Raman Spectra Of Crystals Research Articles

High-pressure Raman spectra of L-isoleucine crystals

Raman spectroscopy investigations of L-isoleucine crystals under high pressures have been carried out up to 7.3 GPa. From this study it was possible to observe modifications on bands associated to both rocking vibrations of r(NH 3 +) and r(CO 2 −) as well as to lattice modes at about 2.3 and 5.0 GPa. These modifications were correlated to either conformational change of molecules or to a solid–solid phase transition undergone by the crystals involving the hydrogen bonds that maintain the molecules held in the unit cell. A comparison with a few results on other amino acid crystals is also given.

Spatially resolved Raman investigation on phase separations of mixed Na2SO4/MgSO4 droplets

AbstractBy using spatially resolved Raman spectroscopy together with in situ microscopy, mixed Na2SO4/MgSO4 aerosol particles with molar ratios of 1:1 and 2:1 deposited on a quartz substrate were carefully examined in their evaporation processes. Upon decreasing the relative humidity (RH), phase separations were found to occur for these droplets. For Na2SO4/MgSO4 droplets with a molar ratio of 1:1, two paths of phase separation were identified, and a large amount of small crystals were observed to disorderly distribute in the residual solutions. By comparisons with the known Raman spectra of crystals, it was concluded that the scattered crystals in the two paths were anhydrous Na2SO4 in metastable phase III and the double salt of Na2SO4· MgSO4 · 4H2O, respectively. For Na2SO4/MgSO4 aerosol droplets with a molar ratio of 2:1, only anhydrous Na2SO4 in metastable phase III precipitated with the decrease of RH. Copyright © 2008 John Wiley & Sons, Ltd.

Assessment of Raman spectroscopy as a tool for the non-destructive identification of organic minerals and biomolecules for Mars studies

Several characteristic geological features found on the surface of Mars by planetary rovers suggest that a possible extinct biosphere could exist based on similar sources of energy as occurred on Earth. For this reason, analytical instrumental protocols for the detection of biomarkers in suitable geological matrices unequivocally have to be elaborated for future unmanned explorations including the forthcoming ESA ExoMars mission. As part of the Pasteur suite of analytical instrumentation on ExoMars, the Raman/LIBS instrument will seek elemental and molecular information about geological, biological and biogeological markers in the Martian record. A key series of experiments on terrestrial Mars analogues, of which this paper addresses a particularly important series of compounds, is required to obtain the Raman spectra of key molecules and crystals, which are characteristic for each biomarker. Here, we present Raman spectra of several examples of organic compounds which have been recorded non-destructively—higher n-alkanes, polycyclic aromatic hydrocarbons, carotenoids, salts of organic acids, pure crystalline terpenes as well as oxygen-containing organic compounds. In addition, the lower limit of β-carotene detection in sulphate matrices using Raman microspectroscopy was estimated.

Light depolarization induced by sharp metallic tips and effects on Tip-Enhanced Raman Spectroscopy

Tip-Enhanced Raman Spectroscopy is based on the field enhancement and confinement provided by sharp metallic tips, used to locally excite the Raman emission from molecules, crystals and nanostructures. This technique has recently demonstrated capabilities for nanometer scale optical resolution in Raman analysis and has enormous potential applications in nano- and bio-technologies. In this paper we describe the state-of-the-art experimental techniques, focusing the attention on the strategies to reduce the far-field background, on new processes to fabricate sharp metallic tips and on the effect of the depolarization produced by metallic tips on Tip-Enhanced Raman spectra of bulk crystals.

Phonon characterization of nano-crystals by Raman spectroscopy

Proposed herein is a method based on the Levenberg–Marquadt technique to retrieve an approximated average phonon dispersion curve from Raman spectra of nano-sized crystals. To this end we deal with a modified form of the well-known phonon confinement model in which the intrinsic linewidth Γ 0 is replaced by an effective linewidth Γ. An application to experimental results obtained on TiO 2 nano-crystals is presented as a test of the method.

