Electron microscopy. Who and when invented the microscope
To study nanoobjects with the resolution of optical microscopes ( even using ultraviolet) is clearly insufficient. As a result, in the 1930s the idea arose to use electrons instead of light, the wavelength of which, as we know from quantum physics, is hundreds of times smaller than that of photons.
As you know, our vision is based on the formation of an image of an object on the retina of the eye by light waves reflected from this object. If, before entering the eye, light passes through the optical system microscope, we see an enlarged image. At the same time, the course of light rays is skillfully controlled by the lenses that make up the objective and the eyepiece of the device.
But how can you get an image of an object, and with a much higher resolution, using not light radiation, but a stream of electrons? In other words, how is it possible to see objects based on the use of particles, not waves?
The answer is very simple. It is known that the trajectory and speed of electrons are significantly affected by external electromagnetic fields, which can be used to effectively control the movement of electrons.
The science of the movement of electrons in electromagnetic fields and the calculation of devices that form the desired fields is called electronic optics.
An electronic image is formed by electrical and magnetic fields about the same as light - optical lenses. Therefore, in an electron microscope, devices for focusing and scattering an electron beam are called “ electronic lenses”.
electronic lens. The coil wires carrying the current focus the electron beam in the same way that a glass lens focuses a light beam.
The magnetic field of the coil acts as a converging or diverging lens. To concentrate the magnetic field, the coil is covered with a magnetic " armor» made of a special nickel-cobalt alloy, leaving only a narrow gap in the inner part. The magnetic field created in this way can be 10-100 thousand times stronger than the Earth's magnetic field!
Unfortunately, our eye cannot directly perceive electron beams. Therefore, they are used for drawing” images on fluorescent screens (which glow when electrons hit). By the way, the same principle underlies the operation of monitors and oscilloscopes.
Exists a large number of various types of electron microscopes among which the scanning electron microscope (SEM) is the most popular. We get its simplified scheme if we place the object under study inside cathode ray tube ordinary TV between the screen and the source of electrons.
In such microscope a thin beam of electrons (beam diameter about 10 nm) runs around (as if scanning) the sample in horizontal lines, point by point, and synchronously transmits a signal to the kinescope. The whole process is similar to the operation of a TV in the scanning process. The source of electrons is a metal (usually tungsten), from which, when heated, electrons are emitted as a result of thermionic emission.
Scheme of operation of a scanning electron microscope
Thermionic emission is the exit of electrons from the surface of the conductors. The number of released electrons is small at T=300K and grows exponentially with increasing temperature.
When electrons pass through a sample, some of them are scattered due to collisions with the nuclei of atoms in the sample, others due to collisions with electrons of atoms, and still others pass through it. In some cases, secondary electrons are emitted, x-rays are induced, and so on. All these processes are recorded by special detectors and in a transformed form are displayed on the screen, creating an enlarged picture of the object under study.
The magnification in this case is understood as the ratio of the size of the image on the screen to the size of the area that the beam runs around on the sample. Due to the fact that the wavelength of an electron is orders of magnitude smaller than that of a photon, in modern SEMs this increase can reach 10 million15, corresponding to a resolution of a few nanometers, which makes it possible to visualize individual atoms.
Main disadvantage electron microscopy- the need to work in a complete vacuum, because the presence of any gas inside the microscope chamber can lead to ionization of its atoms and significantly distort the results. In addition, electrons have a destructive effect on biological objects, which makes them unsuitable for research in many areas of biotechnology.
History of creation electron microscope is a remarkable example of an achievement based on an interdisciplinary approach, when independently developing fields of science and technology came together to create a new powerful tool for scientific research.
pinnacle classical physics was the theory of the electromagnetic field, which explained the propagation of light, electricity and magnetism as the propagation electromagnetic waves. Wave optics explained the phenomenon of diffraction, the mechanism of image formation, and the interplay of factors that determine resolution in a light microscope. good luck quantum physics we owe the discovery of the electron with its specific corpuscular-wave properties. These separate and seemingly independent paths of development led to the creation of electron optics, one of the major inventions which in the 1930s became the electron microscope.
But scientists did not rest on this either. The wavelength of an electron accelerated by an electric field is several nanometers. This is not bad if we want to see a molecule or even an atomic lattice. But how to look inside the atom? What does it look like chemical bond? What does the process look like chemical reaction? For this, today different countries scientists develop neutron microscopes.
