Spectra Of Single-walled Carbon Nanotubes Research Articles

Oxygen Defects in Single-Wall Carbon Nanotubes for Near-Infrared Light Emitters

A new peak with an increased intensity was observed in the photoluminescence (PL) spectra of single-walled carbon nanotubes (SWCNTs), exposed to UV irradiation in the presence of sodium hypochlorite. It was concluded on the basis of the spectroscopic data that the new PL peak is associated with oxygen defects in the SWCNT structure. The impact of environmental acidity on the optical properties of oxygen-doped SWCNTs (O-SWNTs) was investigated. An increased sensitivity of the new PL peak to the pH of the medium was observed. It was concluded that the usage of a pH-neutral medium is of crucial importance for the creation of IR light sources based on O-SWNTs. Keywords: single-walled carbon nanotubes, photoluminescence, localized exciton, acidity, infrared light emitters.

Кислородные дефекты в одностенных углеродных нанотрубках для источников излучения в ближнем инфракрасном диапазоне

A new peak with an increased intensity was observed in the photoluminescence (PL) spectra of single-walled carbon nanotubes (SWCNTs), exposed to UV irradiation in the presence of sodium hypochlorite. It was concluded on the basis of the spectroscopic data that the new PL peak is associated with oxygen defects in the SWCNT structure. The impact of environmental acidity on the optical properties of oxygen-doped SWCNTs (O-SWNTs) was investigated. An increased sensitivity of the new PL peak to the pH of the medium was observed. It was concluded that the usage of a pH-neutral medium is of crucial importance for the creation of IR light sources based on O-SWNTs.

Enhancement of single-walled carbon nanotube accumulation in glioma cells exposed to low-strength electric field: Promising approach in cancer nanotherapy

The objective of the study is to determine the patterns of regulation of single-walled carbon nanotube accumulation, distribution, and agglomeration in glioma cells exposed to an external electric field. C6 glioma cells were treated with 5 μg/ml DNA wrapped single-walled carbon nanotubes and exposed to bi-phasic electric pulses (6.6 V/m, 200 Hz, pulse duration 1 ms). Nanotube accumulation was determined by Raman microspectroscopy and their intracellular local concentration was evaluated using the G-band intensity in Raman spectra of single-walled carbon nanotubes. It was revealed that the low-frequency and low-strength electric field stimulation of glioma cells exposed to single-walled carbon nanotubes led to facilitation and, thus, to amplification of nanotube accumulation inside the cells. The number of nanotubes in intracellular agglomerates increased from (28.8 ± 13.1) un./agglom. and (84.0 ± 28.7) un./agglom. in control samples to (60.6 ± 21.4) un./agglom. and (184.2 ± 53.4) un./agglom. for 1 h and 2 h stimulation, respectively. Thus, the tumor exposure to an external electric field makes it possible to more effectively regulate the accumulation and distribution of carbon nanotubes inside glioma cells allowing to reduce the applied therapeutic doses of carbon nanomaterial delivered anticancer drugs.

Quantitative Detection of the Disappearance of the Antioxidant Ability of Catechin by Near-Infrared Absorption and Near-Infrared Photoluminescence Spectra of Single-Walled Carbon Nanotubes.

We succeeded in quantitatively detecting the disappearance of catechin antioxidant ability as a function of time using near-infrared (NIR) absorbance and NIR photoluminescence (PL) spectra of single-walled carbon nanotubes (SWNTs) wrapped with DNA molecules (DNA–SWNT hybrids). When 15 μg/mL of catechin was added to the oxidized hybrid suspension, the absorbance of SWNTs increased, according to the antioxidant ability of catechin, and the effect was maintained at least for 30 min. When catechin concentrations were less than 0.3 μg/mL, SWNT absorbance gradually decreased, although it increased when catechin is added. The results revealed that disappearance of the catechin effects could be quantitatively detected by NIR absorbance spectra. When NIR PL was employed, the disappearance of PL intensity was also observed in the case of low catechin concentrations. However, time-lapse measurement of the disappearance was difficult because the PL intensity was rapidly quenched. In addition, the optical responses were different due to different chirality of SWNTs. Our results suggested that both NIR absorbance and PL can detect disappearance of catechin antioxidant effects; in particular, slow response of NIR absorbance was effective to detect time dependence of the disappearance of the catechin effects. Contrarily, PL revealed huge and rapid responses in contrast to NIR absorbance. PL might be effective for reversible use of DNA–SWNT hybrids as a nanobiosensor.

