Showing papers on "Raman spectroscopy published in 1995"

Infrared and Raman spectroscopy : methods and applications

Abstract: Early history of vibrational spectroscopy general survey of vibrational spectroscopy tools for infrared and Raman spectroscopy vibrational spectroscopy of different classes and states of compounds evaluation procedures special techniques and applications (Part contents)

Growth and Characterization of Sexithiophene Single Crystals

Abstract: Single crystals of unsubstituted sexithiophene (6T) were produced by a sublimation technique. X-ray structure shows that the molecule has a quasi-planar all-trans configuration, the torsional angles between adjacent rings being lower than their respective standard deviation. The unit cell belongs to space group P21h and presents the herringbone packing common to a great deal of planar molecules. Owing to the well-defined orientation of the molecules in single crystals, the polarization of vibration modes and coupling due to crystalfield effects could be unambiguously determined from polarized light IR spectroscopy. Raman spectra of the single crystals are also presented and compared to that of polycrystalline evaporated films.

Discrete dipole approximation for calculating extinction and Raman intensities for small particles with arbitrary shapes

Abstract: We present a discrete dipole approximation (DDA) method to determine extinction and Raman intensities for small metal particles of arbitrary shape. The Raman intensity calculation involves evaluation of surface electromagnetic fields, and thus is relevant to surface enhanced Raman scattering (SERS) intensities. We demonstrate convergence of the method by considering light absorption and scattering from an isolated spheroid, from an isolated tetrahedron, from two coupled spheroids, and from a spheroid on a flat surface. We also examine comparisons with traditional T‐matrix methods. Extensions and simplifications of the method in studies of clusters and arrays of particles are presented.

The Raman spectrum of brookite, TiO2 (Pbca, Z = 8)

Abstract: The Raman microprobe spectra of natural brookite crystals from Switzerland and Brazil and a synthetic brookite powder exhibit a characteristic intense band at 153 cm−1. In contrast, anatase has a band of similar intensity at 144 cm−1 and rutile lacks a strong band in this region. Polarization experiments with the Brazilian crystal permit 17 out of a predicted 36 vibrational bands to be readily assigned as A1g (127, 154, 194, 247, 412, 640 cm−1), B1g (133, 159, 215, 320, 415, 502 cm−1), B2g (366, 395, 463, 584 cm−1) and B3g (452 cm−1). Eight weaker bands and component sub-bands resolved at 172, 287, 545, 618 cm−1, 254, 329, 476 cm−1 and 497 cm−1 are tentatively assigned to B3g, B2g and A1g modes respectively. Traces of Si and Fe in the Brazilian crystal and Si, Fe, Al and S in the Swiss sample are indicated by spot electron microprobe analysis. Electron spin resonance spectra suggest small amounts of Fe3+ are substituting for Ti4+ in the structure. Concomitant protonation of oxygens to maintain charge balance yields OH groups which give rise to three sharp, low-intensity absorption bands near 3360, 3380 and 3404 cm−1 in the infrared spectra. X-ray photoelectron spectroscopy shows that surface-bound OH/H2O species exist in both crystals but no detectable Ti3+.

Visible photoluminescence from nanocrystallite Ge embedded in a glassy SiO 2 matrix: Evidence in support of the quantum-confinement mechanism

Abstract: Nanocrystallite Ge (nc-Ge) embedded in a glassy ${\mathrm{SiO}}_{2}$ matrix is fabricated and examined by x-ray photoelectron spectrometry, Raman spectrometry, and high-resolution transmission-electron microscopy. The precipitation and growth of nc-Ge are found to be related to a thermodynamical reduction of ${\mathrm{GeO}}_{2}$, the diffusion of Si atoms from the Si substrate into the glassy matrix, and an aggregation of small-sized nc-Ge. The size inhomogeneity can be precisely controlled by a double annealing, and the average size can be changed in the range of 2\char21{}6 nm. Broadband photoluminescence (PL) spectra are observed in the visible wavelength range at room temperature, and they exhibit pronounced blueshifts of the peak energies and broadening of the PL spectra, which can be correlated to the change in the size. The PL excitation spectra show a Stokes energy smaller than 0.1 eV and dependence on the measurement energy. The visible PL also shows a strong correlation to the presence and actual condition (i.e., size) of nc-Ge. Possible origins of the visible PL such as a quantum-confinement model in quantum dots, the presence of luminescent centers (Ge:E') in silica glass, or a structural transition of nc-Ge are discussed. The present experimental data are concluded to be more consistent with a quantum-confinement model than with the other models.

