Cm-1 Raman Band Research Articles

Multimodal Microscopy Characterisation of 2D Materials

Spatial characterisation of transition metal dichalcogenides (TMDCs) is crucial for understanding their optoelectronic properties and optimising film growth conditions. In this work, we present a multimodal microscopy platform combining conventional widefield, Raman, photoluminescence and second harmonic generation imaging for the characterisation of 2D materials.The platform was used to characterise CVD grown WSe2 crystals. Raman imaging identified the presence of both monolayer and multilayer WSe2 through a change in the intensity and position of the E12g / A1g phonon modes. Photoluminescence imaging confirmed the presence of monolayer WSe2 with emission at 780 nm and identified two distinct multilayer regions through changes in the photoluminescence wavelength and intensity. Lastly, second harmonic generation imaging under femtosecond laser excitation was used to obtain the relative growth orientation of the three identified domains in the crystal.Figure 1 highlights Raman imaging of WSe2. (a) Intensity of the E12g / A1g (250 cm-1) Raman band, (b) Peak position of the E12g / A1g (250 cm-1) Raman band, (c) least squares spectral matching revealing three distinct Raman spectral areas, (d) Averaged Raman spectra from areas A, B and C. Figure 1

Time-gated interferometric detection increases Raman scattering to fluorescence signal ratio in biological samples.

Attainable levels of signal-to-background ratio (SBR) in Raman spectroscopy of biological samples is limited by the presence of endogenous fluorophores. It is customary to remove the ubiquitous fluorescence background using postacquisition data processing. However, new approaches are needed to reduce background contributions and maximize the fraction of the sensor dynamical range occupied by Raman photons. Time-resolved detection using pulsed lasers and time-gated measurements can be used to address the signal-to-background problem in biological samples by limiting light detection to nonresonant interaction phenomena with relaxation time scales occurring on sub-nanosecond time scales, thereby excluding contributions from resonant phenomena such as fluorescence. A time-gated Fourier-transform spectrometer was assembled using a commercially available interferometer, a single channel single-photon avalanche diode and time tagging electronics. A time gate of 300 ps increased the signal-to-background-ratio of the 1440 cm-1 Raman band from 36% to 69% in an olive oil sample hereby demonstrating the potential of this approach for autofluorescence suppression.

Noninvasive Detection of Differential Water Content Inside Biological Samples Using Deep Raman Spectroscopy.

Here we conceptually demonstrate the capability of deep Raman spectroscopy to noninvasively monitor changes in the water content within biological tissues. Water was added by injection into an isolated tissue volume (a 20 mm diameter disk of 5 mm thickness) representing a 20% increase in the overall mass, which was equivalent to a 5% increase in the water/tissue content. The elevated water content was detected through a larger volume of tissue with a total thickness of approximately 12 mm and a spiked tissue segment located in its center using transmission Raman spectroscopy (TRS) by monitoring the change of the OH (∼3390 cm-1) Raman band area (3350-3550 cm-1 spectral region) after being normalized to the neighboring CH stretching band. The tissue sample was raster scanned with TRS to yield a spatial map of the water concentration within the sample encompassing the spiked tissue zone. The mapping revealed the presence and location of the spiked region. The results provide the first conceptual demonstration using a deep Raman-based architecture, which can be used noninvasively for the detection of an elevated water content deep within biological tissues. It is envisaged that this concept could play a role in rapid in vivo detection and localization of cancerous lesions (generally exhibiting a higher water content) beneath the tissue surface.

Temperature-Dependent Structure of a High-k (Ba,La)(Ti,Ce)O<sub>3</sub> Ceramic

The lattice structure of a high-k BL3TC5 [(Ba1-xLax)(Ti1-x/4-yCey)O3 (x = 0.03, y = 0.05)] ceramic with cubic structure at room temperature was investigated using temperature-dependent X-ray diffraction (XRD) and Raman spectroscopy. The XRD results indicated that with increasing temperature, the cubic lattice of BL3TC5 showed a lower coefficient of linear expansion (α = 7.7 × 10-6/°C) relative to the cubic BaTiO3 (9.3 × 10-6/°C), but no abnormal change in cell volume occurred around the dielectric peak temperature Tm (= 38 °C). The peak position of the ~835 cm-1 Raman band induced by the substitution of La3+ ions for Ba sites in BL3TC5 was nearly independent of temperature on each side of Tm, whereas a slight fall (3 cm-1) in the peak position around Tm was detectable. The link between the dielectric peak with DPT and the lattice change was discussed.

