Electron Microscope – Principle, Types, Parts, Application, Diagram

An electron microscope is an instrument which produces an image of a sample by means of a beam of electrons. Optical microscopes use light to form an image, electron microscopes have greater resolution, and can be used to observe objects that are too small to be seen with an optical microscope. In a large variety of fields going from biology to materials science, and nanotechnology, electron microscopes are used to study the structure and properties of materials at small scale. Transmission electron microscopes, scanning electron microscopes, scanning transmission electron microscopes are some examples of different electron microscopes, of which different types have their various set of abilities and application.

What is an Electron Microscope?

• What is an electron microscope? Super fancy equipment that uses a beam of accelerated electrons to see at much higher magnifications than ordinary light microscopes. While light-based microscopes utilize visible light to illuminate samples, electron microscopes employ electrons to provide images with resolution far superior to those produced by conventional optical microscopes.
• The basic premise of an electron microscope is actually based on the nature of the electrons. These subatomic particles have wavelengths many thousands of times shorter than visible light, so they can be used to form images with resolutions up to 0.1 nanometer, as opposed to the 200 nanometers obtained with light microscopes. This incredible resolution is what makes electron microscopes invaluable in materials science and biology and in fields requiring detailed visualization at the atomic or molecular level.
• The electron microscope was first developed in 1931 when a German engineer named Ernst Ruska built the first electron microscope. But the fundamental ideas that he introduced still serve as a keystone to current electron microscopes. These instruments use electron optics, similar to the glass lenses used in optical microscopes, to control an electron beam. These optics serve to focus the electrons so that images or electron diffraction patterns with a magnification can be produced.
• There are different kinds of electron microscopes used for different things. Transmission Electron Microscopy (TEM) is a technique that uses an electron beam to pass through a thin sample, producing a 2D view of internal structures. In contrast, scanning electron microscopy uses an electron beam that scans the surface of a specimen, creating 3D images that emphasize surface characteristics. Another method is Scanning Transmission Electron Microscopy (STEM), which utilizes a scanned electron probe for thin samples, combining some characteristics of both the TEM and SEM techniques.
• More recently, developments have led to Low-Energy Electron Microscopy, which focuses on surface imaging, and Photoemission Electron Microscopy, which images electrons emitted from the surface when photons strike the surface. Such variations will increase the scope of potential applications; scientists can, in other words, study material in never-before-seen detail.
• The electron microscope’s capacity to magnify and resolve on the atomic level has transformed science and has opened up the discovery of structures and phenomena that are totally invisible to the human eye. Technology continues to advance, providing even better means of investigating the microcosm.

Principle of Electron Microscope – How does an electron microscope work?

• The working of an electron microscope is based on how an electron beam interacts with the object under study. This helps to closely examine the object’s structure, shape, and materials.
• The process begins with an electron gun, which is an important part that creates the flow of electrons needed for taking pictures. These electrons are then aimed into a narrow, focused beam by two pairs of condenser lenses. This beam improves as it passes through the sample, which has to be very thin-about 200 times thinner than what is used in optical microscopy. Usually, very thin slices between 20 and 100 nanometers are cut and placed on a holder for viewing.
• It enables efficient movement of electrons down the column of the microscope due to the application of the voltage between the tungsten filament and the anode. Usually, it falls within the range of 100 kV to 1000 kV. These high energy electrons are pushed towards the specimen.
• As the electron beam passes through the specimen, it interacts with parts of it that are different in terms of thickness and refractive index. Thicker parts scatter more electrons, hence fewer hit the screen, thus appearing darker in the final image. The clearer parts scatter fewer electrons, and thus more will reach the screen, hence appear brighter in the image.
• After interaction with the specimen, the electron beam proceeds onward to the objective lens. Being one of the highly powerful lenses, the objective lens creates an intermediate magnified image of the specimen. These are further enlarged by ocular lenses so as to produce an accurate and proper view of the sample.

