Showing papers on "Raman spectroscopy published in 1997"

Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering

Abstract: Optical detection and spectroscopy of single molecules and single nanoparticles have been achieved at room temperature with the use of surface-enhanced Raman scattering. Individual silver colloidal nanoparticles were screened from a large heterogeneous population for special size-dependent properties and were then used to amplify the spectroscopic signatures of adsorbed molecules. For single rhodamine 6G molecules adsorbed on the selected nanoparticles, the intrinsic Raman enhancement factors were on the order of 10 14 to 10 15 , much larger than the ensemble-averaged values derived from conventional measurements. This enormous enhancement leads to vibrational Raman signals that are more intense and more stable than single-molecule fluorescence.

Raman microspectroscopy of some iron oxides and oxyhydroxides

Abstract: Hematite (α-Fe2O3), magnetite (Fe3O4), wustite (FeO), maghemite (γ-Fe2O3), goethite (α-FeOOH), lepidocrocite (γ-FeOOH) and δ-FeOOH were studied by Raman microscopy. Such compounds have already been studied by Raman spectroscopy, but there are some disagreements in the reported data. Here, Raman microscopy was employed to investigate the laser power dependence of the spectra of these oxides and oxyhydroxides. Low laser power was used for the reference spectra in order to minimize the risks of spectral changes due to sample degradation. The results obtained show that increasing laser power causes the characteristic bands of hematite to show up in the spectra of most of the compounds studied whereas the hematite spectrum undergoes band broadening and band shifts. © 1997 John Wiley & Sons, Ltd.

Diameter-Selective Raman Scattering from Vibrational Modes in Carbon Nanotubes

Abstract: Single wall carbon nanotubes (SWNTs) that are found as close-packed arrays in crystalline ropes have been studied by using Raman scattering techniques with laser excitation wavelengths in the range from 514.5 to 1320 nanometers. Numerous Raman peaks were observed and identified with vibrational modes of armchair symmetry (n, n) SWNTs. The Raman spectra are in good agreement with lattice dynamics calculations based on C-C force constants used to fit the two-dimensional, experimental phonon dispersion of a single graphene sheet. Calculated intensities from a nonresonant, bond polarizability model optimized for sp2 carbon are also in qualitative agreement with the Raman data, although a resonant Raman scattering process is also taking place. This resonance results from the one-dimensional quantum confinement of the electrons in the nanotube.

Evidence for charge transfer in doped carbon nanotube bundles from Raman scattering

Abstract: Single-walled carbon nanotubes1 (SWNTs) are predicted to be metallic for certain diameters and pitches of the twisted graphene ribbons that make up their walls2. Chemical doping is expected to substantially increase the density of free charge carriers and thereby enhance the electrical (and thermal) conductivity. Here we use Raman spectroscopy to study the effects of exposing SWNT bundles1 to typical electron-donor (potassium, rubidium) and electron-acceptor (iodine, bromine) dopants. We find that the high-frequency tangential vibrational modes of the carbon atoms in the SWNTs shift substantially to lower (for K, Rb) or higher (for Br2) frequencies. Little change is seen for I2 doping. These shifts provide evidence for charge transfer between the dopants and the nanotubes, indicating an ionic character of the doped samples. This, together with conductivity measurements3, suggests that doping does increase the carrier concentration of the SWNT bundles.

Raman Investigation of SiC Polytypes

Abstract: It has been recognized that Raman scattering spectroscopy is a powerful tool to characterize SiC crystals non-destructively. We review recent significant developments in the use of Raman scattering to study structural and electronic properties of SiC crystals. The areas to be discussed in the first part include polytype identification, evaluation of stacking disorder and ion-implantation damages, and stress evaluation. The Raman scattering by electronic transitions is discussed in the second part of this article. We concentrate on the plasmon LO-phonon coupled modes whose spectral profiles are used to evaluate the carrier concentration and mobility. Anisotropic electronic properties of α-SiC and characteristics of heavily doped crystals are discussed. Semiconductor-to-metal transition and Fano interference effect are also treated.

Measurement of Clathrate Hydrates via Raman Spectroscopy

Abstract: Raman spectra of clathrate hydrate guest molecules are presented for three known structures (I (sI), II (sII), and H (sH)) in the following systems: CH4 (sI), CO2 (sI), C3H8 (sII), CH4 + CO2 (sI), CD4 + C3H8 (sII), CH4 + N2 (sI), CH4 + THF-d8 (sII), and CH4 + C7D14 (sH). Relative occupancy of CH4 in the large and small cavities of sI were determined by deconvoluting the ν1 symmetric bands, resulting in hydration numbers of 6.04 ± 0.03. The frequency of the ν1 bands for CH4 in structures I, II, and H differ statistically, so that Raman spectroscopy is a potential tool to identify hydrate crystal structure. Hydrate guest compositions were also measured for two vapor compositions of the CH4 + CO2 system, and they compared favorably with predictions. The large cavities were measured to be almost fully occupied by CH4 and CO2, whereas only a small fraction of the small cavities are occupied by CH4. No CO2 was found in the small cavities. Hydration numbers from 7.27 to 7.45 were calculated for the mixed hydrate.