Study on Defect Complexes in High-K Sr and Zr Co-Doped BaTiO<sub>3</sub> Ceramics

In order to investigate the evidence for the formation of Sr2+-Zr4+ defect complexes in high-k Sr and Zr co-doped BaTiO3 ceramics, BaTiO3, (Ba0.85Sr0.15)TiO3, Ba (Ti0.8Zr0.2)O3, and (Ba0.85Sr0.15)(Ti0.8Zr0.2)O3 (BS15TZ20) ceramics were prepared using a mixed oxide method. The Y5V-type BS15TZ20 shows a high-k diffuse phase transition behavior (ε ́m = 8000) and its Curie peak occurs near room temperature. The evolution in crystal structure and in dielectric property for the four samples gives four evidences for the formation of Sr2+-Zr4+ defect complexes in BS15TZ20. The splitting of the 720 cm-1 Raman band does not relate to Sr2+-Zr4+ defect complexes, but to a multiphonon (A1 (TO2) + A1 (TO3)).

Effect of Additives on Absorption Edge Energy in Bismuth Phosphate Glass

The effect of third-additive oxides on absorption edge energy was studied in binary bismuth phosphate glass. The edge energy was shifted toward lower energies by the substitution of third-additive oxides for P2O5. The substitutions of glass-network-forming (NWF) oxides and glass-network-modifying (NWM) oxides resulted in distinct linear relationships between the edge energy and Duffy's ideal basicity. The energy shift against the calculated basicity was larger for the substitution of NWF oxides than for that of NWM oxides. Moreover, the substitution of intermediate oxides gave a similar tendency to the substitution of NWF oxides rather than to that of NWM oxides for both relationships. It was deduced that electron donation is more sensitive to the substitution of NWF oxides than to that of NWM oxides. On the other hand, a 238 cm-1 Raman band, which is attributed to the Bi–O vibration mode, shifted toward higher wave numbers by the substitution of all of the oxides. The 238 cm-1 band showed a linear relationship with the ideal basicity similar to the relationship between the edge energy and the basicity. Sensitivity against electron donation was consistent with both the Bi–O bond strength and the absorption edge energy.

Micro-spectroscopic characterization of organic and hydrous components in weathered Antarctic micrometeorites

Eight unmelted Antarctic micrometeorite (AMMs) recovered from Kuwagata Nunatak were studied on Al-foils by infrared (IR), Raman, and visible reflection micro-spectroscopy in combination with electron microscopy. Major element abundances of the AMMs studied were found to be similar to solar abundance, although all have the common characteristic of Mg-depletion. Absorption bands around 500 nm were detected for some of the AMMs by the visible micro-spectroscopic method, and these AMMs can be assigned to Fe-hydroxidelike materials. These results suggest that the studied AMMs experienced weathering in Antarctica. Four grains showed the presence of IR H2O and CH bands similar to those of type 2 carbonaceous chondrites, while these were found to be absent in two grains, as in type 3 carbonaceous chondrites. D (disordered: 1360 cm-1) and G (graphite: 1600 cm-1) Raman band features of graphitic carbonaceous materials in these AMMs were not similar to those for type 3 but were rather close to those for type 2 and 1 carbonaceous chondrites, although some data showed a degree of deviation. The genetic classification of individual AMM grains can thus be studied by these methods, although the weathering effects and the atmospheric entry heating on organics and hydrous components need to be evaluated. These multiple micro-spectroscopic reflectance methods are useful for the characterization of precious small samples.