Operating Procedure of Electron microscope

The use of an electron microscope varies depending on the type of microscope and what is being examined. However, there are a few general steps that most electron microscopes follow:

1. Prepare the specimen- The preparation of the specimen is a basic requirement before this sample can be observed with the electron microscope. This can sometimes be in form of thinning the sample and adding a contrasting agent, sometimes mounting the specimen on a stage or holder.
2. Position the sample. The sample will be positioned with the area of interest inside the field of view within the electron beam. In addition, any sample can be moved and then rotated for inspection of several regions or to focus more closely on the desired features.
3. Adjust the electron beam – In this case, the energy as well as the intensity of the electron beam can be tuned to optimize contrast of the image and minimize damage to the sample. Finally, the beam may be focused or rastered over the sample in order to produce a high-resolution image.
4. Collect and process the image data- The electronic detector collects the electrons that bounce off or are reflected by the sample. These electrons help in making an image of the sample. Image data can be processed and analyzed with special software.
5. View and Interpret the Image- An image is first displayed on the screen or recorded by a camera and can, therefore, be viewed and interpreted to study its structure and the properties of a sample. Depending on the available information, such an image could be magnified, rotated, or enhanced with specific features and to help in interpretation.

Parts of an Electron Microscope

The parts of an electron microscope are as follows:

The electron microscope is in reality a complex mechanism made up of several important parts. Here is a summary of the major components:

• Electron Gun– The electron gun is the source of electrons used for imaging. A heated tungsten filament, which gives off electrons when burning, makes up the hot cathode source. These electrons are accelerated down towards the sample.
• Electromagnetic Lenses
  • Condenser Lenses – These focus the electron beam onto the specimen. The first condenser lens of the bunch is used to focus the beam, and the second actually forms it into a thin, tight, and strong (high-velocity) stream so that it can interact optimally with the specimen.
  • Objective Lens– After the specimen, the electron beam proceeds to the objective lens, which has the largest magnification power and produces the intermediate image.
  • Projector: (Ocular) Lens or lenses which further magnify the intermediate image from the objective lens to produce the final image. All of the set of lenses are very important in keeping the resolution and detail very high.
• Specimen Holder– So the sample is in the specimen holder, and it’s supposed to hold the sample while being imaged. It usually comprises an exceptionally thin layer of carbon or collodion, supported by a metal grid. This enables the sample to be held steady against the electron beam.
• An image viewing and recording system– The image is finally magnified onto a fluorescent screen for viewing. Below this screen is usually a camera that captures the image in order to analyze it and document it.
• Detector– Detector used to register the scattered or reflected electrons from the sample and it is essential. Different types of detectors, such as scintillators, phosphor screens, and CCDs, may be used. Such detectors are extremely important in forming a full complete picture of the specimen.
• Electronics and Computer System – This controls the functioning of the electron microscope and performs data. Usually comprises computer, monitor, and specialized software to control the scope and comprehend the resulting images.

No.	Part	Description
1	Electron Gun	Generates and accelerates electrons for imaging. In TEM, it consists of a cathode and an anode; in SEM, it has a cathode for generation and an anode for acceleration of electrons.
2	Electromagnetic Lens System	Comprises different types of lenses to focus and magnify the electron beam: <br> – Condenser lens: Focuses the electron beam on the specimen. <br> – Objective lens: Magnifies the image after the beam passes through the specimen. <br> – Projector (ocular) lenses: Further magnifies the intermediate image to form the final image.
3	Sample Holder/Specimen Holder	A platform with a mechanical arm to hold and position the specimen. In TEM, the sample is placed on a grid or holder; in SEM, it’s mounted on a stage that can be moved and positioned under the electron beam.
4	Image Viewing and Recording System	Projects the final image of the specimen onto a fluorescent screen, with a camera located below the screen to record the image.
5	Detector	Detects scattered or reflected electrons from the sample to create an image. Various types of detectors can be used, including scintillators, phosphor screens, and CCDs.
6	Electronics and Computer System	Controls the microscope’s operation and processes and displays image data. This system may include computers, monitors, and software for microscope control and data analysis.

Types of Electron microscope

Electron Microscopes are divided into three classes;

1. Transmission Electron Microscope (TEM)

The TEM is a device that forms relatively sharp internal images of a thin sample by allowing the transmission of an electron beam through them. It is then employed to visualize samples in excellent detail at the nanoscale.