Ring effect studies: Rayleigh scattering, including molecular parameters for rotational Raman scattering, and the Fraunhofer spectrum.

Abstract: Improved parameters for the description of Rayleigh scattering in air and for the detailed rotational Raman scattering component for scattering by O2 and N2 are presented for the wavelength range 200 ‐1000 nm. These parameters enable more accurate calculations to be made of bulk molecular scattering and of the Ring effect for a variety of atmospheric radiative transfer and constituent retrieval applications. A solar reference spectrum with accurate absolute vacuum wavelength calibration, suitable for convolution with the rotational Raman spectrum for Ring effect calculations, has been produced at 0.01-nm resolution from several sources. It is convolved with the rotational Raman spectra of O2 and N2 to produce an atmospheric Ring effect source spectrum. © 1997 Optical Society of America

Organic Structural Spectroscopy

Abstract: Preface 1 Introduction I NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY 2 Introduction and Experimental Methods 3 The Chemical Shift 4 The Coupling Constant 5 Further Topics in One-Dimensional NMR 6 Two-Dimensional NMR II VIBRATIONAL SPECTROSCOPY 7 Introduction and Experimental Methods 8 Group Frequencies: Infrared and Raman 9 Structure Analysis III ELECTRONIC ABSORPTION AND CHIROPTICAL SPECTROSCOPY 10 Introduction and Experimental Methods 11 UV-Vis, CD, and ORD 12 Structure Analysis IV MASS SPECTROMETRY 13 Ionization and Mass Analysis 14Fragmentation and Ion Chemistry 15 Chemical Analysis Appendix to Part IV V INTEGRATED PROBLEMS 16 Integrated Problems 15 Index

Purification of single-wall carbon nanotubes by microfiltration

Abstract: A purification procedure for single-wall carbon nanotubes (SWNTs) prepared by pulsed laser ablation is discussed, which separates coexisting carbon nanospheres (CNS), metal nanoparticles, polyaromatic carbons, and fullerenes from the SWNT fraction. The process involves the suspension of CNS, metal nanoparticles, and SWNTs in an aqueous solution using a cationic surfactant and the subsequent trapping of SWNTs on a membrane filter. No oxidative treatment is required. Scanning/transmission electron microscopy and Raman scattering were used to evaluate the purification process and the vibrational features of SWNTs. Purity of SWNTs at the final stage sample is in excess of 90% by weight, and no evidence of impurity carbon phases was revealed in the Raman spectrum of the SWNT fraction.

Raman characterization of aligned carbon nanotubes produced by thermal decomposition of hydrocarbon vapor

Abstract: Raman characterization of aligned carbon nanotubes of average diameter 10–15 nm, produced by chemical vapor deposition on a mesoporous substrate, has been carried out. The resonance behavior and higher-order Raman bands up to fourth order have been observed and compared with those of carbon nanotubes produced by arc discharge and highly oriented pyrolytic graphite, as well as pyrolytic graphite. The phonon properties have been analyzed with the help of high-resolution transmission electron microscope studies.

FT-Raman Spectroscopy of Wood: Identifying Contributions of Lignin and Carbohydrate Polymers in the Spectrum of Black Spruce (Picea Mariana):

Abstract: Good-quality Raman spectra of most wood species can now be obtained by using near-infrared Fourier transform Raman spectroscopy. To make effective use of such spectroscopic information, one needs to interpret the data in terms of contributions from various wood components and, for each component polymer, in terms of vibrational modes of its substructural units/groups. In the present work, Raman spectral features of black spruce (Picea mariana) wood were associated with lignin and/or carbohydrate polymers. Lignin's spectral contributions were recognized in several ways. In addition to spectra of milled-wood and enzyme lignins, a spectrum of native lignin was obtained by subtracting the spectrum of acid chlorite delignified black spruce from the spectrum of an untreated wood sample. A comparison of lignin spectra indicated that the Raman features of the three lignins are very similar. Raman contributions of carbohydrate polymers, namely, those of cellulose and hemicellulose, were identified by using authentic and/or isolated samples and, in the case of cellulose, by using previously published spectra. Such an analysis showed that the hemicellulose present in black spruce did not give rise to any new, unique features that were not already present due to cellulose. Therefore, it was concluded that the hemicellulose contribution is broad and is hidden under the Raman contribution of cellulose. Also, peak positions of lignin contributions did not overlap with those of cellulose, and there were spectral regions where either lignin or cellulose contributed.