The short-range structure of zinc polyphosphate glass

Abstract: 31 P magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy and Raman spectroscopy have been used to examine the polyhedral arrangements in x ZnO · (1 − x )P 2 O 5 (0.50 ≤ x ≤ 0.71) glasses. The depolymerization of P metaphosphate chains by the addition of ZnO is quantitatively described by the increase in the concentration of Q 1 -phosphate sites, determined from the 31 P MAS-NMR spectra. When x > 0.60, the NMR and Raman spectra exhibit peaks due to Q 0 and Q 2 tetrahedra, indicating that structures disproportionate in glass melts near the pyrophosphate composition. The splitting of the Raman peak due to the Q 1 terminal oxygen stretching mode indicates that a variety of P-O-Zn bonds participate in the polyphosphate glass structure. The complex mixture of P and Zn polyhedra contributes to the glass-forming tendency of the high ZnO (> 60 mol%) compositions.

Reactions of Organic Monolayers on Carbon Surfaces Observed with Unenhanced Raman Spectroscopy

Abstract: High sensitivity surface Raman spectroscopy was used to acquire spectra from monolayers of nitrophenyl groups covalently bonded to glassy carbon (GC) and ordered graphite. 4-Nitrophenyl radical was generated by electroreduction of 4-nitrophenyl diazonium ion in acetonitrile, then the radical covalently bonded to the carbon surface. Raman spectroscopy with a 488 nm laser and a CCD spectrograph revealed spectral features characteristic of the nitrophenyl group. Voltammetry, XPS, and Raman spectroscopic data established that the surface coverage was 6.5 x mol/cm2 on the basal plane of highly ordered pyrolytic graphite (HOPG). Electrochemical reduction of the nitrophenyl surface yielded significant spectral changes, perhaps due to formation of an amine. The similarity of the Raman results for modified HOPG and GC surface indicates that the nitrophenyl radical is quite aggressive, apparently attacking both edge plane and basal plane sites.

Transformation of stishovite to a denser phase at lower-mantle pressures

Abstract: WHETHER stishovite, the highest-pressure polymorph of SiO2 known from natural samples, transforms to a denser structure at higher pressures has long been of interest. A suggested transition from rutile to the CaCl2 structure driven by a vibrational-mode instability1 was supported by the observation of a frequency decrease (softening) of a Raman mode with increasing pressure2. Subsequent X-ray diffraction measurements provided evidence3 for stability of the CaCl2 phase near 100 GPa. Electronic-structure calculations predict, however, that the transition occurs at much lower pressure, where a shear modulus vanishes and before the Raman mode softens completely4. Here we use in situ Raman spectroscopy and a new theoretical model to investigate the high-pressure behaviour of stishovite. At 50 GPa, the pressure dependence of the soft B1g mode abruptly changes and the Eg mode splits, as predicted for transformation to the CaCl2 structure. Our results demonstrate that any free silica in the, deep mantle (below 1,200-1,500 km) will exist in the CaCl2 structure at considerably lower pressures than previously thought3.

In situ Raman study on electrochemical Li intercalation into graphite

Abstract: Electrochemical lithium intercalation into graphite materials has been extensively studied for use in negative electrodes of secondary lithium batteries. Electrochemical lithium intercalation into highly oriented pyrolytic graphite and natural graphite powder was investigated using in situ Raman spectroscopy. Three plateaus were observed on the charging curve for both samples. From the Raman spectral changes the first plateau was assigned to a phase transition from dilute stage 1 to stage 4, the second from a stage 2 phase to another stage 2 phase, and the third from stage 2 to stage 1, which are in good agreement with Dahn's results by in situ X-ray diffraction. The spectral changes associated with the phase transitions occurred reversibly during a charge and discharge cycle. It was shown from the Raman spectral changes of the HOPG electrode that the electrode potential during the electrochemical intercalation is determined by the surface stage of graphite intercalation compounds.

Polarized Raman spectra in GaN

Abstract: We have measured polarized Raman spectra in a 2.0 mu m GaN epitaxial layer of high quality, grown on a sapphire substrate. All symmetry-allowed optical phonons in GaN have been assigned as follows: A1(LO), 735 cm-1; A1(TO), 533 cm-1; E1(LO), 743 cm-1; E1(TO), 561 cm-1; E2, 144 and 569 cm-1. Using the Lyddane-Sachs-Teller relation, the static dielectric constants of GaN for the ordinary and extraordinary directions have been estimated as epsilon perpendicular to 0=9.28 and E/sub //0/=10.1. We have also observed quasi-LO phonons in GaN. A brief discussion on these will be given.

The intermolecular dynamics of liquid water

Abstract: We present the complete intermolecular dynamical spectrum of liquid water, by merging the data sets from femtosecond nonlinear‐optical polarization spectroscopy with the depolarized, Bose–Einstein corrected Raman spectrum to cover the frequency range from 0–1200 cm−1. The impulse response function for liquid water at room temperature is calculated, including all of the intermolecular motions.

