Transmission Electron Microscope

Transmission electron microscope is a microscope used to form an image of a specimen by transmitting a beam of electrons through it. The specimen is most often an ultrathin section less than 100nm thick or a suspension on a grid. 

An image is formed from the interaction of the electrons with the sample as the beam is transmitted through the specimen. 

The image is then magnified and focused onto an imaging device, such as a fluorescent screen, a layer of photographic film, or a sensor such as a scintillator attached to a charge-coupled device.

Transmission electron microscope is also known by terms Transmitted electron microscope and 

1. What is Microscopy?

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Human eye can distinguish objects down to about 0.2 mm. Beyond that human eye do not have the capability to observe them. To satisfy this curiosity, many inventions have been devised. One of them is the optical microscope. 

The Optical microscopes reveal small objects, which would be otherwise invisible to the human eye, by magnifying them with the help of a combination of glass lenses. 

Optical microscopes use light as the illumination, so they have a limited ability to distinguish small structures (resolution). They cannot distinguish any structure smaller than the wavelength of light.
Engineers invented the “electron microscope”, which uses an electron beam as the illumination source instead of light. 

That enables us to observe small structures at a far better magnification than is possible with optical microscopes. It is now possible to distinguish the arrangement of atoms in materials.
TEMs can magnify objects up to 2 million times. 

In order to get a better idea of just how small that is, think of how small a cell is. It is no wonder TEMs have become so valuable within the biological and medical fields.

Human eye can distinguish objects down to about 0.2 mm. Beyond that human eye do not have the capability to observe them. To satisfy this curiosity, many inventions have been devised. One of them is the optical microscope. 

The Optical microscopes reveal small objects, which would be otherwise invisible to the human eye, by magnifying them with the help of a combination of glass lenses. 

Optical microscopes use light as the illumination, so they have a limited ability to distinguish small structures (resolution). They cannot distinguish any structure smaller than the wavelength of light.

Engineers invented the “electron microscope”, which uses an electron beam as the illumination source instead of light. 

That enables us to observe small structures at a far better magnification than is possible with optical microscopes. It is now possible to distinguish the arrangement of atoms in materials.

TEMs can magnify objects up to 2 million times. In order to get a better idea of just how small that is, think of how small a cell is. It is no wonder TEMs have become so valuable within the biological and medical fields.

Electron microscopes have emerged as a powerful tool for the characterization of a wide range of materials. Their versatility and extremely high spatial resolution render them a very valuable tool for many applications. The two main types of electron microscopes are the Transmission Electron Microscope (TEM) and the Scanning Electron Microscope (SEM)

2. What is Transmission Electron Microscope used for?

The Transmission Electron Microscope is most useful for observing thin specimens such as tissue sections, molecules, etc by passing electrons through, which generate a projection image. 

A transmission electron microscope image is formed by transmitting a beam of electrons through a specimen. The specimen is most often an ultrathin section less than 100nm thick or a suspension on a grid. An image is formed from the interaction of the electrons with the sample as the beam is transmitted through the specimen. 

The image is then magnified and focused onto an imaging device, such as a fluorescent screen, a layer of photographic film, or a sensor such as a scintillator attached to a charge-coupled device.

Further, TEM is used to image the cell interiors, the structure of protein molecules (contrasted by metal shadowing), the organization of molecules in viruses and cytoskeletal filaments (prepared by the negative staining technique), and the arrangement of protein molecules in cell membranes (by freeze-fracture).

Transmission electron microscope can produce high-resolution images. This enables the instrument to capture fine detail even as small as a single column of atoms, which is thousands of times smaller than a resolvable object seen in a light microscope.

