Scanning Electron Microscope (SEM)- Definition, Principle, Parts, Images

Electron microscopes are devices that use a beam of electrons as a source of illumination to magnify and examine the structure of samples with greater detail than the optical microscopes. They use electron optics that are analogous to the glass lenses of an optical light microscope. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light, electron microscopes have a higher resolution of about 0.1 nm, which compares to about 200 nm for light microscopes.

The history of electron microscopy dates back to the early twentieth century when the first electromagnetic lens was developed by Hans Busch in 1926. This opened the door of possibility to use the principles of the lens to invent a microscope that could examine the structure of samples with greater detail. This had the potential to exceed the theoretical limit for optical microscopy, which was about 0.2 micrometers at that time.

The first prototype electron microscope, capable of four-hundred-power magnification, was developed in 1931 by Ernst Ruska and Max Knoll, a physicist and an electrical engineer, respectively, at the Berlin Technical University . In 1933, Ruska built an electron microscope that was more powerful than the optical microscope and could resolve individual atoms. Ruska had a vast knowledge of electron wavelengths and invented the electron microscope while he was studying at the Technical University of Munich.

In 1937, Bodo von Borries and Helmut Ruska joined him to develop ways that the principles could be applied, such as to examine biological samples. In the same year, Manfred von Ardenne developed the first scanning electron microscope. Siemens-Schuckertwerke released the first commercial electron microscope to the public in 1938. From this point onwards, transmission electron microscopes became more readily available in other areas of the world, including North America.

In 1986, Ernst Ruska was awarded the Nobel Prize in Physics for the invention of the electron microscope, in conjunction with Heinrich Rohrer and Gerd Binnig for the development of the scanning tunneling microscope (STM).

Electron microscopes can be classified into two main types: transmission electron microscopy (TEM) and scanning electron microscopy (SEM). TEM uses a beam of electrons that passes through a thin sample and forms an image on a screen or a detector. SEM uses a beam of electrons that scans over a surface of a sample and collects signals such as secondary electrons, backscattered electrons, and X-rays that reflect the topography and composition of the sample. Both types of electron microscopes have various applications in different fields of science and technology.

In this article, we will focus on scanning electron microscopy (SEM), its definition, principle, parts, images, applications, advantages, and limitations. We will also introduce scanning-transmission electron microscopy (STEM), which combines the features of both TEM and SEM.

A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that contain information about the surface topography and composition of the sample. The electron beam is scanned in a raster scan pattern, and the position of the beam is combined with the intensity of the detected signal to produce an image.

The SEM can achieve high magnifications and resolutions, up to 1 nanometer, which is much higher than the conventional light microscope. The SEM can also provide depth of field, which allows for the observation of three-dimensional structures on the sample surface. The SEM can operate in different modes, such as secondary electron mode, backscattered electron mode, cathodoluminescence mode, and X-ray microanalysis mode, depending on the type of signal that is detected and analyzed.

The SEM requires that the sample be placed in a vacuum chamber, as the electrons would be scattered by air molecules. The sample also needs to be electrically conductive, or coated with a thin layer of conductive material, to prevent charging effects that would distort the image. The sample can be observed in its natural state, or after various treatments such as fixation, dehydration, drying, staining, or coating.

The SEM was invented by Manfred von Ardenne in 1937, based on the principle of scanning a raster with a demagnified and finely focused electron beam. He applied this technique to surpass the resolution of the transmission electron microscope (TEM), which uses transmitted electrons to form an image. The first commercial SEM was produced by Cambridge Instruments in 1965. Since then, the SEM has been widely used in various fields of science and engineering, such as biology, materials science, geology, forensics, nanotechnology, and semiconductor analysis.

The scanning electron microscope (SEM) operates on the principle of using a beam of electrons to scan the surface of a sample and generate various signals that contain information about its topography and composition.

The beam of electrons is produced by an electron source, such as a tungsten filament, a lanthanum hexaboride crystal, or a field emission gun. The electron source emits electrons under thermal or electric stimulation and accelerates them towards a positively charged anode. The electrons are then collimated and focused by a series of electromagnetic lenses that form a narrow and convergent beam.

