Showing papers by "S. Roth published in 2005"

Vanishing of the Breit-Wigner-Fano component in individual single-wall carbon nanotubes.

Abstract: In order to decide definitely on the dependence of the intensity of the Breit-Wigner-Fano (BWF) component with the size of the bundle, we have measured the radial breathing modes and tangential modes (TMs) of well defined metallic individual single-wall carbon nanotubes (SWCNTs) and individual SWCNT bundles. In this aim, a complete procedure including the preparation of the substrates, the sample preparation, atomic-force-microscopy imaging and Raman spectroscopy has been developed. From this procedure, we show unambiguously that the BWF component vanishes in isolated metallic SWCNTs. In other words, the observation of a BWF component in the TM bunch is an intrinsic feature of the metallic SWCNT bundle.

Catalyst patterning methods for surface-bound chemical vapor deposition of carbon nanotubes

Abstract: We present three different catalyst preparation and patterning techniques for plasma-enhanced chemical vapor deposition of carbon nanostructures from acetylene and ammonia mixtures. The different merits and potential areas of application are highlighted for each technique as compared to the benchmark of e-beam-lithography patterning. Maskless, focused ion beam written Pt can nucleate aligned carbon nanofibers, thereby allowing a sub-100 nm lateral resolution on non-planar substrate geometries combined with an in-situ monitoring. Ion beam milling additionally enables the pre-shaping and marking of the substrate, which is shown for the growth of individual nanofibers on the apex of commercial scanning probe tips. Pulsed electrochemical deposition was used to form Ni and Fe catalyst islands of controlled size and density. This is also demonstrated on complex substrate geometries such as carbon cloth. Nanocontact printing was employed to deposit a highly purified Co colloid in regular patterns with feature sizes down to 100 nm onto silicon wafers for low cost patterning over large areas. We analyze the catalyst restructuring upon exposure to elevated temperatures for each technique and relate this to the nucleated nanofiber dimensions and array densities. The flexibility in catalyst and substrate material allows a transfer of our achievements to catalyst-assisted growth of nanostructures in general facilitating their hierarchical device integration and future application.

Single-electron transistor mediated by C60 insertion inside a carbon nanotube

Abstract: Electrical properties of C60 encapsulated peapod are investigated at various temperatures from room temperature down to T=1.8K. The current behavior induced by the applied gate and source-drain voltage shows that C60 peapods exhibit single-electron transistor properties. Gate-dependent conductance is enhanced at negative gate voltage and is suppressed and oscillate at positive gate voltage. This behavior is ascribed to the modulation of density of states by the insertion of C60s inside a carbon nanotube.

Visualization and in situ contacting of carbon nanotubes in a scanning electron microscope

Abstract: A method for contacting carbon nanotubes on an insulating surface is presented. Tubes which are already connected to prefabricated electrodes can be visualized in the scanning electron microscope exploiting voltage contrast at low beam energies. Additional connections to tubes identified in this way are fabricated by in situ electron beam induced deposition from metalorganic precursors such as tungsten hexacarbonyl. A resistivity of 0.15Ωcm before annealing has been achieved so far with this material.

Purification and Percolation — Unexpected Phenomena in Nanotube Polymer Composites

Abstract: SWNT were synthesized by arc‐discharge methods. The as produced material was treated by different procedures in order to get purified SWNT. The treatments led to higher conductivity values as measured on bucky papers. The SWNT were incorporated into a polymer matrix of polycarbonate by melt mixing using a small scale compounder. The as produced SWNT material led to electrical percolation between 2 and 3wt%. Unexpectedly, the composites with the treated SWNT showed higher percolation concentrations. This is contradictory to theoretical expectations. These results would suggest that increasing the purity of SWNT does not lead automatically to lower percolation contents when dispersing SWNT in a molten polymer by shear mixing. During the purification the interactions between SWNT bundles might be increased, thus leading to a more agglomerated SWNT material which is more difficult to wet, infiltrate and disperse during the melt mixing procedure.

