| The TEM, developed by Max Knoll and Ernst Ruska in Germany (1931), was the first electron microscope to be built. It was developed to answer the cries of the scientists who found that they had reached the limit of optical microscopy and needed to see even further into their environment. In spite of the fact that TEMs are 62 years old, we still use them today in many aspects of science.
How it works:
The TEM is essentially an optical microscope (a normal microscope that you'd see anywhere) that, instead of light, uses electrons to penetrate the sample. In the early 1900s, optical microscopes could magnify to about 1000x. Unfortunately this was about as far as light microscopes could magnify. This was because light has a wavelength of about 200nm and because of that, the light could 'dodge' any particles smaller than that. Electrons are smaller than an atom and, therefore, are small enough to 'hit' atoms rather than 'dodge' them. That means that theoretically the TEM can see individual atoms! But *how* does it do that? Well lets have a look at some ASCII art!
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| | Tungsten Filament (1)
\/\/\/
. .
BIG | . . . | BIG
FAT | . . . . | FAT (2)
ELECTRO | . . . . | ELECTRO
MAGNETIC| . . . . . | MAGNETIC
LENS | . . . . . | LENS
| . . . . . |
| . . . . . . |
| . . . |
| . . |
| . . . |
| . . |
| . . . . |
| . . . . . | (3)
| . . . . . |
| . . . . . |
| . . . . |
| . . |
| . . |
| . |
| . | (4)
| . |
.
.
:--------: The sample (5)
.
| ... | YET
| ..... | ANOTHER
| ....... | LENS (6)
| ......... |
| ........... |
______________
| |
| Magnified |
| Image |
| of | (7)
| Sample |
|______________|
As you may be able to see, it is very similar to the traditional optical microscope. At position (1) there is a tungsten filament that is heated to about 1000 degrees C. When this filament is heated electrons are flung off into the microscope. At position (2) the electrons are focused by a multitude of electromagnetic lenses into a beam. As you can see at position (3) the lenses focus a few times just to make sure and to allow the beam to be more or less focused until at position (4) the electrons are in a finely focused beam. The beam then passes through the sample where the electrons interact with the atoms. The electrons that make it through the sample (position (6)) are then focused onto a screen and make a picture in the same way your television does. This allows some of the best TEMs to magnify by anywhere between 700x to 1,200,000x!
Uses:
Well you can see really small stuff with one of these in fine detail. And, with a little tweaking, you can also get information about crystal structure!
Limitations:
As you can see in the above picture, the electrons have to pass through a sample. Because of this, your sample has to be VERY thin (about 100nm) otherwise your electrons will be absorbed by the sample and you won't get a picture. Also, the chamber usually has to be in a vacuum thus preventing you from looking at liquids.
These things are really fun to play with and essentially simple to use. In about 20 minutes my instructer was able to show us atomic resolution of some powder that he's working on! That's simply mind boggling...
References:
"Physics for Scientists and Engineers 5th edition", Serway and Beichner.
Lecture notes from Nanotechnology 101 @ Curtin University of Technology
http://www.nuigalway.ie/ncbes/tem.htm
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