The beam of electrons interacts with the atoms in the sample, causing different types of electron-matter interactions. Some of the interactions result in the emission of secondary electrons, backscattered electrons, characteristic X-rays, or cathodoluminescence from the sample surface or subsurface. These signals are detected by various detectors that are placed around the sample chamber. The intensity and distribution of the signals depend on the properties and orientation of the sample, such as its morphology, topography, elemental composition, and crystal structure.

The beam of electrons is scanned over the sample surface in a raster pattern by using scanning coils that deflect the beam horizontally and vertically. The position of the beam and the intensity of the detected signal are synchronized and used to form an image on a display device, such as a cathode ray tube or a computer monitor. The image is composed of pixels that correspond to the scanned points on the sample surface. The brightness of each pixel is proportional to the intensity of the signal detected at that point.

The magnification of the image is controlled by changing the scanning area on the sample surface. A smaller scanning area results in a higher magnification, while a larger scanning area results in a lower magnification. The resolution of the image is determined by the size and shape of the electron beam spot on the sample surface, which depends on factors such as the electron source, the lens system, the beam current, and the working distance. The resolution of SEM can reach up to 1 nanometer for some instruments.

A scanning electron microscope (SEM) is a type of electron microscope that uses a beam of electrons to scan the surface of a solid object and produce an image of its microstructure and morphology. The electron beam is generated by an electron source and focused by electromagnetic lenses in the electron column. The beam stimulates the emission of different signals from the specimen, such as backscattered electrons, secondary electrons, and characteristic X-rays, which are collected by various detectors and used to create a high-resolution image and analyze the chemical composition of the specimen. The SEM operates under high vacuum and can achieve a resolution down to the nanometer scale.

The basic steps of how a SEM works are as follows :

• An electron gun fires a beam of electrons, which then accelerates down the column of the SEM. The electron gun can be either a tungsten filament, a lanthanum hexaboride crystal, or a field emission gun (FEG), depending on the type and quality of the electron source.
• The electron beam passes through a series of lenses and apertures, which act to focus and control it. The lenses are either electrostatic or electromagnetic, and they can be adjusted to change the size, shape, and position of the beam spot on the specimen.
• The scanning coil deflects the beam back and forth over a rectangular area of the specimen surface. The scanning speed and pattern can be varied depending on the desired resolution and image size.
• When the electron beam hits the specimen, it interacts with the atoms in the sample and produces various signals that contain information about the surface topography and composition of the specimen. These signals include backscattered electrons (BSE), secondary electrons (SE), and characteristic X-rays.
• Backscattered electrons are high-energy electrons that are scattered out of the sample after elastic collisions with the nuclei of the atoms. They originate from deep within the sample (a few microns below the surface) and reflect the atomic number contrast of the specimen. The higher the atomic number, the brighter the material appears in the BSE image .
• Secondary electrons are low-energy electrons that are ejected from the outer shells of the atoms after inelastic collisions with the primary electrons. They originate from within a few nanometers of the sample surface and are very sensitive to surface features and topography. The more protruding or tilted the surface is, the more SE are emitted and detected. The SE image shows fine details and texture of the specimen surface .
• Characteristic X-rays are photons that are emitted when an inner-shell electron is knocked out by a primary electron and an outer-shell electron fills its place. The energy difference between the two shells corresponds to a specific wavelength of X-ray radiation, which is characteristic of each element. By measuring the energy and intensity of these X-rays, it is possible to identify and quantify the elements present in the specimen .
• The different signals are collected by various detectors that are placed around or above the specimen chamber. The detectors convert the signals into electrical currents that are then amplified and processed by a computer. The computer generates an image on a monitor by assigning each pixel a brightness value proportional to the signal intensity at each point on the scanned area .

The SEM can produce images with magnifications ranging from 10x to over 500,000x, depending on the type of electron source, lens system, detector, and specimen preparation . The SEM can also perform spot chemical analysis using energy-dispersive X-ray spectroscopy (EDS) or wavelength-dispersive X-ray spectroscopy (WDS), which can provide elemental maps or line profiles of the specimen. The SEM can also be combined with other techniques such as cathodoluminescence (CL), electron backscatter diffraction (EBSD), or scanning transmission electron microscopy (STEM) to obtain additional information about the specimen structure, properties, or orientation.

