Structural modeling of dahlia-type single-walled carbon nanohorn aggregates by molecular dynamics.

The structure of dahlia-type single-walled carbon nanohorn aggregates has been modeled by classical molecular dynamics simulations, and the validity of the model has been verified by neutron diffraction. Computer-generated models consisted of an outer part formed from single-walled carbon nanohorns with diameters of 20-50 Å and a length of 400 Å and an inner turbostratic graphite-like core with a diameter of 130 Å. The diffracted intensity and the pair correlation function computed for such a constructed model are in good agreement with the neutron diffraction experimental data. The proposed turbostratic inner core explains the occurrence of the additional (002) and (004) graphitic peaks in the diffraction pattern of the studied sample and provides information about the interior structure of the dahlia-type aggregates.

Changes in the fluorescence spectrum of individual single-wall carbon nanotubes induced by light-assisted oxidation with hydroperoxide.

Through fluorescence-spectrum measurements, we investigated the effects of light-assisted oxidation with H2O2 (LAOx) on single-wall carbon nanotubes (SWNTs) that were individually dispersed in an aqueous solution of surfactant. The intensities of the fluorescence spectra were decreased remarkably by the LAOx when the light's wavelength was 400-500 nm and a little when 600-700 nm. The spectrum intensity did not recover even when the pH was restored to an original value of 6.5. The spectra changed little when the LAOx wavelength was 500-600 nm or the light was not irradiated. In addition, the effect of LAOx on SWNTs was related to the diameters of SWNTs. We inferred that these phenomena reflected that H2O2 was dissociated by absorbing the fluorescence light emitted from optically excited SWNTs, which, in turn, accelerated the burning out of SWNTs.

Dissociation of electrolytes in a nano-aqueous system within single-wall carbon nanotubes.

Research on material incorporation within single-wall carbon nanotubes (SWNTs) through aqueous solutions of various electrolytes is performed for the purpose of providing a foundation for future application of SWNTs to, for example, drug delivery systems. We have determined that the optical spectra of SWNTs were significantly affected when SWNTs that had opened holes or removed caps were treated through immersion in an aqueous solution of electrolytes, followed by drying at room temperature; however, the spectra of SWNTs without opened holes or removed caps were not subjected to such treatment. We infer that when the sucked solutions remained inside the tubes, even after drying (the nano-aqueous system), the electrolyte was dissociated into ions, which was likely to change the electronic states of SWNTs. On the other hand, when the SWNTs were well-dried under vacuum, no remarkable changes in their optical spectra were observed.

Isolating single-wall carbon nanohorns as small aggregates through a dispersion method.

An approach to isolating small aggregates of single-wall carbon nanohorns (SWNHs) is presented. SWNHs are ultrasonically treated in an aqueous solution of surfactant, resulting in dispersion of SWNH aggregates. Subsequent centrifuging enables the separation of small aggregates from larger aggregates or agglomerations and removal of graphitic particles (GG balls), the main impurity. The SWNHs obtained in this way were purified and formed small aggregates, thus exhibiting characteristics superior to those of SWNHs before treatment. We believe that the ability to isolate small SWNH aggregates in an aqueous solution should contribute to their application in the fields of biological sensing and drug delivery systems.

Employing Raman spectroscopy to qualitatively evaluate the purity of carbon single-wall nanotube materials.

Carbon single-wall nanotubes (SWNTs) have highly unique electronic, mechanical and adsorption properties, making them interesting for a variety of applications. Raman spectroscopy has been demonstrated to be one of the most important methods for characterizing SWNTs. For example, Raman spectroscopy may be employed to differentiate between metallic and semi-conducting nanotubes, and may also be employed to determine SWNT diameters and even the nanotube chirality. Single-wall carbon nanotubes are generated in a variety of ways, including arc-discharge, laser vaporization and various chemical vapor deposition (CVD) techniques. In all of these methods, a metal catalyst must be employed to observe SWNT formation. Also, all of the current synthesis techniques generate various non-nanotube carbon impurities, including amorphous carbon, fullerenes, multi-wall nanotubes (MWNTs) and nano-crystalline graphite, as well as larger micro-sized particles of graphite. For any of the potential nanotube applications to be realized, it is, therefore, necessary that purification techniques resulting in the recovery of predominantly SWNTs at high-yields be developed. It is, of course, equally important that a method for determining nanotube wt.% purity levels be developed and standardized. Moreover, a rapid method for qualitatively measuring nanotube purity could facilitate many laboratory research efforts. This review article discusses the application of Raman spectroscopy to rapidly determine if large quantities of carbon impurities are present in nanotube materials. Raman spectra of crude SWNT materials reveal tangential bands between 1500-1600 cm(-1), as well as a broad band at approximately 1350 cm(-1), attributed to a convolution of the disorder-induced band (D-band) of carbon impurities and the D-band of the SWNTs themselves. Since the full-width-at-half-maximum (FWHM) intensity of the various carbon impurity D-bands is generally much broader than that of the nanotube D-band, an indication of the SWNT purity level may be obtained by simply examining the line-width of the D-band. We also briefly discuss the effect of nanotube bundling on SWNT Raman spectra. Finally, sections on employing Raman spectroscopy, and Raman spectroscopy coupled with additional techniques, to identify the separation and possible isolation of a specific nanotube within purified SWNT materials is provided. Every SWNT can be considered to be a unique molecule, with different physical properties, depending on its (n, m) indices. The production of phase-pure (n, m) SWNTs may be essential for some nanotube applications.

Fe-sapphire and C-Fe-sapphire interactions and their effect on the growth of single-walled carbon nanotubes by chemical vapor deposition.

We previously reported that the quantity of single-walled carbon nanotubes grown on Fe-coated sapphire by chemical vapor deposition depended on the crystallographic faces of sapphires. In this report, we show that the interaction of Fe, sapphire, and carbon depended on the sapphire faces. We deduce that the quantity of Fe available to catalyze the growth of single-walled carbon nanotubes was suppressed by the formation of Fe-Al alloys and whether the Fe-Al alloys were formed on Fe-coated sapphire or not depended on the sapphire-surface structure.

Compressibility and polygonization of single-walled carbon nanotubes under hydrostatic pressure

Single-walled carbon nanotubes show linear elasticity under hydrostatic pressure up to 1.5 GPa at room temperature. The volume compressibility, measured by in situ synchrotron x-ray diffraction, has been determined to be 0.024 GPa (-1). Theoretical calculations suggest that single-walled carbon nanotubes are polygonized when they form bundles of hexagonal close-packed structure and the intertubular gap is smaller than the equilibrium spacing of graphite (002) (d = 3.35 A). It has also been determined that the deformation of the trigonal nanotube lattice under hydrostatic pressure is reversible up to 4 GPa, beyond which the nanotube lattice is destroyed.