Electron microscope composition
Composition of electron microscope
The main components of are:
Electron source: A cathode that releases free electrons, and a ring-shaped anode that accelerates electrons. The voltage difference between the cathode and anode must be very high, typically between thousands of volts and 3 million volts.
Electron: Used to focus electrons. Generally, magnetic lenses are used, and sometimes electrostatic lenses are used. The function of an electron lens is the same as that of an optical lens in an optical microscope. The focus of an optical lens is fixed, while the focus of an electronic lens can be adjusted, so an electronic microscope does not have a movable lens system like an optical microscope.
Vacuum device: A vacuum device used to maintain a vacuum state within a microscope, so that electrons are not 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. The projection of a sample can be directly obtained using a transmission electron microscope (TEM). In this microscope, electrons pass through the sample, so the sample must be very thin. The atomic weight of the atoms that make up the sample, the voltage of the accelerating electrons, and the desired resolution determine the thickness of the sample. The thickness of the sample can range from a few nanometers to a few 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, one can directly magnify the image of the focal point of the objective lens. From this one can obtain electron diffraction images. Using this image can analyze the crystal structure of the sample.
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 within the sample.
The Development Course of Electron Microscope
In 1931, M. Noel and E. Ruska of Germany modified a high-voltage oscilloscope with a cold cathode discharge electron source and three electron lenses, and obtained images that were magnified by more than ten times. They invented a transmission electron microscope, confirming the possibility of magnification imaging by an electron microscope. In 1932, after Ruska's improvement, the resolution ability of the electron microscope reached 50 nanometers, about ten times the resolution ability of the optical microscope at that time, breaking the resolution limit of the optical microscope. Therefore, the electron microscope began to receive attention. In the 1940s, Hill of the United States used an astigmatizer to compensate for the rotational asymmetry of the electron lens, making a new breakthrough in the resolution of the electron microscope and gradually reaching a modern level. In China, a transmission electron microscope with a resolution of 3 nanometers was successfully developed in 1958. In 1979, a large electron microscope with a resolution of 0.3 nanometers was developed.
Construction principle of electron microscope
The electron microscope consists of a lens barrel, a vacuum system, and a power cabinet. The lens barrel mainly includes components such as an electron gun, an electron lens, a sample holder, a fluorescent screen, and a photographic mechanism. These components are usually assembled into a column from top to bottom; The vacuum system consists of a mechanical vacuum pump, a diffusion pump, a vacuum valve, etc., and is connected to the lens barrel through an air extraction pipe; The power cabinet is composed of a high-voltage generator, an excitation current stabilizer, and various adjustment and control units.
The electron lens is the most important component in the lens barrel of an electron microscope. It uses a spatial electric or magnetic field symmetrical to the axis of the lens barrel to bend the electron trajectory towards the axis to form a focus. Its function is similar to that of a glass convex lens to focus the light beam, so it is called an electron lens. Most modern electron microscopes use electromagnetic lenses, which focus electrons with a strong magnetic field generated by a stable DC excitation current flowing through a coil with pole shoes.
The electron gun is a component composed of a tungsten wire hot cathode, a gate electrode, and a cathode. It can emit and form an electron beam with a uniform velocity, so the stability of the acceleration voltage is required to be no less than 1/10000.