Raman and photoluminescence studies of biaxial strain in GaN epitaxial layers grown on 6H–SiC

Abstract: The effect of biaxial strain on optical phonons in high-quality GaN epitaxial layers grown on 6H–SiC substrates by metal organic chemical vapor deposition has been studied. The deformation potential constants for the E2(1), A1(TO), E1(TO), and E2(2) optical phonon modes in hexagonal GaN have been obtained. A method for calculating strain in hexagonal GaN layers from Raman data alone is suggested. A comparative analysis of the strain measured by x-ray diffraction and Raman spectroscopy shows that these data agree well. It is found that the biaxial stress of 1 GPa results in a shift of the excitonic photoluminescence lines of 20±3 meV.

Nitrogen modification of hydrogenated amorphous carbon films

Abstract: The effect of nitrogen addition on the structural and electronic properties of hydrogenated amorphous carbon (a-C:H) films has been characterized in terms of its composition, sp3 bonding fraction, infrared and Raman spectra, optical band gap, conductivity, and paramagnetic defect. The variation of conductivity with nitrogen content suggests that N acts as a weak donor, with the conductivity first decreasing and then increasing as the Fermi level moves up in the band gap. Compensated behavior is found at about 7 at. % N, for the deposition conditions used here, where a number of properties show extreme behavior. The paramagnetic defect density and the Urbach tailwidth are each found to decrease with increasing N content. It is unusual to find alloy additions decreasing disorder in this manner.

Raman spectroscopy as a morphological probe for tio2 aerogels

Abstract: We report on the application of Raman scattering as a probe to follow the structural evolution of TiO2 aerogels. We have traced the evolution of the 142 and 630 cm-1 Eg anatase peaks as a function of morphology. The results have been analyzed in terms of the q vector relaxation mechanism which predicts a unique relation between particle size and features of the Raman spectra. Since the phonon dispersion of anatase is not known, we have used the analogous rutile dispersion. There is good agreement between the particle sizes evaluated from the model and those determined by X-ray diffraction. Our results demonstrate that Raman spectroscopy can be an effective tool in monitoring the evolution of these aerogel systems.

Raman study of layered rock‐salt LiCoO2 and its electrochemical lithium deintercalation

Abstract: Unpolarized and polarized Raman spectra (200–800 cm-1) of LiCoO2 with a layered rock-salt structure were measured. The Raman-active lattice modes of LiCoO2 were assigned by polarized Raman measurements of a c-axis oriented thin film. The variation of the Raman spectra of Li1-xCoO2 powder prepared by electrochemical lithium deintercalation was investigated, and the spectral changes were well correlated with the structural changes determined by x-ray diffraction except that peak splitting by the distortion in the monoclinic phase was not observed. The observed line broadening of the second hexagonal phase and the monoclinic phase indicated that the lithium ions remaining in the lattice after deintercalation randomly occupy the available sites on the lithium planes in the lattice the layered rock-salt structure. © 1997 John Wiley & Sons, Ltd.

Structural design of sealing glasses

Abstract: Requirements for enhanced component performance and reliability have led to the development of novel glass compositions for a variety of hermetic sealing applications. The development of technologically useful glass compositions was based on an understanding of the relationships between the molecular-level glass structure and important physical properties. The properties of the alkaline earth aluminoborate glasses for lithium batteries are sensitive to changes in B- and Al-coordination number, characterized by solid state nuclear magnetic resonance (NMR) spectroscopy. In general, the most useful compositions have structures that are dominated by tetrahedral Band Al-sites. Mixed alkali aluminophosphate glasses were developed for aluminum electrical connectors. The properties of sodium aluminophosphate glasses depend on the O/P ratio and significant property changes (e.g. maxima in T g and refractive index) occur when O/P exceeds the pyrophosphate limit at 3.5. Associated with these property changes is a decrease in the average Al-coordination number, from six to four, at O/P > 3.5. Raman spectroscopy provides additional information about the aluminophosphate network. Finally, zinc borophosphate glasses are developed for seals in flat panel displays. Boron-11 NMR shows that tetrahedral borons are preferred in xB 2 O 3 (1-x)(PO 3 ) 2 and in yB 2 O 3 (1 - y)Zn 2 P 2 O 7 glasses for x < 0.4 and y < 0.2. Raman spectroscopy reveals the concomitant evolution from a phosphate to a borophosphate network with increasing x and y.

