Raman scattering study of calcium hexaboride

We report significant difference in the Raman spectra of two different kinds of CaB₆ single crystals grown from boron purity 99.9% (3N) or 99.9999% (6N), respectively. Our Raman spectra of CaB₆ (3N), which are similar to those of previous measurement [N. Ogita, S. Nagai, N. Okamoto, M. Udagawa, F. Iga, M. Sera, J. Akimitsu, S. Kunii, Phys. Rev. B 68 (2003) 224305], show peaks at 781.3 cm⁻¹ (T_2g), 1140.1 cm⁻¹ (E_g), and 1283.5 cm⁻¹ (A_1g). The E_g mode shows a characteristic double-peak feature due to an additional weak broad peak centered at 1156.0 cm⁻¹. However, the Raman spectra of CaB₆ (6N) show sharp peaks at 772.5 cm⁻¹ (T_2g), 1137.9 cm⁻¹ (E_g), and 1266.6 cm⁻¹ (A_1g). The peak frequencies are down shifted as much as 17 cm⁻¹. In addition, no additional peak feature is observed for the E_g mode so that the mode is symmetric in the case of CaB₆ (6N). The X-ray powder diffraction patterns for both CaB₆ (3N) and CaB₆ (6N) show that the lattice parameters are essentially the same. The majority of the impurity in the 99.9% (3N) boron is assessed to be C. Thus we prepared Ca(B_0.995C_0.005)₆, CaB₆ (6N) doped with C, and looked for the difference in the Raman spectra. The Raman spectra of Ca(B_0.995C_0.005)₆ are nearly identical to those of CaB₆ (6N), indicating that the difference in the Raman spectra of CaB₆ (3N) and CaB₆ (6N) is not due to C impurity. However, presence of impurity, even if small amount, seems to be enough to trigger local-structure changes to lower symmetry inducing the difference in Raman spectra of CaB₆ (3N) and CaB₆ (6N).     

A study on the formation mechanism of graphite fluorides by Raman spectroscopy

Raman spectra were measured of highly fluorinated graphite samples prepared at room temperature, 380 and 515 °C. C_xF prepared at room temperature showed a novel downshifted band at 1555–1542 cm⁻¹ along with G band at 1593–1583 cm⁻¹. Similar behavior is also observed for samples prepared at 380 and 515 °C at early stages of fluorination, after which the Raman shifts completely disappeared. Raman spectra as well as X-ray diffraction (XRD) analysis suggest that graphite fluorides, (CF)_n and (C₂F)_n are formed via fluorine-intercalated phase with planar graphene layers.     

Raman,surface-enhanced Raman scattering and DFT study of para-nitro-aniline

Raman spectra of para-nitro-aniline (pNA), a molecule with high applicability potential in molecular electronics, were recorded in solid state and in ethanol solution. Complete assignment of the experimental spectra was made by using the B3LYP/6-31G(d) theoretical results. The calculated molecular electrostatic potential shows a high negative charge localized on the nitro group of pNA and the surface-enhanced Raman scattering (SERS) spectrum of pNA adsorbed to colloidal silver particles denote the molecule interaction with the silver surface mainly through the nitro group. However, theoretical results obtained by modeling the pNA–4Ag complex also suggest the adsorption of pNA through the amino group or a flattened orientation of pNA with respect to the silver surface.     

Raman scattering study of highly fluorinated graphite

Raman spectra of highly fluorinated C_xF samples (1<x<2) prepared at room temperature and 515°C were measured. C_xF samples prepared at room temperature exhibited two Raman bands at 1593–1583 and 1555–1542 cm⁻¹. Graphite samples fluorinated at 515°C for 1 and 2 min also gave similar bands at 1581–1580 and 1550–1538 cm⁻¹. However, graphite samples fluorinated from 15 min to 10 h at 515°C no longer showed such spectra. The Raman peaks shifted to lower frequencies with increasing fluorine concentration in C_xF. This trend is due to the weakening of the C---C bonds of the graphene layers. Observation of both kinds of Raman bands suggests the coexistence of two highly fluorinated phases, C₂F and C₁F, in the samples. The process of formation of graphite fluoride is discussed on the basis of the Raman spectra of C_xF samples obtained at 515°C.     

