Combined single-molecule electronics and optics

This'll be my last self-referential post for a while. Now that it's out online, I want to write a post about our latest result. As readers of this blog know, I am not a big fan of the hype that accompanies a lot of nano research. I cringe everytime someone claims that a minor development is a breakthrough, and it drives me crazy when people who know better feel compelled to imply that self-reproducing nanobots are going to build spaceships out of single-crystal diamond in five years. That being said, I really do think that this result is a major advance, both in molecular-scale electronics and in ultrasensitive chemical sensing.

The two-sentence summary: we can do simultaneous electronic and optical measurements on single molecules (!) by using our electrodes as optical antennas. This opens up lots of science to be done as well as some very intriguing technological possibilities.

Over the last decade, a number of techniques have been developed to measure electronic conduction through single molecules. There are lots of basic physics and physical chemistry questions that still need to be answered in such systems (e.g., how does dissipation work at these scales? What happens when electronic correlations are strong and the system is driven out of equilibrium?). One long-standing problem, though, has been the lack of any independent (non-transport) way to confirm that conduction is taking place through a particular molecule of interest. Except for scanning tunneling microscopy (great for science, but impractical for some measurements and definitely not scalable for devices), there are no good imaging techniques to see the object (molecule of interest? contaminant? accidental nanoparticle?) through which the current is passing. The resulting approach to these devices has been essentially statistical, requiring the fabrication of large numbers of devices with many control experiments, etc.

Over the same period, as I discussed here in reference to an earlier paper from our group, surface-enhanced Raman spectroscopy (SERS) has been studied extensively. Raman spectroscopy is a very common physical chemistry technique to probe the vibrational spectrum of materials. Light comes in at some frequency, dumps some energy into the vibrational modes of the material (this is called Stokes scattering), and leaves with less energy. By measuring the energy shift between incoming and outgoing light, it's possible to pick out a material's characteristic vibrational modes - a kind of chemical fingerprint. In SERS, nanostructured metal surfaces act like little optical antennas when illuminated, creating so-called hotspots where the local electromagnetic intensity can be as much as a million times greater than the incident intensity, leading to greatly enhanced Raman emission. People have reported SERS capable of measuring single molecules, but demonstrating that conclusively is extremely difficult.

In our new paper, we've been able to kill two birds with one stone. We have been able to perform simultaneous electronic transport and Raman spectroscopy on individual molecules. The same metal electrodes used to push current through the molecules also function as a plasmonic antenna, giving enormous SERS enhancements. Conduction between the electrodes is known to be by tunneling, and tunneling depends so steeply on distance that the total volume through which the current is passing can only contain at most one or two molecules. (This steep distance dependence is the reason STMs work!) At room temperature (and in air), we see that the conduction from one electrode to the other bops around a bit as a function of time. This is due to molecular motion and the changing molecular environment, and isn't surprising. However, we can simultaneously measure the Raman signal from the region between the electrodes. We find that the time variation in the Raman emission correlates extremely well with the time variation in the interelectrode conduction. Since the conduction occurs via tunneling and probes about one molecular volume, the Raman emission must be from the same single molecule in question.

This demonstrates that we can mass-fabricate single-molecule sensitive SERS hotspots in high yield in pre-defined locations. At the same time, this multimodal single-molecule sensing shows via the Raman signature that we are pushing current through the specific molecule of interest in a given device.

We've got lots of ideas on where to go with this - it's very exciting.