Raman Excitation Profile of the G-band Enhancement in Twisted Bilayer Graphene

A resonant Raman study of twisted bilayer graphene (TBG) samples with different twisting angles using many different laser lines in the visible range is presented. The samples were fabricated by CVD technique and transferred to Si/SiO₂ substrates. The Raman excitation profiles of the huge enhancement of the G-band intensity for a group of different TBG flakes were obtained experimentally, and the analysis of the profiles using a theoretical expression for the Raman intensities allowed us to obtain the energies of the van Hove singularities generated by the Moiré patterns and the lifetimes of the excited state of the Raman process. Our results exhibit a good agreement between experimental and calculated energies for van Hove singularities and show that the lifetime of photoexcited carrier does not depend significantly on the twisting angle in the range intermediate angles (?? between 10^° and 15^°). We observed that the width of the resonance window (Γ ≈ 250 meV) is much larger than the REP of the Raman modes of carbon nanotubes, which are also enhanced by resonances with van Hove singularities.     

Raman tensor analysis of (K0.5Na0.5)NbO3–LiSbO3 lead‐free ceramics and its application to study grain/domain orientation

A quantitative polarized Raman analysis of ferroelectric grain/domain orientation in LiSbO₃ (LS‐modified) (K_0.5Na_0.5)NbO₃ (KNN) ceramics is presented, based on the analysis of the complex orientation dependence in space of their Raman‐active modes. Complete sets of Raman tensor elements of A_g, and E_g phonon modes for orthorhombic/tetragonal structures of KNN have been determined. Through this spectroscopic algorithm, quantitative information could be extracted in terms of three Euler angles in space for KNN samples consisting of mixed phases, thus enabling quantitative visualization of the local distribution of grains/domains in the solid angle. As an application of the method, we quantitatively examined the unknown crystallographic grain orientation patterns on the surfaces of pure KNN and of KNN‐0.05LS ceramics. These two samples were useful to clarify a polymorphic phase transition from the orthorhombic to the tetragonal phase taking place in the LS‐modified KNN system. Thus, we demonstrated that polarized Raman spectroscopy is a valuable and efficient tool for nondestructive three‐dimensional assessments of grain/domain orientation in ferroelectric materials with complex polymorphic structures. We believe that the data shown here represent a typical scenario encountered in grain/domain orientation assessments of piezoelectric perovskites. Copyright © 2012 John Wiley & Sons, Ltd.     

Characterization of tetragonal SAT0.3: LA0.075: CAT0.625 perovskite crystal: spectroscopic and microscopic investigations

We report room temperature measurements of X‐ray diffraction (XRD), optical transmission microscopy (OTM), atomic force microscopy (AFM), infrared‐absorption (IR), and micro‐Raman spectroscopy (µ‐RS) of the oriented SAT_0.3: LA_0.075: CAT_0.625 single crystal. The final structure refinement of SAT_0.3: LA_0.075: CAT_0.625 crystal was performed for I4/m space group at room temperature. Vibrational spectra of the crystal were discussed in terms of group‐theoretical predictions for untilted (Fm3 m) and tilted tetragonal (I4/m) perovskite structure. The confocal µ‐Raman measurements of depth profiling of SAT_0.3: LA_0.075: CAT_0.625 crystal suggest a relationship between sensitivities of the ordering‐related Raman‐active modes and the variation of order parameter η. Copyright © 2009 John Wiley & Sons, Ltd.     

Polarization resolved stimulated raman scattering: probing depolarization ratios of liquids

Polarization resolved stimulated Raman scattering (SRS) signal is described in the case of isotropic media and linearly polarized incident fields. The model gives simple expressions for the two perpendicularly polarized SRS signals I_X and I_Y, detected along the X and Y directions, respectively, as a function of the incident pump and Stokes polarization angles. We find that Raman depolarization ratio can be simply obtained from the ratio of the SRS intensities detected along the X and Y axis. These theoretical findings are supported by polarization resolved SRS measurements performed on polarized and depolarized bands of cyclohexane. Copyright © 2011 John Wiley & Sons, Ltd.     

