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Multiwalled carbon nanotubes (MWCNTs) were successfully incorporated inside silica aerogel matrix. Solid composite materials were investigated by high-resolution transmission microscopy, scanning electron microscopy, optical spectroscopy. MWCNTs in the form of small bundles and individual tubes get locked inside the aerogel between its pores resulting in the local solution-free environment for nanotubes. Optical transmission of the composite material can be modified by the amount of the added MWCNTs. Nonlinear optical properties were studied by a Z-scan technique. Composite materials demonstrate saturable absorption for femtosecond laser pulses at 515 nm wavelength that are attributed to the properties of embedded nanotubes. Silica aerogels possess significantly better thermal stability compared to polymer matrices, hosts that are frequently used for saturable absorption applications of carbon nanotubes. Solid and lightweight silica aerogels with embedded nanotubes can be used as optical elements for various photonic devices. Photo of silica aerogel with embedded MWCNTs. Normalized Z-scan transmittance of silica aerogel with nanotubes for two different on-focus intensities.

Metallic and semiconducting fractions of single-walled carbon nanotubes (SWCNTs) sorted by an aqueous two-phase extraction technique and integrated into thin films were used to produce CuCl@SWCNT hybrid material via a gas phase filling the nanotubes with CuCl. As a result of doping, the SWCNTs in the hybrid material possess a higher optical transparency than the pristine ones. The hybrid material properties strongly depend on the metallic/semiconducting tube ratio in a pristine material. In this work, with UV-vis-NIR optical absorption and Raman techniques we show that upon filling with CuCl the solely metallic nanotubes provide much higher doping efficiency than the solely semiconducting SWCNTs. The CuCl@SWCNT hybrid material based on sorted nanotubes is more competitive as a transparent conductive material for the future photovoltaics. A schematic view of electron transfer between CuCl@s-SWCNT and CuCl@m-SWCNT within a bundle of CuCl@SWCNT.

Curved graphene fragments can be considered as building blocks of carbon nanotubes. Thus, the size of graphene fragments may be considered as one of the most important characteristics of their structural disorder. In this paper, we have performed a comparative Raman study of CVD multi-walled carbon nanotubes (MWCNTs) and graphene flakes deposited on MWCNTs. Raman data have been considered in combination with HRTEM characterization of nanotubes. The basic attention has been paid to the behavior of D (disorder-induced), G (tangential mode), and 2D (two-phonon scattering) bands in Raman spectra in order to use them for MWCNT characterization. A ratio of intensities of 2D and D bands (I2D/ID) demonstrates almost a linear dependence on the mean diameter of MWCNTs produced with two different types of catalysts (see abstract figure). It should be mentioned that each type of catalyst provides the linear dependence with its own specific slope. The graphene fragments have been proposed to form a mosaic structure of nanotube walls. I2D/ID ratio depends on the amount of graphene flakes deposited on nanotube surface via ethylene decomposition. A dependence of intensity ratio I2D/ID on the diameter of nanotubes produced with different types of catalysts.

Single-walled carbon nanotubes (SWCNTs) have unique electronic and optical properties, which are attractive in various applications. These properties can be further improved by functionalization. In this work, it is shown that filling nanotubes with CuCl leads to improvement of their electro-conductive and optical transmission properties up to outstanding level. The improvement is due to the significant p-type doping of the functionalized nanotubes. That leads to a huge increase in nanotubes charge carrier density and the considerable decrease of the nanotubes absorption via Pauli blocking. The functionalization was realized via a gas-phase filling of SWCNTs with CuCl. We demonstrate the effect of huge doping on optical properties of nanotubes with different average diameters. The observed sufficient stability of the CuCl@SWCNT composite is proved to be due to the formation of quasi-one-dimensional (1D) CuCl crystal inside nanotubes. AC-HRTEM image of 1D CuCl crystal formed inside SWCNT.

Single-walled carbon nanotubes can serve as a template for formation of various structures inside. Depending on conditions, the nanotube filling with coronene molecules may lead to creation of coronene stacks, graphene nanoribbons, or inner tubes. In this work, the formation of coronene stacks inside single-walled carbon nanotubes was studied using photoluminescence (PL) spectroscopy. We demonstrate the difference in the excitation PL maps between the spectral features of coronene stacks inside the nanotubes and products formed outside the nanotube interior. The optical investigation was supported by calculations of adsorption energy of coronene molecules inside and outside nanotubes. The theoretical calculations predict the most energetically favorable configurations of coronene molecules with respect to the diameter of the outer nanotubes. Orientation of coronene stacks depends on the nanotube diameter. Models of ten coronene molecules placed inside (10,10) and (11,11) nanotubes (upper and lower image part, respectively).

