Raman spectroscopic determination of the structure and orientation of organic monolayers chemisorbed on carbon electrode surfaces

Azobenzene (AB) and 4-nitroazobenzene (NAB) were covalently bonded to carbon surfaces by electrochemical reduction of their diazonium derivatives. The N(1s) features of XPS spectra of modified surfaces had intensities expected for monolayer coverage. However, the Raman spectra were significantly more intense than expected, implying an increase in scattering cross section upon chemisorption. A likely explanation is resonance enhancement of the carbon/adsorbate chromophore analogous to that reported earlier for dinitrophenylhydrazine (DNPH) chemisorption. Vibrational assignments indicate that the C-C vibration between azobenzene and the carbon surface is in the 1240-1280 cm(-1) region, and this conclusion is supported by spectra obtained from [(13)C]graphite. Observation of depolarization ratios for 4-nitroazobenzene and DNPH on graphite edge plane indicate that NAB is able to rotate about the NAB/carbon C-C bond, while chemisorbed DNPH is not. The partial multiple bond character of the DNPH linkage to graphite is consistent with the observation that the DNPH π system remains parallel to the graphitic planes.     

Defect structure of multiwalled carbon nanotubes studied by Raman spectroscopy

The defect structure of multiwalled carbon nanotubes has been studied by transmission electron microscopy and Raman spectroscopy, with particular attention to the shape and intensity of the defect band D and its overtone D*. Electron-microscopic results demonstrate that multiwalled nanotubes typically have multiple bends. The associated short- and long-range disorder influences the Raman spectrum of the nanotubes. The presence of several defect species, differing in scattering probability, results in a stochastic relationship between the intensities of the D*- and D-bands. This relationship is qualitatively interpreted in terms of general mechanisms of elastic/inelastic interactions of π-electrons with phonons and defects.     

Determination of nanotubes properties by Raman spectroscopy

The basic concepts and characteristics of Raman spectra from single-wall carbon nanotubes (SWNTs, both isolated and bundled) are presented. The physical properties of the SWNTs are introduced, followed by the conceptual framework and characteristics of their Raman spectra. Each Raman feature, namely the radial breathing mode, the tangential G band, combination modes and disorder-induced bands are discussed, addressing their physical origin, as well as their capability for characterizing SWNT properties.     

Investigation of the graphitic microstructure in flake and spheroidal cast irons using Raman spectroscopy

It has been demonstrated that Raman spectroscopy is an excellent technique for the characterisation of the graphite in flake and spheroidal cast irons. The Raman spectrum of graphite is highly sensitive to both structural ordering and residual stresses as can be seen from the position, intensity and shape of the Raman bands. The relative intensities and widths of the G Raman bands for flake and spheroidal graphite have been compared and found to correspond to the degree of graphitisation across the graphite. The two Raman bands of the G doublet (G₁ and G₂) change in intensity and width with distance across a graphite spheroid, although their positions remain approximately constant. In contrast, the change in intensity and width of the two Raman bands of the G doublet for flake graphite shows no discernible pattern leading to the conclusion that there is no systematic change of graphitic ordering along a graphite flake. Finally, the fact that the positions of the G₁ and G₂ Raman bands for both flake and spheroidal graphite remain relatively constant with distance implies that there are no residual stresses in the graphite in flake and spheroidal cast iron.     

Determination of technetium and its speciation by surface-enhanced Raman spectroscopy

Technetium-99 (Tc) is an important radionuclide of concern, and there is a great need for its detection and speciation analysis in the environment. For the first time, we report that surface-enhanced Raman spectroscopy (SERS) is capable of detecting an inorganic radioactive anion, pertechnetate (TcO4-), at approximately 10(-7) M concentration levels. The technique also allows the detection of various species of Tc such as oxidized Tc(VII) and reduced and possibly complexed Tc(IV) species by use of gold nanoparticles as a SERS substrate. The primary Raman scattering band of Tc(VII) occurs at about 904 cm-1, whereas reduced Tc(IV) and its humic and ethylenediaminetetraacetic acid (EDTA) complexes show scattering bands at about 866 and 870 cm-1, respectively. Results also indicate that Tc(IV)-humic complexes are unstable and reoxidize to TcO4- upon exposure to oxygen. This study demonstrates that SERS could potentially offer a new tool and opportunity in studying Tc and its speciation and interactions in the environment at low concentrations.     