Structural anisotropy study by resonance Raman scattering in nematics with different dyes

Ionic liquid crystals based on congruent ion pairs composed of mesogenic cations and anions of similar shape provide an attractive tool for the tuning of mesophase properties. Here, the effect of the number and symmetry of lipophilic side chains and the type of head group on the phase type and thermal mesophase properties was probed by the synthesis and investigation of two series of novel guanidinium and imidazolium sulphonates and compared with the corresponding iodides. Their mesomorphic properties were examined via differential scanning calorimetry, optical polarising microscopy and X-ray diffraction. While derivatives bearing only one alkoxy chain in either cation or anion with up to three alkoxy chains in total within the ion pairs display smectic A mesophases, hexagonal columnar mesophases were observed for all other compounds with four or five alkoxy chains totally irrespective of the head group. However, with increasing steric bulk, i.e. with a total of six alkoxy chains, the symmetry of the aryl moiety in the anion as well as the type of head group becomes relevant, resulting in hexagonal columnar or plastic phases for guanidinium sulphonates with symmetrical anions, while those with unsymmetrical anions were non-mesomorphic. In contrast, the corresponding imidazolium sulphonates displayed cubic phases for the combinations of symmetrical cation/symmetrical anion and symmetrical cation/unsymmetrical anion. If both ions are unsymmetrically substituted, the imidazolium sulphonate displayed a hexagonal columnar phase. The results further demonstrate the utility of the congruent ion pairs for tailor-made ionic liquid crystals.     

Structural anisotropy study by resonance Raman scattering in nematics with different dyes

The method for the structural anisotropy determination, which is based on a comparison of the degree of polarization of absorption and resonance Raman bands, is tested with three dyes having different molecular dimensions in one nematic liquid crystal. The experimentally observed correlation between the variation of the structural anisotropy factor and the order parameters deviates from the predictions of the Segre model. This method could also be applied to determine the local field anisotropy for liquid crystals, but we must take into account the observed dependence of the structural anisotropy on the dimensions of the probe molecules.     

Rotational correlation in cyclohexane by Raman scattering

The profile of the 802 cm⁻¹ Raman line in C₆H₁₂ has been measured. Significant deviation from lorentzian line shape is observed. The zeroth moment of the correlation function corresponding to the reorientation of the three-fold symmetry axis of the molecule is determined to be 1.0 ± 0.05 ps.     

Raman scattering study of high-pressure phase transition in thiourea

A high-pressure Raman spectroscopic study of phase transitions in thiourea is reported. The changes in the Raman spectra with increasing and decreasing pressure have been followed to a maximum pressure of approximately 11 GPa. We observe several changes in the spectra including splitting of modes, appearance of new modes, and sudden change in the slope of the frequency-pressure curve at several pressures. On the basis of this study, we propose the existence of three more transitions in this system to phases VII, VIII, and IX at approximately 1, 3, and 6.1 GPa, respectively, in addition to the V-VI phase transition at 0.35 GPa reported earlier. All the transitions have been found to be completely reversible. We interpret these changes in terms of symmetry-lowering phase transitions.     

Theoretical study of helix formation in substituted phenylene ethynylene oligomers

Theoretical investigations of the relative stabilities of helical vs extended forms of phenylene ethynylene oligomers established that MMFF molecular mechanics was more useful than AM1 or DFT for calculating helical structures and for estimating relative energies. At the level of MMFF, theory predicts that for o- or m-oligophenylene ethynylenes, helix formation is enthalpically favored for ester and ether-substituted oligomers. In contrast to simple electron-demand predictions, we predict that the position of substituents can make a substantial difference in the tendency to form helices.     