Assessing the Role of the Active-site Cysteine Ligand in the Superoxide Reductase from Desulfoarculus baarsii

Superoxide reductase is a novel class of non-heme iron proteins that catalyzes the one-electron reduction of O(2)(.) to H(2)O(2), providing an antioxidant defense in some bacteria. Its active site consists of an unusual non-heme Fe(2+) center in a [His(4) Cys(1)] square pyramidal pentacoordination. In this class of enzyme, the cysteine axial ligand has been hypothesized to be an essential feature in the reactivity of the enzyme. Previous Fourier transform infrared spectroscopy studies on the enzyme from Desulfoarculus baarsii revealed that a protonated carboxylate group, proposed to be the side chain of Glu(114), is in interaction with the cysteine ligand. In this work, using pulse radiolysis, Fourier transform infrared, and resonance Raman spectroscopies, we have investigated to what extent the presence of this Glu(114) carboxylic lateral chain affects the strength of the S-Fe bond and the reaction of the iron active site with superoxide. The E114A mutant shows significantly modified pulse radiolysis kinetics for the protonation process of the first reaction intermediate. Resonance Raman spectroscopy demonstrates that the E114A mutation results in both a strengthening of the S-Fe bond and an increase in the extent of freeze-trapping of a Fe-peroxo species after treatment with H(2)O(2) by a specific strengthening of the Fe-O bond. A fine tuning of the strength of the S-Fe bond by the presence of Glu(114) appears to be an essential factor for both the strength of the Fe-O bond and the pK(a) value of the Fe(3+)-peroxo intermediate species to form the reaction product H(2)O(2).

UV Near-Resonance Raman Spectroscopic Study of 1,1‘-Bi-2-naphthol Solutions

The normal and UV near-resonance Raman (UVRR) spectra of 1,1'-bi-2-naphthol (BN) in basic solution were measured and analyzed. Density functional theory (DFT) calculations were carried out to study the ground state geometry structure, vibrational frequencies nu, off-resonance Raman intensities I, and depolarization ratios rho of 1,1'-bi-2-naphtholate dianion (BN(2-)). On the basis of the calculated and experimental results of nu, I, and rho, the observed Raman bands were assigned in detail. The 1612 cm(-1) Raman band of BN in basic solution was found dramatically enhanced in the UV resonance Raman spectrum in comparison with the normal Raman spectrum. Analyzing the depolarization ratios of the 1366 and 1612 cm(-1) bands in the RR spectra manifests that both the symmetric and antisymmetric parts of transition polarizabilities contribute to the 1366 cm(-1) band, but that only the symmetric part contributes to the 1612 cm(-1) band.

Raman scattering as a probe of phonon confinement and surface optical modes in semiconducting nanowires

Raman scattering is shown to be an effective probe of optical and surface optical phonons in highly crystalline semiconducting nanowires (SNWs). We show that the confinement model of Richter et al. well describes the nanowire diameter dependence of the asymmetric broadening of the one-phonon band in Si nanowires observed at ∼520 cm-1. We also show that the use of high laser flux (∼0.1 mW/μm2) leads to a second mechanism that can asymmetrically broaden the 520 cm-1 Raman band. This broadening has nothing to do with confinement, and can qualitatively be understood in terms of inhomogeneous laser heating. A model is presented that supports this explanation. The production of SNWs via the vapor–liquid–solid growth mechanism leads, in many cases, to an instability in the nanowire diameter or cross-sectional area. In the second part of this review, we show that this instability activates the surface optical (SO) phonon Raman scattering. Examples of this phenomenon are shown for GaP and ZnS nanowires. The former and latter have, respectively, cylindrical and rectangular cross sections. We show that the cross-sectional shape of the nanowire is important for a quantitative analysis of these SO modes.

Simultaneous Microfocus Raman and Microfocus XRD: Probing the Deformation of a Single High-Performance Fiber