Evidence for a high-density amorphous form in indomethacin from Raman scattering investigations

Pressure-induced transformation of $\ensuremath{\gamma}$-IMC [1-($p$-chlorobenzoyl)-5-methoxy-2-methylindole-3-acetic acid] is analyzed from Raman scattering investigations in the low-frequency range of $10--250\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ and the high frequency region between 1550 and $1750\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, where $\mathrm{C}\mathrm{O}$ stretching vibrations are usually observed. At room temperature, by pressurization from atmospheric pressure up to $4\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, $\ensuremath{\gamma}$-IMC undergoes a collapse transformation into a high-pressure crystalline form, induced by large rearrangement in the hydrogen-bonded network associated with molecular conformational changes. The Raman spectrum of the high-pressure crystal is similar to that of the $\ensuremath{\alpha}$ form, which is denser than the $\ensuremath{\gamma}$ form and metastable with respect to $\ensuremath{\gamma}$-IMC at atmospheric pressure. Upon further compression a solid-state amorphization is observed via the breakdown of hydrogen bonds. The Raman line shape of the high-pressure amorphous form is different from that of the vitreous state (or thermal glass obtained by quenching the liquid), suggesting the existence of a high-density amorphous state. By release of pressure, this high-density amorphous state transforms into the thermal glass. This transformation can be interpreted as a transformation between a high-density amorphous to a low-density amorphous state, which could be associated with a polyamorphic transformation.

High temperature Raman spectra of L-leucine crystals

Single crystal samples of L-leucine, C6H13NO2, a fundamental aliphatic amino acid of the human body, have been studied by Raman spectroscopy at temperatures from 300 to 430 K over the spectral range from 50 to 3100 cm-1. A tentative assignment of all bands is given. For high temperatures, several modifications on the Raman spectra were observed at about 353 K, giving evidence that the L-leucine crystal undergoes a structural phase transition. An interpretation for this phase transition in terms of group theory analysis is given.

The four polymorphic modifications of the semiconductor dibenzo-tetrathiafulvalene

The organic semiconductor dibenzotetrathiafulvalene (DB-TTF) presents, at ambient conditions, four polymorphic modifications. In addition to the already known monoclinic structure (α) with Z = 2, we have resolved by single crystal X-ray diffraction (XRD) a second crystal structure (β), also monoclinic but with Z = 4. Two other polymorphic modifications (γ and δ) have been found by means of lattice phonon confocal Raman microscopy and XRD. In this paper we present a series of growth experiments, from vapour or from solution, in films and in the bulk phase, describing the variety of transformations which occur among the different phases and their mixing, also at a micrometric scale. Quasi harmonic lattice dynamics calculations satisfactorily reproduce the structures of the α and the β polymorphs and help the phonon assignment of polarized Raman spectra in single crystals.

X‐Ray Single Crystal Structure and Raman Spectrum of Ammonium Hexafluoroarsenate.

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Raman spectroscopic investigation of pure and ytterbium‐doped rare earth silicate crystals

AbstractRaman spectroscopy was used to study the molecular structure of a series of selected rare earth (RE) silicate crystals including Y2SiO5 (YSO), Lu2SiO5 (LSO), (Lu0.5Y0.5)2SiO5 (LYSO) and their ytterbium‐doped samples. Raman spectra show resolved bands below 500 cm−1 region assigned to the modes of SiO4 and oxygen vibrations. Multiple bands indicate the nonequivalence of the REO bonds and the lifting of the degeneracy of the RE ion vibration. Low intensity bands below 500 cm−1 are an indication of impurities. The (SiO4)4− tetrahedra are characterized by bands near 200 cm−1 which show a separation of the components of ν4 and ν2, in the 500–700 cm−1 region which are attributed to the distorting bending vibration and in the 880–1000 cm−1 region which are attributed to the symmetric and antisymmetric stretching vibrational modes. The majority of the bands in the 300–610 cm−1 region of Re2SiO5 were found to arise from vibrations involving both Si and RE ions, indicating that there is considerable mixing of Si displacements with SiO bending modes and REO stretching modes. The Raman spectra of RE silicate crystals were analyzed in terms of the molecular structure of the crystals, which enabled separation of the bands attributed to distinct vibrational units. Copyright © 2007 John Wiley & Sons, Ltd.