Simultaneous acquisition of all four forms of circular polarization Raman optical activity: results for α‐pinene and lysozyme

A new circularly polarized (CP) Raman spectrometer is described that demonstrates simultaneous acquisition of all four forms of circular polarization Raman optical activity (ROA). The instrument is a design extension of a commercially available back scattering circular polarization (SCP) ROA spectrometer. Circular polarization of the incident beam is introduced with a quarter‐wave plate, and a half‐wave plate alternately positioned in and out of the beam controls the modulation between right circular polarization (RCP) or left circular polarization (LCP) states. Combining this modulation with the simultaneous detection of RCP and LCP scattered Raman radiation allows the measurement of incident circular polarization (ICP), SCP, in‐phase dual circular polarization(DCP_I) and out‐of‐phase DCP_II‐ROA. In addition, three different forms of backscattered Raman spectra, namely unpolarized, highly polarized, and depolarized Raman spectra, as well as a degree of circularity spectrum are obtained. The performance of the new all‐CP ROA spectrometer is evaluated with neat α‐pinene and aqueous hen lysozyme solution. Copyright © 2011 John Wiley & Sons, Ltd.     

Probing structural stability of double-walled carbon nanotubes at high non-hydrostatic pressure by Raman spectroscopy

Theoretical calculations predict that the collapse pressure for double-walled carbon nanotubes (DWCNTs) is proportional to 1/R ³, where R is the effective or average radius of a DWCNT. In order to address the problem of CNT stability at high pressure and stress, we performed a resonance Raman study of DWCNTs dispersed in sodium cholate using 532 and 633 nm laser excitation. Raman spectra of the recovered samples show minor versus irreversible changes with increasing I _D/I _G ratio after exposure to high non-hydrostatic pressure of 23 and 35 GPa, respectively. The system exhibits nearly 70% pressure hysteresis in radial breathing vibrational mode signals recovery on pressure release which is twice that predicted by theory.     

Investigation of thermostability and phonon–phonon interactions in Mo6S3I6 nanowires by Raman scattering spectroscopy

Detailed Raman scattering measurements were performed on molybdenum–sulfur–iodine nanowires (Mo₆S₃I₆). At room temperature, 21 well‐resolved Raman modes were experimentally observed for the first time in this new compound. The phase stability and vibrational properties of the nanowires were investigated by different temperature treatments. High‐temperature Raman spectra showed that the phase separation of Mo₆S₃I₆ nanowires took place between 573 and 673 K, followed by appearance of a new mode at 819 cm⁻¹ characteristic of the MoO₃ phase. Low‐temperature Raman scattering spectra showed a significant difference in phonon–phonon interactions between internal and external Raman modes of Mo₆S₃I₆ nanowires. These interesting vibrational properties can give new insights for improved material preparation and achievement of higher conductivity and other functional properties of these otherwise interesting materials. Copyright © 2009 John Wiley & Sons, Ltd.     

Near‐infrared Raman spectroscopy for gastric precancer diagnosis

Raman spectroscopy is a molecular vibrational spectroscopic technique that is capable of optically probing the biomolecular changes associated with neoplastic transformation. The purpose of this study was to apply near‐infrared (NIR) Raman spectroscopy for differentiating dysplasia from normal gastric mucosa tissue. A total of 65 gastric mucosa tissues (44 normal and 21 dysplasia) were obtained from 35 patients who underwent endoscopy investigation or gastrectomy operation for this study. A rapid NIR Raman system was utilized for tissue Raman spectroscopic measurements at 785‐nm laser excitation. High‐quality Raman spectra in the range of 800–1800 cm⁻¹ can be acquired from gastric mucosa tissue within 5 s. Raman spectra showed significant differences between normal and dysplastic tissue, particularly in the spectral ranges of 850–1150, 1200–1500 and 1600–1750 cm⁻¹, which contained signals related to proteins, nucleic acids and lipids. The diagnostic decision algorithm based on the combination of Raman peak intensity ratios of I₈₇₅/I₁₄₅₀ and I₁₂₀₈/I₁₆₅₅ and the logistic regression analysis yielded a diagnostic sensitivity of 90.5% and specificity of 90.9% for identification of gastric dysplasia tissue. This work demonstrates that NIR Raman spectroscopy in conjunction with intensity ratio algorithms has the potential for the noninvasive diagnosis and detection of precancer in the stomach at the molecular level. Copyright © 2009 John Wiley & Sons, Ltd.     