Combined microfocus Raman and microfocus XRD is employed to probe the in situ deformation of a single poly(p-phenylene terephthalamide), PPTA, or Kevlar fiber. The experimental results are obtained from the same point on the sample simultaneously, using coaxial microfocus X-ray and laser beams. The 1610 cm-1 Raman band shifts linearly to a lower wavenumber with macroscopic stress, while the crystal lattice along the fiber axis direction extends linearly. The resulting Raman band shift rate and crystal modulus are −3.3 cm-1/GPa and 214 GPa, respectively. Raman band broadening with increasing stress suggests an increasing stress heterogeneity between the crystalline and amorphous fractions. Meanwhile, meridional layer line narrowing is interpreted as a deformation-induced homogenization of the PPTA fiber's skin-core structure. These results demonstrate the benefits of combining experimental techniques in situ which offer complementary phase and volume selectivity. In this case, macroscopic fiber deformation can be qualitatively explained using low- and high-stress uniform-strain mechanical models. At low stresses, linear Raman band broadening with stress can be attributed to a phase-related difference in mechanical properties. At high stresses, an increasing degree of crystalline domain orientation causes heterogeneous stiffening within the PPTA fiber structure. Consequently, the amorphous fraction remains “understressed” during subsequent deformation.

Effect of Pump and Probe Light Field on Picosecond Time-Resolved Resonance Raman Spectra of S1 trans-Stilbene. Disagreement between Stokes- and Anti-Stokes Scattering Frequencies

Abstract Picosecond time-resolved spontaneous resonance Raman spectra of the first excited singlet (S1) state of trans-stilbene were measured with various power densities of pump or probe light, for both the Stokes and anti-Stokes scattering regions. Careful calibration of the peak positions of the 285 cm-1 Raman bands reveals that the Stokes scattering band and the anti-Stokes scattering band shift to the same direction in absolute wavenumber, or to the opposite directions in Raman shift, when the pump or probe power density is increased. The direction of the wavenumber shift caused by the pump light field is opposite to that caused by the probe. Because of these apparent peak shifts, the Stokes Raman band and the anti-Stokes Raman band are recorded at different Raman shifts. The intrinsic change of the vibrational level spacing and the apparent peak shift caused by the pump or probe light field can be clearly separated by calculating the “symmetric” and “anti-symmetric” components from the Stokes and anti-Stokes peak positions. The experimental results are not explained by the optical Stark effect in its simplest form. Possible mechanisms to account for the results are discussed.

Modification of Kaolinite Surfaces through Intercalation with Deuterated Dimethylsulfoxide

The surfaces of kaolinite have been modified through intercalation with deuterated dimethylsulfoxide (d-DMSO). X-ray diffraction shows the kaolinite to be expanded from 7.2 to 11.19 Å. Modification of the surface has been explored through (a) changes to the hydroxyl surfaces of the kaolinite and (b) through changes to the d-dimethylsulfoxide inserting molecule. Upon intercalation of the kaolinite with d-DMSO, additional infrared bands at 3660, 3538, and 3502 cm-1 and additional Raman bands at 3660, 3537, 3507, and 3480 cm-1 are observed. The first band at 3660 cm-1 is attributed to the inner-surface hydroxyls hydrogen bonded to the d-DMSO and the other bands to water hydroxyl-stretching modes. Both infrared and Raman spectroscopy shows that significant changes in the molecular structure of the d-DMSO occur upon intercalation. First, the CD stretching modes observed for d-DMSO at 2125 and 2249 cm-1 in the DRIFT spectrum lose the degeneracy and split into 2140 and 2127 cm-1 and 2267, 2250, and 2238 cm-1. The Raman spectrum shows this loss of degeneracy through the bands observed at 2272, 2267, 2263, and 2251 cm-1 for the antisymmetric CD stretching vibration and at 2129 and 2141 cm-1 for the symmetric stretching vibrations. Upon intercalation with d-DMSO, the SO stretching region shows bands at 1066, 1023, and 1010 cm-1. The 1066 cm-1 band is assigned to the free monomeric SO group and the 1023 and 1010 cm-1 bands to two different polymeric SO groups. Bands attributed to the CS stretching vibrations, the in-plane and out-of-plane SO bending and the CSC symmetric bends all move to higher frequencies upon intercalation with d-DMSO. It is proposed that intercalation depends on the presence of water and that the additional bands at 3536 and 3501 cm-1 are due to the presence of water. The precise positions of the hydroxyl stretching modes of water at these positions suggest that water is in a well-defined position within the intercalation structure.

