Filling single-wall carbon nanotubes

Single-wall nanotube (SWNT)-based hybrid materials represent a quite recent research field. On the basis of previous works performed on multi-wall carbon nanotubes (MWNTs), the goal is to fill the inner SWNT cavity with various compounds, whose the combination with the surrounding carbon tube is expected to allow peculiar physical phenomena to occur and/or peculiar properties to be obtained. As compared to MWNTs, regular SWNT inner cavity is really nanometric, i.e., in the dimension range where quantum phenomena could occur. However, a major drawback is that filling ∼1-nm wide tubes is less easy (i.e., compared to MWNTs), and the related driving forces not fully understood. Who is working in the field, what and how are the SWNT-based hybrid nano-materials prepared so far, what could possibly be the filling mechanisms, are questions that are discussed in this paper. A review of the existing literature is made, with a focus on C₆₀@SWNTs (peapods), which appear to be the most amazing—and the most promising—SWNT-based hybrid nano-materials to date.     

Photoluminescence intensity of single-wall carbon nanotubes

The photoluminescence (PL) intensity of a single-wall carbon nanotube (SWNT) is calculated for each (n, m) by multiplying the photon-absorption, relaxation and photon-emission matrix elements. The intensity depends on chirality and “type I vs type II” for smaller diameter semiconducting SWNTs (less than 1 nm). By comparing the calculated results with the experimental PL intensity of SWNTs prepared by chemical vapor deposition at different temperatures, we find that the abundance of (n, m) nanotubes with smaller diameters should exhibit a strong chirality dependence, which may be related to the stability of their caps.     

Structure modification of single-wall carbon nanotubes

Various purification processes were applied to single-wall carbon nanotubes synthesized by metal catalyzed laser ablation. Structure modifications introduced by these processes were investigated by high-resolution transmission electron microscopy and Raman spectroscopy. An apparent structure modification after purification was the increase of bundle size although breaking of nanotubes and a change of nanotube diameter distribution were also observed. More vigorous attacking of single-wall carbon nanotube structure was identified by a strong mixed-acid treatment.     

Synthesis of Y-junction single-wall carbon nanotubes

Y-junction single-wall carbon nanotubes (SWNTs) are synthesized using controlled catalysts by chemical vapor deposition. Mo-doped Fe nanoparticles supported by aluminum oxide particles are used as catalysts for growing Y-junction single-wall carbon nanotubes. Distribution of Mo-doped Fe particles plays an important role in Y-junction formation. Transmission electron microscopy confirmed the formation of single-walled structures of Y-junctions with diameters of 2-5 nm. Radial breathing mode peaks in Raman spectra show that our sample has both metallic and semiconducting nanotubes, indicating the possible formation of Y-branching with different electrical properties. The different electrical properties of branch and stem can be utilized in nanoscale three terminal electronic devices. The growth mechanism of Y-junction SWNTs is proposed.     

Purification process for single-wall carbon nanotubes

Single-wall carbon nanotubes (SWNTs) have exceptional strength and stiffness and high thermal and electrical conductivity, making them excellent candidates for aerospace structural materials. However, one of the most fundamental challenges is purifying the SWNTs. The purpose of this study was to develop a simple purification process for SWNTs, along with an understanding of the purification process. In addition, uncomplicated analytical methods were sought to screen and compare various purification methods. In this study, we demonstrate an easy method of cleaning SWNTs and evaluating their purity. The cleaning method, which employed oxidative heat treatment followed by acid reflux, was straightforward, inexpensive, and fairly effective. The purification mechanism was determined to be, first, that much of the non-nanotube carbon and iron catalyst was oxidized and, second, that the acid washing removed the iron oxide, leaving relatively pure SWNTs. Also, it was shown that a combination of thermal gravimetric analysis and Raman spectroscopy, both of which take only a few minutes and require little sample preparation, are sufficient as qualitative screening tools to determine the relative purity of SWNTs. Other analytical techniques were used to verify the validity of the screening techniques.     

Solid-liquid-solid growth mechanism of single-wall carbon nanotubes

Reasons are presented which suggest that the liquefaction of the catalytic particles is a decisive condition for formation of single wall carbon nanotubes (SWNTs) by physical synthesis techniques. It is argued that the SWNT growth mechanism is a kind of solid-liquid-solid graphitization of amorphous carbon or other imperfect carbon forms catalyzed by molten supersaturated carbon-metal nanoparticles. The assumption of low temperature melting of these nanoparticles in contact with amorphous carbon followed by its precipitation in the form of SWNTs allows to explain qualitatively the experimentally observed SWNT growth rates and temperature dependence of the SWNT yield. Guidelines for increasing SWNT yield are proposed.     