3. Structure of Transmission Electron Microscope (TEM)

Transmission electron microscope (TEM), type of electron microscope that has three essential systems:

  • An electron gun, which produces the electron beam, and the condenser system, which focuses the beam onto the object,
  • The image-producing system, consisting of the objective lens, movable specimen stage, and intermediate and projector lenses, which focus the electrons passing through the specimen to form a real, highly magnified image, and
  • The image-recording system, which converts the electron image into some form perceptible to the human eye. The image-recording system usually consists of a fluorescent screen for viewing and focusing the image and a digital camera for permanent records.

In addition, a vacuum system, consisting of pumps and their associated gauges and valves, and power supplies are required.

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a. The Electron gun and Condenser system

The source of electrons, the cathode, is a heated V-shaped tungsten filament or, in high-performance instruments, a sharply pointed rod of a material such as lanthanum hexaboride. 

The filament is surrounded by a control grid, sometimes called a Wehnelt cylinder, with a central aperture arranged on the axis of the column; the apex of the cathode is arranged to lie at or just above or below this aperture. 

The cathode and control grid are at a negative potential equal to the desired accelerating voltage and are insulated from the rest of the instrument. The final electrode of the electron gun is the anode, which takes the form of a disk with an axial hole. 

Electrons leave the cathode and shield, accelerate toward the anode, and, if the stabilization of the high voltage is adequate, pass through the central aperture at a constant energy. The control and alignment of the electron gun are critical in ensuring satisfactory operation.

The intensity and angular aperture of the beam are controlled by the condenser lens system between the gun and the specimen. 

A single lens may be used to converge the beam onto the object, but, more commonly, a double condenser is employed. In this the first lens is strong and produces a reduced image of the source, which is then imaged by the second lens onto the object. 

Such an arrangement is economical of space between the electron gun and the object stage and is more flexible, because the reduction in size of the image of the source (and hence the final size of illuminated area on the specimen) may be varied widely by controlling the first lens. The use of a small spot size minimizes disturbances in the specimen due to heating and irradiation.

b. The image producing system

The specimen grid is carried in a small holder in a movable specimen stage. The objective lens is usually of short focal length (1–5 mm [0.04–0.2 inch]) and produces a real intermediate image that is further magnified by the projector lens or lenses. 

A single projector lens may provide a range of magnification of 5:1, and by using interchangeable pole pieces in the projector a wider range of magnifications may be obtained. 

Modern instruments employ two projector lenses (one called the intermediate lens) to permit a greater range of magnification and to provide a greater overall magnification without a commensurate increase in the physical length of the column of the microscope.

For practical reasons of image stability and brightness, the microscope is often operated to give a final magnification of 1,000–250,000× on the screen. If a higher final magnification is required, it may be obtained by photographic or digital enlargement. 

The quality of the final image in the electron microscope depends largely upon the accuracy of the various mechanical and electrical adjustments with which the various lenses are aligned to one another and to the illuminating system. 

The lenses require power supplies of a high degree of stability; for the highest standard of resolution, electronic stabilization to better than one part in a million is necessary. 

The control of a modern electron microscope is carried out by a computer, and dedicated software is readily available.

c. Image recording

The electron image is monochromatic and must be made visible to eye either by allowing the electrons to fall on a fluorescent screen fitted at the base of the microscope column or by capturing the image digitally for display on a computer monitor. 

Computerized images are stored in a format such as TIFF or JPEG and can be analyzed or image-processed prior to publication. 

The identification of specific areas of an image, or pixels with specified characteristics, allows spurious colours to be added to a monochrome image. This can be an aid to visual interpretation and teaching and can create a visually attractive picture from the raw image.

d. Vacuum System

To increase the mean free path of the electron gas interaction, a standard TEM is evacuated to low pressures, typically on the order of 10−4 Pa. The need for this is twofold: first the allowance for the voltage difference between the cathode and the ground without generating an arc, and secondly to reduce the collision frequency of electrons with gas atoms to negligible levels this effect is characterized by the mean free path. 

TEM components such as specimen holders and film cartridges must be routinely inserted or replaced requiring a system with the ability to re-evacuate on a regular basis. As such, TEMs are equipped with multiple pumping systems and airlocks and are not permanently vacuum sealed.

