What Is Transmission Electron Microscopy?

Transmission Electron Microscope (TEM) can see fine structures smaller than 0.2um that cannot be seen under an optical microscope. These structures are called submicrostructures or ultrastructures. If you want to see these structures clearly, you must choose a light source with a shorter wavelength to improve the resolution of the microscope. In 1932, Ruska invented a transmission electron microscope with an electron beam as the light source. The wavelength of the electron beam is much shorter than visible light and ultraviolet light, and the wavelength of the electron beam is inversely proportional to the square root of the voltage of the emitted electron beam, which means that the higher the voltage The shorter the wavelength. At present, the resolution of TEM can reach 0.2nm.

Transmission Electron Microscope (TEM) can see fine structures smaller than 0.2um that cannot be seen under an optical microscope. These structures are called submicrostructures or ultrastructures. If you want to see these structures clearly, you must choose a light source with a shorter wavelength to improve the resolution of the microscope. In 1932, Ruska invented a transmission electron microscope with an electron beam as the light source. The wavelength of the electron beam is much shorter than visible light and ultraviolet light, and the wavelength of the electron beam is inversely proportional to the square root of the voltage of the emitted electron beam, which means that the higher the voltage The shorter the wavelength. At present, the resolution of TEM can reach 0.2nm.
Chinese name
Transmission electron microscope
Foreign name
Transmission Electron Microscope
Short name
TEM
Inventor
E. Ruska

Introduction to Transmission Electron Microscopy

The imaging principle of an electron microscope and an optical microscope is basically the same, except that the former uses an electron beam as a light source and an electromagnetic field as a lens. In addition, since the penetrating power of the electron beam is very weak, the specimen used for the electron microscope must be made into an ultra-thin section with a thickness of about 50 nm. This section needs to be made with an ultramicrotome. The electron microscope has a magnification of up to nearly one million times. It consists of five parts: the lighting system, the imaging system, the vacuum system, the recording system, and the power supply system. If subdivided: the main part is the electronic lens and the imaging recording system. Electron guns, condensers, objective chambers, objectives, diffractive mirrors, intermediate mirrors, projection mirrors, fluorescent screens and cameras in vacuum.
An electron microscope is a microscope that uses electrons to show the inside or surface of an object. The wavelength of high-speed electrons is shorter than the wavelength of visible light (wave-particle duality), and the resolution of a microscope is limited by the wavelength used. Therefore, the theoretical resolution of an electron microscope (about 0.1 nm) is much higher than that of an optical microscope. Rate (about 200 nm).
Transmission electron microscope (TEM), referred to as transmission electron microscope [1] , is used to project an accelerated and focused electron beam onto a very thin sample. The electrons collide with the atoms in the sample to change the direction, resulting in stereo Angular scattering. The size of the scattering angle is related to the density and thickness of the sample, so different images can be formed. The image will be displayed on imaging devices (such as fluorescent screens, films, and photosensitive coupling components) after being enlarged and focused.
Because the De Broglie wavelength of the electron is very short, the resolution of the transmission electron microscope is much higher than that of the optical microscope, which can reach 0.1 to 0.2 nm, and the magnification is tens of thousands to millions. Therefore, the use of a transmission electron microscope can be used to observe the fine structure of a sample, or even the structure of only one column of atoms, which is tens of thousands of times smaller than the smallest structure that can be observed by an optical microscope. TEM is an important analytical method in many scientific fields related to physics and biology, such as cancer research, virology, materials science, as well as nanotechnology, semiconductor research, and so on.
When the magnification is low, the contrast of TEM imaging is mainly due to the different absorption of electrons caused by the different thickness and composition of the material. When the magnification is high, complicated fluctuations will cause the brightness of the imaging to be different, so professional knowledge is required to analyze the obtained image. By using different modes of TEM, the sample can be imaged by the chemical properties of the substance, the crystal orientation, the electronic structure, the electronic phase shift caused by the sample, and the usual electron absorption.
The first TEM was developed by Max Knorr and Ernst Ruska in 1931. This research group developed the first TEM with a resolution exceeding visible light in 1933, while the first commercial TEM was developed in 1939. success.

Transmission electron microscope

Large-scale transmission electron microscopes (conventional TEM) generally use 80-300kV electron beam acceleration voltage. Different models correspond to different electron beam acceleration voltages. The resolution is related to the electron beam acceleration voltage and can reach 0.2-0.1nm. Level resolution.

Transmission electron microscope

Low-Voltage electron microscope (LVEM) uses an electron beam acceleration voltage (5kV) that is much lower than that of large TEM. The lower acceleration voltage will increase the intensity of the interaction between the electron beam and the sample, thereby improving the contrast and contrast of the image, and it is especially suitable for samples such as polymers and biology. At the same time, the low-voltage transmission electron microscope will cause less damage to the sample. [2]
Larger resolution electron microscope is low, 1-2nm. Due to the low voltage, TEM, SEM and SEM can be integrated on one device

Transmission electron microscope

Cryo-microscopy is usually equipped with a sample freezing device on an ordinary transmission electron microscope to cool the sample to liquid nitrogen temperature (77K), which is used to observe temperature-sensitive samples such as proteins and biological sections. By freezing the sample, the damage of the sample by the electron beam can be reduced, and the deformation of the sample can be reduced, thereby obtaining a more realistic sample morphology.

