Types 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 is used for the study of self-emitting electron surfaces.
(1) Transmission electron microscope
Components of a transmission electron microscope (TEM) include:
1. Electron gun: emits electrons, composed of cathode, grid and anode.
2. Condenser lens: It is an electronic lens, which concentrates the electron beam and can be used to control the illumination intensity and aperture angle.
3. Sample chamber: place the sample to be observed, and is equipped with a rotating table to change the angle of the sample, as well as equipped with heating, cooling and other equipment.
4. Objective lens: It is a short-distance lens with high magnification, and its function is to magnify the electronic image. The objective lens is the key to determine the resolving power and imaging quality of the transmission electron microscope.
5. Intermediate mirror: It is a weak lens with variable magnification, and its function is to re-magnify the electronic image. By adjusting the current of the intermediate mirror, the image or electron diffraction pattern of the object can be selected for amplification.
6. Transmission mirror: It is a high-magnification strong lens, which is used to further enlarge the intermediate image after the second magnification and then form an image on the fluorescent screen.
7. Secondary vacuum pump: vacuumize the sample chamber.
8. Camera device: used to record images. Because electrons are easy to scatter or be absorbed by objects, the penetrating power is low, and the density and thickness of the sample will affect the final imaging quality. Thinner ultrathin sections must be prepared, usually 50-100 nm.
Therefore, the sample needs to be processed very thin when observed with a transmission electron microscope. Usually prepared by thin sectioning or freeze etching:
(1) Thin slice method
The sample is usually fixed with osmic acid and glutaraldehyde, embedded with epoxy resin, and sliced by thermal expansion or spiral propulsion. The slice thickness is 20-50 nm, and stained with heavy metal salts to increase the contrast.
(2) Freeze etching method also known as freeze fracture method
After the specimens were frozen in dry ice at -100°C or liquid nitrogen at -196°C, the specimens were quickly cut off with a cold knife. After the fractured specimen is heated up, the ice sublimates immediately under vacuum conditions, exposing the fractured structure, which is called etching. After the etching is completed, a layer of vaporized platinum is sprayed at a 45o angle to the section, and a layer of carbon is sprayed at a 90o angle to enhance contrast and strength. The sample is then digested with sodium hypochlorite solution, and the carbon and platinum film is peeled off, which is called a complex film, which can reveal the morphology of the etched surface of the specimen. The image obtained under the electron microscope represents the structure at the fractured surface of the cell in the specimen.
(2) Scanning electron microscope
Scanning electron microscope (SEM) came out in the 1960s, and the resolution can reach 6-10 nm at present.
Its working principle is that the finely focused electron beam emitted by the electron gun hits the sample through the two-stage condenser lens, deflection coil and objective lens, scans the surface of the sample and excites secondary electrons. The amount of secondary electrons generated is related to the incident angle of the electron beam, that is, related to the surface structure of the sample. After the secondary electrons are collected by the detector, they are converted into optical signals by the scintillator, and then converted into electrical signals by the photomultiplier tube and amplifier to control the intensity of the electron beam on the fluorescent screen, and display a scanning image synchronized with the electron beam. The image is a three-dimensional image, reflecting the surface structure of the specimen.
Before the inspection, the specimens of the scanning electron microscope need to be fixed, dehydrated, and then sprayed with a layer of heavy metal particles. The heavy metals emit secondary electronic signals under the bombardment of the electron beam.