This work gives data on the stability of dispersions of single wall carbon nanotubes stabilized by sodium dodecylbenzenesulfonate in binary polar solvents “water + antifreeze” (glycerol, polyethyleneglycole) with eutectic compositions. The absorption spectra of the suspensions demonstrate no changes during 1-year storage with temperature spanning from −40 to +40 °C. The systems provide relevant optical power limiting properties, the one with glycerol showing good resistance to optical bleaching effects. We also demonstrate that aqueous dispersions of nanotubes exhibit considerable enhancement of optical limiting parameters alongside an increase of the bundled material populace.

Single-wall carbon nanotubes with polyiodide chains inside are interesting from two points of view. According to predictions, first, the iodine structure type inside the nanotube is determined by the nanotube geometry. Second, after iodination all nanotubes become metallic. In this work, we made an attempt to check both predictions. To study the diameter-dependent properties we have taken for a gas-phase iodination the pristine single-wall carbon nanotubes grown by three different techniques providing a different average diameter: a chemical vapor deposition with a Co/Mo catalyst (CoMoCat) with a diameter range (0.6–1.3) nm, a high-pressure CO decomposition (HiPCO) – a diameter range (0.8–1.5) nm, and an aerosol technique with Fe catalyst – a diameter range (1.3–2.0) nm. The Raman spectra have shown a complication of the polyiodide chain structure while the nanotube diameter increased. The optical spectroscopy data (a suppression of E11 band in the UV–Vis–NIR absorption spectrum) have confirmed the theoretical prediction about transformation of all nanotubes into metallic phase after doping.

The electronic band structure of helical iodine chains was calculated within the empirical tight binding approach. The screw symmetry of the system was used to reduce the size of the problem to a single atom with four atomic orbitals. The overlap parameters were fitted to reproduce the DFT results for a linear iodine chain. The obtained results for helical chains have shown the energy band splitting due to the overlap between p orbitals introduced by the structure twisting. The splitting magnitude depends on the pitch of the helix.

Here we report the data on the few layer graphene films grown by a chemical vapour deposition on nickel foils and transferred onto desirable substrates for different applications. The experimental conditions have been optimized to prepare a really single graphene layer. A strong influence of the growth conditions onto the number of graphene layers has been revealed. Raman spectroscopy was used to confirm the formation of the single graphene layer with a little amount of defects. To estimate a transmittance of graphene films the optical absorption spectra have been measured. A number of graphene layers in different samples has been estimated on the base of an optical absorbance of a monolayer graphene (2.3% of the incident light intensity). (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Raman spectroscopy study of multi-wall carbon nanotubes (MWNTs) of different diameters (with a small controllable number of walls) produced using Fe–Co catalyst with a variable size of the same active component has been performed. We have characterized as produced MWNTs with different diameters (series 1) and two types of tubes with fixed mean diameters (∼10 and 20 nm) heated in a flow of pure argon at various temperatures (2200, 2600 and 2800 °C – series 2). The Raman spectra of MWNTs have been registered in three spectral regions, corresponding to D (disorder-induced), G (graphite) and 2D (two-phonon scattering) bands. A ratio of intensities I2D/ID for tubes of series 1 has demonstrated almost a linear dependence on the nanotube diameter. After heating (series 2), D (disorder-induced) Raman band has shown a substantial decrease in intensity. The variation of the Raman spectra parameters is discussed in terms of defectiveness of nanotubes.

In this work, we report the photoluminescence (PL) studies of the films produced by different methods from pristine singlewall carbon nanotubes (SWNTs). The suspensions of CoMoCATandHiPCOSWNTswere used tomake two different types of films: the polymer films with embedded SWNTs and the SWNTfilms deposited onto quartz substrates through filtration. A density gradient ultracentrifugation (DGU) has been used for enrichment of the initial carbon nanotube suspensions with semiconducting SWNTs and extraction of various diameter nanotubes to reveal a role of the nanotube interaction in the films. Optical diagnostic techniques including Raman spectroscopy, UV–VIS–NIR absorption spectroscopy, and PL have been used for characterization and comparison of the initial suspensions and films formed from them. A PL mapping has revealed an arrangement of carbon nanotubes inside the films. A cross-linkage between the individual SWNTs appeared to be higher inside the films deposited via filtration than in the polymer films with dispersed SWNTs.