Surface-enhanced Raman spectroscopy of soft-landed polyatomic ions and molecules

Surface-enhanced Raman spectroscopy (SERS) was used to detect and characterize polyatomic cations and molecules that were electrosprayed into the gas phase and soft-landed in vacuum on plasma-treated silver substrates. Organic dyes such as crystal violet and Rhodamine B, the nucleobase cytosine, and nucleosides cytidine and 2'-deoxycytidine were immobilized by soft landing on plasma-treated metal surfaces at kinetic energies ranging from near thermal to 200 eV. While enhancing Raman scattering 10(5)-10(6)-fold, the metal surface effectively quenches the fluorescence that does not interfere with the Raman spectra. SERS spectra from submonolayer amounts of soft-landed compounds were sufficiently intense and reproducible to allow identification of Raman active vibrational modes for structure assignment. Soft-landed species appear to be microsolvated on the surface and bound via ion pairing or pi-complexation to the Ag atoms and ions in the surface oxide layer. Comparison of spectra from soft-landed and solution samples indicates that the molecules survive soft landing without significant chemical damage even when they strike the surface at hyperthermal collision energies.     

Determination by infrared absorption of the orientation of molecules in monomolecular layers

The mean orientation of molecules constituting Langmuir-Blodgett monomolecular layers was determined by infrared absorption with variable incidence transmission by polarizing the beam in the plane of incidence. The absorption calculation is completely developed for axial anisotropy layers when their thickness is small with respect to the wavelength. The experimental results show that the axis of the aliphatic chains makes an angle of 25±4° with the normal to the substratum for behenic acid layers, whereas this angle is only 8±5° for calcium behenate layers.     

Mixture analysis and quantitative determination of nitrogen-containing organic molecules by surface-enhanced Raman spectrometry

Surface-enhanced Raman spectrometry (SERS) on a silver-coated filter paper substrate of nitrogen-containing organic molecules is reported. A correction procedure for standardization of measurements is proposed and evaluated to solve the difficult problem of quantitation of adsorbate in SERS. The relative standard deviation obtained through this procedure is around 15%. Linearity (r = 0.999) was achieved up to 50 micrograms/mL aminoacridine. A limited dynamic range is observed, however, due to the limited number of SERS active sites in the substrate. Spectral fingerprinting of three-component mixtures by concentration-dependent selective molecular adsorption on the substrate is also reported.     

Orientation of carbon nanotubes in polymers and its detection by Raman spectroscopy

Small amounts of single-wall carbon nanotubes embedded in a polymer matrix were used to sense the mechanical response of the polymer using microRaman spectral data. A flow orientation method was applied to align the nanotubes in the matrix. The Raman spectra obtained for specimens cut both parallel and perpendicular to the flow direction were found to be significantly different, as a function of mechanical strain. Thus Raman spectroscopy combined with mechanical testing provides a way to probe the alignment of nanotubes in composites. The Raman shift-strain response for samples loaded perpendicular to the flow direction suggests that nanotube reorientation is achieved upon straining the polymer beyond its yield point. Our data suggest that the adhesion between the nanotubes and the polymer exceeds the shear yield strength of the matrix. We show that a stress–strain curve for the polymer may be produced directly by means of Raman spectroscopy.     

