Surface enhanced Raman scattering (SERS) signal fluctuate due to position between near field and adsorbed molecules. According to past works, the frequency of the bright period follows a power law. The purpose of this study is to analyze the blinking behavior of SERS spectra by Density-Based Spatial Clustering Analysis with Noise (DBSCAN) with temporal resolution higher than a conventional spectroscopic method. As a result, it was confirmed that the SERS blinking actually follows the power law even under adverse conditions such as extremely low sample concentration and a very short exposure time of 10 ms.
Plasmonic phenomena have greatly contributed to nanooptics and nanophotonics owing to their features such as light localization and high sensitivity to the surrounding environment. The nanoparticles of poor metals (e.g. Al) exhibit plasmonic properties in the UV range (240-350 nm) where many organic molecules and semiconductors absorb light, which was recently confirmed and utilized in enhanced Raman spectroscopy and UV photocatalysis. The present study demonstrates the efficient TiO2 photocatalysis with indium nanostructures resonant in the near-UV range. Indium (In) nanograins were densely distributed on a TiO2 thin film, where methylene blue (MB) was applied to test the photocatalytic activity. The photocatalytic reaction was initiated by irradiating the samples with UV light, and the time-dependent decay of the MB absorbance was observed. A reaction rate was found to increase by factors as high as 7 while the enhancement of photocatalysis shows particle size dependence. The increase and downward trend in the enhancement shows a good agreement with that in the field intensity simulated by the discrete dipole approximation (DDA). Simulation results also suggest that the largest enhancement of photocatalysis be obtained with In nanograins whose resonance is close to the bandgap of TiO2. It is expected that the light at the absorption edge wavelength confined at plasmonic nanostructures effectively for the photocatalytic reaction.
Wide-band gap semiconductor nanoparticles has been the focus of interest recently, due to their validity for energy creations, decomposition of harmful substances, boosting useful chemical reactions etc. In this work, we will evaluate optical characteristics of a single semiconductor nanoparticle via broadband-UV Rayleigh scattering spectroscopy and photoluminescence (PL) spectroscopy. Rayleigh scattering spectroscopy reveal the bandgap energies while PL spectroscopy provide the information on exciton generation efficiencies as well as existence of surface defects.
In our microscopy setup, a broadband white light source (LDLS) was collimated and obliquely illuminated on the sample to realize dark-field illumination to distinguish the position of individual particles in the microscopic image. Scattering from a single nanocrystal was collected by an reflection-type objective lens (NA0.5) and introduced to a spectrometer and detected by an EMCCD camera. The spectrometer was designed specifically for UV-DUV broadband spectroscopy and imaging. For photoluminescence (PL) measurements, we introduce 320 nm (CW) laser for excitation. The sample is enclosed in a temperature-controlled cell ranging from room temperature to 77K.
We especially focus on titanium dioxide (TiO2), a typical photocatalyst, and tangusten trioxide (WO3) which is one of the candidate for decomposition of water into oxygen and hydrogen by a visible or longer wavelength light. The band structure of nano-particles is changed when the size is smaller than several tens of nanometers, due to crystallinity and quantum size effects. PL of single zinc oxide (ZnO) nanoparticles were also measured together with the temperature effects. The spectra obtained from a single nanoparticle is different from aggregates both for exciton PL and defects PL.
KEYWORDS: Plasmons, Silver, Silicon, Coating, Near field optics, Near field scanning optical microscopy, Polarization, Nanoimaging, Near field, Laser scattering
Plasmon nanofocusing, a phenomenon where plasmons propagate on a tapered metallic structure with compressing its energy into a nanometric volume of the apex to generate localized electric field, holds a great promise for near-field optical imaging techniques due to its background-free nature. Because it does not require to illuminate the tip apex with an incident laser, one can efficiently eliminate scattering background noise by the incident laser, which has been an issue in conventional near-field optical microscopies. To apply plasmon nanofocusing for near-field optical imaging, a tapered metallic tip plays an important role as a base material for plasmon propagation. It is therefore essential to establish an efficient and practical methods of the metallic tip fabrication for plasmon-nanofocusing-based optical imaging techniques. In this study, we propose an optimized tip fabrication for efficient plasmon nanofocusing, which achieved 100% reproducibility in plasmon nanofocusing. Through numerical analysis, we have optimized the tip structure, such as types of material, metal thickness, plasmon coupler structure, etc. Also, the fabrication conditions were well-optimized to obtain smooth metal surface down to 0.5 nm roughness to reduce energy loss of plasmon propagation. Through thorough optimizations, we observed plasmon nanofocusing with 100% reproducibility in more than 20 fabricated metallic tips. Such efficient, reliable and practical tip fabrication opens the doors for many potential scientists working in related fields.
