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MICROSCOPY TECHNIQUES | Scanning Electron Microscopy
134 MICROSCOPY TECHNIQUES / Scanning Electron Microscopy
standard procedure. It should be pointed out that specimen damage during sputter coating is one of the most common artifacts in scanning electron micrographs. There are very few specimens that, if correctly prepared, may not be viewed without coating, provided the SEM is operated at low accelerating voltages and with appropriate care. If the sample contains water, this will have to be removed before examination by SEM. There are basically three methods: air drying, freeze drying, and critical-point drying (CPD). The one used will depend on the sample. Air drying is the quickest but the sample will distort and shrink unless it is protected by a rigid ‘shell’. If the only interest is X-ray analysis, this may not be a problem. Freeze drying, i.e., rapid freezing and then removal of the ice, usually in a specialized vacuum chamber, is ideal for samples such as wood, but critical point drying is most commonly used. The object of this technique is to remove the water initially by solvent extraction (as in thin section), then after the 100% alcohol step to change to acetone, and finally in the CPD apparatus to substitute the acetone for liquid carbon dioxide. Conversion from the liquid phase to the gaseous phase is achieved at elevated temperature and pressure. The critical point is the change from liquid to gas ‘without change of volume’. In reality, the volume change is minimized but it does occur, and this must be considered when finally viewing the sample. It would be preferred that the specimen could be viewed without preparation, and for this reason cryostages and environmental chambers exist. As mentioned previously, it is possible to freeze–fracture tissue to reveal the internal structure, but it is
possible also to fracture the critical-point-dried sample, which will be very brittle. The resultant image is limited only by the fixation method. Only the most commonly used techniques have been described, and it is inevitable that each specialized requirement will modify or create a new method. See also: Adhesives and Sealants. Microscopy Techniques: Specimen Preparation for Light; Scanning Electron Microscopy.
Further Reading Brawman JC and Sinclair R (1984) The preparation of cross-section specimens for transmission electron microscopy. Journal of Electron Microscopy Techniques 1: 53–61. Chapman SK (1986) Working with a Seaming Electron Microscope. Chislehurst, Kent: Lodgemark Press. Glauert AM.(ed.) (1974–1992). Practical Methods in Electron Microscopy. Amsterdam: Elsevier Science Publishers. Goodhew PJ (1984) Specimen Preparation for Transmission Electron Microscopy of Materials, Microscopy Handbooks no. 3. Oxford: Oxford University Press and Royal Microscopical Society. Jacobs EG, Forster IA, Wu Y, Wilson AR, and Pinizzotto RF (1993) Ultramicrotomy: A unique method for preparation of composite solder far transmission electron microscopy. Journal of Material Research 1: 1–20. Orloff J, Swanson L, and Utlaut MW (2002) High Resolution Focused Ion Beams: Fib and its Applications: The Physics of Liquid Metal Ion Sources and Ion Optics and their Applications to Focused Ion Beam Technology. New York: Plenum.
Scanning Electron Microscopy M I Szynkowska, Technical University of Lodz, Lodz, Poland & 2005, Elsevier Ltd. All Rights Reserved. This article is a revision of the previous-edition article by A F Otterloo, pp. 3151–3160, & 1995, Elsevier Ltd.