Structure changes of single-wall carbon nanotubes and single-wall carbon nanohorns caused by heat treatment

Raman spectra and transmission electron microscope images showed that diameter enlargement of HiPco, a kind of single-wall carbon nanotube, accompanied by tube-wall corrugation was caused by heat treatment (HT) at 1000 to 1700 °C. Further enlargement accompanied by straightening of the tube walls and incorporation of carbon fragments within the tubes became obvious after HT at 1800 to 1900 °C. The transformation of some single-wall carbon nanotubes into multi-wall nanotubes was observed after HT at 2000 °C, and most single-wall tubes were transformed into multi-wall ones by HT at 2400 °C. What influence the Fe contained in the HiPco tubes had on these structure changes was unclear; similar changes were observed in single-wall carbon nanohorns that did not contain any metal. This indicates that thermally induced changes in the structure of single-wall carbon nanotubes can occur without a metal catalyst. Heat treatment increased the integrity of the nanotube-papers, and this increase may have been due to tube-tube interconnections created by HT.     

Third-order optical nonlinearity in single-wall carbon nanotubes

The third-order optical nonlinearity in carbon nanotubes (CNT) exposed to an intensive external electromagnetic field has been investigated. The analysis is based on the quantum kinetic equations for the density matrix of π-electrons in CNT. In the regime of weak driving field, the kinetic equations have been solved by the perturbation method and the third-order nonlinear polarizability of different achiral CNTs has been calculated. The theory elaborated has been used for the evaluation of nonlinear susceptibility of CNT-based composites. Comparison with available experimental data has been presented. In the case of high intensive driving field a nonperturbative numerical simulation of the process has been carried out in the time domain. The axial current density in CNT has been calculated. Excitation of π-plasmons in CNTs has been analyzed.     

One-dimensional silver nanostructures on single-wall carbon nanotubes

We report the synthesis and characterization of one-dimensional silver nanostructures using single-wall carbon nanotubes (SWCNT) as a template material. Transmission electron microscopy and scanning tunneling microscopy are consistent with the formation of a one-dimensional array of silver particles on SWCNT. We observe evidence for the excitation of the longitudinal silver plasmon mode in the optical absorption spectra of Ag-SWCNT dispersions, even in the lowest silver concentrations employed. The results indicate that silver deposits on SWCNT may be candidates for light-to-energy conversion through the coupling of the electric field excited in arrays of plasmonic particles.     

Efficient microwave-assisted radical functionalization of single-wall carbon nanotubes

The efficiencies of two methods of functionalizing single wall carbon nanotubes (SWCNTs) are compared, either through a radical addition of 4-methoxyphenylhydrazine hydrochloride by a classical thermally activated procedure, or via a microwave-assisted method. X-ray photoelectron spectroscopy and thermal gravimetric analysis clearly indicate the efficiency of both methods. Raman and absorption spectroscopy further confirm the functionalization and reveal the covalent nature of the bonds created at the carbon nanotube surface. For the microwave-assisted reaction, 5-15 min is enough to functionalize the SWCNTs. Longer microwave exposure times reduce the functionalization yield and lead to a removal of groups which were bonded in a previous stage. An optimal choice of microwave irradiation time allows reducing the reaction time from days to minutes.     

1D-confinement of polyiodides inside single-wall carbon nanotubes

1D-confinement of polyiodides inside single-wall carbon nanotubes (SWCNT) is investigated. Structural arrangement of iodine species as a function of the SWCNT diameters is studied. Evidence for long range one dimensional ordering of the iodine species is shown by X-ray and electron diffraction experiments independently of the tube diameter. The structure of the confined polyiodides is investigated by X-ray absorption spectroscopy. The confinement influences the local arrangement of the chains. Below a critical diameter Φ_c of 1 nm, long linear polyiodides are evidenced leading to a weaker charge transfer than for nanotube diameter above Φ_c. A shortening of the polyiodides is exhibited with the increase of the nanotube diameter leading to a more efficient charge transfer. This point reflects the 1D-confinement of the polyiodides inside the nanotubes.     

Optical absorption matrix elements in single-wall carbon nanotubes

The optical absorption matrix element as a function of one-dimensional (1D) wave vector k, and subband index μ of a single wall carbon nanotube is given analytically for linearly polarized light with polarization parallel to the nanotube axis. For armchair nanotubes, it is found that the optical transitions for non-degenerate A symmetry bands are forbidden over the whole 1D k region and the transitions for all other bands are also forbidden at the k = 0 point. Near the Fermi level, the absorption for all metallic nanotubes is found to be approximately zero. For both metallic and semiconducting nanotubes, it is found that the absorption matrix element has a maximum absolute value at the van Hove singularity (vHS) k point around the Fermi energy for each band. The absorption dependence on diameter and chiral angle is also presented for semiconducting nanotubes. For light polarization perpendicular to the nanotube axis, on the other hand, the absorption for nanotubes is generally weak near a vHS.     

