Raman spectroscopy of HiPCO single-walled carbon nanotubes at 300 and 5 K

Single-walled carbon nanotubes (SWNTs) produced by the high pressure CO disproportionation (HiPCO method) and purified by controlled thermal oxidation in air have been studied by Raman spectroscopy at 300 and 5 K. Raman spectra have been observed at λ_exc=632.8 and 441.6 nm laser excitation in the range of 160-1800 cm⁻¹. In the low-frequency part of the spectra (the radial breathing mode range) eleven narrow lines can be detected at low temperatures, enabling an estimation of nanotube diameters (0.8-1.3 nm) and chirality. The width at half-maximum intensity of these spectral lines is about 3-4 cm⁻¹ at 5 K. The Stokes and anti-Stokes spectra are measured at λ_exc=632.8 nm at room temperature. The most intense lines in these spectra are caused with the resonant Raman-scattering process. With increasing temperature from 5 to 300 K the shift (3-4 cm⁻¹) of the most intense high-frequency component of the tangential mode (G mode) to lower frequency is observed. Based on the analysis of the Stokes/anti-Stokes spectra and the G band shape, the corresponding lines were identified with metallic or semiconducting type of nanotubes.     

Raman spectroscopy and nitrogen vapour adsorption for the study of structural changes during purification of single-wall carbon nanotubes

Raman spectroscopy and nitrogen adsorption measurements were combined to study the surface features of semi-conducting and metallic single-wall nanotubes (SWNTs). The nanotubes were treated chemically and with heat under moderate conditions that more than doubled the mesopore volume of the tested samples, which consistently led to a significant rise in the total surface area of up to 1550 m²/g. The large increase in the number of micropores of less than 1 nm in diameter was associated with the loosening of nanotube bundles as well as the creation of structural flaws on the surface of individual SWNTs due to chemical treatment. Micropores in the 1.0-1.8 nm range were associated with the holes created on the surface of individual tubes. Heating at 1000 °C was shown to restore nanotube diameter to their initial pre-chemical treatment levels with the change in the chirality of SWNTs and diminish the porosity by closing small holes. It was assumed that the intermediate frequency range (500-1100 cm⁻¹) was associated with the degree of imperfection of HiPco SWNTs crystalline structures, and therefore provided information about the degree of tube surface damage due to the presence of functional groups. A hypothesis explaining the transformation of SWNT porous structure during heat treatment is proposed.     

Covalent functionalization of single-walled carbon nanotubes by aniline electrochemical polymerization

Electrochemical polymerization of aniline in an HCl solution on a single-walled carbon nanotubes (SWNTs) film has been studied by Raman and FTIR spectroscopy. It is shown that this method leads to a covalent functionalization of SWNTs with polyaniline (PANI). A careful study in Raman scattering shows that the increase in the intensity of the band at 178 cm⁻¹ associated with radial breathing modes of SWNTs bundles suggests an additional nanotubes roping with PANI as a binding agent. A post chemical treatment with the NH₄OH solution of polymer-functionalized SWNTs involves an internal redox reaction between PANI and carbon nanotubes. As a result, the polymer chain undergoes a transition from the semi-oxidized state into a reduced one.     

Effect of laser intensity on yield and physical characteristics of single wall carbon nanotubes produced by the Nd:YAG laser vaporization method

We report on the effect of the Nd:YAG laser intensity on diameter distribution, yield and physical characteristics of single-wall carbon nanotubes (SWNTs) while comparing three different laser configurations (namely: (i) single 532 nm pulse; (ii) single 1064 nm pulse; and (iii) 532 nm followed by the 1064 nm double pulse). The carbon SWNTs were synthesized at a furnace temperature of 1150 °C and characterized by means of laser micro-Raman spectroscopy and high resolution transmission electron microscopy (HRTEM). Regardless of the laser configuration used, it is found that both the yield and the structural characteristics of the SWNTs are highly sensitive to the laser intensity. Indeed, by combining Raman analyses together with HRTEM observations we were able to point out the existence of an optimal laser intensity which leads not only to the highest yield of SWNTs and the largest bundles but also to the lowest level of amorphous and, or disordered sp² carbon in the deposits. While the optimal laser intensity was found to increase from 1.7 to 2.9×10⁹ W/cm² when the laser wavelength is changed from 1064 to 532 nm, the double pulse configuration offered a larger process latitude since high yield of SWNTs was obtained over the (0.8–3.5)×10⁹ W/cm² laser intensity range centered around the optimal value of 2.3×10⁹ W/cm². Moreover, it is shown that the increase of the laser intensity (from 0.5 to 5.6×10⁹ W/cm²) favors the growth of large nanotubes (1.4 nm-diam.) to the detriment of smaller ones (1.1 nm-diam.). A tendency to form larger nanotubes was also observed when increasing the furnace temperature from 1000 to 1150 °C. Finally, the laser intensity effect is interpreted in terms of near-surface or deep laser energy absorption in the graphite target.     

