The influence of doping on the Raman intensity of the D band in single walled carbon nanotubes

The D band in the Raman spectra of single walled carbon nanotubes is considered as an indicator of defects in carbon nanotubes. However, its dependence on charge-transfer doping is generally ignored, despite the studied samples are often naturally doped. We studied the intensity of the D band, the ratio of the intensities of the D band and TG band (I_D/I_TG) and the ratio of the intensities of the D and G′ band (I_D/I_G′) in the Raman spectra of the single walled carbon nanotubes in dependence on a doping level. We tested two laser excitation energies viz 2.41 and 1.92 eV, which are in resonance with semiconducting and metallic tubes, respectively in our sample. It is shown that the D band intensity is significantly attenuated in doped carbon nanotubes sample for both semiconducting and metallic tubes. The I_D/I_TG ratio is weakly dependent on doping for semiconducting tubes but for metallic tubes the I_D/I_TG ratio exhibits strong dependence on doping. The I_D/I_G′ ratio is suggested for evaluation of the defects in carbon nanotubes samples since it is less sensitive to doping both for semiconducting and metallic tubes. Nevertheless, for highly doped samples even the I_D/I_G′ ratio exhibits significant dependence on doping level.     

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 spectroscopy study of HM carbon fibres: effect of plasma treatment on the interfacial properties of single fibre/epoxy composites

The effect of an oxygen plasma treatment upon the structural and morphological properties of high-modulus carbon fibres has been studied by means of several characterisation techniques. Scanning electron microscopy showed that there were only minor changes of the morphology of the fibres following treatment. X-ray diffraction traces revealed that there were differences in structural parameters between the untreated fibres but no further modifications in the crystalline structure were detected after the plasma oxidation. Raman spectroscopy was used to follow the changes on the fibre surface structure following treatment. The peak positions and widths of the four main Raman bands (D, G, D′ and G′) were determined, with no significant changes observed after the surface treatment. A relationship between the width of the G band and the crystal parameter d₀₀₂ was found, with the magnitudes of both decreasing as the fibre modulus increased. A reference order parameter I_D/(I_D+I_G) ratio was calculated from the intensities of D and G bands. The treated fibres exhibited a more highly disordered surface structure that the untreated ones, as revealed by the increase of I_D/(I_D+I_G) after the plasma oxidation.     

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.     

Synthesis and structural characterization of thin multi-walled carbon nanotubes with a partially facetted cross section by a floating reactant method

Here we describe synthesis of very unusual multi-walled carbon nanotubes through a catalytic chemical vapor deposition method using a floating reactant method and subsequent thermal treatment up to 2600 °C in a large quantity. Main characteristics of these nanotubes are (1) relatively wide distribution of diameters ranging from 20 to 70 nm and linear, long macro-morphology (aspect ratio >100), (2) highly straight and crystalline layers, (3) high purity through removal of metallic impurity, (4) very low interlayer spacing (0.3385 nm) and low R value (I_D/I_G = 0.0717), (5) high G′ intensity over intensity of G band (G′/G = 0.85) and strongly negative magnetoresistance value of −1.08% at 77 K and 1 T. The unusual microstructure of thin multi-walled carbon nanotubes with a partially facetted cross-sectional shape caused by thermal treatment is mainly ascribed to abrupt density changes (from 1.89 to 2.1 g/cm³) within a confined nanosized space, accompanying with the phase separation.     

Probing plasma-induced defect formation and oxidation in carbon nanotubes by Raman dispersion spectroscopy

Plasma treatment of multi-walled carbon nanotubes (MWCNTs) using an atmospheric pressure hydroxyl radical (OH) source has been monitored with micro-Raman spectroscopy. We use dispersion behavior of the intensity ratio, band position, and linewidth of the D, G, D′, and G′ bands to probe the defect formation and oxidation process in MWCNTs. A simple kinetic model is adopted to interpret the observed dispersion trends in plasma-treated MWCNTs. X-ray photoelectron spectroscopic analysis, scanning electron microscopy inspection, and Raman dispersion characterization of MWCNT surfaces suggest that the dominant effect of OH plasma on MWCNTs is reduced π-conjugated states due to creating structural defects and attachment of oxygen-containing functional groups.     

