Electron microscope, principle and application

Introduction
History
Resolution &Magnification of
Electron microscope
Types of electron microscope
1) Transmission electron microscope (TEM)
- Structural parts of TEM
- Principle & Working of TEM
- Sample preparation for TEM
- Advantages & disadvantages of TEM
Scanning electron microscope (SEM)
- Structural parts of SEM
- Principle & Working of SEM
- Sample preparation for SEM
- Advantages & disadvantages of SEM
3) Scanning transmission electron microscope (STEM)
Applications of electron microscope
Conclusion
References

1. PRINCIPLE & APPLICATIONS OF ELECTRON MICROSCOPY 1 By KAUSHAL KUMAR SAHU Assistant Professor (Ad Hoc) Department of Biotechnology Govt. Digvijay Autonomous P. G. College Raj-Nandgaon ( C. G. )

2. CONTENTS Introduction History Resolution &Magnification of Electron microscope Types of electron microscope 1) Transmission electron microscope (TEM) - Structural parts of TEM - Principle & Working of TEM - Sample preparation for TEM - Advantages & disadvantages of TEM   2

3. 2) Scanning electron microscope (SEM) - Structural parts of SEM - Principle & Working of SEM - Sample preparation for SEM - Advantages & disadvantages of SEM 3) Scanning transmission electron microscope (STEM) Applications of electron microscope Conclusion References 3

4. INTRODUCTION Electron microscope work by using an electron beam instead of visible light & an electron detector instead of our eyes & the magnification is obtained by electromagnetic field . Electron beam has the properties of a wave with a wavelength that is much smaller than visible light . The smaller is the wavelength of light , the greater is the resolving power . 4

5. Electron microscope examine objects on a very fine scale & this examination can yield information about the topography , morphology , composition & crystallographic information . The electron microscope is best used for studying biological ultra-structure & the image obtained is called electron micrograph . 5

6. HISTORY The first electromagnetic lens was developed in 1931 by Hans Busch . It was Ernst Ruska & Max Knoll , a physicist & an electrical engineer , respectively from the University of Berlin , who created the first electron microscope in 1931 . The first commercial electron microscope was produced in 1938 by Siemens . Albert Prebus & Siemens produced a transmission electron microscope (TEM) in 1939 . 6

7. RESOLUTION & MAGNIFICATION Resolution power is the ability of an imaging device to see objects distinctly , that are located at a small angular distances . Magnification in terms is defined as “a measure of the ability of a lens or other optical instruments to magnify , expressed as the ratio of the size of the image to that of the object” . This means , that an object of any size is magnified to form an enlarged image . Thus , the resolving power of an electron microscope is 200 times greater than that of a light microscope . It produces useful magnification up to X 400,000 as compared to X 2000 in a light microscope . 7

8. TYPES OF ELECTRON MICROSCOPE There are three types of electron microscope as described below :- 1) Transmission electron microscope (TEM) 2) Scanning electron microscope (SEM) 3) Scanning transmission electron microscope (STEM) 8

9. 1 ) TRANSMISSION ELECTRON MICROSCOPE (TEM) 9

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11. Structural Parts of TEM 11

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13. Principle & Working of TEM Specimen is bombarded by a beam of electrons , the primary electrons . The bombarding electrons are focused onto the object . In areas in the object where these electrons encounter atoms with a heavy atomic nucleus , they rebound . In regions where the material consist of lighter atoms , the electrons are able to pass through . The fine pattern of electrons leaving the object , reaches the objective lens forms the image . 13

14. It is then greatly enlarged by projector lens . Eventually , the tranversing electrons (transmission) reach the scintillator plate at the base of the column of the microscope . The scintillator contains phosphor compounds that can absorb the energy of the stricking electrons & convert it to light flashes . Thus , a contrasted image is formed on this plate . 14

15. Sample Preparation for TEM The material to be studied under electron microscope must be well preserved , fixed , completely dehydrated , ultrathin & impregnated with heavy metals that sharpen the difference among various organelles . The material is preserved by fixation with glutaraldehyde & then with osmium tetroxide . The fixed material is dehydrated & then embedded in plastic (epoxy resin) & sectioned with the help of diamond or glass razor of ultra- microtome . The sections are ultrathin about 50-100 nm thick . 15

