Raman Spectra Of Carbon Nanotubes Research Articles

Interacting Phonons between Layers in Raman Spectra of Carbon Nanotubes inside Boron Nitride Nanotubes.

We present the resonant Raman spectra of a single-wall carbon nanotube inside a multiwall boron nitride nanotube (SWNT@BNNT). At EL = 1.58 eV, SWNT@BNNT exhibited resonant Raman spectra at 807 (ωBN) and 804 cm-1 (ωGr). Their intensities almost disappeared at EL = 2.33 eV. We assigned ωBN to the out-of-plane BN phonon mode that coupled with ωGr. At EL = 4.66 eV, the G+ and G- bands of the SWNT@BNNT red-shifted 3.8 cm-1 compared with the SWNT, suggesting the interwall interactions between the in-plane modes of SWNT and BNNT. Moreover, the E2g mode of the BNNT in SWNT@BNNT appeared at 1370.3 ± 0.1 cm-1, which is undistinguishable for EL < 3 eV because of the overlap with the D band frequency. The assignment of the present Raman spectra was confirmed through the first-principles calculations.

Effect of HF wet-etching and H2-plasma polishing on the low-temperature growth of carbon nanotubes on stainless-steel substrates

Direct growth of carbon nanotubes (CNTs) on 316 stainless-steel (SS) substrates could facilitate versatile commercial applications. For the successful synthesis of CNTs on SS substrate, hydrofluoric acid (HF) etching renders suitable surface roughness to promote CNT growth. For catalyst preparation and also subsequent CNT growth, H2-plasma treatment plays essential roles. Reversing the aging effect of Fe/Ni from its unwanted oxides of reduced catalytic activity and formation of Fe nanoparticles from the sheet are accomplished using H2-plasma. High-density plasma, along with specific gas precursors with its energetic ions, required for uniform CNT growth is attained by microwave plasma-enhanced chemical vapor deposition. By using CO2, providing suitable oxidizer precursors, in (CH4 + H2) diffuse plasma, necessary ambience for low-temperature nucleation (300 °C) and high-yield CNT growth is obtained. Reduction in the deposition rate by highly-energetic ion collisions in high-power plasma is resolved by introducing remote plasma configuration using a specially designed mask assembly. Considering the optimum magnitudes of the intensity ratios of D- and 2D-peaks to the G-peak in the Raman spectra of CNTs, a time span of 2 h of HF-etching appears optimum for proper functioning of the Fe/Ni nanoparticle catalyst. Furthermore, H2-plasma polishing is effectively utilized as the last step of pretreatment, leading to the growth of lower-diameter CNTs. Finally, using a shadow mask assembly and weak oxidizing gas, low-temperature growth of CNTs is successfully accomplished on SS substrates, pretreated by optimum HF-etching and H2-plasma polishing on the Fe-nanoparticle catalyst layer, avoiding the conventional steps of high-temperature annealing for catalyst nanoparticle formation as well as high-temperature nucleation for CNT growth. Even without the use of a separate Fe-catalyst layer, CNT growth is accomplished directly on HF-etched and H2-plasma polished SS substrates, utilizing mostly Fe/Ni nano-grains that originate from the intrinsic composition of SS as the nucleation sites. Use of limited action-steps and of a low-temperature (~300 °C) distinguish this as a realistic avenue for commercial production of CNTs.

Time-dependent degradation of carbon nanotubes correlates with decreased reactive oxygen species generation in macrophages.

Introduction and objective: With the increase in carbon nanotube-based products on the commercial market, public concern regarding the possible toxicity of these nanomaterials has attracted much attention. Although previous studies found no obvious toxicity related to carbon nanotubes (CNTs), their safety has not been established because long-term evaluation is still needed. In vitro assays are used to understand the toxicity of nanomaterials. However, the data published so far were generated in short-term assays in which cells are continuously exposed to CNTs. Therefore, the objective of this study is to quantitatively assess the relative long-term cytotoxicity and degradation of CNTs after uptake by macrophages.Methods: We used macrophage cell line of RAW 264.7 and primary rat Kupffer cells to investigate macrophage uptake of CNTs as well as their quantity changes up to a relatively late time point after uptake (7 days) by measuring optical absorbance in the near infrared region and Raman spectra of CNTs in the cell lysates. The time-dependent cytotoxicity was evaluated by measuring reactive oxygen species (ROS), glutathione, cell viability, and caspase 3/7 activity in 1–7 days.Results: CNTs were degraded by approximately 25–30% within first 4 days after uptake; however, and no additional degradation occurred for the remainder of the 7-day test period. Generation of ROS by macrophages decreased as CNT degradation occurred, returning to control levels by Day 7. In the meantime, the glutathione level gradually recovered over time. There were no changes in cell viability or caspase 3/7 activation during CNT degradation.Conclusion: These results confirm that degradation of CNTs by macrophages is associated with ROS generation. The data also suggest that CNT cytotoxicity decreases as they are degraded.

