Tip-enhanced Near-field Raman Spectroscopy Research Articles

The controlled fabrication of “Tip-On-Tip” TERS probes

Tip-enhanced near-field Raman spectroscopy has exhibited a great ability to detect in situ chemical and structural information on a sample surface with the highest lateral resolution combined with a high sensitivity at the same time. A key challenge in the TERS field is the development and improvement of the fabrication of reproducible metal tip geometries with a sharper apex. In this study, one novel apertureless “Tip-On-Tip” TERS probe with a special plasmonic nanostructure is designed, and a simple Ar+-ion sputtering route has been developed to fabricate silver nanoneedle arrays on the scanning probe microscopy (SPM) cantilevers for tip-enhanced Raman spectroscopy (TERS) probes. These silver nanoneedles possess a very sharp apex with an apex diameter of 15 nm and an apex angle of 20°. The novel TERS probes exhibit enhanced near-field Raman signals compared to an Ag-coated SPM probe, which are attributed to the intense electromagnetic field around the apexes of the Ag nanoneedles and the periodic structure of the Ag nanoneedles.

Nano‐scale analysis of graphene layers by tip‐enhanced near‐field Raman spectroscopy

AbstractWe demonstrate nano‐scale optical analysis of graphene layers by tip‐enhanced near‐field Raman spectroscopy (TERS). In this technique, the spatial resolution ∼30 nm is realized by the near‐field probe which acts as a nano‐light source. From the intensity change of the Raman band of silicon generated from the near‐field probe, we can conveniently estimate the edge boundaries and the number of stacking layers. TERS measurement across the layer edges reveals the nano‐scale properties of the material as well as the existence of local defects and edge boundaries. The intensity change of the G‐band shows the step‐like behavior that follows the layer boundary, whereas the two components in 2D peak show more complex behaviors even inside layers. The peak fluctuation in the 2D band also suggests the local stress distribution due to interlayer interactions. An excess charge effect is observed through the correlation between the peak position and the width of the G‐band and their nano‐scale distribution within a layer is revealed. Besides the vibrational analysis, we successfully performed the estimation of the number of layers in two‐dimensional imaging by the same experimental platform, which allows us high‐throughput nondestructive identification of graphene layers critical for the evaluation of this material especially in future device applications. Copyright © 2009 John Wiley & Sons, Ltd.

Tip-enhanced near-field Raman spectroscopy with a scanning tunneling microscope and side-illumination optics

Conventional Raman spectroscopy (RS) suffers from low spatial resolution and low detection sensitivity due to the optical diffraction limit and small interaction cross sections. It has been reported that a highly localized and significantly enhanced electromagnetic field could be generated in the proximity of a metallic tip illuminated by a laser beam. In this study, a tip-enhanced RS system was developed to both improve the resolution and enhance the detection sensitivity using the tip-enhanced near-field effects. This instrument, by combining RS with a scanning tunneling microscope and side-illumination optics, demonstrated significant enhancement on both optical sensitivity and spatial resolution using either silver (Ag)-coated tungsten (W) tips or gold (Au) tips. The sensitivity improvement was verified by observing the enhancement effects on silicon (Si) substrates. Lateral resolution was verified to be below 100 nm by mapping Ag nanostructures. By deploying the depolarization technique, an apparent enhancement of 175% on Si substrates was achieved. Furthermore, the developed instrument features fast and reliable optical alignment, versatile sample adaptability, and effective suppression of far-field signals.

Towards atomic site-selective sensitivity in tip-enhanced Raman spectroscopy

Depending on each nitrogen atom of adenine molecule to which a silver atom of a metallic tip approaches, tip-enhanced near-field Raman spectroscopy may show a potential to achieve atomic site-selective detection sensitivity. Molecular vibrational calculations show that silver atoms and adenine molecule create several isomers generating specific vibrational modes of each isomer that are shifted or not observable in isolated adenine molecule itself. Here, the authors observe the specific vibrational modes and spectral shifts of isomers experimentally and are in good agreement with their calculations.

Optical Nano-Imaging of Materials: Peeping Through Tip-Enhanced Raman Scattering

High-resolution optical nano-imaging, with a spatial resolution far beyond the diffraction limits of the probing light, has been shown by means of tip-enhanced near-field Raman spectroscopy. The capabilities of imaging the physical and chemical properties of materials have been demonstrated by imaging the diameter distribution of the nanotubes within an isolated bundle of single wall carbon nanotube. Further, by combining the nonlinear effects in the scattering process, it has been shown that the tip-enhanced coherent anti-Stokes Raman scattering imaging of DNA networks is capable of even higher spatial resolution and contrast.

Tip-enhanced near-field Raman spectroscopy probing single dye-sensitized TiO2 nanoparticles

The correlated metallic tip-enhanced Raman spectroscopy and atomic force microscopy (AFM) technique was used to characterize dye-sensitized titanium oxide (TiO2) nanoparticles. We have obtained the near-field Raman spectra that are associated with the photo-induced charge transfer reaction in Ru(4,4’-dicarboxy-2,2’-bipyridine)2(NCS)2-sensitized TiO2 single nanoparticles. This method demonstrates that tip-enhanced near-field Raman spectroscopy is an effective approach for understanding inhomogeneous interfacial electron transfers with nanoscale spatial resolution.

Fabrication of Silver Probes for Localized Plasmon Excitation in Near-field Raman Spectroscopy

Abstract We present a new method for preparing a silver probe for tip-enhanced near-field Raman spectroscopy. In this method, the probe is coated with silver colloids by a simple chemical reaction known as a mirror reaction. Silver particles ≈40 nm in diameter, which is ideal for localized surface plasmon excitation in the visible wavelength range, are formed at the tip apex with high reproducibility. Futhermore, tip-enhanced Raman spectra of single-walled carbon nanotubes were measured by using the probe.

Near-field optics and spectroscopy for molecular nano-imaging.

Application of near-field optical microscopy with a sharp metallic probe to Raman spectroscopy brings microanalysis of materials to their nano-identification and imaging. The local plasmon polariton excitation on the probe tip results in the localization and amplification of the optical field at the vicinity of the tip. The tip-enhanced near-field Raman spectroscopy has analyzed DNA base molecules and single-walled carbon nanotubes (SWNTs) with the nanometric spatial resolution and sufficient sensitivity. Combined with tip pressurization and nonlinear effects, the tip-enhanced near-field Raman spectroscopy gives additional spectral information or improves the spatial resolution and sensitivity. This article introduces the recent progresses on the tip-enhanced near-field Raman spectroscopy and imaging.

Detection of an individual single-wall carbon nanotube by tip-enhanced near-field Raman spectroscopy

A tip-enhanced near-field Raman microscope has been applied to the detection of an individual single-wall carbon nanotube (SWNT). The detected information is a color (Raman-shift) image of molecular distribution without having to resort to staining non-fluorescent molecules of interest. In addition to nanometric-sensing and -imaging capability, local field-enhancement of the metallic tip has been utilized to detect a weak Raman scattering from nanometer region, which cannot be observed by conventional micro-Raman configuration (far-field Raman). The experimental results are shown with analysis of distinct vibration modes of both radial breathing mode and G-band.