Raman spectroscopy is a powerful technique for analyzing atomic structure based on the inelastic scatter of light from molecules, with diverse applications including medical imaging and chemical sensing.
Researchers have found that nanostructures can enhance the effect of Raman scattering and so improve the sensitivity of the Raman technique. Acoustic vibrations can provide further augmentation of the Raman scattering effect by exciting collective electron oscillations, known as surface plasmons, that contribute to light scatter. In particular, it has been shown that scattering might be intensified by vibrating nanoparticles alongside purpose-built resonators, but until now there has been a limited understanding of the interactions that occur during such vibrations.
Sudhiranjan Tripathy and co-workers from the A*STAR Institute of Material Research and Engineering, collaborating with Adnen Mlayah and colleagues from the National Center of Scientific Research (CNRS) in France, have now discovered how interactions between the surface plasmons produced by a triad of gold nanodiscs and acoustic vibrations in spherical gold nanoparticles can enhance Raman scattering effects1.
"We investigated the dynamic properties of metallic nanoscale objects," says Tripathy. "The coupling mechanisms between acoustic vibrations and surface plasmons are not well understood and need to be investigated both theoretically and experimentally, so we focused on this area, combining our expertise in nanofabrication with optical spectroscopy."
Tripathy and his co-workers measured Raman scattering from the gold nanoparticles with and without the gold ‘trimers' in place. The team observed a significant increase in scattering intensity when the vibrating nanoparticles were coupled with the trimer resonators. The trimers effectively act like loudspeakers, amplifying the scattering of light from the particles (see image). "The enhancement in the scattering of light allowed us to measure eight Raman features associated with the vibrational modes of the spherical gold nanoparticles," says Mlayah.
"Usually, only two or three features are observed in standard low-frequency Raman experiments."
The enhanced scattering intensity has been attributed to ‘hot spots' in the trimers' electric field, which leads to further excitation of the plasmons. Previous studies have only focused on nanoparticles as the source of surface plasmons, and monitored the scattering reaction to vibration on that object alone. This is the first study to use two different sources-acoustic vibrations and surface plasmons-to produce beneficial effects.
By engineering surface plasmons in this way, the researchers can gain tight control over scattering properties. "Our future work in this area will involve applications in chemical sensing and biosensors," says Tripathy.
Tripathy, S. et al.: Acousto-plasmonic and surface-enhanced Raman scattering properties of coupled gold nanospheres/nanodisk trimers. Nano Letters 11, 431-437 (2011).