Pressure dependence of Raman modes in double wall carbon nanotubes filled with 1D Tellurium

The preparation of highly anisotropic one-dimensional (1D) structures confined into carbon nanotubes (CNTs) in general is a key objective in nanoscience. In this work, capillary effect was used to fill double wall carbon nanotubes (DWCNTs) with trigonal Tellurium. The samples are characterized by high resolution transmission electronic microscopy and Raman spectroscopy. In order to investigate their structural stability and unravel the differences induced by intershell interactions, unpolarized Raman spectra of radial and tangential modes of DWCNTs filled with 1D nanocrystalline Te excited with 514 nm were studied at room temperature and high pressure. Up to 11 GPa we found a pressure coefficient of 3.7 cm⁻¹ GPa⁻¹ for the internal tube and 7 cm⁻¹ GPa⁻¹ for the external tube. In addition, the tangential band of the external and internal tubes broaden and decrease in amplitude. All findings lead to the conclusion that the outer tube acts as a protection shield for the inner tube (at least up 11 GPa). No pressure-induced structural phase transition was observed in the studied range.     

Comparative study of first- and second-order Raman spectra of MWCNT at visible and infrared laser excitation

Comparative studies of first- and second-order Raman spectra of multi-walled carbon nanotubes (MWCNT) and three other graphitic materials - carbon fiber, powdered graphite and highly ordered pyrolytic graphite - are reported. Three laser excitation wavelengths were used: 514.5, 785 and 1064 nm. In first-order Raman spectra, the positions of the bands D, G and D′ (1100-1700 cm⁻¹) presented very similar behavior, however the intensity (I) ratio I_D/I_G ratio showed differed behaviors for each material which may be correlated to differences in their structural ordering. In the second-order spectra, the G′ band varied strongly according to structure with the infrared laser excitation.     

Raman spectroscopic evidence for interfacial interactions in poly(bithiophene)/single-walled carbon nanotube composites

Electrochemical polymerization of 2,2′-bithiophene (BTh) on single-walled carbon nanotube (SWCNT) films has been studied by Raman scattering and infrared absorption spectroscopy. Covalent functionalization of SWCNTs with poly(bithiophene) (PBTh) in its un-doped and doped states is demonstrated. The occurrence of a charge transfer process at the interface of PBTh and SWCNTs, is shown by: (i) an up-shift of the Raman lines associated with the radial breathing modes of SWCNTs that reveals both a doping process and an additional twisting together as a rope with the conducting polymer as binding agent; (ii) a new Raman band in the range 1430-1450 cm⁻¹ indicating the functionalization of SWCNTs with PBTh in doped and un-doped states; (iii) strong absorption bands situated in the interval 600-800 cm⁻¹ resulting from steric hindrance produced by the nanotube binding to the polymeric chain. Treatment of the PBTh/SWCNT composite with aqueous NH₄OH solution forms un-doped PBTh covalently functionalized SWCNTs. At the resonant excitation of the metallic tubes, an additionally enhanced Raman process is generated by plasmon excitation in the metallic nanotubes. It is evidenced by a particular behavior in the Stokes and anti-Stokes branch of the PBTh Raman line at 1450 cm⁻¹.     

Spectroscopic and SEM studies of SWNTs: Polymer solutions and films

Raman spectra of SWNTs suspended in aqueous solutions containing fragmented single-stranded DNA (SWNT:DNA), and films obtained from this suspension have been obtained. SEM study of the dried films indicated that the nanotubes tend to aggregate into bundles which results in the enhancement of the Raman intensity of the G⁻ tangential band, and an upshift and broadening of the G⁺ band. The intensity of radial breathing modes of metallic SWNTs is higher in the SWNT:DNA films as compared to that of the SWNT:DNA solution. The Raman spectra of SWNT:PVP and SWNT:agaroza samples exhibit similar changes as the SWNT:DNA samples when films are cast from the corresponding solutions. Both films and the solution forms of SWNT:DNA yield luminescence spectra which indicates the presence of individual tubes or small bundles in the films. The luminescence bands of SWNT:DNA films are relatively wider and is attributed to the interaction of DNA with the nanotube surface in the solid state.     

