Showing papers by "Mildred S. Dresselhaus published in 2006"

Fabrication of high-purity, double-walled carbon nanotube buckypaper

Abstract: Recently, much attention has been paid to double-walled carbon nanotubes (DWNTs). This article highlights a recent advance in the fabrication of high-purity DWNT buckypaper through a combination of a catalytic chemical vapor deposition and an optimized two-step purification process. The DWNTs have small diameters (below 2 nm) and are packed in a hexagonal array. Their highly enhanced thermal stability (up to 2300 C) is believed to derive from their coaxial structure. DWNT buckypaper exhibits superior mechanical and electrical properties, accessible surface area, and structural integrity, and may eventually replacing single-walled carbon nanotubes in numerous applications. (Abstract Copyright [2006], Wiley Periodicals, Inc.)

In Situ Raman Study on Single‐ and Double‐Walled Carbon Nanotubes as a Function of Lithium Insertion

Abstract: We investigated the electrochemical lithium ion (Li(+)) insertion/desertion behavior on highly pure and bundled single- and double-walled carbon nanotubes (SWNTs and DWNTs) using an in situ Raman technique. In general, two storage sites could host Li(+) in SWNT and DWNT bundles when varying an external potential: a) the outer surface sites, and b) the interstitial spaces within the bundles. The most sensitive changes in the tangential mode (TM) of the Raman spectra upon doping with Li(+) can be divided into two regions. The first region was found from 2.8 to 1.0 V (the coverage of Li(+) on the outer surface of a bundled nanotube) and was characterized by the loss of resonant conditions via partial charge transfer, where the G(+) line of the SWNT and the TM of the outer tube of DWNTs experienced a highly depressed intensity, but remained almost constant in frequency. The appearance of a Breit-Wigner-Fano (BWF) profile provided strong evidence of metallic inner tubes within DWNTs. The second region was observed when the applied potentials ranged from 0.9 to 0 V and was characterized by Li(+) diffusion into the interstitial sites of the bundled nanotube material. This phenomenon invoked a large downshift of the G(-) band in SWNTs, and a small downshift of the TM of the inner tube of DWNTs caused by expansion of the C--C bonds due to the charge transferred to the nanotubes, and the disappearance of the BWF profile through the screening effect of the interstitial Li(+) layers.

Selection rules for one- and two-photon absorption by excitons in carbon nanotubes

Abstract: Recent optical absorption and emission experiments showed that the lower energy optical transitions in carbon nanotubes are excitonic in nature, as predicted by theory. These experiments were based on the symmetry aspects of free electron-hole states and bound excitonic states. The present work shows, however, that group theory does not predict the selection rules needed to explain the two photon experiments. We obtain the symmetries and selection rules for the optical transitions of excitons in single-wall carbon nanotubes within the approach of the group of the wave vector, thus providing important information for the interpretation of theoretical and experimental optical spectra of these materials.

Raman characterization of electronic transition energies of metallic single-wall carbon nanotubes

Abstract: Using a confocal micro-Raman system, spectra showing the splitting of optical transitions due to trigonal warping effect are presented for metallic single-wall carbon nanotubes SWNT’s. Our results indicate that the intensity variations between different optical transitions can be attributed primarily to the differences in the magnitude of the electron-phonon coupling matrix elements. Our approach will allow the study of the magnitude of electron-phonon matrix elements as well as quantum interference effects between different transitions in metallic SWNT’s.

Generating Suspended Single-Walled Carbon Nanotubes Across a Large Surface Area via Patterning Self-Assembled Catalyst-Containing Block Copolymer Thin Films

Abstract: Using self-assembled block copolymers as templates, catalytically active nanostructures with controlled size and space have been produced A self-assembled polystyrene-b-polyferrocenylsilane thin film and monolayer of surface micelles of cobalt-complexed polystyrene-b-poly(2-vinylpyridine) are fully compatible with novolac-based conventional photoresists Combining bottom-up self-assembly of catalyst-containing block copolymers with top-down microfabrication processing, plateaus covered with arrays of catalytically active nanostructures have been generated Spatially selective growth of suspended single-walled carbon nanotubes over a large surface area has been achieved Greatly enhanced Raman signals have been detected from the suspended tubes This facile method of creating highly ordered catalyst nanostructures on top of posts enables the rational synthesis of suspended carbon nanotubes, thus facilitating the study of CNT properties by optical methods and enabling the fabrication of devices based on su

Resonance Raman spectroscopy in one-dimensional carbon materials

Abstract: Brazil has played an important role in the development and use of resonance Raman spectroscopy as a powerful characterization tool for materials science. Here we present a short history of Raman scattering research in Brazil, highlighting the important contributions to the field coming from Brazilian researchers in the past. Next we discuss recent and important contributions where Brazil has become a worldwide leader, that is on the physics of quasi-one dimensional carbon nanotubes. We conclude this article by presenting results from a very recent resonance Raman study of exciting new materials, that are strictly one-dimensional carbon chains formed by the heat treatment of very pure double-wall carbon nanotube samples.