The SEM is widely used in various fields such as materials science, nanotechnology, biology, geology, metallurgy, engineering, forensics, medicine, and archaeology. It can provide detailed information about surfaces of solid objects that cannot be obtained by other methods . However, it also has some limitations such as requiring high vacuum conditions, special sample preparation, and high cost and maintenance .

A scanning electron microscope (SEM) is composed of several components that work together to produce high-resolution images of the surface and composition of a specimen. The main parts of an SEM are:

• Electron gun: This is the source of the electrons that form the beam that scans the specimen. The electron gun can be either a thermionic gun, which uses a heated filament (usually made of tungsten) to emit electrons, or a field emission gun, which uses a strong electric field to extract electrons from a metal tip. The electron gun operates at a high voltage (1-40 kV) and generates a large and stable current of electrons .
• Lenses: These are electromagnetic coils that focus and control the electron beam as it travels through the column of the microscope. The lenses are not made of glass, but of magnets that can bend the path of the electrons. There are several types of lenses in an SEM, such as condenser lenses, objective lenses, and intermediate lenses. The lenses can adjust the spot size, magnification, and resolution of the image .
• Sample chamber: This is where the specimen is mounted and scanned by the electron beam. The sample chamber must be very sturdy and insulated from vibration, as any movement of the specimen can affect the quality of the image. The sample chamber also has a mechanism to manipulate the specimen, such as rotating it or tilting it at different angles. The sample chamber must be under a high vacuum to prevent interference from air molecules .
• Detectors: These are devices that collect the signals generated by the interaction of the electron beam with the specimen. There are different types of detectors for different types of signals, such as secondary electrons, backscattered electrons, X-rays, and photons. The detectors convert the signals into electrical impulses that are sent to a display device or a computer. The detectors can provide information about the morphology, topography, and composition of the specimen .
• Vacuum chamber: This is the part of the microscope that maintains a high vacuum in the column and the sample chamber. A vacuum is necessary for an SEM to operate, as air molecules can scatter or absorb the electrons and reduce the quality of the image. The vacuum chamber consists of pumps and valves that remove air from the system and prevent leaks .
• Scanning coil: This is a pair of coils that deflects the electron beam in a raster pattern over the specimen surface. The scanning coil controls the speed and direction of the scanning motion, which determines the size and resolution of the image. The scanning coil works in synchrony with the detectors to produce a point-to-point correspondence between the signals and the image pixels .

Some other parts of an SEM that help with its operation are:

• Power supply: This is a device that provides electricity to all parts of the microscope, such as the electron gun, the lenses, the detectors, and the scanning coil. The power supply must be stable and consistent to ensure optimal performance of the microscope.
• Display device: This is a device that shows the image produced by the microscope on a screen or a monitor. The display device can be either an analog device, such as a cathode ray tube (CRT), or a digital device, such as a computer. The display device can also have software that allows image processing and analysis.
• Anode: This is a positively charged plate that attracts and accelerates the electrons emitted by the electron gun. The anode is located between the electron gun and the first lens in an SEM.
• Sample stage: This is a platform that holds and supports the specimen in the sample chamber. The sample stage can be moved up or down to adjust the working distance between the specimen and the objective lens.
• Controllers: These are devices that allow users to adjust various parameters of the microscope, such as voltage, current, magnification, focus, brightness, contrast, and scanning speed.
• Heater: This is a device that heats up the filament in thermionic guns to produce electrons.

• Water chiller: This is a device that cools down the lenses and other parts of the microscope that generate heat during operation.

  Images produced by Scanning Electron Microscope (SEM)

One of the main advantages of the SEM is that it can produce high-resolution images of the surface of a specimen, revealing details of its morphology, topography and composition. The images are formed by the signals generated by the interaction of the electron beam with the atoms in the sample. The most common signals are secondary electrons and backscattered electrons, which provide information about the surface features and contrast, respectively.

