Near-field Raman Imaging and Spectroscopy
Nanoscale Vibrational Analysis

Raman scattering cross-sections are usually extremely small, being approximately 14 orders of magnitude smaller than the typical fluorescence cross-section of single molecules. We overcome this difficulty by carefully placing a sharp metal probe in the vicinity of a laser spot in order to enhance the amplitude of the Raman scattering strength (surface enhanced Raman scattering, SERS). The signal enhancement is highly localized to the apex of the metal probe (~20nm in diameter). The metal probe is positioned into a tightly focused laser beam and the sample is raster scanned. A Raman scattering spectrum is acquired for each image pixel from which different Raman images can be extracted by integrating the intensity associated with different modes. The technique allows us to acquire multidimensional vibrational images with spatial resolution of 10-20nm.



Fig. 1: Near-field Raman image (intensity of the G band) and simultaneously
recorded topographic image of SWNTs on glass (scan area 2 x 2 um).


But not all SWNTs are wiggly



Fig. 2: Near-field Raman image recorded over the graphite-like G band (1590cm-1) for a CVD grown SWNT.


Studying phonon localization in single-walled carbon nanotubes

We use near-field Raman scattering to study the localization of vibrational modes along individual single-walled carbon nanotubes (SWNTs). The well-defined size and shape of SWNTs offers the possibility for simultaneously localizing individual nanotubes both topographically and optically using our tip-enhanced imaging technique. Probing the Raman scattering spectrum of SWNTs renders a unique chemical fingerprint from which detailed information can be extracted, i.e. tube structure (n, m) (RBM), defects (D band), metallic or semiconducting (G band & RBM).



Fig. 3: Near-field Raman image (G' band) of a spatially isolated SWNT using a sharp gold tip. Also shown is a Gaussian fit to the line section shown. The FWHM was determined to be 14nm.

The spatial resolution is limited solely by the size of the metal probe. As shown in the figure above, we achieve spatial resolutions on the order of ~ 15nm. To date our best resolution is ~ 10nm. This was achieved using a gold wire electrochemically etched to a sharp apex.


Fig. 4: Raman spectrum acquired from an individual SWNT with (green) and without (red) a metal tip present. From the increase in the Raman signal we determine the enhancement factor to be on the order of 10^6.




Fig. 5: Near-field spectral images of a CVD grown SWNT. We clearly observe a uniform spatial profile for both the RBM (260cm-1) and graphite-like G band (1587cm-1). However, we observe significant localization associated with defect-induced (1280cm-1) and IFM (835cm-1) Raman scattering at the same location along the nanotube shown.




Studying exciton localization in single-walled carbon nanotubes

In addition to studying phonon localization in carbon nanotubes we use our tip-enhanced technique to study the spatial extent of photoluminescence (PL) emission from SWNTs. Studies of SWNTs deposited directly on glass substrates reveal highly confined photoluminescence (PL) emission from short segments of about 20nm in length. For SWNTs embedded in micelles resting on MICA substrates we find that the PL emission typically extends over several hundreds of nanometers. By acquiring simultaneous near-field Raman and PL images we aim to study possible correlations between structural defects and the PL properties of individual SWNTs.



Fig. 6: (a) Near-field PL spectra acquired at the positions 1 -6 indicated in the near-field PL image of the SWNT in (b)




Localized Stress Analysis in Semiconductor Nanostructures

We are also exploring ways to characterize stress in silicon devices with nanoscale resolution. Localized stress affects device performance and also limits the lifetime of integrated circuits. Being able to image stress with nanoscale resolution is of great importance for improved circuit design and for further miniaturization of semiconductor circuits.


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