Analysis and Application of Electron Microscope in Nanomaterials
As the name implies, a microscope is an instrument used to magnify tiny objects for observation. Through an electron optical system composed of three electromagnetic lenses, the electron beam is focused into a small electron beam of about several nm to irradiate the surface of the test piece. The end lens is equipped with a scanning coil, which is mainly used to deflect the electron beam, so that it can scan the two-dimensional space on the test piece, and this scanner is synchronized with the scanning on the cathode ray (CRT). When the electron beam hits Secondary electrons (secondary electrons) and reflected electrons are excited when the test piece is tested. When these electrons are detected by the detector, the signal is sent to the CRT through the amplifier. Since the current on the scanning coil is synchronized with the current of the picture tube, the signal generated at any point on the surface of the test piece corresponds to the picture tube. Therefore, the test piece It is an analytical instrument that can express the topography and characteristics of the surface one by one by means of synchronous imaging. Electron microscopes are divided into many types, and the appropriate selection is made according to the needs. The image resolution or magnification produced by different microscope technologies is also different, such as: SEM scanning electron microscope, TEM transmission electron microscope, STM scanning Transmission electron microscope, AFM atomic force microscope, etc.
The material properties of the test piece are also a very important part, basically determined by three factors: structural composition and bonding, in order to observe the small scale, and then develop the electron microscope, these tools are limited to the surface of the material, and cannot provide the internal information of the material. Structural composition and bonding information, but material scientists must know the structural composition and bonding information inside the material, so the TEM transmission electron microscope has high-energy electrons (100kM~1MeV) to drive the electron beam into the test piece, through After the sample, because of the Coulomb potential energy interaction between the electrons and the atoms inside the sample, there is no loss of energy, which is commonly known as the "elastic scattering" phenomenon. We can obtain information about the internal microstructure and atomic structure from elastic and inelastic scattering electrons. Elastically scattered and inelastically scattered electrons will be imaged on the image plane through the objective lens. The electron beam input with different energies will affect the volume of the test piece, and the relationship is proportional. When the voltage is high, some secondary electrons come from below 0.2 μm from the surface (the thickness of the mica sheet). Therefore, it is necessary to use a lower voltage to observe the polymer material such as nanometer, so as not to lose the information on the upper surface, but pay attention to the discharge effect on the non-conductive test piece.
The influence of the surface of the test piece on EDS, if the SEM test piece itself is metal or has good conductivity, it can be directly detected without prior treatment. However, if it is a non-conductor, it must be coated with a metal film with a thickness of 50-200Å on the surface. The metal film should be evenly coated on the surface to avoid disturbing the surface of the test piece. The metal film is usually gold or Au. - Pd alloy or platinum. The more commonly used test piece preparation operations include: cutting, cleaning, embedding, grinding, polishing, erosion, powder coating, gold plating, etc. Large test pieces need to be cut into appropriate sizes for observation, while small test pieces need to be embedded for observation. Some principles must be paid attention to in the preparation of SEM test pieces: the position to be analyzed should be revealed, the conductivity of the surface should be good, heat-resistant, liquid or gel-like substances should be contained to avoid volatilization, non-conductive surfaces should be plated with gold, because we cannot determine the material elements The source, the proportion of the signal generated by the backscattered electrons, is analyzed qualitatively and quantitatively by analyzing the characteristics released by the test piece.
Another electron microscope, TEM, can not only observe the dislocation structure in the crystal and after processing and heat treatment, but also directly observe the formation of secondary crystals, cornering, recrystallization, creep, and dislocation in multiphase crystals. Many phenomena that are closely related to the mechanical properties of substances, such as the interaction with precipitates, the electron beam interacts with the test piece, forms a diffraction pattern on the backfocal plane after the objective lens, and generates a magnified image on the imaging plane. . When operating an electron microscope, the intermediate mirror is often focused on the focal plane or imaging plane behind the objective lens by changing the current of the intermediate mirror, and then the diffraction pattern or magnified image is observed respectively. The two images generated by the different diffraction conditions of the various parts of the test piece irradiated by the electron beam are bright field image and dark field image. The difference between them is that the aperture of the objective lens blocks the Electron beam (or direct electron beam), only let the direct electron beam pass through imaging (diffraction electron beam), observe and photograph the three-dimensional structure or slice on the surface of the test piece, especially suitable for the research of biological samples, but with electron Shoots through objects, revealing their internal state. TEM can analyze features as small as 1 Å, provided that the specimen must be sliced with a thickness not exceeding 1000 Å. Therefore, TEM cannot present a magnified image of a mosquito, but it can reveal the virus hidden in insect cells.