Molecular orientation evolution during low‐density polyethylene blown film extrusion using real‐time Raman spectroscopy

Real‐time polarized Raman spectroscopy was used in this study to measure the molecular orientation evolution during blown film extrusion of low‐density polyethylene (LDPE). Spectra were obtained at different locations along the blown film line, starting from the molten state near the die and extending up to the solidified state near the nip rolls. The trans C C symmetrical stretching vibration of polyethylene (PE) at 1132 cm⁻¹ was analyzed for films possessing uniaxial symmetry. For the given peak, the principal axis of the Raman tensor is coincident with the c‐axis of the orthorhombic crystal, and was used to solve a set of intensity ratio equations to obtain second (〈P₂(cosθ)〉) and fourth (〈P₄(cosθ)〉) moments of the orientation distribution function. The orientation parameters (P₂, P₄) were found to increase along the axial distance in the film line even past the frost‐line height (FLH). The P₂ values also showed an increasing trend with crystalline evolution during extrusion, consistent with past observations that molecular orientation takes place even after the blown film diameter is locked into place. It was also found that the integral ratio (I₁₁₃₂/I₁₀₆₄) obtained from a single, ZZ‐back‐scattered mode can provide a reasonable estimate of molecular orientation. These results indicate the potential of real‐time Raman spectroscopy as a rapid microstructure monitoring tool for better process control during blown film extrusion. Copyright © 2008 John Wiley & Sons, Ltd.     

Raman analysis of amorphous silicon ruthenium thin films embedded with nanocrystals

We report the Raman analysis of both as‐deposited and annealed amorphous silicon ruthenium thin films embedded with nanocrystals. In the Raman spectra of as‐deposited films, variations of TO peak indicate a short‐range disorder of a‐Si network with an increase of Ru concentration. The substitutional Ru atoms lower the concentration of Si―Si bonds and suppress the intensity of TO peak, but have less effect on TA, LA and LO peaks. In the Raman spectra of annealed films, characteristic parameters confirm the upgrade of a‐Si network at a low annealing temperature and the emergence of both ruthenium silicide and silicon nanocrystals at 700 °C. Although ruthenium silicide nanocrystals present no Raman peaks in the Raman spectra of as‐deposited samples, the non‐linear variations of intensity ratios I_{LA + LO}/I_TO and I_TA/I_TO still suggest their existence, and these nanocrystals are subsequently verified by high‐resolution transmission electron microscopy. Copyright © 2015 John Wiley & Sons, Ltd.     

Multipeak fitting analysis of Raman spectra on DLCH film

The hydrogenated diamond‐like carbon (DLCH) film with 1‐µm thickness is deposited by direct hydrocarbon gas ion beam method on silicon wafer and annealed at 400 °C. Detailed Raman spectra feature are fitted from nine sets of different peak fitting functions, including Gaussian, Lorentzian and Breit‐Wigner‐Fano (BWF) functions. These fitting results obtained from a two‐peak combination show some specific variances on the G peak position, FWHM_G and I_D/I_G ratio for as‐deposited and as‐annealed DLCH films. The most popular two‐peak fitting method with full Gaussian function tends to exhibit a higher ratio of the G peak position shift and higher I_D/I_G ratio than others fitting methods, the drastic difference among the most popular G (G) & G (D) and B (G) & L (D) schemes also have brought out in I_D/I_G ratio. However, for a more complex four‐peak Gaussian function fitting Raman spectra, the I_D/I_G ratio is close to that of a two‐peak fitting function with a mixture functions of BWF (G) and L (D). Furthermore, a series of systematic peak fitting procedures and comparisons of Raman spectra have been discussed in this study. Copyright © 2009 John Wiley & Sons, Ltd.     

Polarized Raman scattering and lattice eigenmodes of antiferromagnetic NdFeO3

The first‐ and second‐order Raman‐active phonons in the orthorhombic Pbnm NdFeO₃ single crystals were studied by means of polarized Raman scattering and lattice dynamics computations (LDC). The zone‐center phonons of A_g symmetry were distinguished from the B_1g eigenmodes by performing polarized Raman scattering experiments using two parallel polarization configurations, X′(ZZ)X′ and Z(X′X′)Z. With the help of LDC, we were able to assign most of the observed Raman‐active modes, including phonons of B_2g and B_3g symmetry. The LDC results indicated that among the 16 force constants employed, the one corresponding to the stretching vibration between the central Fe cation and the axial oxygen atom in a FeO₆ octahedron unit had the largest value. This suggests that the B‐site Fe cation is more tightly bound to the axial O1 ion than the other two equatorial O2 ions. It was further shown that at higher wavenumbers, the displacement of oxygen atoms contributed dominantly to the zone‐center vibrations. Copyright © 2008 John Wiley & Sons, Ltd.     