We investigated two-dimensional lipid bilayers by spectroscopic imaging with surface enhanced Raman spectroscopy (SERS). A DSPC lipid bilayer incubated on a glass substrate was coated with a thin layer of silver. Due to the strong electromagnetic enhancement of the silver film and the affinity to lipid molecules, the Raman spectrum of a single bilayer was obtained in a 1 s exposure time with 0.1 mW of incident laser power. In the C-H vibrational region of the spectra, which is sensitive to bilayer configurations, a randomly stacked area was dominated by the CH3 asymmetric-stretch mode, whereas flat areas including double bilayers showed typical SERS spectra. The spectral features of the randomly stacked area are explained by the existence of many free lipid molecules, which is supported by DFT calculations of paired DSPC molecules. Our method can be applied to reveal the local crystallinity of single lipid bilayers, which is difficult to assess by conventional Raman imaging.
We propose a new nano-imaging technique for intrinsic absorption properties of materials under a platform of conventional aperture-less near-field scanning optical microscopy (NSOM). In aperture-less NSOM, when a silicon nanotip is utilized and illuminated by the visible light instead of a metallic tip, Raman scattering of silicon from the tip apex can be obtained. Since the wavelength of this Raman scattered light is shifted to 520cm-1 from the one of the excitation light, far-field background signal excited by the diffraction limited focus spot of the incident light, which is one of the major problems in aperture-less NSOM, can be avoided. When the silicon nano-tip is on the sample and illuminated, the Raman signal of silicon can be partially absorbed by the sample while passing through it, so that measuring the intensity of the Raman signal of silicon enables us to observe the absorption behavior of the sample at nano-scale. Because the absorbance of light is dependent on the absorption coefficient of the sample as well as its sample topography, it is needed to eliminate the effect of the sample topography from the absorption measurement to technically evaluate the absorption coefficient of the sample. For this purpose, we simultaneously employed two different incident lasers and utilized absorbance ratio between two wavelengths to monitor the absorption coefficient of the sample. As an example, we demonstrated that two types of carbon nanotubes, which have different absorption properties, could be clearly distinguished with nano-scale resolution by our technique.
Deep-UV (DUV) plasmonics can expand the possibilities of DUV-based techniques (i.e. UV lithography, UV spectroscopy, UV imaging, UV disinfection). Here we present that indium is useful for research of DUV plasmonics. According to dielectric function, indium and aluminum are low-loss, DUV plasmonic metals, of which the imaginary parts are far smaller than those of other metals (i.e. rhodium, platinum) in the DUV range. Additionally, the real parts in the whole DUV range are close to but smaller than -2, allowing efficient generation of surface plasmon polaritons on an indium or aluminum nanosphere. In comparison to aluminum, indium provides a distinctive feature for fabricating DUV-resonant substrates. It is highly apt to form a grainy deposition film on a standard, optically transparent substrate (i.e. fused silica). The surface plasmon resonance wavelength becomes promptly tailored by simply varying the deposition thickness of the films, resulting in different grain sizes. Thus, we fabricated indium-coated substrates having different plasmon resonance wavelengths by varying the deposition thicknesses from 10 to 50 nm. DUV resonance Raman scattering of adenine molecules was best enhanced using the 25 nm deposition thickness substrates by the factor of 2. Furthermore, the FDTD calculation simulated the electromagnetic field enhancement over a grainy, indium-coated fused silica substrate. Both results indicate how indium plays an indispensable role in study of DUV plasmonics.