Stress dependence of the Raman spectrum of polycrystalline barium titanate in presence of localized domain texture

The stress dependence of the Raman spectrum of polycrystalline barium titanate (BaTiO3, BT) ceramics has been examined with microprobe polarized Raman spectroscopy. The angular dependence of the Raman spectrum of the tetragonal BT crystal has been theoretically established, enabling us to assess the stress dependence of selected spectral modes without the influence of crystallographic domain orientation. Upon considering the frequency shift of selected Raman modes as a function of orientation between the crystallographic axis and the polarization vector of incident and scattered light, a suitable instrumental configuration has been selected, which allowed a direct residual stress measurement according to a modified piezospectroscopic procedure. The analysis is based on the selection of mixed photostimulated spectral modes in two perpendicular angular orientations.

X–ray Single Crystal Structure and Raman Spectrum of Ammonium Hexafluoroarsenate

AbstractThe structure of ammonium hexafluoroarsenate, NH4AsF6, has been determined by X‐ray diffraction using a single crystal grown from saturated solution in anhydrous HF. NH4AsF6 crystallizes rhombohedral with the KOsF6 structure type, with a = 7.459(3) Å, c = 7.543(3) Å (at 200 K), Z = 3, space group $R{\bar 3}$ (No. 148). No phase transition was observed in the 100 K–296 K temperature range. The structure is dominated by regular AsF6 octahedra and disordered NH4+ cations. Raman spectrum of a single crystal of NH4AsF6 shows the bands at 372 cm−1, 572 cm−1, 687 cm−1 (AsF6−) and at 3240 cm−1 and 3360 cm−1 (NH4+).

The behavior of NH 3 torsional vibration of l-alanine, l-threonine and taurine crystals under high pressure: A Raman spectroscopic study

Raman spectra of l-alanine, l-threonine and taurine amino acids crystals under high-pressure conditions at room temperature were investigated in a diamond anvil cell in order to observe the behavior of the torsional mode of NH 3 unit. It was observed for l-threonine and taurine crystals that increasing pressure produces an increase in the wave number of the band associated to the torsional vibration of ammonium group, while for l-alanine crystal a decrease in the wave number is observed. The anomalous behavior of l-alanine is explained in terms of the length and geometry of the hydrogen bonds within the crystalline structure: due to a short average length the effect of pressure is to deform the hydrogen bond, while for l-threonine and taurine, pressure approximates the N and O atoms in the N–H⋯O intermolecular bond.

Preparation and Characterization of Sr, Ba Filled Type-I Ge Clathrate Single Crystals

Sr or Ba filled type-I Ge clathrate single crystals were prepared using high pure elemental Ga, Ge, and Sr or Ba as the starting materials, and Ga as a flux. Powder X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), differential scanning calorimeter (DSC) were used to characterize the crystals, respectively. The results show that large and well-crystallized single crystals can be prepared under optimum synthesis condition. The composition of the crystals is in good agreement with the nominal composition (M8Ga16Ge30, M=Sr, Ba). Raman spectra of the single crystals were studied at room temperature and several of the Raman active vibrational modes in the crystals have been identified.

Observation of surface polarity dependent phonons in SiC by deep ultraviolet Raman spectroscopy

Backscattering Raman spectra of SiC polytype crystals with SiC{0001} polar faces have been measured using deep ultraviolet (DUV), UV, and visible (VIS) excitation sources. We have found that for DUV excitation the intensity profiles of zone-folded modes differ markedly for Si and C faces. This Raman spectral feature is attributed to the presence of nonpropagating phonon modes confined in the near-surface region. It is concluded that the surface-bound phonon modes created with DUV photon field extend over a region a few hundred nanometers in depth, and that the displacements of the phonon modes are anisotropic in the direction of the polar axis. This surface-orientation-dependent Raman spectrum can be used to identify the surface polarity of SiC polytypes.

Raman and infrared spectroscopic characterization of LiNbO3 crystals grown from alkali metal oxide solvents

AbstractLiNbO3 crystals grown from Na2O, Rb2O, and Cs2O containing fluxes were characterized by infrared and Raman spectroscopic methods. Pure, nearly stoichiometric crystals with 49.7–50.0 mol% Li2O content were obtained from solvents containing Rb2O and Cs2O, while Na2O entered the crystal as a dopant. The intensity ratio of two OH– absorption peaks at 3480 and 3465 cm–1 in thermal equilibrium was shown to be a useful and accurate parameter for the determination of the lithium content of pure LiNbO3 crystals close to the stoichiometric composition. From the remarkable change of both infrared and Raman spectra in crystals grown from Na2O solvent, the incorporation of Na on Li sites was assumed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Polarised vibrational studies of the α-glycine single crystal: Part I. Polarised Raman spectra—the Problem of effective local Raman tensors for the glycine zwitterions

