Raman Intensity Ratio Research Articles

Study of both fingerprint and high wavenumber Raman spectroscopy of pathological nasopharyngeal tissues

High wavenumber (HW) Raman spectroscopy has weaker fluorescence background compared with fingerprint (FP) region. This study aims to evaluate the discrimination feasibility of nasopharyngeal non-cancerous and nasopharyngeal cancer (NPC) tissue with both FP and HW Raman spectroscopy. HW Raman spectra of nasopharyngeal tissue were obtained for the first time. Raman spectra were collected to differentiate nasopharyngeal non-cancerous (n = 37) from NPC (n = 41) tissues in FP (800–1800cm−1), HW (2700–3100cm−1), and integrated FP/HW region. First, to assess the utility of this method, the averaged Raman spectral intensities and intensity ratios of corresponding Raman bands were analyzed in HW and FP regions, respectively. The results show that intensities as well as the ratios of specific Raman peaks might be helpful in distinguishing nasopharyngeal non-cancerous from NPC tissue with the HW Raman spectroscopy, as with FP Raman reported before. The multivariate statistical method based on the combination of principal component analysis–liner discriminant analysis (PCA-LDA), together with leave-one-patient-out, cross-validation diagnostic algorithm, was used for discriminating nasopharyngeal non-cancerous from NPC tissue, generating sensitivities of 87.8%, 85.4%, and 95.1% and specificities of 86.5%, 91.9%, and 89.2%, respectively, with Raman spectroscopy in the FP, HW, and integrated FP/HW regions. The posterior probability of classification results and receiver operating characteristic curves were utilized to evaluate the discrimination of PCA-LDA algorithm, verifying that HW Raman spectroscopy has a positive effect on the differentiation for the diagnosis of NPC tissue by integrated FP/HW Raman spectroscopy. What's more, the potential of Raman spectroscopy used for differentiating different pathology NPC tissues was also discussed. The results demonstrate that both FP and HW Raman spectroscopy have the potential for diagnosis and detection in early nasopharyngeal carcinoma, and HW Raman spectroscopy may improve the discrimination of NPC tissue compared with FP region alone, providing a promising diagnostic tool for the diagnosis of NPC tissue. Copyright © 2015 John Wiley & Sons, Ltd.

Origin of van Hove singularities in twisted bilayer graphene

In this work, we present a resonance Raman spectroscopy study of more than 100 samples of twisted bilayer graphene (TBG) with a continuous distribution of twisting angles from 0° to 30°, using three different laser excitation energies (1.96, 2.33 and 2.54eV). From the Raman images of all investigated samples we could observe giant enhancements of the G band for samples with twisting angles between 9° and 17°, in agreement with previous Raman studies of TBGs. However, although we investigated a large number of samples with low (<9°) or high (>17°) twisting angles, we could not observe the G-band enhancement that was predicted for these samples from the zone-folding of the single layer graphene electronic structure. Our results allow us to conclude that the van Hove singularities in the density of states are associated with the Moiré pattern, which does not necessarily exhibit a translational symmetry. Moreover, we could observe that the Raman intensity ratio ITBG/ISLG in non-resonance cases is always smaller than two, and this value depends on the laser energy. This result is discussed in terms of the relaxation of photo-excited carriers in TBG, which is expected to increase with increasing laser excitation energy.

Multiphonon Raman spectroscopy properties and Raman mapping of 2D van der Waals solids: graphene and beyond

Strong in‐plane bonding (covalent) and weak van der Waals (vdW) interplanar interactions characterize a number of layered solids, as epitomized by graphite. The advent of graphene (Gr), individual atomic two‐dimensional (2D) layers, isolated from mineral graphite via micromechanical exfoliation enabled the ability to pick, place and stack of arbitrary compositions. Moreover, this discovery implicated an access to other 2D vdW solids beyond graphene and artificially stacking atomic layers forming heterostructures/superlattices. Raman spectroscopy (RS) is a fast reliable non‐destructive analytical tool and an integral part for lattice dynamical structural characterization of crystalline solids at nanoscale, revealing not only the collective atomic/molecular motions but also localized vibrations/modes and specifically used to determine the number of graphene layers and of other 2D vdW solids. We present Raman spectroscopy in first‐, second‐ and higher‐order vibrational modes involving 3 and 4 phonons (overtones and combination) and mapping of graphene (mono‐, bi‐, tri‐ and few‐) layers, semiconducting transition metal dichalcogenides (TMDs) [molybdenum disulfide (MoS2) and tungsten disulfide (WS2)] and wide bandgap hexagonal boron nitride (h‐BN) dispersed monolayers, revealing various molecular vibrations and structural quality/disorder. First‐ and higher‐order phonon modes are observed and analyzed in terms of Raman intensity (spatial inhomogeneity or thickness variation), band position (intrinsic mechanical strain) and intensity ratio (structural disorder) as a function of graphene layer (n). An empirical relation for G band position with n is corroborated. All of the higher order modes are observed to upshift almost linearly with n, betraying the underlying interlayer vdW interactions. These findings exemplify the evolution of structural parameters in layered materials in changing from 3‐ to 2‐ or low‐dimensional regime. The results are presented in view of applications of graphene by itself and in combination that help better understanding of physical and electronic properties for nano‐/optoelectronics. Copyright © 2015 John Wiley &amp; Sons, Ltd.

