Scanning Electron Microscopy Advantages, Application and Disadvantages
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The document provides an overview of Scanning Electron Microscopy (SEM), covering its introduction, historical development, principles, and working mechanisms. It details the components of SEM, the process of image formation, and its various applications, advantages, and disadvantages. SEM is a critical tool for analyzing material microstructures with high magnification and resolution, although it is limited by the requirement for vacuum compatibility and potential artifacts in imaging.
Scanning Electron Microscopy Advantages, Application and Disadvantages
- 1. Scanning Electron Microscopy Dr. Bhanu Krishan
- 2. Topics to be discussed • Introduction to SEM • History • Principle Involved • Working of SEM • Image Formation • Advantages and Disadvantages
- 3. Introduction to SEM • The most routinely utilized instrument for imaging the microstructure and morphology of the materials. • It is used for inspecting topographies of specimens at very high magnifications using a piece of equipment called the scanning electron microscope. • SEM magnifications can go to more than 300,000 X. • In SEM, an electron beam with low energy is radiated to the material and scans the surface of the sample (Omidi, et.al., 2017).
- 4. History • 1926 – Hans Busch demonstrates that charged particles can be bent in a magnetic field as glass lenses bend visible light. • 1931 – Ernst Ruska built the first transmission electron microscope with resolution higher than a light microscope (~12,000x). • 1938 – Manfred von Ardenne developed the first scanning transmission electron microscope, with an electron beam diameter on target of ~10 nm. His first image was of a zinc oxide crystal at 8000x magnification. • 1965 – Cambridge Scientific instruments released the first commercial SEM • 1985 – ZEISS launches the first fully digital SEM, the DSM 950.
- 5. Figure:Cambridge Scientific Instruments releases the first commercial SEM, Figure: ZEISS launches the first fully digital SEM, the DSM 950. Figure: Manfred von Ardenne develops the first scanning transmission electron microscope Source: ScienceMAg©
- 6. Principle • The scanning electron microscope (SEM) uses a focused beam of high-energy electrons to generate a variety of signals at the surface of solid specimens • Accelerated electrons in an SEM carry significant amounts of kinetic energy, and this energy is scattered as a variety of signals produced during electron-sample i.e. when the incident electrons are decelerated in the solid sample. • Such signals then produce secondary electrons (that produce SEM images), backscattered electrons (BSE), diffracted backscattered electrons (EBSD that are used to determine crystal structures and orientations of minerals), photons (characteristic X-rays that are used for elemental analysis and continuum X-rays), visible light (cathodoluminescence--CL), and heat. • Secondary electrons and backscattered electrons are commonly used for imaging samples: secondary electrons are most valuable for showing morphology and topography on samples and backscattered electrons are most valuable for illustrating contrasts in composition in multiphase samples
- 7. Working of SEM • The main components of a typical SEM are electron column, scanning system, detector(s), display, vacuum system and electronics controls. • The electron column of the SEM consists of an electron gun and two or more Electromagnetic lenses operating in vacuum. • The electron gun gun located at the top of the column where free electrons are generated by thermionic emission from a tungsten filament at ~2700K. The filament is inside the Wehnelt which controls the number of electrons leaving the gun. • Electrons are primarily accelerated toward an anode that is adjustable from 200V to 30 kV (1kV=1000V) (Cheney, 2007). Figure: Schematic diagram of a scanning electron microscope (JSM—5410, Source:
- 8. • Condenser Lenses: After the beam passes the anode it is influenced by two condenser lenses that cause the beam to converge and pass through a focal point. The electron beam is essentially focused down to 1000 times its original size. ▫ In conjunction with the selected accelerating voltage the condenser lenses are primarily responsible for determining the intensity of the electron beam when it strikes the specimen • Apertures: Depending on the microscope one or more apertures may be found in the electron column. The function of these apertures is to reduce and exclude extraneous electrons in the lenses. ▫ The final lens aperture located below the scanning coils determines the diameter or spot size of the beam at the specimen. ▫ The spot size on the specimen will in part determine the resolution and depth of field. Decreasing the spot size will allow for an increase in resolution and depth of field with a loss of brightness (Postek,et.al., 1980). Source: Nanoscience Instruments ©
- 9. • Scanning System: Images are formed by rastering the electron beam across the specimen using deflection coils inside the objective lens. ▫ The stigmator or astigmatism corrector is located in the objective lens and uses a magnetic field in order to reduce aberrations of the electron beam. ▫ The electron beam should have a circular cross section when it strikes the specimen however it is usually elliptical thus the stigmator acts to control this problem • Specimen Chamber: The lower portion of the column the specimen stage and controls are located. The secondary electrons from the specimen are attracted to the detector by a positive charge (Watt,1985). Source: Researchgate
- 10. • Vacuum system: the ability for a SEM to provide a controlled electron beam requires that the electronic column be under vacuum at a pressure of at least 5x10-5 torr. • A High vacuum pressure is required for a variety of reasons. • First, the current that passes through the filament causes the filament to reach temperatures around 2700k. A hot tungsten filament will oxidize and burn out in the presence of air at atmospheric pressure. • Secondly, the ability of the column optics to operate properly requires a fairly clean, dust-free environment. • Third, air particles and Dust inside the column can interfere and block the electrons before the ever reach the specimen in the sample chamber
- 11. Image Formation • Complex interactions occurs when the electron beam in an SEM impinges on the specimen surface and various signals are produced for SEM observation. • The secondary electrons, BSEs, transmitted electrons, or the specimen current electrons might all be collected and displayed for the image formation. • Backscattered Electrons or BSE reflect off the sample surface like light from a mirror. As backscattering efficiency depends on atomic number, BSE can recognize differences in sample composition, such as the presence of a silver particle coating on the synthetic fiber of an antimicrobial dressing. • Secondary Electrons (SE) or Low-energy (<50eV) secondary electrons (SE) result from “inelastic” interaction of the primary electron beam (and backscattered electrons) with the sample, reveals topography and electrical properties of the sample. Figure: BSE image of Ag particles on antimicrobial dressing. Figure: SE image of the arms of octopus.