Controllable Synthesis of Special Reed-Leaf-Like Carbon Nanostructures Using Copper Containing Catalytic Pyrolysis for High-Performance Field Emission

Special reed-leaf-like carbon nanostructures have been realized by using chemical vapor deposition (CVD) under the combined action of copper containing catalytic pyrolysis and ammonia (NH3) gas. The nucleation and growth mechanisms of CNLs based on growth parameters are discussed. The Raman spectra of carbon nanotubes (CNTs), CNLs and CNT-CNL composites were measured and found to be strongly influenced by the type of gas. Field emission (FE) properties of CNL-CNT composites were observed with a lower turn-on electric field of 0.73 V/µm, and a higher current density of 18.0 mA/cm2 at an electric field of 2.65 V/µm, which are superior to those of CNTs and flower-like CNLs. This is because there are more field emitters in CNLs inter-planted in CNTs. We consider that the unique FE stability of CNTs and defects in CNLs play a synergetic role on the improved FE properties.

Application of Raman Spectroscopy for Analysis of Carbon Nanotube Distribution in Living Cells

We have used Raman spectroscopy combined with confocal microscopy to study suspensions of single-wall and double-wall carbon nanotubes of different lengths and also multiwall carbon nanotubes. We have shown that the intensity of the G mode in the Raman spectrum of carbon nanotubes is directly proportional to the nanotube concentration, the exposure time, the exciting radiation power, and depth of focus in the transparent sample under study. We have established that the Raman spectra of longer carbon nanotubes (~1 μm) are characterized by higher intensity of the G mode compared with short carbon nanotubes (~250–450 nm). The dependences obtained were used to determine the local intracellular concentration of carbon nanotubes within the waist of the exciting laser beam, with the aim of mapping the carbon nanotube distribution inside the cells.

Carbon Nanotubes as Etching Masks for the Formation of Polymer Nanostructures.

We investigate the interaction of carbon nanotubes (CNTs) embedded in a polymer matrix [poly(methyl methacrylate) (PMMA)] with Ar plasma, which results in the formation of PMMA nanostructures, as CNTs act as an etching mask. Because of the large differences in the Ar ion sputtering yields between CNTs and PMMA, PMMA lines with the width comparable to that of CNTs and as high as 20 nm (for single-walled CNTs) or 80 nm (for multiwalled CNTs) can be obtained after repeated exposure of CNT/PMMA films to Ar plasma. We also follow the etching process by investigating changes in the IV characteristics and Raman spectra of CNTs after each exposure to Ar plasma, which shows progressive defect generations in CNTs while they maintain structural integrity long enough to act as the etching mask for PMMA underneath. We demonstrate that the PMMA nanostructure patterns can be transferred to a different polymer substrate using nanoimprinting.

Raman spectra of carbon nanotubes with monovacancy

By using the empirical Brenner potential, we have calculated the structural and vibrational properties of single walled carbon nanotubes (SWCNTs) with monovacancy, based upon which their nonresonant Raman spectra have been calculated by the empirical bond polarizability model. It is found that a new pentagon ring coupling with a dangling bond will form in the SWCNTs with monovacancy and the structural stability is responsible for the orientation of the new bond comparing with the tube axis. More importantly, it is found that the monovacancy-induced Raman peaks lie out of the SWCNT's G-band and can be considered as the intrinsic characteristic of the SWCNTs with monovacancy, which can be used to detect the monovacancy experimentally.

Note: Automatic laser-to-optical-fiber coupling system based on monitoring of Raman scattering signal

We developed an automatic laser-to-optical-fiber coupling (ALOC) system that is based on the difference in the Raman scattering signals of the core and cladding of the optical fiber. This system can be easily applied to all fields of fiber optics since it can perform automatic optical coupling within a few seconds regardless of the core size or the condition of the output end of the optical fiber. The coupling time for a commercial single-mode fiber for a wavelength of 632.8 nm (core diameter: 9 μm, cladding diameter: 125 μm) is ~1.5 s. The ALOC system was successfully applied to single-mode-fiber Raman endoscopy for the measurement of the Raman spectrum of carbon nanotubes.