Microstructural change of nano-sno2 grain assemblages with the annealing temperature

Abstract: The microstructural changes of nano-SnO2 have been systematically studied using x-ray diffraction and Raman spectroscopy. The nano-SnO2 grain assemblage possesses some features of the rutile structure, but has a large amount of defects: vacancies of oxygen, vacancy clusters, and local lattice disorder at the interface and interior surface, which lead to a significant lattice distortion and an evident space-symmetry reduction of D-4h(14) and the appearance of a group of new Raman peaks. Two major Raman peaks N-1 and N-2 in accordance with Matossi's force constant model have been found. When the annealing temperature is close to 673 K, the density of vacant lattice sires and local lattice disorders decreased rapidly in the grains, and the lattice distortion and Raman peaks N-1 and N-2 almost disappeared at the same time. It suggests that N-1 and N-2 are closely related to the microstructural change of the nanocluster grains, or in other words, N-1 and N-2 peaks mark an additional characteristic of space symmetry of the grain assemblage of nano-SnO2. The Raman peak N-3 may be related to local SnO2 clusters and vacancy clusters.

Effect of hydrogen dilution on the structure of amorphous silicon alloys

Abstract: We investigate why high levels of hydrogen dilution of the process gas lead to enhanced light soaking stability of amorphous silicon (a-Si) alloy solar cells by studying the microstructural properties of the material using high-resolution transmission electron microscopy (TEM) and Raman spectroscopy. The TEM results show that a-Si alloy (with or without hydrogen dilution) is a heterogeneous mixture of amorphous network and linear-like objects that show evidence of order along their length. The volume fraction of these ordered regions increases with increasing hydrogen dilution.

Characterization of the structure of binary zinc ultraphosphate glasses by infrared and Raman spectroscopy

Abstract: The Raman and infrared spectra of zinc phosphate glasses, xZnO·(1−x)P2O5 with 0≤x≤0.5, and the crystalline compounds, o′-P2O5, ZnP4O11, α1-, α2-, and β-Zn(PO3)2, were measured. An analysis of the vibrational spectra is discussed in comparison with the results of the investigation of the same glasses by 31 P -MAS–NMR spectroscopy and X-ray diffraction. The structure of the zinc phosphate glasses can be described by substructures of the high-temperature modifications of the three crystalline compounds with x=0, 0.33, and 0.5.

Direct observation of sp3 bonding in tetrahedral amorphous carbon using ultraviolet raman spectroscopy

Abstract: The vibrational modes of the sp3 sites in tetrahedral amorphous carbon (ta-C) thin films are revealed directly using ultraviolet Raman spectroscopy at 244 nm excitation and are shown to produce a Raman peak centered around 1100 cm−1. In addition, the main Raman peak associated with sp2 vibrational modes is shifted upward in frequency by 100 cm−1 relative to its position in spectra excited at 514 nm. The spectra are interpreted in terms of the bonding in ta-C.

Raman-scattering study of exciton-phonon coupling in PbS nanocrystals

Abstract: The exciton-phonon coupling strength of PbS nanocrystals that strongly confine both charge carriers is investigated using resonant Raman scattering. The strength of this coupling (Huang-Rhys parameter S\ensuremath{\sim}0.7) is four orders of magnitude greater than that predicted theoretically for the intrinsic states of the nanocrystal, but is the same order of magnitude as that measured for Cd(S,Se) nanocrystals. The large exciton-phonon coupling is consistent with one charge carrier being localized at the surface of the nanocrystal.

Diagnosis of Basal Cell Carcinoma by Raman Spectroscopy

Abstract: Skin cancers are the most common form of malignant neoplasms in man. In this work, near-infrared Fourier transform (NIR-FT) Raman spectroscopy was used to study the molecular alterations in the most common skin cancer, basal cell carcinoma (BCC). Biopsies from 16 histopathologically veri‐ed BCC and 16 biopsies from normal skin were harvested and analysed by NIR-FT-Raman spectroscopy using a 1064 nm Nd : YAG laser as a radiation source. Di†erences in Raman spectra between BCC and normal skin indicated alterations in protein and lipid structure in skin cancer samples. Spectral changes were observed in protein bands, amide I (1640‐1680 cm~1), amide III (1220‐1300 cm~1) and m(C‐C) streching (probably in the amino acids proline and valine, 928‐940 cm~1), and in bands characteristic of lipids, scissoring vibration (1420‐1450 cm~1) and in-phase CH 2 E(CH 2 ) n E twist vibration around 1300 cm~1. Moreover, possible changes in polysaccharide structure were found in the region 840‐860 cm~1. Analysis of the band intensities in the regions 1220‐1360, 900‐990 and 830‐900 cm~1 allowed for a complete separation between BCC and normal skin spectra. In addition to a direct assessment of spectral intensities, a neural network analysis was performed, which con‐rmed the di†erences in spectra between BCC and normal skin. In conclusion, Raman spectra from BCC di†er considerably from those of normal skin. Hence, Raman spectroscopy can be viewed as a promising tool for the diagnosis of skin cancer. 1997 by John Wiley &