Identification of individual isotopes in a polymer blend using tip enhanced Raman spectroscopy

For the first time, tip enhanced Raman spectroscopy (TERS) blinking measurements are used to identify the individual isotopes of non‐Raman resonant polystyrene in a miscible, binary blend. This demonstrates the sensitivity and selectivity required for nanoscale chemical imaging and broadens the types of surface components potentially identifiable with TERS. Copyright © 2015 John Wiley & Sons, Ltd.     

Extending nanoscale spectroscopy with titanium nitride probes

We present a means of controlling the stoichiometry of titanium nitride (TiN) coatings on probes for tip‐enhanced Raman spectroscopy measurements made using sputtering so that outstanding enhancements can be obtained. This provides a more robust alternative to gold‐coated tips that also has potential for tuning the plasmon resonance and working in new environments. Proof of concept measurements on poly(3,4‐ethylenedioxythiophene)/poly(styrenesulfonate) thin films demonstrate increases in the observed intensity with contrast values up to 3.1. TiN is mechanically, chemically, and thermally robust. When deposited under appropriate conditions it has optical properties, including a plasmon resonance, very similar to those of gold. However, the spontaneous formation of a surface TiN_xO_y layer with relatively high values of y has prevented attaining enhancements in tip‐enhanced Raman spectroscopy beyond that provided by the lightning rod effect. Depositing a thin layer of aluminum to form a passivating Al₂O₃ layer over the TiN plasmonic structure allows the stoichiometry achieved in the vacuum deposition to be maintained. Copyright © 2016 John Wiley & Sons, Ltd.     

Exploring the origin of tip‐enhanced Raman scattering; preparation of efficient TERS probes with high yield

Tip‐enhanced Raman scattering (TERS) spectroscopy is a promising technique for nanoscale chemical analysis. However, there are several challenges preventing widespread application of this technology, including reproducible fabrication of efficient TERS probes. These problems reflect a lack of clear understanding of the origins of, and the parameters influencing TERS. It is believed that the coating characteristics at the apex of the tip have a major effect on the near‐field optical enhancement and thus the TERS activity of a metalized probe. Here we show that the aspect ratio of the tip can play a significant role in the efficiency of TERS probes. We argue that the electrostatic field arising from the lightning‐rod effect has a substantial role in the observed TERS effect. This argument is supported by ‘edge‐enhanced Raman scattering’ which is shown for a noble metal film. Furthermore, it is reported that an associated tip‐surface‐enhanced Raman scattering effect can be achieved by using a TERS‐inactive metalized probe on a surface‐enhanced Raman spectroscopy‐inactive roughened surface. This observation can be explained by an interparticle enhancement of the electromagnetic field. Copyright © 2011 John Wiley & Sons, Ltd.     

Understanding tip‐enhanced Raman spectra of biological molecules: a combined Raman,SERS and TERS study

Although conventional Raman, surface‐enhanced Raman (SERS) and tip‐enhanced Raman spectroscopy (TERS) have been known for a long time, a direct, thorough comparison of these three methods has never been carried out. In this paper, spectra that were obtained by conventional Raman, SERS (on gold and silver substrates) and TERS (in ‘gap mode’ with silver tips and gold substrates) are compared to learn from their differences and similarities. Because the investigation of biological samples by TERS has recently become a hot topic, this work focuses on biologically relevant substances. Starting from the TER spectra of bovine serum albumin as an example for a protein, the dipeptides Phe–Phe and Tyr–Tyr and the tripeptide Tyr–Tyr–Tyr were investigated. The major findings were as follows. (1) We show that the widely used assumption that spectral bands do not shift when comparing SER, TER and conventional Raman spectra (except due to binding to the metal surface in SERS or TERS) is valid. However, band intensity ratios can differ significantly between these three methods. (2) Marker bands can be assigned, which should allow one to identify and localize proteins in complex biological environments in future investigations. From our results, general guidelines for the interpretation of TER spectra are proposed. Copyright © 2012 John Wiley & Sons, Ltd.     