• Electron Beam Production– The TEM uses a very energetic electron beam generated by an electron gun, which is essentially a tungsten wire known as the cathode. The anode accelerates this electron beam; the voltage applied is usually set at +100 keV, though it can be anywhere between 40 and 400 keV depending on the requirements of the imaging.
• Sample Preparation and Mounting– The specimen is made very thin, usually a few nanometers in thickness, so the electrons can easily pass through. It is mounted on a holder or grid that is placed inside the microscope.
• Focusing the Electron Beam– The electron beam is directed through the specimen by special lenses that apply electricity and magnetism. Such lenses direct the beam, so it goes right through the specimen.
• Transmission and Scattering – As the electron beam moves through the specimen, it encounters all the different parts of the sample. Electrons may be transmitted through partially transparent regions or scattered by denser regions. This differential transmission and scattering provide contrast in the resulting image.
• Magnification and Imaging – The transmitted electrons are further magnified by the objective lens system of TEM. This magnification enhances the detail and resolution of the image. The magnified image is projected onto a fluorescent viewing screen coated with a phosphor or scintillator material such as zinc sulfide.
• Capture and Display – The final image is taken by using a digital camera. This recorded image can analyze the internal structures of the specimen, including its cellular components, protein molecules, and molecular arrangement.
• Applications and Usefulness – TEM is used a lot in many areas like materials science, biology, and nanotechnology. It helps to study the inside of cells, look at protein and virus structures, and check for defects and impurities in materials. The high detail that TEM provides makes it a strong tool for looking at things at the atomic and molecular levels.

2. Scanning electron microscope (SEM)

The Scanning Electron Microscope (SEM) is a kind of electron microscope that gives clear pictures of the surface of samples. It works by moving a focused electron beam over the sample and examining the secondary electrons that come out to make high-quality images.

• Making the Electron Beam– The SEM starts with a heated electron gun that produces a thin, focused beam of electrons. This beam is important for taking images because it interacts with the surface of the specimen.
• Specimen Exposure and Scanning– The specimen is exposed to the thin electron beam, which scans its surface in a grid pattern. This scanning process means moving the electron beam quickly over the specimen to make sure it covers everything.
• Secondary Electron Emission– When the electron beam strikes the specimen, it knocks out secondary electrons and other kinds of radiation from the surface of the specimen. The number and strength of the secondary electrons are dependent on the shape and chemical nature of the surface.
• Finding and Changing Signals– The released secondary electrons are collected by a detector. They are converted to electronic signals for image production. The effectiveness and precision of the detector ensure a clear picture on the surface of the specimen, showing minute details.
• Images are produced, stored, and transferred – The electronic signals are scanned and then displayed on a cathode ray tube, which is an equivalent of a television system. Then, this CRT image is recorded either directly from the CRT or through a digital camera in the modern SEMs.
• Applications and Advantages– SEM provides clear images with a large area in focus, making it very good for looking closely at the surfaces of cells, living things, and different materials. Also, SEM is used to count particles, measure their sizes, and help control processes, providing more options than Transmission Electron Microscopy (TEM).

3. Scanning and Transmission Electron Microscope (STEM)

The basic concept of the Scanning Transmission Electron Microscope is derived from scanning and transmission electron microscopy. A STEM benefit of viewing at the nanoscale will be discussed. STEM combines the strengths of the transmission electron microscopes and scanning electron microscopes as it provides complete imaging options.

• Interaction of Electron Beam with Sample– STEMs use a thin beam of electrons that is allowed to pass through a very thin sample. Upon striking the sample, the electron beam gets scattered by the different layers of atoms within the material. This scattering is crucial in attaining clear information about the inner structure of the specimen.
• Detection of Scattered Electrons – The electrons are captured by a detector that accumulates these emissions as they are scattered. The detection system converts the scattered electrons into electronic signals used to make an image of the sample. The image is shown on a screen or photographed by a camera.
• High Resolution Imaging – It is particularly good at giving clear images of the outside and inside structures of samples. For materials that have atomic numbers with relatively high values, such as metals and semiconductors, clear images of small structural details are of major importance.
• Sample Sensitivity and Energy Adjustment– One of the benefits of STEM is that it can analyze samples sensitive to high-energy electrons. The energy of the electron beam can be controlled carefully to minimize damage to fragile samples, so STEM is suitable for observing materials that could be damaged by more intense electron impacts.
• Scanning Mechanism– In a STEM, the sample is placed on a stage and scanned by a focused beam of electrons in a pattern like a grid. This scanning process makes sure that the whole surface of the sample is looked at. The way the beam interacts with the sample produces detailed information, which is then used to make the image.
• Applications – STEM is applied in materials science, nanotechnology, and biology. It is used to analyze a wide variety of samples: metals, semiconductors, polymers, and biological materials. This technique is excellent for the detection of defects in structure and impurities at a molecular level and properties in materials at an atomic level.

