Science Questions

What does imaging atoms really show us?

Tue, 29th Nov 2011

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Question

Luciano Medrano via Facebook asked:

There are certain types of microscopy, that can, apparently, show us atoms but what are we really seeing? Are we seeing atoms? Are we seeing electron clouds? What does it really show us?Ē

Answer

Dave -  Pretty much all of the kinds of microscopy which can see atoms are forms of electron microscopy.  Some of them are actually atomic microscopy way or either firing electrons at a surface and getting them to bounce off or you're firing atoms at a surface and getting them to bounce off and you can draw a picture from that but actually, the more common one is actually called scanning electron microscopy. 

These have a very, very sharp point and they just scan this point across the surface on an atomic resolution so you build up lots of lines. And the point is moved up and down to either produce a constant force which is called atomic force microscopy or a constant electric current which is called scanning tunnelling electron microscopy. 

The point moves along and it goes up and down to get this constant current.  So what you're seeing is something to do with the electrons on the surface, either itís how hard they're pushing the point up and down or itís to do with how well they conduct and how well electricity can move up into the point.  By changing the voltage, you can actually get different pictures and you can look at electrons at different parts of the atom and you can actually do some very cunning bits of science on that.

Ben -   So although weíre looking at different properties of the atom itself, what we build up with is an image built by inference.  Weíre inferring that because of the change in electrical properties or the change in mechanical properties, there is therefore something there and itís on such a small scale that we can actually infer the exact position of an atom.

Dave -   Yes, that's right.  And something which is very neat which has done a while ago which I saw in the news was that they were doing things on a sheet of graphene which is such a regular repeating structure that you can subtract away this background pattern which you're getting in your picture, and just leaving the atom or molecule on the surface, so you could actually see hydrogen atoms attached to carbon atoms which is absolutely minute and you couldnít possibly see it in other ways.

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