Characterizing unusual metal substrates for gap‐mode tip‐enhanced Raman spectroscopy

In this article, the electromagnetic (EM) field in gap‐mode tip‐enhanced Raman spectroscopy (TERS) is investigated theoretically and experimentally for a range of commonly used and unusual metal and nonmetal substrates. By approaching a metal tip to a substrate, both form a coupled system that confines the EM field created at the tip apex. The influence of the substrate onto the EM field enhancement is observed in a top‐illumination gap‐mode TERS setup for different metal substrates. These include Au, the most commonly used substrate, and also a wide range of rarely or previously unused TERS substrates (Cu, Ag, Al, Pd, Pt, Ni, Ti, Mo, W, stainless steel, Al₂O₃, SiO₂). Self‐assembled monolayers of thiols and brilliant cresyl blue thin film samples are investigated experimentally on nine metal substrates, all showing considerable TERS enhancement. With finite difference time domain and finite element simulations used, the article provides a good estimate of the EM field enhancement for a wide range of substrates for users to estimate how well a substrate of choice will perform in a gap‐mode TERS experiment. The reduction in EM field strength |E²| compared with Au is less than an order of magnitude for many metals (Calculations: Cu 92%, Ag 81%, Ni 53%). This article experimentally shows that a wide variety of conductive substrates can be used, when one is willing to trade a fraction of the EM field enhancement. TERS was seen on all metal substrates including stainless steel, yet quantification was not always possible. These qualitative results were complemented with intensities from calculations. The wider variety of substrates will increase the applicability of TERS and evolve it one step further towards use in standard analytics. Copyright © 2012 John Wiley & Sons, Ltd.     

Subdiffraction‐limited chemical imaging of patterned phthalocyanine films using tip‐enhanced near‐field optical microscopy

A tip‐enhanced near‐field optical microscope, based on a shear‐force atomic force microscope with plasmonic tip coupled to an inverted, confocal optical microscope, has been constructed for nanoscale chemical (Raman) imaging of surfaces. The design and validation of the instrument, along with its application to near‐field Raman mapping of patterned organic thin films (coumarin‐6 and Cu(II) phthalocyanine), are described. Lateral resolution of the instrument is estimated at 50 nm (better than λ/10), which is roughly dictated by the size of the plasmonic tip apex. Additional observations, such as the distance scaling of Raman enhancement and the inelastic scattering background generated by the plasmonic tip, are presented. Copyright © 2016 John Wiley & Sons, Ltd.     

Enhancement of lattice defect signatures in graphene and ultrathin graphite using tip‐enhanced Raman spectroscopy

Understanding the role of defects in graphene is the key to tailoring the properties of graphene and promoting the development of graphene‐based devices. Defects can affect the electronic properties of a device while also offering a means by which to functionalize the local properties. Using tip‐enhanced Raman spectroscopy (TERS), heightened defect sensitivity was demonstrated on graphene edges, folds, and overlapping regions. Measurements confirm that TERS can provide simultaneous structural and spectral information on a localized scale, hence offering defect characterization on a scale that is not obtainable using conventional Raman spectroscopy. This study observed preferential enhancement of the D band signal on multilayered graphene and ultrathin graphite; in addition, other key defect signatures were also enhanced and detected. We present our findings in relation to theoretical predictions of graphene defect signatures and an analysis of the sensitivity of TERS in measuring two‐dimensional structures. Copyright © 2013 John Wiley & Sons, Ltd.     

Bacterial mixture identification using Raman and surface‐enhanced Raman chemical imaging

The ability of normal Raman and surface‐enhanced Raman scattering (SERS) to identify and detect bacteria has shown great success in recent studies. The addition of silver nanoparticles to bacterial samples not only results in an enhanced Raman signal, but it also suppresses the native fluorescence associated with biological material. In this report, Raman chemical imaging (RCI) was used to analyze individual bacteria and complex mixtures of spores and vegetative cells. RCI uses every pixel or a binned pixel group (BPG) of the Raman camera as an independent Raman spectrograph, allowing collection of spatially resolved Raman spectra. The advantage of this technique resides primarily in the analysis of samples in complex backgrounds without the need for physically isolating or purifying the sample. Using a chemical imaging Raman microscope, we compare normal RCI to SERS‐assisted chemical imaging of mixtures of bacteria. In both cases, we are able to differentiate single bacterium in the Raman microscope's field of view, with a 60‐fold reduction in image acquisition time and a factor of 10 increase in the signal‐to‐noise ratio for SERS chemical imaging over normal RCI. Copyright © 2010 John Wiley & Sons, Ltd.     