Development of Fe-doped carbon electrode for mass-producing high-yield single-wall carbon nanotubes

High crystallinity single-wall carbon nanotubes (SWNTs) can be synthesized by arc discharge evaporation of Fe-doped carbon electrode in hydrogen mixed gas, but the purity of as-grown SWNTs is strongly affected by the kinds of Fe-doped carbon electrode. Various carbon materials (artificial graphite powder, carbon black, calcined coke, etc.) have been tried to prepare Fe-doped carbon electrodes. The calcined coke, a kind of graphitizing carbon, is suitable for preparing high-quality Fe-doped carbon electrode. Moreover, the heat-treatment of Fe-doped carbon electrode in vacuum at the temperature of 1600 °C also plays an important role. At present, the best carbon electrode containing 1 at.% Fe catalyst is capable of continuously generating SWNTs at a production rate of 8 mg/min, which can be easily purified to obtain high purity SWNTs.     

Formation of trans-polyacetylene from single-wall carbon nanotubes

We report the formation of trans-polyacetylene from single-wall carbon nanotubes (SWCNTs). The SWCNTs including metal catalysts were exposed to air with water vapor for a long time, and then irradiated with laser light. The formation of trans-polyacetylene in the irradiated SWCNTs was observed by means of Raman spectroscopy. The formation process can be related to the cutting of SWCNTs due to the catalytic hydrogenation by laser heating.     

Oxygen-driven surface segregation of lithium from single-wall carbon nanotubes

We have investigated the effects of oxygen exposure on lithium-intercalated single-wall carbon nanotube bundles at room temperature, and the evolution of surface configuration versus temperature. X-ray and ultraviolet photoelectron spectroscopy measurements show a remarkable increase of surface lithium concentration, associated to modifications suffered by both core-level O 1 s line shape and valence band spectra. These results have been attributed to the occurrence of lithium surface segregation induced by interaction with oxygen species. Moreover, lithium oxide and peroxide related features are observed indicating that lithium oxidation takes place at the nanotube bundle surface.     

Fluorination of single-wall carbon nanotubes and subsequent derivatization reactions

This Account focuses on the most recent and systematic efforts in the area of functionalization chemistry of the single-wall carbon nanotubes (SWNTs) which utilizes direct fluorination for the preparation of "fluoronanonotubes" and their subsequent derivatization. The results obtained prove that the addition of fluorine drastically enhances the reactivity of the nanotube side walls. The use of this strategy as a versatile tool for preparation and manipulation of SWNTs with variable side-wall functionalities has been demonstrated. The functionalized SWNTs have shown an improved solubility in selected solvents and significantly altered electrical, mechanical, and optical properties. An overview of new synthetic methods for preparation and a discussion of characterization data for the functionalized SWNTs are provided.     

Characterization of doped single-wall carbon nanotubes by Raman spectroscopy

Single-wall carbon nanotubes were grown by thermal chemical vapor deposition using either boron- or nitrogen-containing feedstocks or both. Carrier doping was evidenced by hardenings of the G band in Raman spectra, and the estimated carrier concentration reached ∼0.4%. In the G′ and D band spectra, a doping-induced component was observed at the high- or low-energy side of the original one. However, the appearance of the new component did not always coincide with the carrier doping. The doped SWCNTs often show radial breathing mode peaks in the off-resonance region, indicating a defect-induced modification of absorption spectrum.     

Direct observation of spin-injection in tyrosinate-functionalized single-wall carbon nanotubes

In this work, we report on the interaction of a tyrosinate radical with single wall carbon nanotubes (CNT). The tyrosinate radical was formed from tyrosine (ester) by Fenton’s reagent and, reacted in situ with carbon nanotubes resulting in novel tyrosinated carbon nanotube derivatives. The covalent attachment of tyrosine on the external surface of the CNTs resulted in the appearance of a free radical, localized in the graphitic surface. The ‘electron injection’ (delocalization) of the free radical from the tyrosine ring onto the carbon nanotubes was studied and characterized by a combination of electron paramagnetic resonance, Raman and X-ray photoelectron spectroscopies, thermogravimetric analysis, as well as transmission electron and atomic force microscopies. The experiments, complemented by computer simulations, give insight into the formation process and structural details of the produced hybrid structures.     

Characterization of single-wall carbon nanotubes by N2 adsorption

N₂ adsorption isotherms at 77 K of single-wall carbon nanotubes (SWNTs), multi-wall carbon nanotubes (MWNTs), and mixtures of these carbon nanotubes (CNTs) were analyzed for differences in their pore size distributions (PSDs). The PSDs, calculated in the microporous region by the Horvath–Kawazoe method and in the mesoporous region by the BJH method, are in agreement with the structures of both types of CNTs deduced from high-resolution transmission electron microscopy. A characteristic peak in the microporous region in the PSD of SWNTs is not present in the PSDs of MWNTs and impurities such as amorphous carbon, metal residues of catalysts, etc. The evaluation of this peak is proposed as a convenient tool for the quantitative characterization of SWNT purity in carbon nanotube-containing samples.