Determination of water concentration in brain tissue by Raman spectroscopy

Brain edema is one of the most common morbidity factors in patients with intracranial neoplasms and cerebrovascular pathology. Monitoring of intracranial pressure gives only an indirect and global measure of brain swelling. We have made an assessment of the applicability of Raman spectroscopy as an alternative method for assessing brain edema, which measures the water concentration in the tissue directly. Partial least-squares models were developed on the basis of Raman spectra measured in the 2600-3800-cm(-1) region, which predict the water fraction of brain tissue in the 0.75-0.95 range, with an accuracy better than 0.01.     

Discrimination of bacteria and bacteriophages by Raman spectroscopy and surface-enhanced Raman spectroscopy

Detection of pathogenic organisms in the environment presents several challenges due to the high cost and long times typically required for identification and quantification. Polymerase chain reaction (PCR) based methods are often hindered by the presence of polymerase inhibiting compounds and so direct methods of quantification that do not require enrichment or amplification are being sought. This work presents an analysis of pathogen detection using Raman spectroscopy to identify and quantify microorganisms without drying. Confocal Raman measurements of the bacterium Escherichia coli and of two bacteriophages, MS2 and PRD1, were analyzed for characteristic peaks and to estimate detection limits using traditional Raman and surface-enhanced Raman spectroscopy (SERS). MS2, PRD1, and E. coli produced differentiable Raman spectra with approximate detection limits for PRD1 and E. coli of 10(9) pfu/mL and 10(6) cells/mL, respectively. These high detection concentration limits are partly due to the small sampling volume of the confocal system but translate to quantification of as little as 100 bacteriophages to generate a reliable spectral signal. SERS increased signal intensity 10(3) fold and presented peaks that were visible using 2-second acquisitions; however, peak locations and intensities were variable, as typical with SERS. These results demonstrate that Raman spectroscopy and SERS have potential as a pathogen monitoring platform.     

Determination of organic and inorganic carbon in forest soil samples by mid-infrared spectroscopy and partial least squares regression

Analyses of organic and inorganic carbon are of great interest in the field of soil analyses. Soil samples from a national monitoring project were provided for this study, including more than 130 forest sites from Austria. We investigated the humus layers (if present undecomposed litter (L), of mixed samples of F- (intermediate decomposed organic matter) and H-(highly decomposed organic matter) (FH)) and upper mineral soil layers (0-5 and 5-10 cm) of the samples. Mid-infrared spectra were recorded and evaluated by their band areas; subsequently we calculated models with the partial least squares approach. This was done by correlating calculated data of the mid-infrared spectra with gas-volumetrically determined carbonate values and measurements of organic carbon from an elemental analyzer. For carbonate determination, this approach gave satisfying results. For measurements of organic carbon, it was necessary to discriminate into humus layers and mineral soils or even more groups to obtain satisfactory correlations between spectroscopically determined and conventionally measured values. These additional factors were the presence of carbonate, the forest type, and the dominant tree species. In mineral soils, fewer subdivisions were necessary to obtain useful results. In humus layers, groupings of sites with more similar characteristics had to be formed in order to obtain satisfying results. The conclusion is that the chemical background of soil organic matter leading to different proportions of functional groups, especially in the less humified organic matter of the humus layers, plays a key role in analyses with mid-infrared spectroscopy. Keeping this in mind, the present approach has a significant potential for the prediction of properties of forest soil layers, such as, e.g., carbonate and organic carbon contents.     

Ammunition identification by means of the organic analysis of gunshot residues using Raman spectroscopy

The ability to unequivocally identify a gunshot residue (GSR) when a firearm is discharged is a very important and crucial part of crime scene investigation. To date, the great majority of the analyses have focused on the inorganic components of GSR, but the introduction of "lead-free" or "nontoxic" ammunitions makes it difficult to prevent false negatives. This study introduces a fast methodology for the organic analysis of GSR using Raman spectroscopy. Six different types of ammunition were fired at short distances into cloth targets, and the Raman spectra produced by the GSR were measured and compared with the spectra from the unfired gunpowder ammunition. The GSR spectrum shows high similarity to the spectrum of the unfired ammunition, allowing the GSR to be traced to the ammunition used. Additionally, other substances that might be found on the victim's, shooter's, or suspect's clothes and might be confused with GSR, such as sand, dried blood, or black ink from a common ballpoint pen, were analyzed to test the screening capability of the Raman technique. The results obtained evidenced that Raman spectroscopy is a useful screening tool when fast analysis is desired and that little sample preparation is required for the analysis of GSR evidence.     