Distinguishing defect induced intermediate frequency modes from combination modes in the Raman spectrum of single walled carbon nanotubes

Understanding of the origin of some of the intermediate frequency modes (IFMs) in the Raman spectrum of carbon nanotubes has remained controversial in the literature. In this work, through controlled introduction and elimination of defects in carbon nanotubes, we attempt to isolate the IFMs due to structural defects from that of the combination modes in single walled carbon nanotubes (SWCNTs). Our investigations on pristine and defect engineered SWCNTs using ion-irradiation, thermal annealing, and laser processing show systematic changes in the IFMs in the range 400–1200 cm−1 and its manipulation with the processing parameters. In particular, we found that the intensity of IFM at 929 cm−1 scale up with the increasing defect concentration, while that at 668 cm−1 follows opposite behavior. New IFM peaks were observed upon the creation of a controlled amount of structural defects through 30 keV N+ ion irradiation. Elimination of defects through vacuum annealing results into reduction of intensity of some IFMs identified as defect related, while the intensity of characteristic combination modes correspondingly increases. Our results show that the IFMs observed at 709, 805, 868, 926, and 1189 cm−1 are due to structural defects in the SWCNTs, while those in the range 400–550 cm−1 and at 669 cm−1 are due to the combination modes. Our x-ray photoelectron spectroscopy analysis on ion irradiated SWCNTs supports the Raman results.

碳纳米管和金属纳米粒子复合结构的拉曼光谱特性

碳纳米管和金属纳米粒子复合结构的拉曼光谱特性

Effect of plasma treatment on electrical conductivity and Raman spectra of carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) were treated with a radio-frequency discharge. We found that MWCNTs showed opposite trends in electrical conductivity when treated with oxygen and hydrogen plasmas. MWCNTs showed enhanced electrical conductivity when placed at cathode with oxygen plasma treatment, whereas MWCNTs treated at positive column did not show such a trend. In contrast, the conductivity of MWCNTs dropped sharply with hydrogen plasma treatment. The measured conductivity trends of MWCNTs are correlated with observed Raman spectral shift. The possible mechanisms of the change in electrical conductivity in plasma-treated MWCNTs are discussed.

Surface‐induced changes in the vibrational spectra of conducting polymer – carbon nanotube hybrid materials

AbstractWe present infrared (IR) and Raman spectra of carbon nanotubes (CNTs) coated by the conducting polymers polyaniline (PANI), polycarbazole (PCz), and melanin (polydopamine). These hybrid materials have been prepared by electrochemical polymerization and exhibit increased conductivity and enhanced electron transfer from the electrolyte to the electrode. Interaction of the polymers with the nanotube surface causes attenuation of some IR modes [surface‐attenuated infrared absorption (SAIRA) effect], while the Raman modes are enhanced by a surface‐enhanced Raman scattering (SERS) mechanism. The magnitude of the attenuation depends on the strength of the nanotube–polymer interaction. We compared the spectra of the hybrids and found the most outspoken effect in the case of polyaniline, in accordance with the results of ultraviolet–visible (UV–Vis) and impedance spectroscopy.

An efficient and inexpensive method for the production of multi-wall carbon nano tubes by the thermal plasma route

An extended argon arc plasma reactor has been utilised for efficiently producing multi-walled carbon nano tubes (MWCNT) with an inexpensive raw material such as the calcined petroleum coke. In this investigation, CPC powder was dry mixed with 2.5 wt %, Ni catalyst in a ball mill and the resulting homogeneous raw material mixture was heat treated in a graphite crucible in a vacuum argon arc plasma reactor for the duration of 30 minutes. It was noted that carbon nano tubes (CNTs) were deposited on a relatively cooler portion of the plasma reactor, with a yield of 15 wt %, with respect to the total weight of the charge. The extensive characterization of the carbon nano-tubes was carried out through micro-Raman, X-ray diffraction and transmission electron microscopy techniques. The XRD studies on the CNTs, produced by the above method showed 100% peak at d002 value of 3.39 A and other peaks of 4-H graphite. The d- band and g- band reflections observed in the Raman spectrum of CNTs also showed graphite as a major and cubic diamond as a minor phase. The TEM studies on the plasma processed and purified CNTs, showed aspect ratio of 15 to 16 and an average diameter of 3.4 nm.

Energy ofK-Momentum Dark Excitons in Carbon Nanotubes by Optical Spectroscopy

Phonon sideband optical spectroscopy determines the energy of the dark K-momentum exciton for (6,5) carbon nanotubes. One-phonon sidebands appear in absorption and emission, split by two zone-boundary (K-point) phonons. Their average energy locates the E11 K-momentum exciton 36 meV above the E11 bright level, higher than available theoretical estimates. A model for exciton-phonon coupling shows the absorbance sideband depends sensitively on the K-momentum exciton effective mass and has minimal contributions from zone-center phonons, which dominate the Raman spectra of carbon nanotubes.