The formation mechanisms of multi-wall carbon nanotubes over the Ni modified MCM-41 catalysts

Multi-wall carbon nanotubes (MWCNTs) were grown by thermal chemical vapor deposition (thermal CVD) of CH₄ by using Ni-MCM-41 as the catalyst. Methane pyrolysis has been performed in a quartz tube reactor over the catalyst surface to form carbon atoms via dehydrogenation process. The migration and rearrangement of the surface carbon atoms result in the formation of MWCNTs. Transmission electron microscope (TEM) and scanning electron microscope (SEM) were used to determine the morphologies and structures of CNTs, and Raman spectroscopy was exploited to analyze their purity with the relative intensity between the D-band (Disorder band) in the vicinity of 1,350 cm⁻¹ which is characteristic of the sp³ structure and G-band (Graphitic band) in vicinity of 1,580 cm⁻¹ which is characteristic of the sp² structure. In addition, the controlling factors of methane pyrolysis such as the catalyst composition; the reaction temperature, and the methane flow rate on the formation of MWCNTs were investigated to optimize the structure and yield of MWCNTs. SEM/TEM results indicate that the yield of the CNTs increases with increasing Ni concentration in the catalyst. The optimized reaction temperature to grow CNT is located between 640 and 670 °C. The uniform and narrow diameter MWCNTs form at lower flow rate of methane (∼30 sccm), and non-uniform in diameter and disorder structure of MWCNTs are observed at higher flow rate of methane. This is consistent with Raman analysis that the relative intensity of I _D/I _G increases with increasing methane flow rate. The formation mechanisms of the MWCNTs on the Ni-MCM-41 catalyst have been determined to be a Tip-Growth mode with a nanoscale catalyst particle capsulated in the tip of the CNT.     

Structure of and stress transfer in fibres spun from carbon nanotubes produced by chemical vapour deposition

One of the most promising routes for exploiting the axial properties of carbon nanotubes (CNTs) is in a macroscopic fibre consisting of CNTs aligned parallel to each other and to the fibre axis. In this work, we study the structure of and stress transfer in CNT fibres produced by direct spinning from the gas-phase during CNT growth. Using Raman spectroscopy, X-ray diffraction (XRD) and high-resolution electron microscopy we show that the fibres are mostly composed of collapsed double-wall nanotube stacks highly aligned and uniformly oriented along the fibre axis. In situ Raman and XRD measurements during tensile deformation indicate that the transfer of stress between bundles in the fibre is not uniform, with the Raman peak downshift rates for the D, G and G′ bands varying by as much as a factor of 2 in different areas of the fibre. The infiltration of polymer into the CNT fibre increases stress transfer, the highest value of the G′ band downshift rate observed being −33.6 cm⁻¹/% fibre strain; however, with a non-uniform stress distribution still observed after polymer ingress.     

Nanoparticle decoration of carbon nanotubes by sputtering

Vapor phase growth of gold, nickel and titanium metal nanoparticles on multiwall carbon nanotube (MWCNT) buckypaper by sputtering was investigated. The size and distribution of nanoparticles was dependent on the intrinsic binding energy of the metal elements, but could be altered to mimic that of metals with different binding energies by in situ modification of the MWCNT surfaces by energetic metal ions or annealing of the buckypaper. A range of average gold particle diameters from approximately 5–30 nm could be produced depending on the intrinsic sputter process parameters (especially metal ion flux and kinetic energy) and defect density of the MWCNT surfaces, which could also be controlled by annealing prior to sputtering. The diameter of the MWCNTs had a significant influence on the geometry of the nanoparticles. Particles were elongated along the nanotube axis for tube diameters <30 nm. Remarkably strong alignment of the particles along the nanotube axis was observed, especially for MWCNTs with higher defect densities.     

Surface-enhanced Raman scattering of suspended monolayer graphene

Abstract

The interactions between phonons and electrons induced by the dopants or the substrate of graphene in spectroscopic investigation reveal a rich source of interesting physics. Raman spectra and surface-enhanced Raman spectra of supported and suspended monolayer graphenes were measured and analyzed systemically with different approaches. The weak Raman signals are greatly enhanced by the ability of surface-enhanced Raman spectroscopy which has attracted considerable interests. The technique is regarded as wonderful and useful tool, but the dopants that are produced by depositing metallic nanoparticles may affect the electron scattering processes of graphene. Therefore, the doping and substrate influences on graphene are also important issues to be investigated. In this work, the peak positions of G peak and 2D peak, the I_2D/I_G ratios, and enhancements of G and 2D bands with suspended and supported graphene flakes were measured and analyzed. The peak shifts of G and 2D bands between the Raman and SERS signals demonstrate the doping effect induced by silver nanoparticles by n-doping. The I_2D/I_G ratio can provide a more sensitive method to carry out the doping effect on the graphene surface than the peak shifts of G and 2D bands. The enhancements of 2D band of suspended and supported graphenes reached 138, and those of G band reached at least 169. Their good enhancements are helpful to measure the optical properties of graphene. The different substrates that covered the graphene surface with doping effect are more sensitive to the enhancements of G band with respect to 2D band. It provides us a new method to distinguish the substrate and doping effect on graphene.

