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August 21, 2008

Electron Microscopy And Graphene

perforated sieve having holes of fixed diameterLook at the sieve on the left. The perforations have a fixed diameter. We can study these holes by throwing balls at it. If the balls pass through, then the diameter of the holes are obviously more than those of the balls, (at least in one axis, if the balls were a bit oblong). Likewise, when the balls bounce off, we can conclude that the diameter of the holes are less than those of the ball.

Electron Microscopy (EM) involves the use of electrons to visualize atoms and molecules. Optical microscopy does not yield much high resolution, since the wavelength of light is more, allowing them to cover ‘more than one hole’ in the above analogy. If we employ a 1 centimeter ruler to measure an object 12 cm long, we will do much better than if we employed a 6 cm ruler, isn't it? The resolution will be much better then, 12 pixels vs 2 !

Using electrons allow us to do just that! Electrons can be thought of as wave, whose wavelength is related to the Planck's constant divided by its momentum (de Broglie hypothesis). schematic diagram of scanning electron microscopy and transmission electron microscopy: SEM and TEMThe more you accelerated an electron, its wavelength became smaller and smaller. But you can't play dice with light, Einstein might book you for breaking speed limit then. You can't go faster than light, they say.

Now look at the diagram, provided courtesy of Opensource Handbook of Nanoscience and Nanotechnology. [Real marvelous illustration, by Kristian Molhave.] While optical microscopy used lenses for magnification of the image; in electron microscopy, electromagnetic and electrostatic lenses are used. You have seen how your hair stood on end when you combed your hair: thats electrostatic force. I did another hair-raising experiment during my childhood. I brought a strong magnet near the picture tube of my color TV (not LCD type but a CRT one). I saw a rainbow kind of pattern near the magnet. Its electromagnetic force! (DON'T do it yourself, else you may ruin your TV and a procedure called de-gaussing may be necessary).

Next we need a platform on which the sample should be placed: strong enough to tolerate the fast and furious electrons, yet transparent enough to let the electrons pass through, in case of transmission electron microscopy (TEM). Graphene, a one atom, one dimensional layer of carbon
is now being given a serious consideration as the candidate substrate. Now, Jannik Meyer, Alex Zettl and colleagues at the University of California, Berkeley have been able to visualize single atoms, such as hydrogen and carbon atoms, using this graphene substrate. However, speeding electrons sometimes drilled holes in the layer, even imparted some of its energy to make some hydrocarbon molecules move. They even watched one such hole being repaired using carbon from the atmosphere. One interesting thing is that, the carbon atoms in the graphene layer don't show them up, the orderly lattice structure takes care of that.

In scanning electron microscopy, the electron beam is scanned, as shown. The deflected (not transmitted!) beam containing information of the object, is picked up, and amplified by an electronic op-amp circuit. Thus, while you see whats inside of a cell in TEM, SEM allows you to have a pick of what goes on on the surface. But, how do you generate electrons in the first place? Simple! a heated tungsten filament will happily do it for you. And to speed them up, attract them with a bait, a high voltage anode. Don't forget to regulate the voltage and be careful of stray magnetic fields.

Looks like there's plenty of room at the bottom.
Last modified: Jun 26, 2010
Reference: Silcox, J. (2008). Microscopy: Spot the atom Nature, 454 (7202), 283-284 DOI: 10.1038/454283a

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