Raman study of molecular aggregation in liquid dimethyl sulfoxide

Parallel I _∥(ν) and perpendicular I _⊥(ν) polarized components of the S=O band in the Raman spectrum of liquid dimethyl sulfoxide (DMSO) were found to consist of at least three lines each. The depolarization ratios of these lines are different. Only the high-frequency line remains in both components after high dilution of DMSO with CCl₄ 10 mol %. All three lines are present in both components even after high dilution of DMSO with nitromethane, with their intensities being changed only slightly. These facts were explained by the occurrence of various molecular aggregates along with individual (monomeric) molecules present in DMSO and its nitromethane solutions. The monomeric molecules are predominant in a highly diluted CCl₄ solution. The polarized components of the S=O band of an aqueous DMSO solution point to the presence of H-aggregated formations of two types. The similarity in the molecular structure of DMSO and acetone makes it possible to assume that there are aggregated formations in liquid acetone as well. The small difference in wavenumbers as well as the overlap of the lines belonging to various aggregated formations may lead to the noncoincidence of the peaks of unresolved I _∥(ν) and I _⊥(ν) components of the C=O band of acetone.     

In vivo study on the protection of indole‐3‐carbinol (I3C) against the mouse acute alcoholic liver injury by micro‐Raman spectroscopy

Micro‐Raman spectroscopy (MRS) was utilized for the first time to evaluate the effect of indole‐3‐carbinol (I3C) on acute alcoholic liver injury in vivo. In situ Raman analysis of tissue sections provided distinct spectra that can be used to distinguish alcoholic liver injury as well as ethanol‐induced liver fibrosis from the normal state. Sixteen mice with liver diseases including acute liver injury and chronic liver fibrosis, and eight mice with normal liver tissues, and eight remedial mice were studied employing the Raman spectroscopic technique in conjunction with biomedical assays. The biochemical changes in mouse liver tissue when liver injury/fibrosis occurs such as the loss of reduced glutathione (GSH), and the increase of collagen (α‐helix protein) were observed by MRS. The intensity ratio of two Raman peaks (I₁₄₅₀/I₆₆₆) and in combination with statistical analysis of the entire Raman spectrum was found capable of classifying liver tissues with different pathological features. Raman spectroscopy therefore is an important candidate for a nondestructive in vivo screening of the effect of drug treatment on liver disease, which potentially decreases the time‐consuming clinical trials. Copyright © 2008 John Wiley & Sons, Ltd.     

Experimental study of the Raman strain rosette based on the carbon nanotube strain sensor

This work presents a new technique named Raman strain rosette for the micro‐strain measurement of both Raman active and Raman inactive materials. The technique is based on the theoretical model of the carbon nanotube (CNT) strain sensor that applies the resonance and polarization Raman properties of CNTs and calculates the synthetic contributions of uniformly dispersed CNTs to the entire Raman spectrum. In our work, the proposed technique is applied in different experiments on the Raman inactive materials, such as step‐by‐step uniaxial tensile and Raman mapping around a circular hole. The experimental results reached by the Raman strain rosette are consistent with the actual values as a whole. This study verifies that the Raman strain rosette is applicable to quantitative measurement of all the in‐plane components of the strain tensor (including both normal and shear strains) by three polarized Raman detections for each sampling spot on a microscale. The technique is further applicable to achieving the strain fields through Raman mapping. Copyright © 2010 John Wiley & Sons, Ltd.     

Vibrational properties of Cs4W11O35 and Rb4W11O35 systems: high pressure and polarized Raman spectra

Cs₄ W₁₁O₃₅ (CW) and Rb₄ W₁₁O₃₅ (RW) belong to the class of hexagonal bronzes whose structure originates from the K_xWO₃ superconductor hexatungstate. Charge‐imbalanced tungsten bronzes are dielectric materials with rich polymorphism, ferroelectric properties and second‐harmonic generation. In this work, we report the polarized Raman spectra results for both CW and RW, as well as results of high‐pressure Raman scattering experiments (0.0–11.0 GPa) for the Cs₄ W₁₁O₃₅ system, in which we have observed two structural phase transitions at ∼4 and 7.5 GPa. We discuss these transformations and polarized Raman spectra on the basis of lattice dynamics calculation in the related system KNbW₂O₉. Polarized Raman spectra provide strong indication that the highest wavenumber modes observed in these systems originate from tungsten or oxygen vacancies. The observation of a soft‐like mode indicates that the observed phase transitions exhibit a displacive‐type behavior, thus further indicating that these transformations are likely related to reorientations of the octahedral units. The soft mode nature is discussed as well. PACS: 77.80.Bh; 78.30.Hv; 78.30.‐j. Copyright © 2010 John Wiley & Sons, Ltd.     