PACS

78.67.Wj (optical properties of graphene); 74.25.nd (Raman and optical spectroscopy); 63.22.Rc (phonons in graphene)     

Diameter dependence of interwall separation and strain in multiwalled carbon nanotubes probed by X-ray diffraction and Raman scattering studies

We have made a systematic study on the diameter dependent spectral features in X-ray diffraction (XRD) and Raman scattering studies of multiwalled carbon nanotubes (MWCNTs) of various diameters in the range 5?100 nm. High resolution transmission electron microscopy (HRTEM) imaging reveals a systematic decrease in the interwall separation from 3.8 Å down to 3.2 Å as the diameter of nanotubes increases from 5.8 nm to 63.2 nm. Analysis of the XRD patterns shows an exponential decrease in d₀₀₂ interlayer spacing with increasing tube diameter, in close agreement with the HRTEM results. Further, XRD profile line width shows inverse diameter dependence and exponential increase in intensity as the diameter of the MWCNTs increases. Raman spectra of different diameter nanotubes show different evolutions of metallic and semiconducting components in the G-band, as found from spectral deconvolution. The frequency and full width at half maximum (FWHM) of the semiconducting component of the G^? band gradually decreases as the tube diameter increases. Ratio of intensities of G^? band to D-band first shows a sharp fall as the tube diameter increases from 7 nm to 15 nm and then slowly increases with increasing diameter. On the other hand, G′ mode frequency shows large up shift when average diameter is increased from 7 nm to 15 nm and then saturates for higher diameter tubes. Analysis of Raman and XRD data reveals that the lowest diameter (7 nm) MWCNTs have features similar to those of the single walled nanotubes, while the spectral features are distinctly different for higher diameter MWCNTs due to the interaction among tube walls that is very significant for large diameter MWCNTs. Observed diameter dependence of the spectral features is explained in terms of nanotube curvature and atomic vibrations involving interaction among the walls in MWCNTs. The present study demonstrates the power of XRD for nondestructive evaluation of diameter distribution and interwall separation in MWCNTs with wide range of diameters.     

Evaluation of bisacrylate terminated epoxy resins as coatings

The paper describes the effect of backbone flexibility on the coating performance of vinyl ester resin. For this purpose four resins of varying flexibility were prepared. Bisacrylate terminated epoxy resin (D) was prepared by reacting 2 mol of acrylic acid with 1 mol of diglycidyl ester of hexahydrophthalic anhydride. The flexibility of the backbone was increased by chain extension of mono acryloxy derivative of diglycidyl ester of hexahydrophthalic anhydride with sebacic acid (resin G′). Partial cross-linking of resin D containing pendant hydroxyl group was also done by reacting with small amounts of isophorone diisocyanate (D₂I₁₀, D₂I₄).     

Oxygen functionalization of multiwall carbon nanotubes by Ar/H2O plasma treatment

To increase the applicability of multiwall carbon nanotubes (MWCNTs), oxygen-containing functional groups were introduced on the surfaces of MWCNTs by using microwave-excited Ar/H₂O surface-wave plasma. X-ray photoelectron spectroscopy and Raman spectroscopy were used to determine dependencies of Ar/H₂O gas partial pressure, treatment time and microwave power. The oxygen functionalization of MWCNTs by plasma can be achieved very rapidly, about 10 min. The C-O and O-C═O fractions firstly increase and then decrease with increasing Ar partial pressure. The C-O and O-C═O fractions increase with increasing microwave power from 400 W to 700 W. A slight increase of the R (I_D/I_G ratio) value for the treated MWCNTs indicated disordering in the surface microstructure of MWCNTs coincident with the introduction of surface oxygen. The oxygen-containing groups introduced on the surfaces of MWCNTs by plasma treatment are hydrophilic. The dispersion of plasma treated MWCNTs is therefore improved.     

Char structure characterised by Raman spectroscopy and its correlations with combustion reactivity

Raman spectroscopy was applied to characterise the microstructure of coal chars generated under various heat treatment conditions, which was correlated with the combustion reactivity measured by thermogravimetric analysis. The Raman spectra were fitted with the combination of 4 Lorentzian bands and 1 Gaussian band. It was found that the increase of char microstructural order under heat treatment can be characterised by Raman parameters, in particular the band area ratios, indicated by the increase in I_G/I_All and the decrease in I_D1/I_G, I_D2/I_G, I_D3/I_G and I_D4/I_G with increasing treatment temperature and/or time. The combustion reactivity of the chars from demineralised coals was found to have good correlations with the band area ratios, independent of coal type and heat treatment condition. It was found that the presence of inorganic matter in coal chars marginally affected the evolution of the average char microstructure. However, it did affect the char reactivity evolution. It was confirmed that the thermal deactivation of coal char during heat treatment was dependent not only on the ordering of char crystalline structure but also on the loss of catalytic activity of the inorganic matter.     

