Nanoscopic imaging of oxidized graphene monolayer using tip-enhanced Raman scattering

Nano Research - Tip-enhanced Raman scattering (TERS) can be used for the structural and chemical characterization of materials with a nanoscale resolution, and offers numerous advantages compared...     

Nanoscale chemical imaging using top-illumination tip-enhanced Raman spectroscopy

We present a new top-illumination scheme for tip-enhanced Raman spectroscopy (TERS) in a gap-mode configuration with illumination and detection in a straightforward fashion perpendicular to the sample surface. This illumination focuses the light tightly around the tip end, which effectively diminishes far-field background contributions during TERS measurements. The setup maintains the entire functionality range of both the scanning probe microscopy and the confocal optical microscopy of the setup. For the first time, we show large (64 × 64 up to 200 × 200 pixels), high-resolution TERS imaging with full spectral information at every pixel, which is necessary for the chemical identification of sample constituents. With a scanning tunneling microscope tip and feedback, these TERS maps can be recorded with a resolution better than 15 nm (most likely even less, as discussed with Figure 6). An excellent enhancement (～10(7)×, sufficient for detection of few molecules) allows short acquisition times (<1 s/pixel) and reasonably low laser power (in the microwatt regime) yielding spectroscopic images with high pixel numbers in reasonable time (128 × 128 pixels in <25 min). To the best of our knowledge, no Raman maps with similar pixel numbers and full spectral information have ever been published.     

Raman spectroscopy of crystals for stimulated Raman scattering

Raman frequency shift, line width, integral and peak Raman scattering cross sections were measured in various crystals using spontaneous Raman spectroscopy. The highest Raman gain coefficient in steady state Stimulated Raman Scattering (SRS) regime was proved to be in barium nitrate crystal; for transient SRS it is expected to be in lithium niobate and tungstate crystals. Barium molybdate crystal is proposed as a new highly efficient Raman material.OCIS: 300.6450; 290.5910; 190.2640     

Nanoscale roughness on metal surfaces can increase tip-enhanced Raman scattering by an order of magnitude

We studied the influence of nanosteps on signal intensity in gap-mode tip-enhanced Raman spectroscopy (TERS). A benzenethiol monolayer adsorbed on an Au substrate was investigated. The correlation between the TERS signal and the local topography on the substrate shows that a 2 nm high sharp step on the Au surface can significantly increase the enhancement. Furthermore, theoretical models were built, and the numerical simulation results were consistent with our experimental results. The findings provide evidence that nanoscale roughness can play a crucial role in the "hot sites" corresponding to single-molecule surface-enhanced Raman spectroscopy (SERS).     

Stimulated Raman scattering of laser radiation in Raman crystals

Experimental study of stimulated Raman scattering (SRS) in barium nitrate and potassium gadolinium tungstate crystals are presented. Main features of crystalline single and multipass Raman shifters, Raman lasers with external and intracavity pumping are investigated and compared. Optimization of the optical schemes resulted in development of compact solid state Raman lasers for new important near infrared and eye safe spectral region of 1.2–1.5 μm.     

Separation free DNA detection using surface enhanced Raman scattering

Surface enhanced raman scattering (SERS) based molecular diagnostic assays for the detection of specific DNA sequences have been developed in recent years to compete with the more common fluorescence based approaches. Current SERS assays either require time-consuming separation steps that increase assay cost and can also increase the risk of contamination or they are negative assays, where the signal intensity decreases in the presence of target DNA. Herein, we report a new separation free SERS assay with an increase in signal intensity when target DNA is present using a specifically designed SERS primer. The presence of specific bacterial DNA from Staphylococcus epidermidis was detected using polymerase chain reaction (PCR) and SERS and indicates a new opportunity for exploration of SERS assays requiring minimal handling steps.     

