Near-field Raman Spectroscopy Research Articles

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.

Light depolarization induced by metallic tips in apertureless near-field optical microscopyand tip-enhanced Raman spectroscopy

We have investigated the depolarization effects of light scattered by sharp tips used forapertureless near-field optical microscopy. Dielectric and metal coated tips have beeninvestigated and depolarization factors between 5 and 30% have been measured, changingas a function of the incident light polarization and of the tip shape. The experimentalresults are in good agreement with theoretical calculations performed by the finite elementmethod, giving a near-field depolarization factor close to 10%. The effect of depolarizationhas been investigated in polarized tip-enhanced Raman spectroscopy (TERS)experiments; the depolarization gives rise to forbidden Raman modes in Si crystals.

Subwavelength-resolution near-field Raman spectroscopy

The resolution capabilities of near-field Raman spectroscopy based on a giant enhancement of the electric field near a nanosized metal probe are studied. As a test sample, bundles of single-walled carbon nanotubes deposited on glass substrates are used. It is shown that this method ensures a subwavelength spatial resolution of about 50 nm and demonstrates a Raman scattering enhancement of the order of 104.

Investigation of Apertureless NSOM for Measurement of Stress in Strained Silicon

Strain in blanket and patterned silicon-on-insulator (SOI) structures have been investigated via apertureless near-field scanning Raman spectroscopy, specifically, to investigate the efficacy of so-called tip-enhanced Raman scattering (TERS) for Si strain characterization and metrology for integrated circuit (IC) devices. The current study compares TERS generation using Ag-coated W tips in a 45o incident beam, fixed-tip geometry and Ag-coated SiO2 capillary tips in a normal incident, scanning-tip geometry. The latter demonstrates superior performance and is used in a differential scheme to investigate strain profiles in patterned SOI structures. Specifically, a blanket device layer with an engineered strain of 0.075%, is patterned to form arrays of isolated mesa structures (2 μm diameter). These mesa structures exhibit a stress relaxation of 117 MPa over a region extending ~ 200 nm from the island edge.

Chirality Changes in Carbon Nanotubes Studied with Near-Field Raman Spectroscopy

We report on the direct visualization of chirality changes in carbon nanotubes by mapping local changes in resonant RBM phonon frequencies with an optical resolution of 40 nm using near-field Raman spectroscopy. We observe the transition from semiconducting-to-metal and metal-to-metal chiralities at the single nanotube level. Our experimental findings, based on detecting changes in resonant RBM frequencies, are complemented by measuring changes in the G-band frequency and line shape. In addition, we observe increased Raman scattering due to local defects associated with the structural transition. From our results, we determine the spatial extent of the transition region to be Ltrans approximately 40-100 nm.

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.

Investigation of the liquid–liquid interface with high spatial resolution using near-field Raman spectroscopy

Local spectroscopic investigation of liquid–liquid interfaces and their surroundings is experimentally challenging. Highly localized illumination by a near-field optical probe is a possibility, but precise positioning of the probe very close to such an interface prior to measurement has been very difficult. Here, we show how this difficulty is overcome with greatly improved instrumentation. Near-field Raman spectroscopy was used to study the water–carbon tetrachloride interface with a spatial resolution of 200 nm. A blue shift of the OH stretching vibration of water was observed when approaching the interface, indicative a weaker H-bond in the vicinity of the apolar CCl 4.

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.

Nanoscale Vibrational Analysis of Single-Walled Carbon Nanotubes

We use near-field Raman imaging and spectroscopy to study localized vibrational modes along individual, single-walled carbon nanotubes (SWNTs) with a spatial resolution of 10-20 nm. Our approach relies on the enhanced field near a laser-irradiated gold tip which acts as the Raman excitation source. We find that for arc-discharge SWNTs, both the radial breathing mode (RBM) and intermediate frequency mode (IFM) are highly localized. We attribute such localization to local changes in the tube structure (n, m). In comparison, we observe no such localization of the Raman active modes in SWNTs grown by chemical vapor deposition (CVD). The direct comparison between arc-discharge and CVD-grown tubes allows us to rule out any artifacts induced by the supporting substrate.

Nano-Raman spectroscopy with side-illumination optics

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 103–104 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. Copyright © 2005 John Wiley & Sons, Ltd.

NEAR-FIELD RAMAN SPECTROSCOPY OF INDIVIDUAL SINGLE-WALLED CARBON NANOTUBES

Near-field Raman spectroscopy with high spatial resolution is used to study single-walled carbon nanotubes (SWNTs) within dense samples. A sharp, laser-irradiated metal tip acts as a near-field excitation source causing an enhanced Raman signal within close proximity of the tip. We present optical images of different Raman modes with a spatial resolution better than 15 nm. Local spectroscopy allows us to identify and distinguish tubes with different structures on the nanometer-scale and to observe variations within the spectrum of individual tubes.

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.

Near-field Raman spectroscopy using optical fiber and silicon cantilever

Near-field Raman spectroscopy using optical fiber and silicon cantilever

A near-field optical method for probing liquid–liquid interfaces

A high resolution method to study the interface between two immiscible liquids is presented. The technique is based on the combination of a near-field optical probe and Raman spectroscopy. This combination brings together high local resolution in tip direction and the capability for molecular analysis at the interface. First trans-phase boundary Raman profiles on the p-xylene/ethylene glycol system measured with a spatial resolution of 1 μm are presented. Changes in the Raman bands due to the influence of the phase boundary extend as far as 10 μm from the phase boundary itself. Studies with coated and uncoated near-field probes demonstrate the important role of the optical near field in the experiments.

Near-field Raman Spectroscopy using the Local Field-Enhancement Technique

Near-field Raman Spectroscopy using the Local Field-Enhancement Technique

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.

Optimization of SERS-active substrates for near-field Raman spectroscopy

As a first step towards near-field Raman, we chose to study surface enhanced Raman scattering (SERS)-active substrates to cope with the weakness of Raman scattering (small cross-section and low concentration). We concentrated our work on localized surface plasmon (LSP) since they turned out to play a great part in SERS and we put forward the relation between LSP resonance and Raman enhancement. Roughness of our samples is controlled either by annealing process or electron-beam lithography (EBL); this latter technique proved to best suit to our study. Substrates are characterized by extinction spectroscopy which determines the LSP resonance and then Raman spectrum of a probe molecule, trans-1,2-bis(4-pyridyl)ethylene (BPE) is recorded. We show that maximum of enhancement is obtained when the LSP resonance is red-shifted (50 nm) compared to the excitation laser line (632.8 nm).

Large-area topography analysis and near-field Raman spectroscopy using bent fibre probes.

We present a method for combined far-field Raman imaging, topography analysis and near-field spectroscopy. Surface-enhanced Raman spectra of Rhodamine 6G (R6G) deposited on silver nanoparticles were recorded using a bent fibre aperture-type near-field scanning optical microscope (NSOM) operated in illumination mode. Special measures were taken to enable optical normal-force detection for control of the tip-sample distance. Comparisons between far-field Raman images of R6G-covered Ag particle aggregates with topographic images recorded using atomic force microscopy (AFM) indicate saturation effects due to resonance excitation.