Depolarization effect in reflection-mode tip-enhanced Raman scattering for Raman active crystals

We present a tip-enhanced Raman spectroscopy (TERS) system that permits efficient illumination and detection of optical properties in the visible range to obtain high contrasts Raman signals from strained silicon (e-Si) assembled on silicon germanium substrate using an edge filter. The cut-off investigation in a depolarized TERS configuration. We overshadow wavelength of the edge filter is tuned by changing the angle of incident beam to deliver high incident power and effectively collect scattered near-field signals for nanoscopic strong far-field background signals from Raman active materials by utilizing the results obtained from depolarized surface-enhanced Raman scattering experiments in conjunction with silicon Raman tensor calculation to quantify which polarizer, analyzer and sample azimuth combination gives the minimum far-field signals. Here, we utilize the s-polarization instead of p-polarized light in conjunction with polarization properties of e-Si to obtain a high contrast Raman signal. We found that for imaging Raman active and bulk crystalline materials such as silicon, background signal suppression (s-illumination) is more important than the field enhancement with strong far-field signal levels (p-polarization). The utilization of an edge filter for shorter collection time, depolarized configuration for higher contrast and tip heating for higher resolution are discussed.

Tip-enhanced Raman scattering (TERS) is a powerful near-field spectroscopic tool to measure Raman spectra of materials with nanometer spatial resolution and even with sensitivity down to single-molecule level [1]. The optical enhancement in TERS relies on lightning-rod effect and plasmonic coupling of input electric field into sharp metal tips. In general, the presence of strong electric field component along the tip axis (i.e., longitudinal field) is required for efficient coupling and several coupling schemes (e.g., use of radial polarization) have been suggested to enhance the interaction [1, 2].We propose a new approach for coupling light efficiently into the tip structures, which relies on coupling two metal tips together with a metal bridge. Due to the bridge, no strong longitudinal field is anymore needed for efficient coupling. The structures are manufactured using UV-nanoimprint lithography (UV-NIL) combined with electron-beam evaporation, which makes the technique fast, simple and relatively cheap [3]. In addition, large arrays of structures can be manufactured e.g. for lab-on-a-chip or optical tweezing applications.We measured Raman spectra from aqueous solution of crystal violet molecules using arrays of bridged and single gold and silver nanocones [4]. Almost ten-fold increase in the Raman signal for the bridged nanocones was observed, demonstrating the enhanced coupling of light.

While surface‐enhanced Raman scattering (SERS) is an established technique to significantly enhance the Raman signal from samples bound to metal surfaces, a quantitative interpretation is very difficult because of the rough and arbitrary nature of the substrate. A distinction between concentration and enhancement effects is virtually impossible. Tip‐enhanced Raman scattering (TERS), a scattering type of near‐field optical microscopy technique, circumvents the problem of random enhancement by concentrating the roughness to only a small region directly at the apex of a scanning probe tip. In this paper, TERS spectra of DNA pyrimidine bases have been collected and compared with standard SERS and Raman measurements of the nucleotides and the pure bases. The results are discussed in terms of electromagnetic and chemical effects of the SERS enhancement of the TERS probes. Copyright © 2006 John Wiley & Sons, Ltd.

Discrimination between enantiomers is achieved by tip‐enhanced Raman scattering (TERS) using a silver tip that is chemically modified by an achiral para‐mercaptopyridine (pMPY) probe molecule. Differences in the relative intensities of the pMPY spectra were monitored for three pairs of enantiomers containing hydroxy (−OH) and/or amino (−NH2) groups. The N: or N+−H functionality of the pMPY‐modified tip participates in hydrogen‐bond interactions with a particular molecular orientation of each chiral isomer. The asymmetric arrangement of silver atoms at the apex of the tip induces an asymmetric electric field, which causes the tip to become a chiral center. Differences in the charge‐transfer (CT) states of the metal‐achiral probe system in conjunction with the asymmetric electric field produce different enhancements in the Raman signals of the two enantiomers. The near‐field effect of the asymmetric electric field, which depends on the number of analyte functional groups capable of hydrogen‐bond formation, improves the degree of discrimination.