Reinterpretation of Single-Wall Carbon Nanotubes by Raman Spectroscopy

Raman spectra of single-wall carbon nanotubes (SWNTs) exhibit a unique radial breathing mode (RBM) band (∼100–300 cm–1) and a G– peak (∼1570 cm–1), along with a D band (∼1350 cm–1). We show that th...

Correlative force and tip-enhanced Raman microscopy

Tip-enhanced Raman scattering (TERS) microscopy attracts increasing attention for visualization and characterization of strain distribution on crystalline samples at a nanoscale due to nano-sized fields localized at a metal tip. However, as the metal tip approaches close to a sample surface, a force acts between the tip and sample, which can induce unwanted perturbation on the local strain of the sample. Analysis and evaluation of intrinsic strains on samples with a high reliability demands a technique to correlate TERS spectra with the tip-sample force. Here, we present a TERS microscope based on a frequency-modulation atomic force microscope (FM-AFM) using a quartz tuning fork (QTF) as a force sensor. By continuously monitoring a shift in the resonance frequency of the QTF during TERS, the tip-sample force can be directly measured in both attractive and repulsive force regions, which is not possible by other AFMs. TERS spectra of single-walled carbon nanotubes (CNTs) were measured simultaneously while changing the tip-sample distance and hence the tip-sample force acting on the CNTs. We found that TERS occurs at the tip-sample distance where the repulsive force dominantly works and mostly decays out in the attractive force region.

Differences in the response of the near-infrared absorbance spectra of single-walled carbon nanotubes; Effects of chirality and wrapping polymers.

We detected antioxidant activity of catechin, one of the main components of tea, using SWNTs surface coated with two different biomolecules. Compared to coating with DNA already reported, it can hardly be detected when coated with carboxymethyl cellulose. For nanobiosensing using SWNTs, its sensitivity is not determined only by SWNTs, we found that biomolecules covering the surface are extremely important. In this experiment, we measured the near-infrared absorption spectra of SWNTs coated separately with two different water-soluble polymers; DNA (double-stranded DNA-SWNT complexes) and carboxymethyl cellulose (CMC, CMC-SWNT complexes), and uncovered the differences in their antioxidant properties against the flavonoid catechin. Each dispersion was oxidized with H2O2 at 0.03% (final concentration), following which catechin solutions were added to reduce the samples. Our results showed that the magnitude of the change in the absorbance spectra for dsDNA-SWNT complexes in response to oxidation and reduction was superior to that for CMC-SWNT complexes. The CMC-SWNT complexes exhibited almost no change in their spectra even though the same SWNT powder (produced by the high-pressure carbon monoxide (HiPco) method) was used. On the other hand, when (6, 5)-enriched SWNT powder produced by the ComoCat method was used, no significant change in the absorbance was observed, even though (6, 5)-enriched SWNTs are frequently used for nanobiosensing. Our results revealed that both the SWNT chirality and type of polymer for wrapping SWNTs are important factors for establishing nanobiosensing methods utilizing SWNTs.

ZA-derived phonons in the Raman spectra of single-walled carbon nanotubes

We report the observation of four Raman modes in individual, aligned single-walled carbon nanotubes, with frequencies between the RBM and D modes, and above the G± modes. By studying the diameter and excitation-energy dependence, we find that these strongly dispersive and comparatively intense modes share the lowest-frequency helical phonon branch as the origin, which can be derived from the out-of-plane acoustic (ZA) phonon branch of graphene. This ZA phonon is observed due to a defect-assisted, double-resonant process, similar to the D mode; its two-phonon overtones and combinations with transverse optical (TO) phonons do not require defects. Our assignment is supported by a theoretical model based on sixth-nearest neighbour tight-binding and a force-constant approach. We present a complete theoretical evaluation of the diameter and excitation-energy dependence of the ZA, 2ZA, and TO±ZA modes, which fits very well to our experimental results and previous studies of the so-called region of intermediate frequency modes.

