Single-Pulsed SERS with Density-Based Clustering Analysis

We developed a new method for obtaining surface-enhanced Raman scattering (SERS) spectra with extremely high sensitivity and spectral resolution. In this method, thousands of SERS spectra are acquired, followed by a data selection procedure based on density-based spatial clustering of applications with noise (DBSCAN). Each spectrum is recorded by exposure to a single nanosecond laser pulse to avoid the effect of time averaging. The reconstructed spectrum consists of the data that belong to the clusters. The method was applied to a crystal violet aqueous solution with a concentration of 10–7 mol/L. The results suggest that several minor Raman peaks were successfully recovered, which cannot be detected in conventional SERS measurements. Moreover, the method is also effective for separately observing Raman peaks that overlap with other neighboring peaks. This method extends the possibilities of SERS and will contribute to future high-resolution spectroscopy in condensed phases

Quantitative Analysis of Polarization-Controlled Tip-Enhanced Raman Imaging through the Evaluation of the Tip Dipole

Polarization analysis in tip-enhanced Raman spectroscopy (TERS) is of tremendous advantage, as it allows one to study highly directional intrinsic properties of a sample at the nanoscale. However, neither evaluation nor control of the polarization properties of near-field light in TERS is as straightforward as in usual far-field illumination, because of the random metallic nanostructure attached to the tip apex. In this study, we have developed a method to successfully analyze the polarization of near-field light in TERS from the scattering pattern produced by the induced dipole in the metallic tip. Under dipole approximation, we measured the image of the dipole at a plane away from the focal plane, where the information about the direction of the dipole oscillation was intact. The direction of the dipole oscillation was determined from the defocused pattern, and then the polarization of near-field light was evaluated from the oscillation direction by calculating the intensity distribution of near-field light through Green’s function. After evaluating the polarization of some fabricated tips, we used those tips to measure TERS images from single-walled carbon nanotubes and confirmed that the contrast of the TERS image depended on the oscillation direction of the dipole, which were also found in excellent agreement with the calculated TERS images, verifying that the polarization of the near-field was quantitatively estimated by our technique. Our technique would lead to better quantitative analysis in TERS imaging with consideration of polarization impact, giving a better understanding of the behavior of nanomaterials

Indium for Deep-Ultraviolet Surface-Enhanced Resonance Raman Scattering

The dielectric constant of indium in the deep-ultraviolet (DUV) region satisfies the conditions for localized surface plasmon resonance with low absorption loss. We report that indium acts as an agent of efficient surface-enhanced resonance Raman scattering (SERRS) in the DUV. Indium-coated SERRS substrates were prepared by depositing indium on fused silica glass substrates with control of the deposition thickness to tailor the plasmon resonance in the DUV. With excitation at 266 nm, SERRS was observed from thin adenine films deposited on the indium-coated substrates, and the signal intensity was up to 11 times higher than that of a bare fused silica glass substrate. FDTD calculations showed that an enhanced electromagnetic field can be locally generated on the indium-coated substrates. Considering the volume of the enhanced field region in the excitation spot, we estimated the average enhancement factor to be 10² or higher. Our results indicate that indium is a promising and easy-to-use metal for efficiently exciting DUV-SERRS of samples containing a small number of molecules

Intermolecular Interaction between Single-Walled Carbon Nanotubes and Encapsulated Molecules Studied by Polarization Resonance Raman Microscopy

In the present study, we investigated the intermolecular interactions between single-walled carbon nanotubes (SWCNTs) and encapsulated molecules by polarization resonance Raman microscopy. C70 encapsulated in SWCNTs is investigated under incident laser polarization parallel and perpendicular to the tube axis. We employed two excitation laser wavelengths 442 and 532 nm, which are in resonance with different electronic states of C70. Under 532 nm excitation, no distinct polarization dependence is found in the Raman spectral pattern, while under 442 nm excitation, a peak not previously seen for this excitation wavelength was clearly observed for parallel excitation. This result can be explained by the modulation of the resonance Raman process via a charge transfer contribution between C70 and the SWCNTs, which is sensitive to the incident polarization as well as the excitation wavelength. The intensity of the local electronic field inside a SWCNT is higher for the parallel excitation than the perpendicular excitation when the nanotubes are in a bundle. The results can be explained by field localization effects at the nanotube walls, qualitatively supported by finite-difference time-domain simulations

Direct and Indirect Interlayer Excitons in a van der Waals Heterostructure of hBN/WS₂/MoS₂/hBN

A van der Waals (vdW) heterostructure composed of multivalley systems can show excitonic optical responses from interlayer excitons that originate from several valleys in the electronic structure. In this work, we studied photoluminescence (PL) from a vdW heterostructure, WS2/MoS2, deposited on hexagonal boron nitride (hBN) flakes. PL spectra from the fabricated heterostructures observed at room temperature show PL peaks at 1.3–1.7 eV, which are absent in the PL spectra of WS2 or MoS2 monolayers alone. The low-energy PL peaks we observed can be decomposed into three distinct peaks. Through detailed PL measurements and theoretical analysis, including PL imaging, time-resolved PL measurements, and calculation of dielectric function ε­(ω) by solving the Bethe–Salpeter equation with G0W0, we concluded that the three PL peaks originate from direct K–K interlayer excitons, indirect Q−Γ interlayer excitons, and indirect K−Γ interlayer excitons