History of transmission electron microscopy

Ernst Abbe initially pointed out that the resolution of object details is limited by the wavelength of light waves used for imaging, so the use of optical microscopes can only magnify micron-scale structures. By using an ultraviolet light microscope developed by August Kohler and Moritz von Rolle, the ultimate resolution can be doubled. However, because commonly used glass absorbs ultraviolet light, this method requires more expensive quartz optics. At that time, it was believed that due to the limitation of the optical wavelength, sub-micron resolution images could not be obtained.
The first TEM in action
In 1858, Julius Prück realized that the cathode rays could be bent by using a magnetic field. This effect was used by Ferdinand Braun to make a measuring device called a cathode-ray oscilloscope as early as 1897. In fact, as early as 1891, Rick realized that using a magnetic field could focus the cathode rays . Later, Hans Booth published his work in 1926, demonstrating that the lens-maker's equation can be used for electron rays under appropriate conditions.
In 1928, Adolf Mattias, a professor of high-voltage technology at the Technical University of Berlin, asked Max Knorr to lead a research group to improve a cathode-ray oscilloscope. The research team consists of several doctoral students, including Ernst Ruska and Bodo von Boris. This group of researchers considered the lens design and the column arrangement of the oscilloscope in an attempt to find a better oscilloscope design in this way, while developing an electronic optics component that can be used to produce low magnifications (close to 1: 1). In 1931, the research group successfully produced an enlarged electronic image of a grid placed on the anode aperture. This device uses two magnetic lenses to achieve higher magnifications, so it is called the first electron microscope. In the same year, Reinhold Ludenburg, director of Siemens' research laboratory, patented the electrostatic lens of an electron microscope.

Improved TEM resolution

In 1927, a paper published by Giovanni Broglie revealed the wave characteristics of electrons, particles of matter that were supposed to be charged. The TEM research group did not know this paper until 1932, and then they quickly realized that the wavelength of the electron wave was several orders of magnitude smaller than the wavelength of the light wave, theoretically allowing people to observe matter at the atomic scale. In April 1932, Ruska suggested constructing a new electron microscope to look directly at the sample inserted into the microscope instead of looking at the image of the grid or aperture. With this device, people have successfully obtained diffraction images and normal images of aluminum flakes. However, their characteristics that exceed the resolution of optical microscopes have not been fully proven. It was not until 1933 that the high resolution of the TEM was officially demonstrated by imaging cotton fibers. However, because electron beams can damage cotton fibers, the imaging speed needs to be very fast.
In 1936, Siemens continued its research on electron microscopy. Their research purpose was to improve the imaging effect of TEM, especially the imaging of biological samples. At this time, the electron microscope has been manufactured by different research groups, such as the EM1 equipment manufactured by the British National Physical Laboratory. In 1939, the first commercial electron microscope was installed in the Physics Department of I. G Farben-Werke. Further research on electron microscopy was hampered because the new laboratory set up by Siemens was destroyed in an air strike in World War II and two researchers were killed.

Further study by transmission electron microscope

After World War II, Ruska continued his research at Siemens. Here he continued his research on electron microscopes, producing the first microscope capable of magnifying 100,000 times. The basic design of this microscope is still used in today's modern microscopes. The first international conference on electron microscopy was held in Delft in 1942 with more than 100 participants. Subsequent conferences included the Paris Conference in 1950 and the London Conference in 1954.
With the development of TEM, the corresponding scanning transmission electron microscope technology was re-researched. In 1970, Albert Crow of the University of Chicago invented the field emission gun and added high-quality objectives to invent modern scanning transmission Electron microscope. This design allows imaging of atoms by annular dark-field imaging. Crewe and his colleagues invented a cold-field electron emission source and built a scanning transmission electron microscope capable of observing heavy atoms on a thin carbon substrate.

TEM background

Transmission electron microscope electron

Theoretically, the maximum resolution that can be achieved by an optical microscope, d , is limited by the wavelength of the photon irradiated on the sample and the numerical aperture of the optical system, NA :
In the early twentieth century, scientists discovered that the use of electrons could theoretically break the limit of the wavelength of visible light waves (wavelengths about 400-700 nm). Like other substances, electrons have wave-particle duality, and their wave characteristics mean that a beam of electrons has properties similar to a beam of electromagnetic radiation. The wavelength of an electron can be obtained by using the kinetic energy of the electron through the Broglie formula. Since the velocity of electrons in TEM is close to the speed of light, they need to be relativistically corrected:
Where h is the Planck constant, m 0 is the static mass of the electron, and E is the energy of the electron after acceleration. Electrons in an electron microscope are usually emitted from a tungsten filament through an electron thermal emission process, or are obtained by field electron emission. The electrons are then accelerated by the potential difference and focused on the sample by the electrostatic field and the electromagnetic lens. The transmitted electron beam contains electron intensity, phase, and periodic information, which will be used for imaging.

