Duets by molecules and plasmons

(Nanowerk News) Light waves trapped on a metal's surface can interrogate the nearby molecules about their chemical identity through the molecule's characteristic vibrations, which act as fingerprints. Scientists at Pacific Northwest National Laboratory (PNNL) "saw" the interaction between the molecules and the trapped light waves or surface plasmons. Their fundamental endeavor could lead to breakthroughs in interpreting ultrasensitive chemical images for characterizing new materials with potential uses in catalysis and energy conversion.
The article is featured in the American Chemical Society (ACS) Editors' Choice collection ("Frequency-Resolved Nanoscale Chemical Imaging of 4,4'-Dimercaptostilbene on Silver"). The decision was made by the journal editors, who select one article a day from across the 55 ACS journals to appear in this special collection. As such, it was made freely available to the public. The article was the editors' choice for November 12, 2014.
Frequency resolved TERS images of the laser-irradiated region, reconstructed using four different Raman-allowed vibrations of DMS. The representative images are obtained from the same pass of the AFM probe through the laser-illuminated region of the substrate. Namely, this is the same image, reconstructed at four different energies. The 2D contours have a color bar starting at white (brightest signals), passing through yellow to black (lowest signals). (© American Chemical Society)
Why It Matters
To detect only a few molecules, researchers using ultrasensitive spectroscopic and microscopic techniques rely on localized surface plasmons. This is particularly the case when the molecular identity must be determined. These plasmons are essentially light waves trapped at the surface of nanoscale noble metal structures. In surface plasmon-enhanced vibrational spectroscopy, the recorded optical signals result from the interaction between molecular and plasmonic states. This was revealed by PNNL scientists, who set out to understand the information content encoded in nanoscale chemical images, recorded by taking advantage of the unique properties of localized surface plasmons. They found that each pixel in their recorded images reports on the distinct local environments in which each probed molecule resides.
"This is really fundamental science that takes us a step closer to understanding what one actually measures with this type of spectroscopy," said Dr. Patrick El-Khoury, a PNNL chemical physicist who carried out the measurements.
In their work, El-Khoury and his colleagues employed a gold tip of an atomic force microscope to record chemical images (25 nm spatial resolution) of a silver surface coated with 4,4'-dimercaptostilbene. They scanned the microscope's tip over a laser-illuminated area of the surface. They recorded molecular Raman scattering at every tip position. The images were then reconstructed at different energies, corresponding to different vibrational resonances of 4,4'-dimercaptostilbene. The technique is termed tip-enhanced Raman spectroscopy (TERS).
The results were rather unusual.
The simultaneously recorded images reconstructed at different vibrational resonances exhibit optimal TERS activity at different sites of the metal surface. "The recorded images map the unique interaction between the different vibrational modes of the molecule and the varying enhanced electric fields sustained at different positions of the microscope's tip," said Dr. Wayne P. Hess, a PNNL chemical physicist and co-author of the study.
The recorded images can only be understood by understanding the nature of the interaction between molecule and plasmon. In effect, the pixelated chemical images broadcast intimate details about plasmons, as well as plasmon-molecule interactions taking place on much finer scales.
What's Next? The team's study opens way for local electric field imaging using TERS. "Molecular spectroscopists have an obsession with molecules," said El-Khoury. "We are reversing the problem and exploiting molecular properties to learn more about plasmons and molecule-plasmon coupling."
Source: Pacific Northwest National Laboratory
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