On the oxidation of VS2 2D platelets using tip-enhanced Raman spectroscopy

2D materials are enabling disruptive advancements in electronic and photonic devices yielding to the development of sensing and wearable materials and in the field of energy production and storage as key components of photovoltaic technology and batteries. Nevertheless, little attention has been paid to TMDs and oxides that contain vanadium, as it is the case of vanadium disulfide (VS₂) and vanadium dioxide (VO₂). In this study we review the synthesis and characterization using Raman spectroscopy of VS₂ and its oxidized states. Laser-induced oxidation occurring during the Raman experiments in ambient conditions is described and plateau values of laser power levels to induce oxidation are provided. Furthermore, tip-enhanced Raman spectroscopy (TERS) spectra and maps are conducted to reveal at the single flake level the onset of oxidation mechanisms at the surface of the 2D platelets.     

Angle-resolved surface-enhanced Raman scattering on metallic nanostructured plasmonic crystals

Surface-enhanced Raman scattering is an ideal tool for identifying molecules from the "fingerprint" of their molecular bonds; unfortunately, this process lacks a full microscopic understanding and, practically, is plagued with irreproducibility. Using nanostructured metal surfaces, we demonstrate strong correlations between plasmon resonances and Raman enhancements. Evidence for simultaneous ingoing and outgoing resonances in wavelength and angle sheds new light on the Raman enhancement process, allowing optimization of a new generation of reproducible Raman substrates.     

Two-phonon Raman scattering of light in rare gas crystals

In this paper we calculate the two-phonon efficiency for light scattering from rare gas crystals at all temperatures. We use a Lennard-Jones potential and the Lorentz-Lorenz model following previous theoretical work and use the first-order self-consistent phonon theory, supplemented by a simple model giving the phonon anharmonic widths and shifts. Our results, though larger than the experimental results, agree well with earlier calculations at 0 K and related molecular dynamics work. The calculated spectra show a rich structure that varies with temperature. We suggest that further experiments could lead to significant new insights into the nature of excitations in rare gas solids as a function of temperature.Work submitted in partial fulfillment of the requirements of the Ph.D. degree at Rutgers University.     

Finding a needle in a chemical haystack: tip-enhanced Raman scattering for studying carbon nanotubes mixtures

Tip-enhanced Raman scattering (TERS) has emerged as a powerful analytical tool for measuring chemical images with nanometre spatial resolution. In this paper, the application of TERS to study differentiation of single-walled carbon nanotubes (SWCNTs) with 14?nm spatial resolution is demonstrated by the measurement of a mixture of two different types of SWCNTs as the model sample. The results demonstrate that TERS is a viable tool for the detection and localization of different SWCNTs and amorphous carbon in mixed SWCNTs based on the spectral differences in the radial breathing mode and the D bands.     

Silver-embedded zeolite crystals as substrates for surface-enhanced Raman scattering

This paper reports the preparation of a type of Ag-embedded zeolite crystals as surface-enhanced Raman spectroscopy (SERS) substrates by chemical reduction of Ag⁺-exchanged ZSM-5. Ag⁺ ions were loaded into the zeolite framework by ion exchange. Then the exchanged-Ag⁺ ions were reduced and metallic silver clusters formed inside the zeolite channel. The resulting Ag-embedded zeolite crystals are characterized by using a number of techniques including X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy to confirm silver formed inside the crystal channel. The fabricated Ag-embedded ZSM-5 zeolite substrates displayed strong and reproducible SERS activity for different Raman probe molecules such as Tris(2,2′-bipyridyl) ruthenium(II) chloride (RuBpy) and rhodamine 6G (R6G). Since silver embedded into the zeolite channel without changing the crystal surface property, the Ag–ZSM-5 zeolite crystal can be used to prepare different SERS-active substrate (SERS-tags), in which different probe molecules may be detected. Such Ag-embedded zeolite substrate would be useful in chemical and biological sensing and in the development of SERS-based analytical devices.     

Catalytic processes monitored at the nanoscale with tip-enhanced Raman spectroscopy

Heterogeneous catalysts play a pivotal role in the chemical industry, but acquiring molecular insights into functioning catalysts remains a significant challenge. Recent advances in micro-spectroscopic approaches have allowed spatiotemporal information to be obtained on the dynamics of single active sites and the diffusion of single molecules. However, these methods lack nanometre-scale spatial resolution and/or require the use of fluorescent labels. Here, we show that time-resolved tip-enhanced Raman spectroscopy can monitor photocatalytic reactions at the nanoscale. We use a silver-coated atomic force microscope tip to both enhance the Raman signal and to act as the catalyst. The tip is placed in contact with a self-assembled monolayer of p-nitrothiophenol molecules adsorbed on gold nanoplates. A photocatalytic reduction process is induced at the apex of the tip with green laser light, while red laser light is used to monitor the transformation process during the reaction. This dual-wavelength approach can also be used to observe other molecular effects such as monolayer diffusion.     

