Scanning Electron Microscope Operation - Essay Example

? Scanning Electron Microscope Scanning Electron Microscope The scanning electron microscope utilizes abeam of focused high-energy electrons for the generation of various signals on the surface of specimens (Reimer, 2008 p97). The signals derived from the interaction between the sample and the electron reveals information regarding the sample inclusive of texture, material orientation in the sample, crystalline structure, and chemical composition. During most of its applications, data collected over the selected sample surface area is generated as a 2-D image which displays the spatial variations of the properties being investigated. Areas that range from a width of one centimetre to as minute as five microns can be seen in scanning mode using techniques in scanning electron microscopy. The magnification ranges from 20X to 30,000X with spatial resolution of fifty to one hundred nanometres. The SEM also has the capability of analysing specific locations found on the sample, with this approach being particularly useful in semi-quantitative and qualitative determination of crystal orientations, crystalline structure, and chemical compositions. A scanning electron microscope images samples via scanning them with electron beams with the sample in a raster scan pattern (Reimer, 2008 p 97). The electrons released by the SEM interact with sample atoms and produce signals that consist of information revealing the composition and topography, as well as electrical conductivity. In its functioning, accelerated electrons carry specific amounts of kinetic energy which is dissipated in form of various signals derived from interactions between the sample and the electrons (Reimer, 2008 p97). This occurs when the electrons incident on the sample are decelerated on contact with the sample. The received signals could be in form of secondary electrons, heat, visible light, photons, and diffracted backscattered electrons. Backscattered electrons and secondary electrons are usually utilized for sample imaging, with secondary electrons also used for showing the topography and morphology of the sample with backscattered electrons are used for contrast illustration of multiphase sample composition. X-rays are produced by through inelastic collisions of electrons incident on the sample and the electrons present in the sample atom’s discrete shells. During these electrons return to their lower energies, they give out fixed wavelength X-rays. Each element in the mineral being investigated produces X-rays which are characteristic to it when excited by the beam of electrons. This process is non-destructive as these X-rays do not cause any change in volume of the sample when they are lost. Thus, the same material can be investigated continuously. A scanning electron microscope has the following essential components (Reimer, 2008 p90): Source of electrons Electron lenses Stage for the sample Sensors for detecting all signals required Devices for data output and display Power supply, cooling system, vacuum system, electric and magnetic field free room, and a vibration free floor SEMs will always possess one detector usually for detection of secondary electrons, with most having more detectors. The accommodated detectors critically determine the instrument’s specific capabilities. Scanning electron microscopes are normally used for the generation of high-resolution images of various samples and their chemical spatial variations (Goldstein, 2009 p63). They aid in the acquisition of elemental maps or EDS assisted spot chemical analysis and phase discrimination using the sample atoms’ mean atomic number. They also give the compositional maps which they base on trace element differences. The scanning electron microscope is also used for the identification of phases with basis on crystalline structure and/or qualitative chemical analysis (Goldstein, 2009 p63). Specific measurement of extra small features which could be to fifty nanometres in size can also be done using scanning electron microscopy. Images formed from backscattered electrons are used for quick phase discrimination of multiphase samples. Microscopes which are fitted with detectors for electrons can be utilized for the examination of crystallographic and micrographic orientation. Magnification in scanning electron microscopes is controllable over ranges of close to six magnitude orders of approximately ten to five hundred thousand times (Goldstein, 2009 p65). They are different from transmission and optical electron microscopes in that their magnification of images is not a function of the objective lens’s power. While they may be fitted with objective lenses and condensers, their function has to do with the focusing of the electron beam on to a specific spot and not to produce images of the specimen. As long as a scanning electron microscope has an electron gun which can generate electron beams with adequately small diameter, the microscope could work in principle without the objective lens or the condenser. This would, however, make it less versatile and unable to achieve extra resolution. In scanning electron microscopes, magnification comes about due to the dimensional ratios of the specimen raster on the device. Making the assumption that the screen has a fixed display size, reduction of the specimen raster size will result in increased magnification. Therefore, magnification is controlled by supplied current to the y and x scanning coils or the supplied voltage to y and x deflector plates, and not by the power of the objective lens. A study carried out by Brazilian Oral research (2008) found out that dental lesions such as tooth wear had increased due to simultaneous erosion, abrasion, and attrition. Erosion is caused by hard dental tissue