A. Graff

Solid–Liquid–Solid Growth Mechanism of Single-Wall Carbon Nanotubes

Abstract 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.

Efficient production of B-substituted single-wall carbon nanotubes

We present an efficient method to achieve high-purity boron doped single-walled carbon nanotubes (SWCNT) using an adapted substitution reaction. We observe that around 15 at.% of the carbon atoms are substituted by boron, with local concentrations of up to 20 at.% (higher than previous report). The as-prepared material was characterised by local scale method: transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) as well as by bulk sensitive methods like optical spectroscopy in the infrared energy range (IR), X-ray photoemission spectroscopy (XPS) and bulk sensitive high-resolution EELS.

Reduced diameter distribution of single-wall carbon nanotubes by selective oxidation

We report an easy way to narrow the diameter distribution of single-walled carbon nanotubes (SWNT) by oxidization treatments. Both a chemical treatment in 2 M HNO3 as well as oxidation in a reduced O2 atmosphere lead to a selective burning of the narrower SWNT in bulk samples and to a diameter distribution which is smaller by a factor of two. This is a first important step towards a selective production of SWNT with a defined diameter on a bulk scale.

Impact of catalyst coarsening on the formation of single-wall carbon nanotubes

The single-wall carbon nanotube (SWCN) yield as a function of the gas flow velocity for different catalyst contents in a furnace-based pulsed laser evaporation method is shown to depend sensitively on the size distribution and growth conditions of the condensed catalyst nanoparticles in the gas phase. In particular, accelerated particle coarsening should be avoided. Consequently, a high number density of small catalyst nanoparticles leads to a high nanotube yield within the timescale of a few hundred milliseconds. Hence, the attainment of enhanced particle growth control will enable a high yield evaporation-based synthesis of high-quality SWCNT.

Infrared response of multiwalled boron nitride nanotubes

We report the infrared (IR) response of bulk samples of multiwalled boron nitride nanotubes, produced by a substitution reaction from single walled carbon nanotubes, which is dominated by two characteristic BN-vibrations at 800 and 1372 cm-1.

Electronic structure and optical properties of boron doped single-wall carbon nanotubes

We present a study of the electronic structure and the optical properties of boron doped single walled carbon nanotubes which have been produced by a substitution reaction from nanotube templates. The morphology and crystal structure of the samples have been characterized by transmission electron microscopy and electron energy‐loss spectroscopy. Clean boron doped SWCNT with an average boron content of 15 at% have been produced. The B1s and C1s core level spectra reveal that boron is in an sp2 configuration and that the effective charge transfer is about 0.5 holes per boron to the C‐derived states. The boron substitution also leads to new features in the optical absorption spectra which can be attributed to the appearance of an acceptor band about 0.1 eV above the top of the valence band of the SWCNT. These changes in the electronic structure and in the optical properties upon boron substitution are in good agreement with state of the art ab initio calculations.

Production and characterization of MWBNNT and B‐doped SWCNT

We present a study of mutiwalled boron nitride nanotubes (MWBNNT) and boron doped single wall carbon nanotubes ( SWCNT) which have been produced by substitution reaction from SWCNT templates. Pure MWBNNT with a 1:1 BN atomic ratio as well as clean SWCNT with B‐substitution levels up to 20% have been produced. The morphology and crystal structure of the samples have been characterized by transmission electron microscopy (TEM), electron diffraction and electron energy‐loss spectroscopy (EELS).

Electronic Properties of Multiwall Boron Nitride Nanotubes

We report on infra‐red and electron energy‐loss spectroscopy studies of high purity multiwall boron nitride nanotubes synthesised by substitution reactions. The IR pattern of the BN tubes shows the presence of both tangential (∼800 cm−1) and longitudinal (∼1400 cm−1) modes characteristic of h‐BN. We show that the different energy positions and the LO‐TO splitting observed for the BN‐tubes can be explained by geometrical considerations. The dielectric function e of the BN‐tubes obtained from the electron energy‐loss function reveals an intense π‐π* interband transition at 5.4 eV, which is shifted to lower energies by 0.6 eV when compared to hexagonal BN. In addition, the absorption onset of the optically allowed transitions is less abrupt and begins at lower energies than that of h‐BN. We ascribe this effect to the presence of tubes with low inner diameter (below 3 nm) in good agreement with recent theoretical band structure calculations.

Optimization of purification and selective burning of single‐wall carbon nanotubes

Single wall carbon nanotubes produced by laser ablation were purified and selectively O2 treated. In this contribution we present a comparative study of the success of different multistep purification attempts. After optimization of the mentioned purification methods we can remove almost all catalyst metals and amorphous carbon in the samples. Chemical treatments and O2 burning lead to a selective creation of different SWNT. The results of the purification effort and selective burning were monitored by optical spectroscopy, TEM, SEM and atomic absorption spectroscopy (AAS).