Neutrons are usually part of atomic nuclei along with protons and have almost 2000 times more mass than an electron. Those who have not forgotten de Broglie's formula from the quantum chapter will immediately realize that the wavelength of a neutron is as many times smaller, that is, it is picometers thousandths of a nanometer! Then the atom will appear to researchers not as a blurry spot, but in all its glory.
Neutron microscope has many advantages - in particular, neutrons reflect hydrogen atoms well and easily penetrate into thick layers of samples. However, it is very difficult to build it: neutrons do not have an electric charge, therefore they calmly ignore magnetic and electric fields and so they strive to elude the sensors. In addition, it is not so easy to expel large clumsy neutrons from atoms. Therefore, today the first prototypes of the neutron microscope are still very far from perfection.
electrOmicroskOP(English - electron microscope) – This is a device for observing and photographing a multiply (up to 1 x 10 6 times) enlarged image of objects, in which, instead of light rays, electron beams accelerated to high energies (30 - 100 keV and more) in deep vacuum are used.
The transmission electron microscope (TEM) has the highest resolution, surpassing light microscopes in this parameter by several thousand times. The so-called resolution limit, which characterizes the ability of the device to display separately small as close as possible details of the object, for TEM is 2 - 3 A°. Under favorable conditions, individual heavy atoms can be photographed. When photographing periodic structures, such as the atomic planes of crystal lattices, it is possible to realize a resolution of less than 1 A°.
To determine the structure of solids, it is necessary to use radiation with a wavelength λ shorter than the interatomic distances. In an electron microscope, electron waves are used for this purpose.
de Broglie wavelength λ B for an electron moving at a speed V
Where p- his momentum h is Planck's constant, m 0 - electron rest mass, V- its speed.
After simple transformations, we obtain that the de Broglie wavelength for an electron moving in an accelerating uniform electric field with a potential difference U, is equal to
. (1)
In expressions for λ B, the relativistic correction is not taken into account, which is significant only at high electron velocities V>1 10 5 V.
The value of λ B is very small, which makes it possible to provide a high resolution of the electron microscope.
For electrons with energies from 1 eV up to 10,000 eV, the de Broglie wavelength lies in the range from ~1 nm to 10 −2 nm, that is, in the wavelength range x-ray radiation. Therefore, the wave properties of electrons should manifest themselves, for example, when they are scattered by the same crystals on which diffraction x-rays. [
Modern microscopes have a resolution of (0.1 - 1) nm at an electron energy of (1 10 4 - 1 10 5) eV, which makes it possible to observe groups of atoms and even individual atoms, point defects, surface relief, etc.
Transmission electron microscopy
The electron-optical system of a transmission electron microscope (TEM) includes: electron gun I and condenser 1, designed to provide the illumination system of the microscope; objective 2, intermediate 3 and projection 4 lenses that display; observation and photographing camera E (Fig. 1).
Fig.1. Ray path in TEM in image observation mode
The source of electrons in the electron gun is a tungsten thermionic cathode. A condenser lens makes it possible to obtain a spot with a diameter of several microns on an object. With the help of a display system, an electron microscopic image of the object is formed on the TEM screen.In the plane associated with the object, the objective lens forms the first intermediate image of the object. All electrons emanating from one point of the object fall into one point of the conjugate plane. Then, using intermediate and projection lenses, an image is obtained on a fluorescent microscope screen or photographic plate. This image conveys the structural and morphological features of the specimen.
TEM uses magnetic lenses. The lens consists of a winding, a yoke and a pole piece, which concentrates the magnetic field in a small volume and thereby increases the optical power of the lens.
TEMs have the highest resolution (PC), surpassing light microscopes in this parameter by several thousand times. The so-called resolution limit, which characterizes the ability of the device to display separately small, as close as possible, details of an object, for a TEM is 2–3 A°. Under favorable conditions, individual heavy atoms can be photographed. When photographing periodic structures, such as the atomic planes of crystal lattices, it is possible to realize a resolution of less than 1 A°. Such high resolutions are achieved due to the extremely short de Broglie wavelength of electrons. Optimum iris can reduce the spherical aberration of the lens, which affects the PC TEM, with a sufficiently small diffraction error. No effective methods for correcting aberrations have been found. Therefore, in TEM, magnetic electron lenses (ELs), which have smaller aberrations, have completely replaced electrostatic ELs. PEMs for various purposes are produced. They can be divided into 3 groups:
simplified TEM,
high resolution TEM,
TEM with increased accelerating voltage.
