Raman spectroelectrochemistry of a single-wall carbon nanotube bundle

Raman microscopy and spectroelectrochemistry with polymer electrolyte gating is developed to study the effect of charging on Raman spectra of individual single-wall carbon nanotubes (SWCNTs) and bundles. The Raman spectra of a small bundle, consisting of well-separated features from a metallic and a semiconducting SWCNT, have been obtained at different electrochemical charging levels. The broad Fano peak of the metallic SWCNT exhibits an appreciable frequency upshift and simultaneous line narrowing when the charging level, either positive or negative, is increased, in agreement with the presence of a Kohn anomaly in metallic SWCNTs. The radial breathing mode of the metallic tube also shows a similar but much weaker dependence on the charging potential. While the G mode frequencies of the semiconducting SWCNT also increase with the increasing charging level, the magnitude of such change is much smaller than in the metallic SWCNT. At high negative charging potentials the G⁻ peak of the semiconducting SWCNT exhibits a larger upshift than its G⁺ peak, leading to the observation of merging of these two peaks. However, both G⁺ and G⁻ peaks of the semiconducting SWCNT become broader at high charging levels, which are not predicted from previous theoretical studies.     

Electrochemical doping of single-walled carbon nanotubes in double layer capacitors studied by in situ Raman spectroscopy

The electrochemical doping of single-walled carbon nanotubes (SWCNTs) in 1 M Et₄NBF₄ in acetonitrile was investigated by in situ Raman spectroscopy. The capacitance was determined to be 82 F/g for the positive and 71 F/g for the negative SWCNT electrode, respectively, which approaches the typical values for microporous activated carbons used in supercapacitors. The changes in the Raman intensities and shifts of the D and G⁺ bands as well as of the radial breathing modes (RBMs) during electron and hole injection were studied as a function of the electrode potential. For the D and G⁺ bands, hole doping leads to strong upshifts which can be attributed to a stiffening of C-C bonds and the corresponding phonon modes. Electron doping results in much less pronounced changes in the band positions. The intensity attenuation of the RBM bands was found to be markedly different for semi-conducting and metallic SWCNTs, whereby sufficiently high doping leads to a loss of Raman intensity due to bleaching of electronic transitions. The main RBM bands upshift upon both electron and hole doping, which is attributed to changes in the chemical environment of individual SWCNTs upon charging and discharging of the electrochemical double layer within SWCNT bundles.     

Optimisation of reaction conditions for the synthesis of single‐walled carbon nanotubes using response surface methodology

The effects of three preparation variables, i.e. reaction temperature, reaction time and reaction gas (methane/nitrogen) flow rate, on the ratio of the intensity of the Raman D band to the intensity of the G band (I_D/I_G), carbon mass and the presence of radial breathing mode (RBM) peaks were investigated by using a central composite design to develop two linear models. The most influential factor in each experimental design‐response was identified using the analysis of variance. The predicted I_D/I_G ratio, carbon mass and presence of RBM peaks determined during the process optimisation were found to agree satisfactorily with the experimental values. The optimum conditions for synthesising single‐walled carbon nanotubes were determined to be a reaction temperature of 900°C, a reaction time of 59 min and a reaction gas flow rate of 54 mL/min. © 2011 Canadian Society for Chemical Engineering     

Origin of radial breathing mode in multiwall carbon nanotubes synthesized by catalytic chemical vapor deposition

The origin of radial breathing mode (RBM) in the Raman spectra of multiwall carbon nanotubes (MWNCTs) is discussed. In general, RBM is characteristics of single wall carbon nanotube (SWCNT). With the help of transmission electron microscope (TEM) and Raman spectroscopic studies, it is established that the presence of SWCNT in the cavity of MWCNT is responsible for the appearance of RBM in MWCNT (synthesized by low temperature catalytic chemical vapor deposition technique). The estimated diameter of 8.2 Å (from Raman study) of SWCNT is almost same as that observed (∼8.3 Å) in TEM studies.     