KEYWORDS: Nanorods, Lithography, Nanolithography, Polymethylmethacrylate, Gold, Super resolution, Color imaging, Nanostructuring, Particles, Control systems
Earlier, our group proposed a lens made of metallic nanorods, stacked in 3D arrays tapered in a conical shape. This nanolens could theoretically realize super-resolution color imaging in the visible range. The image could be magnified and transferred through metallic nanorods array. Lithography or self-assembly are common ways to fabricate such nanostructured devices. However, to precisely arrange nanorods is challenging due to the limitations to scale down components, and to increase accuracy of assembling particles in large area.
Here we experimentally demonstrated 2D nanolens with long chains of metallic nanorods placed at tapered angles in a fan-like shape to magnify images. In the fabrication, we chemically synthesized gold nanorods coated with CTAB surfactant to ensure a 10 nm gap between the rods for the resonance control of nanolens. And we prepared trenches patterned by FIB lithography on a PMMA coated glass substrate. The different hydrophobicity of PMMA and CTAB coats enabled to optimize capillary force in gold nanorod solution and selectively assemble nanorods into hydrophilic trenches. Finally, we obtained 2D nanolens after lift-off of the PMMA layer.
We numerically estimated the resonance property of nanorods chain and found a broad peak in the visible range located at a wavelength of 727 nm. The broadness of this peak (~178 nm) confirms that a broad range of wavelength can be resonant with this structure. This phenomenon was also confirmed experimentally by optical measurements. These results show that the combination of lithography and self-assembly has the potential to realize plasmonic nanolens.
TERS has emerged over the past decade as a powerful tool for Raman spectroscopy that shows high sensitivity and capability of nano-scale imaging with high spatial resolution. TERS utilizes a metallic nano-tip, which confines and enhances the propagating light into near-field in the close vicinity of the apex. Besides the nano-scale spatial resolution, polarization analysis in TERS is of tremendous advantage, as it allows one to study highly directional intrinsic properties of a sample at the nanoscale. In this study, we have developed a method to 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 We used those evaluated tips to measure nano-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.
Modern farming relies highly on pesticides to protect agricultural food items from insects for high yield and better
quality. Increasing use of pesticide has raised concern about its harmful effects on human health and hence it has become
very important to detect even small amount of pesticide residues. Raman spectroscopy is a suitable nondestructive
method for pesticide detection, however, it is not very effective for low concentration of pesticide molecules. Here, we
report an approach based on plasmonic enhancement, namely, particle enhanced Raman spectroscopy (PERS), which is
rapid, nondestructive and sensitive. In this technique, Raman signals are enhanced via the resonance excitation of
localized plasmons in metallic nanoparticles. Gold nanostructures are promising materials that have ability to tune
surface plasmon resonance frequency in visible to near-IR, which depends on shape and size of nanostructures. We
synthesized gold nanorods (GNRs) with desired shape and size by seed mediated growth method, and successfully
detected very tiny amount of pesticide present on food items. We also conformed that the detection of pesticide was not
possible by usual Raman spectroscopy.
Tip-enhanced Raman spectroscopy (TERS) offers one of the best techniques for analysis and imaging of molecule structures at nanoscale spatial resolution. An important issue in TERS is to improve the detection sensitivity of inherently weak Raman scattering so as to observe varieties of materials. For enhancement of the Raman signal, fully metallized tips are utilized in TERS, which enhance signals through plasmon oscillation at the tip apex. However, length of metal along the tip axis is on the order of a few to a few tens of micrometers, which means the plasmon resonant wavelength is much longer than the wavelength of the visible light used in TERS. From that point, if the tip has a metallic nanostructure on the apex, it would give better enhancement in the visible range compared with fully metallized tips. In this research, we employed photoreduction as a new fabrication method to grow a metallic nanostructure at the tip apex. We found a particular property of photoreduction that it occurs selectively at sharp corners, such as the tip apex of silicon cantilevers. Through this property, we succeeded in growing silver nanoparticles selectively at the tip apex. One of the advantages of the photoreduction is that the size of metal nanostructures is well controlled by optimizing various parameters. We controlled the size of silver nanoparticles from 100 to 400 nm by changing the laser exposure time. Furthermore, we obtained an order of magnitude higher enhancement from our fabricated tip compared with fully metalized tips through TERS measurements.