The deviation of growth model for transparent conductive graphene

An approximate growth model was employed to predict the time required to grow a graphene film by chemical vapor deposition (CVD). Monolayer graphene films were synthesized on Cu foil at various hydrogen flow rates from 10 to 50 sccm. The sheet resistance of the graphene film was 310Ω/□ and the optical transmittance was 97.7%. The Raman intensity ratio of the G-peak to the 2D peak of the graphene film was as high as ~4 when the hydrogen flow rate was 30 sccm. The fitting curve obtained by the deviation equation of growth model closely matches the data. We believe that under the same conditions and with the same setup, the presented growth model can help manufacturers and academics to predict graphene growth time more accurately.

Decay dynamics of α,β-carboxylic methyl esters (CH3OCOCH:CHR) in the lower-lying excited states--resonance Raman and complete active space self-consistent field calculation study.

The photophysics of two α,β-carboxylic methyl esters after excitation to the light absorbing S2(ππ(*)) state were studied by using the resonance Raman spectroscopy and complete active space self-consistent field (CASSCF) method calculations. The vibrational spectra were assigned on the basis of the experimental measurements and the B3LYP/6-31G(d) computations, as well as the normal mode analysis. The A-band resonance Raman spectra of methyl 2,4-pentadienoate (M24PDA) and methyl trans cronoate (MTCA) were measured to probe the structural dynamics in Franck-Condon region. CASSCF calculations were done to obtain the minimal excitation energies and geometric structures of the lower-lying singlet and triplet excited states, and the curve-crossing points. It was revealed that the short-time structural dynamics of M24PDA was dominated by the Cα=Cβ-C4=C5 stretch coordinate, while that of MTCA was mostly along the Cα=Cβ and the C=O stretch motion. Comparison of the structural dynamics of M24PDA and MTCA with that of 3-methyl-3-pentene-2-one (3M3P2O) indicated that the structural dynamics of MTCA is similar to that of 3M3P2O but different than that of M24PDA in that the variation of the Raman intensity ratios for ν7/ν8, (ν7+ν8)/2ν8, (ν7+2ν8)/3ν8, (ν7+3ν8)/4ν8 of MTCA is similar to that of 3M3P2O but different from that of M24PDA. It is found that the substitution of methyl group in the α(')-position of α,β-enones by methoxyl group does not substantially affect the short-time structural dynamics, while the substitution of vinyl group in the β-position changes significantly the short-time structural dynamics and the subsequent decay processes. A detailed decay mechanism is proposed. Two sub-processes which consider the reconjugation and the subsequent charge-transfer reaction of O=C-Cα=Cβ chromophore were postulated to describe the variation of short-time structural dynamics with the different substitution.

Raman spectroscopic investigation on aqueous NaCl solutions at temperatures from 273 to 573 K: Effect of NaCl on water structure

The Raman spectra of aqueous NaCl solutions (0–25.0mass%) were investigated at temperatures from 273 to 573K and a pressure of approximately 30MPa. It was known that the dissolution of NaCl could significantly alter the water structure by changing the relative abundance of fully hydrogen-bonded water (FHW) and partially hydrogen-bonded water (PHW). Specifically, NaCl induced blue shifts of the Raman main peak above 3390cm−1 (vPHW) at temperatures below 433K, which could be attributed to the strengthened H2O–Cl− interaction (HCI) and the weakened H2O–H2O interaction (HHI) in the hydration shell of Cl−. The red shifts of vPHW at temperatures above 433K could be explained by the strengthened HHI and weakened HCI. NaCl also showed contrary effects on Raman intensity ratio of O-H stretching vibration of FHW and PHW below and above the temperature of 513K, which was due to the further strengthened HHI and weakened HCI in the hydration shell of Cl−, and the intensified ion–ion interaction (III) promoted the formation of ion pairs with rising temperature. The test results also showed a significant decrease of the water structure breaking capability of NaCl with an increasing temperature. The energy change due to the transition from FHW to PHW was determined by Gaussian fitting. It was found that ∆H (kJ·mol−1), ∆S (J·mol−1·K−1) and salinity (C, mass%) were linearly correlated with the following relationships: ∆H=−0.14C+7.01, ∆S=−0.22C+27.64 (T≤430±9K, depending on salinity) and ∆H=−0.35C+13.43, ∆S=−0.70C+42.34 (T>430±9K), respectively from 273 to 573K.