Transmission electron microscope electron source

From top to bottom, the TEM contains an electron emission source that may be made of tungsten wire or lanthanum hexaboride. For tungsten filament, the shape of the filament can be
Basic TEM optical element layout.
It can be pin-shaped or small spike-shaped. Lanthanum hexaboride uses a very small single crystal. By connecting the electron gun to a high-voltage source of up to 100,000 volts to 300,000 volts, the electron gun will emit electrons into the vacuum through thermionic emission or field electron emission mechanism when the current is sufficiently large. This process usually uses a grid to accelerate electron generation. Once the electrons are generated, the lens above the TEM requires the electron beam to be formed at the required size to interact with the sample.
The control of the electron beam is mainly achieved through two physical effects. Moving electrons will be subjected to Lorentz force in the magnetic field according to the right-hand rule, so the magnetic field can be used to control the electron beam. Magnetic fields can be used to form magnetic lenses with different focusing capabilities. The shape of the lens is determined by the distribution of magnetic flux. In addition, the electric field can deflect electrons by a fixed angle. The electron beam can be translated by performing two consecutive reverse skew operations on the electron beam. This effect is used as a means of electron beam movement in TEM, and plays a very important role in scanning electron microscope. With these two effects and the use of an electronic imaging system, the electron beam path can be adequately controlled. Unlike an optical microscope, the optical configuration of a TEM can be very fast, because the lens located on the electron beam path can be opened, changed, and closed by a fast electronic switch. The speed of change is only affected by the hysteresis effect of the lens.

Transmission electron microscope electron optics equipment

Generally, a TEM contains a tertiary lens. These lenses include focusing lenses, objective lenses, and projection lenses. The focusing lens is used to shape the original electron beam, and the objective lens is used to focus the electron beam that passes through the sample so that it passes through the sample. Focus). A projection lens is used to project an electron beam on a fluorescent screen or other display device, such as a film. The magnification of TEM is determined by the ratio of the sample to the image plane distance of the objective lens. Another quadrupole or hexapole lens is used to compensate for the asymmetric distortion of the electron beam and is called astigmatism. It should be noted that the optical configuration of the TEM is very different from the actual implementation. Manufacturers will use custom lens configurations, such as spherical aberration compensation systems or use energy filtering to correct electronic chromatic aberrations.

Transmission electron microscope imaging equipment

TEM's imaging system includes a phosphor screen, possibly made of extremely fine particles (10-100 microns) of zinc sulfide, which provides the operator with a direct image. In addition, film-based or CCD-based image recording systems can be used. Usually these devices can be removed from the electron beam path or inserted into the path by the operator as needed.

Structural principle of transmission electron microscope

The overall working principle of a transmission electron microscope is: the electron beam emitted by the electron gun passes through the condenser lens along the optical axis of the lens body in a vacuum channel, and is condensed by the condenser lens into a thin, bright and uniform light spot, which is illuminated in the sample chamber. After passing through the sample, the electron beam passing through the sample carries the internal structural information of the sample. There is less electron passing through the dense part of the sample, and more electron passing through the sparse part. After the focusing and focusing of the objective lens and primary magnification, The electron beam enters the lower intermediate lens and the first and second projection mirrors for comprehensive magnification imaging, and the enlarged electronic image is finally projected on a fluorescent screen in the observation room; the fluorescent screen converts the electronic image into a visible light image for users to observe. This section will introduce the main structure and principle of each system.

Transmission electron microscope imaging method

The electron beam carries information about the sample as it passes through it, and the imaging equipment of the TEM uses this information to image. The projection lens projects the electron wave distribution in the correct position on the observation system. The observed image intensity, I, is approximately proportional to the time-averaged amplitude of the electron wave function, assuming that the imaging device is of high quality. If the electron wave function emitted from the sample is expressed as , then
Different imaging methods try to obtain the information related to the sample by modifying the wave function of the electron beam emitted from the sample. According to the previous equation, it can be deduced that the observed image intensity depends on the amplitude of the electron wave, and also depends on the phase of the electron wave. Although the effect of phase is negligible when the amplitude of the electron wave is low, phase information is still very important. High-resolution images require the sample to be as thin as possible and the energy of the electron beam to be as high as possible. Therefore, it can be considered that the electrons will not be absorbed by the sample, and the sample cannot change the amplitude of the electron wave. Since the sample only affects the phase of the wave in this case, such a sample is called a phase-only object. The effect of a phase-only object on the phase of the wave far exceeds the effect on the amplitude of the wave, so complex analysis is required to obtain the observed image intensity. For example, in order to increase the contrast of an image, the TEM needs to be slightly out of focus. This is because if the sample is not a phase object, convolution with the contrast transfer function of the TEM will reduce the contrast of the image.

TEM contrast information

The contrast information in TEM has a lot to do with the mode of operation. Complex imaging techniques constitute many operating modes by changing the strength of the lens or eliminating a lens, etc. These models can be used to obtain specific information of interest to researchers.

TEM bright field

The most common mode of operation for TEM is the bright-field imaging mode. In this mode, classic contrast information is obtained based on the sample's absorption of the electron beam. The thicker regions in the sample or the regions with more atoms absorb more electrons, so they appear darker on the image, and the regions with less electron absorption appear brighter. This is also the term bright field . origin. The image can be considered as a two-dimensional projection of the sample along the direction of the optical axis, and can be approximated using Beer's law. A more complex analysis of the bright-field mode needs to take into account the phase information of the electron wave as it passes through the sample.

