Transmission electron microscope (TEM) is the transmission of an accelerated and concentrated electron beam onto a very thin specimen, and the electrons hit the atoms in the specimen and change their direction, forming solid angle scattering. The scattering angle is related to the density and thickness of the specimen, so it can form an image of varying brightness and darkness, which is magnified and focused and displayed on the microscope of imaging devices such as phosphor screens, films, and photosensitive coupling components.
1. Background knowledge
0 cannot be seen under a light microscopeMicrostructures below 2 microns are called submicrostructures or ultrafine structures. In order to clearly observe these structures, a shorter wavelength light source is required to increase the microscope resolution. In 1932, Ruska invented the transmission electron microscope with an electron beam as a light source, the wavelength of the electron beam is shorter than that of visible light and ultraviolet light, and the wavelength of the electron beam is inversely proportional to the square root of the voltage emitted by the electron beam, that is, the higher the voltage, the shorter the wavelength. TEM now has a resolution of up to 02 nm. After transmission, the electron beam contains information such as electron intensity, phase, and periodicity, which is used in imaging.
Diagram of the interaction between the electron beam and the sample.
2. TEM system components
The TEM system mainly includes the following parts:
Electron gun: one that emits electrons. It includes a cathode, a gate, an anode. The electrons emitted from the cathode tube pass through the gate hole to generate a ray beam and accelerate ** into the condenser under the anode voltage, which plays the role of accelerating the electron beam and pressurizing the electron beam.
Condenser: Concentrates the electron beams to obtain a collimated light source.
Sample holder: Loads the sample to be observed.
Objective: One-time magnified focused imaging.
Intermediate Mirror: Secondary magnification and control of imaging mode (image or electron diffraction mode).
Projector mirror: magnify 3 times.
Phosphor screen: converts electronic signals into visible light for the operator to observe.
CCD camera: A charge-coupled element used to convert optical images into digital signals.
Schematic diagram of the basic structure of a transmission electron microscope.
3. Principle
The transmission electron microscope and the optical microscope each lens and the optical path diagram are basically the same, the light source is irradiated on the specimen after the condenser converges, the beam passes through the sample and then enters the objective lens, and the image is converged into an image through the objective lens, and then the single magnification image formed by the objective lens enters the observer's eye through the second magnification of the objective lens under the light microscope, and finally forms a phosphor screen projection after the second relay amplification of the intermediate mirror and the projection mirror under the electron microscope, which is observed by the observer. The imaging optical path of the electron microscope objective is the same as that of the optical convex lens.
Optical path diagram of electron microscope and light microscope and imaging principle of electron microscope objective.
4. Sample preparation
Since transmission electron microscopy collects information about the electron beam that passes through the sample, the sample must be thin enough for the electron beam to pass through.
Sample classification: duplicate samples, ultra-microscopic particle samples, material film samples, etc.
Sample preparation equipment: vacuum coater, ultrasonic cleaner, slicer, grinding machine, electrolytic double sprayer, ion thinning instrument, ultra-thin microtome, etc.
5. Image Categories
1) Light and dark field contrast images.
Bright Field Image: The back focal plane of the objective lens allows the transmitted beam to pass through the diaphragm of the objective lens, blocking the diffraction beam to obtain image contrast.
Dark Field Image: The direction of the incident beam is oblique to 2 angles, and the diffraction beam passes through the diaphragm of the objective lens to block the transmitted beam to obtain image contrast.
Bright-dark field diagram of dislocations inside silicon.
2) High-resolution TEM (HRTEM).
Image hrtem can obtain a lattice fringe image (reflecting the crystal plane spacing information);Higher-resolution image information, such as structural images and individual atom images (reflecting the configuration of atoms or clusters of atoms in a crystal structure). However, the sample thickness is required to be less than 1 nanometer.
Schematic diagram of the HRTEM optical path.
HRTEM image of silicon nanowires.
3) Electron diffraction image.
Selected Area Diffraction (SAD): Structural characteristics of tiny regions at the micron scale.
Convergent beam electron diffraction (CBED): Structural characterization of tiny regions at the nanoscale.
Microbeam Electron Diffraction (MED): Structural characterization of tiny regions at the nanoscale.
Schematic diagram of the electron diffraction optical path.
Electron diffraction pattern of single-crystal zinc oxide.
Amorphous silicon nitride electron diffraction pattern.
Electron diffraction pattern of zirconium-nickel-copper alloys.