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Physics term paper on TRANSMISSION ELECTRON MICROSCOPE
Friday, January 21, 2011 by STUDY PORTAL in Labels: TERM PAPERS

Acknowledgement
It is a pleasure of mine to find myself penning down these lines to express my sincere thanks to my teacher to give me this opportunity of preparing this Term Paper, to enhance my professional practice. I express my deep sense of gratitude to my physics teacher to give me knowledge about the topic and concept related to this particular Term Paper. Without his guidance I cannot imagine to complete my Term Paper on time.
In the last, I want to thank my parents, without whom nothing was possible.

Contents

1. What is TEM.
2. Principle of TEM.
3. Working of TEM.
4. Uses of TEM
5. Limitations of TEM.
6. References

The Transmission Electron Microscope:
The transmission electron microscope (TEM) is a scientific instrument that uses electrons instead of light to scrutinize objects at very fine resolutions. They were developed in the 1930s when scientists realized that electrons can be used instead of light to "magnify" objects or specimens under study.

View inside cell via sections magnification 120,000 * 50,000X

Principle:
The transmission electron microscope (TEM) operates on the same basic principles as the light microscope but uses electrons instead of light. What you can see with a light microscope is limited by the wavelength of light.
TEMs use electrons as “light source” and their much lower wavelength make it possible to get a resolution a thousand times better than with a light microscope.
You can see objects to the order of a few angstroms (10-10 m). For example, you can see small details in the cell or different materials down to near atomic levels
. The possibility for high magnifications has made the TEM a valuable tool in both medical, biological and materials research.

Working:
An projector works by projecting light through a film layer. As the light passes through the film, it interacts with the film and specific areas of the film lets light pass through unobstructed, other areas absorb light and doesn't let it pass through, and still some absorb part of the light and lets only a fraction get through. The light that does go through is hits the lenses found on the other side and the resulting image is projected onto a screen.
The TEM works similarly. In the case of the TEM, though, a beam of electrons are focused on a single, pinpoint spot or element on the sample being studied. The electrons interact with the sample and only those that go past unobstructed Glossary Link hit the phosphor screen on the other side. At this point, the electrons are converted to light and an image is formed.
Uses of TEM:
The transmission electron microscope is used to characterize the microstructure of materials with very high spatial resolution, as it is not possible with a simple microscope which uses light instead of electrons.
Samples of skin can be taken from people and studied with an electron microscope, known as biopsies, which is used to find out diseases such as cancer, and there are many more uses of the transmission electron.

Limitations:
There are a number of drawbacks to the TEM technique. Many materials require extensive sample preparation to produce a sample thin enough to be electron transparent, which makes TEM analysis a relatively time consuming process with a low throughput of samples
Also the field of view is relatively small, raising the possibility that the region analyzed may not be characteristic of the whole sample. There is potential that the sample may be damaged by the electron beam, particularly in the case of biological materials.

Basic principles
The design of a transmission electron microscope (TEM) is analogous to that of an optical microscope. In a TEM high-energy (>100 kV) electrons are used instead of photons and electromagnetic lenses instead of glass lenses. The electron beam passes an electron-transparent sample and a magnified image is formed using a set of lenses. This image is projected onto a fluorescent screen or a CCD camera. Whereas the use of visible light limits the lateral resolution in an optical microscope to a few tenths of a micrometer, the much smaller wavelength of electrons allows for a resolution of 0.2 nm in a TEM.

Electron wavelength @ 200KeV = 2x10-12 m
Resolution normally achievable @ 200KeV = 2 x 10-10 m = 2Å

Imaging
Image contrast is obtained by interaction of the electron beam with the sample. Several contrast effects play a role. In the resulting TEM image denser areas and areas containing heavier elements appear darker due to scattering of the electrons in the sample. In addition, scattering from crystal planes introduces diffraction contrast. This contrast depends on the orientation of a crystalline area in the sample with respect to the electron beam. As a result, in a TEM image of a sample consisting of randomly oriented crystals each crystal will have its own grey-level. In this way one can distinguish between different materials, as well as image individual crystals and crystal defects. Because of the high resolution of the TEM, atomic arrangements in crystalline structures can be imaged in large detail.

TEM column or Layers

1) Electron Gun
An electron gun (also called electron emitter) is an electrical component that produces an electron beam that has a precise kinetic energy and is most often used in televisions and monitors which use cathode ray tube technology, as well as in other instruments, such as electron microscopes and particle accelerators.

2) Anode
An anode is an electrode through which electric current flows into a polarized electrical device.
An electrode through which current flows the other way (out of the device) is termed a cathode.
The flow of electrons is always from anode to cathode outside of the cell or device
In a battery or galvanic cell, the anode is the negative electrode from which electrons flow out towards the external part of the circuit.
In electrochemistry, the anode is where oxidation occurs and is the positive polarity contact in an electrolytic cell.
3) Condenser Lens
A condenser is one of the main components of the optical system of many transmitted light compound microscopes. A condenser is a lens that serves to concentrate light from the illumination source that is in turn focused through the object and magnified by the objective lens. It is a basic component of almost all compound light microscopes manufactured since the 19th Century. An equivalent condenser, which focuses an electron beam, is a basic component of both transmission electron microscopes. Objectives can be single lenses or mirrors, or combinations of several optical elements. In an optical instrument, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image.
4) Specimen
A specimen is a portion or quantity of material for use in testing, examination, or study.
Biology
A laboratory specimen is an individual animal, part of an animal, plant, part of a plant, or microorganism used as a representative to study the properties of the whole population of that species or subspecies
Medical laboratory * A sample taken from a patient, most frequently blood, urine, or semen.
Geology
* A piece of a type of rock or mineral taken from the earth. This can be anything from jasper to a diamond.

Typography * A type specimen is a publication that shows the available glyphs in a typeface, including variations used for ligatures and special ornaments.

Specimen Preparation for TEM àHigh pressure freezing: Plant tissue is flash frozen in a pressure bomb -197 C àWater in the tissue is replaced with acetone over 5 day period àAcetone saturated tissue is embedded in resin àResin is cut in thin sections, 80 nm thick

5) Optical Aperture lens
The objective aperture is to be used to allow the un-deflected beam to pass but to stop diffracted electrons.

6) Intermediate lens
The first intermediate lens magnifies the initial image that is formed by the objective lens:
The lens can be focused on: * Initial image formed by the objective lens, or * Diffraction pattern formed in the back focal plane of the objective lens.
This determines whether the viewing screen of the microscope shows a diffraction pattern or an image.

7) Projector Lens
Magnification in the electron microscope can be varied from hundreds to several hundred thousands of times.
This is done by varying the strength of the projector and intermediate lenses. Not all lenses will necessarily be used at lower magnifications.

8) Fluorescent Screen
A fluorescent screen includes a substrate made of a light-scattering material and a mixture of an organic fluorescent substance and a binder dispersed therein. Since the luminescent light is scattered by the light-scattering material, the light emitted from the screen is increased to a level higher than that of a conventional fluorescent screen to enhance brightness thereof. v Light beam excites the electron, raising it to a higher energy state, v As it falls back to its original state, it releases energy in the form of a light of lower E and longer wavelength than original beam of light

References:-
Websites:
http://www.nobelprize.org/educational/physics/microscopes/tem/index.htm www.matter.org.uk/tem www.springerlink.com/index/h828n03h0057r1vj.pdf

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