Raman Studies on Oxygen Doped GaN Grown by Molecular Beam Epitaxy

AbstractHigh quality Ga-polarity GaN films were grown by plasma-assisted molecular beam epitaxy to study strain effects due to oxygen incorporation. Oxygen concentrations up to 2 × 1022 cm-3 were studied. Layers containing oxygen at levels above 1022 cm-3 exhibit severe cracking while oxygen concentrations less than 1021 cm-3 apparently does not introduce significant strain. Raman spectra of O2-doped films were evaluated with respect to spectra of unintentionally doped GaN films (n=4x1014 cm-3) grown under the same conditions except for the O2-flux. Analysis of the E2 (high frequency phonon mode near 570 cm-1) Raman band indicated the heaviest doped samples exhibit compressive strain. Frequency-shifts between 0.7 and 0.9 cm-1 were observed as a function of the distance from the crack edges in the heavily doped samples, implying a strain (da/a) of the order of 5x10-4.

Interaction of Polycrystalline Silver with Oxygen, Water, Carbon Dioxide, Ethylene, and Methanol: In Situ Raman and Catalytic Studies

In situ Raman spectroscopy was used to study the interaction of polycrystalline silver with 16O2, 18O2, H2O, D2O, CO2, C2H4, and CH3OH at atmospheric pressure and temperatures between 25 and 500 °C. Raman bands at 956 and 800 cm-1 were observed upon exposure of silver to 16O2, and replacement of 16O2 with 18O2 shifted these bands to 931 and 778 cm-1, respectively. The 956 cm-1 Raman band was assigned to the stretching vibration of surface atomic oxygen with a AgO double bond, which sits on top of a Ag atom. This species is stable up to 250 °C and does not appear to interact with water, carbon dioxide, ethylene, or methanol. The 800 cm-1 Raman band was assigned to the stretching vibration of surface atomic oxygen with bridging Ag−O−Ag bonds, which sits on the interstices of three Ag atoms. This oxygen species is desorbed above 300 °C and interacts with water, ethylene, and methanol. Upon exposure of preoxidized silver to H2O and D2O, a new major Raman band at 866 cm-1 was observed, which was also assigned to surface atomic oxygen species, but not surface OH because of the absence of a H/D isotopic effect. The selective oxidation of ethylene and methanol was determined to occur primarily on the surface atomic Ag−O−Ag oxygen species corresponding to the 800 cm-1 Raman band.

The shape and width of the 818 cm-1 Raman band of propan-2-ol in solutions

The width and shape of the 818 cm-1 band of propan-2-ol in the Raman spectra in aqueous and other solutions were studied. In neutral solvent (1,2-dichloroethane) the band at low contents of the alcohol (0.3 mole fraction) obviously has a dual character. The bandwidth in aqueous solutions passes through a maximum with change in concentration, and the maximum of broadening occurs in the same range of concentrations as described in the literature for the maximum of the intensity of isotropic Rayleigh scattering of light. Dilution of propan-2-ol in acetonitrile leads to the disappearance of only the low-wavenumber asymmetry of band, accompanied by its narrowing. The maximum of line broadening in aqueous solutions is explained by reconstruction of the solution structure. As a result of this reconstruction, the hydrogen atoms of the OH group of the alcohol molecules are free from H-bonding. © 1998 John Wiley & Sons, Ltd.