Integrins are important membrane receptors that form focal adhesions with the extracellular matrix and are transmembrane signaling proteins. We demonstrate that nanoparticles functionalized with c-RGDfC ligands bind to intact cell membranes and selectively enhance the amino acid signals of the integrin receptor when coupled with tip-enhanced Raman scattering (TERS) detection. Controlling the plasmonic interaction between the functionalized nanoparticle and the TERS tip provides a clear Raman signal from αVβ3 integrins in the cell membrane that matches the signal of the purified integrin receptor. Random aggregation of nanoparticles on the cell does not provide the same spectral information. Chemical characterization of membrane receptors in intact cellular membranes is important for understanding membrane signaling and drug targeting. These results provide a new method to investigate the chemical interactions associated with ligand binding to membrane receptors in cells.

The ability to confine light in small volumes, associated to low background signals, is an important technological achievement for a number of disciplines such as biology or electronics. In fact, decoupling the source position from the sample area allows an unprecedented sensitivity which can be exploited in different systems. The most direct implications are however related to either Surface Enhanced Raman Scattering (SERS) or Tip Enhanced Raman Scattering (TERS). Furthermore, while the combination with super-hydrophobic patterns can overcome the typical diffusion limit of sensors, focused surface plasmons decaying into hot electrons can be exploited to study the electronic properties of the sample by means of a Schottky junction. Within this paper these techniques will be briefly described and the key role played by both surface and localized plasmons will be highlighted. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Optical and spectroscopic techniques offer unique possibilities for non destructive or micro-destructive characterization of surfaces, with widespread applications, starting from in-line monitoring of industrial processes. A significant group of industrial applications concerns nuclear grade material characterization and plasma diagnostics relevant to the thermonuclear fusion process. However, technologies and methodologies originally developed for specific in-vessel utilization, can easily find additional in-situ and remote application to environmental and cultural heritage (CH) diagnostics addressed to preventive conservation and restoration of surfaces. At ENEA Frascati different prototypes have been developed and patented to collect reflectance and fluorescence images excited at different ultraviolet and visible laser wavelengths, Raman and LIBS signals. Portable integrated instruments suitable for operation at different distances from a 1.5 to 30 m, have been assembled and operated in laboratory on multilayered samples and in field campaigns for Security and on CH painted surfaces. Significant results relevant to the cross fertilization among different applications will be presented and discussed.

Influence of natural radioactive aerosols on artificial radioactivity detection in the Spanish surveillance networks

Raman spectroscopy is a set of techniques based on Raman scattering properties, widely applied to analyze the composition of various substances. The techniques consist of 1) backscattering, which collects Raman signals scattered from the surface of a sample; and 2) transmission, which collects Raman signals transmitted by the surface of a sample in the opposite direction of a light source (which triggers less fluorescence than backscattering). In this paper, we will measure the Raman spectrum of acetone solution by the transmission Raman spectroscopy and the backscattering Raman spectroscopy systems that we set up. Those accessories we use are a 532-nanometer diode laser with 100 milliwatt power as the light source, a focusing lens, an objective lens used for amplifying the Raman signals scattered from the sample, and a long-pass filter used to block light with a wavelength shorter than 532 nanometers. For the experimental samples, there are acetone solutions each prepared at 1M, 3M, 5M, 7M, 9M, 11M, and 13M. In the results of this experiment, we found that the intensity of Raman peaks for each Raman shift of acetone and the molar concentrations of acetone measured by both systems have a linear function in the backscattering Raman system and have a quadratic function in the transmissions Raman system because in backscattering system have noise by fluorescence effect more than in transmission system and it might be the effect of the Beer-Lambert law.

This study reports about the influence of titania on the structure and bioactivity of 60SiO2 ⋅ (32 − x)CaO · 8P2O5 · xTiO2 glasses with 0 ≤ x ≤ 12 mol%. In order to analyse the evolution of the apatite structure self‐assembled on the glass surface after immersion in simulated body fluid, a complementary approach of different techniques was applied such as X‐ray diffraction, Fourier transform infrared spectroscopy (FT‐IR), scanning electron microscopy and Raman spectroscopy. It was evidenced that the crystalline hydroxyapatite amount and the size of the nanostructured apatite phase, i.e. crystalline and amorphous one, decrease with the rise of TiO2 content. FT‐IR spectra recorded in reflection mode showed the increase of the apatite‐like structure amount as the titania content became higher. On the other hand, the behaviour of the full width at half maximum of the most intense Raman signal originating from the apatite‐like structure confirms the size decrease of the nanostructured apatite phase with the progressive addition of TiO2 and shows the existence of a less ordered apatite‐like structure. The herein presented investigation demonstrates that the FWHM of the Raman signal combined with the FT‐IR data recorded in reflection mode can be used as key spectral indicators for elucidating the evolution of apatite‐like structures developed on the bioactive material surface. Copyright © 2016 John Wiley & Sons, Ltd.