Secondary electrons are low-energy electrons that are emitted from the specimen surface when they are hit by the primary electron beam. They are sensitive to the shape and texture of the surface, and can produce images with a three-dimensional appearance. The brightness of a pixel in a secondary electron image depends on the angle and orientation of the surface relative to the electron beam. For example, raised surfaces will appear brighter than depressed surfaces, as they emit more secondary electrons to the detector.

Backscattered electrons are high-energy electrons that are reflected from the specimen surface when they collide with the nuclei of the atoms in the sample. They are sensitive to the atomic number and density of the elements in the sample, and can produce images with compositional contrast. The brightness of a pixel in a backscattered electron image depends on the average atomic number of the region being scanned. For example, heavier elements will appear brighter than lighter elements, as they scatter more electrons back to the detector.

In addition to secondary and backscattered electrons, other signals can be detected by the SEM, such as characteristic X-rays, cathodoluminescence and electron backscatter diffraction. These signals can provide information about the elemental composition, chemical bonding and crystal structure of the sample, respectively. However, these signals require special detectors and analytical techniques that are beyond the scope of this article.

The following are some examples of images produced by SEM using different signals and modes:

• A scanning electron micrograph of Tradescantia pollen and stamens. This image was taken using secondary electrons, showing the surface morphology and texture of the pollen grains and filaments.

• A low-temperature scanning electron micrograph of soybean cyst nematode and its egg. This image was taken using secondary electrons at a low temperature (-120°C) to preserve the biological structure and prevent dehydration of the specimen.

• A scanning electron micrograph of a hederelloid from the Devonian of Michigan. This image was taken using backscattered electrons, showing the compositional contrast between the calcareous skeleton (bright) and the organic matrix (dark) of this fossilized colonial organism.

• A scanning electron micrograph of photoresist. This image was taken using backscattered electrons, showing the contrast between different layers of materials used in photolithography for semiconductor fabrication.

• A scanning electron micrograph of the surface of a kidney stone. This image was taken using secondary electrons, showing the tetragonal crystals of weddellite (calcium oxalate dihydrate) emerging from the amorphous central part of the stone.

• A scanning electron micrograph of stomata on the lower surface of a leaf. This image was taken using secondary electrons, showing the openings (stomata) surrounded by guard cells that regulate gas exchange in plants.

Applications of the Scanning Electron Microscope (SEM)

The scanning electron microscope (SEM) is a powerful and versatile tool that can be used for various applications in different fields of science and technology. Some of the main applications of the SEM are:

• Material science: The SEM can reveal the microstructure, morphology, crystallography, and chemical composition of various materials, such as metals, ceramics, polymers, composites, and nanomaterials. The SEM can also be used to study the effects of mechanical stress, corrosion, fracture, wear, and fatigue on materials. The SEM can also perform elemental analysis using energy-dispersive X-ray spectroscopy (EDS) or wavelength-dispersive X-ray spectroscopy (WDS), which can identify and quantify the elements present in a sample.

• Biology and medicine: The SEM can provide detailed images of biological specimens, such as cells, tissues, organs, microorganisms, parasites, and biomolecules. The SEM can also be used to study the surface features, interactions, and functions of biological materials, such as membranes, receptors, enzymes, DNA, and proteins. The SEM can also be used for biomedical applications, such as orthodontics, implantology, tissue engineering, drug delivery, and diagnosis of diseases. The SEM can also perform immunogold labeling, which can detect specific antigens or antibodies on the surface of biological samples using gold nanoparticles.

• Forensic science: The SEM can provide valuable information for forensic investigations, such as identification of gunshot residues, fingerprints, hair, fibers, drugs, explosives, paint chips, soil particles, and other trace evidence. The SEM can also be used to determine the origin, history, and authenticity of documents, artworks, coins, and other objects. The SEM can also perform ballistic analysis, which can compare the characteristics of bullets and firearms using backscattered electron imaging.

