Showing papers by "Mildred S. Dresselhaus published in 2002"

Temperature dependence of radial breathing mode Raman frequency of single-walled carbon nanotubes

Abstract: Recent high-temperature studies of Raman-active modes in single-walled carbon nanotube (SWNT) bundles report a softening of the radial and tangential band frequencies with increasing sample temperature. A few speculations have been proposed in the past to explain the origin of these frequency downshifts. In the present study, based on experimental data and the results of molecular dynamics simulations, we estimate the contributions from three factors that may be responsible for the observed temperature dependence of the radial breathing mode frequency $[{\ensuremath{\omega}}_{\mathrm{RBM}}(T)].$ These factors include thermal expansion of individual SWNTs in the radial direction, softening of the C-C (intratubular) bonds, and softening of the van der Waals intertubular interactions in SWNT bundles. Based on our analysis, we find that the first factor plays a minor role due to the very small value of the radial thermal expansion coefficient of SWNTs. On the contrary, the temperature-induced softening of the intra- and intertubular bonds contributes significantly to the temperature-dependent shift of ${\ensuremath{\omega}}_{\mathrm{RBM}}(T).$ For nanotubes with diameters $(d)g~1.34\mathrm{nm},$ the contribution due to the radial thermal expansion is \ensuremath{\leqslant}4% over the temperature range used in this study. Interestingly, this contribution increases to \ensuremath{\geqslant}10% in the case of nanotubes having $dl~0.89\mathrm{nm}$ due to the relatively larger curvature of these nanotubes. The contributions from the softening of the intra- and intertubular bonds are approximately equal. These two factors together contribute a total of about \ensuremath{\sim}95% and 90%, respectively, for SWNTs having $dg~1.34\mathrm{nm}$ and \ensuremath{\leqslant}0.89 nm.

Single Nanotube Raman Spectroscopy

Abstract: A review is presented on the observation of the resonant Raman spectra from one isolated single wall carbon nanotube, focusing on the important structural information that is provided by single nanotube spectroscopy including the (n, m) determination of the individual tubes. The special sensitivity of the radial breathing mode to the (n, m) determination is emphasized, and the corroboration of this (n, m) assignment by diameter- and chirality-dependent phenomena in other Raman modes, such as the G-band, D-band, and G‘-band features is also discussed. The significance of single nanotube spectroscopy for future nanotube research in general is briefly reviewed.

Stokes and anti-Stokes double resonance Raman scattering in two-dimensional graphite

Abstract: In this work, we discuss all possible double resonance processes which explain the observed features of the disorder-induced D band and its overtone, the ${G}^{\ensuremath{'}}$-band, in Stokes (S) and anti-Stokes (AS) Raman spectra of two-dimensional graphite. It is shown that the D band is composed by two peaks,which are not associated with resonances with the incident and scattered photons, but rather are related to whether the first scattering of an electron is by a phonon or by a defect. The model explains the experimental results concerning the frequency shift of the D and ${G}^{\ensuremath{'}}$ bands in the S and AS spectra, as well as the fact that the ${G}^{\ensuremath{'}}$ band is not centered at two times the center of the D-band frequency.

Making electrical contacts to nanowires with a thick oxide coating

Abstract: Techniques are presented for making ohmic contacts to nanowires with a thick oxide coating. Although experiments were carried out on Bi nanowires, the techniques described in this paper are generally applicable to other nanowire systems. Metal electrodes are patterned to individual Bi nanowires using, electron beam lithography. Imaging the chemical reaction on the atomic scale with in situ high-resolution transmission electron microscopy shows that annealing in H-2 or NH3 can reduce the nanowires' oxide coating completely. The high temperatures required for this annealing, however, are not compatible with the lithographic techniques. Low-resistance ohmic contacts to individual bismuth nanowires are achieved using a focused ion beam (FIB) to first sputter away the oxide layer and then deposit Pt contacts. By combining electron beam lithography and FIB techniques, ohmic contacts stable from 2 to 400 K are successfully made to the nanowires. A method for preventing the burnout of nanowires from electrostatic discharge is also developed.

