Tuesday, January 19, 2010

Inelastic electron tunneling spectroscopy

Motivated in part by this recent paper and ensuing conversation here, I thought it might be useful to say a few words about inelastic electron tunneling spectroscopy (IETS).  Mysterious kinks in the current as a function of voltage were first observed over 40 years ago in oxide tunnel junctions between superconductors.  As the voltage passed certain threshold values, the conductance (slope of I vs. V) increased suddenly.  A kink in I vs. V could also be plotted as a step in dI/dV vs. V, or as a peak in d2I/dV2 vs. V.  When plotted this way, and converting V into units of energy, Jaklevic and Lamb realized that what they saw looked remarkably like an infrared or Raman spectrum of some organic compound.  They were right - using inelastic electron tunneling, they had measured the vibrational spectrum of organic compounds that had been trapped in their tunnel barrier during the fabrication process.  IETS has undergone a major resurgence in the last decade, in part because of Wilson Ho's group's beautiful demonstration that one can see these effects at the single molecule level, and because it's a way of confirming that fabricated molecular junctions actually contain what they're supposed to.

In IETS, current flows via a second-order tunneling process, in which an electron tunnels on to the vibrational ground state of a molecule, and in the same coherent process tunnels of the vibrationally excited state of that molecule, leaving behind a vibrational quantum of energy.  This can only happen of the voltage applied is large enough to supply the necessary energy; hence the thresholds seen in experiment.  The voltage positions of the features correspond directly with the energies of the modes being excited.  (In the single-electron transistor world, this process would be called "inelastic cotunneling" via vibrationally excited states.)  The requirement that there be a nonzero amplitude for this process gives rise to selection rules, so that not every mode can be pumped this way.  More recently, it's been realized that IETS may not necessarily always lead to simple peaks in d2I/dV2 vs. V, because the IETS process can interfere coherently with other tunneling processes.  This is supported by data in the paper mentioned at the top of this post.

IETS is pretty amazing, when you think about it.  Even though the tunneling electrons never "really" occupy the molecule (such a state is classically forbidden due to energy conservation), nonetheless the molecule "feels" the effects of the electrons as they tunnel past.    


Don Monroe said...

Thanks, Doug.

For someone who truly gets Raman scattering, IETS shouldn't be so amazing, right? But it does seem like a really good tool for the single-molecule electronics field. Maybe it should be demanded whenever people make claims about the electrical properties of structures they can't see?

Doug Natelson said...

Don, IETS is nice, but there are two issues. First, b/c it's a second order process, it's weak, meaning that seeing it in single-molecule structures can be difficult. If the conductance itself is low, lots of averaging may be necessary. Second, IETS tells you that tunneling electrons are interacting with the molecules; it doesn't tell you how. In the original Jaklevic & Lamb paper, they have some random physisorbed amount of organics in their oxide - hardly an ordered, bound monolayer, for example. IETS is good to have, but not a cure-all.

I actually worry a bit about the carbon contamination problem from the Raman world rearing its head in IETS. That is, many of the strong modes that people see in IETS (C-C ring modes; CH stretch and wag modes) are going to be there in lots of systems, including carbon contamination and damaged molecules. Still, IETS is certainly much better than nothing.

Anonymous said...

Hi Douglas

Nice post. A few comments/questions:

- this might be the first experiment to show a clear gate coupling in a SMT that operates outside the Coulomb blockade regime ?? The characteristic shape of the Fowler-Nordheim plots seems to be a fingerprint of coherent transport (see e.g. Nano Lett., 2009, 9 (11), pp 3909–3913).

- in molecular single-electron transistors cotunneling lines due to vibrational excitations are to my knowledge rarely seen. How come they appear so clearly in IETS then ?

K. Kaasbjerg

Doug Natelson said...

Hello Kristen - I need to look more carefully at this to decide just how different this particular result is than the CB regime. I think that there is still some question about the proper interpretation of transition voltage spectroscopy (your reference, for example), but it is highly suggestive of a coherent process. I also agree that IETS, as a second-order process, indicates coherent transport. As for why IETS is hard to see in single-molecule transistors, I think it's largely a question of overall conductance, for practical experimental reasons. Inelastic cotunneling in SMTs has been seen - by us, as well as by the Delft group of van der Zant and in experiments by Ralph et al. at Cornell and Park et al. at Harvard.

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