Near-field Raman Spectra Research Articles

Correlated KPFM and TERS imaging to elucidate defect-induced inhomogeneities in oxygen plasma treated 2D MoS2 nanosheets

Modulating physical and chemical properties of two-dimensional (2D) transition metal dichalcogenides (TMDCs) by defect engineering induced by oxygen plasma is actively pursued. In this work, exfoliated 2D MoS2 layers treated by medium power oxygen plasma for different times (0, 10, 20, 40, and 60 s) are investigated using Kelvin probe force microscopy and tip-enhanced Raman spectroscopy (TERS) besides micro-Raman and photoluminescence (PL) spectroscopy. Under oxygen plasma, defects (mono- and di-sulfur vacancies) and chemical oxidation are predominant from 0 (native defects) up to 40 s, while etching becomes dominant beyond 40 s for mono- (1L), bi- (2L), and tri- (3L) layer MoS2 with optimal defect density for four- (4L) and more layers. While Raman spectra exhibited lattice distortion (broadening of phonon bands) and surface oxidation by the presence of sub-stoichiometric molytrioxide MoO3 (i.e., MoO3–x or MoSxO2–x), the increased spectral weight of trions and quenching in PL spectra are observed with treatment time. The localized nanodomains (∼20–40 nm) and aggregated vacancies as nanovoids and intermixed MoS2/MoO3–x alloy are identified in near-field Raman spectra. The atomic force microscopy also showed defects aggregation, and Kelvin probe force microscopy revealed the work function (WF) increase from 4.98 to 5.56 eV, corroborating the existence of MoO3–x phase which enables doping and shift Fermi level. We also highlight the unique interaction between the gold substrate and the formed MoO3–x facilitating Mo6+ cation reduction to lower oxidation (i.e., Mo4+), thereby yielding intermediate oxidation states responsible for lower WF (ca. theoretical 6.3 eV for stoichiometric MoO3). Strong correlations among the work function and vibrational and optical responses are established while exploring the oxygen plasma-induced defects and changing the landscape on oxygen doping at the nanoscale with varying MoS2 layers, which are useful for heterogeneous electrocatalysis and applicable to other 2D-TMDCs.

High-resolution Raman imaging of bundles of single-walled carbon nanotubes by tip-enhanced Raman spectroscopy

Bundles of single-walled carbon nanotubes (SWCNTs) prepared by plasma torch method and further purified, are deposited over a glass coverslip to estimate the spatial resolution of tip-enhanced Raman spectroscopy measurements. For this purpose, near-field Raman maps and spectra of isolated bundles of carbon nanotubes are collected using optimized experimental conditions such as a tightly focused beam using a 1.4 numerical aperture oil immersion microscope objective and a gold coated atomic force microscope probe illuminated by a radially polarized 632.8 nm wavelength to selectively excite the localized surface plasmon confined at the extremity of the tip. The near-field nature of the collected Raman signals is evaluated through measuring the decay of the Raman signal with respect to the tip-sample separation.

Tip-Enhanced Raman Spectroscopy of Graphene Nanoribbons on Au(111)

We report tip-enhanced Raman spectroscopy of graphene nanoribbons (GNRs) fabricated on Au(111) by the on-surface polymerization technique under ultrahigh-vacuum conditions. The 0.74 nm wide armchair GNRs are directly observed by scanning tunneling microscopy at room temperature, and the characteristic vibration modes of GNRs appear in both the far- and near-field (tip-enhanced) Raman spectra. The Raman scattering is enhanced by up to 4 × 105 in the near-field, while a strong intensity fluctuation (blinking) frequently emerges in the time series of the near-field spectra. From the STM observation of a stable adsorption structure of GNRs under the laser illumination and statistical analysis of the intensity fluctuation, we attribute the origin predominantly to thermal fluctuations of the effective radius of the Au tip apex that induces the localized plasmonic field. The intensity distribution is qualitatively reproduced with a simple theoretical model in which the tip apex is approximated by ideal metal sphere.

Comparative Study of Far-Field and Near-Field Raman Spectra from Silicon-Based Samples and Biological Nanostructures

Raman spectroscopic characterization of biological nanostructures requires near-field techniques, which provide nanoscale resolution and high sensitivity simultaneously. Tip-enhanced Raman spectroscopy provides the required sensitivity to obtain chemical and structural information from such small structures. However, near-field spectra typically show significant intensity variations and band shifts when comparing the spectroscopic information acquired from sample positions even a few nm apart. In the present study, we compare far-field and near-field Raman spectra of silicon-based samples and biological nanostructures like avipox virus or amyloid fibrils. It is found that the width of the bands in tip-enhanced spectra is typically narrower than in the corresponding far-field spectra. Additionally, the observed spectral variations in near-field Raman spectra are strongly influenced by the structural and chemical heterogeneity of the sample.