Dimensional scale effects on surface enhanced Raman scattering efficiency of self-assembled silver nanoparticle clusters

A study of the Surface Enhanced Raman Scattering (SERS) from micrometric metallic nanoparticle aggregates is presented. The sample is obtained from the self-assembly on glass slides of micro-clusters of silver nanoparticles (60 and 100 nm diameter), functionalized with the organic molecule 4-aminothiophenol in water solution. For nanoparticle clusters at the micron scale, a maximum enhancement factor of 109 is estimated from the SERS over the Raman intensity ratio normalized to the single molecule contribution. Atomic force microscopy, correlated to spatially resolved Raman measurements, allows highlighting the connection between morphology and efficiency of the plasmonic system. The correlation between geometric features and SERS response of the metallic structures reveals a linear trend of the cluster maximum scattered intensity as a function of the surface area of the aggregate. On given clusters, the intensity turns out to be also influenced by the number of stacking planes of the aggregate, thus suggesting a plasmonic waveguide effect. The linear dependence results weakened for the largest area clusters, suggesting 30 μm2 as the upper limit for exploiting the coherence over large scale of the plasmonic response.

Determination of critical micelle concentration of cationic surfactants by surface-enhanced Raman scattering

A novel approach, based on surface-enhanced Raman scattering (SERS) and 5,5′-dithiobis-(2-nitrobenzoic acid) (DTNB) functionalized silver nanoparticles, was developed to determine the critical micelle concentration (CMC) of cationic surfactants. A graph was generated by plotting the Raman intensity ratio between the aromatic ring vibration of DTNB at 1558 cm−1 and its symmetric nitro stretching at 1333 cm−1 as a function of surfactant concentration. An abrupt change of slope at a particular concentration was shown to reliably predict the CMC of cetyltrimethylammonium bromide (CTAB).

Investigation of porosity and atmospheric gas diffusion in microcrystalline silicon fabricated at high growth rates

The effects of postdeposition air exposure of microcrystalline silicon films, prepared at varied deposition rates, are investigated. The changes in the oxygen content, evaluated from Fourier transform infrared spectroscopy measurements, were studied over a period of time after deposition (up to 180 days) depending on deposition rate and Raman intensity ratio. Two types of behavior were identified: in the case of highly crystalline samples, an oxygen uptake increases with increasing Raman intensity ratio; while less crystalline samples were found to be more stable against oxygen incorporation. These observations are related to the film microstructure and porosity and are linked to the variations in Raman intensity ratio and growth rate of microcrystalline silicon films.

FT-Raman study of deferoxamine and deferiprone exhibits potent amelioration of structural changes in the liver tissues of mice due to aluminum exposure

The present study inform the alterations on major biochemical constituents such as lipids, proteins, nucleic acids and glycogen along with phosphodiester linkages, tryptophan bands, tyrosine doublet, disulfide bridge conformations, aliphatic hydrophobic residue, and salt bridges in liver tissues of mice using Fourier transform Raman spectroscopy. In amide I, amide II and amide III, the area value significant decrease due structural alteration in the protein, glycogen and triglycerides levels but chelating agents DFP and DFO upturned it. Morphology changes by aluminium induced alterations and recovery by chelating agents within liver tissues known by histopathological examination. Concentrations of trace elements were found by ICP-OES. FT-Raman study was revealed to be in agreement with biochemical studies and demonstrate that it can successfully specify the molecular alteration in liver tissues. The tyrosyl doublet ratio I899/I831 decreases more in aluminum intoxicated tissues but treatment with DFP and DFO+DFP brings back to nearer control value. This indicates more variation in the hydrogen bonding of the phenolic hydroxyl group due to aluminum poisoning. The decreased Raman intensity ratio (I3220/I3400) observed in the aluminum induced tissues suggests a decreased water domain size, which could be interpreted in terms of weaker hydrogen-bonded molecular species of water in the aluminum intoxicated liver tissues. Finally, FT-Raman spectroscopy might be a useful tool for obtained successfully to indicate the molecular level changes.