Transmission electron microscope diffraction contrast

Because the Bragg scattering occurs when the electron beam enters the sample, the diffraction contrast information of the sample is carried by the electron beam. For example, a crystal sample will scatter the electron beam to discrete points on the back focal plane. By placing the aperture on the back focal plane, you can select the appropriate reflected electron beam to observe the desired Bragg scattering image. Usually only a very small sample of electron diffraction is projected on the imaging device. If the selected reflected electron beam does not include the unscattered electron beam located at the focal point of the lens, then in the image where the sample does not scatter the electron beam, that is, the area without the sample will be dark. Such an image is called a dark field image.
TEM image of lattice dislocation at the atomic scale in steel.
Modern TEMs are often equipped with a fixture that allows the operator to tilt the sample at a certain angle to obtain specific diffraction conditions, and the aperture is also placed above the sample to allow the user to choose an electron beam that can enter the sample at an appropriate angle.
This imaging method can be used to study crystal lattice defects. By carefully selecting the orientation of the sample, not only the location of the crystal defect, but also the type of defect can be determined. If a particular crystal plane of the sample is only slightly smaller than the strongest diffraction angle, any crystal plane defect will produce a very strong contrast change. However, atomic dislocation defects do not change the Bragg scattering angle and therefore do not produce strong contrast.

Transmission electron microscope electron energy loss

By using a spectrometer using advanced technology such as electron energy loss spectroscopy, appropriate electrons can be separated based on their voltage. These devices allow the selection of electrons with a specific energy. Since the electrons have the same charge, a specific energy means a specific voltage. In this way, these electrons of a specific energy can have a specific influence on the sample. For example, different elements in a sample can cause different electron energies to be emitted from the sample. This effect usually results in dispersion, but this effect can be used to produce an informational image of the elemental composition, based on the electron-electron interaction of the atom.
Electron energy loss spectrometers usually operate in spectral mode and image mode, which can isolate or exclude specific scattered electron beams. In many images, the inelastic scattered electron beam contains a lot of information that the operator does not care about, which reduces the observability of useful information. In this way, the electron energy loss spectroscopy technology can effectively improve the contrast between the bright field observation image and the dark field observation image by excluding unnecessary electron beams.

Transmission electron microscope phase contrast technology

The crystal structure can be studied by high-resolution transmission electron microscopy. This technique is also called phase contrast microscopy. When a field emission electron source is used, the observation image is reconstructed from the difference in the phase of the electron wave caused by the interaction between the electron and the sample. However, because the image also depends on the number of electrons shot on the screen, the recognition of phase contrast images is more complicated. However, the advantage of this imaging method is that it can provide more information about the sample.

TEM diffraction mode

As mentioned before, by adjusting the magnetic lens so that the imaging aperture is at the back focal plane of the lens rather than the image plane, a diffraction pattern will be generated. For single crystal samples, the diffraction pattern appears as a set of regularly arranged points, and for polycrystalline or amorphous solids a set of rings will be generated. For single crystals, the diffraction pattern is related to the direction of the electron beam on the sample and the atomic structure of the sample. Usually, only the position of the point on the diffraction pattern and the symmetry of the observation image can be used to analyze the space group information of the crystal sample and the relative relationship between the crystal direction of the sample and the direction of the electron beam path.
Twin-crystal diffraction pattern of face-centered cubic austenitic stainless steel
The dynamic range of a diffraction pattern is usually very large. For crystal samples, this dynamic range usually exceeds the maximum range that a CCD can record. Therefore TEM is usually equipped with a film cassette to record these images.
The point-to-point analysis of diffraction patterns is very complicated. This is because the image and the thickness and direction of the sample, the defocus of the objective lens, spherical aberration and chromatic aberration have a close relationship. Although it is possible to quantitatively explain the contrast of grid images, the analysis is very complex in nature and requires a lot of computer simulations to calculate.
Diffraction planes have more complicated manifestations, such as Kikuchi lines caused by multiple diffraction of crystal lattice points. In the convergent electron beam diffraction technology, the convergent electron beam forms an extremely thin probe on the surface of the sample, resulting in a non-parallel convergent wavefront. The function of the convergent electron beam and the sample can provide information outside the sample structure, such as the sample Thickness and so on.

TEM TEM imaging principle of transmission electron microscope

The imaging principle of transmission electron microscope [3] can be divided into three cases:
  • Absorption image: When electrons strike a sample with a high density, the main phase-forming effect is scattering. The place where the mass thickness is large on the sample has a large scattering angle for electrons, less electrons pass through, and the brightness of the image is darker. Early transmission electron microscopes were based on this principle.
  • Diffraction image: After the electron beam is diffracted by the sample, the amplitude distribution of the diffracted wave at different positions of the sample corresponds to the different diffractive power of each part of the crystal in the sample. When a crystal defect occurs, the diffractive power of the defective part is different from the complete area, so that the diffracted wave The uneven amplitude distribution reflects the distribution of crystal defects.
  • Phase image: When the sample is thinner than 100Å, the electrons can pass through the sample, the amplitude change of the wave can be ignored, and the imaging comes from the phase change.