Synthesis and characterization of copper-doped carbon aerogels

Abstract: We have prepared carbon aerogels (CAs) doped with copper through sol−gel polymerization of formaldehyde with the potassium salt of 2,4-dihydroxybenzoic acid, followed by ion exchange with Cu(NO3)2,...

XPS Study of Copper-Doped Carbon Aerogels

Abstract: Copper-doped carbon aerogels were investigated by X-ray photoelectron spectroscopy to determine the chemical nature and distribution of the copper species in the aerogel framework. The Cu2p spectra of both the organic and carbon aerogels show a fairly uniform distribution of copper species in the aerogel network, with a slight increase in copper content going from the edge to the center of the monolith. The O1s spectra of the copper-doped organic aerogel indicate that both the carboxyl and hydroxyl groups of the aerogel framework are involved in chelation of the copper ions. After carbonization, the content of the copper detected by XPS decreases significantly as the copper ions are reduced into metallic copper nanoparticles. These nanoparticles are difficult to detect by XPS because they are coated by a thin carbon layer and migrate into the carbon matrix. The carbon skeleton of the copper-doped carbon aerogels is mainly composed of a uniform micro-graphite-like crystalline network, and no copper−carbon ...

Thick porous anodic alumina films and nanowire arrays grown on a solid substrate

Abstract: The presently disclosed invention provides for the fabrication of porous anodic alumina (PAA) films on a wide variety of substrates. The substrate comprises a wafer layer and may further include an adhesion layer deposited on the wafer layer. An anodic alumina template is formed on the substrate. When a rigid substrate such as Si is used, the resulting anodic alumina film is more tractable, easily grown on extensive areas in a uniform manner, and manipulated without danger of cracking. The substrate can be manipulated to obtain free-standing alumina templates of high optical quality and substantially flat surfaces. PAA films can also be grown this way on patterned and non-planar surfaces. Furthermore, under certain conditions, the resulting PAA is missing the barrier layer (partially or completely) and the bottom of the pores can be readily accessed electrically. The resultant film can be used as a template for forming an array of nanowires wherein the nanowires are deposited electrochemically into the pores of the template. By patterning the electrically conducting adhesion layer, pores in different areas of the template can be addressed independently, and can be filled electrochemically by different materials. Single-stage and multi-stage nanowire-based thermoelectric devices, consisting of both n-type and p-type nanowires, can be assembled on a silicon substrate by this method.

Intercalation Compounds of Graphite

Abstract: A broad review of recent research work on the preparation and the remarkable properties of intercalation compounds of graphite, covering a wide range of topics from the basic chemistry, physics and materials science to engineering applications.

First and Second-Order Resonance Raman Process in Graphite and Single Wall Carbon Nanotubes

Abstract: Resonant Raman scattering for one single wall carbon nanotube spectroscopy is overviewed. First-order resonance Raman spectra of the radial breathing mode of one carbon nanotube is of importance for assigning (n,m) values to the nanotube. This assignment of (n,m) values is confirmed by the chirality dependence of other phonon modes. Second-order, one-phonon emission, and the inter-valley scattering processes of two dimensional graphite and of single wall carbon nanotubes are relevant to disorder-induced D-band Raman spectra. The dispersive nature of the D-band Raman spectra is explained by double resonance processes. Many weak Raman spectra appearing in the intermediate frequency range, which have been observed for a long time but never assigned so far, have recently been assigned as double resonance Raman peaks. The second-order Raman phonon frequencies can be used as a new fundamental tool for determining the phonon energy dispersion relations, especially for disordered materials and for zone boundary phonons.