• Nanoscience and nanotechnology: The SEM can image and manipulate nanoscale objects and structures, such as nanoparticles, nanowires, nanotubes, nanofibers, nanocomposites, and nanodevices. The SEM can also be used to fabricate nanostructures using focused ion beam (FIB) milling or electron beam lithography. The SEM can also perform scanning probe microscopy (SPM), which can measure the physical and electrical properties of nanomaterials using various probes attached to the electron beam.

• Other applications: The SEM can also be used for various other applications in different fields of science and technology. For example:

  • In geology and mineralogy: The SEM can study the morphology, composition, texture, and origin of rocks, minerals, fossils, and meteorites.
  • In environmental science: The SEM can analyze the pollution, contamination, and degradation of air, water, soil, and biota.
  • In electronics and engineering: The SEM can inspect the quality, performance, and reliability of electronic components, circuits, devices, and systems.
  • In chemistry and physics: The SEM can investigate the structure, properties, and reactions of molecules, atoms, ions, and electrons.
  • In agriculture and food science: The SEM can examine the structure, quality, safety, and nutrition of crops, plants, animals, and food products.
  • In cosmetics and pharmaceuticals: The SEM can evaluate the formulation, stability, efficacy, and delivery of cosmetic products and drugs.

These are some of the main applications of the scanning electron microscope (SEM). However the list is not exhaustive and new applications are constantly being developed as the technology advances and new challenges arise in different fields of science and technology.

Advantages of the Scanning Electron Microscope (SEM)

The scanning electron microscope (SEM) is a powerful and versatile tool for studying the surfaces and structures of various materials. Some of the advantages of using SEM are:

• Resolution: The SEM can produce images with a resolution as low as 15 nanometers, which is much higher than that of optical microscopes. This allows the SEM to reveal fine details and features of microstructures, such as fractures, corrosion, grains, and grain boundaries.
• Magnification: The SEM can achieve a wide range of magnifications, from 10x to over 500,000x, depending on the type of electron source and lens system. This enables the SEM to examine specimens at different scales and levels of detail.
• Depth of field: The SEM has a large depth of field, which means that more of the specimen can be in focus at one time. This gives the SEM images a three-dimensional and topographical appearance, which can help to understand the shape and texture of the specimen.
• Chemical analysis: The SEM can be combined with energy dispersive spectroscopy (EDS) to provide qualitative and quantitative information on the elemental composition of the specimen. EDS can detect elements from boron to uranium, and can generate maps and line scans to show the spatial distribution and variation of elements. This can be useful for identifying defects, foreign materials, coatings, and particles in the specimen.
• Ease of operation: The SEM is relatively easy to operate and has user-friendly interfaces. Some modern SEMs can also generate digital data that can be stored and transferred easily. It is also possible to acquire data from the SEM within a short period of time, about 5 minutes.
• Versatility: The SEM can be used for a variety of applications and fields, such as industrial quality control, nanoscience, biomedical research, microbiology, geology, archaeology, and forensics . The SEM can also accommodate different types of specimens, such as metals, ceramics, polymers, biological materials, and minerals. Some specimens may require special preparation, such as fixation, dehydration, drying, or coating, but others can be examined directly without any treatment.

Limitations of the Scanning Electron Microscope (SEM)

Despite its many advantages, the SEM also has some limitations that need to be considered before using it for materials analysis. Some of the main limitations are:

• Vacuum environment: The SEM operates in a high vacuum, which means that the samples must be compatible with this condition. Live specimens, volatile materials, or hydrated samples cannot be observed directly by the SEM, as they would evaporate or collapse under the vacuum. Some samples may require special preparation techniques, such as fixation, dehydration, drying, or coating, to make them stable and conductive for SEM observation. However, these techniques may introduce artifacts or alter the natural morphology of the samples. Alternatively, some modern SEMs can use variable pressure or low vacuum modes to accommodate samples that are sensitive to high vacuum .
• Size and cost: The SEM is a large and expensive instrument that requires a dedicated space in the laboratory and a stable power supply. It also needs to be isolated from any sources of vibration, magnetic fields, or electrical interference that may affect the quality of the images. The maintenance of the SEM involves keeping a constant voltage, current, and cooling system for the electron source and the lenses . The SEM also requires trained personnel to operate it properly and safely.
• Resolution and depth of field: The resolution and depth of field of the SEM depend on several factors, such as the electron source, the accelerating voltage, the spot size, the working distance, and the detector type. Generally, higher resolution images require higher accelerating voltages and smaller spot sizes, but this also reduces the depth of field and the signal-to-noise ratio. Conversely, lower accelerating voltages and larger spot sizes increase the depth of field and the signal-to-noise ratio, but decrease the resolution. Therefore, there is a trade-off between resolution and depth of field that needs to be optimized for each sample and application .