Introduction Scanning electron microscopy (SEM) is one of the most versatile and widely used of the surface analytical techniques as it allows both the morphology and composition of various materials in modern science to be studied. It is considered a relatively rapid,
inexpensive, and basically nondestructive approach to surface analysis. It covers a number of areas of application like biology, geology, metallurgy, semiconductor research, and catalysis. The first scanning electron microscopes-used to examine thick specimens was described by Zworykin and co-workers in 1942, but the first commercial scanning electron microscopes, based on the work of Oatley and coworkers at Cambridge University, became available only in 1965. Since then, many advances have been made owing to improvements in lens design, highbrightness electron sources, new detectors, and electronic signal processing. Nowadays there are many different types of scanning electron microscope
standard procedure. It should be pointed out that specimen damage during sputter coating is one of the most common artifacts in scanning electron micrographs. There are very few specimens that, if correctly prepared, may not be viewed without coating, provided the SEM is operated at low accelerating voltages and with appropriate care. If the sample contains water, this will have to be removed before examination by SEM. There are basically three methods: air drying, freeze drying, and critical-point drying (CPD). The one used will depend on the sample. Air drying is the quickest but the sample will distort and shrink unless it is protected by a rigid ‘shell’. If the only interest is X-ray analysis, this may not be a problem. Freeze drying, i.e., rapid freezing and then removal of the ice, usually in a specialized vacuum chamber, is ideal for samples such as wood, but critical point drying is most commonly used. The object of this technique is to remove the water initially by solvent extraction (as in thin section), then after the 100% alcohol step to change to acetone, and finally in the CPD apparatus to substitute the acetone for liquid carbon dioxide. Conversion from the liquid phase to the gaseous phase is achieved at elevated temperature and pressure. The critical point is the change from liquid to gas ‘without change of volume’. In reality, the volume change is minimized but it does occur, and this must be considered when finally viewing the sample. It would be preferred that the specimen could be viewed without preparation, and for this reason cryostages and environmental chambers exist. As mentioned previously, it is possible to freeze–fracture tissue to reveal the internal structure, but it is
possible also to fracture the critical-point-dried sample, which will be very brittle. The resultant image is limited only by the fixation method. Only the most commonly used techniques have been described, and it is inevitable that each specialized requirement will modify or create a new method. See also: Adhesives and Sealants. Microscopy Techniques: Specimen Preparation for Light; Scanning Electron Microscopy.
Further Reading Brawman JC and Sinclair R (1984) The preparation of cross-section specimens for transmission electron microscopy. Journal of Electron Microscopy Techniques 1: 53–61. Chapman SK (1986) Working with a Seaming Electron Microscope. Chislehurst, Kent: Lodgemark Press. Glauert AM.(ed.) (1974–1992). Practical Methods in Electron Microscopy. Amsterdam: Elsevier Science Publishers. Goodhew PJ (1984) Specimen Preparation for Transmission Electron Microscopy of Materials, Microscopy Handbooks no. 3. Oxford: Oxford University Press and Royal Microscopical Society. Jacobs EG, Forster IA, Wu Y, Wilson AR, and Pinizzotto RF (1993) Ultramicrotomy: A unique method for preparation of composite solder far transmission electron microscopy. Journal of Material Research 1: 1–20. Orloff J, Swanson L, and Utlaut MW (2002) High Resolution Focused Ion Beams: Fib and its Applications: The Physics of Liquid Metal Ion Sources and Ion Optics and their Applications to Focused Ion Beam Technology. New York: Plenum.
Scanning Electron Microscopy M I Szynkowska, Technical University of Lodz, Lodz, Poland & 2005, Elsevier Ltd. All Rights Reserved. This article is a revision of the previous-edition article by A F Otterloo, pp. 3151–3160, & 1995, Elsevier Ltd.
Introduction Scanning electron microscopy (SEM) is one of the most versatile and widely used of the surface analytical techniques as it allows both the morphology and composition of various materials in modern science to be studied. It is considered a relatively rapid,
inexpensive, and basically nondestructive approach to surface analysis. It covers a number of areas of application like biology, geology, metallurgy, semiconductor research, and catalysis. The first scanning electron microscopes-used to examine thick specimens was described by Zworykin and co-workers in 1942, but the first commercial scanning electron microscopes, based on the work of Oatley and coworkers at Cambridge University, became available only in 1965. Since then, many advances have been made owing to improvements in lens design, highbrightness electron sources, new detectors, and electronic signal processing. Nowadays there are many different types of scanning electron microscope