Chemical Method for Improving Both the Electrical Conductivity and Mechanical Properties of Carbon Nanotube Yarn via Intramolecular Cross-Dehydrogenative Coupling

Chemical post-treatment of the carbon nanotube fiber (CNTF) was carried out via intramolecular cross-dehydrogenative coupling (ICDC) with FeCl3 at room temperature. The Raman intensity ratio of the G band to the D band (IG/ID ratio) of CNT fiber increased from 2.3 to 4.6 after ICDC reaction. From the XPS measurements, the AC═C/AC-C ratio of the CNT fiber increased from 3.6 to 4.8. It is of keen interest that both the electrical conductivity and tensile strength of CNT yarn improved to 3.5 × 10(3) S/cm and 420 MPa, which is 180 and 200% higher than that of neat CNT yarn.

Defect healing of reduced graphene oxide via intramolecular cross-dehydrogenative coupling

A chemical defect healing of reduced graphene oxide (RGO) was carried out via intramolecular cross-dehydrogenative coupling (ICDC) with FeCl3 at room temperature. The Raman intensity ratio of the G-band to the D-band, the IG/ID ratio, of the RGO was increased from 0.77 to 1.64 after the ICDC reaction. From XPS measurements, the AC=C/AC−C ratio, where the peak intensities from the C=C and C–C bonds are abbreviated as AC=C and AC−C, of the RGO was increased from 2.88 to 3.79. These results demonstrate that the relative amount of sp2-hybridized carbon atoms is increased by the ICDC reaction. It is of great interest that after the ICDC reaction the electrical conductivity of the RGO was improved to 71 S cm−1, which is 14 times higher than that of as-prepared RGO (5 S cm−1).

Resonance Raman Overtone Intensities and Electron–Phonon Coupling Strengths in Semiconductor Nanocrystals

For linear electron-phonon coupling, the Huang-Rhys factor, S, gives the intensity ratio of the one-quantum vibronic transition (0 → 1) to the purely electronic origin transition (0 → 0) in a vibrationally resolved, zero-temperature absorption or emission spectrum. It is often assumed that the overtone to fundamental integrated intensity ratio in resonance Raman scattering of semiconductor nanocrystals is equal to or proportional to S, or that S may be determined from the overtone intensity in some other straightforward manner. In fact, this is not generally possible because of different excitation profiles for overtones and fundamentals, differential sensitivity of overtones and fundamentals to electronic dephasing, and interference effects from partially overlapping electronic transitions. Here we examine the relationship between the Huang-Rhys factor and the overtone to fundamental intensity ratio through spectroscopic simulations using parameters appropriate to II-VI semiconductor nanocrystals such as CdSe. A simple equation relating the overtone to fundamental Raman intensity ratio to the Huang-Rhys factor is obtained only in the case of a single resonant electronic state, excitation at the maximum of the inhomogeneously broadened absorption band, and a homogeneous line width small compared with the phonon frequency.

Graphene synthesis by thermal chemical vapor deposition using solid precursor

We report single layer to few layer graphene on polycrystalline nickel by chemical vapor deposition at ambient pressure using solid precursor, camphor. Investigating at a wide range of temperature, it was observed that 870 °C is better for the deposition of single layer graphene on nickel substrate. The percentage of single layer on the substrate reduced significantly with decreasing the deposition temperature. The full width half maximum of the synthesized single layer graphene was 21 cm−1 and Raman intensity ratio of 2D to G peak was almost nine. The film was transferred to insulating substrate and measured transmittance was 85 %. Raman spectroscopy, Raman mapping, SEM and UV–visible spectrometer measurement were performed for characterization.

Raman spectroscopy for the study of reduction mechanisms and optimization of conductivity in graphene oxide thin films

Highly reduced few-layer graphene oxide films with conductivities of up to 500 S cm−1 are obtained. The thin films with an optimized compromise between sheet resistance (3.1 kΩ sq−1) and transparency (around 80% to 90%) are suitable for touch screens and transparent electrodes in OLEDs. We discuss the effects of low temperature annealing and chemical reduction on the properties of the films and present an optimized reduction process that allows the original 2D/G Raman intensity ratio of few-layer graphene to be recovered. The Raman spectrum of graphene oxide is found to be only related to oxygen-free graphene-like regions with Raman bands at 1130 and 3155 cm−1 that probably involve C–H vibrations of rings and edges, while a band at 1700 cm−1 is assigned to irregular rings such as Stone Wales defects. All the bands involve resonant Raman processes and disappear in highly reduced samples. Clear correlations of the D band width with the sp2 content in thin films and resistivity have been observed, indicating that this is a good Raman parameter for evaluating the quality of the samples. The structural defects produced by the release of embedded water and some of the oxygen functional groups during annealing are detrimental for intra-grain conductivity but greatly enhance inter-grain connectivity.