16. These sections are placed on a copper grid & exposed to electron dense materials like lead acetate , uranylacetate , palladium vapours , phosphotungstate etc . Now the sections can be viewed in the TEM . The coating with electrons dense materials enables the specimen to withstand electric bombardment . 16

17. Advantages of TEM TEMs offer very powerful magnification & resolution . TEMs provide information on element & compound structure . Images are high – quality & detailed . 17

18. Disadvantages of TEM TEMs are large & expensive . Laborious sample preparation . Operation & analysis requires special training . Samples are limited to those that are electron transparent . TEMs require special housing & maintainance . Images are black & white . 18

19. 2) SCANNING ELECTRON MICROSCOPE (SEM) 19

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21. Structural Parts of SEM 21

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23. Principle & Working of SEM The electron gun produces an electron beam when tungsten wire is heated by current . This beam is accelerated by the anode . The beam travels through electromagnetic fields & lenses , which focus the beam down toward the sample . A mechanism of deflection coils enables to guide the beam so that it scans the surface of the sample in a rectangular frame . When the beam touches the surface of the sample , it produces : - Secondary electrons (SE) - Back scattered electrons (BSE) - X- Rays … 23

24. The emitted SE is collected by SED & convert it into signal that is sent to a screen which produces the final image . These signals are scanned in the manner of a television system to produce an image on a cathode ray tube (CRT) . The image is recorded by capturing it from the CRT . Additional detectors collects the X-rays , BSE & produce corresponding images . A secondary electron detector (SED) attracts the SE & depending on the number of electrons that reach the detector , registers different levels of brightness on a monitor . 24

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27. Sample Preparation for SEM The specimen is first fixed in liquid propane at-180 degree Celsius & then dehydrated in alcohol at-70 degree Celsius . The dried specimen is the coated with a thin film of heavy metal , such as platinum or gold , by evaporation in a vacuum provides a reflecting surface of electrons . The surface of the specimen when scanned by the electron beam release secondary electrons that form a three-dimensional image of the specimen on a television screen . Holes & fissures appear dark , & knobs & ridges appear light . Complete scanning from top to bottom usually takes only a few seconds 27

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29. Advantages of SEM It gives detailed 3D & topographical imaging & the versatile information generated from different detectors . Modern SEMs allow for the generation of data in digital form . Most SEM samples require minimal preparation actions . 29

30. Disdavantages of SEM SEMs are expensive & large . Special training is required to operate an SEM . SEMs carry a small risk of radiation exposure associated with the electrons that scatter from beneath the sample surface . 30

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34. 3 ) SCANNING TRANSMISSION ELECTRON MICROSCOPE (STEM) 34

35. A scanning transmission electron microscope or STEM combines the capabilities of both a SEM & a TEM. The electron beam is transmitted across the sample to create an image (TEM) while it also scans a small region on the sample (SEM) . 35

36. APPLICATIONS OF ELECTRON MICROSCOPE Electron microscope is being used today in research laboratories around the world to explore the molecular mechanisms of disease , to visualize the 3D structure of tissues & cells . Forensic science uses electron microscopy to analyze criminal evidence such as gunshot residue , clothing fibres , soil samples etc. 36

37. Inorganic particles – both natural & manmade including soil , coal , cement , fly ash etc . can be analyzed to provide more detailed understanding of impact of waste pollution on environment & health . In medical field electron microscope is used to compare healthy & unhealthy blood & tissue samples . 37

38. CONCLUSION Since its invention , electron microscope has been a valuable tool in the development of scientific theory . Its wide spread is because it permit the observation of material on a nanometer (nm) to micrometer scale . Although SEMs & TEMs are large & expensive , they remain popular among researchers due to the high-resolution & detailed images the produce 38

39. References Electron microscope – Wikipedia - https ://en.m.wikipedia.org>wiki>Electr... Electron microscope : Principles & types – Biology Discussion - www.biologydiscussion .com>microscope… Working principle of an electron microscope - www.yourarticlelibrary.com>working-p... 39