Smooth SERS support

The judicious use of SERS-active nanoparticles directly or indirectly can surmount the inherent obstacle in the way of the more widespread adoption of surface-enhanced Raman scattering (SERS) studies. Proof of principle in the current work involves activating an organic monolayer by attaching silver nanoparticles.

Noble metal nanoparticles can enhance scattering cross sections dramatically for any molecules adsorbed on the particles enabling single-molecule spectroscopy and hinting at enhancement factors as high as 10¹⁴ to 10¹⁵. Theory suggests that at least 8 to 10 orders of magnitude are due to electromagnetic surface plasmon excitation and the chemical enhancement associated with charge transfer transitions. Other studies suggest that this latter factor could be made to be the dominant one and so allow spectroscopists to improve their spectra still further by careful choice of active nanoparticles.

Given the nature of the electromagnetic and chemical enhancement mechanisms, SERS is particularly sensitive to the first layer of adsorbate, a point that is not lost on those exploiting the technique in chemical analysis, studies of corrosion and lubrication, and in heterogeneous catalysis. However, SERS is not perfect and suffers from two important drawbacks, according to Korean researchers.

The first drawback is that large enhancements are limited to two noble metals, silver and gold. This limits the wider application of SERS to other metallic materials that are both fundamental and practical importance. The second drawback is that even for silver and gold, the particles must have a surface morphology with characteristics on the scale of 50?200 nanometres otherwise a large enhancement factor is not obtained. This scale of roughness is apparently critical, as some researchers have made progress towards using even transition metals to induce SERS effects with careful control of the surface morphology. But, that's perhaps irrelevant as atomically flat surfaces, commonly used in the area of surface science and nanotechnology, have until now proved unsuitable for SERS investigations.

Now, Kwan Kim, Hyang Bong Lee, Jae Keun Yoon, Dongha Shin, and Kuan Soo Shin of the Department of Chemistry, at Seoul National University, hope to change all that.

Writing in the Journal of Physical Chemistry C, the team explains how they and others are searching for a way to mimic tip-enhanced Raman spectroscopy (TERS) in which the gap between a silver or gold tip on a scanning probe microscope and a metal substrate is illuminated using a laser of suitable wavelength. Instead of using a SPM, researchers have suggested that using silver or gold nanoparticles with a gap between them and a planar metal substrate could similarly function as a hot site for SERS.

Kim and others have previously demonstrated such an effect and obtained very intense SERS spectra for the organosulfur compound 4-aminobenzenethiol (4-ABT) assembled on a macroscopically flat silver or gold substrate simply by attaching the nanoparticles to the surface. "On the basis of these observations, we have investigated in this work whether unaggregated silver nanoparticles can also induce SERS for 4-ABT assembled on a macroscopically smooth platinum substrate."

The team has now carried out Raman scattering measurements for 4-ABT) monolayers assembled on a macroscopically smooth platinum substrate. In the absence of silver nanoparticles no Raman peak is detected, but attaching the nanoparticles to the amine groups distinct Raman spectra emerge. This suggests that a SERS effect is occurring through an electromagnetic (EM) coupling of the localized surface plasmon of the silver nanoparticles with the surface plasmon polariton of the platinum substrate, the team explains.

SERS intensity gradually decreases as the excitation wavelength is raised from 488 to 514.5, 568, and 632.8 nm, the team adds. They explain that a similar trend was seen in a finite-difference time-domain calculation, which suggests that the EM coupling should be strong at short-wavelength excitation too. As such, enhancement per silver nanoparticle is estimated at 7900 at 514.5 nm.

"The present observation clearly demonstrates that the inherent obstacles to the more widespread use of SERS can be overcome by the judicious use of SERS-active nanoparticles directly or indirectly," the team concludes.