Thursday, January 16, 2014

Self-promotion - two papers, one post

Time for one of my comparatively rare scientific self-promotion posts.  I'm economizing by writing about two new papers in one post.  They're both fun results, and hopefully they're both reasonably accessible to a broad audience.

The first paper is this one.  I've written about plasmons before.   Light can come in and hit a metal nanostructure and be absorbed by exciting a plasmon (a sloshing of the electronic fluid, technically a coherent bunch of electron-hole excitations).  Over time, the energy in that plasmon eventually ends up as heat, slightly broadening the energy distribution of the electrons, and making the atoms vibrate.  A lot of people have been using the plasmon response of metal nanoparticles as a way to generate heat locally, with applications like cooking tumors or boiling water.  It's a real challenge, though, to measure the local increase in temperature, and to tell the difference between plasmon-based absorption and just ordinary absorption (which also dumps energy initially into the electrons, but not in a coherent way).   In our paper, my (now former) postdoc Joseph Herzog was able to do some clever measurements looking at plasmon-based heating in nanowires, with the wire itself being used as a resistive thermometer.  We could separate out the plasmon-based contribution because it has a very strong dependence on the polarization of the incident light, while ordinary absorption doesn't care much about that.  Mark Knight then did some really great optical + thermal modeling, and the results match the experiments very nicely.  Hopefully this will be a useful resource as people work on studying and engineering this kind of plasmon-based heating.  As a bonus, this tells us that in our other optics experiments on similar structures, the heating from having the laser on is probably only a few degrees.

The second paper is here, with a news release here.  Using special optical antenna structures, we have been able to do vibrational spectroscopy on single- or few-molecule junctions while driving current through them.  Previously we have shown that you can see when the electrons have enough energy to pump the molecular vibrations.  Recently, my student Yajing Li found, when looking at junctions containing C60, that the energies of vibrational states (that is, the natural frequencies of the molecular vibrations) were systematically lower when a decent voltage was applied across the junction.  Initially, we thought that this was an example of something called the vibrational Stark effect, and we turned to theorist colleagues (Jeff Neaton and his student Peter Doak, and Leeor Kronik) to see if that explanation held water.  It turns out, no, this is not the vibrational Stark effect (which is too small and also does not systematically lower vibrational energies).  Instead, when we apply a voltage across the junction, we slightly increase how much electron density is sitting on the molecule.  That slight increase is enough to soften some of the molecular bonds a little, and therefore lower the vibrational frequencies.  In chemistry lingo, we are partly filling an antibonding orbital, so that weakens the bonds.  The theory does a nice job explaining the shape and magnitude of what we see (though there is still plenty to do in terms of understanding the details).  For the science fiction fans in my readership:  Unfortunately there is no obvious way to run this the other direction and arbitrarily dial up the strength of molecular bonds, so I will not be opening a company called General Products that sells unbreakable spacecraft hulls. 


Unknown said...

Dear Prof. Natelson

Congratulations for these papers,

Regarding the PNAS: I might have missed something but wouldn't a HOMO-conducting molecule experience a strengthening of the bonds according to the model?

Incidentally: Thanks for your work (and your blog!)

Douglas Natelson said...

Hello Pierre - thanks for the kind words! I think it's not necessarily clear; it depends on the net bonding vs antibonding character of the orbitals. In principle, though, sure, you could have that situation.