Showing papers by "Mildred S. Dresselhaus published in 2000"

Phonons in carbon nanotubes

Abstract: A broad review of the unusual one-dimensional properties of phonons in carbon nanotubes is presented, including phonons in isolated nanotubes and in crystalline arrays of nanotubes in nanotube bundles. The main technique for probing the phonon spectra has been Raman spectroscopy and the many unique and unusual features of the Raman spectra of carbon nanotubes are reviewed. Also included is a brief review of the thermal properties of carbon nanotubes in relation to their unusual phonon dispersion relations and density of states.

Trigonal warping effect of carbon nanotubes

Abstract: Chirality-dependent van Hove singularities (vHs) of the one-dimensional electronic density of states (DOS) are discussed in connection with resonant Raman spectroscopy. The effect of trigonal warping on the energy dispersion relations near the Fermi energy splits the peaks of the density of states for metallic nanotubes, and the magnitude of this effect depends on the chiral angle of the carbon nanotube. The width of the peak splitting has a maximum for metallic zigzag nanotubes, and no splitting is obtained for armchair nanotubes or semiconducting nanotubes. We also find an additional logarithmic singularity in the electronic DOS for carbon nanotubes that is related to a two-dimensional singularity, which does not depend on either the diameter or the chirality.

Polarized raman study of aligned multiwalled carbon nanotubes

Abstract: Polarized Raman spectra of high purity aligned arrays of multiwalled carbon nanotubes, prepared on silica substrates from the thermal decomposition of a ferrocene-xylene mixture, show a strong dependence of the graphitelike G band and the disorder-induced D band on the polarization geometry employed in the experiments. The experimental G-band intensity exhibits a minimum at straight theta(m) = 55 degrees in the VV configuration, in good agreement with theoretical predictions of a characteristic minimum at 54.7 degrees for A(1g) modes in single wall nanotubes, where straight theta(m) denotes the angle between the polarization direction and the nanotube axis.

Polarized raman study of single-wall semiconducting carbon nanotubes

Abstract: Polarized Raman spectra were obtained from a rope of aligned semiconducting single-wall nanotubes (SWNTs) in the vicinity of the D band and the G band. Based on group theory analysis and related theoretical predictions, the G-band profile was deconvolved into four intrinsic SWNT components with the following symmetry assignments: 1549 cm(-1) [E-2(E-2g)], 1567 cm(-1) [A(A(1g)) + E-1(E-1g)], 1590 cm(-1) [A(A(1g)) + E-1(E-1g)] and 1607 cm(-1) [E-2(E-2g)]. The frequency shifts of the tangential G modes from the 2D graphitelike E-2g2 frequency are discussed in terms of the nanotube geometry.

Hydrogen Storage in Single-Walled Carbon Nanotubes at Room Temperature.

Abstract: Masses of single-walled carbon nanotubes (SWNTs) with a large mean diameter of about 1.85 nanometers, synthesized by a semicontinuous hydrogen arc discharge method, were employed for hydrogen adsorption experiments in their as-prepared and pretreated states. A hydrogen storage capacity of 4.2 weight percent, or a hydrogen to carbon atom ratio of 0.52, was achieved reproducibly at room temperature under a modestly high pressure (about 10 megapascal) for a SWNT sample of about 500 milligram weight that was soaked in hydrochloric acid and then heat-treated in vacuum. Moreover, 78.3 percent of the adsorbed hydrogen (3.3 weight percent) could be released under ambient pressure at room temperature, while the release of the residual stored hydrogen (0.9 weight percent) required some heating of the sample. Because the SWNTs can be easily produced and show reproducible and modestly high hydrogen uptake at room temperature, they show promise as an effective hydrogen storage material.

Aharonov-Bohm spectral features and coherence lengths in carbon nanotubes

Abstract: The electronic properties of carbon nanotubes are investigated in the presence of disorder and a magnetic field parallel or perpendicular to the nanotube axis. In the parallel field geometry, the ${\ensuremath{\varphi}}_{0}$-periodic $({\ensuremath{\varphi}}_{0}=hc/e)$ metal-insulator transition induced in metallic or semiconducting nanotubes is shown to be related to a chirality-dependent shifting of the energy of the van Hove singularities (VHS's). The effect of disorder on this magnetic-field-related mechanism is considered with a discussion of mean free paths, localization lengths, and magnetic dephasing rate in the context of recent experiments.

Novel Electronic Properties of a Nano-Graphite Disordered Network and Their Iodine Doping Effects.

Abstract: Activated carbon fibers (ACFs) are microporous carbons consisting of a three-dimensional disordered network of nano-graphites with a mean in-plane size of about 30 A. We investigated the structure, electronic properties and iodine doping effects for ACF samples heat-treated up to 2800° C. The samples heat-treated below 1000° C exhibit Coulomb gap variable-range hopping conduction and the presence of localized spins, suggesting the importance of charging effects and the edge-inherited non-bonding states in nano-graphites, the latter being predicted theoretically. Iodine doping reduces the charging effect due to the dielectric constant enhanced by the iodine that is accommodated in the micropores. Heat treatment above 1300° C changes ACFs from an Anderson insulator to a disordered metal by the development of an infinite inter-nano-graphite percolation path network for electron transport, accompanied by a change from localized-spin magnetism to itinerant electron magnetism. In the metallic regime, carrier sc...