Depth profiling of porcine adipose tissue by Raman spectroscopy

Raman spectroscopy was applied on a depth profile of porcine adipose tissue (from skin to meat) with the purpose of (1) discriminating between fat layers and (2) estimating the variation in fatty acid composition as a function of fat depth and fat layer: total degree of unsaturation (iodine value), fractions of saturated, and monounsaturated and polyunsaturated fatty acids. The thickness and composition of the outer layer of porcine adipose tissue influences the final quality of backfat. A too‐thick outer layer is associated with problems such as oily appearance, rancidity development, and difficulties in separating muscle and adipose tissue when cutting. From principal component analysis on standard normal variate preprocessed Raman spectra (1800–800 cm^–1), it was possible to discriminate between the outer and the inner backfat layer. Principal component analysis loadings showed that the separation of layer was mainly explained by variation in the bands originating from vibration of double bond C = C stretching plus = C–H twisting and rocking. In the prediction of iodine value a three‐component partial least squares regression model based on full range Raman spectra showed a root mean square error of cross validation of 2.00 and R² = 0.69. Applying Cauchy–Lorentz band fitting proved that information regarding fat unsaturation was found not only in band intensity, but also in band parameters such as location and width. The results suggest Raman spectroscopy as a potential measurement technique for rapid grading of pork carcasses. Copyright © 2011 John Wiley & Sons, Ltd.     

Measurement and calculation of the Q‐branch spectrum of nitrogen using inverse Raman spectroscopy and cw Raman‐induced polarization spectroscopy

The techniques of inverse Raman spectroscopy, Raman‐induced polarization spectroscopy (RIPS), and optical heterodyne RIPS (OHD‐RIPS) are compared by probing the Q‐branch of the nitrogen molecule. The signal is measured employing either a photomultiplier tube (low background level–RIPS) or a photodetector (high background level–IRS and OHD‐RIPS). The measurements are performed using atmospheric mixtures of N₂ Ar with concentrations varying from 0 to 79% N₂. This strategy permits estimation of detection limits using the different techniques. Pump and probe energy levels are varied independently to study signal dependence on laser irradiance. A theoretical treatment is presented on the basis of the Raman susceptibility equations, which permits the calculation of spectra for all three techniques. Calculated Q‐branch spectra are compared with the measured spectra for the interactions of a linearly polarized probe beam with a linearly or circularly polarized pump beam. The polarizer angle in the detection path for OHD‐RIPS has a dramatic effect on the shape of the spectrum. The calculated and experimental OHD‐RIPS spectra are in good agreement over the entire range of investigated polarizer angles. Detection limits using these techniques are analyzed to suggest their applicability for measuring other species of importance in combustion and plasma systems. Copyright © 2007 John Wiley & Sons, Ltd.     

How does the substrate affect the Raman and excited state spectra of a carbon nanotube?

We study the optical properties of a single, semiconducting single-walled carbon nanotube (CNT) that is partially suspended across a trench and partially supported by a SiO₂-substrate. By tuning the laser excitation energy across the E ₃₃ excitonic resonance of the suspended CNT segment, the scattering intensities of the principal Raman transitions, the radial breathing mode (RBM), the D mode and the G mode show strong resonance enhancement of up to three orders of magnitude. In the supported part of the CNT, despite a loss of Raman scattering intensity of up to two orders of magnitude, we recover the E ₃₃ excitonic resonance suffering a substrate-induced red shift of 50 meV. The peak intensity ratio between G band and D band is highly sensitive to the presence of the substrate and varies by one order of magnitude, demonstrating the much higher defect density in the supported CNT segments. By comparing the E ₃₃ resonance spectra measured by Raman excitation spectroscopy and photoluminescence (PL) excitation spectroscopy in the suspended CNT segment, we observe that the peak energy in the PL excitation spectrum is red-shifted by 40 meV. This shift is associated with the energy difference between the localized exciton dominating the PL excitation spectrum and the free exciton giving rise to the Raman excitation spectrum. High-resolution Raman spectra reveal substrate-induced symmetry breaking, as evidenced by the appearance of additional peaks in the strongly broadened Raman G band. Laser-induced line shifts of RBM and G band measured on the suspended CNT segment are both linear as a function of the laser excitation power. Stokes/anti-Stokes measurements, however, reveal an increase of the G phonon population while the RBM phonon population is rather independent of the laser excitation power.