- 12. Figure: EDS map image of a paint sample from original artwork. Figure: Cathodoluminescence (CL) When electrons hit luminescent materials they produce light. A CL detector picks up those photons, producing “live-color” images of minerals in geology applications—for instance, in oil and gas research and mining—or of luminescent proteins in biology.
- 13. Figure: SE, BSE, EDX detecting images differently
- 14. Applications of SEM • Topography: To determine the surface structure of the organism or any material • Morphology: Size and Shape of the particles making up the object. • Composition: The elements and compounds that the object is composed of can be determined. • Crystallographic information: Arrangement of atomes in the object and direct relation between these arrangements. • Characterisation: the most routinely utilized instruments for the characterization of nanomaterials. With an SEM it is possible to obtain secondary electron images of organic and inorganic materials with nanoscale resolution
- 16. Figure: Morphology of marine tidal tardigrade Echiniscoides sigismundi Figure: SARS CoV 2 effect on host cell and the adhesion of virus to the host cell
- 17. Advantages • 1. Resolution. This test provides digital image resolution as low as 5 nanometers, providing instructive data for characterizing microstructures such as fracture, corrosion, grains, and grain boundaries. • 2. Traceable standard for magnification. Because all imaging is calibrated to a traceable standard, it’s easy to apply analysis—such coating thicknesses, grain size determinations, and particle sizing—to saved images. • 3. Chemical analysis. SEM with EDS provides qualitative elemental analysis, standard less quantitative analysis, x-ray line scans, and mapping. This data can be used to examine product defects, identify the elemental composition of foreign materials, assess the thickness of coatings, and determine grain and particle size.
- 18. Disadvantages • 1. Vacuum environment. In most cases, SEM samples must be solid and vacuum-compatible. However, higher pressures can be used for imaging of vacuum-sensitive samples that are nonconductive and volatile. • 2. Artifacts are possible. Samples that are strong insulators must be coated—usually with gold or carbon— before testing. Thus, this process can result in artifacts. • 3. Less Resolution than TEM • The resolution of SEM is less than the TEM i.e. of 6nm and thus ultra structures cannot be seen.
- 19. References • Omidi, M., Fatehinya, A., Farahani, M., Akbari, Z., Shahmoradi, S., Yazdian, F., . . . Vashaee, D. (2017). Characterization of biomaterials. Biomaterials for Oral and Dental Tissue Engineering, 97-115. doi:10.1016/b978-0-08-100961-1.00007-4 • Scanning Electron Microscopy (SEM). (2004). Techniques. https://serc.carleton.edu/research_education/geochemsheets/techniques/SEM.html • Cheney, B. (2007). Introduction to scanning electron microscopy. Materials Engineering department San Jose State University. • M.T. Postek, K.S. Howard, A.H. Johnson and K.L. McMichael. (1980). Scanning Electron Microscopy: A Student’s Handbook, Ladd Research Ind., Inc. Williston, VT. • Rautela, A., Rani, J., & Das, M. D. (2019). Green synthesis of silver nanoparticles from Tectona grandis seeds extract: characterization and mechanism of antimicrobial action on different microorganisms. Journal of Analytical Science andTechnology,10(1),1-10. • Jacobs, J. B., Arai, M., Cohen, S. M., & Friedell, G. H. (1976). Early lesions in experimental bladder cancer: scanning electron microscopy of cell surface markers. Cancer Research, 36(7 Part 2), 2512-2517. • Hygum, T. L., Clausen, L. K., Halberg, K. A., Jørgensen, A., & Møbjerg, N. (2016). Tun formation is not a prerequisite for desiccation tolerance in the marine tidal tardigrade Echiniscoides sigismundi. Zoological Journal of the Linnean Society, 178(4), 907-911.