Fiber optic probes for linear and nonlinear Raman applications – Current trends and future development

This review focuses on fiber optic probes for linear and nonlinear Raman spectroscopy, especially for medical applications. It aims at providing an overview over contemporary technology, recent first clinical trials, and helps identifying future developments necessary to bring the emerging technology to clinical end users. After a short introduction to linear and nonlinear Raman spectroscopic modalities, general design considerations will be discussed and compared to common fiber probe setups. Subsequently, examples for medical applications of fiber optic Raman probes will be given concentrating on probes for linear Raman spectroscopy as these devices are technologically more mature compared to their counterparts based on nonlinear Raman spectroscopy. The review also includes a brief summary of first multimodal fiber optic probes and highlights their benefits for clinical applications. Finally, probes are introduced which employ either nonlinear Raman spectroscopy or surface enhanced spectroscopy.     

Nano‐imaging through tip‐enhanced Raman spectroscopy: Stepping beyond the classical limits

The spatial resolution in optical imaging is restricted by so‐called diffraction limit, which prevents it to be better than about half of the wavelength of the probing light. Tip‐enhanced Raman spectroscopy (TERS), which is based on the SPP‐induced plasmonic enhancement and confinement of light near a metallic nanostructure, can however, overcome this barrier and produce optical images far beyond the diffraction limit. Here in this article, the basic phenomenon involved in TERS is reviewed, and the high spatial resolution achieved in optical imaging through this technique is discussed. Further, it is shown that when TERS is combined with some other physical phenomena, the spatial resolution can be dramatically improved. Particularly, by including tip‐applied extremely localized pressure in TERS process, it has been demonstrated that a spatial resolution as high as 4 nm could be achieved.     

Plasmonic effects and the morphology changes on the active material P3HT:PCBM used in polymer solar cells using Raman spectroscopy

There is no consensus yet that the enhancement effects of plasmonic device are predominantly caused by plasmonic effects or induced morphology changes in the optoelectronic `materials. Herein, we present a detailed Raman characterization of a typical organic P3HT:PCBM system comprising silver nanowires (Ag NWs) with different size, which can simultaneously study the plasmonic effects and the morphology changes. The direct comparison of the Raman spectra of non‐annealed and annealed samples indicates that the morphology of plasmonic samples has changed before annealing and the morphology of plasmonic samples and reference sample after annealing is not distinguishable. This indicates that the interaction between P3HT and Ag NWs with different size can be explained by plasmonic effects after annealing. Moreover, in‐situ Raman spectroscopy is used to study the morphology changes in plasmonic samples with different diameters of Ag NWs during heating process. This method can distinguish the plasmonic effects and morphology changes of plasmonic device. Copyright © 2016 John Wiley & Sons, Ltd.     

Halide‐ion‐assisted increase of surface‐enhanced hyper‐Raman scattering: a clear observation of the chemical effect

We show that the increase of surface‐enhanced hyper‐Raman scattering (SEHRS) intensity of organic dye molecules adsorbed on single silver (Ag) colloid aggregate in the presence of halide ions is a direct evidence of the chemical effect in the enhancement mechanism. Time‐dependent SEHRS measurements before and after adding halide ions enabled us to distinctly observe the chemical effect. The presence of the halide ions results to a more stable chemical interaction between metal and dye molecule, making it more resistant against photodegradation effects. This study can contribute in elucidating the chemical effect mechanism and aid in the development of SEHRS as a useful spectroscopic tool. Copyright © 2008 John Wiley & Sons, Ltd.     

VHF-PECVD低温制备微晶硅薄膜的拉曼散射光谱和光发射谱研究

采用拉曼散射光谱和PR650光谱光度计对VHF-PECVD制备的微晶硅薄膜进行了结构表征和在线监测研究.结果表明：功率对材料的晶化率（χc）有一定的调节作用，硅烷浓度大，微调作用更明显；SiH*的强度只能在一定的范围内表征材料的沉积速率，功率大相应的速率反而下降；I［Hα*］/I［SiH*］强度比值反映了材料晶化程度，此结果和拉曼散射光谱测试结果显示出一致性；I［Hβ*］/I［Hα*］的强度比表明氢等离子体中的电子温度随功率的增大而逐渐降低. 关键词： 甚高频等离子体增强化学气相沉积 微晶硅 拉曼散射谱 光发射谱     

Vibrational spectra,X‐ray and molecular structure of 1H‐ and 3H‐imidazo[4,5‐b]pyridine and their methyl derivatives: DFT quantum chemical calculations