Employing Raman spectroscopy to qualitatively evaluate the purity of carbon single-wall nanotube materials

Carbon single-wall nanotubes (SWNTs) have highly unique electronic, mechanical and adsorption properties, making them interesting for a variety of applications. Raman spectroscopy has been demonstrated to be one of the most important methods for characterizing SWNTs. For example, Raman spectroscopy may be employed to differentiate between metallic and semi-conducting nanotubes, and may also be employed to determine SWNT diameters and even the nanotube chirality. Single-wall carbon nanotubes are generated in a variety of ways, including arc-discharge, laser vaporization and various chemical vapor deposition (CVD) techniques. In all of these methods, a metal catalyst must be employed to observe SWNT formation. Also, all of the current synthesis techniques generate various non-nanotube carbon impurities, including amorphous carbon, fullerenes, multi-wall nanotubes (MWNTs) and nano-crystalline graphite, as well as larger micro-sized particles of graphite. For any of the potential nanotube applications to be realized, it is, therefore, necessary that purification techniques resulting in the recovery of predominantly SWNTs at high-yields be developed. It is, of course, equally important that a method for determining nanotube wt.% purity levels be developed and standardized. Moreover, a rapid method for qualitatively measuring nanotube purity could facilitate many laboratory research efforts. This review article discusses the application of Raman spectroscopy to rapidly determine if large quantities of carbon impurities are present in nanotube materials. Raman spectra of crude SWNT materials reveal tangential bands between 1500-1600 cm(-1), as well as a broad band at approximately 1350 cm(-1), attributed to a convolution of the disorder-induced band (D-band) of carbon impurities and the D-band of the SWNTs themselves. Since the full-width-at-half-maximum (FWHM) intensity of the various carbon impurity D-bands is generally much broader than that of the nanotube D-band, an indication of the SWNT purity level may be obtained by simply examining the line-width of the D-band. We also briefly discuss the effect of nanotube bundling on SWNT Raman spectra. Finally, sections on employing Raman spectroscopy, and Raman spectroscopy coupled with additional techniques, to identify the separation and possible isolation of a specific nanotube within purified SWNT materials is provided. Every SWNT can be considered to be a unique molecule, with different physical properties, depending on its (n, m) indices. The production of phase-pure (n, m) SWNTs may be essential for some nanotube applications.     

Raman spectroscopy as a probe of graphene and carbon nanotubes

Progress in the use of Raman spectroscopy to characterize graphene samples for the number of graphene layers and doping level they contain is briefly reviewed. Comparisons to prior studies on graphites and carbon nanotubes are used for inspiration to define future promising directions for Raman spectroscopy research on few layer graphenes.     

Raman spectroscopy and structure of crystalline gallium phosphide nanowires

Gallium phosphide nanowires with a most probable diameter of approximately 20.0 nm and more than 10 microns in length have been synthesized by pulsed laser vaporization of a heated GaP/5% Au target. The morphology and microstructure of GaP nanowires have been investigated by scanning electron microscopy and transmission electron microscopy. Twins have been observed along the crystalline nanowires, which have a growth direction of [111]. Raman scattering shows a 4 cm-1 downshift of the longitudinal optical phonon peak in the nanowire with respect to the bulk; the transverse optical phonon frequency and line width are, however, the same as in the bulk. The quantum confinement model first proposed by Richter et al. cannot explain the observed behavior of the Raman modes.     