A low-cost Raman spectrometer design used to study Raman scattering from a single-walled carbon nanotube

The paper discusses the design of a low cost Raman spectrometer. Single-walled nanotubes (SWNT) have been studied to demonstrate the reach of such a system. We observe both the radial-breathing mode (RBM) and the tangential mode from the SWNT. The tube diameters of the SWNT used in these experiments have been determined using RBM to be predominantly 1.4 and 1.6 nm. These are consistent with the TEM images taken of the same sample. The new method of producing SWNT using Ni-Y catalyst in electric-arc discharge method produces nanotubes with very small dispersion in diameter and high yields. The chirality of the SWNT can be deduced from their radial breathing modes, and it suggests that they are metallic in nature. Dedicated to Professor C N R Rao on his 70th birthday     

Decylammonium chloride/ammonium chloride aqueous solutions A Raman scattering study

Aqueous solutions of decylammonium chloride and decylammonium chloride/ammonium chloride between 20 and 50 per cent of DACl have been studied by Raman scattering. The spectral behaviour was followed as a function of temperature and phase transitions were observed for all samples. We have focused our study on the intensity variations of both the symmetric and the antisymmetric stretching vibration modes of CH₂ groups situated in the region 2800-3000cm^-1. The study of the high frequency part of the spectrum showed the characteristics of order disorder transitions already studied for other materials. We have observed that the existence of a nematic phase in the ternary systems is caused by the addition of the salt.     

Isotropic and anisotropic Raman scattering study in liquid p-dioxane

The isotropic and anisotropic profiles of the 835 and 2965 cm⁻¹ Raman lines of p-dioxane in the neat liquid and in solution have been studied as a function of temperature and concentration. From the correlation functions obtained by Fourier inversion of band contours, the possible interaction responsible for the vibrational dephasing of the oscillators and their reorientational relaxation are considered. It is shown that the p-dioxane molecule tumbling about the C₂ axis in the molecular plane perpendicular to the oxygen-oxygen direction proceeds by small-step Brownian diffusion associated with an Arrhenius activation energy of 9.0 kJ mol⁻¹. The vibrational relaxation mechanism of the two modes is interpreted in terms of pure dephasing due to weak collisions.     

Plasma-induced formation of Ag nanodots for ultra-high-enhancement surface-enhanced Raman scattering substrates

We report here plasma-induced formation of Ag nanostructures for surface-enhanced Raman scattering (SERS) applications. An array of uniform Ag patterned structures of 150 nm diameter was first fabricated on a silicon substrate with imprint lithography; then the substrate was further treated with an oxygen plasma to fracture the patterned structures into clusters of smaller, interconnected, closely packed Ag nanoparticles (20-60 nm) and redeposited Ag nanodots ( approximately 10 nm) between the clusters. The substrate thus formed had a uniform ultrahigh SERS enhancement factor (1010) over the entire substrate for 4-mercaptophenol molecules. By comparison, Au patterned structures fabricated with the same method did not undergo such a morphological change after the plasma treatment and showed no enhancement of Raman scattering.     

Transient dimer formation in supercritical carbon dioxide as seen from Raman scattering

The polarized and depolarized Raman profiles of supercritical CO(2) have been measured in the region of the nu(2) bending mode (forbidden transition at about 668 cm(-1)) and for the Fermi dyad (1285 and 1388 cm(-1)) along the isotherms 307, 309, 313, and 323 K in a reduced density domain 0.04相似文献   

A study of DFT and surface enhanced Raman scattering in silver colloids for thymine

The vibrational spectrum of crystal thymine is calculated by density functional theory (DFT) at the B3LYP complex function. Considering the effect of intermolecular H-bonds, we add two water molecules that can form H-bonds with the CO and NH groups of thymine. The experimental spectra of normal Raman of thymine in solid state and surface enhanced Raman (SERS) of thymine adsorbed in silver colloids are presented in this study. The calculated Raman spectrum of thymine by DFT is in agreement with the experimental result of normal Raman spectrum. The appearance of new bands of thymine in SERS shows that molecules of thymine are adsorbed in the surface of silver nanoparticles with a perpendicular orientation through an oxygen atom (O7).     

