Systematic nano Raman

Scientists in Israel and the US have demonstrated systematic differences in the Raman spectra of individual molecules adsorbed on small as opposed to large nanoparticles. The discovery has the potential to open up a new approach to single molecule studies.

Tali Dadosh, Joseph Sperling, Timur Shegai, M. Dyshel, Gilad Haran, and Israel Bar-Joseph of the Weizmann Institute of Science, Rehovot, Israel, Garnett Bryant of the National Institute of Standards and Technology, Gaithersburg, Maryland, Ronald Breslow of Columbia University, New York, have studied surface-enhanced Raman scattering (SERS) of individual organic molecules embedded in dimers of two metal nanoparticles.

Raman spectroscopy is a powerful tool for obtaining the spectroscopic fingerprints of different molecules. However, it is difficult to use it to study single molecule samples due to the small cross section of the process. Dadosh and colleagues have found that silver nanoparticles can be used to control SERS shape.

Haran points out that single-molecule SERS using nanoparticles has been demonstrated previously. "What we achieved was the systematic generation of dimers with one molecule, and the demonstration of size-dependent shaping of spectra," he told SpectroscopyNOW.

They have used pre-prepared junctions in dimeric structures consisting of silver nanoparticles of a relatively narrow size distribution, and a short organic molecule that bridges them. Because silver is relatively reactive the team synthesised their nanoparticles using gold cores to allow them to control growth rate and thus size more precisely than with silver alone.

In choosing the test molecule, the team had to ensure that it had chemical groups on each end that have a high affinity for silver allowing them to create a stable dimeric structure. The team also points out that the molecular absorption has to have a peak near the silver nanoparticle plasmon resonance. Two molecules in particular satisfy both of these conditions: rhodamine123 (Rh123), and a polythiophene T4.

"We found that the plasmon absorption spectrum of the dimer exhibits a strong red shift as the nanoparticle size increases," the researchers say, "and thereby increases the overlap between the plasmon and Raman spectra." Since SERS enhancement depends very much on this overlap, the researchers observe an increased enhancement with nanoparticle size. For the larger nanoparticles total Raman intensity increases and the lower energy Raman modes become dominant facilitating single-molecule studies.

The team has also performed electromagnetic calculations on this enhancement effect and shown that it can be explained in terms of the overlap between the plasmonic modes of the dimeric structure being investigated and the Raman spectrum itself.

"As the nanoparticle size increases, the plasmonic dipolar mode shifts to longer wavelength and thereby its overlap with the Raman spectrum changes," the team explains. "This suggests that the dimer structure can provide an external control of the emission properties of a single molecule. Indeed, clear and systematic differences are observed between Raman spectra of individual molecules adsorbed on small versus large particles."

The team points out that the phenomenon they have observed is analogous to recent work on shaping the emission spectra of fluorescent molecules using plasmonic nanoresonators. Control was achieved in that other work by varying the distance between the nanoparticles rather than particle size. The end result of both the Raman and the fluorescent approach is similar: a change in the distance or size leads to a red shift in the spectrum, which allows detail in the fluorescence an Raman spectrum to emerge that would otherwise not be seen. There is, however, one important advantage to the size-control approach, however, in that it allows manipulation of the spectrum at the single molecule level.

Previously, the researchers had demonstrated that similar dimeric structures could be conveniently used for making electrical contacts for a single organic molecule. This hints at the possibility of using the pronounced Raman signal to carry out measurements of the coupling between electrical transport and inelastic scattering by individual molecules.