We describe an apertureless near-field Raman spectroscopy setup that has successfully produced substantial enhancements for a wide variety of samples and achieved a high contrast. The tremendous potential of tip-enhanced Raman spectroscopy (TERS) for nanoscale chemical characterization has been demonstrated by various groups by measuring organic dyes, biological molecules, single-walled carbon nanotubes and silicon. Keys to rapid advances in the application of TERS to pressing scientific problems include the optimization of the method to achieve greater reproducibility and greater enhancement factors if possible, but more importantly, greater imaging contrast. Using a side-illumination geometry, we demonstrate reproducible enhancements of the Raman signal per volume on the order of 10 3 -10 4 using silver-and gold-coated tips on various molecular, polymeric and semiconducting materials as well as on carbon nanotubes. We have experimentally verified localization of the enhancement to a depth of ∼20 nm. Most importantly, optimization of the polarization geometry makes possible a contrast between the near-field and far-field signals of 900% in the case of silicon -a level that makes the technique attractive for various applications.

We have demonstrated that scanning nano-Raman spectroscopy (SNRS), generally known as tip-enhanced Raman spectroscopy (TERS), with side illumination optics can be effectively used for analysis of siliconbased structures at the nanoscale. Even though the side illumination optics has disadvantages such as difficulties in optical alignment and shadowing by the tip, it has the critical advantage that it may be used for the analysis of nontransparent samples. A key criterion for making SNRS effective for imaging Si samples is the optimization of the contrast between near-field and far-field (background) Raman signals. This has been achieved by optimizing the beam polarization, resulting in an order of magnitude improvement in the contrast. We estimate the lateral resolution of our Raman images to be ∼ 20 nm.

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