• Chemical analysis: The SEM can provide qualitative and quantitative chemical analysis of the samples using EDS or other techniques, but it has some limitations in this aspect as well. For example, EDS cannot detect elements with atomic numbers lower than 5 (boron), and it has poor sensitivity for light elements (carbon to oxygen). EDS also suffers from peak overlaps, matrix effects, and background noise that may affect the accuracy and precision of the analysis. Moreover, EDS only provides information about the surface composition of the samples, not their bulk composition .

  Introduction to Scanning-Transmission Electron Microscope (STEM)

A scanning-transmission electron microscope (STEM) is a type of electron microscope that combines the features of both scanning electron microscope (SEM) and transmission electron microscope (TEM). As with a conventional TEM, images are formed by electrons passing through a sufficiently thin specimen. However, unlike TEM, in STEM the electron beam is focused to a fine spot (with the typical spot size 0.05 – 0.2 nm) which is then scanned over the sample in a raster illumination system constructed so that the sample is illuminated at each point with the beam parallel to the optical axis.

The rastering of the beam across the sample makes STEM suitable for analytical techniques such as Z-contrast annular dark-field imaging, and spectroscopic mapping by energy dispersive X-ray (EDX) spectroscopy, or electron energy loss spectroscopy (EELS). These signals can be obtained simultaneously, allowing direct correlation of images and spectroscopic data. A typical STEM is a conventional transmission electron microscope equipped with additional scanning coils, detectors, and necessary circuitry, which allows it to switch between operating as a STEM, or a CTEM; however, dedicated STEMs are also manufactured.

The first STEM was built in 1938 by Baron Manfred von Ardenne, working in Berlin for Siemens. However, the technique was not developed further until the 1970s, when Albert Crewe at the University of Chicago developed the field emission gun and added a high-quality objective lens to create a modern STEM. He demonstrated the ability to image atoms using an annular dark field detector.

The addition of an aberration corrector to STEMs enables electron probes to be focused to sub-angstrom diameters, allowing images with sub-angstrom resolution to be acquired. This improves the spatial resolution and sensitivity of STEM imaging and spectroscopy.

The main advantages of STEM over TEM are:

• The ability to form images with different contrast mechanisms by using different detectors
• The ability to perform elemental analysis and chemical mapping by EDX or EELS
• The ability to image thicker specimens than TEM due to the parallel illumination
• The ability to image specimens at low voltages for reduced beam damage

The main disadvantages of STEM over TEM are:

• The lower signal-to-noise ratio due to the smaller beam current
• The increased sensitivity to specimen drift and vibration due to the scanning mode
• The increased complexity and cost of the instrument due to the additional components

The scanning electron microscope (SEM) is a powerful and versatile tool that can be used for various applications in different fields of science and technology. Some of the main applications of the SEM are:

• Material science: The SEM can reveal the microstructure, morphology, crystallography, and chemical composition of various materials, such as metals, ceramics, polymers, composites, and nanomaterials. The SEM can also be used to study the effects of mechanical stress, corrosion, fracture, wear, and fatigue on materials. The SEM can also perform elemental analysis using energy-dispersive X-ray spectroscopy (EDS) or wavelength-dispersive X-ray spectroscopy (WDS), which can identify and quantify the elements present in a sample.