Molecular structure, vibrational energy levels and potential energy distribution of 1H‐imidazo[4,5‐b]pyridine, 3H‐imidazo[4,5‐b]pyridine, 5‐methyl‐1H‐imidazo[4,5‐b]pyridine, 6‐methyl‐1H‐imidazo[4,5‐b]pyridine and 7‐methyl‐3H‐imidazo[4,5‐b]pyridine were determined using density functional theory (DFT) at the B3LYP/6‐31G(d,p) level. The optimised bond lengths and bond angles are in good agreement with the X‐ray data of 5‐methyl‐1H‐imidazo[4,5‐b]pyridine obtained in the present work (Pbca space group; a = 8.660(2), b = 11.078(2), c = 11.078(3) Å, Z = 8). The N⁺H group plays the role of a proton donor in a medium strong hydrogen bond of the type N H…N, linking the N‐atom of the pyridine with the adjacent molecule related by the symmetry operation: 1/2 − x, y − 1/2, z(N…N = 2.869(25) Å). The presence of hydrogen bond is confirmed by appearance in the IR spectra of a very broad and strong contour in the 2000–3100 cm⁻¹ range. The place of substitution of the methyl group at the pyridine ring influences the proton position of the NH group at the imidazole unit. Copyright © 2007 John Wiley & Sons, Ltd.     

Raman spectroscopy of polyconjugated molecules with electronic and mechanical confinement: the spectrum of Corallium rubrum

The chemical nature of the red pigment of Corallium rubrum (CR) has not yet been clearly identified. We have recorded the Raman spectra of CR, canthaxantin, and parrot feather, and we propose an interpretation with the help of quantum chemistry and of the effective conjugation coordinate theory, which accounts for the main Raman lines (ν₁, ν₂, ν₃) as originating from the polyene backbone. In this study, two additional lines in the 1000 cm⁻¹ range (ν₄ and ν₅) are considered as well as a series of overtones and combinations in the second order Raman spectrum (2000–3000 cm⁻¹). Density Functional Theory calculations predict that, moving with successive methylation from a simple unsubstituted polyene chain with 9 C═C bonds (as psyttacofulvins) to a 9, 9′, 13, 13′ tetramethylated chain (as carotenoids), the geometric structures of the molecules and their Raman spectra show changes that can be rationalized if the effective conjugation coordinate theory is extended to account for mechanical confinement. This turns out to be a new concept that helps the interpretation of the Raman spectra of partially methylated conjugated oligoenes. Evidence is found that the main component of the pigment of CR does not posess a fully demethylated polyene chain (psyttacofulfine) nor a tetramethylated chain (carotenoid), thus indicating that the polyene chain is partially methylated. Because we consider resonance Raman spectra, the target of this study is the structure of the polyene backbone; no direct spectroscopic information can be obtained in this way on the chemical nature of the groups attached to both chain ends. Copyright © 2012 John Wiley & Sons, Ltd.     

Surface enhanced Raman spectroscopic investigation of orchil dyed wool from Roccella tinctoria and Lasallia pustulata

In this work Raman spectroscopic techniques have been utilized to characterize the vibrational spectral features of orchil dyed wool samples. Specifically, it is noted by surface enhanced Raman spectroscopy that wool dyed purple with two historically used orchil species (Roccella tinctoria and Lasallia pustulata) show spectral differences possibly owing to their specific dye‐precursor constituents. The additional natural dyestuff woad (Isatis tinctoria L.) overdyeing the R. tinctoria orchil dyed wool is a further challenge when distinguishing the mixed dye components given by the co‐adsorption of the dyestuffs as permitted by the selection rules of surface enhanced Raman spectroscopy. Furthermore, the effects of dilution of the L. pustulata species in its spectral detection have been assessed along with the evaluation of subsequent lichen extract boiling before dyeing which resulted in the detection of a degraded form of the orchil dye. Proof of concept included the surface enhanced Raman spectroscopy (SERS) investigation of a purple dyed tapestry (XVI century) which permitted an aged orchil dye to be determined. This contribution utilizes SERS as a fast, reproducible and specific method for both orchil dye detection and alteration induced by degradation. Copyright © 2014 John Wiley & Sons, Ltd.     