Photoluminescence Side Band Spectroscopy of Individual Single-Walled Carbon Nanotubes

Photoluminescence spectra of single-walled carbon nanotubes (SWCNTs) have been recorded and analyzed for selected individual nanotubes and structurally sorted bulk samples to clarify the nature of secondary emission features. Room temperature spectra show, in addition to the main peak arising from the E11 bright exciton, three features at lower frequency, which are identified here (in descending order of energy difference from E11 emission) as G1, X1, and Y1. The weakest (G1) is interpreted as a vibrational satellite of E11 involving excitation of the ∼1600 cm–1 G mode. The X1 feature, although more intense than G1, has a peak amplitude only ∼3% of E11. X1 emission was found to be polarized parallel to E11 and to be separated from that peak by 1068 cm–1 in SWCNTs with natural isotopic abundance. The separation remained unchanged for several (n,m) species, different nanotube environments, and various levels of induced axial strain. In 13C SWCNTs, the spectral separation decreased to 1023 cm–1. The measured...

Manifestation of Structure of Electron Bands in Double-Resonant Raman Spectra of Single-Walled Carbon Nanotubes.

Micro-Raman spectra of single-walled carbon nanotubes in the range of two-phonon 2D bands are investigated in detail. The fine structure of two-phonon 2D bands in the low-temperature Raman spectra of the mixture and individual single-walled carbon nanotubes is considered as the reflection of structure of their π-electron zones. The dispersion behavior of 2D band fine structure components in the resonant Raman spectra of single-walled carbon nanotube mixture is studied depending on the energy of excitating photons. The role of incoming and outgoing electron-phonon resonances in the formation of 2D band fine structure in Raman spectra of single-walled carbon nanotubes is analyzed. The similarity of dispersion behavior of 2D phonon bands in single-walled carbon nanotubes, one-layer graphene, and bulk graphite is discussed.

Features of the chemical bonding in fluorinated and hydrogenated carbon: Nanotubes analysis of polarized NEXAFS spectra

The carbon K-edge NEXAFS spectra of initial and fluorinated and hydrogenated single-wall carbon nanotubes (SWCNTs) are analyzed theoretically with regard to the polarization dependence. Differences in the polarized theoretical spectra of SWCNTs with zigzag and chair chiralities are shown. As a result of the comparison of experimental and theoretical carbon K-edge NEXAFS spectra of hydrogenated and fluorinated SWCNTs it is found that it is most probable that hydrogen atoms attach to the outer surface of the tube wall and fluorine atoms to the outer and inner surfaces of the SWCNT wall. Based on the analysis of polarized theoretical spectra of fluorinated and hydrogenated SWCNTs differences in the symmetry of the final states of C-H and C-F bonds are shown.

Raman Spectra of Single-Walled Carbon Nanotubes Synthesized by Aerosol CVD-Method Using Ferrocene and CuNi Nanoparticles

Properties of single-walled carbon nanotubes (SWCNTs) obtained by aerosol method of chemical deposition from the gas phase using ethanol, ferrocene, and CuNi nanoparticles are studied. The structural and vibrational characteristics of synthesis products are determined by Raman spectroscopy. The influence of the catalyst nanoparticles introduced into the reaction mixture on the properties of the synthesized SWCNTs is discussed.

Phonon Energy Spectra and Stationary Elastic Waves in Single-Walled Carbon Nanotubes and Graphite Bulk Crystals

Micro-Raman spectra of single-walled carbon nanotubes, graphite bulk crystals, one-layer and two-layer graphenes are investigated in detail. The structure of Davydov multiplets in the energy spectra of electronic states of one-layer and two-layer graphenes and Bernal graphite bulk crystals is established. The energy spectra of vibrational and electronic states of nanotubes and graphite bulk crystals are shown to be described by the dispersion curves, which pairwise joint at points A of the Brillouin zones of these structures. The forms of stationary elastic waves in single-walled carbon nanotubes at Γ and A points of their Brillouin zones and in one-layer and two-layer graphenes and bulk graphite at Γ, K, and M points of their Brillouin zones are analytically calculated.

