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 The microspheres are formed 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 that 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.
The top of the transmission electron microscope lens barrel is an electron gun. The electrons are emitted by the tungsten hot cathode, and the electron beams are focused by the first and second condensers. After passing through the sample, the electron beam is imaged on the intermediate mirror by the objective lens, and then enlarged step by step through the intermediate mirror and projection mirror, and then imaged on the fluorescent screen or the photocoherent plate.
The magnification of the intermediate mirror can be continuously changed from tens of times to hundreds of thousands of times mainly through the adjustment of the excitation current; by changing the focal length of the intermediate mirror, electron microscopic images and electron diffraction images can be obtained on the tiny parts of the same sample . In order to study thicker metal slice samples, the French Dulos Electron Optics Laboratory developed an ultra-high voltage electron microscope with an accelerating voltage of 3500 kV.
The electron beam of the scanning electron microscope does not pass through the sample, but only scans and excites secondary electrons on the surface of the sample. The scintillation crystal placed next to the sample receives these secondary electrons, amplifies and modulates the electron beam intensity of the picture tube, thereby changing the brightness on the screen of the picture tube. The deflection coil of the picture tube keeps synchronous scanning with the electron beam on the surface of the sample, so that the fluorescent screen of the picture tube displays the topographic image of the sample surface, which is similar to the working principle of an industrial TV.
The resolution of a scanning electron microscope is mainly determined by the diameter of the electron beam on the sample surface. The magnification is the ratio of the scanning amplitude on the picture tube to the scanning amplitude on the sample, which can be continuously changed from tens of times to hundreds of thousands of times. Scanning electron microscopy does not require very thin samples; the image has a strong three-dimensional effect; it can use information such as secondary electrons, absorbed electrons, and X-rays generated by the interaction of electron beams and substances to analyze the composition of substances.
The electron gun and condenser lens of the scanning electron microscope are roughly the same as those of the transmission electron microscope, but in order to make the electron beam thinner, an objective lens and an astigmatizer are added under the condenser lens, and two sets of mutually perpendicular scanning beams are installed inside the objective lens. coil. The sample chamber below the objective lens is equipped with a sample stage that can move, rotate and tilt.