Purity Evaluation of Bulk Single Wall Carbon Nanotube Materials

Abstract: We report on our experience using a preliminary protocol for quality control of bulk single wall carbon nanotube (SWNT) materials produced by the electric arc‐discharge and laser ablation method. The first step in the characterization of the bulk material is mechanical homogenization. Quantitative evaluation of purity has been performed using a previously reported procedure based on solution phase near‐infrared spectroscopy. Our results confirm that this method is reliable in determining the nanotube content in the arc‐discharge sample containing carbonaceous impurities (amorphous carbon and graphitic particles). However, the application of this method to laser ablation samples gives a relative purity value over 100 %. The possible reason for that might be different extinction coefficient meaning different oscillator strength of the laser ablation tubes. At the present time, a 100 % pure reference sample of laser ablation SWNT is not available, so we chose to adopt the sample showing the highest purity as a new reference sample for a quantitative purity evaluation of laser ablation materials. The graphitic part of the carbonaceous impurities has been estimated using X‐ray diffraction of 1:1 mixture of nanotube material and C60 as an internal reference. To evaluate the metallic impurities in the as prepared and homogenized carbon nanotube soot inductive coupled plasma (ICP) has been used.We report on our experience using a preliminary protocol for quality control of bulk single wall carbon nanotube (SWNT) materials produced by the electric arc‐discharge and laser ablation method. The first step in the characterization of the bulk material is mechanical homogenization. Quantitative evaluation of purity has been performed using a previously reported procedure based on solution phase near‐infrared spectroscopy. Our results confirm that this method is reliable in determining the nanotube content in the arc‐discharge sample containing carbonaceous impurities (amorphous carbon and graphitic particles). However, the application of this method to laser ablation samples gives a relative purity value over 100 %. The possible reason for that might be different extinction coefficient meaning different oscillator strength of the laser ablation tubes. At the present time, a 100 % pure reference sample of laser ablation SWNT is not available, so we chose to adopt the sample showing the highest purity as...

The Influence of Sulfur Promoter on the Production of SWCNTs by the Arc‐Discharge Process

Abstract: The influence of sulfur addition on the production of SWCNTs by the arc‐discharge method with Fe/Y catalysts was investigated. It was found that the yield of SWCNTs is strongly influenced by the presence of sulfur. The dependence of the SWCNTs yield on the concentration of sulfur in the anode reaches its maximum around 1.5 at%. The yield of SWCNTs was estimated by thermal gravimetry and by optical absorption.

Random Telegraph Noise in Carbon Nanotube Peapod Transistors

Abstract: We investigated the switching of resistance between two discrete values, known as random telegraph noise (RTN), observed in carbon nanotube peapod transistors [single‐walled carbon nanotubes (SWNTs), C60‐peapods, and Cs‐encapsulated SWNTs (so‐called Cs‐peapods)]. By analyzing the features of the RTN, we suggest that this noise for SWNTs is due to the random transition of defects between two metastable states, activated by inelastic scattering with ballistic electrons. The noise for C60‐peapods (Cs‐peapods) is attributed to the motion of C60s (Cs) in the nanotubes.

Inertness of Near‐Armchair Carbon Nanotubes Towards Fluorination

Abstract: Reactivity of single‐wall carbon nanotubes (SWNTs), obtained by HiPco method, and double‐wall carbon nanotubes (DWNTs), grown during a chemical vapor deposition process, towards fluorinating agent BrF3 has been examined using optical absorption spectra measurements. The spectra of the fluorinated samples exhibited sets of distinctly resolved peaks with energies having close values for DWNTs and HiPco nanotubes. It was found that the peaks can be associated with energy transitions in (n,n‐l)‐type carbon nanotubes, which structure remained unchanged with fluorination.

Electron Transport — from Buckypaper to Thin Single‐Wall Nanotube Networks

Abstract: The mechanism of electronic transport in a buckypaper and in thinner networks of single‐wall carbon nanotubes (SWNTs) of different densities is studied. A systematic trend in changes of the temperature dependences of the electrical conductance is observed as the thickness of the networks changes. Nevertheless, the behaviour typical for conductive networks is present even for very thin networks. The character of the temperature dependences of the electrical conductance of SWNT networks is different from that of an individual SWNT.Various models of electronic transport developed for conductive networks were tested to fit the temperature dependences of the conductance for all samples. In the case of SWNT buckypaper, a model of metallic conduction interrupted by barriers accounts for the data, while for the less metallic thin SWNT networks, the Variable Range Hopping (VRH) model seemed to be more suitable.