1. Simplified TEM designed for research that does not require a high PC. They are simpler in design (including 1 condenser and 2–3 lenses for enlarging the image of an object), they are distinguished by a lower (usually 60–80 kV) accelerating voltage and its lower stability. PCs of these instruments range from 6 to 15. Other applications are object preview, routine research, teaching purposes. The thickness of an object that can be "enlightened" by an electron beam depends on the accelerating voltage. In a TEM with an accelerating voltage of 100 kV, objects with a thickness from 10 to several thousand A° are studied.
2. High resolution TEM(2 - 3 Å) - as a rule, universal multi-purpose devices (Fig. 2, a). With the help of additional devices and attachments, it is possible to tilt an object in different planes at large angles to the optical axis, heat, cool, deform it, carry out X-ray structural analysis, studies using electron diffraction methods, etc. The voltage accelerating electrons reaches 100-125 kV, it is regulated step and is highly stable: in 1 - 3 minutes it changes by no more than 1 - 2 millionths of the original value. A deep vacuum is created in its optical system (column) (pressure up to 1 10 -6 mm Hg). Scheme optical system TEM - in Fig. 2, b. The electron beam, the source of which is the hot cathode, is formed in the electron gun and then twice focused by the first and second condensers, which create an electronic "spot" on the object, the spot diameter of which can be varied from 1 to 20 μm. After passing through the object, some of the electrons are scattered and retained by the aperture diaphragm. Unscattered electrons pass through the diaphragm opening and are focused by the objective in the object plane of the intermediate lens. Here the first enlarged image is formed. Subsequent lenses create a second, third, etc. image. The last lens forms an image on a fluorescent screen that glows when exposed to electrons.
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Rice. 2 a. TEM: 1 – electron gun; 2 - condenser lenses; 3 - lens; 4 - projection lenses; 5 - light microscope, additionally magnifying the image observed on the screen: 6 - tube with viewing windows through which the image can be observed; 7 - high-voltage cable; 8 - vacuum-smart system; 9 - control panel; 10 - stand; 11 - high-voltage power supply; 12 - lens power supply. |
Rice. 2 b. Optical scheme of TEM. 1 - V-shaped cathode made of tungsten wire (heated by current passing through it up to 2800 K); 2 - focusing cylinder; 3 - anode; 4 - the first (short-focus) condenser, which creates a reduced image of the electron source; 5 - the second (long-focus) condenser, which transfers a reduced image of the electron source to the object; 6 - object; 7 - aperture diaphragm; 8 - lens; 9, 10, 11 - system of projection lenses; 12 - cathodoluminescent screen on which the final image is formed. |
The magnification of the TEM is equal to the product of the magnifications of all lenses. The degree and nature of electron scattering are not the same at different points of the object, since the thickness, density and chemical composition objects change from point to point. Accordingly, the number of electrons delayed by the aperture diaphragm after passing through various points of the object changes, and, consequently, the current density in the image, which is converted into light contrast on the screen. Under the screen is a store with photographic plates. When photographing, the screen is removed and the electrons act on the photoemulsion layer. The image is focused by changing the current that excites the magnetic field of the lens. The currents of the other lenses are adjusted to change the magnification of the TEM.
3. FEM with increased accelerating voltage(up to 200 kV) are designed to study thicker objects (2-3 times thicker) than conventional TEMs. Their resolution reaches 3 – 5 Å. These devices differ in design electron gun: in order to ensure electrical strength and stability, it has two anodes, one of which is supplied with an intermediate potential, which is half the accelerating voltage. The magnetomotive force of the lenses is greater than in a TEM with an accelerating voltage of 100 kV, and the lenses themselves have increased dimensions and weight.
4. Ultrahigh voltage electron microscopes(SVEM) - large-sized devices (Fig. 3) with a height of 5 to 15 m, with an accelerating voltage of 0.50 - 0.65; 1 - 1.5 and 3.5 MV.
They build special rooms for them. SVEM are intended for research of objects with thickness from 1 · to · 10 microns. The electrons are accelerated in an electrostatic accelerator (so-called direct-acting accelerator) located in a tank filled with electrically insulating gas under pressure. A high-voltage stabilized power supply is located in the same or in an additional tank. In the future - the creation of a TEM with a linear accelerator, in which electrons are accelerated to energies of 5 - 10 MeV. When studying thin objects, PC SVEM is lower than that of TEM. In the case of thick objects, the PC SVM is 10–20 times superior to the PC TEM with an accelerating voltage of 100 kV. If the sample is amorphous, then the contrast of the electronic image is determined by the thickness and absorption coefficient of the sample material, which is observed, for example, when studying the surface morphology using plastic or carbon replicas. In crystals, in addition, electron diffraction takes place, which makes it possible to determine the crystal structure.