Optical Absorption Spectra of Single-Walled Carbon Nanotubes Synthesized by an Aerosolchemical Vapor Deposition Method

The optical absorption spectra of thin films (networks) based on single-walled carbon nanotubes (SWCNT) produced by an aerosol-chemical vapor deposition method using ferrocene and sulfur with different concentrations are measured, and the chiral indices and diameters of the grown nanotubes are calculated. The optimal ferrocene concentration in the reaction mixture which provides an increase in the SWCNT yield by a factor of up to 1.5 in comparison with unoptimized mixture composition is determined. It is demonstrated that sulfur additives with concentrations in the range 1/72–1/64 g/L cause additional increase in the SWCNT yield by a factor of 2–2.5 in comparison with synthesis without sulfur.

Optical heating and temperature determination of core-shell gold nanoparticles and single-walled carbon nanotube microparticles.

The real-time temperature measurement of nanostructured materials is particularly attractive in view of increasing needs of local temperature probing with high sensitivity and resolution in nanoelectronics, integrated photonics, and biomedicine. Light-induced heating and Raman scattering of single-walled carbon nanotubes with adsorbed gold nanoparticles decorating silica microparticles are reported, by both green and near IR lasers. The plasmonic shell is used as nanoheater, while the single-walled carbon nanotubes are Raman active and serve as a thermometer. Stokes and Anti-Stokes Raman spectra of single-walled carbon nanotubes serve to estimate the effective light-induced temperature rise on the metal nanoparticles. The temperature rise is constant with time, indicating stability of the adsorption density. The effective temperatures derived from Stokes and Anti-Stokes intensities are correlated with those measured in a heating stage. The resolution of the thermal experiments in our study was found to be 5-40 K.

Energy spectrum and optical absorption spectra of carbon nanotubes with chiralities of (10, 10), (11, 9), and (12, 8)

The energy spectra of single-wall carbon nanotubes (CNTs) with chiralities of (10, 10), (11, 9), and (12, 8) are calculated in the static fluctuation approximation for the Hubbard model. The choice of these systems for investigation was dictated by the fact that these systems are the most typical of heterogeneous samples obtained by synthesis. It is shown that in the chosen model and approximation, the type of conduction is independent of chirality, which contradicts the generally accepted opinion concerning the critical dependence of the CNT conduction current on the chirality indices. The optical absorption spectra of the CNTs under investigation are calculated using the resultant energy spectra. The shape of the optical absorption spectrum averaged over the known weight composition is in good agreement with experimental data. The results of investigation suggest that the rule according to which CNTs exhibit metal-type conductivity when the difference in the chirality indices is a multiple of three; otherwise, they have the semiconductor-type conductivity, is in all probability not general, but has applicability limits that can be established by rigorous calculations based on the Hubbard model.

Structural modification of single-walled carbon nanotube induced by X-ray irradiation and subsequent annealing studied by Raman scattering spectroscopy in radial breathing mode

The formation of X-ray-induced defects changes the spectral shape of the radial breathing mode (RBM) and defect-induced mode (D band) in the Raman spectra of single-walled carbon nanotubes (SWNTs). X-ray-induced defects have been found to be annealed by thermal treatment, indicating that they are Frenkel pairs (vacancy and interstitial pairs). We found that the spectral shape of RBM is not entirely recovered after post-irradiation annealing. The temperatures for the complete annealing of X-ray-induced defects were within the range of 200–600 °C depending on the tube geometry. From these results, we suggest that the stability of X-ray-induced defects depends on the tube geometry and that the combination of X-ray irradiation and post-irradiation annealing causes a chirality change in SWNTs.

