Nano-Raman spectroscopy with side-illumination optics

Abstract

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.

High contrast scanning nano‐Raman spectroscopy of silicon

“…The penetration depth of the 514.5 nm laser light in silicon is 27 approximately 0.68 µm, and using this depth V far is estimated as ¾2 µm 3 . The localization of the tipenhanced signal in depth has been estimated in our previous publication 22 For practical nano-Raman analysis of Si samples, a contrast of 0.5 may not be enough. In order to increase the contrast, we have used the approach proposed by Poborchii et al 21 to minimize the contribution of far-field Raman intensity to the contact signal by suppressing the far-field signal more than near-field signal.…”

“…The details of tip preparation are described in a previous publication. 22 Silver-coated tips were used for scanning and imaging measurements and gold-coated tips were used for the other measurements. The enhanced signal was measured in contact mode in AFM with a normal force below 2 nN.…”

Performing tip‐enhanced Raman spectroscopy in liquids

“…Second, EtS has a length of only ∼4.5 Å, far below the extent of the enhanced field, which is known to extend 10-30 nm from the tip into the surrounding medium. [44,45] Finiteelement simulations predict an e −z/R dependence of the field enhancement of a tip with radius R over the distance z from its surface. [46] The extension of the enhanced field is on the order of the tip radius and can thus be even smaller than 10 nm for very sharp tips.…”

Imaging and spectroscopy through plasmonic nano-probe

“…However, transmission-mode ANSOM is not applicable to opaque or thick samples, as the incident light has to pass through the sample. In a reflection-mode, the tip is illuminated directly from the same side of the tip [27][28][29][30][31], which is advantageous and promising for observing opaque samples. In addition, an important requirement for attaining efficient enhancement under the tip is the polarization of the electric field of the incident light along the tip axis (see Sect.…”

“…They reported a one dimensional TERS scanning of a silicon grating. In 2005, Mehtani et al constructed a side illumination system [29]. This nearly horizontal confocal microscope used a long working distance (LWD) objective (×50, 0.42NA, 20.5 mm WD) to facilitate the optical coupling of the Raman spectrometer with AFM stage.…”