Raman scattering study of NaNO2-infiltrated opal photonic crystals

We have studied light scattering in synthetic opal crystals infiltrated with ferroelectric sodium nitrite, NaNO₂, and have analyzed simple models for the energy band structure of photonic crystals. Expressions have been derived for the group velocity of photons whose energy is close to the photonic band gap. Our results indicate that the infiltration of photonic crystals with NaNO₂ markedly increases the Raman scattering intensity.     

Detection of the potential pancreatic cancer marker MUC4 in serum using surface-enhanced Raman scattering

Pancreatic cancer (PC) is one of the most lethal malignancies. It has a 5-year survival rate of only 6%, owing in part to the lack of a reliable tumor marker for early diagnosis. Recent research has shown that the mucin protein MUC4 is aberrantly expressed in pancreatic adenocarcinoma cell lines and tissues but is undetectable in normal pancreas and chronic pancreatitis. Thus, the level of MUC4 in patient sera has the potential to function as a diagnostic and prognostic marker for PC. However, the measurement of MUC4 in sera using conventional test platforms (e.g., enzyme linked immunosorbent assay (ELISA) and radioimmunoassay (RIA)) has been unsuccessful. This has prevented the assessment of the utility of this protein as a possible PC marker in sera. In addressing this obstacle, the work herein examines the potential to create a simple diagnostic test for MUC4 through the development of a surface-enhanced Raman scattering (SERS)-based immunoassay, which was then used to demonstrate the first ever detection of MUC4 in cancer patient serum samples. Importantly, these measurements showed that sera from patients with PC produced a significantly higher SERS response for MUC4 compared to sera from healthy individuals and from patients with benign diseases. These results indicate that a SERS-based immunoassay can monitor MUC4 levels in patient sera, representing a much needed first step toward assessing the potential of this protein to serve as a serum marker for the early stage diagnosis of PC. This paper details these and other findings (i.e., the detection of the mucin protein CA19-9), which demonstrate that our SERS assay outperforms conventional assays (i.e., RIA and ELISA) with respect to limits of detection, readout time, and required sample volume.     

Plasmonic dimer antennas for surface enhanced Raman scattering

Electron beam induced deposition (EBID) has recently been developed into a method to directly write optically active three-dimensional nanostructures. For this purpose a metal-organic precursor gas (here dimethyl-gold(III)-acetylacetonate) is introduced into the vacuum chamber of a scanning electron microscope where it is cracked by the focused electron beam. Upon cracking the aforementioned precursor gas, 3D deposits are realized, consisting of gold nanocrystals embedded in a carbonaceous matrix. The carbon content in the deposits hinders direct plasmonic applications. However, it is possible to activate the deposited nanostructures for plasmonics by coating the EBID structures with a continuous silver layer of a few nanometers thickness. Within this silver layer collective motions of the free electron gas can be excited. In this way, EBID structures with their intriguing precision at the nanoscale have been arranged in arrays of free-standing dimer antenna structures with nanometer sized gaps between the antennas that face each other with an angle of 90°. These dimer antenna ensembles can constitute a reproducibly manufacturable substrate for exploiting the surface enhanced Raman effect (SERS). The achieved SERS enhancement factors are of the order of 10? for the incident laser light polarized along the dimer axes. To prove the signal enhancement in a Raman experiment we used the dye methyl violet as a robust test molecule. In future applications the thickness of such a silver layer on the dimer antennas can easily be varied for tuning the plasmonic resonances of the SERS substrate to match the resonance structure of the analytes to be detected.     