Growth of few-wall carbon nanotubes with narrow diameter distribution over Fe-Mo-MgO catalyst by methane/acetylene catalytic decomposition

Few-wall carbon nanotubes were synthesized by methane/acetylene decomposition over bimetallic Fe-Mo catalyst with MgO (1:8:40) support at the temperature of 900°C. No calcinations and reduction pretreatments were applied to the catalytic powder. The transmission electron microscopy investigation showed that the synthesized carbon nanotubes [CNTs] have high purity and narrow diameter distribution. Raman spectrum showed that the ratio of G to D band line intensities of I_G/I_D is approximately 10, and the peaks in the low frequency range were attributed to the radial breathing mode corresponding to the nanotubes of small diameters. Thermogravimetric analysis data indicated no amorphous carbon phases. Experiments conducted at higher gas pressures showed the increase of CNT yield up to 83%. Mössbauer spectroscopy, magnetization measurements, X-ray diffraction, high-resolution transmission electron microscopy, and electron diffraction were employed to evaluate the nature of catalyst particles.     

Effective control of nanodefects in multiwalled carbon nanotubes by acid treatment

The defect-type evolution and gradual increase in nanodefects in the outer walls of multiwalled carbon nanotubes (MWCNTs) in a chemically oxidizing environment were thoroughly investigated using a mixture of sulfuric acid (H₂SO₄) and nitric acid (HNO₃). A fairly low temperature of 323 K was employed for the acid treatment, and this limited the reaction rate to provide a mild acidic environment for gradual chemical oxidation compared to commonly used treatment conditions. High-resolution transmission electron microscopy (HRTEM) observations clearly demonstrated the formation of groove-type defects in the outer walls of MWCNTs in the early period around 0.5–3 h, followed by a morphological change into circumference-type defects and further into unzipped graphene nanoribbons. A unique parabolic variation in the intensity ratio of D and G bands (I_D/I_G) was observed. This observation supports the gradual oxidation of graphitic walls with increasing acid treatment time and corresponds well with the formation and evolution of nanodefects. Herein, the appropriate acid treatment time that would result in an effective number of nanodefects via suitable carboxyl functionalization, providing preferential nanocarbide sites for interfacial improvement as well as uniform dispersibility of MWCNTs, has been discussed.     

Characterization of epitaxial GaAs MOS capacitors using atomic layer-deposited TiO2/Al2O3 gate stack: study of Ge auto-doping and p-type Zn doping

Few-wall carbon nanotubes were synthesized by methane/acetylene decomposition over bimetallic Fe-Mo catalyst with MgO (1:8:40) support at the temperature of 900°C. No calcinations and reduction pretreatments were applied to the catalytic powder. The transmission electron microscopy investigation showed that the synthesized carbon nanotubes [CNTs] have high purity and narrow diameter distribution. Raman spectrum showed that the ratio of G to D band line intensities of I _G/I _D is approximately 10, and the peaks in the low frequency range were attributed to the radial breathing mode corresponding to the nanotubes of small diameters. Thermogravimetric analysis data indicated no amorphous carbon phases. Experiments conducted at higher gas pressures showed the increase of CNT yield up to 83%. M?ssbauer spectroscopy, magnetization measurements, X-ray diffraction, high-resolution transmission electron microscopy, and electron diffraction were employed to evaluate the nature of catalyst particles.     

Pulsed laser heating process of multi-walled carbon nanotubes film

The prepared multi-walled carbon nanotubes (MWCNTs) film was mounted on the holder and the film surface was flashed with a single pulse of Nd:YAG laser (λ = 532 nm) in the air. The dynamics of pulsed nanosecond laser heating process was simulated by the solution of the one-dimensional heat conduction equation. The finite element method (FEM) was applied to solve the equation. At the laser fluence of 1 J/cm² with Nd:YAG laser, the surface reached the maximum temperature 1503 °C at 13 ns. Moreover, the Raman spectroscopy of MWCNTs films before and after irradiation were measured. The intensity of the two characteristic Raman shifts I_D (defect-mode) and I_G (graphite-mode) was measured by the Raman spectroscopy. The maximum surface temperature was calculated and compared with the I_G/I_D ratio of MWCNTs film. The graphitization occurred on the sample after irradiation.     

Electrochemical growth of self-organized TiO2�CWO3 composite nanotube layers: effects of applied voltage and time

The present work demonstrates that morphology of TiO₂?CWO₃ composite nanotubes formed by alloy anodization can be tuned by controlling applied voltages and time. Distinctive tube morphology can be formed by applying a voltage of more than 80?V. Nanotube diameter and length have a linear relationship with the anodization voltage with a current efficiency of almost 100?%. Furthermore, compared to pure TiO₂, the composite nanotubes show a very uniform tube diameter along the tube axis even at the extended anodization time.