Surface-enhanced Raman scattering studies on C60 fullerene self-assemblies

Surface-enhanced Raman scattering (SERS) was used to investigate C₆₀ self-assembling in solvents like pyrrolidine (Py) and N-methyl-2-pyrrolidinone (NMP) as well as in binary mixtures of o-dichlorobenzene (DCB)/acetonitrile (ACN) and DCB/NMP. For a correct evaluation of the modifications of Raman spectra induced by the C₆₀ aggregation, we have also presented the variations due to the measuring method, i.e., the signal dependence of the metallic support type and the surface roughness. The interaction between C₆₀ and the Au substrate, appearing as a chemical component in SERS generation, is mainly evidenced by a band at ∼342 cm⁻¹. In the aggregated phase, the intermolecular interactions lead to a reduction in the parent I_h C₆₀ symmetry as observed by a modified phonon spectrum. As a general feature, the spectral range below 800 cm⁻¹ is the most diagnostic for the aggregate assignment, the main indicative being the disappearance of the Raman bands associated to the radial vibration modes. SERS measurements have revealed two stages in the self-assembling of C₆₀ in NMP. In the beginning, charge-transfer molecular complexes that associate slowly in stable aggregates are formed by the binding of an NMP molecule to the C₆₀ cage. These complexes are noticed in the SERS spectrum by the replacement of the original H_g(1) band at ∼269 cm⁻¹ with two others at ∼255 and ∼246 cm⁻¹. In the aggregated phase, when using NMP and P as a solvent, the Raman spectrum reveals new bands that appear around 94 and 110-118 cm⁻¹, which are associated with the interball interactions. In a DCB/ACN solvent mixture, the self-assembling process is driven by weak van der Waals type forces and resembles a precipitation, yielding C₆₀ clusters of different size.     

FT-Raman spectroscopic study of the evolution of char structure during the pyrolysis of a Victorian brown coal

FT-Raman spectroscopy with a 1064 nm laser was used to investigate chemical structural changes of char during the pyrolysis of Victorian Loy Yang brown coal samples. The chars were diluted with KBr in order to record Raman spectra with acceptable quality. The interpretation of the Raman spectral data for these highly disordered and heterogeneous chars differs distinctly from that for the highly condensed/graphitised carbon materials. The FT-Raman spectra of chars in this study over the range of 800–1800 cm⁻¹ were curve-fitted with 10 bands representing major structures in the chars. This has given information about the size of aromatic rings and the nature of substitutional groups and cross-links in char. The observed Raman intensity of a char is governed by its Raman scattering ability and its light absorptivity for both excitation laser and Raman scattering. The overall Raman intensity (peak area) as well as the ratios among the intensities of some major Raman bands has allowed some semi-quantitative evaluation of changes in char structure with increasing temperature during pyrolysis. The presence of ion-exchangeable Na and Ca in brown coal greatly affects the char-forming reactions during pyrolysis.     

Raman microspectroscopy of soot and related carbonaceous materials: Spectral analysis and structural information

Experimental conditions and mathematical fitting procedures for the collection and analysis of Raman spectra of soot and related carbonaceous materials have been investigated and optimised with a Raman microscope system operated at three different laser excitation wavelengths (514, 633, and 780 nm). Several band combinations for spectral analysis have been tested, and a combination of four Lorentzian-shaped bands (G, D1, D2, D4) at about 1580, 1350, 1620, and 1200 cm⁻¹, respectively, with a Gaussian-shaped band (D3) at ∼1500 cm⁻¹ was best suited for the first-order spectra. The second-order spectra were best fitted with Lorentzian-shaped bands at about 2450, 2700, 2900, and 3100 cm⁻¹. Spectral parameters (band positions, full widths at half maximum, and intensity ratios) are reported for several types of industrial carbon black (Degussa Printex, Cabot Monarch), diesel soot (particulate matter from modern heavy duty vehicle and passenger car engine exhaust, NIST SRM1650), spark-discharge soot (Palas GfG100), and graphite. Several parameters, in particular the width of the D1 band at ∼1350 cm⁻¹, provide structural information and allow to discriminate the sample materials, but the characterisation and distinction of different types of soot is limited by the experimental reproducibility of the spectra and the statistical uncertainties of curve fitting. The results are discussed and compared with X-ray diffraction measurements and earlier Raman spectroscopic studies of comparable materials, where different measurement and fitting procedures had been applied.     