• Biology and medicine: The SEM can provide detailed images of biological specimens, such as cells, tissues, organs, microorganisms, parasites, and biomolecules. The SEM can also be used to study the surface features, interactions, and functions of biological materials, such as membranes, receptors, enzymes, DNA, and proteins. The SEM can also be used for biomedical applications, such as orthodontics, implantology, tissue engineering, drug delivery, and diagnosis of diseases. The SEM can also perform immunogold labeling, which can detect specific antigens or antibodies on the surface of biological samples using gold nanoparticles.

• Forensic science: The SEM can provide valuable information for forensic investigations, such as identification of gunshot residues, fingerprints, hair, fibers, drugs, explosives, paint chips, soil particles, and other trace evidence. The SEM can also be used to determine the origin, history, and authenticity of documents, artworks, coins, and other objects. The SEM can also perform ballistic analysis, which can compare the characteristics of bullets and firearms using backscattered electron imaging.

• Nanoscience and nanotechnology: The SEM can image and manipulate nanoscale objects and structures, such as nanoparticles, nanowires, nanotubes, nanofibers, nanocomposites, and nanodevices. The SEM can also be used to fabricate nanostructures using focused ion beam (FIB) milling or electron beam lithography. The SEM can also perform scanning probe microscopy (SPM), which can measure the physical and electrical properties of nanomaterials using various probes attached to the electron beam.

• Other applications: The SEM can also be used for various other applications in different fields of science and technology. For example:

  • In geology and mineralogy: The SEM can study the morphology, composition, texture, and origin of rocks, minerals, fossils, and meteorites.
  • In environmental science: The SEM can analyze the pollution, contamination, and degradation of air, water, soil, and biota.
  • In electronics and engineering: The SEM can inspect the quality, performance, and reliability of electronic components, circuits, devices, and systems.
  • In chemistry and physics: The SEM can investigate the structure, properties, and reactions of molecules, atoms, ions, and electrons.
  • In agriculture and food science: The SEM can examine the structure, quality, safety, and nutrition of crops, plants, animals, and food products.
  • In cosmetics and pharmaceuticals: The SEM can evaluate the formulation, stability, efficacy, and delivery of cosmetic products and drugs.

These are some of the main applications of the scanning electron microscope (SEM). However the list is not exhaustive and new applications are constantly being developed as the technology advances and new challenges arise in different fields of science and technology.

The scanning electron microscope (SEM) is a powerful and versatile tool for studying the surfaces and structures of various materials. Some of the advantages of using SEM are:

• Resolution: The SEM can produce images with a resolution as low as 15 nanometers, which is much higher than that of optical microscopes. This allows the SEM to reveal fine details and features of microstructures, such as fractures, corrosion, grains, and grain boundaries.
• Magnification: The SEM can achieve a wide range of magnifications, from 10x to over 500,000x, depending on the type of electron source and lens system. This enables the SEM to examine specimens at different scales and levels of detail.
• Depth of field: The SEM has a large depth of field, which means that more of the specimen can be in focus at one time. This gives the SEM images a three-dimensional and topographical appearance, which can help to understand the shape and texture of the specimen.
• Chemical analysis: The SEM can be combined with energy dispersive spectroscopy (EDS) to provide qualitative and quantitative information on the elemental composition of the specimen. EDS can detect elements from boron to uranium, and can generate maps and line scans to show the spatial distribution and variation of elements. This can be useful for identifying defects, foreign materials, coatings, and particles in the specimen.
• Ease of operation: The SEM is relatively easy to operate and has user-friendly interfaces. Some modern SEMs can also generate digital data that can be stored and transferred easily. It is also possible to acquire data from the SEM within a short period of time, about 5 minutes.
• Versatility: The SEM can be used for a variety of applications and fields, such as industrial quality control, nanoscience, biomedical research, microbiology, geology, archaeology, and forensics . The SEM can also accommodate different types of specimens, such as metals, ceramics, polymers, biological materials, and minerals. Some specimens may require special preparation, such as fixation, dehydration, drying, or coating, but others can be examined directly without any treatment.

