Molecular junction, what’s your function?
Nanoscale gaps used for Raman spectroscopy.
Improved understandings of electronic conduction through individual small molecules are important to basic scientific research as well as to future technology development. Techniques such as mechanical break junctions, single-molecule transistors, nanoparticle dimers, noise characterization and thermopower measurements already have advanced our understanding considerably. However, without sufficient imaging or spectroscopy tools capable of monitoring the environment and identifying the molecule of interest, interpreting the results obtained with these techniques often is very difficult.
Over the past decade, advances in surface-enhanced Raman spectroscopy (SERS) — a vibrational spectroscopy technique capable of molecular identification — have enabled single-molecule sensitivity. A number of metal substrate configurations have been shown to allow successful single-molecule SERS, including, for example, nanostructures produced using bottom-up patterning or traditional lithographic methods. Researchers have found, however, that the precise, reproducible, localized formation of such structures can be especially challenging, as changes in the local molecular environment lead to intensity fluctuations (“blinking”) and spectral diffusion in SERS emission.
Researchers have described a simple SERS substrate that takes advantage of nanoscale gaps between extended electrodes. Shown here are a transmission electron microscopy image of an electromigrated nanogap (left) and a Raman microscope image demonstrating that the Raman image from the molecule assembled on the substrate is localized to a diffraction-limited region around the nanoscale gap.
Tip-enhanced Raman spectroscopy, in which incident light excites an interelectrode plasmon resonance between a metal probe tip and a metal surface, has been demonstrated as an alternative method for single-molecule detection. This technique is not scalable, however, nor is it easily integrated with other sensing modalities.
Researchers at Rice University in Houston recently described a simple, scalable and reliable SERS substrate based on nanoscale gaps between extended electrodes. They showed that such gap structures produce localized “hot spots” — electric fields confined to a nanoscale region that yield enhanced Raman scattering. In a Nano Letters paper published March 3, they reported simultaneous measurements of electronic transport and SERS in single molecules using the nanoscale gap structures, an approach that could help boost understandings of electronic transduction.
“My group has been studying electronic transport in single molecules for several years,” said Douglas Natelson, the principal investigator of the study. “One outstanding issue in these molecular electronic experiments is the challenge of knowing unambiguously the molecule through which current is flowing. Knowing that tip-enhanced Raman spectroscopy is possible, and that this geometry is rather like two tips pointing toward one another, we decided to try to see whether the nanojunctions would be good substrates for SERS.”
The researchers fabricated the nanogap structures on a silicon wafer roughly 1 cm2 with a 200-nm layer of thermal silicon oxide on top. Using electron beam lithography, they patterned structures consisting of two larger pads connected by a number of constrictions, each 100 to 180 nm in width. They created nanometer-scale gaps in the constrictions using electromigration, in which the momentum from electrons carrying current is transferred to the lattice, thus helping to rearrange the atomic positions. Electromigration is a well-established means of creating electrodes for single-molecule conduction studies.
Previous studies have reported that nanogaps contain a single molecule. Changes in the movement or bonding of the molecule, or of the molecule’s orientation in the gap, lead to different configurations in “tunneling” gaps and, consequently, to changes in conductance.
The investigators conducted simultaneous measurement of conductance and Raman spectra from the nanogaps. They performed the electrical measurements by sourcing an AC signal into one of the pads using a digital lock-in amplifier; the other pad was attached to a current-to-voltage converter. They measured the AC and its second harmonic using lock-in amplifiers and sampled the DC component using a current-to-voltage amplifier. For the optical measurements, they used a scanning confocal Raman microscope from WiTec GmbH of Ulm, Germany, equipped with a 785-nm diode laser and a 100× ultralong-working-distance objective.
The experiments showed that the combination of measurements could yield information about the bonding, orientation and local environment of single molecules. The researchers noted that, with the help of certain calculations and theoretical estimates, they should be able to use the measurements to infer junction geometries and chemical structure likely associated with each type of Raman spectrum.
“The measurements demonstrate multimodal sensing at the single-molecule level,” Natelson said, “and will enable studies of some outstanding questions both in the fundamentals of SERS and in vibrational effects on molecular-scale electronic properties.” He added that the nanogap structures also are well suited to highly sensitive chemical detection.
“The fact that we can place these SERS hot spots with great reliability in predetermined locations is a major advantage and favors integration for lab-on-a-chip applications,” he said.
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