Tip-enhanced Raman spectroscopy reveals rich nanoscale adsorption chemistry of 2-mercaptopyridine on Ag

Adsorption of 2-mercatopyridine (2MPy) on Ag surfaces was studied by tip-enhanced Raman spectroscopy (TERS), which allows the measurement of Raman spectra with nanometer scale spatial resolution on flat surfaces that themselves do not show any surface-enhancement Raman scattering (SERS) activity. We found that the adsorption behavior of 2MPy was affected by the parameters of the preparation for the adsorbate layers, i.e., solution concentration, solution volume, and the exposure time. Besides that, variation of the TERS spectra at randomly chosen sample positions was observed. Only some of the bands appearing in SERS experiments showed up in each TERS measurement. We propose that this is caused by different local adsorption behavior of 2MPy on the Ag surfaces. This observation perfectly demonstrates the advantage of TERS over SERS, i.e., TERS can give localized chemical information on the nanometer scale, whereas SERS can only afford average spectra with micrometer scale resolution. Finally, TERS mapping with a spatial resolution of 24 nm was demonstrated.     

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.     

Ultraviolet and visible Raman spectroscopies of the graphitic BCx phases

Ultraviolet (UV) Raman and visible Raman spectroscopies were applied to study the graphitic BC_x (g-BC_x) phases. The Raman spectra of the g-BC_x phases excited with UV laser at 244 nm have one main peak: a G peak (approximately at 1590 cm^? 1), and do not have the D peak (around 1350 cm^? 1) characteristic for Raman spectra of disordered graphitic phases. The D peak can be detected in all g-BC_x phases when green (534 nm) or near-infrared (785 nm) lasers are used for Raman scattering excitation. The positions of the G and D peaks were found to be independent (within the experimental errors) of the B/C ratio. The pattern of the peaks in UV Raman spectra of g-BC_2.1 phase indicates that the additional peaks centered at 1089 cm^? 1 should be assigned to the E_g mode of B₄C vibration rather than to the T mode characteristic to amorphous graphite. The high signal-to-noise (S/N) ratio and lack of fluorescence of the UV Raman spectra allow an accurate measure of bandwidth and frequency of the G peaks.     

Resonant Raman studies of tetrahedral amorphous carbon films

Resonant Raman scattering has been used to study the tetrahedral amorphous carbon films deposited by the filtered cathodic vacuum arc technique. The excitation wavelengths were 244, 488, 514 and 633 nm, corresponding to photon energies of 5.08, 2.54, 2.41 and 1.96 eV, respectively. In the visible Raman spectra only vibrational modes of sp²-bonded carbon (G and D peaks) are observed, while a wide peak (called the T peak) can be observed at approximately 1100 cm⁻¹ by UV-Raman spectra which is associated with the vibrational mode of sp³-bonded carbon. Both the position and the width of the G peak decrease almost linearly with increasing excitation wavelength, which is interpreted in terms of the selective ππ* resonant Raman scattering of sp²-bonded carbon clusters with various sizes. The G peak position in the UV-Raman spectra, the T peak position and the intensity ratios of I_D/I_G and I_T/I_G all exhibit maximum or minimum values at the carbon ion energy of 100 eV. The changes of these spectral parameters are discussed and correlated with the sp³ fraction of carbon atoms in the films.     

Low temperature growth of carbon nanotubes on Si substrates in high vacuum

Carbon nanotube (CNT) growth was carried out on SiO₂/Si substrates using an alcohol gas source in a high vacuum without any carbon decomposition processes. In the Raman spectra of the grown CNTs, both the G/Si peak intensity ratio and G/D peak intensity ratio indicated that the optimum growth temperature became lower as the pressure decreased. By reducing the pressure to 1 × 10^− 4 Pa, CNTs could be grown at 400 °C, and the G/D ratio was about 16, indicating that the quality of the grown CNTs was good, taking into account the low growth pressure. In addition, the Raman spectra in the radial breathing mode (RBM) region showed that the diameter distribution of the grown CNTs was dependent on both the growth pressure and temperature, and the relative intensity of the RBM peaks from small-diameter CNTs increased as the growth pressure and/or temperature was reduced.     

Stress transfer in polyacrylonitrile/carbon nanotube composite fibers

Gel spun polyacrylonitrile/carbon nanotube (PAN/CNT) composite fibers have been produced, and the stress-induced G′ Raman band shifts in the CNTs have been monitored to observe stress transfer during fiber strain. Improvements in CNT quality, CNT dispersion, and post-processing fiber drawing are shown to increase the stress transfer from the matrix to the CNT. Radial breathing mode (RBM) intensity of specific CNT chiralities confirms CNT debundling during fiber processing. During PAN/CNT fiber straining, there reaches a plateau in the CNT G′ downshift, signifying that the stress on the CNT is maintained despite continued straining of the PAN/CNT fiber. Correlating CNT strain with CNT modulus and volume fraction allows for the interfacial shear strength (τ_i) of the PAN-CNT interface to be determined. The as-spun and fully drawn PAN/CNT-A (99/1) nano composite fibers exhibit τ_i of 13.1 and 30.9 MPa, respectively, while an improved CNT dispersion (PAN/CNT-A (99.9/0.1)) results in τ_i equal to 44.3 MPa.     

