Raman spectroscopy of single molecules and nanostructures
The rich structural information provided by the vibrational Raman response, compared to molecular fluorescence, makes Raman spectroscopy highly desriable for nanoscale analysis. In this study we showed that the simple geometry of the sharp metallic tip located above molecules adsorbed on planar metal surfaces result in sufficient field enhancement to allow for single molecule Raman spectroscopy with the additional advantage provided by the scanning probe microscope:
The following figure shows the tip-scattered Raman signal during tip approach of the planar gold surface covered with ∼ 1 ML of malachite green. The enhancement is confined to a tip-sample spacing of just several nanometers and correlated with the apex radius of the tip:
The characteristic differences between the tip-scattered and -enhanced Raman response and and the far-field spectrum is the result of the strong optical field localization, and related to different selection rules due to the field-gradient.
In our experiment we expect the enhancement to be purely electromagnetic in origin. From a direct comparision of the tip-scattered response with the far-field signal of the same surface molecules we can estimate a Raman enhancement of up to 5Ã—109 compared to the free molecule response in our experiment. Together with the sensitivity of our detection system this suffices for probing the single molecule Raman response:
The coupling of the axial plasmon resonance of the tip to the underlying substrate accross the nanometer tip-sample spacing gives rise to the strong lateral field confinement and related to a local field enhancement about one order of mangitude larger compared to the corresponding values for free standing tip:
This tip-sample coupling provides the necessary local field enhancement of order 70 - 130 deduced from the Raman experiment and necessary to observe a single molecule response.
With the demonstrated potential of TERS to probe molecules at the single emitter level, it is highly desirable to extend the technique to probe other classes of materials such as crystalline nanostructures. By taking advantage of the intrinsic symmetry selectivity of the Raman response in combination with the polarization-selective enhancement of the tip we are applying TERS to identify the crystallographic orientation and -domains in nanostructures.
In Raman scattering, the scattering response is given by I = |es R ei|2, where R is the Raman tensor and es and ei are the scattered and incident light polarizations, respectively. Each normal mode oscillation (phonon in crystalline structures) will have a corresponding Raman tensor, generally containing few nonzero elements. By appropriate selection of the incident and scattered polarization directions, specific phonon modes can be isolated and studied. Conversely, with a priori knowledge of the phonon mode frequencies and Raman tensors, we can determine the crystallographic orientation of a nanostructure.