4. Other types of Electron Microscope

Besides the well-known Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), and Scanning Transmission Electron Microscope (STEM), there are other special types of electron microscopes that offer unique features and benefits for different scientific uses.

• Reflection Electron Microscope (REM)
  • Principle: The Reflection Electron Microscope makes use of an electron beam directed at the surface of a sample. The microscope picks up the reflected beam of elastically scattered electrons.
  • Techniques: REM is mostly used with Reflection High-Energy Electron Diffraction (RHEED) and Reflection High-Energy Loss Spectroscopy (RHELS). These techniques improve the examination of surface structures and yield highly detailed information regarding surface shape and crystal structure.
• Scanning Transmission Electron Microscope (STEM)
  • Principle: STEM uses some components of both SEM and TEM. It scans a focused electron beam across a thin sample, detecting electrons that bounce off the sample.
  • Function: This technique serves to form an image of both the exterior and interior parts of samples. The STM technique is ideal for examining tiny samples and has a minimal chance of damaging materials by altering the energy of the electron beam.
• Scanning Tunneling Microscope (STM)
  • Principle: STM employs an electrically conductive tip. This tip is held at some voltage and placed near the surface of a sample. The probability that electrons move from the tip into the sample varies with distance. This variation allows for the production of a surface profile.
  • Application: This technique is highly effective for studying surfaces at the atomic level. It provides sharp information about the surface and has been widely used in surface science and nanotechnology.
• Environmental Electron Microscopes
  • Principle: Environmental electron microscopes are designed to observe samples in their natural environments, such as in a gas or liquid. They enable us to study materials in real-world environments.
  • Function: Such a microscope is very essential in the examination of materials that are sensitive to vacuum conditions, usually required by other electron microscopes. It makes it possible to view samples in conditions as close as possible to their natural environment.
• Low-Energy Electron Microscopes (LEEMs)
  • Principle: LEEM operates by a low-energy electron beam imaging the surface of a specimen. This is especially useful for samples sensitive to high-energy electrons.
  • Application: LEEM is highly useful for the study of organic materials and biological samples. It gives very high-resolution images of surfaces without damaging the specimens considerably.

Differences Between Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), and Scanning Transmission Electron Microscope (STEM)

Feature	Scanning Electron Microscope (SEM)	Transmission Electron Microscope (TEM)	Scanning Transmission Electron Microscope (STEM)
Principle of Imaging	Scans a focused electron beam across the specimen surface; detects secondary electrons emitted from the surface.	Transmits a beam of electrons through a thin specimen; detects electrons that pass through the specimen.	Combines scanning of a focused electron probe with transmission through a thin specimen; detects scattered electrons.
Sample Thickness	Can image surfaces of bulk samples or thin sections.	Requires very thin samples to allow electron transmission.	Requires thin samples, similar to TEM, but can also analyze surface and internal structure.
Resolution	Moderate to high, typically in the nanometer range.	High, with atomic resolution possible.	High, with capabilities for both surface and internal imaging at the atomic scale.
Image Formation	Produces 3D images of surface topography.	Produces 2D images of internal structures.	Produces high-resolution images of both surface and internal structures.
Electron Interaction	Electrons interact with the surface, generating secondary electrons.	Electrons interact with the entire thickness of the specimen, revealing internal structures.	Electrons interact with both surface and internal structures, depending on the scan.
Sample Preparation	Can be less stringent; specimens need to be coated with a conductive layer if insulating.	Requires ultra-thin sections of the specimen, often prepared by slicing.	Requires thin samples similar to TEM, but scanning adds additional requirements.
Applications	Surface morphology, particle size, and texture analysis; suitable for bulk samples.	Internal structure of cells, organelles, and molecular arrangements; ideal for thin sections.	High-resolution imaging of both surface and internal features; used for detailed analysis of nanostructures.
Image Detection	Detected by secondary electron detectors; images displayed on a screen or recorded.	Detected by transmitted electrons; images displayed on a fluorescent screen or captured by a camera.	Detected by transmitted and scattered electrons; images displayed on a screen or captured by a camera.
Sample Environment	Typically operates in a high vacuum; environmental SEMs can study samples in their native state.	Operates in a high vacuum to avoid electron scattering by air; some variants allow partial pressure.	Operates in a high vacuum; specialized STEMs may accommodate samples in different environments.