Raman scattering characterization of SF-PECVD-grown hydrogenated microcrystalline silicon thin films using growth surface electrical bias

A series of hydrogenated microcrystalline films were grown by a novel thin film deposition method using the Saddle Field Plasma Enhanced Chemical Vapour Deposition system. We show that the surface potential during growth strongly affects the microcrystalline character of the films, as quantified by Raman scattering. This effect can be reproduced on both conductive and non-conductive substrates. Films grown close to the threshold for microcrystalline growth exhibit laser-induced crystallization at low laser intensities.     

Relationship between photorefractive activity and Raman scattering in lithium niobate crystals

We performed Raman scattering experiments in pure, Mg-doped and Fe-doped single crystals of lithium niobate. The measurements were carried out in 90° geometry at several laser beam intensities, and in several polarization arrangements. The possible occurrence of light-induced polarization-anisotropic scattering, and its effects on Raman spectra, were checked by simultaneously measuring the time dependence of the laser beam intensity transmitted by the sample with unchanged polarization, and the time dependence of the intensity of Raman lines from vibrational modes of known symmetry. The results give no indication of any effect of photorefractive activity on Raman observations in our pure and Mg-doped samples. On the contrary, in Fe-doped lithium niobate the build-up of light-induced o–e scattering strongly affected the relative intensities of Raman lines in specific experimental arrangements. We discuss the possible application of these effects to the characterization of lithium-niobate-based materials with strong photovoltaic and photorefractive activity.     

Detection of small surface defects of nontransparent scattering materials by using dynamic speckles

We present a novel method for small surface defect detection based on the spatially filtered dynamic speckles. This method possesses high fidelity and versatility. Factors defining resolution of the proposed method is estimated and it is shown that resolution depends solely on the geometrical parameters of the optical system. Experimental results demonstrate feasibility of the proposed method for surface defect detection and are in good agreement with the theoretical estimations. A prototype for online detection of the defects with diameter of 400 μm is presented. Ways to improve prototype performance and the method resolution are discussed.     

Metal-coated magnetic nanoparticles for surface enhanced Raman scattering studies

We report the optimization and usage of surfactantless, water dispersible Ag and Au-coated gboldsymbolgamma–Fe₂_{boldsymbol 2}O₃_{boldsymbol 3} nanoparticles for applications in surface-enhanced Raman scattering (SERS). These nanoparticles, with plasmonic as well as super paramagnetic properties exhibit Raman enhancement factors of the order of 10⁶ (10⁵) for Ag (Au) coating, which are on par with the conventional Ag and Au nanoparticles. Raman markers like 2-naphthalenethiol, rhodamine-B and rhodamine-6G have been adsorbed to these nanoparticles and tested for nonresonant SERS at low concentrations. Further, to confirm the robustness of Ag-coated nanoparticles, we have performed temperature-dependent SERS in the temperature range of 77–473 K. The adsorbed molecules exhibit stable SERS spectra except at temperatures $boldsymbol >$boldsymbol >323 K, where the thermal desorption of test molecule (naphthalenethiol) were evident. The magnetic properties of these nanoparticles combined with SERS provide a wide range of applications.     

Determination of butanol distribution on salol-butanol crystals using Micro Raman Spectroscopy

We carried out several analytical studies in order to determine the butanol distribution on a salol-butanol crystal. This work is required for a research using microgravity condition focused upon the in-situ observation with an interferometer of the temperature and concentration field for the organic transparent crystal (salol-butanol). In order to evaluate the concentration field for the liquid phase with an interferometer in case of crystal growth from solution in space, it is significant to determine the distribution of solute (butanol) on starting crystal before sample launching using a non-destructive analytical method. The Micro Raman Spectroscopy (MRS) was choice as the most appropriate analytical method among several analytical methods. We prepared a salol-butanol crystal enclosed into the 1mm thick quartz glass ampoule in order to verify the propriety for MRS. Obtained Raman spectrums for salol, butanol and salolbutanol crystal show that the butanol 2D-distribution on salolbutanol crystal can be determined by MRS. These results also demonstrate that there are no influences of thick glass cell upon measurements and that 3D-measurement is possible. In conclusion, we argue that MRS is the most appropriate method for determination of the 2D- and/or 3D- distribution of solute on the crystal among several non-destructive analytical methods.