TEM Transmission electron microscope TEM system components

The TEM system consists of the following parts [4]
  • Electron gun: emits electrons and consists of cathode, grid and anode. The electrons emitted by the cathode tube form a ray beam through a small hole in the grid, and after being accelerated by the anode voltage, they are directed to a condenser, which accelerates and pressurizes the electron beam.
  • Condenser: Focuses the electron beam, and can use the controlled illumination intensity and aperture angle.
  • Sample room: Place the sample to be observed, and install a tilting table to change the angle of the sample, as well as equipment for heating and cooling.
  • Objective lens: a short-distance lens with a high magnification, which is used to magnify electronic images. Objective lens is the key to determine the resolution and imaging quality of transmission electron microscope.
  • Intermediate lens: a weak lens with variable magnification, which is used to magnify the electronic image twice. By adjusting the current of the intermediate mirror, you can choose the image or electron diffraction pattern of the object to zoom in.
  • Transmission lens: a strong lens with a high magnification, which is used to magnify the intermediate image and image it on a fluorescent screen.
  • In addition, there is a secondary vacuum pump to evacuate the sample chamber, and a camera device to record images.

Transmission electron microscope illumination system

The lighting system includes two main components: an electron gun and a condenser lens. Its function is mainly to provide a sample and imaging system with a light source with sufficient brightness and electron beam current. The requirements for it are a single and stable output electron beam wavelength, uniform brightness and easy adjustment , Astigmatism is small.

electronic gun Transmission electron microscope electron gun

It consists of a cathode, an anode, and a grid. Figure 4-14 shows a schematic cross-sectional structure and a physical decomposition diagram.
(1) Cathode The cathode is the source of generating free electrons. Generally, there are two types of direct heating and indirect heating. The indirect heating cathode separates the heating body and the cathode and keeps them independently. In the electron microscope, the heating filament (filament) and the cathode is usually called the direct heating cathode. The material is mostly made of metal tungsten wire. Its characteristics are low cost, but low brightness and short life. The diameter of the filament is about 0.10 0.12mm. When a few amps of heating current flows, free electrons can be emitted, but a high vacuum must be maintained around the filament, otherwise the heated filament will be tilted like a leaky light bulb. Time was burned by oxidation. The shape of the filament is most often a hair fork, or an arrow axe or a dot (Figure 4-15). The latter two filaments have high luminous brightness and a sharp beam focus, which is suitable for high-resolution electron microscope photos. Shoot, but shorter life.
The cathode filament is installed on a highly insulated ceramic lamp holder (Figure 4-16), which can both be insulated and withstand high temperatures of several thousand degrees.
It can also be easily replaced. The heating current value of the filament is continuously adjustable.
Within a certain limit, the amount of free electrons emitted by the filament is proportional to the intensity of the heating current. However, after exceeding this limit, the current continues to increase, which can only reduce the life of the filament, but cannot increase the amount of free electrons emitted. We call this critical point the filament saturation point, which means that the amount of free electrons has reached "full" and cannot be added. In normal use, the heating current adjustment of the filament is often set to a position close to saturation, which is called "undersaturation point". In this way, under the condition that a large amount of free electron emission can be obtained, the service life of the filament can be maximized. The normal service life of tungsten filament is about 40h. In modern electron microscope, a new material, lanthanum hexaboride (LaB6), is sometimes used to make the filament. The life is much longer than that of tungsten filament, which can reach 1000h, which is a very good new material.
(2) The anode is a metal cylinder with a hole in the center, which is located below the cathode. When the anode is applied with a positive voltage of tens of kilovolts or hundreds of kilovolts, it will be free to emit heat from the cathode when the voltage is accelerated The electrons produce a strong gravitational effect, and change it from a disordered state to an ordered directional movement. At the same time, the free electrons are accelerated to a certain speed (related to the acceleration voltage, which has been discussed earlier), forming a beam jet. To the anode target surface. All electron beams moving in the axis will be emitted from the electron gun through the circular hole in the center of the anode and become the light source for illuminating the sample.
(3) The grid is located between the cathode and the anode, close to the top of the filament, and is a hat-shaped metal object. There is also a small hole in the center to pass the electron beam. A negative voltage of 0 to 1000V (for the cathode) is applied to the grid. This negative voltage is called the grid bias VG, and its height is different. It can be adjusted by the user as required. The grid bias can make the electron beam generate The central axis converges, and at the same time it also has a certain regulation and suppression effect on the amount of free electrons on the filament.
(4) Working principle Figure 4-17 shows that under the action of the filament power supply VF, when the current IF flows through the filament cathode and heats it above 2500 ° C, free electrons can be generated and escape the filament surface. The accelerating voltage VA accumulates dense positive charges on the anode surface, forming a strong positive electric field. Free electrons fly out of the electron gun under the action of this positive electric field. Adjusting the VF can make the filament work at the point of undersaturation. During the use of the electron microscope, the size of the grid bias VG can be adjusted according to the need for brightness to control the amount of electron beam flow.
The acceleration voltage VA in the electron microscope is also adjustable. When the VA is increased, the wavelength of the electron beam is shortened, which is beneficial to the improvement of the resolution of the electron microscope. At the same time, the penetrating ability is enhanced, and the thermal damage to the sample is small. However, at this time, due to the collision of the electron beam with the sample, the scattering angle of the elastically scattered electrons will increase accordingly, and the imaging contrast will decrease accordingly. In high-resolution observation applications, choosing a lower acceleration voltage can achieve a larger imaging contrast, especially for biological samples with a small contrast contrast, and it is sometimes advantageous to choose a lower acceleration voltage.
There is also a new type of electron gun field emission type electron gun (see Figure 4-18), which is composed of a cathode and two anodes. A slightly lower (relative to the second anode) adsorption voltage is applied to the first anode to use the cathode The free electrons above are attracted, and the extremely high voltage on the second anode accelerates the free electrons to a very high speed and emits an electron beam. This requires ultra-high voltage and ultra-high vacuum as the working conditions. It requires a vacuum of 10-7Pa during operation, minimal heat loss, and a service life of 2000 h. The light spot of the electron beam spot is more sharp and the diameter can reach Below 10nm, it is 3 orders of magnitude smaller than a tungsten wire cathode (greater than 10nm). Due to its high luminous efficiency, the brightness of its emitted light spot can reach 10 A / cm · s, which is also higher than a tungsten wire cathode (106 A / cm · s) 3 orders of magnitude. Field emission electron guns are only used in high-end high-resolution electron microscopes because of their advanced technology and high cost.

