Transmission electron microscope

more use full to basic science students

1. TRANSMISSION ELECTRON MICROSCOPE K. SENTHILARASAN Edayathangudy G.S.Pillay Arts And Science College. Nagapattinam-02

2. OVERVIEW Introduction Principle Components Construction Working Advantage Disadvantage application

3. TRANSMISSION ELECTRON MICROSCOPE Introduction Scanning electron microscope the resolution of the image is limited only up to 10-20nm. This will not be useful to view the internal features of an atom or the morphology of a sample of size say 0.2nm. Thus to examine the sample of size up to 0.2nm, the transmission electron microscope can be used. In this microscope the image is obtained by transmitting the electrons through the specimen.

4. PRINCIPLE The electrons are allowed to pass through the specimen and the image is formed on the fluorescent screen either by using transmitted electron beam (bright field image) or by using diffracted electron beam (dark field image) from the specimen.

5. Components Electron Gun Magnetic Condensing Lenses Fluorescent Screen Or Charge Coupled Device .

6. Electron gun A high energy electron beam is produced by thermionic emission from directly heated tungsten filament. This electron beam is accelerated by the anode towards the specimen. Magnetic condensing lenses T he electron beam can be converged or diverged by the lenses. Theses lenses are usually of magnetic type ie ., current carrying coils. The focal length can be controlled by the current through the coil of the magnetic lens.

7. CONSTRUCTION It consist of an electron gun to produce electrons . Magnetic condensing lens is used to condense the electron and is also used to adjust the size of the electron that falls onto the specimen. The specimen is placed in between the condensing lens and objective lens. The magnetic objective lens is used to block the high angle diffracted beams and the aperture is used to eliminate the diffracted beam and in turn it increases the contrast of the image. The magnetic projector lens is placed above the fluorescent screen in order to achieve higher magnification. The image can be recorded by using a fluorescent screen or CCD.

8. Working Stream of electrons are produced by the electron gun and is made to fall over the specimen using the magnetic condensing lens. Based on the angle of incidence, the beam is partly transmitted and partly diffracted. Both the transmitted beam and the diffracted beams are recombined at the E-Wald sphere(sphere of reflection which encloses all possible reflections from the crystal, satisfying Bragg’s law), to form the image. The combined image is called the phase contrast image.

9. In order to increase the intensity and the contrast of the image, an amplitude contrast image has to be obtained. This can be achieved only by using the transmitting beam, and thus the diffracted beam has to be eliminated. Now in order to eliminate the diffracted beam, the resultant beam is passed through the magnetic objective lens and the aperture. The aperture is adjusted in such a way that the diffracted image is eliminated. Thus the final image obtained due to the transmitted beam alone is passed through the projector lens for further magnification

10. The bright filed image obtained is purely due to transmitted beam alone. T he dark filed image obtained due to diffracted image.

11. Advantage It can be used to examine the specimen of size upto 0.2nm. The magnification is 1,000000 times greater than the size of the object. It has high resolution. The resolving power 1 to 2

12. Disadvantage The specimen should be very thin It is not suitable for thick samples 3-d image cannot be obtained There are changes for the structural change, during the sample preparation. Incase of biological samples, the electrons may interact with the samples, which may even damage the samples .

13. Applications The main application of TEM is in nano -science used to find the internal structure of nano materials. It is used to find the 2 D image of very small biological cells, virus, bacteria .

14. Thank you