O. Jost

Diameter grouping in bulk samples of single-walled carbon nanotubes from optical absorption spectroscopy

The influence of the synthesis parameters on the mean characteristics of single-wall carbon nanotubes in soot produced by the laser vaporization of graphite has been analyzed using optical absorption spectroscopy. The abundance and mean diameter of the nanotubes were found to be most influenced by the furnace temperature and the cobalt/nickel catalyst mixing ratio. Via an analysis of the fine structure in the optical spectra, the existence of preferred nanotube diameters has been established and their related fractional abundance could be determined. The results are consistent with nanotubes located mainly around the armchair axis.

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.

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.

Third-order optical nonlinearities of carbon nanotubes in the femtosecond regime

Femtosecond pump–probe experiments have been carried out on an ensemble of single-wall carbon nanotubes deposited on a glass substrate. Measurements of transient changes of transmission and reflection provide an estimate of the real and imaginary parts of the second-order hyperpolarizability of carbon nanotubes. These values are compared with previous measurements and are discussed in the light of a simple model of the optical nonlinearities near the optical band-gap.

Carbon nanotube transistor optimization by chemical control of the nanotube–metal interface

Most carbon nanotube transistors work as Schottky barrier transistors. We show that chemical treatment of operational p-type nanotube transistors by trifluoro-acetic acid (TFA) leads to the drastic improvement of all the key device parameters. This effect is due to the highly polar nature of the TFA molecule which, once adsorbed at the metal–nanotube interface, lowers the Schottky barrier for the holes and thus favors their injection.

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.

Photocreated carrier dynamics in isolated carbon nanotubes

We report the first combined photoluminescence and pump–probe measurements on single wall carbon nanotubes isolated in micelles. The bandgap photoluminescence is observed at room temperature and is concomittent with a drastic change in the photocreated carrier dynamics as compared to the one observed in usual macroscopic samples. Probing the fundamental bandgap leads to a non-exponential dynamics which is related to a lifetime distribution within the macroscopic nanotube population. Probing the second van Hove transition leads to a strong increase of the excitation lifetime which demonstrates that some non-radiative relaxation channels are quenched for isolated nanotubes. These observations constitute an important step in the understanding of the existence of photoluminescence in isolated nanotubes.

The spectroscopic investigation of the optical and electronic properties of SWCNT

The spectroscopic investigation of bulk samples of SWCNT is reviewed with examples taken from electron energy-loss spectroscopy in transmission, UV-vis and photoemission spectroscopy. It is shown that, given the insight obtained from EELS studies, UV-vis spectroscopy is a powerful express tool for the characterization of the NT yield and diameter distribution in bulk SWCNT samples. In addition, using photoemission spectroscopy, apart from showing the use of annealing in UHV to remove many of the contaminants commonly found in purified SWCNT material and determining the work function of pristine SWCNT to be 4.65±0.1 eV, we illustrate the similarity of the occupied electronic structure of SWCNT with that of graphite. Open questions, however, remain, such as the appropriate model for the changes of the electronic structure upon intercalation and the reasons for the absence of the singularities in the occupied density of states in the photoemission spectra.

Mechanism of carbon nanotube synthesis by laser ablation

Qualitative analysis of the gas-dynamical phenomena by the laser ablation of graphite under conditions of single-wall carbon nanotubes (SWNTs) suggests to reconsider the currently assumed mechanisms of SWNT formation in this technique and to conclude that it is a form of solid-liquid- solid catalytic graphitization of amorphous carbon or other imperfect carbon forms. The prosed mechanism of melting of catalyst particles followed by precipitation of SWNTs allows to explain the observed dependencies of the SWNT yield on the synthesis temperature and pressure. Critical inspection of the literature data shows that it might also play a role in other physical and chemical deposition techniques of SWNT synthesis.

Metal Oxides and Low Temperature SWCNT Synthesis via Laser Evaporation

Studies using metal oxides show they are active as catalysts for single wall carbon nanotubes synthesis in laser ablation, even at room temperature. In addition when combined with Ni and Co, large diameter SWCNT can be synthesized at temperatures below those required when only using Ni and Co as a binary catalyst. The results suggest a nucleation mechanism previously not identified.