Despite its many advantages, the SEM also has some limitations that need to be considered before using it for materials analysis. Some of the main limitations are:

• Vacuum environment: The SEM operates in a high vacuum, which means that the samples must be compatible with this condition. Live specimens, volatile materials, or hydrated samples cannot be observed directly by the SEM, as they would evaporate or collapse under the vacuum. Some samples may require special preparation techniques, such as fixation, dehydration, drying, or coating, to make them stable and conductive for SEM observation. However, these techniques may introduce artifacts or alter the natural morphology of the samples. Alternatively, some modern SEMs can use variable pressure or low vacuum modes to accommodate samples that are sensitive to high vacuum .
• Size and cost: The SEM is a large and expensive instrument that requires a dedicated space in the laboratory and a stable power supply. It also needs to be isolated from any sources of vibration, magnetic fields, or electrical interference that may affect the quality of the images. The maintenance of the SEM involves keeping a constant voltage, current, and cooling system for the electron source and the lenses . The SEM also requires trained personnel to operate it properly and safely.
• Resolution and depth of field: The resolution and depth of field of the SEM depend on several factors, such as the electron source, the accelerating voltage, the spot size, the working distance, and the detector type. Generally, higher resolution images require higher accelerating voltages and smaller spot sizes, but this also reduces the depth of field and the signal-to-noise ratio. Conversely, lower accelerating voltages and larger spot sizes increase the depth of field and the signal-to-noise ratio, but decrease the resolution. Therefore, there is a trade-off between resolution and depth of field that needs to be optimized for each sample and application .

• Chemical analysis: The SEM can provide qualitative and quantitative chemical analysis of the samples using EDS or other techniques, but it has some limitations in this aspect as well. For example, EDS cannot detect elements with atomic numbers lower than 5 (boron), and it has poor sensitivity for light elements (carbon to oxygen). EDS also suffers from peak overlaps, matrix effects, and background noise that may affect the accuracy and precision of the analysis. Moreover, EDS only provides information about the surface composition of the samples, not their bulk composition .

  Introduction to Scanning-Transmission Electron Microscope (STEM)

A scanning-transmission electron microscope (STEM) is a type of electron microscope that combines the features of both scanning electron microscope (SEM) and transmission electron microscope (TEM). As with a conventional TEM, images are formed by electrons passing through a sufficiently thin specimen. However, unlike TEM, in STEM the electron beam is focused to a fine spot (with the typical spot size 0.05 – 0.2 nm) which is then scanned over the sample in a raster illumination system constructed so that the sample is illuminated at each point with the beam parallel to the optical axis.

The rastering of the beam across the sample makes STEM suitable for analytical techniques such as Z-contrast annular dark-field imaging, and spectroscopic mapping by energy dispersive X-ray (EDX) spectroscopy, or electron energy loss spectroscopy (EELS). These signals can be obtained simultaneously, allowing direct correlation of images and spectroscopic data. A typical STEM is a conventional transmission electron microscope equipped with additional scanning coils, detectors, and necessary circuitry, which allows it to switch between operating as a STEM, or a CTEM; however, dedicated STEMs are also manufactured.

The first STEM was built in 1938 by Baron Manfred von Ardenne, working in Berlin for Siemens. However, the technique was not developed further until the 1970s, when Albert Crewe at the University of Chicago developed the field emission gun and added a high-quality objective lens to create a modern STEM. He demonstrated the ability to image atoms using an annular dark field detector.

The addition of an aberration corrector to STEMs enables electron probes to be focused to sub-angstrom diameters, allowing images with sub-angstrom resolution to be acquired. This improves the spatial resolution and sensitivity of STEM imaging and spectroscopy.

The main advantages of STEM over TEM are:

• The ability to form images with different contrast mechanisms by using different detectors
• The ability to perform elemental analysis and chemical mapping by EDX or EELS
• The ability to image thicker specimens than TEM due to the parallel illumination
• The ability to image specimens at low voltages for reduced beam damage

The main disadvantages of STEM over TEM are:

• The lower signal-to-noise ratio due to the smaller beam current
• The increased sensitivity to specimen drift and vibration due to the scanning mode
• The increased complexity and cost of the instrument due to the additional components

We are Compiling this Section. Thanks for your understanding.