Vibrational spectroscopy, including Raman spectroscopy can be used for identifying molecular species, which is
not possible by a scanning probe microscopy or an electron microscopy. Moreover, vibrational spectra contain structural
information, such as intermolecular interactions, molecular orientations, and symmetry distortions of each species.
Therefore, Raman spectroscopy is a powerful tool for studying the chemical composition of matter.
By employing Tip-enhanced Raman spectroscopy (TERS), we can perform Raman spectroscopy with nano-scale
spatial resolution. Our approach relies on the enhanced filed near a laser irradiated metal tip which works as the Raman
excitation source. We have investigated nano-composite materials by TERS. Near-field Raman spectra revealed the
nano-scale properties of molecules encapsulated in single-wall carbon nanotubes (SWNT). The enhanced field act on
encapsulated molecules through the wall of SWNT to extract chemical information inside. &bgr;-carotene which has strong
Raman intensities under visible light illuminations is used as an encapsulated molecule. The advantage of Raman
spectroscopy is that the information of both SWNT and &bgr;-carotene can be obtained at the same time. So, it is possible to
discuss the interaction between SWNT and the encapsulated molecules. Near-field Raman spectra measured at several
different positions on SWNT bundle show that &bgr;-carotenes inside the tube are not uniformly distributed. We also find
that the filling rates and the peak positions of the radial breathing mode of SWNT are linearly correlated.
Strained silicon (ε-Si), the fundamental material of integrated circuit, is finding tremendous attention not only
because it boosts the speed but also reduces the power consumptions of electronic devices. Carrier mobility in a ε-Si thin
layer is enhanced compared to unstrained layers. However, strain distribution in ε-Si layers is inhomogeneous in the
nano-scale, which can degrade performance of electronic devices. Raman spectroscopy can be used to study strain
fluctuations in silicon because the optical phonons in Raman spectra are strongly influenced by strain. Though silicon are
Raman active devices, the Raman efficiency of a nanometer layer of strained silicon is extremely weak and is often
eclipsed under the Raman scattering of underlying buffer substrates. Here, we utilized surface enhancement in Raman
scattering to overcome weak emission problems and to suppress averaging effect. Thin ε-Si layers were covered with
thin silver layer to invoke surface enhanced Raman spectroscopy. This technique is promising but it lacks the spatial
resolution in the nano-scale due to diffraction limit from the probing light. In order to achieve nano-scale spectroscopy,
point-surface-enhancement was used, rather than a large surface enhancement. We used a silver-coated sharp tip, just
like SERS, but only the sample region very close to the tip apex is characterized. This technique, known as the tipenhanced
Raman spectroscopy, provides nanometric resolution in our measurement. For further improvement of SNR,
we introduce several approaches mainly for the suppression of background signals arising from crystalline bulk materials.
The characterization techniques describe above is applicable to other nano-materials.
Nanoscale characterization of strained silicon is essential for developing reliable next generation integrated circuits. Vibration mode of Si-Si in strained silicon was selectively enhanced to be observed by surface enhanced Raman spectroscopy technique. Covering the silver island film on a strained silicon layer Raman signal from the strained silicon can be detected with a high sensitivity against the overwhelming background signal from the underlying silicon layer. This technique allowed us for micro-Raman spectroscopy on strained silicon, and is straightforward to nano-Raman spectroscopy by tip-enhanced Raman microscope in which a sharpened metallic tip is used instead. We observe localized strains in strained silicon by tip-enhanced near-field Raman spectroscope in reflection-mode. The tip-enhanced Raman spectra show that the Raman frequency and intensity of strained silicon were different within a crosshatch pattern induced by lattice-mismatch. Micro Raman measurements, however, show only uniform features because of averaging effect due to the diffraction limit of light.
When human tear film’s constituents are identified, it is diagnostically useful. Surface enhanced Raman scattering (SERS) can quickly measure, the components at their low concentrations. Reproducible spectra were collected from evaporated gold thin film and silver mirror reaction glass substrates. Synthetic tears were measured on optimised substrates as proof of principle. Further analysis was used to discriminate between diseased and normal tears.
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