Bismuth Nanowires for Potential Applications in Nanoscale Electronics Technology

Abstract: Nanowires of bismuth with diameters ranging from 10 to 200 nm and lengths of 50 microm have been synthesized by a pressure injection method. Nanostructural and chemical compositional studies using environmental and high resolution transmission electron microscopy with electron stimulated energy dispersive X-ray spectroscopy have revealed essentially single crystal nanowires. The high resolution studies have shown that the nanowires contain amorphous Bi-oxide layers of a few nanometers on the surface. In situ environmental high resolution transmission electron microscopy (environmental-HRTEM) studies at the atomic level, in controlled hydrogen and other reducing gas environments at high temperatures demonstrate that gas reduction can be successfully applied to remove th oxide nanolayers and to maintain the dimensional and structural uniformity of the nanowires, which is key to attaining low electrical contact resistance.

Thermoelectric transport properties of single bismuth nanowires

Abstract: We present a novel technique for measuring the power factor (Seebeck coefficient and resistivity) of a single isolated Bi nanowire. Electron beam lithography is used to pattern electrodes on top of a 40 nm diameter Bi nanowire along with a microscopic heater and thermocouples. Running current through the heater generates a temperature gradient of 0.5 K over a distance of 10 /spl mu/m across the nanowire. Measurement of the Seebeck voltage of the nanowires was not possible due to the highly resistive and non-ohmic contacts. The non-linearity in the i(V) characteristics of the electrical contacts are understood by detailed modeling of the tunneling mechanism made through the electrical contacts. The prospect of using the poor contacts to perform tunneling spectroscopy of the electronic density of states of the nanowires is evaluated using this model.

Arrays of nanowires on silicon wafers

Abstract: Nanowires made of thermoelectric-relevant materials were grown in the pores of alumina templates fabricated on silicon wafers. This architecture combines the nanometer-scale, self-assembly nature of the anodic alumina with the micro-scale, versatile nature of integrated circuits processing. The nanowires can be made by the pressure injection technique, and even more conveniently by electrochemical deposition. The geometry is adequate for 2-point transport measurements on the nanowire arrays, and for fabrication of nanowire-based devices made of several materials and several components. In this context, a fabrication scheme for a thermoelectric device, containing both n-type and p-type legs, is suggested.

Raman Analysis of Carbon Nanotube Bundles for Bio-electronic Applications

Abstract: Three different types of carbon nanotubes being considered for bio-recognition experiments were studied using resonance Raman spectroscopy. Raman spectroscopy, taken using several laser excitation energies, has been shown to provide an effective characterization tool for these carbon nanotubes. The technique yields structural information that both complements and corroborates structural information obtained using electron microscopy techniques, such as TEM, SEM, and AFM.

Tem Observation of Metal-Loaded Carbon Aerogels Prepared by an Ion-Exchange Method

Abstract: Cu-loaded carbon aerogel was prepared by soaking sol-gel production, obtained from K-salt of 2,4-dihydroxybenzoic acid, into Cu(NO 3 ) 2 solution. XPS measurement demonstrated that Cu concentration is higher in the outside region of bulk samples, indicating that Cu may be incorporated through an immersing process rather than ion-exchanging. TEM observation showed that, like an aerogel sample with K ions, Cu-loaded organic aerogels had porous texture in which gel particles about 20-30nmo were interconnected with each other. Metal-loading process used in this study was considered as a promising method to disperse metals without changing original structure of carbon aerogels.

Transport Properties of Superlattice Nanowires and Their Potential for Thermoelectric Applications

Abstract: A theoretical model for the electronic structure and transport properties of superlattice (SL) nanowires is presented, based on the electronic tunneling between quantum dots. Due to the periodic potential perturbation, SL nanowires exhibit unusual features in the electronic density of states that are absent in homogeneous nanowires. Transport property calculations of PbSe/PbS SL nanowires are presented, showing improved thermoelectric performance compared to homogeneous nanowires because of a lower lattice thermal conductivity and an enhanced Seebeck coefficient, indicating that SL nanowires are promising systems for thermoelectric applications.

Chirality dependent G-band Raman intensity of an individual single wall carbon nanotube

Abstract: The chirality-dependent G-band Raman intensity of an individual single wall carbon nanotube is presented both by a non-resonant theory for the Raman tensor and by confocal micro-Raman measurements. Theory predicts six or three intense Raman modes, respectively, for chiral or achiral nanotubes whose relative intensities depend on the chiral angle of the nanotube.

