Single-molecule spectroscopy

Raman spectroscopy is a common chemical characterization technique in which incident light interacts inelastically with a system, either losing energy toa vibrational or electronic mode of the system ("Stokes scattering"), or gaining energy from the system ("anti-Stokes scattering"). Raman cross-sections for single molecules tend to be small (~ \(10^{-29}\) cm2). However, nanostructured metal surfaces can act like little optical antennas due to electronic excitations called plasmons. The local electric field in the presence of plasmons can be enhanced by a factor \(g\) over that of the incident light. This translates into an enhancement of Raman emission by roughly \(g^4\) for molecules in the region of this near-field effect. The result is surface-enhanced Raman scattering (SERS), which can have single-molecule sensitivity.

In 2007 we demonstrated that the nanoscale electrodes used for the SMT experiments are outstanding plasmonic antennas and therefore wonderful substrates for SERS. This work was picked up by Nature Photonics.

(Here and here), we successfully performed simultaneous electronic transport and SERS measurements on single molecules. This confirms that transport is through the molecule of interest (via the unique Raman spectroscopic signature) and demonstrates that we can make single-molecule sensitivity Raman "hotspots" in predefined locations. This opens up many exciting experimental possibilities!

When light shines on a metal nanojunction and excites the local plasmon modes responsible for the enhancement, the plasmons lead to a voltage across the nanojunction, oscillating at optical frequencies (exceeding \(10^{14}\) Hz). Combining optical and electronic transport measurements in junctions without molecules, we have used optical rectification to determine this voltage experimentally. Using the simultaneously measured tunneling conductance, we can then infer quantitatively the enhanced electric field in the junction. Consistent with our Raman results, we find that field enhancements can exceed 1000x. Recently we have gained new insights into the interesting plasmon modes responsible for this enhancement.

We have used Raman scattering to examine the pumping of vibrational and electronic populations as current flows through a molecule-containing junction. This work demonstrates that it is possible to access experimentally the energetic distributions of electrons and vibrational modes in driven junctions, in situ, at the single molecule scale. While there has been much theoretical discussion of these distributions, attaining experimental information about the situation is very difficult. We have reviewed such experiments by our group and others here. More recently, we found that a voltage bias may actually be used to tune molecular vibrational frequencies in some circumstances.

Current efforts are looking at controlling electron-vibrational inelastic processes at the molecular scale, discerning between electronic and lattice heating processes, and leveraging our understanding of plasmonic modes for other surface-enhanced spectroscopies. For example, we have examined surface-enhanced infrared absorption spectroscopy in other nanogap structures.

Various components of the nano-optic work have been supported by the Robert A. Welch Foundation, the II-VI Foundation, Lockheed Martin Corporation, and the Army Research Office.