Development course of electron microscope
Composition of electron microscope
Electron source: it is a cathode that releases free electrons, and an annular anode accelerates electrons. The voltage difference between cathode and anode must be very high, usually between several thousand volts and 3 million volts.
Electrons: used to focus electrons. Generally, magnetic lenses are used, and sometimes electrostatic lenses are used. The function of electronic lens is the same as that of optical lens in optical microscope. The focus of the optical lens is fixed, while the focus of the electronic lens can be adjusted, so the electron microscope does not have a movable lens system like the optical microscope.
Vacuum device: the vacuum device is used to ensure the vacuum state in the microscope, so that electrons will not be absorbed or deflected in their path.
Sample rack: the sample can be stably placed on the sample rack. In addition, there are often devices that can be used to change the sample (such as moving, rotating, heating, cooling, stretching, etc.).
Detector: A signal or secondary signal used to collect electrons. Species The projection of a sample can be obtained directly by transmission electron microscope. In this microscope, electrons pass through the sample, so the sample must be very thin. The thickness of the sample is determined by the atomic weight of the atoms that make up the sample, the voltage of accelerating electrons and the desired resolution. The thickness of the sample can range from several nanometers to several microns. The higher the atomic weight and the lower the voltage, the thinner the sample must be.
By changing the lens system of the objective lens, people can directly enlarge the image of the focus of the objective lens. From this, people can obtain electron diffraction images. Using this image, the crystal structure of the sample can be analyzed.
In the energy filtered transmission electron microscope (EFTEM), people measure the speed change of electrons as they pass through the sample. From this, we can infer the chemical composition of the sample, such as the distribution of chemical elements in the sample.
Development course of electron microscope
In 1931, German M. Noel and E. ruska modified a high-voltage oscilloscope with a cold cathode discharge electron source and three electronic lenses, and obtained an image with a magnification of more than ten times. The transmission electron microscope was invented, which confirmed the possibility of magnifying imaging by the electron microscope. In 1932, after ruska's improvement, the resolution of the electron microscope reached 50 nanometers, which was about ten times that of the optical microscope at that time, breaking through the resolution limit of the optical microscope, so people began to pay attention to the electron microscope. In the 1940s, Hill of the United States used an astigmatism device to compensate the rotational asymmetry of the electron lens, which made a new breakthrough in the resolution of the electron microscope and gradually reached the modern level. In China, a transmission electron microscope was successfully developed in 1958 with a resolution of 3 nm, and a large electron microscope with a resolution of 0.3 nm was made in 1979.