The anomalous dispersion of the disorder-induced and the second-order Raman bands in carbon nanotubes

Abstract: In this work we have studied the dispersion of the disorder-induced (D) and the second-order (G') Raman bands in single wall carbon nanotubes using several laser excitation energies (E laser) in the range 1.5-3.0 eV. An anomalous step-like behavior was observed in the E laser dependence of the G'-band frequency. This result is interpreted as a manifestation of the one-dimensional (1D) behavior of the phonon spectrum in carbon nanotubes.

Comparison of power dependence of microwave surface resistance of unpatterned and patterned YBCO thin film

Abstract: The effect of the patterning process on the nonlinearity of the microwave surface resistance R/sub s/ of YBCO thin films is investigated in this paper. With the use of a sapphire dielectric resonator and a stripline resonator, the microwave R/sub s/ of YBCO thin films was measured before and after the patterning process, as a function of temperature and the RF peak magnetic field in the film. The microwave loss was also modeled, assuming a J/sub rf//sup 2/ dependence of Z/sub s/(J/sub rf/) on current density J/sub rf/. Experimental and modeled results show that the patterning has no observable effect on the microwave residual R/sub s/ or on the power dependence of R/sub s/.

Superlattice structures having selected carrier pockets and related methods

Abstract: A carrier pocket engineering technique used to provide superlattice structures having relatively high values of the three-dimensional thermoelectric figure of merit (Z3DT) is described. Also described are several superlattice systems provided in acordance with the carrier pocket engineering technique. Superlattice structures designed in accordance with this technique include a plurality of alternating layers of at least two different semiconductor materials. First ones of the layers correspond to barrier layers and second ones of the layers correspond to well layers but barrier layers can also work as well layers for some certain carrier pockets and vice-versa. Each of the well layers are provided having quantum well states formed from carrier pockets at various high symmetry points in the Brillouin zone of the structure to provide the superlattice having a relatively high three-dimensional thermoelectric figure of merit.

Nanofabrication Using Self-Assembled Alumina Templates

Abstract: A new approach for the use of porous alumina films as a template for nanofabrication is presented. In this process the porous films are prepared on silicon substrates, simplifying both the template fabrication and subsequent processing, and improving the quality of the films and their surfaces. Structural analysis of the film was carried out. Bismuth and bismuth telluride nanowires were prepared by pressure injection and electrochemical deposition, respectively, in alumina films 5-10 µ thick with parallel ordered pores 40 nm in diameter. The films were also patterned by lithography, offering new opportunities for area-selective anodization of non-planar structures. The new approach offers a straightforward method for the fabrication of arrays of nanostructures and their incorporation into electronic and optical devices.

4-Point Resistance Measurements of Individual Bi Nanowires

Abstract: : We have synthesized single crystal bismuth nanowires by pressure injecting molten Bi into anodic alumina templates. By varying the template fabrication conditions nanowires with diameters ranging from 10 to 200nm and lengths of approx. 50 microns can be produced. We present a scheme for measuring the resistance of a single Bi nanowire using a 4-point measurement technique. The nanowires are found to have a 7nm thick oxide layer which causes very high contact resistance when electrodes are patterned on top of the nanowires. The oxide is found to be resilient to acid etching, but can be successfully reduced in high temperature hydrogen and ammonia environments. The reformation time of the oxide in air is found to be less than 1 minute. Focused ion beam milling is attempted as an alternate solution to oxide removal.

Chemical Reaction of Intercalated Atoms at the Edge of Nano-Graphene Cluster

Abstract: The chemical reactions of halogen molecules on a nano-graphite cluster are calculated using a semi-empirical calculational method. A real time calculation of the chemical reaction of halogen molecules up to 200 fs which is performed by the dynamic reaction coordinates (DRC) method of the quantum chemistry library shows that the iodine molecule exhibits a special reaction involving the removal of hydrogen atoms from edge carbon atoms. This result might be relevant to recent experiments on the graphitization of pitch at a low temperature of 400°C. We discuss the deformation of the nano-graphite cluster upon the intercalation of halogen atoms.

