Showing papers by "Mildred S. Dresselhaus published in 2004"

The Hydrogen Economy

Abstract: If the fuel cell is to become the modern steam engine, basic research must provide breakthroughs in understanding, materials, and design to make a hydrogen-based energy system a vibrant and competitive force

Family behavior of the optical transition energies in single-wall carbon nanotubes of smaller diameters

Abstract: Using the extended tight-binding model that allows bond lengths and angles to vary, the optical transition energies Eii in single-wall carbon nanotubes are calculated as a function of inverse tube diameter. After geometrical structure optimization, the 2n+m=constant family behavior observed in photoluminescence (PL) experiments is obtained, and detailed agreement between the calculations and PL experiments is achieved after including many-body corrections.

Carbon Nanotubes: Continued Innovations and Challenges

Abstract: This article outlines the content of the April 2004 issue of MRS Bulletin on Advances in Carbon Nanotubes. Essentially, carbon nanotubes are self-assembling nanostructures constructed of sheets of hexagonal-shaped carbon atoms rolled up into cylinders. Carbon nanotubes have attracted a great deal of attention as model systems for nanoscience and for potential applications. The special interest in carbon nanotubes stems from their unique structure and properties: their very small size (down to ∼0.42 nm in diameter); the possibility for carbon nanotubes to be metallic or semiconducting, depending on their geometrical structure; their exceptional properties of ballistic transport; their extremely high thermal conductivity and high optical polarizability; and the possibilities of high structural perfection. Research in the carbon nanotube field has now advanced to the stage where a good understanding of the structure and many of the basic properties are in place, together with much appreciation of their interrelation. On the other hand, major gaps in basic knowledge remain, with the major obstacles confronting the carbon nanotube field being the lack of a detailed understanding of the nanotube growth mechanism and control of the synthesis process to produce nanotubes with a desired diameter and chirality. The brief review of the carbon nanotube field by leading experts in this issue comes at an opportune time. Many exciting results on the structural, electronic, optical, and transport properties of these tiny well-ordered structures have already been achieved, and the research is well enough developed to assess present progress and identify new research directions waiting to be explored.

High performance of cup-stacked-type carbon nanotubes as a Pt–Ru catalyst support for fuel cell applications

Abstract: The potential of cup-stacked-type carbon nanotubes (CSCNTs) as a catalyst support for the direct methanol fuel cells has been investigated by the electrochemical oxidation of methanol at various temperatures. The CSCNT-supported platinum–ruthenium (Pt–Ru) bimetallic catalyst exhibited twice as high a power density as the Pt–Ru catalyst supported on Vulcan XC-72 carbon, which is widely used as a catalyst support for the DMFC electrodes. The microscopic analysis of the CSCNT-supported Pt–Ru catalysts revealed that the bimetallic electrocatalysts were well dispersed on the CSCNT supports, and the particle size of the electrocatalysts was ca.5nm . The results of this work indicate that the performance of the carbon support materials is largely influenced by their electrical properties, morphology and crystallographic structures.

Structural systematics in boron-doped single wall carbon nanotubes

Abstract: We report atomic level high resolution transmission electron microscopy (HRTEM), electron nanodiffraction and nano-electron energy loss spectroscopy (nano-EELS) of boron-doped carbon nanostructures obtained by laser ablation of Co/Ni/B-doped carbon targets. The observations provide direct evidence for structural systematics and atomic structural defects as a function of the B content in the target. Targets with low B concentration (below 3 at%), produced ropes of single wall carbon nanotubes (SWCNTs) with no detectable boron present in the SWCNT ropes. However, unintended N-doping of the curved honeycomb lattice was observed in the 2.5 at% B sample (which is attributed to the possible presence of small amounts of N in the targets or reaction environments), with striking consequences for doping of heteroatoms within the hexagonal lattice of the graphene layer. At higher B concentration (3.5 at% and higher), there are significant changes in the nanostructure, which exhibits defective graphite layers and a small number of double wall carbon nanotubes (DWCNTs). At the higher B concentration, boron-doping is evidenced in the form of very small amorphous B clusters trapped in graphite-like defective sites.

Stokes and anti-Stokes Raman spectra of small-diameter isolated carbon nanotubes

Abstract: Department of Physics, Tohoku Universityand CREST, JST, Sendai 980-8578, Japan~Received 10 May 2003; revised manuscript received 29 September 2003; published 26 March 2004!By measuring the anti-Stokes~AS! and Stokes ~S! Raman spectra on the same isolated single-wall carbonnanotube ~SWNT!, we here determine the electronic transition energies E

Coalescence of double-walled carbon nanotubes: Formation of novel carbon bicables

Abstract: Here, we demonstrate that by coalescing double-walled carbon nanotubes (DWNTs), a novel and stable structure consisting of flattened tubules containing two single-walled tubes (SWNTs) is created. The process occurs due to the coalescence and reconstruction of the outer shells of DWNTs, leaving the inner cylinders almost intact, the latter being encapsulated inside the large diameter coalesced tube (bicable). We propose that the coalescence process is due to: (1) the thermal activity of the outer shells, which is driven by a surface energy minimization process, and (2) a zipping mechanism followed by atom reconstruction, in which two outer shells interact and anneal.

