Taka-aki Yano

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.

Diameter-selective near-field Raman analysis and imaging of isolated carbon nanotube bundles

Tip-enhanced near-field Raman scattering has been utilized to demonstrate the measurement of the distribution of single-walled carbon nanotubes (SWNTs) with a spatial resolution far beyond the diffraction limit of the probing light. This was done by measuring the radial breathing mode (RBM) of SWNTs in the near-field Raman spectra, which corresponded to the diameters of various SWNTs in the immediate vicinity of the tip. Further, near-field Raman imaging of the RBM provided a super-resolved color mapping corresponding to the diameter distribution of SWNTs within a bundle, which is not possible to realize by conventional topographic imaging methods.

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.

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 raman spectroscopic measurements in the sub-nanometric vicinity of a metallic tip

By developing a new time-gated technique, we demonstarte sub-nanometric control over the tip-sample distance in tip-enhanced Raman spectrscopy, which allows us to precisely control the chemical and mechanical effect and to realize super-high resolution imaging.

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.

Plasmonic color nano-imaging of strain distribution in nanomaterials

We demonstrate plasmonic nano-imaging of color-coded strain distribution in nanomaterials. Tip-enhanced Raman scattering microscopy is utilized to image and characterize locally-distributed strain at a spatial resolution of ~20 nm.

TERS in the Sub‐Nanometric Vicinity of a Metallic Tip

The dominant mechanism for the tip-sample interaction in TERS experiment is electromagnetic (EM), with interaction length of the order of a few tens of nanometers. However, due to the presence of metallic layer on the tip, chemical interaction between the sample and the metal on the tip starts to become significant, if the sample is very close to the tip, particularly at molecular distances [1]. Further, if the distance between the sample and the tip is reduced so much that the tip comes in physical contact with the sample, then mechanical interaction also starts to play an important role [2]. In general, the three interactions are simultaneously present in a TERS experiment. However, since they have slightly different interaction ranges, it could be possible to study them individually, if the distance between the tip and the sample in a TERS experiment is precisely regulated with accuracies of the order of sub-nanometer scale. Here, we demonstrate how we can effectively control the tip-sample distance in TERS experiments with extremely high accuracy, even though the AFM tip in our experiment oscillates in tapping mode operation [3]. The chemical as well as the mechanical interaction distinctly shows up in TERS spectra, when the tip is maintained very close to, or pressed against the sample. We achieve this result by time-gating the illumination, so that the sample is selectively illuminated only when the tip is at a certain distance from the sample during the repeated cycles of its tapping-mode oscillation. The measurement precision that we have achieved in this technique is of sub-nanometer order. With this system, we studied two different samples, an isolated single walled carbon nanotube (SWCNT) and an adenine nanocrystal, which were measured with precise control over the tip-sample distance. As the distance between the tip and the sample was decreased, we clearly observed the signatures of EM interaction, chemical interaction and mechanical interaction in different ranges. A precise control over the tip-sample distance does not only provide the subnanometric information in the vertical direction, but can also provide nanometric resolution in the lateral direction. In order to justify our claim, we demonstrated that this technique could provide extremely high spatial resolution in TERS imaging. We scanned the tip under a constant pressure across an isolated SWCNT, and performed a one-dimensional imaging of the sample. An extremely high spatial resolution of about 3 nm was obtained.

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.

Tip-enhanced near-field Raman scattering and imaging of carbon nanostructures

Near-field Raman scattering has been successfully utilized to study the interaction between a metal-coated nano-tip and carbon nanostructures, such as carbon-60 molecules and single walled carbon nanotubes. The enhanced and localized light field in the vicinity of the tip apex provides high resolution imaging as well as the detection of weak vibrational modes. Apart from the electromagnetic and chemical interactions, a mechanical interaction between tip molecules and the sample molecules has also been investigated.