Field emission characteristics of multi-emitter flat cathodes prepared from NanoBuds, few-layer graphite flakes (nanographite, NG) and pure or N-doped single-wall carbon nanotubes (SWNTs) have been examined. The cathodes demonstrated a low-voltage electron emission with the threshold field values of few Volts per micron, which were ascribed to the high-aspect ratio of nanocarbons. The films of NanoBuds and NG possess excellent emission pattern homogeneity with an emission site density of more than 10 5 cm -2 . This is much higher than for cathodes made of conventional SWNT films. It was found that the achievable maximal current density depends on the type of carbon material and reaches 10 A/cm 2 for NanoBuds film, while for the NG film cathodes, it is about one order of magnitude less. Possible mechanisms responsible for the experimentally observed differences are discussed. The materials examined are of great interest due to their high-emission characteristics and are promising for the development of new types of vacuum electronic devices.

A density gradient ultracentrifugation (DGU) technique recently applied for separation of single-wall carbon nanotubes (SWNTs) over diameter also appeared to be efficient for the nanotube separation according to their electronic structure. In this work we used DGU for extraction of a highly metallic (>97 wt%) nanotube fraction from a raw soot of arc-discharge SWNTs. The process parameters (the surfactant type and concentration, the treatment time, the approach to the gradient formation) have been optimized. A variation of surfactant concentration allowed obtaining the highly metallic nanotube fractions with different dominant diameter distribution resulting in the "blue" or "green" top fractions. Sodium taurodeoxycholate (TDOC) was efficiently used for DGU separation of raw, non-purified SWNTs. The fractions have been studied with the UV-Vis-NIR absorption and Raman spectroscopy. Photograph of the SWNT solution after DGU.

In this work we present the results on preparation and analysis of structural and electronic properties of graphene flakes with different number of layers. Since the material electronic properties depend strongly on the layer number, a simple and reliable way to estimate this parameter should be found. In this work we propose to perform a quantitative analysis based on the atomic force microscopy (AFM) which was exploited by another authors rather as an illustrative technique. The graphene flakes were prepared by a micromechanical cleavage of highly oriented pyrolytic graphite (HOPG). Numerous flakes have been studied with AFM in order to estimate the number of graphene layers in each of them. The thickness distribution over the individual graphene flake, averaged over the all flakes investigated, has exhibited a row of maxima: 0.7 nm, 1.1 nm, 1.4 nm, 1.7 nm, 2.1 nm, etc. These values correspond perfectly to the one-, two, three-, and etc layers of graphene. The flakes characterized by AFM were studied afterwards with a two-phonon Raman spectroscopy. This method is known to be sensitive to the number of layers due to a "double resonance" mechanism. To interpret the Raman spectra the electronic structure of the graphene flakes of different thickness has been calculated using the density functional method. A perfect correspondence between the number of graphene layers in different flakes measured by AFM and by Raman techniques has been obtained. This demonstrates a possibility to estimate correctly the graphene layer number using the atomic force microscopy supplied with the statistical analysis.

Heterophase BN:C single-wall nanotubes are prospective as a material with a predicted possibility to vary the bandgap via changing a relative content of BN and C fractions in the nanotube walls. The challenge is both to find the ways of synthesis of such nanotubes and, in case of success, to confirm BN embedding. In this work field emission studies have been performed for revealing the difference between arc-produced pure carbon nanotubes and nanotubes grown from BN:C mixtures. The relative BN content in the mixtures was varied from 0% up to 50% (by mass). The materials have been characterized by a high resolution transmission electron microscopy, Raman scattering and UV-VIS-NIR optical absorption techniques. The single-wall nanotubes have been revealed in all samples synthesized, but their composition remained questionable. The field emission properties of the samples have been examined in a vacuum diode configuration. It has been found that the threshold fields and slopes of the Fowler-Nordheim plot, evaluated from the measured current-voltage' dependences, increased with an enrichment of the starting mixtures with h-BN. This increase could be attributed to the work function rise due to h-BN embedding into the carbon nanotube walls. This result opens a way to use the field-emission characterization for an indirect confirmation of the heterophase BN:C nanotube formation.

In this paper, a principal calculation of the electronic structure of single-wall carbon nanotubes at different pressure values has been performed. The simulations have been done for nanotubes aggregated in ropes. The pressure has been modeled by changing the distances between neighbor nanotubes in the rope. The results of the calculations show that the pressure increase leads to a decrease of energy of allowed optical transitions in nanotubes. This result corresponds to the experimental data published.