Quantitative evaluation of the octadecylamine-assisted bulk separation of semiconducting and metallic single-wall carbon nanotubes by resonance Raman spectroscopy

Abstract: The selective stabilization of octadecylamine (ODA) on semiconducting (S) single-wall carbon nanotubes (SWNTs) has been reported to provide a means for the bulk separation of S from metallic (M) SWNTs. Utilizing resonance Raman spectroscopy and, in particular, the relative changes in the integrated intensities of the radial-breathing mode region, a generic method has been developed to provide quantitative evaluation of the separation efficiency between M and S SWNTs along with diameter separation. The ODA-assisted separation is shown to provide S enrichment by a factor of 5 for SWNTs prepared by high pressure CO decomposition and greater S enrichment for SWNTs with diameters below 1nm.

Electrochemical gating of individual single-wall carbon nanotubes observed by electron transport measurements and resonant Raman spectroscopy

Abstract: Metal electrodes patterned lithographically on top of individual single-wall carbon nanotubes are used to gate the nanotubes with respect to a reference electrode in an electrolyte drop. The gating is found to have a dramatic effect on both the Raman spectra and electron transport of the nanotubes. Current through metallic nanotubes is found to increase sharply with electrochemical gate voltage, indicating that the Fermi energy reaches valence and conduction band van Hove singularities. Using resonant confocal micro-Raman spectroscopy, we observe a 9 cm−1 upshift of the tangential mode vibrational frequency, as well as a 90% decrease in intensity, by applying 1 V between an individual nanotube and a silver reference electrode in a dilute H2SO4 solution. The mechanisms for the shifts of the Raman mode frequencies are discussed on the basis of changes in the lattice constant of heavily charged nanotubes.

PVDC-Based Carbon Material by Chemical Activation and Its Application to Nonaqueous EDLC

Abstract: An organic solvent-type electrolytic solution was employed for application on an electric double-layer capacitor (EDLC) made of a polyvinylidene chloride (PVDC) based carbon material. Although the PVDC-based carbon material showed an excellent capacitance over a 100 F/g in an aqueous solvent-type electrolytic solution, it showed that a very small capacitance value was obtained in an organic solvent-type electrolyte solution, which did not even exceed the value of 5 F/g. It also confirmed the most effective temperature to get a capacitance shifted was from 700 to 900°C. At this time, chemical activation with KOH was tried for improving of the performance of the capacitance on PVDC-based carbon. The double-layer capacitance affected by variations of the pore size distribution was explained on the basis of the conventional pore analysis by means of gas adsorption. It was confirmed that the chemical activation with KOH was quite effective for improving capacitance. After the activation with KOH, especially by the 400 wt % addition, the capacitance obtained a value as high as 55 F/g. which is equivalent to 220 F/g for a conventional three-compartment system with a reference electrode. It was confirmed that the impregnation of KOH is effective for widening the pore diameter and affect the improvement of the capacitance. The effect of the functional groups on the surface of PVDC-carbons is also mentioned.

Applied physics Nanotube antennas

Abstract: An antenna array that is metres high and wide can detect and transmit radio waves. This effect has now been demonstrated at much smaller electromagnetic wavelengths in a nanoscale array of carbon nanotubes.

The preparation of carbon aerogels based upon the gelation of resorcinol–furfural in isopropanol with organic base catalyst

Abstract: Carbon aerogels with high BET surface area were developed by sol–gel polycondensation of resorcinol and furfural in isopropanol using hexamethylenetetramine (HMTA) as a catalyst, and then directly drying the organic gels under isopropanol supercritical conditions, followed by carbonization under a nitrogen atmosphere. The preparation conditions of carbon aerogels were explored by changing the mole ratio of resorcinol to basic catalyst HMTA (R/C), the ratio of resorcinol to isopropanol (R/I), and the mole ratio of resorcinol to furfural (R/F). The effect of these preparation conditions on the porous structure of the carbon aerogels obtained was studied by nitrogen adsorption isotherms. According to the characterizations of TEM, SEM and nitrogen adsorption, the carbon aerogels obtained have a three-dimensional network that consists of carbon nano-particles with size from 20 to 30 nm, which define numerous micropores, mesopores and macropores. HMTA reacts not only as a catalyst but also as a reagent in the gelation polymerization. XRD characterization indicates that carbon aerogels have disordered nanocrystalline structures similar to activated carbon.

