Electron Microscope Notes PDF

**ELECTRON MICROSCOPE** Electron Microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. It is a special type of microscope having a high resolution of images, able to magnify objects in nanometers, which are formed by controlled use of electrons in a vacuu...

**ELECTRON MICROSCOPE** Electron Microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. It is a special type of microscope having a high resolution of images, able to magnify objects in nanometers, which are formed by controlled use of electrons in a vacuum captured on a phosphorescent screen. Ernst Ruska (1906-1988), a German engineer and academic professor, built the first Electron Microscope in 1931, and the same principles behind his prototype still govern modern EMs. Electron microscope differs from the optical microscope. The electron microscope provides tremendous useful magnification, because of the much higher resolution obtainable with the extremely short wavelength of the electron beam used to magnify the specimen. Electron microscope uses electron beam and magnetic field to produce the image, whereas the light microscope uses the light waves and glass lenses. Why was electron microscope build? Electron microscope was built to overcome the limitation of light microscope. **DIFFERENCE BETWEEN LIGHT AND ELECTRON MICROSCOPE**  When electron microscope employing 60 to 80 kV electrons the wavelength is only 0.005 Å. Å is the abbreviations for angstrom, 1Å equals 1/100,000,000 (10^-8^) cm or 1/10,000 (10^-4^) µm. It is possible to resolve objects as small as 10 Å. The resolving power of the electron microscope is more than 100 times that of the light microscope, and it produces useful magnification upto X400,000. For electron microscope, the specimen to be examined is prepared as an extremely thin dry film on small screens and is introduced into the instrument at a point between the magnetic condenser and the magnetic objective. The magnified image may be viewed on a fluorescent screen through an airtight "window" or recorded on a photographic plate by a camera built into the instrument. Electron microscope is a valuable tool used to obtain high-resolution images in a variety of applications, including biomedical research, forensics, and technology. Electron microscopes can capture much higher resolution images than light microscopes, contributing information that is otherwise unattainable.\ Every electron microscope works by accelerating a focused stream of electrons in a vacuum towards a sample. Interactions between the electron beam and the sample create an image, similar to how optical microscopes use light to capture images. The image created reveals details of a sample's surface or internal composition, depending on the type of electron microscope that is used **PARTS AND FUNCTIONS OF ELECTRON MICROSCOPE** Electron Microscope is in the form of a tall vacuum column that is vertically mounted. It has the following components: Electron gun, electromagnetic lenses - condenser lens, specimen holder, objective lens, intermediate lens, projector (ocular) lenses and fluorescent screen. **ELECTRON MICROSCOPE** 1\. Electron gun: The electron gun is a heated tungsten filament, which generates electrons. 2\. Electromagnetic lenses: The condenser lens focuses the electron beam on the specimen. A second condenser lens forms the electrons into a thin tight beam. 3\. Specimen Holder: The specimen holder is an extremely thin film of carbon or collodion held by a metal grid. 4\. Objective lens: The electron beam coming out of the specimen passes down the objective lens, which magnified the image. 5\. Intermediate lense: Also, magnified the image. 6\. Projector (ocular) lenses: Produce the final further magnified image and projects it on a fluorescent screen. **PRINCIPLE OF ELECTRON MICROSCOPE WORKING** Electron microscopes use signals arising from the interaction of an electron beam with the sample to obtain information about structure, morphology, and composition. 1\. The electron gun generates electrons. 2\. Two sets of condenser lenses focus the electron beam on the specimen and then into a thin tight beam. 3\. To move electrons down the column, an accelerating voltage (mostly between 100 kV-1000 kV) is applied between the tungsten filament and anode. 4\. The specimen to be examined is made extremely thin, at least 200 times thinner than those used in the optical microscope. Ultra-thin sections of 20-100 nm are cut which is already placed on the specimen holder. 5\. The electronic beam passes through the specimen and electrons are scattered depending upon the thickness or refractive index of different parts of the specimen. 6\. The denser regions in the specimen scatter more electrons and therefore appear darker in the image since fewer electrons strike that area of the screen. In contrast, transparent regions are brighter. 7\. The electron beam coming out of the specimen passes to the objective lens, which has high power and forms the intermediate magnified image. 8\. The third set of magnetic lenses called projector (ocular) lenses produce the final further magnified image. Each of these lenses acts as an image magnifier all the while maintaining an incredible level of detail and resolution. 9\. The final image is projected on a fluorescent screen. 10. Below the fluorescent screen is a camera for recording the image. **APPLICATION OF ELECTRON MICROSCOPE** 1. Electron microscopes are used to investigate the ultrastructure of a wide range of biological and inorganic specimens including microorganisms, cells, large molecules, biopsy samples, metals, and crystals. 2. Industrially, electron microscopes are often used for quality control and failure analysis. 3. Modern electron microscopes produce electron micrographs using specialized digital cameras and frame grabbers to capture the images. 4. The science of microbiology owes its development to the electron microscope. The study of microorganisms like bacteria, virus, and other pathogens have made the treatment of diseases very effective. 