Boron nitride nanotubes nucleation on the catalyst's surface after the formation of metal particle alloys oversaturated with boron and nitrogen atoms is the most important step for any type of the catalytic processes of BN nanotubes formation. A thermodynamic analysis of the boron-nitride nanotubes nucleation on the catalysts surface was performed. The master equations for the dependence of critical size of BN nucleus on reaction parameters, such as reaction temperature, supersaturation degree of catalyst particles with B and N, work of adhesion of catalyst to BN were obtained. These equations combined with the phase diagram approach can be used for the description of different scenarios of BN deposits formation and for the development of the main principles of catalysts design for BN nanotube production.

The present work reports the studies of the solubilization of single-wall carbon nanotubes in aqueous solutions of calix[4]resorcinarene derivatives and p-sulfonato-calix[n]arenes (n = 4, 6) modified by alkyl (butyl and dodecyl) substituents. The highest dispersion ability is demonstrated by calix[4]resocinarene, which bears aminomethyl groups on the upper rim and phosphorus-bearing alkyl fragments on the lower rim of calixarene. Optical absorption spectroscopy justifies the formation of stable suspensions of SWNT in aqueous solutions of calixarenes.

In this work the thin polymer films (based on polyvinyl alcohol or carboxymetylcellulose) containing single-wall carbon nanotubes synthesized by different methods have been fabricated by a novel technology and studied in detail by UV-VIS-NIR optical absorption and Raman scattering techniques. The films are prospective as an optical material with a high non-linearity. This property allows using them as ultra-fast saturable absorbers in different laser schemes, especially, in fiber lasers. The optical spectroscopy has been used for optimization of several film parameters: an optical density, a thickness, a level of scattered light and proximity of the film absorption bands to the operation wavelengths of different solid state lasers.

We demonstrate that irradiation with nano-second light pulses results in significant enhancement of the electron emission from nano-graphite films. The observed emission current density is as high as 10 A/cm 2 at applied field of 2 V/pm. The duration of the emission pulse depends on the applied DC voltage and laser intensity. However, in our experimental conditions, the temporal profile of the electron pulse nearly reproduces that of the incident laser pulse.

The temperature-induced changes in Raman spectra of two BN-based materials: a hexagonal boron nitride and soot with single-wall BN nanotubes, have been measured in temperature range 77-600 K in course of heating in the oven/cryostat cell. A nonlinear temperature dependence of the tangential Raman mode position (being at 1366 cm -1 at room temperature) has been observed for both materials. In the temperature range 350-600 K a linear approximation of the tangential Raman mode shifting was applicable. The measured rates were -0.014 cm -1 /K (for h-BN) and -0.027 cm -1 /K (for the nanotube-containing sample). The effects of the oven and laser heating have been compared. Under the laser heating with the same power density the nanotube-containing soot demonstrated much bigger shifts (up to 30 cm -1 ) than h-BN. The data obtained are useful for in-situ diagnostics of different BN-based phases in course of synthesis.

In situ measurements were executed in the continuous wave CO 2 -laser vaporization synthesis of single-walled carbon nanotubes (SWCNTs). The results were compared with the findings of the ex situ analysis of the product. Different catalysts (Co, Ni, Co/Ni, Ni/Y and Co/Y) were used and the temperature of the target surface (T s ) changed to compare the efficiency of the synthesis process. Our main results are related to the SWCNTs diameter which is depending on T, and on the catalysts nature. The targets containing Y with Co or Ni atoms produce more and larger SWCNTs than Co/Ni targets. The SWCNTs' diameter is seen to increase with T s .

We have investigated a wide variety of surfactants for their efficiency in dissolving isolated single-walled carbon nanotubes (SWNTs) in water. In doing so, we have completely avoided the harsh chemical or mechanical conditions, such as acid or ultrasonic treatments, that are known to damage SWNTs. Bile salts in particular are found to be exceptionally effective in dissolving individual tubes, as evidenced by highly resolved optical absorption spectra, bright bandgap fluorescence, and the unprecedented resolution (∼ 2.5 cm - 1 ) of the radial breathing modes in Raman spectra. This is attributed to the formation of very regular and stable micelles around the nanotubes providing an unusually homogeneous environment. Quantitative information concerning the degree of solubilization is obtained from absorption spectroscopy.

The formation of active component of Fe–Mo and Co–Mo catalysts for carbon nanotube growth with variable Mo content is studied with in situ XRD, XPS, and STEM&EDX. The activation of bimetallic Fe–Mo catalysts under the growth conditions leads to the formation of Fe–Mo alloy particles in contrast to Co-containing catalysts in which active alloy incorporates only a small portion of Mo (

Field electron emission from thin films composed of solely metallic or semiconducting single-walled carbon nanotubes is investigated using a scanning tip anode technique. Metallic and semiconducting nanotubes are separated by aqueous two-phase extraction. Local field emission centers observed on nanotube films of both types showed non-linear Fowler–Nordheim (FN) plots bent downwards. The curving of FN plots is much stronger for semiconducting nanotubes which is explained by their higher electrical resistance and stronger field penetration effect compared to metallic nanotubes. Particular nanotubes of both types revealed oscillations in current–voltage characteristics. The periodic oscillations indicated that the field emission current is modulated either by resonant tunnelling from confinement states in nano-objects formed by adsorbates or by the Coulomb blockade effect that can occur for emission from short carbon nanotubes.