Application of Electron Microscopes

Electron microscopes’ main uses are:

• Bioresearch
  • Bacteria, viruses, and fungus may be examined under electron microscopes. Microbiology has enhanced illness therapy and pathogenic mechanism comprehension due to this capabilities.
  • TEMs reveal cell organelles and molecular complexes. Cell biology and medical studies benefit from this precise imaging of cellular processes and disorders.
  • Electron microscopes can image proteins and nucleic acids. Negative staining and metal shadowing raise biomolecule visibility for structural biology and drug development.
• Science of Materials
  • Electron microscopes are essential for metal and alloy microstructure analysis. Materials engineering and quality control require help spotting flaws, grain boundaries, and phase distributions.
  • The semiconductor industry inspects and characterizes nanoscale materials with electron microscopes. This involves testing semiconductor devices and production processes.
  • STEMs examine crystal atom arrangements to understand material properties and design novel materials with specialized qualities.
• Quality and Failure Analysis
  • In industrial applications, electron microscopes identify defects, pollution, and structural abnormalities in manufactured items, ensuring quality. Aerospace, automotive, and electronics employ them to assure product dependability and performance.
  • Electron microscopes examine fracture surfaces, corrosion patterns, and other failure modes to determine the reasons of material or component failure. Manufacturing and product design are improved by this study.
• Science of Environment– Electron microscopes analyze airborne contaminants, particles, and environmental materials. This software helps comprehend how pollution affect health and ecosystems.
• Advanced Research and Development– Electron microscopes help nanotechnology researchers observe and manipulate tiny materials and devices. This aids nanomaterials, nanomedicine, and nanofabrication research.

Advantages of Electron Microscopes

Below is an in-depth review of their important advantages:

• Very High Magnification
  • Capability: Electron microscopes can produce magnifications many, many times larger than light microscopes. Their high magnification enables one to see extremely minute structures and tiny details within specimens.
  • Resolution: The high magnification is also accompanied by extremely high resolution; thus, one may see features down to the atomic level. This is extremely important in examining nanoscale phenomena and materials.
• Incredibly High Resolution
  • Resolution Power: Electron microscopes have higher resolution than optical microscopes because they use electron beams instead of light. This allows us to view structures with very small detail, even smaller than a nanometer.
  • Detail: The high resolution helps to create clear images of internal structures, defects, and how molecules are arranged, giving us information that lower-resolution techniques cannot provide.
• Material Rarely Distorted by Preparation
  • Sample Integrity: Electron microscopes are designed to minimize alterations of samples when they are being prepared. Techniques such as cryo-electron microscopy maintain biological samples in their native state, which minimizes errors that may occur with other techniques.
  • High-resolution Imaging: Maintaining the integrity of the material ensures that the images captured are actual representations of the sample, which means more reliable scientific information.
• Depth of Field Ability to Investigate
  • Depth of Field: The depth of view for an electron microscope is larger than that for a light microscope. This makes them better able to view thicker samples, hence getting a more realistic view of the three-dimensional structure of the sample.
  • Applications: It is quite vital in material and biological science as knowing the entire structure of the sample is highly important in those fields.
• Varied Applications
  • Electron microscopes are versatile tools finding applications in the fields of biology, materials science, and nanotechnology. These are used for the study of cellular structures, materials properties, and nanomaterials.
  • Industry and Research: Applications include quality control and failure analysis; advanced research and its relevance for both academia and industry.