5. It gives information about morphology, topography, composition and crystallography nature of an object. **ADVANTAGE OF ELECTRON MICROSCOPE** 1. Very high magnification 2. Incredibly high resolution 3. Material rarely distorted by preparation 4. It is possible to investigate a greater depth of field 5. Diverse applications **DISADVANTAGE OF ELECTRON MICROSCOPE** 1. The live specimen cannot be observed. 2. As the penetration power of the electron beam is very low, the object should be ultra-thin. For this, the specimen is dried and cut into ultra-thin sections before observation. 3. As the EM works in a vacuum, the specimen should be completely dry. 4. Expensive to build and maintain 5. Requiring researcher training 6. Image artifacts resulting from specimen preparation. 7. This type of microscope is large, cumbersome extremely sensitive to vibration and external magnetic fields. **TYPES OF ELECTRON MICROSCOPE**. 1. Scanning Electron Microscope (SEM) 2. Transmission Electron Microscope (TEM) are the two most common types of electron microscope. TEM and SEM differ in how they work and what types of images they are able to capture. compare to one another. **SCANNING ELECTRON MICROSCOPE OR SCANNING ELECTRON MICROSCOPY (SEM)** Is a kind of electron microscope that uses a fine beam of focused electrons to scan a sample's surface. The microscope records information about the interaction between the electrons and the sample, creating a magnified image. SEM has the potential to magnify an image up to 2 million times. SEM images give insight into a sample's topography and elemental composition. SEM is able to capture 3-D black-and-white images of thin or thick samples. The [sample's size is limited](https://www.fei.com/introduction-to-electron-microscopy/sem/) only by the size of the electron microscope chamber. **STRUCTURE OF THE SEM** 1. Lens: here are not the optical materials (like glass), but electrical field. Electron optics : - Condenser lens: focusing the electron beam to the objective lens. - Objective lens: responsible for size of electron beam impinging on sample surface 2. Electron beam. 3. Transducers (detectors). **BASIC WORKING PRINCIPLE OF SEM** To obtain a high-resolution image, an electron source (also known as an electron gun) emits a stream of high-energy electrons towards a sample. The electron beam is focused using electromagnetic lenses. Once the focused stream reaches the sample, it scans its surface in a rectangular raster.\ The interaction between the electron beam and the sample creates secondary electrons, backscattered electrons, and X-rays. These interactions are captured to create a magnified image. **SEM** **TRANSMISSION ELECTRON MICROSCOPE OR TRANSMISSION ELECTRON MICROSCOPY (TEM)** Is a type of electron microscope that uses a broad beam of electrons to create an image of a sample's internal structure. A beam of electrons is transmitted through a sample, creating an image that details a sample's morphology, composition, and crystal structure. Samples must be incredibly thin, often [less than 150 nm](https://blog.phenom-world.com/sem-tem-difference) thick, to allow electrons to pass through them. After the transmission of the electrons through the sample, they arrive at a detector below and a 2-D image is created. TEMs have an incredible [magnification potential](https://blog.phenom-world.com/sem-tem-difference) of 10-50 million times. The details provided are at the atomic level, the highest resolution of any electron microscope. TEMs are often used to examine molecular and cellular structures. **STRUCTURE OF THE TEM** 1\. The electron gun emits electrons. 2\. The condenser lenses focus the electron beam on the specimen. 3\. The objective lens forms a focused image which is enlarged by the projector (ocular) lenses. 4\. The image is viewed on the fluorescent screen or a computer monitor. **BASIC WORKING PRINCIPLE OF *TEM*** An electron source sends a beam of electrons through an ultrathin sample. When the electrons penetrate the sample, they pass through lenses below. This data is used to create images directly on a fluorescent screen or onto a computer screen using a charge-coupled device (CCD) camera.  **TEM** **SEM vs TEM** SEM and TEM are both valuable tools in the biological, physical, and chemical sciences. By understanding the differences between these two electron microscopes, scientists can choose the correct type of microscope for their needs. **SEM vs TEM ADVANTAGES** Scanning Electron Microscopes and Transmission Electron Microscopes each contain unique advantages when compared to the other. In comparison to TEMs, SEMs: - Cost less - Take less time to create an image - Require less sample preparation - Accept thicker samples - Can examine larger samples In comparison to SEMs, TEMs: - Create higher resolution images - Provide crystallographic and atomic data - Create 2-D images that are often easier to interpret than SEM 3-D images - Allow users to examine more characteristics of a sample **SEM vs TEM SIMILARITIES AND DIFFERENCES** There are many similarities between SEMs and TEMs. The components of these two high-resolution microscopes are very similar. Each has an electron source/gun that emits an electron stream towards a sample in a vacuum, and each contains lenses and electron apertures to control the electron beam and capture images. But the differences in function between the two are vast. They differ in how they work, the types of samples that they require, the resolution of images that they create, and more. The below table summarizes the differences between Scanning Electron Microscopes and Transmission Electron Microscopes. Scanning Electron Microscopes (SEM) Transmission Electron Microscopes (TEM) -------------------- ------------------------------------- ----------------------------------------- Electron stream Fine, focused beam Broad beam Image taken Topographical/surface Internal structure Resolution Lower resolution Higher resolution Magnification Up to 2,000,000 times  Up to 50,000,000 times Image dimension 3-D 2-D Sample thickness Thin and thick samples okay Ultrathin samples only Penetrates sample No Yes Sample restriction Less restrictive More restrictive Sample preparation Less preparation required More preparation required Cost Less expensive More expensive Speed Faster Slower