loss due to acidic substances like soft drinks in the mouth. This acidification causes demineralization rendering the tooth surface more susceptible to abrasion. The study aimed to discover what role saliva plays in tooth wear minimization. There exist a relationship between slowed rates of saliva flow and the mouths ability to flush out dietary acids (Rios, 2008 136). The study set out to evaluate, via the use of scanning electron microscopy, whether rate of salivary flow stimulated by gum chewing had any influence in the demineralization process of enamel exposed to erosion then abrasion after consumption of a soft drink. There was also qualitative study and comparison of human and bovine enamel substrates during abrasive/ erosive lesions. During the electron microscopy stage, nail varnish used to cover at least a half of every slab was removed with swabbed off using a cotton swab dipped in acetone to try and avoid the contamination of the surface. The specimens were then mounted in a cathodic evaporator and sputter-coated using palladium gold. The specimens were then photographed and examined in a SEM at a voltage of 1.5kV (Rios, 2008 136). The SEM allowed visualization of images with a distinct line of demarcation at the margin of test-control in all investigated groups (Rios, 2008 136). Specimens whose surface had been varnished showed no signs of alteration. For those surfaces that were uncoated, and were only submitted to the erosion, there was observation of core dissolution of the enamel prism, while for those exposed to both abrasion and erosion, amore homogenous surface of the enamel was observed, which was most probably caused by the removal of prism layer that had been superficially altered. For Enamel substrates and salivary stimulation tested for other variables, the observed pattern was similar in all specimens. In another study conducted by A. K. Johansson et al (2001) on the dental erosion in deciduous teeth, the study’s objective was to give a report on dental erosion and the etiological factors that it was associated with in deciduous teeth in children living in Saudi Arabia. It also sought to either reject or confirm clinical diagnosis via use of scanning electron microscope. Yet another aim was the study and reporting of erosion progression utilizing healthy permanent and deciduous teeth. A clinical examination and a questionnaire study were to be completed with acquisition of medical histories and examination of eroded exfoliated teeth using SEM. The specimens were first immersed in a solution of 2% citric acid for five minutes at 370 then rinsed off using water. They were then tested for micro hardness at which point they were re-immersed under the same conditions. This was repeated several times. After the last erosion, the specimens were dried, mounted, and covered with gold layers then studied under a JEOL JSM 35 scanning electron microscope utilizing an accelerated voltage of 15kV (Johanssona et al, 2001 p335). The investigation revealed that increased intake of fruits and acidic drinks, frequent medications, and problems of the upper respiratory tract most probably constituted aggravating and etiological factors which caused severe erosion of Saudi children dentals. The enamel of deciduous teeth was also revealed to be softer than that of permanent teeth. The scanning electron microscope did not show evidence of any differences between enamel specimen ultra structures. Then clinical diagnosis was studied via use of scanning electron microscopy and found to be reliable. A third study by A. Gray et al (1998) had the aim of presenting a report on the relationship between dental erosion and wine tasting. There had been reports of widespread erosion of teeth by individuals who had worked for at least ten years in wine industries. The occupation normally involved tasting of wine daily of approximately twenty wines or even more than that. The erosion was normally manifested as sensitivity of the teeth with additional enamel loss around restorations, occlusal pitting, and cervical erosion. The effect of immersing human teeth which are unerupted in wine, specifically white with a pH of 3.2, was put under examination using a scanning electron microscope. Two freshly unerupted, third molar, freshly extracted teeth were sagitally sectioned utilizing a cutting wheel made of diamond (Gray et al, 1998 p33). Every one of the samples was cleaned in distilled water before a half of each was put inside a jar with white wi9ne of pH 3.2 for 24 hours at 37o. The rest of the half was put in ajar with distilled water with exactly the same conditions. The two were then incubated and rinsed then dehydrated in 70%, then 90%, and finally 100% ethanol in series. After this, the samples were coated with a gold layer while placed in a sputter coater (Gray, 1998 p33). Finally, the samples were placed in an SEM and examined at accelerating voltage of 10kV. The study found that exposure to white wine from Switzerland with a pH of 3.6 for approximately twenty minutes caused a reduction in micro hardness as compared to fifty mmmol/L of lactic acid. Bibliography Gray A, Ferguson M. M, Wall J. G. "Wine tasting and dental erosion. Case report." Australian Dental Journal, 1998: 32-34. Goldstein J. Scanning electron microscopy and x-ray microanalysis. New York: Kluwer Academic/Plenum Publishers, 2009. Johanssona A, Sorvaric R, Birkheda D. Meurmand J.H. "Dental erosion in deciduous teeth?an in vivo and in vitro study." Journal, 2001: 333-340. Reimer L. Scanning electron microscopy : physics of image formation and microanalysis. Berlin : Springer, 2008. Rios D. "Scanning electron microscopic study of the in situ effect of salivary stimulation on erosion and abrasion in human and bovine enamel." PEDIATRIC DENTISTRY, 2008: 132-139. Read More