New effects in the resonance Raman features in one‐dimensional systems: isolated single‐wall carbon nanotube studies

Abstract: New experimental results on the resonance Raman spectra of isolated single wall carbon nanotubes (SWNTs) are reported and are explained in terms of the one‐dimensional (1D) physics of SWNTs. The lineshape of both the non‐dispersive and dispersive Raman features depend on (n,m) and Elaser. In addition to the structural (n,m) information about isolated SWNTs, the lineshape analysis of the resonance Raman features of isolated SWNTs gives detailed information not otherwise available about the 1D electronic structure of an isolated SWNT.

Characterization of nanographite and carbon nanotubes by polarization dependent optical spectroscopy

Abstract: The optical absorption for ? electrons as a function of the electron wavevector k is investigated by first order perturbation theory in graphite and single wall carbon nanotubes (SWNTs). The matrix element connecting two states in the valence and conduction bands is found to be significantly anisotropic in k-space and polarization dependent. In the case of graphite, the absorption shows a node around the equi-energy contour, and in the case of SWNTs we obtain selection rules that allow only transitions between certain pairs of subbands. The strength of the optical absorption is not only diameter dependent but also chirality dependent. The implications of the optical absorption matrix element on the resonant conditions are discussed.

Anisotropy in the Phonon Dispersion Relations of Graphite and Carbon Nanotubes Measured by Raman Spectroscopy

Abstract: The possible semiconducting device use of single wall carbon nanotubes (SWNTs) requires a technique for the determination of the exact structure of the nanotubes assembled in the device configuration. Raman spectroscopy has been established as a precise and non-destructive tool for the characterization of graphitic nanostructures. Double resonance theory, which is used to explain the dispersive nature of the Raman bands, has attracted much attention for its potential use for the characterization of the electronic and phonon spectra of these nanostructures. Dispersive features in the Raman spectra of low dimensional graphitic materials, such as carbon nanotubes, can be used to measure directly the anisotropy, or the trigonal warping effect, in the phonon dispersion relations about the hexagonal corner of the Brillouin zone (BZ) of graphite.

Thermoelectric Nanowires By Template Synthesis: Fabrication, Contacts and Properties

Abstract: Using the technique we have developed to grow porous anodic alumina (PAA) templates on the surface of silicon wafers, we have fabricated arrays of nanowires of thermoelectric materials. By this method we can control both (1) the in-plane geometry by the design of the template and the substrate, and (2) the out-of-plane dimension by control over the electrochemical nanowire growth process. We use several straightforward methods to make electrical contacts to the nanowires. Our transport studies show that both the structure of the nanowire and the contact region have a strong influence on the observed properties of the arrays.

Transport Properties and Observation of Semimetal-Semiconductor Transition in Bi-based Nanowires

Abstract: Temperature-dependent resistance measurements of Bi-related nanowire arrays with different wire diameters and Sb concentrations are performed. The variation in the measured R(T) curves of these nanowires is closely related to the unique semimetal-semiconductor transition in Bi, and the results are explained by theoretical simulations. It is found that the special feature of the maximum in the resistance ratio R(10 K)/R(100 K) can be employed to experimentally identify the conditions for the semimetal-semiconductor transition.

Analysis of Stacking Structure of Cup-Stacked Type Carbon Nanofibers

Abstract: The lattice images of the new type (cup-stacked type) carbon nanofibers were observed by high resolution transmission electron microscopy (HRTEM) and the detailed structure was analyzed by image processing. The angle of opposed graphitic basal planes was measured from power spectrum obtained by 2 dimensional first Fourier transform. The structures of the slanted (00·1) lattice planes, which appear at the cross section of the fiber wall, were shown by means of this image analysis technique. Defects were observed in the ordered structure of this material. The (00·1) lattice planes show large fluctuation in alignment of them and the (10·0) lattice planes make several angles with the (00·1) lattice planes. The average repeat distance d 002 was estimated on the basis of d 100 by HRTEM image and image analysis.