IN
Fig.4. Aperture position D with bright field ( A) and dark-field ( b) images: P - transmitted beam; D- diffracted beam; arr - sample; I - electron gun
the image is formed by the transmitted beam P, the diffracted beam D is cut off by the aperture diaphragm D (Fig. 4, A), this is a bright-field image;
aperture diaphragm D transmits diffracted D beam, cutting off the past P, this is a dark-field image (Fig. 4, b);
to obtain a diffraction pattern, the rear focal plane of the objective lens is focused on the microscope screen (Fig. 4). Then a diffraction pattern from the translucent portion of the sample is observed on the screen.
To observe the image in the rear focal plane of the lens, an aperture stop is installed, as a result, the aperture of the rays forming the image is reduced and the resolution is increased. The same diaphragm is used to select the observation mode (see Fig. 2 and 5).
Fig.5. Ray path in TEM in microdiffraction mode D - aperture; And - the source of electrons; arr - sample; E - screen; 1 - condenser, 2 - objective, 3 - intermediate, 4 - projection lens
wave length at voltages used in TEM, is about 1∙10 –3 nm, that is, much less than the lattice constant of crystals A, so the diffracted beam can propagate only at small angles θ to the passing beam (
). The diffraction pattern from a crystal is a set of individual dots (reflexes). In TEM, in contrast to the electron diffraction pattern, it is possible to obtain a diffraction pattern from a small area of the object using a diaphragm in the plane conjugated with the object. The size of the region may be about (1×1) μm 2 . You can switch from the image observation mode to the diffraction mode by changing the optical power of the intermediate lens. The history of the electron microscope
In 1931, R. Rudenberg received a patent for a transmission electron microscope, and in 1932, M. Knoll and E. Ruska built the first prototype of a modern instrument. This work by E. Ruska was awarded in 1986 Nobel Prize in physics, which was awarded to him and the inventors of the scanning probe microscope, Gerd Karl Binnig and Heinrich Rohrer. The use of the transmission electron microscope for scientific research began in the late 1930s, and at the same time, the first commercial instrument built by Siemens appeared.
In the late 1930s - early 1940s, the first scanning electron microscopes appeared, which form an image of an object by sequentially moving an electron probe of a small cross section over the object. The widespread use of these devices in scientific research began in the 1960s when they achieved significant technical sophistication.
A significant leap (in the 70s) in development was the use of Schottky cathodes and cathodes with cold field emission instead of thermionic cathodes, but their use requires a much larger vacuum.
In the late 90s and early 2000s, computerization and the use of CCD detectors greatly increased stability and (relatively) ease of use.
In the last decade, modern advanced transmission electron microscopes have used correctors for spherical and chromatic aberration(which introduce the main distortion in the resulting image), however, their use sometimes significantly complicates the use of the device.
Types of electron microscopes
Transmission electron microscopy
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The original view of the electron microscope. The transmission electron microscope uses a high-energy electron beam to form an image. The electron beam is created by means of a cathode (tungsten, LaB 6 , Schottky or cold field emission). The resulting electron beam is usually accelerated to +200 keV (various voltages from 20 keV to 1 meV are used), focused by a system of electrostatic lenses, passes through the sample so that part of it passes through scattering on the sample, and part does not. Thus, the electron beam passed through the sample carries information about the structure of the sample. Next, the beam passes through a system of magnifying lenses and forms an image on a luminescent screen (usually made of zinc sulfide), a photographic plate, or a CCD camera.
TEM resolution is limited mainly by spherical aberration. Some modern TEMs have spherical aberration correctors.
The main disadvantages of TEM are the need for a very thin sample (on the order of 100 nm) and the instability (decomposition) of the samples under the beam. aaaaa
Transmission scanning (scanning) electron microscopy (SEM)
Main article: Transmission scanning electron microscope
One of the types of transmission electron microscopy (TEM), however, there are instruments that operate exclusively in the TEM mode. An electron beam is passed through a relatively thin sample, but, unlike conventional transmission electron microscopy, the electron beam is focused to a point that moves across the sample along the raster.