condonser lens Transmission electron microscope condenser lens

The condenser lens is located below the electron gun, and generally consists of 2 to 3 levels, which are called the first and second condenser lenses in order from top to bottom (represented by C1 and C2). The structure and working principle of the electromagnetic lens have been introduced in the previous section. The purpose of the condenser lens in the electron microscope is to focus the electron beam emitted by the electron gun into a spot with uniform brightness and adjustable illumination range, and project it on the sample below. The structures of C1 and C2 are similar, but the shape and working current of the pole shoes are different, so the magnetic field strength and application are different. C1 is a strong magnetic field lens and C2 is a weak magnetic field lens. Condenser lenses of various levels are used together to adjust the diameter of the illumination beam spot, thereby changing the intensity of the illumination brightness. Generally, corresponding adjustments are provided on the control panel of the electron microscope. Twist. C1 and C2 work by changing the current in the condenser lens coil to achieve changes in the magnetic field strength formed by the lens. The change in magnetic field strength (that is, the refractive index changes) can move the convergence point of the electron beam up and down. The smaller the beam spot converges on the sample surface, the more concentrated the energy and the greater the brightness; otherwise the beam spot diverges and the illuminated area becomes larger, the brightness decreases. The method of changing the illumination brightness by adjusting the condenser current is actually an indirect adjustment method. The maximum brightness is limited by the electron beam flow. If you want to change the illumination brightness to a greater extent, you can only fundamentally change the size of the electron beam current by adjusting the grid bias in the aforementioned electron gun. C2 is usually equipped with a movable diaphragm to change the aperture angle of the beam illumination. On the one hand, the illumination area projected on the surface of the sample can be limited, and the unobserved part of the sample is protected from the electron beam bombardment. It can reduce the influence of unfavorable signals such as scattered electrons.

Transmission electron microscope imaging system

specimen room Transmission electron microscope sample room (specimen room)

The sample chamber is located under the condenser and contains a sample stage on which the sample is placed. The sample stage must be able to move in the X and Y directions on the horizontal plane to select and move the viewing field. It is equipped with 2 joysticks or rotating handwheels. This is a precise adjustment mechanism. Each joystick rotates 10 times. In this case, the sample stage can move about 3mm in a certain direction. Modern high-end electronic microscopes can be equipped with a computer-driven motor-driven sample stage, which strives to sample accurately when moving and stable when fixed; and can make a label-type positioning mark on the sample by the computer so that users can make retrospective comparisons Relying on computer positioning and finding, this is difficult to achieve in manual selection operations. When biomedical samples are observed by transmission electron microscopy, the original samples are basically embedded in epoxy resin, and then cut into thin slices using a very precise ultra-thin microtome. The knife is a special glass knife or diamond knife. The thickness of the cut biomedical sample is usually only a few tens of nanometers (nm), which cannot be directly seen by the naked eye under normal circumstances. The sections must be floated on the water surface. Specially illuminated by skilled technicians Light and a special angle to see such a thin slice. The cut flakes are put on a copper mesh and can be used for observation only after dyeing and drying. The preparation of TEM samples is a long, complicated and precise process, which is very technical. However, as we mentioned earlier, in order to obtain good electron microscope images, making good sample specimens is a very important first step.
The copper mesh that holds the samples can be varied according to needs, and the diameter is generally 3mm. Generally, we call it as many meshes as there are grids on the copper mesh. The reason why copper is used to make the sample net is that it does not interact with the electron beam and electromagnetic field. Similarly, other metal materials with low magnetic permeability (such as nickel) can be used to make the sample net. The sample net is a consumable, and the copper net Easy processing and low cost, so it is very popular.
There are two common sample stages for TEM: Top-entry sample stage requires a large space in the sample chamber, and multiple (usually six) sample nets can be placed at one time. The sample net contains cups arranged in a ring. When in use, you can rely on a robotic device for sequential replacement. The advantage is that it is convenient and time-saving to break the vacuum of the sample chamber when changing samples after observing multiple samples. However, the space required is too large, which causes the sample to be far away from the objective lens below, which is not suitable for shortening the objective lens focal length. Will affect the improvement of the resolution of the electron microscope. Side-inserted sample stage. The sample stage is made into a rod shape. The sample net is placed on the front end and can only hold 1 or 2 copper nets. The sample stage has a small volume and a small space, and can be set at the upper half of the objective lens, which is conducive to the improvement of the resolution of the electron microscope. The disadvantage is that multiple sample nets cannot be placed at the same time, and the vacuum in the sample chamber must be destroyed once every time the sample is replaced, which is slightly inconvenient.
In high-performance transmission electron microscopes, the above-mentioned side-insertion sample stage is mostly used in order to maximize the resolution of the electron microscope.O1010