The shape and width of the 818 cm1 Raman band of propan‐2‐ol in solutions

The shape and width of the 818 cm1 Raman band of propan‐2‐ol in solutions

Raman microspectrometry of accumulated non-graphitized solid bitumens

Solid bitumens (SB) occur accumulated in sedimentary, metasedimentary and volcanic rocks from the central and western part of the Bohemian Massif (Czech Republic). Structural features of asphaltic-like (ALSB) and hard (HSB) solid bitumens from sedimentary and metasedimentary rocks of Paleozoic and Precambrian age have been obtained using Raman microspectrometry. Structural and microtextural imaging were carried out using the lattice fringes mode of transmission electron microscopy (TEM). Weakly carbonized ALSB of sedimentary origin are characterized by a relatively high H/C atomic ratio (about 1) and differ from highly carbonized HSB from basaltic rocks (H/C atomic ratio about 0.1) which cross-cut a black shale formation of Upper Proterozoic age. Both these groups of accumulated forms of SB differ from dispersed kerogens, isolated from regionally metamorphosed black shales of the same primary sedimentary basin. First-order Raman spectra of both ALSB and HSB differ considerably from those obtained on dispersed carbonaceous matter forms from black shales and schists (from non-metamorphic to medium range biotite metamorphic zone). SB display a higher surface ratio of the 1350 to the 1582 cm-1 Raman bands (0.8–2.2) and higher half-width of the 1582 cm-1 (E2g) Raman band (65–73 cm-1) in comparison with dispersed forms (0.1–1.2 and 21–55 cm-1, respectively). Only HSB display second-order Raman spectra: the first band at 2680 cm-1 is broad and the second band at 2930 cm-1 is sharp. These features of the Raman spectra confirm the low structural order of SB, which is in agreement with X-ray and TEM results. © 1997 John Wiley & Sons, Ltd.

1064 nm FT-Raman spectra of hog kidney mitochondrial outer membranes: effects of NaCl and temperature on 1100 cm-1 region spectra

In order to obtain the experimental basis for the cause of the previously reported intensity decrease of the 1080 cm-1 band in the presence of NaCl, FT-Raman spectra were measured with 1064 nm excitation for aqueous dispersions of hog kidney mitochondrial outer membrane preparations in phosphate and Tris–HCl buffer (pH 7.5) in the absence and presence of NaCl. It was found that the relative intensity of the ∽1084 cm-1 Raman band with respect to the ∽1064 cm-1 band decreased considerably in the presence of NaCl in phosphate buffer and that the intensity decrease in the presence of NaCl was not appreciable in the spectrum in Tris–HCl buffer. It was also found that the presence of NaCl decreased the relative intensities of the ∽1126 cm-1 band with respect to the ∽1064 and ∽1134 cm-1 bands in both phosphate and Tris–HCl buffer. The spectral changes in the same region due to the temperature increase were studied for the membrane preparation of ultracentrifuged pellets for the elucidation of the vibrational assignments. It was found that the intensity ratio of the 1084 cm-1 band with respect to the ∽1064 cm-1 band was increased with a temperature increase from 8 to 20°C. An unexpected increase was observed in the relative intensity of the ∽1126 cm-1 band with respect to the ∽1064 cm-1 band with temperature elevation. It was concluded that the decrease in I1084/I1064 in phosphate buffer (pH 7.5) in the presence of NaCl could be attributed to the decrease in the gauche conformation of phospholipid acyl chains as predicted previously. The NaCl-caused decrease of the gauche conformation was found to be small in Tris–HCl buffer (pH 7.5). The increase in I∽1126/I∽1064 in the temperature increase experiment was interpreted in terms of a fairly large contribution of another chain conformation containing gauche bonds, the so-called almost fully extended form against the fully extended form, i.e. all-trans conformation, to the ∽1126 cm-1 Raman band in the fluid state, in which the usual natural biomembranes exist. The observed decrease in I∽1126/I∽1064 and I∽1126/I∽1134 in the presence of NaCl in both phosphate and Tris–HCl buffers could result from a decrease in this conformation in the acyl chains due to the interchain tighter packing induced by the non-specific screening effect on the membrane surface charges in the presence of NaCl. © 1997 John Wiley & Sons, Ltd.

Tetrahedral Displacement: The Molecular Mechanism behind the Debye Relaxation in Water.

The arguments for and against a single-molecule rotation mechanism for dielectric relaxation of water are surveyed. It is concluded that two distinct molecular mechanisms are operative in water. Single-molecule rotation is faster than the Debye relaxation time, τD, and possesses a smaller activation energy. It governs the abnormally fast proton mobility in water. The temperature dependence of τD agrees with that of water self-diffusion assuming a water hopping distance of 3.3 A, the separation between an occupied and unoccupied corners of a cube binding the pentawater tetrahedron. This slower translational mechanism controls the ordinary transport phenomena in water. “Tetrahedral displacement” correlates with two tetrahedral normal modes: the antisymmetric stretch in extended tetrahedral structures at low temperatures and a torsion mode in loosely bound tetrahedra at high temperatures. The temperature dependence of the 180 cm-1 Raman band is in quantitative agreement with the activation energy for water ...