Limitations of Electron microscope

• No live dissection
  • Vacuum Environment- Electron microscopes must have a very controlled high-vacuum environment so the electrons will not be bounced from their true path by air molecules. As a result, living specimens cannot be observed because they need to be put into dry, vacuum-sealed conditions.
  • Biological Limitation- limitation on being able to observe living processes and interactions, which places a restriction on the kinds of applications of biological research.
• Sample Preparation Challenges
  • Ultrathin Sections : Because electrons cannot penetrate most biological samples, they are cut into ultrathin sections that may take an unreasonable amount of time to prepare. Dividing the waters dies ( sections the specimen), which can create artifacts and result in a loss of naturalness of the sample.
  • Image artifacts are artifacts that occur as a result of sample preparation and can be an issue when it comes to the precision of the images. These artifacts require a mastery of preparing techniques in order to reduce their influence and allow for confident results.
• Cost and Maintenance
  • Expensive: Electron microscopes are very expensive to buy, construct, and maintain. They are so complex in terms of technology, and we need special parts to make them, therefore they are very expensive. That is indeed a huge hurdle for a research project with a small budget.
  • Operational expenses – While operational costs are probably similar to those of other cutting-edge microscopy methods, the cost of initial setup and continuing maintenance are high.
• Size and Sensitivity
  • Big, heavy equipment: it is big and clumsy, and you need a cooling laboratory for something like an electron microscope.
  • Sensitive to External Factors – As a result, these microscopes are extremely sensitive to vibrations, as are external magnetic fields. Proper installation and tight control of the environment are required for use to ensure clear images.
• Training Requirements
  • Requires: I guess it takes a lot of training to use. In addition, to successfully use these tools and to understand the highly complex images that these machines create, researchers need to develop specific skills.
  • Operational Expertise: Due to the complicated nature of the technology, trained personnel can best exploit the merit of electron microscopy and steer clear of pitfalls in image acquisition and analysis.
• Black and White Imaging
  • Color Limitation: The images are always black and white. So the images must be artificially colored in for it to look good – which oftentimes is deceiving.

Why is electron microscope better than light?

Generally speaking, electron microscopes are more powerful and have a greater resolution than light microscopes—which create images using light. This results from numerous factors:

• Higher resolution: Using electron microscopes to see items at the nanoscale allows one to see objects far more clearly than with light microscopes. This makes them a strong tool for researching the structure and characteristics of materials at the atomic and molecular level.
• Greater depth of field: Electron microscopes have a higher depth of field than light microscopes, hence they may provide pictures of things with more depth or thickness. For examining thick or three-dimensional material, this makes them helpful.
• Ability to investigate a broad spectrum of samples: Metals, semiconductors, polymers, and biological materials among other things may be examined with electron microscopes. Studying materials too tiny, too clear, or too opaque for a light microscope is especially benefited from their use.
• Ability to research samples under a number of situations: Electron microscopes may be used to investigate samples under a variety of conditions, such as in a vacuum, in a gas, or in a liquid. This makes them valuable for research on sensitive to the environment or natural state samples that must be examined in their original form.
• Greater contrast: Electron microscopes may generate pictures with a high degree of contrast, therefore facilitating the distinction between many characteristics in a material. Examining samples with a complicated structure or spotting flaws and contaminants in materials would notably benefit from this.

Electron Microscope: Definition, Types, Parts, Application, Advantages, Disadvantages – Video

Electron microscope images

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Reference

• https://www.umassmed.edu/cemf/whatisem/
• https://www.microscopemaster.com/electron-microscope.html
• https://www.wikilectures.eu/w/Electron_microscopy/principle
• https://www.biologydiscussion.com/microscope/electron-microscope/electron-microscope-principle-components-specimen-preparation-and-uses/16595
• https://www.slideshare.net/gangahuvin/electron-microscopy-16995175
• https://getrevising.co.uk/grids/electron_microscopes_2
• https://www.yourarticlelibrary.com/microeconomics/working-principle-of-a-electron-microscopes-with-diagram/26479
• https://www.news-medical.net/life-sciences/Advantages-and-Disadvantages-of-Electron-Microscopy.aspx
• https://www.horiba.com/ind/cathodoluminescence-spectroscopy-electron-microscope/
• https://www.hitachi-hightech.com/in/en/products/microscopes/sem-tem-stem/