Characterization of Small-Diameter Carbon Nanotubes and Carbon Nanocaps on SiC(0001) Using Raman Spectroscopy

The Raman spectra of carbon nanotubes (CNTs) formed on a 6H–SiC(0001) C-face substrate by surface decomposition were acquired using a 1064 nm Nd:YAG laser as the excitation source. Intense radial-breathing-mode Raman bands were successfully detected under resonant excitation from CNTs formed by the surface decomposition of SiC. Fine CNTs having diameters between 0.7 and 1.3 nm were observed on SiC after heating at 1700 °C in a vacuum electric furnace. These diameters are considerably smaller than those previously observed for CNTs grown by this technique.

Double resonance features in the Raman spectrum of carbon nanotubes

A weak feature is observed in the Raman spectra of single-wall carbon nanotube bundles at a frequency between 1410 and $1430\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The band shifts slightly toward lower frequencies as the excitation energy increases from 2.17 to $2.71\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. An inverse correlation is found between the frequency of the band and that of the average radial-breathing mode, indicating a dependence on the nanotube diameter. The frequency can be extrapolated quadratically to $1487\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ for infinite diameter. Two different hypotheses are considered, ascribing the band to modes around either $K$ or $M$ points of the graphite dispersion relations. The appearance of a band close to $1480\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in defective graphite and its dependence with the laser energy, as well as the strong sensitivity of high-frequency $K$-point modes to structural or impurity defects, support the interpretation of the mode as arising from for the upper phonon branch of graphite around the $K$ point activated by the double-resonance mechanism.

Optical Transition Energies for Carbon Nanotubes from Resonant Raman Spectroscopy: Environment and Temperature Effects

This Letter reports the laser energy dependence of the Stokes and anti-Stokes Raman spectra of carbon nanotubes dispersed in aqueous solution and within solid bundles, in the energy range 1.52-2.71 eV. The electronic transition energies (E(ii)) and the radial breathing mode frequencies (omega(RBM)) are obtained for 46 different (18 metallic and 28 semiconducting) nanotubes, and the (n,m) assignment is discussed based on the observation of geometrical patterns for E(ii) versus omega(RBM) graphs. Only the low energy component of the E(M)(11) value is observed from each metallic nanotube. For a given nanotube, the resonant window is broadened and down-shifted for single wall carbon nanotube (SWNT) bundles compared to SWNTs in solution, while by increasing the temperature, the E(S)(22) energies are redshifted for S1 [(2n+m) mod 3=1] nanotubes and blueshifted for S2 [(2n+m) mod 3=2] nanotubes.

Temperature dependence of the raman spectra of carbon nanotubes with 1064 nm excitation

In this report, the near infrared 1064nm line of an Nd:YAG laser, which has strong thermal effect, was used as the excitation. A temperature dependence of the Raman spectra of carbon nanotubes was observed at different temperatures by varying the incident laser power. The results show that the relative Raman intensities to the tangential stretching mode (G mode) of the higher-order Raman modes within 2500–3500cm−1 increase with increasing excitation laser power at the sample and the changes in the relative Raman intensities are linear in the excitation laser power. This has not been reported elsewhere. Thorough analysis shows that this is a temperature dependence of double-phonon Raman scattering and maybe provide important information for the studying of CNTs and double-phonon Raman scattering.

Raman spectra of carbon nanotubes and nanofibers prepared by ethanol flames

Raman spectra of carbon nanotubes and nanofibers prepared by ethanol flames

Growth, Nitrogen Doping and Characterization of Isolated Single-Wall Carbon Nanotubes using Liquid Precursors

ABSTRACTIsolated single wall carbon nanotubes (SWNTs) were prepared on bare quartz and SiO2/ Si substrates using chemical vapor deposition (CVD) in which a liquid precursor, such as xylene, was used as the carbon source. The coverage of isolated SWNTs on the substrates was controlled by adjusting the concentration of iron (III) nitrate nonahydrate/2-propanol solution which provided the Fe seed catalyst particles. Micro-Raman spectra were obtained using the 514.5 nm excitation which showed the typical tangential band around 1590 cm−1 for semiconducting nanotubes. The radial breathing mode (RBM) frequencies ranged between 150 to 240 cm−1 and the estimated tube diameters are in good agreement with those obtained from atomic force microscope (AFM) images. In our synthesis approach controlled doping of isolated SWNTs with nitrogen was achieved by mixing appropriate amount of acetonitrile with xylene. As the nitrogen concentration in the feed was increased from 0 – 33 at. %, the RBM intensity decreased dramatically while the intensity of the D-band increased gradually relative to that of the G-band. Interestingly, the D' band was observed for the first time in the Raman spectrum of carbon nanotubes when the nitrogen concentration reached ∼2–3 at. %.