object lens

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1
2

intemediate lensprojection lens
12
M=MO·MI·MP1·MP2
1

13X
-1520cm
10cm45°

2050/82.5mm×118mm82.5mm×101.6mm90mm×120mm

CRT

CRT

10-310Pa10Pa

160L0.10.01Pa

570L1010Pa
10Pa
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8412221
22

2O1234XY
3C220200m10100m 50400m

Turn on circulating water. Since the circulating water of the new electron microscope is not closed, this step can be saved. But pay attention to whether the water temperature is normal. Turn on the power switch. IN / OUT. It's always on. This step can be saved. Turn on the power of the screen; check the first page of the screen: make sure the voltage is 120KV. Make sure the sample position "specimen position" is the origin: <x, y, z> = 0, 0, 0. If it is not the origin, use the N key on the "SPEC CONTROLLER" control panel on the left side of the observation window to restore (note that there is no When using the sample lever, it is strictly forbidden to use the "N" key! Therefore, it should be reset before each time the sample lever is pushed out; -selector is 2 and key in P3 with the keyboard to display the third page of the screen. In the case: p1: 25, p2: 25, p3: 29, p4,28, p5,100, the observation valves V1, V2 and V4, V5B, V8, V13, V17 and V 21 are in total open. Look at the vacuum panel at the bottom right, and make sure the vacuum enters the 10-5Pa range. Ideally, the pointer is centered, and it should be lower when liquid nitrogen is injected. If it does not reach this range, please contact the duty teacher; the filament is turned off. (The switch of the filament is not ON.) Check whether the condenser light bar is fully open and the objective light bar is fully open. If not, please open all light bars. Check the right panel to see if the electron microscope is in MAG1 (this light is on). Open the left door, and turn the "lens" switch to the ON position (please be very careful, make sure it is the lens switch, do not open the wrong switch. Turn on the filament , Pull the switch slightly outward and lift it up to open.). Observe the screen again, if the voltage is 120KV, if not, the following operations are strictly prohibited. Press the switch HT on the left side of the panel to raise the pressure. Pay attention to the Beam Current value displayed on the left panel. Generally, the current value should be 60-62 at 120KV. If there is a large deviation, please contact the teacher on duty; (press HT. To make sure BEAM CURRENT is stable, just check whether the value in the table is stable. The current is half the voltage plus one or half plus two.) Wait for the Beam Current value to stabilize at 60-62 for at least 5 minutes, and use the keyboard to start the boost program . If the voltage value is not stable, please contact the teacher on duty; use the keyboard to type P1 so that the silver screen displays the first page, and then type RUN. The silver screen will ask "start HT", type 120, and then press "enter". ", Type 160, press the" enter "key, the silver screen will display" ?? ", type 10, the electron microscope will automatically start boosting, then press the right HT wobbler. After this step is completed, press HT wobbler to stop the webbler operation. Pay attention to whether the Beam Current value on the left is stable at 80-82, such as unstable for 10-20 minutes. If it is stable, repeat the previous operation, but change the start HT to 160 and end HT to 180. When the voltage is increased to 180KV, the Beam Current should be around 92 at this time. If it is stable, continue to boost, change the start HT to 180 and end HT to 200. At the end of the boost, the Beam Current should be 102. (This step is to increase the pressure. 120-200Kv, use the left button to set. Apply voltage to the screen, open the keyboard and enter run; enter HT start, enter 120; enter HT stop, enter 160. Increase the high voltage three times to 200kv. Total time, lose 10min; HT step, lose 10.) Observe the third page of the silver screen again to check whether V1-V3 has been opened normally and whether P1-P5 values are in the normal range. If the vacuum is at level 10-5, everything is normal, press the ON button on the left panel filament, turn on the filament, wait for the filament current to stabilize (about 2-4 minutes), and the final filament current should be about 105. Observe whether there are normal light spots.