Energy Spectrum and Optical Properties of Single-Wall Carbon Nanotubes

The energy spectrum and the optical absorption spectra of single-wall carbon nanotubes with chiralities (5,5), (10,0), (9,0), (12,0), and (15,0) is calculated within the framework of the Shubin-Wonsowskii-Hubbard model taking into account distant (adjacent to the nearest) interstitial electron transfer. It is demonstrated that all of them, irrespective of the chirality, are narrow-gap semiconductors with a gap of ~0.1 eV that coincides with the available experimental data. The optical absorption spectra are also in good agreement with the experimentally measured spectra. orbitals of deeply lying bands forming the skeleton of the structure and the nonhybrid p-orbital forming the so-called wandering π -bonds. The states of these electrons are partially localized, and they can be considered in the Wannier representation. It then follows that the electrical conductivity of the CNT must be completely determined by the π -electron subsystem. At present there is a widespread opinion that the electrical conductivity and other properties of the CNT depend critically on the chirality indices. Thus it is believed that the CNT with chiralities (n, m) are metals if the difference between the chiralities n - m is a multiple of three; otherwise, they are semiconductors or dielectrics. This opinion is based on the energy spectrum of the π -electron subsystem obtained by Dresselhaus et al. (2-4) using the results of calculations carried out by Wallace (5) as far back as 1947. In (5) only the electron transfer from site to the neighboring site was taken into account. However, the experimental material accumulated by the present time demonstrates that there is no unambiguous correspondence between the chirality and the electrical conductivity of the CNT (6). This is most likely due to the fact that the Coulomb interaction in the π -electron subsystem whose energy, according to (7), can reach 10 eV and higher, is not taken into account. Results of calculations carried out in (8, 9) in which the Coulomb interaction in the same site was considered demonstrated that all CNT, irrespective of chiralities, were semiconductors with a gap of ~1 eV. However, experimental data on the electrical conductivity (10-12) and direct measurement (13) of the density of states for the CNT with chiralities (15,0), (12,0), and (9,0) demonstrated that the CNT, irrespective of the chiralities, are narrow-gap semiconductors whose gap was in the range ~0.01-0.1 eV. This circumstance calls for a more detailed study of the examined problem.

Phonon spectrum and interaction between nanotubes in single-walled carbon nanotube bundles at high pressures and temperatures

The Raman spectra of single-walled carbon nanotubes at temperatures up to 730 K and pressures up to 7 GPa have been measured. The behavior of phonon modes and the interaction between nanotubes in bundles have been studied. It has been found that the temperature shift of the vibrational G mode is completely reversible, whereas the temperature shift of radial breathing modes is partially irreversible and the softening of the modes and narrowing of phonon bands are observed. The temperature shift and softening of radial breathing modes are also observed when samples are irradiated by laser radiation with a power density of 6.5 kW/mm2. The dependence of the relative frequency Ω/Ω0 for G + and G − phonon modes on the relative change A 0/A in the triangular lattice constant of bundles of nanotubes calculated using the thermal expansion coefficient and compressibility coefficient of nanotube bundles shows that the temperature shift of the G mode is determined by the softening of the C-C bond in nanotubes. An increase in the equilibrium distances between nanotubes at the breaking of random covalent C-C bonds between nanotubes in bundles of nanotubes is in my opinion the main reason for the softening of the radial breathing modes.

Manifestation of auger processes in C1s-satellite spectra of single-walled carbon nanotubes

Using the equipment of the Russian-German beamline of the synchrotron radiation at the BESSY II electron storage ring, satellite spectra accompanying the C1s core lines in the cases of single-walled carbon nanotubes and highly ordered pyrolytic graphite have been measured with a high energy resolution. The Auger spectra corresponding to shaking of the valence system of carbon by the core vacancy have been found and investigated. The Auger spectra of the studied single-walled carbon nanotubes and highly ordered pyrolytic graphite are caused by annihilation of the excited π* electron with a hole in the π subband. It has been established that the electron states in the conduction band have 3π* (gT, K, M) symmetry; i.e., they correspond to flat 3π* subband, which is localized by 12–13 eV above the Fermi level. It has been revealed that the general regularities of the distribution of electron states in the valence system insignificantly change during its shake-up by the excited core.