Components of an electron microscope
Electron source: It is a cathode that releases free electrons, and a ring-shaped anode accelerates electrons. The voltage difference between the cathode and anode must be very high, typically between several thousand volts and three million volts.
Electrons: Used to focus electrons. Generally, magnetic lenses are used, and sometimes electrostatic lenses are also used. The function of the electron lens is the same as that of the optical lens in the optical microscope. The focus of the optical lens is fixed, but the focus of the electronic lens can be adjusted, so the electron microscope does not have a movable lens system like an optical microscope.
Vacuum device: The vacuum device is used to ensure the vacuum state inside the microscope, so that electrons will not be absorbed or deflected on their path.
Sample holder: Samples can be placed on the sample holder stably. In addition, there are often devices that can be used to change the sample (such as moving, rotating, heating, cooling, elongating, etc.).
Detector: A signal or secondary signal used to collect electrons. The projection of a sample can be obtained directly by using a transmission electron microscope (Transmission Electron Microscopy TEM). Electrons pass through the sample in this microscope, so the sample must be very thin. The atomic weight of the atoms making up the sample, the voltage at which the electrons are accelerated, and the desired resolution determine the thickness of the sample. The thickness of the sample can vary from a few nanometers to a few micrometers. The higher the atomic mass and the lower the voltage, the thinner the sample must be.
By changing the lens system of the objective, one can directly magnify the image at the focal point of the objective. From this one can obtain electron diffraction images. Using this image, the crystal structure of the sample can be analyzed.
In Energy Filtered Transmission Electron Microscopy (EFTEM), people measure changes in the speed of electrons as they pass through a sample. From this, the chemical composition of the sample can be inferred, such as the distribution of chemical elements in the sample.
Uses of Electron Microscopes
Electron microscopes can be divided into transmission electron microscopes, scanning electron microscopes, reflection electron microscopes, and emission electron microscopes according to their structures and uses. 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 the 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. The thinner or lower-density part of the sample has less electron beam scattering, so more electrons pass through the objective diaphragm and participate in 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.