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.     

OCRE Domains of Splicing Factors RBM5 and RBM10: Tyrosine Ring-Flip Frequencies Determined by Integrated Use of 1H NMR Spectroscopy and Molecular Dynamics Simulations

The 55-residue OCRE domains of the splicing factors RBM5 and RBM10 contain 15 tyrosines in compact, globular folds. At 25 °C, all 15 tyrosines show symmetric ¹H NMR spectra, with averaged signals for the pairs of δ- and ϵ-ring hydrogens. At 4 °C, two tyrosines were identified as showing ¹H NMR line-broadening due to lowered frequency of the ring-flipping. For the other 13 tyrosine rings, it was not evident, from the ¹H NMR data alone, whether they were either all flipping at high frequencies, or whether slowed flipping went undetected due to small chemical-shift differences between pairs of exchanging ring hydrogen atoms. Here, we integrate ¹H NMR spectroscopy and molecular dynamics (MD) simulations to determine the tyrosine ring-flip frequencies. In the RBM10-OCRE domain, we found that, for 11 of the 15 tyrosines, these frequencies are in the range 2.0×10⁶ to 1.3×10⁸ s⁻¹, and we established an upper limit of <1.0×10⁶ s⁻¹ for the remaining four residues. The experimental data and the MD simulation are mutually supportive, and their combined use extends the analysis of aromatic ring-flip events beyond the limitations of routine ¹H NMR line-shape analysis into the nanosecond frequency range.     

Behavior of the high frequency Raman modes of double-wall carbon nanotubes after doping with bromine or iodine vapors

The effects of two different halogen dopants (bromine and iodine) at different concentrations on the higher frequency modes (the so-called G and G′ bands) of the Raman spectra of double-wall carbon nanotube (DWCNT) “buckypaper” are investigated. The effects of dopants on different DWCNT configurations (metallic inner/semiconducting outer and vice versa) are studied by changing the laser excitation energy. The doping causes the loss of the Breit–Wigner–Fano line shape and the appearance of less metallic behavior. An increase of the relative intensity of the G⁺ band, which is more sensitive for the outer metallic tubes, is clearly observed with increasing Br₂ concentration in the sample. By analysis of the G⁺ band and the G′ band it is possible to measure the changes in the electron–phonon coupling, due to the charge-transfer between the dopant (Br₂ or I₂) and the tubes in the DWCNT. The doping effect causes an upshift of the G⁺ band and a suppression of the contribution of the inner tubes to the G′ band signal and as a consequence, the observed G′ band is dominated by the contribution from the outer tubes.     

Tunable plasmon resonances in a metallic nanotip-film system

Tip-enhanced Raman spectroscopy (TERS) has emerged as a powerful tool for optical imaging at nanoscale spatial resolution, and for investigating the vibrational properties of molecules adsorbed on a substrate. Plasmonic enhancement of the electromagnetic fields near a metallic nanostructure plays a very important role in TERS, where resonant excitation of plasmons is crucial. When two metallic nanostructures are placed at a gap of nanometric distance, their plasmons can interact with one other and result in hybridized shifted plasmon modes. Here, we apply this idea to TERS and demonstrate a significant tunability of the plasmon resonance enabling large electric field enhancement at a desired excitation wavelength. This finding paves the way for efficient optimization of TERS in imaging and spectroscopy applications.     

Direct observation of the formation of linear C chain/carbon nanotube hybrid systems

Multi-wall carbon nanotubes (MWCNTs) containing linear C chains have been synthesised by arc discharge in liquid nitrogen. The experimental conditions used allow one to obtain nanotubes with a very thin innermost diameter, as evidenced by the radial breathing mode (RBM) features in the Raman spectra. A correlation between the RBM and the features of the C chains is reported, which gives a direct indication of how these linear carbon chain/carbon nanotube hybrid systems form. C chains are inserted only in CNTs having the innermost diameter equal to 0.7 nm, as expected.     

