolution of light microscopes, which usually magnify up to 1000X, and sometimes up to 1500X. Electrons have a much shorter wavelength, about 0.005 nm, so they can produce resolution as fine as 0.05 nm. An electron microscope can also magnify images up to 2,000,000X and reveal subcellular structures and some molecular structures, including single...

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...

transmission electron microscopy

How TEM Reveals Tiny Structures

Transmission electron microscopy uses electrons to reveal very small structures inside a specimen. The technique grew from the first electron microscope prototype developed in 1931 by physicist Ernst Ruska. He built on the idea that magnetic fields can direct an electron beam, just as lenses direct light in an optical microscope. In a transmission electron microscope, a hot tungsten element produces electrons. An electron gun then accelerates them with a potential difference, giving the beam...

preparation of samples for electron microscopy

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Electron Microscopy Sample Prep Methods

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Immunogold Labeling in Electron Microscopy

Immunogold labeling in electron microscopy uses antibodies and tiny gold particles to find specific proteins in cells and tissues. The gold particles are very electron-dense, which means they show up clearly in electron microscope images. This gives researchers a high-resolution way to see where proteins are located and to compare their amounts under different stimulation conditions. The method can reveal proteins on the cell surface and inside cells. It has been used to study proteins...

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Cryo-EM for Native Biomolecule Imaging

Cryo-EM, or cryo-electron microscopy, lets scientists image biomolecules in their frozen, native state. It avoids the dehydration, fixation, and staining used in conventional electron microscopy, which can distort samples and create artifacts. It also reduces damage from the electron beam by using gentler imaging conditions. This method is useful when other structure tools have limits. X-ray diffraction needs a molecule to form a crystal, and that is not always possible. Even when...

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3D Imaging with Electron Microscopy

Transmission electron microscopy (TEM) can build 3D images of biological samples. It does this with electron microscope tomography and single-particle reconstruction. Tomography can show cell parts or small cells in vivo, while single-particle reconstruction is used for macromolecules and macromolecular complexes in vitro only. Electron tomography can be done in TEM or in STEM, which stands for scanning transmission electron microscopy. STEM is mainly used for thick biological specimens. It...

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 Cell Biology illustrates complex cellular processes in easy-to-understand yet comprehensive animated video lessons. This collection focuses on the essential components and functions of cells, including organelles and membranes, cellular transport, and cell energetics. It is complemented by scientist-in-action videos that demonstrate the application of cell biology in real-world research experiments.

How SEM Reveals Surface Details

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