Principle and application of electron microscope

According to the principle of electron optics, electron beam and electron lens are used to replace light beam and optical lens, so that the fine structure of matter can be imaged at very high magnification.  

The resolution of an electron microscope is indicated by the minimum distance between two adjacent points that it can distinguish.  In the 1970s, transmission electron microscopes had a resolution of about 0.3 nanometers (the resolution of the human eye is about 0.1 millimeters).  

Now the maximum magnification of the electron microscope is more than 3 million times, and the maximum magnification of the optical microscope is about 2000 times, so the atoms and crystals of some heavy metals can be directly observed through the electron microscope in an orderly atomic lattice.  

In 1931, Knoll and Ruska in Germany modified a high voltage oscilloscope with a cold cathode discharge electron source and three electron lenses and obtained an image magnified more than ten times, which confirmed the possibility of electron microscope magnifying imaging.  In 1932, after Ruska’s improvement, the resolution of the electron microscope reached 50 nanometers, about ten times that of the optical microscope at that time, so the electron microscope began to attract people’s attention.  

electron microscope

In 1940s, Hill in the United States used an astrogator to compensate for the rotation asymmetry of the electronic lens, which made a breakthrough in the resolution of the electron microscope and gradually reached the modern level.  In China, a transmission electron microscope with a resolution of 3 nanometers was successfully developed in 1958, followed by a large electron microscope with a resolution of 0.3 nanometers in 1979.  

Although the resolution power of an electron microscope is far better than that of an optical microscope, it is difficult to observe living organisms because se electron microscope needs to work under vacuum conditions, and the irradiation of the electron beam will also cause irradiation damage to biological samples.  Other problems, such as improving the brightness of electron guns and the quality of electron lenses, also need further research.  

The resolution is an important index of the electron microscope, which is related to the cone angle and wavelength of the electron beam passing through the sample.  The wavelength of visible light is about 300 to 700 nanometers, while the wavelength of the electron beam is related to the acceleration voltage.  When the acceleration voltage is 50 ~ 100 kV, the electron beam wavelength is about 0.0053 ~ 0.0037 nm.  Because the wavelength of the electron beam is much smaller than that of visible light, the resolution power of an electron microscope is much better than that of an optical microscope even if the cone angle of the electron beam is only 1%.  

The electron microscope consists of a mirror tube, vacuum system, and power cabinet.  The lens barrel is mainly composed of the electron gun, electron lens, sample holder, fluorescent screen and photographic mechanism, etc. These parts are usually assembled into a cylinder from top to bottom.  The vacuum system is composed of a mechanical vacuum pump, diffusion pump, and vacuum valve, and is connected with the mirror barrel through the suction pipe.  The power cabinet is composed of a high voltage generator, excitation current stabilizer, and various control units.  

The electron lens is the most important part of the tube of an electron microscope. It uses a space electric field or magnetic field symmetrical to the axis of the tube to bend the electron track to the axis and form a focus. Its function is similar to that of a glass convex lens to focus the light beam, so it is called the electron lens.  Most modern electron microscopes use electromagnetic lenses, in which electrons are focused by a strong magnetic field generated by a very steady dc excitation current passing through a coil with pole boots.  

The electron gun is composed of tungsten wire hot cathode, gate, and cathode components.  It can emit and form an electron beam with uniform velocity, so the stability of the acceleration voltage is required to be no less than one part in ten thousand.  

An electron microscope can be divided into transmission type electron microscope, scanning type electron microscope, reflection type electron microscope, and emission type electron microscope by structure and use.  Transmission electron microscopy is often used to observe the microscopic structures of substances that cannot be distinguished by ordinary microscopes.  A scanning electron microscope is mainly used to observe the morphology of solid surfaces. It can also be combined with an X-ray diffractometer or electron energy spectrometer to form an electron microprobe for substance composition analysis.  Emission electron microscopy is used for the study of self-emitting electron surfaces.  

Projective electron microscopes get their name from the fact that after an electron beam penetrates a sample, an electron lens is used to image and magnify it.  Its flight path is similar to that of a light microscope.  In this type of electron microscope, the contrast of the details of the image is created by the scattering of the sample’s atoms against the electron beam.  

When the sample is thinner or less dense, the electron beam scatters less, so that more electrons pass through the light bar of the objective lens to participate in the imaging and appear brighter in the image.  Conversely, thicker or denser parts of the sample appear darker in the image.  If the sample is too thick or dense, the contrast of the image will deteriorate and may even be damaged or destroyed by absorbing the energy of the electron beam.  

At the top of the barrel of a transmission electron microscope is an electron gun. Electrons are fired from a tungsten hot cathode and are focused through a second or two converging mirrors.  After passing through the sample, the electron beam is imaged on the intermediate mirror by the objective lens, and then magnified step by step through the intermediate mirror and projection mirror, and imaged on the fluorescent screen or photographic plate.  

The intermediate mirror mainly adjusts the excitation current, and the magnification can continuously change from dozens of times to hundreds of thousands of times.  By changing the focal length of the intermediate mirror, the electron micrograph and electron diffraction image can be obtained at a small part of the same sample.  To study thick metal slice samples, the Ultrahigh voltage electron microscope with an acceleration voltage of 3500 kV was developed at the Laboratory of Electron optics in Duros, France.  

The electron beam in the scanning electron microscope does not pass through the sample, but only excites the secondary electrons by scanning the surface of the sample.  Scintillation crystals placed next to the sample receive these secondary electrons and, by amplifying them, modulate the intensity of the electron beam in the picture tube, changing the brightness on the screen of the picture tube.  The tube’s deflecting coil scans in sync with an electron beam on the surface of the sample so that the tube’s fluorescent screen displays a morphologic image of the sample’s surface, similar to how industrial television sets work.  

The resolution of sem is mainly determined by the diameter of the electron beam on the sample surface.  Magnification is the ratio of the scanning amplitude on the picture tube to the scanning amplitude on the sample, which can vary continuously from tens to hundreds of thousands of times.  Scanning electron microscopes do not require very thin samples;  The image has a strong three-dimensional sense;  It can analyze the composition of the substance by using the information of secondary electrons, absorbed electrons, and X-ray generated by the interaction of electron beam with the substance.  

The electron gun and condenser of the scanning electron microscope are roughly the same as those of the transmission electron microscope, but to make the electron beam finer, an objective lens and an instigator are added under the condenser lens, and two sets of scanning coils are installed inside the objective lens that is perpendicular to each other.  The sample chamber below the objective is equipped with a sample stand that can be moved, turned, and tilted.  

Share this post