The structure and electronic properties of the N-doped bamboo-like carbon nanotubes with different nitrogen content (N-CNTs) are studied by the use of transmission electron microscopy, electron energy loss spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy supplemented by measurements of the temperature dependence of conductivity. XPS data are used to estimate the content of nitrogen represented by pyridinic, pyrrolic, graphitic, and oxidized N species and encapsulated molecular nitrogen. The graphene blocks surrounded by borders containing hole defects and N-species are considered as building blocks of N-CNT walls. The intensity ratio of D, G, and 2D bands in Raman spectra and XPS data on the concentration of N-species are used to analyze the size of defect-free graphene blocks in N-CNTs. The size of such blocks depends on the N-defect content and is estimated to be 1–2 nm. The increase of N-species content resulting in a decrease of graphene block size correlates with the decrease of current carrier concentration (nES) estimated within the Efros–Shklovskii model.

Single-walled carbon nanotubes (SWCNTs) demonstrate a high efficiency as optical elements for mode-locking in fiber lasers. A thin polymer film with dispersed nanotubes is a suitable approach to insert the element into the laser resonator. The main problem is that the polymer oxidative destruction temperature is much lower than that for SWCNTs. To form polymer–nanotube composites with a high threshold of thermal degradation, surviving under the relatively intense laser beams, a set of polyimides has been used. It has been shown that for the synthesized polyimides the temperature of degradation was in the range 500–530 °C, depending on the polymer. In the near-infrared range the polyimides were transparent, and the main absorption bands were detected at wavelengths less than 400 nm. A procedure of the fabrication of composite SWCNT/polyimide films was developed. The films were formed on transparent substrates, as quartz and glass slides.

Linear carbon chains are synthesized by laser ablation of graphite flakes suspended in ethanol. The optical absorption spectra of the material are measured, and the typical absorption lines at the positions 216, 226, 239, 252, 261, 276, 287, 306 and 325 nm are attributed to the linear carbon chains consisting of 8–16 atoms of C. The surface-enhanced Raman scattering (SERS) spectra are registered from the material in the presence of silver nanoparticles. While the Raman spectra of linear carbon chains in ethanol demonstrate only the Raman lines of ethanol, the addition of silver nanoparticles lead to the appearance of a carbon chain signal with enormous intensity. The bands at 1850–2200 cm−1 originating from CC vibrations in polyynic chains dominated in the SERS spectra. The gain of the signal of linear carbon chains exceeded 105. SERS spectra are excited by various wavelengths in the visible range. The resonant re-distribution of the lines attributed to chains with different length is revealed.

The present paper is devoted to the influence of the growth temperature of multi-walled carbon nanotubes (MWCNTs) on their defective structure. The MWCNTs obtained within the range of 610–750 °C have been studied using two methods: Raman spectroscopy and the analysis of the temperature dependence of conductivity. This approach allows us to obtain independent data on the concentration of the defects within the MWCNTs via the intensity ratio of 2D and D bands, on one hand, and charge carrier concentration, on the other hand. The results obtained using Raman spectroscopy and the analysis of the conductivity have provided asymptotic and volcano-like curves, respectively, of defect concentration within the temperature range studied. This can be attributed to the difference in the probing depth of each method (∼50 nm for Raman spectroscopy, and ∼2–3 nm for conductivity measurements) providing different sensitivity to surface impurities or defects. The secondary factors (the size of the active component, the amount of the lateral carbon deposits) have been shown to mask the influence of the growth temperature. Nevertheless, as these secondary factors and the synthesis temperature independently affect the effective nanotube defectiveness, we have shown the defect concentration to fall with increasing MWCNT growth temperature.

We report a detailed study of changes in optical properties of single-walled carbon nanotubes caused by doping in acid medium and their comparison with those induced by other doping techniques. In this work, a hydrochloric acid was added to the aqueous suspension of nanotubes wrapped with sodium cholate. Additionally to the shift and suppression of several modes in the Raman spectrum and the suppression of the first optical transition in the optical absorption spectrum, we have observed an appearance of a trion band in the photoluminescence spectrum with the energy about 0.2 eV less than the main photoluminescence band.