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My chemistry textbook was right all along!

As reported in Physics World today, a team of Swiss and Dutch physicists have developed a novel tip for an atomic force microscope (AFM) using a single carbon monoxide molecule. The tip is stable enough to image individual atoms and bonds within molecules.

The first image below is from the group's AFM of a single pentacene (C22H14) molecule on a copper (Cu(111)) substrate. The second picture shows the conventional chemical structure diagram (the vertices are carbon, and one hydrogen hangs off of each exterior corner), and the third is a "ball" model, with carbon atoms in black and hydrogen in white. As discussed in the article, the whole molecule is just 1.4 nm long, with a spacing of 0.14 nm between adjacent carbon atoms.

The paper is out in today's Science, but it's pay-per-view.

Picture of My chemistry textbook was right all along!
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lemonie7 years ago
Save me the effort of trying to find out, but is the tip moved by something like electrostatics - how do you move it atoms? (how do you even find a molecule with it?) It might just be a good link I'm after...

L

kelseymh (author)  lemonie7 years ago
Wikipedia is a really good source for basic summaries of scientific concepts. For political stuff, they're crap, but the science articles are mostly quite good.
Typing "AFM" into their search box gets you a disambiguation page, and a link to http://en.wikipedia.org/wiki/Atomic_force_microscope.

An AFM tip is generally mounted on a "nanoscale" (god, I hate buzzwords) cantilever, usually made via silicon lithography. That system in turn gets mounted on a microscopic piezoelectric stage, mounted on a larger macro stage.

The tip itself is etched silicon which forms a tetrahetral pyramidal structure. Picture the canonical >cough< stack of cannon balls, but in this case each one is a single silicon atom. What this research group did that was new was to attach a carbon monoxide molecule right to the atom at the tip of the pyramid.

So how does it work? The atom at the tip is attracted to an atom in the sample because of electrostatic (van der Waals) forces. That attraction causes the cantilever to bend, which can be measured (e.g. via resistance or capacitance).

The van der Waals force falls of like 1/r6 or so, so you get an incredibly sensitive measurement of the separation between the probe and the sample. Scanning the tip across a crystal plane, the force will vary up and down as you get close to atoms vs in between them.
Its possible to build the older, but related STM as a DIY-able project. I haven't seen anyone try an AFM. Steve
I thought along those lines, thanks for the explanation (I'll go looking later) L
kelseymh (author) 6 years ago
Bump! The same research group has used their technique to directly image a molecule (cephalandole A) with an unknown structure, thereby determining the structure by observation, rather than by inference and calculation.
I'm even going to try to understand this.
kelseymh (author)  Rock Soldier7 years ago
Good! Please feel free to ask questions as you go along. I'll do my best to answer them (or give you some sarcastic suggestion to read Wikipedia ;-> ).
Thanks. So to start, what does the first and second paragraph mean?
kelseymh (author)  Rock Soldier7 years ago
Do you know what an atomic force microscope is? You can follow the link to the Wikipedia article I gave Lemonie (below). AFMs use tips made with microchip fabrication techniques to have one single atom at the point. This team took a standard AFM tip and modified it to have, instead of a silicon atom at the tip, a molecule of carbon monoxide. You can read the rest of my reply to Lemonie (below) for details on how AFMs work, as well as reading the Wikipedia article.

My second paragraph is merely describing the pictures. The second and third pictures are standard from chemistry textbooks, which are just diagrams of what we assume this particular molecule (pentacene) looks like. After all nobody has ever seen a molecule, right?

This team used their AFM to map out the bumps and structure of the atoms and electron orbitals (positions) within one single molecule of pentacene, which was sitting on top of a carefully made sheet of copper. That is what the first picture is, an actual molecule, not a diagram.
The first picture, and the whole point of this posting, is an actual image of a single molecule of pentacene, made with the modified AFM I described above.
umm....
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