Taro Ichimura

Confinement of enhanced field investigated by tip-sample gap regulation in tapping-mode tip-enhanced Raman microscopy

The authors developed a tip-enhanced near field Raman microscope that can precisely regulate longitudinal distance between a metallic tip and sample molecules. This was done by employing a time-gated photoncounting scheme that enabled us to observe exponentially decaying near field Raman intensity with the tip-sample distance. The exponential decay shows a characteristic of the enhanced field generated by the localization of the surface plasmon polaritons near the tip apex. This microscope was applied to evaluate metal-coated tips and also to investigate confinement of the field generated at a gap between two metal nanostructures from the decay curves.

Nanoanalysis of crystalline properties of GaN thin film using tip-enhanced Raman spectroscopy

Nanoscaled variation in crystalline properties was demonstrated through tip-enhanced Raman scattering (TERS) for a thin GaN sample, which apparently showed good and uniform crystalline properties at microscale in conventional micro-Raman scattering. The observation was attributed to the field enhancement and the super-resolution associated with the near-field technique. The enhancement factor of the TERS intensity was assessed to be larger than 2.8×104. Variations in crystalline properties were observed at spatial resolution beyond the diffraction limit of the probing light. The authors propose TERS to be an efficient technique for nanoscale characterization of crystalline properties for the nitride-based semiconductor devices.

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.

Direct Evidence of Chemical Contribution to Surface-enhanced Hyper-Raman Scattering

We report on halide-ion-assisted chemical effect in surface-enhanced hyper-Raman scattering of crystal violet (CV) adsorbed on single silver aggregate. A dramatic increase in spectral intensity was observed in the presence of halide ions as compared to their absence. By measuring the hyper-Rayleigh scattering from single aggregate treated with and without halide ions, we established that it was chemical effect, rather than electromagnetic effect that was responsible for this strong enhancement. We attribute the enhancement to a charge transfer mechanism between CV and metal surface mediated by the halide ions similar to surface-enhanced Raman scattering.

TIP-ENHANCED NEAR-FIELD CARS MICROSCOPY

We apply the field enhancement effect due to plasmon polariton excitation on a metallic nanostructure in order to improve the diffraction limited spatial resolution of coherent anti-Stokes Raman scattering (CARS) microscopy. A cantilever probe tip coated with a 25 nm-thick gold film is utilized as a near-field light source to locally excite the CARS polarizations near the tip. Our CARS microscope has effectively enhanced the CARS signals and realized vibrational imaging of single-wall carbon nanotubes (SWNTs) beyond the spatial resolution of far-field CARS microscopy.

Tip-enhanced near-field CARS microscopy for molecular nano-imaging

Optical microscopy that can visualize the molecular vibration with a nanometric spatial resolution has been realized by a combination of near-field optics and coherent anti-Stokes Raman scattering (CARS) spectroscopy. A metallic probe with a sharp tip is used to strongly enhance optical near-field in the local vicinity of the tip owing to the excitation of local surface plasmon polariton. CARS signals of molecules in the local area can be strongly induced by the plasmonic field. We have visualized DNA molecules and single-walled carbon nanotubes (SWNTs) with a spatial resolution far beyond the diffraction limit by the tip-enhanced near-field CARS microscopy.

Experimental Identification of Chemical Effects in Surface Enhanced Raman Scattering of 4-Aminothiophenol †

We report on the experimental identification of Raman modes that are enhanced through the chemical effect in surface enhanced Raman spectroscopy of 4-aminothiophenol (also known as p-mercaptoaniline) adsorbed on gold substrate. Introduction of a thin spacer layer between the metal and the sample can prevent any possible chemical bonding between metal atoms and sample molecules, hence such a sample shows only those Raman modes that are enhanced through the electromagnetic effect. Alternatively, a significant increase in the chemical effect could be observed in the presence of halide ions as compared to their absence. This result provides another way to experimentally identify those Raman modes that undergo chemical enhancement. In addition, apart from the electromagnetic-based resonance in SERS, chemical enhancement also shows a resonance with varying wavelength of the excitation light, which provides yet another way to experimentally identify chemically enhanced Raman modes in SERS. Some new chemically en...

Tip-enhanced NSOM

A light microscope capable to show images of molecules in nanometer scale has been a dream of scientists, which, however, is difficult due to the strict limitation of spatial resolution due to the wave nature of light. While there have been attempts to overcome the diffraction limit by using nonlinear response of materials, near-field optical microscopy could provide better detecting accuracy. In this paper, we present molecular distribution nano-imaging colored by Raman-scattering spectral shifting, which is probed with a metallic tip. The metallic probe tip has been used to enhance the optical field only in the vicinity of probe tip. The effect is similar to the one seen in the detection of molecules on the metal-island film, known as surface-enhanced Raman spectroscopy (SERS), while in this case a single metallic tip works for the field enhancement in nanometer scale.

Toward single molecule detection through tip-enhanced near-field Raman spectroscopy

When Raman scattering is excited from the evanescent light field created by illuminating the apex of a sharp metallic nano-tip, it achieves new aspects with strong enhancement of scattering efficiencies and super resolving capabilities. The primary mechanism of tip-sample interaction is electromagnetic, which is based on the excitation of localized surface plasmon polaritons. However, when the tip is close enough to the sample, typically at molecular distances, the chemical interactions between the tip and the sample become important. Strong temporal fluctuations of Raman scattering, including fluctuations of peak frequencies and peak intensities, together with extraordinary enhancement of several peaks, were observed. These temporal fluctuations, which are typical signature of single molecule detection, were attributed to the changes of molecular orientations of the sample molecules in the upper layer of the nanocluster, which got chemically adsorbed at the tip molecules.

Near-field effects in tip-enhanced Raman scattering

Publisher Summary The chapter analyzes near-field effects in tip-enhanced Raman scattering (TERS). TERS is based on the same phenomenon as surface-enhanced Raman scattering (SERS), which occurs on island structures consisting of metallic nanoparticles. TERS occurs on a single metallic nanotip that specifies position for observation and analysis with nanometric accuracy, while SERS is induced on plenty of metallic nanostructures. The SERS spectra are measured when C60 molecules are pressurized by an uncoated silicon tip and then, the gap-mode spectra is measured when C60 molecules are pressurized by a silver-coated silicon tip. The tip-force effect is analyzed by measuring the tip-enhanced gap-mode Raman spectra from uniaxially pressurized C60 molecules. The tip-enhanced near-field microscopy also gets benefited from nonlinear effects. The volume of the light-matter interaction can be further confined to a tiny volume at the tip end because of the nonlinearity. The tip-enhanced coherent anti-Stokes Raman scattering is also elaborated in the chapter.