Raster (scanning) electron microscopy
It is based on the television principle of sweeping a thin electron beam over the sample surface.
Low voltage electron microscopy
Fields of application of electron microscopes
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Semiconductors and storage
Biology and biological sciences
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Scientific research
Industry
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The main world manufacturers of electron microscopes
see also
Notes
Links
- Top 15 Electron Microscope Images of 2011 The images on the recommended site are randomly colored, and are of artistic rather than scientific value (electron microscopes produce black and white images rather than color).
Wikimedia Foundation. 2010 .
Table of contents of the subject "Electron Microscopy. Membrane.":Electron microscopes appeared in the 1930s and came into widespread use in the 1950s.
The figure shows a modern transmission (translucent) electron microscope, and the figure shows the path of the electron beam in this microscope. In a transmission electron microscope, electrons pass through the sample before an image is formed. Such an electron microscope was constructed first.
Electron microscope upside down compared to the light microscope. Radiation is applied to the sample from above, and the image is formed from below. The principle of operation of an electron microscope is essentially the same as light microscope. The electron beam is directed by condenser lenses onto the sample, and the resulting image is then magnified by other lenses.
The table summarizes some of the similarities and differences between light and electron microscopes. At the top of the column of an electron microscope is a source of electrons - a tungsten filament, similar to that found in an ordinary light bulb. A high voltage (for example, 50,000 V) is applied to it, and the filament emits a stream of electrons. Electromagnets focus the electron beam.
A deep vacuum is created inside the column. This is necessary in order to minimize the scattering electrons due to collision with air particles. Only very thin sections or particles can be used for studying in an electron microscope, since the electron beam is almost completely absorbed by larger objects. The relatively denser parts of the object absorb electrons and therefore appear darker in the formed image. Heavy metals such as lead and uranium are used to stain the sample to increase contrast.
Electrons invisible to the human eye, so they are directed to the fluorescent, which reproduces the visible (black and white) image. To take a photograph, the screen is removed and electrons are directed directly onto the film. A photograph taken with an electron microscope is called an electron micrograph.
The advantage of the electron microscope:
1) a high resolution(0.5 nm in practice)
Disadvantages of an electron microscope:
1) the material prepared for the study must be dead, since in the process of observation it is in a vacuum;
2) it is difficult to be sure that the object reproduces a living cell in all its details, since fixation and staining of the material under study can change or damage its structure;
3) the electron microscope itself and its maintenance are expensive;
4) preparation of material for work with a microscope takes a lot of time and requires highly qualified personnel;
5) the studied samples are gradually destroyed under the action of the electron beam. Therefore, if required detailed study sample, it is necessary to photograph it.
How does an electron microscope work? What is its difference from an optical microscope, is there any analogy between them?
The operation of an electron microscope is based on the property of inhomogeneous electric and magnetic fields, which have rotational symmetry, to exert a focusing effect on electron beams. Thus, the role of lenses in an electron microscope is played by a set of suitably calculated electric and magnetic fields; the corresponding devices that create these fields are called "electronic lenses".
Depending on the type of electronic lenses electron microscopes are divided into magnetic, electrostatic and combined.
What type of objects can be examined with an electron microscope?
Just as in the case of an optical microscope, objects, firstly, can be "self-luminous", i.e., serve as a source of electrons. This is, for example, an incandescent cathode or an illuminated photoelectron cathode. Secondly, objects that are "transparent" for electrons with a certain speed can be used. In other words, when operating in transmission, the objects must be thin enough and the electrons fast enough to pass through the objects and enter the electronic lens system. In addition, by using reflected electron beams, the surfaces of massive objects (mainly metals and metallized samples) can be studied. This method of observation is similar to the methods of reflective optical microscopy.
By the nature of the study of objects, electron microscopes are divided into transmission, reflection, emission, raster, shadow and mirror.
The most common at present are electromagnetic microscopes of the transmission type, in which the image is created by electrons passing through the object of observation. It consists of the following main components: an illumination system, an object camera, a focusing system, and a final image registration unit consisting of a camera and a fluorescent screen. All these nodes are connected to each other, forming the so-called microscope column, inside which pressure is maintained. The lighting system usually consists of a three-electrode electron gun (cathode, focusing electrode, anode) and a condenser lens (we are talking about electronic lenses). It forms a beam of fast electrons of the desired cross section and intensity and directs it to the object under study located in the object chamber. The electron beam passing through the object enters the focusing (projection) system, which consists of an objective lens and one or more projection lenses.
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