Complete (full) set of the polarised Raman spectra for the α-glycine single crystal at room temperature are presented. For the strongest bands arising from the ν sCH 2, ν aCH 2, δCH 2, τCH 2, νCOO −, δCOO and ν sCCN vibrations effective local Raman tensors are determined applying the Tsuboi approach. The obtained results are compared to literature data available on this problem. Unfortunately, significant differences are observed between our data and other published results. This is due to the improper single crystal sample preparation for polarised Raman spectra measurements in the past. The obtained results are presented in forms which allow to apply these data for the polarised Raman spectra of other crystals of glycine compounds.

Molecular beam epitaxial growth of GaN thin film on Si substrate with InN as interlayer

Low-temperature interlayer InN was used to fabricate GaN thin film on Si (1 1 1) substrate by molecular beam epitaxy method. Surface scanning electron microscopy image shows a uniform polycrystalline structure. Transmission electron microscopy and high-angle annular detector dark-field scanning transmission electron microscopy images indicate column GaN crystal microstructure with the diameter between 100 and 150 nm. Air-gap structure is formed between the high-temperature GaN film and the underlying buffer layer. Photoluminescence measurement shows a small peak of 357 nm at room temperature and an increase of peak intensity about 40 times at temperature 8 K. Room temperature Raman spectra of the GaN column crystals shows narrow full-width at half-maximum of 2 cm −1 and E 2 mode peak line of 565.9 cm −1. A shift of 1.1 cm −1 from the intrinsic position may suggest relaxed residual strain of the GaN column crystals. The liquid In metal derived from the decomposed InN interlayer diffuses into the GaN buffer layer and high-temperature GaN thin film, forming InGaN layer. The application of InN as interlayer offers another effective way for the heteroepitaxial growth of GaN thin film on Si substrate.

Lattice dynamics study and specific heat ofCsH2PO4andCsD2PO4

This paper reports the results of lattice dynamics simulation of ${\mathrm{CsH}}_{2}{\mathrm{PO}}_{4}$ and ${\mathrm{CsD}}_{2}{\mathrm{PO}}_{4}$ crystals in paraelectric and ferroelectric phase using a semiempirical atomistic model based on the Coulomb, short range, covalent, and van der Waals interactions. The peculiarity of the two types of hydrogen bonds is taken into consideration too. The tunneling of protons (deuterons) on the one type of hydrogen bonds above the phase transition point was simplified through keeping the $H(D)$ atom position at the middle in hydrogen bonds. Phonon density of states, partial density of states, isotropic temperature factors, and specific heat were calculated. The computed phonon frequencies are particularly useful in interpreting the complicated Raman and IR spectra of these hydrogen-bonded crystals. The specific heat experimental data, reported by two groups of authors, contradict each other. The results of the given simulation may be helpful in establishing the real temperature behavior of specific heat.

Infrared and Raman spectroscopic characterization of the hydrogen-bonding network in l-serine crystal

The IR spectra (4000–400 cm −1) of neat and isotopically substituted (ND/OD ≤ 10% D and ≅30% D) polycrystalline l-serine (α-amino-β-hydroxypropionic acid; HO–CH 2–CH(NH 3) +–COO −) were recorded in the temperature range 300–10 K and assigned. The isotopic-doping/low-temperature methodology, which allows for decoupling of individual proton vibrational modes from the crystal bulk vibrations, was used for estimating the lengths and energies of the different H-bonds present in l-serine crystal. To this end, the frequency shifts observed in both the NH/OH stretching and out-of-plane bending spectral regions (relatively to reference values for these vibrations in non-hydrogen-bonded l-serine molecules) were used, together with previously developed empirical correlations between these spectral parameters and the H-bond properties. In addition, the room-temperature Raman spectrum (4000–150 cm −1) of a single crystal of neat l-serine was also recorded and interpreted. A systematic comparison was made between the spectroscopic data obtained currently for l-serine and previously for dl-serine, revealing that the vibrational spectra of the two crystals reflect well the different characteristics of their hydrogen-bond networks, and also correlate accurately with the different susceptibility of the two crystals to pressure-induced strain.