Annealing of ion irradiation damage in nuclear graphite

Changes in Raman spectra of polished and ion-irradiated Gilsocarbon and Highly Orientated Pyrolytic Graphite (HOPG) during annealing have been investigated and compared with changes reported during stored energy release in fast neutron irradiated graphite. It is postulated that the change in the Raman spectra of polished, ion-irradiated and neutron irradiated graphite can be attributed to crystalline structural changes due to the annealing of lattice defects. This is illustrated in this work by changes in the kinetic parameters, in terms of the decrease of the Raman ID/IG ratio, during the thermal annealing of polished and ion-irradiated Gilsocarbon and HOPG graphite. Several kinetic models are investigated in terms of activation energy and order of reaction. The most suitable model to explain Raman annealing kinetics was found to be, a two reaction model for polished Gilsocarbon graphite, and a two and possibly three reaction model for ion-irradiated Gilsocarbon graphite and ion-irradiated HOPG. The kinetics parameters obtained using both the two and three reaction models reveal similarities with kinetic models obtained for neutron irradiated graphite. The assumption that the Raman intensity ratio ID/IG is proportional to the square root of the defect quantity provided a better fit than the assumption of direct proportionality.

Strain-induced D band observed in carbon nanotubes

We report the emergence of the D band Raman mode in single-walled carbon nanotubes under large axial strain. The D to G mode Raman intensity ratio (ID/IG) is observed to increase with strain quadratically by more than a factor of 100-fold. Up to 5% strain, all changes in the Raman spectra are reversible. The emergence of the D band, instead, arises from the reversible and elastic symmetry-lowering of the sp2 bonds structure. Beyond 5%, we observe irreversible changes in the Raman spectra due to slippage of the nanotube from the underlying substrate, however, the D band intensity resumes its original pre-strain intensity, indicating that no permanent defects are formed. Open image in new window

Graphene-enhanced Raman imaging of TiO2 nanoparticles

The interaction of anatase titanium dioxide (TiO2) nanoparticles with chemical vapour deposited graphene sheets transferred on glass substrates is investigated by using atomic force microscopy, Raman spectroscopy and imaging. Significant electronic interactions between the nanoparticles of TiO2 and graphene were found. The changes in the graphene Raman peak positions and intensity ratios indicate that charge transfer between graphene and TiO2 nanoparticles occurred, increasing the Raman signal of the TiO2 nanoparticles up to five times. The normalized Raman intensity of TiO2 nanoparticles per their volume increased with the disorder of the graphene structure. The complementary reason for the observed enhancement is that due to the higher density of states in the defect sites of graphene, a higher electron transfer occurs from the graphene to the anatase TiO2 nanoparticles.

Polarized Raman study on phase transitions in 0.24Pb(In1/2Nb1/2)O3–0.43Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 single crystal

Polarized Raman spectroscopy was performed to investigate the local lattice structure and phase transitions of unpoled 0.24Pb(In1/2Nb1/2)O3–0.43Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 (0.24PIN–0.43PMN–0.33PT) single crystal in the temperature range from 30°C to 260°C. MA- and MC-type monoclinic phases were detected by micro-Raman spectra measured in different micro areas. Temperature dependence of Raman intensities, frequency shifts, mode merge and intensity ratios in the VV and VH geometries were investigated. Our results indicated that the monoclinic–tetragonal (M–T) phase transition of the ternary relaxor-based ferroelectric single crystal 0.24PIN–0.43PMN–0.33PT occurs at 85°C, which is verified by the mode merging from 520cm−1 and 580cm−1 to 500cm−1, and the tetragonal–cubic (T–C) phase transition happens at 200°C based on the vanishing mode at 780cm−1 measured in the VH polarization.

Defect Evolution in Multiwalled Carbon Nanotube Films Irradiated by Ar Ions

Controlling defect structure in multiwalled carbon nanotube (MWCNT) is essential to realization of MWCNT devices. Here, we show that the diagram of the Raman intensity ratio of the G to D peaks and the G peak width can reveal two damaging stages of MWCNT films. In a transition period, additional peaks appeared in the X-ray absorption spectra, thereby indicating some significant change in the electronic structure. Also, a remarkable increase occurred in the diameter of the MWCNTs in the latter stage, suggesting the formation of dislocation dipoles which may relate to the change in the properties of field-emission devices.