Estimation of crystallinity of isotactic polypropylene using Raman spectroscopy

A method to estimate the degree of crystallinity in isotactic polypropylene has been developed. The method is based on integrated intensities of the Raman bands at 808 and 841 cm⁻¹. From the observation of correlation splitting, Raman bands related to different conformational states were identified. This analysis indicates the existence of three different conformational states. The 808 cm⁻¹ band was assigned to helical chains within crystals. The 840 cm⁻¹ band was shown to be composed of a band at 840 cm⁻¹, assigned to shorter chains in helical conformation, and a broader band at 830 cm⁻¹ assigned to chains in non-helical conformation. In order to establish a quantitative relation between Raman scattering intensity and crystallinity samples subjected to different cooling rates and crystallisation temperatures were analysed. These results correlate well with those of differential scanning calorimetry.     

Chemical, microstructural and thermal analyses of a naphthalene-derived mesophase pitch

A detailed characterisation of a synthetic naphthalene-derived mesophase pitch, in its as-received state and during pyrolysis, has been performed. The study has been conducted by means of various techniques and with a particular attention to Raman microspectroscopy. The Raman spectra show features in common with the naphthalene precursor, i.e., a broad and complex band at 1150-1500 cm⁻¹ and a multicomponent G band at 1600 cm⁻¹. These features correspond to the vibration modes of the molecules of the pitch and more especially to the non-aromatic C-C bonds involved in alkyl groups, aryl-aryl bonds or naphthenic rings. The pyrolysis of the pitch into coke takes place within a narrow temperature range (480-500 °C) through the elimination of hydrogen and light alkanes resulting from the breaking of homolytic C-H bonds and naphthenic cycles, respectively. This process initiates a swelling of the pitch. The analysis of the Raman features shows that the structure of the pitch is only slightly affected within this temperature range. Conversely, significant structural changes of the material (as shown by the vanishing of the multicomponent bands at 1600 and 1150-1500 cm⁻¹) are evidenced beyond 750 °C, simultaneously with a hydrogen release and an increase of the true density. This phenomenon corresponds to the extension of the graphene layers of the coke and the formation of a distorted carbon network.     

Origin of the 2450 cm Raman bands in HOPG, single-wall and double-wall carbon nanotubes

The second order Raman signals around the G′-band region of graphite and carbon nanotubes have been investigated at more than 15 excitation laser lines. Two distinct Raman bands have been observed around 2700 cm⁻¹; a prominent one is due to the so-called G′-band and the other is a weak band around 2450 cm⁻¹. Both two bands can be from the double resonance process involving two phonons around the K-point in the phonon dispersion of a two-dimensional graphite. The 2450 cm⁻¹-band has exhibited little power dependence, whereas the intensity of G′-band has shown large photon energy dependence as already reported. The 2450 cm⁻¹-band and the G′-band correspond to non-dispersive q = 0 and fully-dispersive q = 2k, respectively. From the phonon dispersion and the corresponding phonon frequency, the 2450 cm⁻¹-band can be assigned as an overtone mode of LO phonon (i.e. 2LO). This is revealed by calculated Raman spectra of graphite with proper electron-phonon matrix elements. The present study is the first report on the origin and assignment of the 2450 cm⁻¹-band, which is based on the double resonance Raman scattering.     

Characterization by Fourier transform infrared spectroscopy of hydroxyapatite co-doped with zinc and fluoride

Pure and Zn²⁺ and/or F⁻ doped hydroxyapatite (HA) were synthesized by the precipitation method and detection of ion incorporations into the HA structure was investigated by a non invasive Fourier transform infrared (FTIR) spectroscopic technique. The synthesized materials were sintered at 1100 °C for 1 h. The Zn²⁺ addition amount was kept constant at 2 mol% whereas F⁻ amount was changed. The weight fractions of the HA and CaO were calculated by Rietveld analysis by using GSAS. Co-doping of Zn²⁺ and F⁻ ions increased the stability of HA. A detailed analysis of FTIR spectroscopy was performed to observe whether HA structure was formed or not. The bands corresponding to the (PO₄³⁻) functional group and (OH⁻) functional group were observed. Moreover, the ion incorporation into the HA structure and the amount of the ions were analyzed by FTIR spectroscopy. The OH…F bands were observed at 711 cm⁻¹ and 3543 cm⁻¹. The Zn–O stretching band was observed at 3403 cm⁻¹ and 433 cm⁻¹. The area calculation under the OH…F bands and (OH⁻) stretching and librational modes of the bands revealed that as the F⁻ amount increased, the area under the bands at 711 cm⁻¹ and 3543 cm⁻¹ increased whereas the area under the (OH⁻) stretching and librational modes of the bands decreased due to the fact that F⁻ ion replaced with (OH⁻) ion in HA structure. All these results showed that Zn²⁺ and F⁻ ions were successfully incorporated into the HA structure. Moreover, the amount of F⁻ ions in the HA structure was successfully confirmed by determination of the area under the F⁻ and (OH⁻) related bands.     

