Friday, June 13, 2014

"Seeing" chemical bonds with sub-molecular resolution

Chemists (and physicists) often draw molecular bonds as little lines connecting atoms, but actually imaging the bonds themselves is very hard.  With the advent of the scanning tunneling microscope, it's become almost commonplace to be able to image the position of atoms.  STM images the ability of electrons to enter or leave a conducting surface, and since an atom on the surface carry electrons within itself, the presence of an atom on the surface strongly modulates the STM signal.  This doesn't show anything direct about bonding between atoms, however.

Wilson Ho's group at UC Irvine has published another gem.  The paper is here (unfortunately behind the Science paywall), and the news release is here.  The new STM-based imaging technique, "itProbe", is based in inelastic tunneling, which I've described before.  (One advantage in being an ancient blogger - I can now refer back to my old stuff, with google helping me remember what I wrote.)  The Ho group deliberately attaches a CO molecule to their STM tip.  The CO molecule has a couple of very sharp vibrational (and "hindered translational") modes at low energies that can be seen electrically through inelastic electron tunneling spectroscopy (IETS) - basically sharp features in (the second derivative of) the tunneling current-voltage curve.  In the itProbe technique, the experimenters map out spatially what happens to those modes.  The idea is, as the CO molecule interacts with the sample close by, the precise energies of those vibrational modes shift - the environment of the CO molecule tweaks the effective spring constant for the CO's motion.  Imaging in this way, they find that maps of the inelastic signal seem to show the bonds between the atoms in an underlying molecule, rather than the atom positions themselves.  I admit I don't understand the precise mechanism here, but the images are eye-popping.  A similar idea, involving atomic force microscopy with CO attached to an AFM tip, was demonstrated before (here and here, for example).  In those experiments, the investigators looked at how interactions between the CO on the tip and the sample affected the mechanical properties of the tip as a whole.   

This is an example of a tour de force experiment that can be accomplished by long, sustained effort - the Ho group has been refining their IETS measurements for nearly two decades, and it's really paid off.  Hopefully these kinds of efforts will not become even less common as research funding seems to be focused increasingly on short time horizons and rapid changes in fashion.


Anonymous said...

"I admit I don't understand the precise mechanism here, but the images are eye-popping. "

Why is this any more important than Wolframs useless computer graphics.

Douglas Natelson said...

Anon, assuming you actually want an answer and that wasn't rhetorical: Wolfram, in his "useless computer graphics", is solving equations. This work is tied to an actual physical measurement. There is a real physical process at work that allows us to map chemical bonds spatially. The fact that I personally don't understand precisely why the bonding interaction has a particular effect on the vibrational energy of a nearby CO molecule is a reflection of my ignorance, nothing more. The physics at work here can be important for understanding chemical processes at surfaces (like heterogeneous catalysis); this is a new way to study molecular interactions on surfaces and provides a stringent test of our theoretical calculational abilities for these molecule-on-surface systems.

Anonymous said...

Regarding hte effect on the vibrational energy I thought this was due to Van der Waals bonding with the molecule? That certainly should change its vibrational states - although I am too ignorant to estimate order of magnitudes (and hence experimental feasibility based on this principle).

James Middlebrock said...

Nice description but went over head to me. I will try to figure it out and then Comment my views. Thanks for the post.