In liquid cell (LC) annular dark-field scanning transmission electron microscopy (ADF-STEM), the spatial resolution is limited by the low ratio of signals from samples to background signals from the silicon nitride window membranes and liquid. We report the development of a double-tilt LC holder for atomic-resolution LC-ADF-STEM imaging of single-crystal samples under zone-axis incidence conditions. A SrTiO3 <001> lamellar sample, approximately 100 nm thick, was prepared using the focused ion beam technique and transferred onto a silicon nitride window membrane of an LC chip via a glass probe pick-up method in air, which avoids Ga ion beam-induced damage to the window membrane. The sample adhered to the high-flatness window membrane and remained immobile, even when embedded in a water droplet. The sample and pure water were enclosed in an LC and observed under <001> zone-axis incidence conditions using aberration-corrected ADF-STEM. Electron channeling along the atomic columns enabled atomic-resolution LC-ADF-STEM imaging with high contrast, sufficiently overcoming background signals from window membranes and liquids. This high-contrast imaging technique could lower the probe current and is expected to mitigate electron-beam-induced radiolysis and minimize undesired sample damage, particularly under high-magnification imaging conditions.

A Versatile Tetraphenylethene Derivative Bearing Excitation Wavelength Dependent Emission, Multistate Mechanochromism, Reversible Photochromism, and Circularly Polarized Luminescence and Its Applications in Multimodal Anticounterfeiting

Surface-enhanced Raman scattering (SERS) tags have been intensively applied in biological detection and imaging in recent years. However, both applications often suffer from high Raman background signals from containers such as 96-well plates or autofluorescence signals from biological tissues. Here, we greatly mitigate the influence of these high background Raman or fluorescent signals in both biological detection and imaging using two multivariate curve resolution (MCR) methods including negative matrix factorization and classical least squares. The limit of detection is lowered by one order of magnitude after applying MCR methods to detect target SERS tags in a 96-well plate. Additionally, in a multiplexed cell imaging assay, both false-negative and false-positive results were eliminated with the aid of MCR methods. Accordingly, we suggest a wider application of MCR methods during both biological detection and imaging of SERS tags with high background signals.

Although the homeotropic alignment of liquid crystals is widely used in LCD TVs, no easy method exists to measure its anchoring coefficient. In this study, we propose an easy and convenient measurement technique in which a polarizing optical microscope is used in the reflective mode with an objective lens having a low depth of focus. All measurements focus on the reflection of light near the interface between the liquid crystal and alignment layer. The change in the reflected light is measured by applying an electric field. We model the response of the director of the liquid crystal to the electric field and, thus, the change in reflectance. By adjusting the extrapolation length in the calculation, we match the experimental and calculated results and obtain the anchoring coefficient. In our experiment, the extrapolation lengths were 0.31 ± 0.04 μm, 0.32 ± 0.08 μm, and 0.23 ± 0.05 μm for lecithin, AL-64168, and SE-5662, respectively.

Recent studies on signal enhancement of spontaneous Raman scattering for developing of sensitive Raman gas detectors are reported. Raman scattering is a gas detecting method with high feasibility, but usually its signal is very low. To improve the level and the quality of the Raman signal, the effects of pumping laser source, sample cell, and optical arrangement are studied in detail. It is found that not only the wall of sample cell will give a wide Raman or fluorescence background which will decrease the sensitivity, but the dichroic beam splitter will also contribute considerable background if it is not aligned properly. The sample cell of hollow fiber is characterized by its high responsibility as well as its high background and low signal contrast. When the hollow fiber is replaced by a free-space sample cell consisted of metal-coated parabolic reflector, the wide background is largely suppressed. If there is no common optical elements between the pumping and collecting optical systems, the wide background will be cut down obviously, which is proved by the intracavity-enhanced Raman scattering in a He-Ne laser. These experimental results will be helpful for the research and developing of highly sensitive Raman gas detectors.