Nano-Scale Observation Using Near-Field Raman Microscope

In order to observe the Raman scattering spectra in nano-scale region, the near-field Raman scattering system was developed by combining an atomic force microscopy with a micro-probed Raman scattering spectroscopy. The cantilever with aperture was fabricated by a focused ion beam micro-fabrication technique. The surface of the cantilever was coated by Ag in order to enhance the Raman signal owing to the surface enhanced Raman scattering. By using the cantilever, surface enhanced near-field Raman spectra of the Gd2(MoO4)3 single crystal and the thin film of tri-glycine sulphate were observed in the frequency region of internal modes.

Tip-Enhanced Raman Spectroscopy and Imaging: An Apical Illumination Geometry

Results are presented illustrating the use of tip-enhanced Raman spectroscopy (TERS) and imaging in a top-illumination geometry. A radially polarized beam is used to generate an electric field component in the direction of beam propagation, normal to the surface, resulting in a 5x increased enhancement compared to a linearly polarized beam. This multiplicative enhancement facilitates a discrimination of the near-field signal from the far-field Raman background. The top illumination configuration facilitates the application of TERS for investigating molecules on a variety of surfaces, such as Au, glass, and Si. The near-field Raman spectra of Si(100), rhodamine B, brilliant cresyl blue, and single wall carbon nanotubes are presented. Sufficient enhancement is obtained to permit a sub-diffraction-limited resolution Raman imaging of the surface distribution of large bundles of carbon nanotubes of various diameters.

Characterization of Si nanostructures using a noncontact mode scanning near-field optical Raman microscope, with 100nm spatial resolution and 5nm depth resolution, using ultraviolet resonant Raman scattering

The authors developed a noncontact mode scanning near-field Raman microscope (NC-SNORM) with a hollow pyramidal probe, using ultraviolet resonant Raman scattering, measured the stress distribution in very-large-scale integration standards made of silicon dioxide (SiO2) film and Si, and characterized Si nanodot structures. The authors found that the areas covered by SiO2 are under tensile stress and the areas not covered by SiO2 are under compressive stress. Compressive stress concentrates on the interface, about 100nm wide, between the covered and noncovered areas. They measured near-field Raman spectra of the Si nanodot structures and found that the Raman intensities change periodically approximately every 100nm.

Nanoscale Uniaxial Pressure Effect of a Carbon Nanotube Bundle on Tip-Enhanced Near-Field Raman Spectra

In situ measurement of tip-enhanced near-field Raman spectra of an isolated single-wall carbon nanotube (SWNT) bundle has been demonstrated by applying a uniaxial pressure up to approximately 2 GPa to the bundle via a metal-coated atomic force microscope tip. We investigated the pressure dependences of Raman frequencies and the intensity of the radial breathing mode bands, the D-band and the G-band, which were related to deformation of SWNTs caused by the tip pressure.

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.

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.

Tip-enhanced near-field Raman analysis of tip-pressurized adenine molecule

We report on near-field Raman spectra of a single nanocrystal of DNA-base adenine molecules using a silver-layer-coated apertureless probe tip of an atomic force microscope. The tip-enhanced near-field Raman spectrum shows eight Raman bands that are assigned to the normal modes of adenine molecules based on the density-functional theory ~DFT! calculations. The vibrational frequencies of several bands are observed to have unambiguously shifted to the values of the corresponding bands, observed using the conventional surfaceenhanced Raman-scattering spectrum of adenine molecules adsorbed on colloidal silver particles. The DFT vibrational calculations of adenine complexes involving a silver atom suggest that the Raman band shifts occur due to the deformation of adenine molecules by the silver atoms coated in the probe tip.

Near-Field Raman Spectra of Ferroelectric Single Crystals

Raman scattering spectra of ferroelectric single crystals has been observed with high spatial resolution using a near-field optical microscope. A Si cantilever with aperture of about 150 nm in diameter was used both as an illumination nano-source and a nano-detector of scattering light. The near-field Raman spectrum obtained by the subtraction of the background is in agreement with that of the micro-Raman scattering. Near the lattice defect in GMO, the sample was scanned in nano-scale and near-field Raman spectra were observed as a function of position on the surface.

Scanning Near-Field Raman Microscopy on Ferroelectrics

By combining scanning atomic force microscopy with micro-probed Raman scattering spectroscopy, we have developed a new type of scanning near-field Raman scattering system. A special designed cantilever with aperture of about 150 nm in diameter was used both as an illumination nano-source and a nano-detector of scattering light. Near-field Raman spectra of internal modes in TGS and GMO single crystals were observed at their cleavage surfaces at room temperature. The results are compared with those obtained by a conventional Raman microscope.