Tip‐enhanced Raman spectroscopy – an interlaboratory reproducibility and comparison study

Since its first experimental realization, tip‐enhanced Raman spectroscopy (TERS) has emerged as a potentially powerful nanochemical analysis tool. However, questions about the comparability and reproducibility of TERS data have emerged. This interlaboratory comparison study addresses these issues by bringing together different TERS groups to perform TERS measurements on nominally identical samples. Based on the spectra obtained, the absolute and relative peak positions, number of bands, peak intensity ratios, and comparability to reference Raman and surface‐enhanced Raman spectroscopy (SERS) data are discussed. Our general findings are that all research groups obtained similar spectral patterns, irrespective of the setup or tip that was used. The TERS (and SERS) spectra consistently showed fewer bands than the conventional Raman spectrum. When comparing these three methods, the spectral pattern match and substance identification is readily possible. Absolute and relative peak positions of the three major signals of thiophenol scattered by 19 and 9 cm⁻¹, respectively, which can probably be attributed to different spectrometer calibrations. However, within the same group (but between different tips), the signals only scattered by 3 cm⁻¹ on average. This study demonstrated the suitability of TERS as an analytical tool and brings TERS a big step forward to becoming a routine technique. Copyright © 2014 John Wiley & Sons, Ltd.     

Raman spectroscopy assisted with XRF and chemical simulation to assess the synergic impacts of guardrails and traffic pollutants on urban soils

Urban soils are potential reservoirs of toxic metals as a consequence of traffic emissions. Sources like brake linings, tyres, road pavement, exhaust fumes, guardrail, traffic signals and other galvanised steel structures are used in a large variety of external constructions in the modern urban areas. Their beneficial properties from a corrosion and oxidation perspective are well‐known but less is known about their contribution to the environmental fate of corrosion‐induced released zinc. In this work, the impact of guardrails and other traffic pollutants on urban soils has been studied by means of Raman spectroscopy (molecular speciation) and thermodynamic speciation to understand the mechanisms of metal release and uptake by the soils. Hydrozincite, Zn₅(CO₃)₂(OH)₆, was identified by means of Raman spectroscopy as the degradation compound of the galvanised zinc layer from guardrails which leads to the formation of soluble zinc, by acidic attack of the urban atmosphere, that drops and accumulate (zinc nitrate was identified) in soils. This fact shows the environmental risk of zinc release from the guardrails because zinc nitrate can be easily mobilised by water runoff, affecting the surrounding areas or groundwater. Other traffic pollutant that reaches guardrail and soil by atmospheric deposition, such as barium, was also identified in soil as well as in the guardrail in its carbonate form, BaCO₃. Because of its low solubility, barium will accumulate in urban soils. Copyright © 2012 John Wiley & Sons, Ltd.     

Micro‐Raman and stratigraphic studies of the paintings on the ‘Cembalo’ model musical instrument (A.D. 1650) and laser‐induced degradation of the detected pigments

Micro‐fragments of the painted part of the ‘Cembalo’ model by Michele Todini (1625–1689) are investigated. The technique used for painting the terracotta base was studied via the stratigraphic analyses. No background layer of inorganic materials, e.g. gypsum, was found. To prevent absorption effects due to the terracotta porosity, a very thin layer of proteinaceous material was probably used. The micro‐Raman analyses have revealed the use of pigments currently used in the post‐Renaissance period (lead white, indigo, yellow of iron hydroxide, gypsum, hematite and carbon black) mixed with a pigment, the Prussian blue, discovered in A.D . 1704. This raises the authenticity problem of the work of art, a problem analysed and discussed in presenting the history of the work of art, and after the pigment study. The presence of degraded lead white is recognized via the laser‐induced degradation of the irradiated material. The possibility of a restoring action of the painted parts, as opposite to the non‐originality of the work, is considered and discussed. Since most part of the investigated pigments shows laser‐induced effects, a careful study of this phenomenon is performed by using the modern counterparts of the ancient pigments. For different laser powers, the temperatures of the investigated zones have been obtained via the detailed balance principle and connected to the laser‐induced degradation effects. Copyright © 2007 John Wiley & Sons, Ltd.     

Tip‐based plasmonics: squeezing light with metallic nanoprobes

Nanofabricated metallic tips are at the core of important research in single‐molecule imaging, near‐field scanning optical microscopy, tip‐enhanced Raman spectroscopy, as well as potential commercial applications such as heat‐assisted magnetic recording. While challenging to fabricate, much progress has been made towards the reliable production of extremely sharp (10 nm) metallic probes. In this review, the various factors that go into the design of metallic tips, their fabrication, packaging and system integration, characterization, passivation, and eventual use are discussed. Fabrication challenges, implementation issues, optical excitation schemes, and various current and emerging applications are also discussed. For the rapidly emerging fields of plasmonics and nanophotonics, the use of sharp metallic tips has generated significant scientific progress and will play an integral role well into the future.