Application of Image Analysis for TEM Image of Acceptor Graphite Intercalation Compound

Abstract: A new approach to study of micro texture and structure of graphite intercalation compounds (GICs) is shown using high resolution transmission electron microscopy (TEM) combined with image analysis and fuzzy reasoning As a result of application of the frequency analysis using 2-dimensional (2D) fast Fourier transform (FFT) to the CuCl2-GICs, a characteristic power spectrum pattern called streak, which is similar to an electron diffraction pattern, is obtained The images correspond to the specific frequencies are reconstructed by 2D inverse FFT (IFFT) Then the stage structure of CuCl2-GICs is investigated

Fundamental Properties of Fullerenes

Abstract: Fullerenes are cage-like all-carbon molecules which under special circumstances, such as exposure to high intensity photon or electron irradiation, ion plasma excitation, pressure, or doping, may form polymeric solids. This chapter reviews the properties of the fullerene molecules, the types of solids they normally form and their potential for forming polymeric solids. Fullerenes are also used to form fullerene-polymer composites, where their special molecular properties are exploited for specific applications.

Anodic Performance of Polyparaphenylene (PPP)-Based Carbons Heat-Treated at Various Temperatures

Abstract: The microstructure and electrochemical properties of polyparaphenylene (PPP)-based carbons prepared by the Kovacic and the Yamamoto methods have been comparatively studied after heat-treatment up to 3000°C. The results obtained by X-ray diffraction can clarify the specific electrochemical properties of PPP-based carbons and graphites.

Carrier Pocket Engineering for the Design of Low Dimensional Thermoelectrics with High Z3DT

Abstract: The concept of carrier pocket engineering applied to Si/Ge superlattices is tested experimentally. A set of strain-symmetrized Si(20A)/Ge(20A) superlattice samples were grown by MBE and the Seebeck coefficient S, electrical conductivity σ, and Hall coefficient were measured in the temperature range between 4K and 400K for these samples. The experimental results are in good agreement with the carrier pocket engineering model for temperatures below 300K. The thermoelectric figure of merit for the entire superlattice, Z3DT, is estimated from the measured S and σ, and using an estimated value for the thermal conductivity of the superlattice. Based on the measurements of these homogeneously doped samples and on model calculations, including the detailed scattering mechanisms of the samples, projections are made for δ-doped and modulation-doped samples [(001) oriented Si(20A)/Ge(20A) superlattices] to yield Z3DT ≈ 0.49 at 300K.

Structure and Anode Performance of Pristine and B-Doped Graphites for Li-ion Batteries

Abstract: The microstructure and electrochemical properties of pristine graphitized and boron-doped materials have been comparatively analyzed by x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), and electrochemical measurements. The electrochemical properties in a Li ion secondary battery of boron-doped graphitized materials depends strongly on the structural geometry and chemical composition of the pristine host materials.

Microstructure of Milled Mesophase Pitch-Based Carbon Fibers as an Anode Material for Li-ion Batteries

Abstract: The microstructure of milled mesophase pitch-based carbon fibers (mMPCFs) that have been developed as an anode material for Li ion batteries have been studied as a function of heat treatment temperature (HTT), by SEM, X-ray diffraction, and Raman spectroscopy. And the results obtained are compared with those by X-ray diffraction (XRD) and SEM observations, for the characterization of specific structural features of mMPCFs as a promising anode material.

Breit-Wigner-Fano Lineshape Analysis of the angential G -band of Metallic Nanotubes

Abstract: We present a resonant Raman study of the tangential G-band in metallic SWNTs. By measuring the Raman spectra for isolated SWNTs, we show that the two different lineshapes observed for semiconducting and metallic SWNTs in bundles also occur for isolated SWNTs. A lineshape analysis of the tangential G-band feature for metallic SWNT bundles is presented, showing that only two components are present, the higher frequency component having a Lorentzian lineshape, and the lower one having a Breit–Wigner–Fano (BWF) line-shape. Through comparisons of the Raman tangential G-band spectra from three different diameter distributions of carbon nanotubes, we find that both the frequency and linewidth of the BWF component are diameter dependent.

A Structural Analysis Method for Graphite Intercalation Compounds

Abstract: Study of the microstructure of electronic materials can be enhanced by using high resolution transmission electron microscopy (TEM) combined with the technique of digitized image analysis. We show here a practical image analysis method for the microstructures of acceptor graphite intercalation compounds (GICs) with CuCl2 and FeCl3 intercalates. The two dimensional fast Fourier transform (2D-FFT) was used for the frequency analysis of the TEM pictures. It is found that the lattice images of CuCl2-GICs consist of different frequency images corresponding to specific frequencies. The detailed features of the stage-1 structure of the FeCl3-GICs is extracted quantitatively by this method from a relatively indistinct TEM picture. The stage structure of the CuCl2- and FeCl3-GICs are further investigated by analyzing the reconstruction of the TEM images by means of the two dimensional inverse FFT (2D-IFFT).

Polarized Raman Spectra of Carbon Nanotubes

Abstract: P olarizedresonant Raman spectra for the G-band were obtained from a rope of aligned semiconducting SWNTs and from nonaligned semiconducting and metallic SWNTs. Based on group theory analysis and related theoretical predictions, we assign the symmetry for the modes in the G-band of both metallic and semiconducting SWNTs. he frequency shifts of the tangential G modes from the 2D graphite-like E2g2 frequency are discussed in terms of the nanotube geometry.