Application of principal component analysis to detect outliers and spectral deviations in near-field surface-enhanced Raman spectra

A recently developed technique measures near-field surface-enhanced Raman spectra with 100-nm resolution, enabling a fast survey on the sample surface. This technique has two bottlenecks. One is a general problem: signal changes are attributed to either the sample composition or the substrate morphology. Therefore, it is mandatory to detect even small signal changes in order to distinguish between these two effects. Secondly, huge data amounts make the spectrum interpretation tedious. How to find the interesting and important information? To investigate these problems, a sample, containing dye-labeled DNA-fragments that are drop-coated onto a silver island substrate, is measured. The enhanced Raman spectra yield indirect information on the DNA-fragments. The goal of this investigation is to provide a tool that allows a fast and reliable spectral analysis. Is it possible to distinguish local differences in the sample composition and to correlate them with the sample morphology? A general explorative data analyses tool, principal component analysis (PCA), is used for a first investigation. PCA has a useful side-effect: spikes, well-known artifacts, are also detected. After removing these artifacts, PCA facilitated the detection of three neighboring spectra, clearly deviating from the others. Probably, the DNA double-strand unfolded and generated a direct Raman-signal. The automated PCA-procedure gives identical results. It is concluded that a general explorative tool can solve two major difficulties. Application of dedicated chemometrical tools could improve the results. The combination of chemometrics and this new technique is powerful and promising.

The comparative study on diamond film by near-field Raman spectroscopy and micro-Raman spectroscopy

By combining a Dilor XY Raman spectrograph with a homemade scanning near-field optical microscope, we studied the near-field Raman spectrum of a diamond film prepared by hot filament chemical vapor deposition. Several quite different near-field Raman spectra were observed in different positions of a diamond film. The results were compared with micro-Raman spectra at the same positions. We found the top layer of the film gave more contribution to the near-field Raman spectra.

Near‐field Raman spectroscopy

An optical device was constructed to collect near-field Raman spectra (NFRS) and far-field scattering Raman spectra simultaneously on the same sasample area. In Raman applications where the studied samples do not enjoy any advantages of resonance effects, the nanoprobe may be used only as a nanodetector in order to avoid damage of the tip by the necessary laser powers. The silicon-based samples, which are either calibrated grid or a single crystal, were studied by NFRS and conventional scattering Raman spectroscopy. The near-field Raman signal of silicon is divided by at least four when the tip–silicon substrate distance is separated by a 100 nm layer of oxide. Moreover, between the NFR spectrum and the far-field spectrum, on the single crystal, a shift of about 10 cm−1 is displayed. Some interpretations are discussed. These experiments confirm the potential of the NFRS already emphasized previously in the literature. Copyright © 1999 John Wiley & Sons, Ltd.

Near-Field Raman Spectral Measurement of Polydiacetylene

Two-dimensional near-field Raman spectra and topographic images of the polydiacetylene surface are measured simultaneously by using a near-field Raman spectrometer. The spectra (located 100 nm apart) have different spectral features. The peak intensity ratio of two C=C peaks at 1520 and 1457 cm−1 does not correlate with the topographic image and shows differences in the subwavelength scale. These differences can be interpreted as spatial differences in the number of successive bonds on the polydiacetylene surface. In contrast, the near-field Raman intensity of the C=C bond at 1457 cm−1 correlates strongly with the topographic image. This phenomenon can be interpreted as a change in the efficiency of collecting Raman scattering light.

Near-Field Surface-Enhanced Raman Spectroscopy on Single Silver Nanoparticles

We report the development of a near-field Raman microscope for nanometer-scale vibrational spectroscopy at room temperature. With the use of surface-enhanced Raman scattering and tapered single-mode fiber probes, near-field Raman spectra have been obtained from single Ag colloidal nanoparticles with excitation intensities as low as 10 nW. Far-field confocal studies have also been carried out on spatially isolated, single Ag nanoparticles. A direct comparison of the near-field and confocal results shows that near-field optical excitation does not lead to significantly different selection rules in Raman spectroscopy. This work further demonstrates that single metallic nanoparticles can be attached to the tip of a near-field probe for potential application in surface-enhanced optical microscopy and nanofabrication.

Development of a scanning near-field optical probe for localised Raman spectroscopy

The scanning near-field optical probe technique has made it possible to perform conventional optical spectroscopies with unprecedented spatial resolution. We present the development of an instrument for scanning near-field Raman spectroscopy and present near-field Raman linescans and spectra of a range of materials with subwavelength resolution.