Raman spectroelectrochemical study of self-doped copolymers of aniline and selected aminonaphthalenesulfonates

Novel self-doped polyaniline-like copolymers have been prepared by electrochemical copolymerization of aniline with four aminonaphthalenesulfonates. All copolymer films prepared show their electrochemical redox activity even in pH-neutral solutions at a midpoint potential around 0.0 V versus Ag/AgCl. Raman spectroelectrochemical study of the copolymers prepared has been done with a red laser excitation (632.8 nm) within a broad electrochemical potential window of 0.0-1.0 V, and specific Raman features have been identified. Raman bands within the range of 1300-1400 cm⁻¹ have been discussed regarding localized or delocalized polaronic ν_s(CN⁺) vibrations. The influence of sulfonate group position in aminonaphthalenesulfonates on the parameters of polaronic bands has been demonstrated.     

Raman scattering study of the thermal conversion of bundled carbon nanotubes into graphitic nanoribbons

Raman scattering is used to study the temperature-driven structural transformations of bundled single-walled carbon nanotubes (SWCNTs) observed in HiPCO and ARC synthesis by electron microscopy, i.e., tube-tube coalescence ∼1300-1400 °C, coalesced tubes to multi-walled tubes (MWCNT) at ∼1600-1800 °C and finally (only ARC tubes) MWCNT to graphitic nanoribbons (GNRs) at ∼1800 °C. All these transformations occurred in vacuum. Here, we present the details of these transformations as seen through the “eyes” of Raman scattering via changes in the radial (R) SWCNT band, the G-band (and its substructure) and the relative intensity of the disorder-induced D- and D′-band scattering. The Raman spectrum of GNRs is also discussed in detail. For 514.5 nm laser excitation, five relatively broad GNR Raman bands are observed: 1350, 1580, 1620, 2702 and 3250 cm⁻¹. A Knight plot is used to estimate the GNR width and we find w ∼ 9 nm, which is in reasonable agreement with the estimate of 7.6 nm based on TEM and the model that a GNR is a collapsed MWCNT.     

Effects of Ni-catalyst characteristics on the growth of carbon nanowires

Carbon nanowires (CNWs) were grown on amorphous Ni thin film catalysts in a microwave plasma-enhanced chemical vapor deposition system under a methane and hydrogen gas mixture. The resulting CNWs were found to be polycrystalline but not amorphous as commonly found elsewhere. In general, a catalyst could be seen at the tip of each CNW. However, multiple CNWs grown from a single catalyst were also regularly observed. The use of amorphous Ni thin film catalyst also resulted in the formation of an interlayer between the CNWs and the substrate. The appearance of this interlayer, however, depends on the Ni film thickness. The carbon nanowires obtained were found to exhibit an unusual microstructure in that the basal planes were perpendicular to the wire axial direction. The Raman signatures of the CNWs consist of two peaks near 1322 cm⁻¹ (D-band) and 1578 cm⁻¹ (G-band), similar to that of carbon nanotubes. The broadening of the G-band peak was found to be greater than 60 cm⁻¹ and the I_D/I_G ratio was found to decrease with FWHM as in a-C:H.     

A comparison of Raman signatures and laser-induced incandescence with direct numerical simulation of soot growth in non-premixed ethylene/air flames

The predictions of “soot” concentrations from numerical simulations for nitrogen-diluted, ethylene/air flames are compared with laser-induced incandescence and Raman spectra observed from samples thermophoretically extracted using a rapid insertion technique. In some flame regions, the Raman spectra were obscured by intense, radiation that appeared to peak in the near infrared spectral region. There is a good agreement between spatial profiles of this ex situ laser-induced incandescence (ES-LII) and the “traditional” in situ laser-induced incandescence (IS-LII). Raman signatures were observed from low in the flame and extended into the upper flame regions. The spectra consisted of overlapping bands between 1000 and 2000 cm⁻¹ dominated by the “G” band, near ≈1580 cm⁻¹, and the “D” band in the upper 1300 cm⁻¹ range. Several routines are explored to deconvolve the data including 3- and 5-band models, as well as a 2-band Breit–Wigner–Fano (BWF) model. Because the Raman signals were observed at heights below those where in situ LII was observed, we postulate that these signals may be attributable to smaller particles. The results suggest that the observed Raman signals are attributable to particulate with modest (≈1 nm) crystallite sizes. This observation is discussed in the context of current models for nascent particle formation.