Optical principle and application scope of electron microscope
An electron microscope is an instrument that uses electron beams and electron lenses instead of light beams and optical lenses to image the fine structures of substances at very high magnifications based on the principle of electron optics.
The resolving power of an electron microscope is represented by the minimum distance between two adjacent points that it can resolve. In the 1970s, the resolution of the transmission electron microscope was about 0.3 nanometers (the resolution of the human eye is about 0.1 mm). Now the maximum magnification of the electron microscope exceeds 3 million times, and the maximum magnification of the optical microscope is about 2000 times, so the atoms of some heavy metals and the neatly arranged atomic lattices in the crystal can be directly observed through the electron microscope.
In 1931, Knorr-Bremse and Ruska of Germany refitted 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 magnified imaging. In 1932, after Ruska's improvement, the resolution of the electron microscope reached 50 nanometers, which was about ten times the resolution of the optical microscope at that time, so the electron microscope began to receive people's attention.
In the 1940s, Hill in the United States used an astigmatizer to compensate the rotational asymmetry of the electron lens, which made a new breakthrough in the resolving power 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, and a large electron microscope with a resolution of 0.3 nanometers was manufactured in 1979. Although the resolving power of the electron microscope is far better than that of the optical microscope, it is difficult to observe living organisms because the electron microscope needs to work under vacuum conditions, and the irradiation of the electron beam will also cause the biological samples to be damaged by radiation. Other issues, such as the improvement of the brightness of the electron gun and the quality of the electron lens, are still to be studied. Resolving power is an important index of electron microscope, which is related to the incident cone angle and wavelength of the electron beam passing through the sample. The wavelength of visible light is about 300-700 nanometers, while the wavelength of electron beam is related to the accelerating voltage. When the accelerating voltage is 50-100 kV, the wavelength of electron beam is about 0.0053-0.0037 nanometers. Since the wavelength of the electron beam is much smaller than the wavelength of visible light, even if the cone angle of the electron beam is only 1% of that of the optical microscope, the resolving power of the electron microscope is still far superior to that of the optical microscope. Electron microscope consists of three parts: lens barrel, vacuum system and power cabinet. The lens barrel mainly includes electron guns, electron lenses, sample holders, fluorescent screens, and camera mechanisms. These components are usually assembled into a column from top to bottom; the vacuum system is composed of mechanical vacuum pumps, diffusion pumps, and vacuum valves. The gas pipeline is connected with the lens barrel; the power cabinet is composed of a high voltage generator, an excitation current stabilizer and various adjustment control units.
The electron lens is the most important component in the electron microscope barrel. It uses a space electric field or magnetic field symmetrical to the axis of the lens barrel to bend the electron trajectory to the axis to form a focus, and its function is similar to that of a glass convex lens to focus the beam, so it is called an electronic lens. Most modern electron microscopes use electromagnetic lenses, which focus electrons through a strong magnetic field generated by a very stable DC excitation current passing through a coil with pole pieces.
The electron gun is composed of tungsten hot cathode, grid and cathode.
pieces. It can emit and form electron beams with uniform speed, so the stability of the accelerating voltage is required to be no less than one ten thousandth.
Electron microscopes can be divided into transmission electron microscopes according to their structures and uses.
Microscopes, Scanning Electron Microscopes, and Emission Electron Microscopes, etc. Transmission electron microscopes are often used to observe the fine material structures that cannot be resolved by ordinary microscopes; scanning electron microscopes are mainly used to observe the morphology of solid surfaces, and can also be combined with X-ray diffractometers or electron energy spectrometers to form electronic Microprobes for material composition analysis; emission electron microscopy for the study of self-emitting electron surfaces.
The transmission electron microscope is named after the electron beam penetrates the sample and then magnifies the image with an electron lens. Its optical path is similar to that of an optical microscope. In this type of electron microscope, the contrast in image detail is created by the scattering of the electron beam by the atoms of the sample. Parts of the sample that are thinner or less dense are less scattered by the electron beam, so that more electrons pass through the objective diaphragm 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 too dense, the contrast of the image will deteriorate, or even be damaged or destroyed by absorbing the energy of the electron beam.
The top of the transmission electron microscope lens barrel is the electron gun, and the electrons are emitted by the tungsten hot cathode, and the electron beam is focused by the first and second condenser lenses. After passing through the sample, the electron beam is imaged on the intermediate mirror by the objective lens, and then enlarged step by step by the intermediate mirror and projection mirror, and then imaged on the fluorescent screen or the photocoherent plate.
The magnification of the intermediate mirror can be continuously changed from tens of times to hundreds of thousands of times mainly through the adjustment of the excitation current; changing the focal length of the intermediate mirror can obtain an electron microscopic image on a tiny part of the same sample
and electron diffraction images. In order to be able to study thicker metal slice samples, the Electron Optics Laboratory in Dulos, France has developed an ultra-high voltage electron microscope with an accelerating voltage of 3500 kV.
The electron beam of the scanning electron microscope does not pass through the sample, but only scans and excites secondary electrons on the surface of the sample. The scintillation crystal placed next to the sample receives these secondary electrons, amplifies and modulates the electron beam intensity of the picture tube, thereby changing the brightness on the picture tube fluorescent screen. The deflection coil of the kinescope keeps scanning synchronously with the electron beam on the surface of the sample, so that the fluorescent screen of the kinescope displays the topographic image of the sample surface, which is similar to the working principle of an industrial television.
The resolution of a scanning electron microscope is primarily determined by the diameter of the electron beam on the sample surface. The magnification is the ratio of the scanning amplitude on the picture tube to the scanning amplitude on the sample, which can be continuously changed from tens of times to hundreds of thousands of times. Scanning electron microscopy does not require very thin samples; the image has a strong three-dimensional effect; it can use information such as secondary electrons, absorbed electrons, and x-rays generated by the interaction between electron beams and substances to analyze the composition of substances.
The electron gun and condenser lens of a scanning electron microscope are roughly the same as those of a transmission electron microscope, but in order to make the electron beam thinner, an objective lens and an astigmatizer are added under the condenser lens, and two sets of mutually perpendicular scanning beams are installed inside the objective lens. coil. The sample chamber below the objective lens is equipped with a sample stage that can be moved, turned and tilted.