Transmission electron microscope sample preparation

I. Sample requirements
1. Basic requirements for powder samples
(1) The size of a single powder is preferably less than 1 m;
(2) Non-magnetic;
(3) It is mainly composed of inorganic components, otherwise it will cause serious pollution of the electron microscope, high voltage jump off, and even damage the high voltage gun;
2. Basic requirements for block samples
(1) Electrolytic thinning or ion thinning is required to obtain a thin area of tens of nanometers to observe;
(2) If the grain size is less than 1 m, it can also be made into powder by mechanical methods such as crushing for observation;
(3) Non-magnetic;
(4) The preparation of bulk samples is complicated, time-consuming, and has many procedures, which require the guidance or preparation of experienced teachers; the preparation of the samples directly affects the observation and analysis of the electron microscope. Therefore, before the preparation of block samples, it is best to communicate with the teacher of the TEM, or ask for advice.
Preparations before sending samples
1. The purpose should be clear: (1) what to do (such as determining the growth direction of nanorods, specific observation and analysis of defects on a crystal plane, phase structure analysis, orientation relationship between the main phase and the second phase, interface lattice matching, etc.) ; (2) what problems we hope to solve;
2. After the sample passes the X-Ray powder diffraction (XRD) test and the structure is determined, it is decided whether to perform HRTEM; this can save time and obtain more microstructure information on the basis of XRD.
3 Before doing HRTEM, please bring the XRD data and other experimental results, and communicate with the HRTEM teacher to determine whether you can achieve the purpose; at the same time, the HRTEM teacher will also provide you with good suggestions based on your other experimental data. Can meet your requirements, even make the test content deeper and improve the grade of the paper.
Preparation of powder samples
1. Selecting a high-quality microgrid grid (3mm diameter) is the first step related to the ability to take high-quality high-resolution electron microscopy pictures. , The price is 20 yuan / only; ordinary carbon film copper mesh is provided free of charge.)
2. Carefully remove the microgrid with tweezers, with the membrane side facing up (the glossy side, that is, the membrane side when viewed under light), and lay it flat on the white filter paper;
3 Take an appropriate amount of powder and ethanol into a small beaker and sonicate for 10 to 30 minutes. After 3 to 5 minutes, use a glass capillary to suck the uniform mixture of powder and ethanol, and then drop 2 to 3 drops of the mixed liquid onto the microgrid. (If the powder is black, when the surface of the white filter paper around the microgrid becomes slightly black, it will be moderate at this time. Too much dripping will cause the powder to not spread, which is not conducive to observation, and the probability of the powder falling into the electron microscope is greatly increased. , Which seriously affects the service life of the electron microscope; too little drip, it is not good for the observation of the electron microscope, it is difficult to find the powder particles required for the experiment. It is recommended to prepare by the teacher or under the guidance of the teacher.)
4 Wait for more than 15 minutes for the ethanol to evaporate as much as possible; otherwise, insert the sample on the sample stage and insert the electron microscope, which will affect the vacuum of the electron microscope.
Four, block sample preparation
1. Electrolytic thinning method
For the preparation of metal and alloy samples. (1) Cut the block sample into approximately 0.3mm thick uniform slices; (2) Mechanically grind with emery paper to a thickness of about 120 ~ 150m; (3) Polish and grind to a thickness of about 100m; (4) Punch into a 3mm disc (5) Select the proper electrolyte and the working conditions of the dual-spray electrolyzer, and thin the center of the 3mm wafer to make a small hole; (6) Quickly remove the thinned sample and rinse it in absolute ethanol.
Precautions:
(1) The electrolyte used for electrolytic thinning is highly corrosive, and it is necessary to pay attention to personnel safety and equipment cleaning;
(2) The sample thinned by electrolysis needs to be handled lightly, lightly, lightly, and lightly, otherwise it will be easily broken, leading to the loss of previous work;
2. Ion thinning method
For the preparation of ceramic, semiconductor, and multilayer film cross sections. Block sample preparation (1) Cut the block sample into about 0.3mm thick uniform slices; (2) Paste the uniform slices on the glass slide on the sample holder of the ultrasonic cutting machine with paraffin; (3) punch into 3mm with an ultrasonic cutting machine (4) mechanically grind to a thickness of about 100 m with emery paper; (5) use a pit grinder to grind a pit in the center of the wafer, the pit depth is about 50 ~ 70 m, the purpose of the pit is mainly to reduce Subsequent ion thinning process time to improve the final thinning efficiency; (6) Carefully put a clean, pitted 3mm wafer into the ion thinning instrument, and choose the appropriate ion thinning according to the characteristics of the sample material Thinning parameters are used for thinning. Generally, the ion thinning time of general ceramic samples takes 2 ~ 3 days; the whole process takes about 5 days.
Precautions:
(1) The sample in the pit process needs to be accurately centered, rough grinding and fine grinding and polishing first, and the load of the grinding wheel should be moderate, otherwise the sample will be easily broken;
(2) After the pit is completed, the grinding wheel and shaft of the pit meter should be cleaned;
(3) The pitted samples need to be soaked in acetone, washed and dried;
(4) During the process of loading and removing the sample with the ion thinned sample, very careful and meticulous actions are required, because the center of the 3mm thin sample is very thin and the force is uneven. Or too large, it can easily cause the sample to break.
(5) Needs good patience, but haste. [5]

Transmission electron microscope applications

Transmission electron microscope is widely used in materials science [6] and biology. Because the electrons are easily scattered or absorbed by the object, the penetrating power is low. The density and thickness of the sample will affect the final imaging quality. Thinner ultra-thin sections must be prepared, usually 50-100nm. Therefore, the samples used for observation with a transmission electron microscope need to be processed very thinly. Common methods are: ultra-thin sectioning method, frozen ultra-thin sectioning method, freezing etching method, freezing fracture method and the like. For liquid samples, observation is usually performed on a pre-treated copper wire.

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