Vibration and orientation of diatomic molecules flowing through small carbon nanotubes

With their unique long cylindrical shape, carbon nanotubes may one-day form nozzles for nano-scale printing or flow into a chamber. Since the scale of the flowing molecules is similar to the diameter of the nanotubes, molecular vibration, orientation and density become influenced by the confinement during flow. We have studied the flow of diatomic molecules through carbon nanotube nozzles using non-equilibrium molecular dynamics simulations, in an effort to gain a greater understanding about the fundamental properties of such molecules in such a setting. The frequency of vibration of the molecules is shown to be dependent on the density inside the nanotubes and follow the same relation as an experimental micro-scale density-frequency study suggests, although only for nanotubes above a certain diameter. Meanwhile no relation is found between the frequency of vibration and the flow rate. The effect of nanotube diameter on the orientation of the molecules is also examined in detail, showing the transition between axial and radial orientation, with "pull" and "push" effects determining the orientation.     

Stable and controlled amphoteric doping by encapsulation of organic molecules inside carbon nanotubes

Single-walled carbon nanotubes (SWNTs) have strong potential for molecular electronics, owing to their unique structural and electronic properties. However, various outstanding issues still need to be resolved before SWNT-based devices can be made. In particular, large-scale, air-stable and controlled doping is highly desirable. Here we present a method for integrating organic molecules into SWNTs that promises to push the performance limit of these materials for molecular electronics. Reaction of SWNTs with molecules having large electron affinity and small ionization energy achieved p- and n-type doping, respectively. Optical characterization revealed that charge transfer between SWNTs and molecules starts at certain critical energies. X-ray diffraction experiments revealed that molecules are predominantly encapsulated inside SWNTs, resulting in an improved stability in air. The simplicity of the synthetic process offers a viable route for the large-scale production of SWNTs with controlled doping states.     

Review Polarised Raman spectroscopy for the study of molecular orientation distributions in polymers

Polarised Raman spectroscopy is a vibrational spectroscopic technique that is used widely for the chemical and physical analyses of materials since it is both non-destructive and suitable for remote analysis. In particularly over the last 40 years, the technique has been developed and applied for the study of molecular orientation distributions in polymers. Compared to other analytical techniques, polarised Raman spectroscopy has the following advantages, (1) quantitative and precise measurement of molecular orientation distributions, and (2) study of these distributions in both the crystalline and amorphous phases. Knowledge obtained from the technique is of both academic and industrial interest to study relationships between microstructure and macroscopic physical properties in polymers. In this paper, polarised Raman spectroscopy is reviewed with regard to the study of molecular orientation distributions in polymeric materials. The basis of polarised Raman scattering is first described, and this is followed by the procedure for obtaining spectra. It is shown how Raman scattering intensities for different polarised scattering geometries can be interpreted to give parameters and functions representing quantitative measures of the degree of molecular orientation. Factors affecting the evaluation of these parameters are also summarised. Finally, the usefulness of the technique is demonstrated by practical applications including a study of molecular orientation distributions in poly(ethylene terephthalate) (PET) fibres.     

Resonance Raman spectroscopy characterization of single-wall carbon nanotube separation by their metallicity and diameter

Several techniques were recently reported for the bulk separation of metallic (M) and semiconducting (S) single wall carbon nanotubes (SWNTs), using optical absorption and resonance Raman spectroscopy (RRS) as a proof of the separation. In the present work, we develop a method for the quantitative evaluation of the M to S separation ratio, and also for the SWNT diameter selectivity of the separation process, based on RRS. The relative changes in the integrated intensities of the radial-breathing mode (RBM) features, with respect to the starting material, yield the diameter probability distribution functions for M and S SWNTs in the separated fractions, accounting for the different resonance conditions of individual SWNTs, while the diameter distribution of the starting material is obtained following the fitting procedure developed by Kuzmany and coworkers. Features other than the RBM are generally less effective for characterization of the separation process for SWNTs.