Raman scattering from nanometer-sized diamond

Abstract: We have measured Raman spectra of diamond with nanometer size, called cluster diamond. The Raman bands assigned to sp2 and sp3 clusters have been observed at around 1600 and 1322 cm−1, respectively. This result suggests that the cluster diamond slightly contains the sp2 cluster. The Raman band assigned to sp3 cluster is found to shift by −10 cm−1, compared with that of bulk crystal and to be asymmetric with some tailing toward lower Raman frequency. The observed Raman spectrum agrees well with that calculated by a phonon confinement model. The crystallite size of the cluster diamond estimated from the phonon confinement model agrees approximately with that estimated from x‐ray measurement. Raman spectroscopy gives some information about the crystallite size of diamond particles with nanometer size.

High-pressure synthesis, characterization, and equation of state of cubic C-BN solid solutions.

Abstract: Synthesis of several samples across the cubic ${\mathrm{C}}_{\mathit{x}}$(BN${)}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ solid solution (x=0.3--0.33,0.5,0.6) at pressures in excess of 30 GPa and temperatures above 1500 K indicates that they are isostructural with diamond and cubic BN (borazon). Measurement of the lattice parameters of C-BN samples quenched to ambient conditions shows that the solid solution between C (diamond) and cubic BN is nonideal, with unit-cell volumes up to 1% larger than predicted based on ideal mixing (Vegard's law). In addition, we have measured the zero-pressure, 300-K vibrational spectra for ${\mathrm{C}}_{0.3}$(BN${)}_{0.7}$. In the midinfrared absorption spectrum, we observe a reststrahlen band ranging from 1000--1120 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, and the Raman spectrum has a longitudinal optic mode at 1323(\ifmmode\pm\else\textpm\fi{}2) ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Based on a comparison of the spectrum with that of diamond and cubic BN, we conclude that the bonding in cubic C-BN is partially ionic. In addition to these measurements on quenched samples, we have measured the isothermal bulk modulus of ${\mathrm{C}}_{0.33}$(BN${)}_{0.67}$ by x-ray diffraction through a diamond cell to over 100 GPa at 300 K. The bulk modulus is 355(\ifmmode\pm\else\textpm\fi{}19) GPa, which is lower than those of diamond, cubic BN and the value predicted from ideal mixing between the end members, but is consistent with the nonideal expansion observed for the cubic C-BN lattice parameters.

Raman spectroscopy of amorphous and microcrystalline silicon films deposited by low‐pressure chemical vapor deposition

Abstract: In this work we used Raman spectroscopy to investigate the structural characteristics of as‐deposited amorphous and micro‐crystalline silicon films. For amorphous silicon films, the order (or disorder) of the silicon network was quantified using properties of the Raman spectra that were related to key deposition conditions. We found that a strong relationship exists between the structural order of the silicon matrix and the deposition temperature and deposition rate. A quantitative model was proposed relating the intensity ratio of transverse optical phonon peak to longitudinal optical phonon peak to the surface diffusion length, a parameter that was calculated from available data. It was found that optimization of the as‐deposited silicon microstructure is possible by selecting deposition conditions yielding peak–ratio values in the vicinity of 0.53. For as‐deposited micro‐crystalline silicon films, Raman spectroscopy was used to estimate the initial crystalline fraction of the film and monitor the cryst...

A Highly Ordered Self-Assembled Monolayer Film of an Azobenzenealkanethiol on Au(111): Electrochemical Properties and Structural Characterization by Synchrotron in-Plane X-ray Diffraction, Atomic Force Microscopy, and Surface-Enhanced Raman Spectroscopy

Abstract: The synthesis and characterization of p-HS(CH{sub 2}){sub 11}OC{sub 6}H{sub 4}N=NC{sub 6}H{sub 5}, compound 1d, is reported. Compound 1d self-assembles onto Au(111) substrates into highly ordered monolayer films. Self-assembled monolayer films (SAMs) of 1d on Au(111)/mica have been characterized by ellipsometry, surface-enhanced Raman spectroscopy (SERS), and atomic force microscopy (AFM). We also report the characterization of SAMs of 1d on bulk single crystal Au(111) by synchrotron in-plane X-ray diffraction (XRD) measurements. AFM and in-plane XRD suggest that a SAM of 1d is comprised of domains of 1d which form a hexagonal lattice (4.50 {+-} 0.06 A nearest neighbor spacing) that is incommensurate with the underlying Au(111) lattice. A model is proposed to describe the SAM structure. In such a model, small bundles (nearly 80 molecules) of ordered azobenzene moieties that rest over a set of inward tilting alkyl surface tethering groups make up the individual domains. Thermal annealing of the as-deposited SAM of 1d results in a modest increase in domain size from nearly 45 to around 55 A and a change in azobenzene tilt angle from 20-30{degree} to approximately 0{degree} with no change in nearest neighbor spacing. The redox activity of the azobenzene group is significantly affected by monolayer filmmore » structure. Only 2% of the azobenzene groups within a SAM of 1d are electrochemically accessible through cyclic voltammetry in a THF/0.1 M n-Bu{sub 4}NPF{sub 6} electrolyte. 46 refs., 13 figs.« less