Carbon nanotube photophysics

Abstract: In single-walled carbon nanotubes (SWNTs), their electronic and vibrational structure as well as their charge-carrier dynamics are crucial for potential ultrasmall optical device applications. SWNT properties have now been obtained from optical absorption and time-resolved photoemission and, at the single-nanotube level, by resonance Raman scattering and photoluminescence studies. This article presents an overview of SWNT photophysics, discussing important findings for the characterization of carbon nanotube properties and directions for future research and potential applications. The unique optical properties observed in SWNTs are due to the one-dimensional confinement of electronic states, resulting in van Hove singularities in the nanotube density of states. Optical measurements of phonons, charge-carrier dynamics, and the electronic transition energy van Hove singularities are discussed.

Environment effects on the Raman spectra of individual single-wall carbon nanotubes: Suspended and grown on polycrystalline silicon

Abstract: An enhanced Raman signal is observed from individual suspended single-wall carbon nanotubes (SWNTs) and from isolated SWNTs grown on an n-doped polycrystalline silicon film used in standard silicon processing. The radial breathing modes of the Raman spectra taken from suspended SWNTs exhibit narrow linewidths, which indicate a relatively unperturbed environment for suspended SWNTs. Clear Raman signals from intermediate frequency modes in the frequency range from 520to1200cm−1 are presented, which might allow a detailed study of the phonon band structure of individual SWNTs.

Surface-enhanced Raman scattering on single-wall carbon nanotubes.

Abstract: Exploiting the effect of surfaceenhanced Raman scattering (SERS), the Raman signal of singlewall carbon nanotubes (SWNTs) can be enhanced by up to 14 orders of magnitude when the tubes are in conta...

A step in synthesis.

Abstract: The extraordinarily high strength and stiffness of single-walled carbon nanotubes promises a myriad of unique applications, but many of these are reliant on the growth of ultralong, continuous nanotubes. A new synthetic procedure takes us a step closer to this goal.

Studies of the Chemical and Pore Structures of the Carbon Aerogels Synthesized by Gelation and Supercritical Drying in Isopropanol

Abstract: The carbon aerogels prepared by a new method through gelation and supercritical drying in isopropanol were characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, and a surface area analyzer. Their chemical structure, morphology, and pore structure are discussed. We found that all of these carbon aerogel (CA-IPA) samples have almost the same carbon and oxygen elemental states, as well as similar oxygen-containing groups. The curve fitting of the C1s XPS spectra of the samples for characterizing oxygen-containing surface groups can be performed by assuming the peak type to be a Gaussian–Lorentzian Cross Product, but we cannot obtain good results using a Gaussian lineshape. When the mass density of the CA-IPA decreases, the mesopores and macropores of the samples are found to grow, but the size and the shape of individual carbon nanoparticles in various CA-IPA samples do not apparently change. The micropore volume of the CA-IPA samples increases with a decrease in the mass density, while the mesopore volume has a maximum at a certain mass density. The CA-IPA samples have a very narrow micropore distribution at about 0.5 nm. The mesopore distribution of the CA-IPA is widened and the average pore size increases as the mass density of the sample decreases. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3060–3067, 2004

Raman Spectroscopy of Graphitic Foams

Abstract: To better understand the very high thermal conductivity to weight ratio of the graphitic foams recently developed at the Oak Ridge National Laboratory, a Raman spectroscopy study was performed. It was also shown that the Raman scattering can be useful for the characterization of the graphitic foam, being able to evaluate the quality of the samples with respect to the density and location of lattice defects.

Corrections to the Optical Transition Energies in Single-Wall Carbon Nanotubes of Smaller Diameters

Abstract: Optical spectroscopy characterization of carbon nanotube samples requires accurate determination of their band structure and exciton binding energies. In this paper, we present a non-orthogonal density-functional based tight-binding calculation for the electronic transition energies in single-wall carbon nanotubes. We show that the curvature-induced rehybridization of the electronic orbitals, long-range atomic interactions, and geometrical structure relaxation all have a significant impact on the electronic transition energies in the small diameter limit. After including quasiparticle corrections and exciton binding energies, the calculated electronic transition energies show good agreement with the experimental transition energies observed by photoluminescence and resonance Raman spectroscopy.

Resonance Raman Spectroscopy to Study and Characterize Defects on Carbon Nanotubes and other Nano-Graphite Systems

Abstract: The use of resonance Raman spectroscopy (RRS) to study and characterize single wall carbon nanotubes (SWNTs) is discussed, focusing on preliminary efforts for the development of the RRS to characterize defects in SWNTs. The disorder-induced D-band, disorder-induced peaks just above the first-order allowed graphite G-band, as well as the intermediate frequency modes (IFMs) appearing between the RBM and the D/G spectral region are addressed. RRS on nanographite ribbons and on a step-like defect in highly ordered pyrolytic graphite (HOPG) sheds light into the problem of characterizing specific defects in nano-related carbons.