DFT and surface-enhanced Raman scattering study of tryptophan-silver complex

Electric field-induced assembly of biological and synthetic particles has proven useful in two- and three-dimensional fabrication of composite materials, microwires, photonic crystals, artificial tissues, and more. Biological particles are typically irregularly shaped, and using non-spherical synthetic particles has the ability to expand current applications. However, there is much to be understood about the dielectrophoretic (DEP) interaction that takes place between particles of general shape. In this work, we numerically study the DEP interaction between two prolate spheroid particles suspended in an unbounded fluid. The boundary-element method (BEM) is applied to solve the coupled electric field, Stokes flow, and particle motion, and the DEP forces are obtained by integrating the Maxwell stress tensor over the particle surfaces. Effects of the initial configuration and aspect ratio are investigated. Results show that the particles go through a self-rotation process, that is, electro-orientation, while translating slowly to form a chain pair. The final formation resembles the chaining pattern observed previously in experiments using densely distributed ellipsoidal particles. Thus, the transient behavior and particle-particle interaction exhibited in the current study could be used as the fundamental mechanism to explain the phenomenon in the experiment.     

Single molecule analysis by surfaced-enhanced Raman scattering

Our main objective in this tutorial review is to provide insight into some of the questions surrounding single molecule detection (SMD) using surface-enhanced Raman scattering (SERS) and surface-enhanced resonance Raman scattering (SERRS). Discovered thirty years ago, SERS is now a powerful analytical tool, strongly tied to plasmonics, a field that encompasses and profits from the optical enhancement found in nanostructures that support localized plasmon excitations. The spectrum of the single molecule carries the quantum fingerprints of the system modulated by the molecule-nanostructure interactions and the electronic resonances that may result under laser excitation. This information is embedded in vibrational band parameters. The dynamics and the molecular environment will affect the bandwidth of the observed Raman bands. In addition, the localized surface plasmon resonances (LSPR) empower the nanostructure with a number of optical properties that will also leave their mark on the observed inelastic scattering process. Therefore, controlling size, shape and the formation of the aggregation state (or fractality) of certain metallic nanostructures becomes a main task for experimental SERS/SERRS. This molecule-nanostructure coupling may, inevitably, lead to spectral fluctuations, increase photobleaching or photochemistry. An attempt is made here to guide the interpretation of this wealth of information when approaching the single molecule regime.     

Near-infrared excitation of Raman scattering by chromophoric proteins

Fourier-transformed Raman spectra of bacteriorhodopsin, the photosynthetic reaction center, and myoglobin in aqueous solution excited at 1064 nm are presented. These proteins are representative of three important classes of chromophoric proteins. The observed vibrational modes are assigned and discussed based on the known resonance Raman spectra of these proteins. In each case, chromophore vibrations dominate the Raman scattering, with little or no contribution from other protein vibrations. However, the limitations encountered in resonance Raman studies of chromophoric proteins due to sample fluorescence or sample photolability are circumvented. The relative intensities in the bacteriorhodopsin Raman spectrum excited at 1064 nm are nearly identical to the relative intensities previously observed by resonance excitation. The Raman spectrum of the reaction center of the photosynthetic bacterium Rhodobacter sphaeroides excited at 1064 nm contains contributions from both bacteriochlorophyll and bacteriopheophytin pigments, with possible preresonance enhancement of bacteriochlorophyll modes. The 1064-nm-excited Raman spectrum of myoglobin displays several marker bands that have been characterized previously in resonance Raman investigations with excitation in both the Soret and Q-band regions.     

Raman scattering study of toluene vibrational relaxation in benzene-toluene liquid system

The polarizes and depolarized profiles of the 520 cm^?1 Raman line of toluene in benzene solutions have been measured as a function of concentration. The vibrational correlation functions were obtained by Fourier inversion of Raman band contours. The concentration dependence of the experimental vibrational correlation time has been compared with that computed from the Fischer-Laubereau vibrational dephasing model modified for use in binary mixtures.