High-voltage TEMs require ultra-high vacuums on the range of 10−7 to 10−9 Pa to prevent the generation of an electrical arc, particularly at the TEM cathode. 

As such for higher voltage TEMs a third vacuum system may operate, with the gun isolated from the main chamber either by gate valves or a differential pumping aperture – a small hole that prevents the diffusion of gas molecules into the higher vacuum gun area faster than they can be pumped out. For these very low pressures, either an ion pump or a getter material is used.

Poor vacuum in a TEM can cause several problems ranging from the deposition of gas inside the TEM onto the specimen while viewed in a process known as electron beam induced deposition to more severe cathode damages caused by electrical discharge. 

The use of a cold trap to adsorb sublimated gases in the vicinity of the specimen largely eliminates vacuum problems that are caused by specimen sublimation.

4. How does Transmission Electron Microscope (TEM) work?

EMs employ a high voltage electron beam in order to create an image. An electron gun at the top of a TEM emits electrons that travel through the microscope’s vacuum tube. 

Rather than having a glass lens focusing the light (as in the case of light microscopes), the TEM employs an electromagnetic lens which focuses the electrons into a very fine beam. 

This beam then passes through the specimen, which is very thin, and the electrons either scatter or hit a fluorescent screen at the bottom of the microscope. 

An image of the specimen with its assorted parts shown in different shades according to its density appears on the screen.

5. Transmission electron microscope vs scanning electron microscope

The main difference between Scanning vs Transmission Electron Microscope is that the Scanning Electron Microscope creates an image by detecting reflected or knocked-off electrons while Transmission Electron MicroscopeM uses transmitted electrons (electrons which are passing through the sample) to create an image. 

As a result, Transmission Electron Microscope offers valuable information on the inner structure of the sample, such as crystal structure, morphology and stress state information, while Scanning Electron Microscope provides information on the sample’s surface and its composition.

6. Differences Between Transmission Electron Microscope and Light Microscope

Although TEMs and light microscopes operate on the same basic principles, there are several differences between the two. 

The main difference is that TEMs use electrons rather than light in order to magnify images. The power of the light microscope is limited by the wavelength of light and can magnify something up to 2,000 times. Electron microscopes, on the other hand, can produce much more highly magnified images because the beam of electrons has a smaller wavelength which creates images of higher resolution.

7. What are the applications of TEM?

TEMs provide topographical, morphological, compositional and crystalline information. The images allow researchers to view samples on a molecular level, making it possible to analyze structure and texture. 

This information is useful in the study of crystals and metals, but also has industrial applications. TEMs can be used in semiconductor analysis and production and the manufacturing of computer and silicon chips.

Technology companies use TEMs to identify flaws, fractures and damages to micro-sized objects; this data can help fix problems and/or help to make a more durable, efficient product. Colleges and universities can utilize TEMs for research and studies. 

Although electron microscopes require specialized training, students can assist professors and learn TEM techniques. Students will have the opportunity to observe a nano-sized world in incredible depth and detail.

8. What are the Advantages and Disadvantages of TEM?

Advantages of transmission electron microscope

  1. TEMs offer the most powerful magnification, potentially over one million times or more
  2. TEMs have a wide range of applications and can be utilized in a variety of different scientific, educational and industrial fields
  3. TEMs provide information on element and compound structure
  4. Images are high-quality and detailed
  5. TEMs can yield information of surface features, shape, size and structure
  6. They are easy to operate with proper training

Disadvantages of transmission electron microscope

  1. Some cons of electron microscopes include:
  2. TEMs are large and very expensive
  3. Laborious sample preparation
  4. Potential artifacts from sample preparation
  5. Operation and analysis require special training
  6. Samples are limited to those that are electron transparent, able to tolerate the vacuum chamber and small enough to fit in the chamber
  7. TEMs require special housing and maintenance
  8. Images are black and white
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