Introduction to the main applications of scanning electron microscopy
Scanning electron microscope is a multifunctional instrument with many superior properties and is one of the most widely used instruments, which can perform the following basic analyses:
(1) Observation and analysis of three-dimensional morphology;
(2) Compositional analysis of micro-regions while observing morphology.
(1) Observation of nanomaterials. The so-called nanomaterials are solid materials that are obtained by pressure moulding under the condition of keeping the surface clean when the size of the particles or microcrystals composing the material is within the range of 0.1 to 100 nm. Nanomaterials have many unique physicochemical properties that are different from those of the crystalline and amorphous states. Nanomaterials have a broad development prospect and will become the key direction of future materials research. An important feature of scanning electron microscope is its high resolution, which is now widely used to observe nanomaterials.
② To carry out the analysis of material fracture. Another important feature of the scanning electron microscope is the large depth of field, the image is rich in three-dimensional sense. Scanning electron microscope depth of focus than the transmission electron microscope 10 times larger than the optical microscope hundreds of times. As the image depth of field is large, so the scanning electron image is rich in three-dimensional sense, with three-dimensional form, can provide much more information than other microscopes, this feature is very valuable to the user. Scanning electron microscope shows the fracture morphology from the deep, high depth of field angle presents the essence of the material fracture, in teaching, scientific research and production, has an irreplaceable role in the analysis of the cause of the material fracture, analysis of the cause of accidents and process reasonableness is a powerful means of determination.
③ Direct observation of the original surface of a large specimen. It can directly observe the specimen with a diameter of 100 mm, a height of 50 mm, or larger sizes, without any restriction on the shape of the specimen, and rough surfaces can also be observed, which eliminates the trouble of preparing samples, and can truly observe the specimen itself as a material component of the different lining (back-reflected electron image).
④Observation of thick specimens. When observing thick specimens, it is possible to obtain high resolution and the most realistic appearance. The resolution of scanning electron microscopy is between that of optical microscopy and transmission electron microscopy. However, when comparing the observation of a thick specimen, because in the transmission electron microscope, it is necessary to use the compound film method, and the resolution of the compound film is usually only 10 nm, and the observation is not of the specimen itself, therefore, it is more favourable to observe the thick specimen with the scanning electron microscope to get the real information of the specimen's surface.
⑤ Observe the details of each area of the specimen. The specimen has a very large movable range in the sample chamber. The working distance of other microscopes is usually only 2-3cm, so the specimen is only allowed to move in two degrees of space. However, in the scanning electron microscope is different, because the working distance is large (can be more than 20 mm), the focal depth is large (10 times larger than the transmission electron microscope), and the space of the sample chamber is also large, so the specimen can be allowed to have six degrees of freedom of movement in three degrees of space (i.e., three degrees of space translation, three degrees of space rotation), and the range of movement is large, which is extremely convenient for the observation of irregularly shaped specimens of the details of the various regions.
(vi) Observation of samples under large field of view and low magnification. The field of view for observing specimens with a scanning electron microscope is large. In a scanning electron microscope, the field of view F, which allows simultaneous observation of specimens, is determined by the following formula: F = L/M [8].
Where F - field of view range;
M - the magnification of the observation;
L - the size of the fluorescent screen of the tube.
If the scanning electron microscope using a 30 cm (12 inch) tube, magnification 15 times, its field of view range of up to 20 mm. large field of view, low magnification observation of the shape of the sample is necessary for some areas, such as criminal investigation and archaeology.
(7) Continuous observation from high magnification to low magnification. The range of magnification is very wide and there is no need to focus frequently. Scanning electron microscope magnification range is very wide (from 5 to 200,000 times continuously adjustable), and a good focus can be from high to low times, from low to high times continuous observation, without refocusing, which is particularly convenient for accident analysis.
⑧ observation of biological specimens. Due to electronic irradiation and the occurrence of specimen damage and contamination is very small. Comparison with other modes of electron microscopy, because the observation of the electron probe used in the current is small (generally about 10-10 ~ 10-12A) electron probe beam spot size is small (usually 5 nm to tens of nanometers), the electron probe energy is also relatively small (accelerating voltage can be as small as 2 kV), and is not a fixed point of irradiation of the specimen, but a raster scanning method of irradiation of the specimen, therefore, due to the Therefore, the degree of damage and contamination of the specimen due to electron irradiation is very small, which is particularly important for the observation of some biological specimens.