Raman investigation of single oxidized carbon nanotubes

The oxidation process of single-walled carbon nanotubes via nitric acid treatment was followed by IR-, UV-Vis-NIR, and single bundle Raman spectroscopy. The introduction of functional, oxygen-containing groups is revealed by an additional absorption band at 1725 cm⁻¹, characteristic of carbonyl stretch vibrations. No significant shift of the optical absorption bands could be detected after oxidation. The combination of atomic force microscopy and confocal scanning resonance-enhanced Raman microscopy was used to investigate thin bundles and, eventually, individual nanotubes in detail. These experiments enabled determination of the dependence of the Raman intensity of the G-line (around 1590 cm⁻¹) on the bundle height for both non-oxidized and oxidized tubes. The Raman cross-section of the oxidized tubes was found to be reduced by a factor of ˜4, compared to the pristine tubes. This observation is ascribed to all tubes within a bundle that are oxidized to the same degree.     

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.     

New developments in the growth of 4 Angstrom carbon nanotubes in linear channels of zeolite template

We report new developments on the chemical vapor deposition growth of 0.4 nm single-walled carbon nanotubes (SWCNTs) inside the linear channels of the aluminophosphate zeolite, AlPO₄-5 (AFI), single crystals (0.4 nm-SWCNT@AFI). Ethylene (C₂H₄) and carbon monoxide (CO) were used as the feedstock. Polarized Raman spectroscopy was used to analyze the structure and quality of SWCNTs, both the radial breathing mode and G-band are much clearer and stronger than the samples grown by the old process which used template tripropylamine molecules for growing SWCNT@AFI. From the Raman spectra, it is clearly seen that the RBM is composed of two peaks at 535 and 551 cm⁻¹. By using the pseudopotential module in Material Studio to calculate the Raman lines, the 535 cm⁻¹ peak is attributed to the (5,0) SWCNTs and the 551 cm⁻¹ peak to the (3,3) SWCNTs. The abundance of (4,2) is relatively small. Thermal gravity analysis showed that while the samples grown by CO display less than 1 wt% of carbon, for the samples heated in C₂H₄ atmosphere the weight percentage of SWCNTs is around 10%, which implies ∼30% of the AFI channels are occupied with SWCNTs, a significant increase compared with the previous samples.     

Synthesis of Novel Porphyrin and its Complexes Covalently Linked to Multi-Walled Carbon Nanotubes and Study of their Spectroscopy

Novel covalent porphyrin and its complexes (Co²⁺, Zn²⁺) functionalized multi-walled carbon nanotubes (MWNTs) have been successfully synthesized by the reaction of the carboxyl on the surface of MWNTs which was synthesized to use carbon radicals generated by the thermal decomposition of azodiisobutyronitrile (AIBN) with 5-p-hydroxyphenyl-10,15,20-triphenyl-porphyrin and its complexes (Co²⁺, Zn²⁺). Three resulting nanohybrids were characterized by spectroscopy (FT-IR, Raman, and UV-vis), TGA, and TEM. The quality of porphyrin attached to the MWNTs was determined from thermogravimeric analysis (TGA) of the MWNTs, which showed a weight loss of about 60%. The Raman and absorption spectroscopy data showed that the electronic properties of modified MWNTs were mostly retained, without damaging their one-dimensional electronic properties. From fluorescence measurements, it was observed that the porphyrin and its complexes (Co²⁺, Zn²⁺) were nearly quenched by MWNTs, indicating that this covalently modified mode facilitated the effective energy or electron transfer between the excited porphyrin moiety and the extended π-system of MWNTs.     

The exfoliation of SWCNT bundles examined by simultaneous Raman scattering and photoluminescence spectroscopy

Simultaneous measurements of Raman scattering and photoluminescence (PL) prove to be a powerful method for quantifying the bundling states of single-walled carbon nanotube (SWCNT). This paper presents physical analysis and experimental evidence to establish that the G-band normalized photoluminescence, which is determined from the simultaneously acquired Raman scattering and PL emission spectra, can serve as a good indicator for quantifying the degree of exfoliation of SWCNT dispersions. Without introducing the complications of sampling geometry and instrumental correction, this indicator directly relates to the intrinsic physical properties of a given SWCNT sample, namely, the absorption cross-section, differential Raman scattering cross-section, and PL quantum yield of SWCNT. An inverse linear relationship between the G-band normalized 267 cm^?1 RBM intensity and the PL emission intensities for SWCNT dispersions with different degrees of exfoliation was experimentally observed, indicating this can be used for quantitative characterization of the degree of exfoliation for a given SWCNT sample. An in-depth analysis of the indicators of the degree of exfoliation in various as-sonicated SWCNT dispersions highlights a two-stage exfoliation mechanism of SWCNT bundles under sonication.