Carbon nanotubes and nucleic acids: tools and targets

Abstract: Nucleic acids; with their intrinsic structural properties as well as their high specificity, are playing an important role in the rapid development of nano-technologies. In turn, these new technologies and their efficient performance enable fast and precise methods for detection of nucleic acids, improving the diagnosis of diseases and identification of pathogens. We discuss the use of nucleic acids to disperse and sort single walled carbon nanotubes (SWNTs), and carbon nanotube-based field effect transistors (CNT-FETs) to electrically detect specific nucleic acid sequences. Both DNA and RNA are efficient agents for dispersion and separation of SWNTs by diameter and chirality. Fractions enriched in a narrow band gap distribution of DNA: SWNT hybrids do not alter the electronic performance of field effect transistors. A CNT-FET fulfills the requirements for a nanosensing device that can greatly exceed the existing technologies. Electrical detection of specific nucleic acid sequence could potentially overcome the current limitations of optical detection, by increasing sensitivity and speed, while reducing sample manipulation, size, and cost.

Characterizing the chirality distribution of single‐walled carbon nanotube materials with tunable Raman spectroscopy

Abstract: We have developed a methodology to use resonant Raman spectroscopy with a tunable excitation source to characterize the chirality distribution of a synthesized single-walled carbon nanotube (SWNT) material. Isolated SWNTs are either dispersed or directly grown on substrates with markers. The samples are then placed on an automatic scanning stage of a confocal Raman microscope. Resonant Raman signals from SWNTs are collected while the laser spot scans across the substrate and a two-dimensional mapping is obtained at each laser excitation energy. From the resonant Raman signal the chiralities of the SWNTs can be assigned and a chirality distribution of the material can therefore be obtained by examining hundreds to thousands SWNTs on the substrate.

Sodium chloride-catalyzed oxidation of multiwalled carbon nanotubes for environmental benefit.

Abstract: A sodium chloride (NaCl) catalyst (0.1 w/w %) lowers the oxidation temperature of graphitized multiwalled carbon nanotubes: MWCNT-20 (diameter: 20-70 nm) and MWCNT-80 (diameter: 80-150 nm). The analysis of the reaction kinetics indicates that the oxidation of MWCNT-20 and MWCNT-80 mixed with no NaCl exhibits single reaction processes with activation energies of E(a) = 159 and 152 kJ mol(-1), respectively. The oxidation reaction in the presence of NaCl is shown to consist of two different reaction processes, that is, a first reaction and a second reaction process. The first reaction process is dominant at a low temperature of around 600 degrees C, while the second reaction process becomes more dominant than the first one in a higher temperature region. The activation energies of the first reaction processes (MWCNT-20: E(a1) = 35.7 kJ mol(-1); MWCNT-80: E(a1) = 43.5 kJ mol(-1)) are much smaller than those of the second reaction processes (MWCNT-20: E(a2) = 170 kJ mol(-1); MWCNT-80: E(a2) = 171 kJ mol(-1)). The comparison of the kinetic parameters and the results of the spectroscopic and microscopic analyses imply that the lowering of the oxidation temperature in the presence of NaCl results from the introduction of disorder into the graphitized MWCNTs (during the first reaction process), thus increasing the facility of the oxidation reaction of the disorder-induced nanotubes (in the second reaction process). It is found that the larger nanopits and cracks on the outer graphitic layers are caused by the catalytic effect of NaCl. Therefore, the NaCl-mixed samples showed more rapid and stronger oxidation compared with that of the nonmixed samples at the same residual quantity.

Band-Gap Modulation and Kohn Anomalies in Two-Dimensional Graphite and Single-Wall Carbon Nanotubes

Abstract: The electron-phonon coupling in two-dimensional graphite (graphene sheet) and metallic single-wall carbon nanotubes (SWNTs) is analyzed. In the graphene sheet the G-band phonon mode induces oscillations of the Fermi points, while the G′-band phonon mode opens a dynamical (oscillating with the phonon frequency) band gap, and accordingly, both phonon modes exhibit Kohn anomalies. Similarly, truly metallic armchair SWNTs undergo Peierls transitions driven by the G−- and G′-band phonon modes both of which open a dynamical band gap. In addition, the dynamical band gap induces a non-linear dependence of the phonon frequencies on the doping level and gives rise to strong anharmonic effects in the graphene sheet and metallic SWNTs.