Showing papers by "Mildred S. Dresselhaus published in 2005"

Thermal conductivity of simple and tubular nanowire composites in the longitudinal direction

Abstract: This work establishes a generic model to study phonon transport and the thermal conductivity of periodic two-dimensional nanocomposites in the longitudinal direction (along the wire axis direction) More specifically, the generic model is applied to study the thermal conductivity of silicon-germanium composites with simple silicon nanowire and tubular silicon nanowire inclusions in a germanium matrix, and cylindrical nanoporous silicon materials The results show that the effective thermal conductivity changes not only with the volumetric fraction of the constituents but also with the radius of the nanowires and cylindrical pores due to the nature of the ballistic phonon transport The smaller the wire/pore diameter, the smaller is the thermal conductivity of the periodic two-dimensional nanocomposites for a given volumetric fraction Composites with tubular nanowire inclusions have a lower effective thermal conductivity than simple nanowire composites due to the introduction of additional surface scattering through the pores associated with tubular nanowires Results of this study can be used to direct the development of both high-efficiency thermoelectric materials and thermal interface material containing high-thermal-conductivity particle or wire inclusions

Thermal conductivity modeling of core-shell and tubular nanowires.

Abstract: The heteroepitaxial growth of crystalline core-shell nanostructures of a variety of materials has become possible in recent years, allowing the realization of various novel nanoscale electronic and optoelectronic devices. The increased surface or interface area will decrease the thermal conductivity of such nanostructures and impose challenges for the thermal management of such devices. In the meantime, the decreased thermal conductivity might benefit the thermoelectric conversion efficiency. In this paper, we present modeling results on the lattice thermal conductivity of core-shell and tubular nanowires along the wire axis direction using the phonon Boltzmann equation. We report the dependence of the thermal conductivity on the surface conditions and the core-shell geometry for silicon core-germanium shell and tubular silicon nanowires at room temperature. The results show that the effective thermal conductivity changes not only with the composition of the constituents but also with the radius of the nanowires and nanopores due to the nature of the ballistic phonon transport. The results in this work have implications for the design and operation of a variety of nanoelectronic devices, optoelectronic devices, and thermoelectric materials and devices.

Formation of graphitic structures in cobalt- and nickel-doped carbon aerogels.

Abstract: We have prepared carbon aerogels (CAs) doped with cobalt or nickel through sol−gel polymerization of formaldehyde with the potassium salt of 2,4-dihydroxybenzoic acid, followed by ion exchange with M(NO3)2 (where M = Co2+ or Ni2+), supercritical drying with liquid CO2, and carbonization at temperatures between 400 and 1050 °C under a N2 atmosphere. The nanostructures of these metal-doped carbon aerogels were characterized by elemental analysis, nitrogen adsorption, high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Metallic nickel and cobalt nanoparticles are generated during the carbonization process at about 400 and 450 °C, respectively, forming nanoparticles that are ∼4 nm in diameter. The sizes and size dispersion of the metal particles increase with increasing carbonization temperatures for both materials. The carbon frameworks of the Ni- and Co-doped aerogels carbonized below 600 °C mainly consist of interconnected carbon pa...

Electron-phonon matrix elements in single-wall carbon nanotubes

Abstract: We have developed the electron-phonon matrix element in single-wall carbon nanotubes by using the extended tight-binding model based on density functional theory. We calculate this matrix element to study the electron-phonon coupling for the radial breathing mode (RBM) and the $G$-band $A$ symmetry modes of single-wall carbon nanotubes. Three well-defined family patterns are found in the RBM, longitudinal optical (LO) mode and transverse optical (TO) mode. We find that among the RBM, LO, and TO modes, the LO mode has the largest electron-phonon interaction. To study the electron-phonon coupling in the transport properties of metallic nanotubes, we calculate the relaxation time and mean free path in armchair tubes. We find that the LO mode, ${A}_{1}^{\ensuremath{'}}$ mode, and one of the ${E}_{1}^{\ensuremath{'}}$ modes give rise to the dominant contributions to the electron inelastic backscattering by phonons. Especially, the off-site deformation potential gives zero matrix elements for ${E}_{1}^{\ensuremath{'}}$ modes while the on-site deformation potential gives rise to nonzero matrix elements for the two ${E}_{1}^{\ensuremath{'}}$ modes, indicating that the on-site deformation potential plays an important role in explaining the experimentally observed Raman mode around $2450\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in carbon.

Nanocomposites with high thermoelectric figures of merit

Abstract: The present invention is generally directed to nanocomposite thermoelectric materials that exhibit enhanced thermoelectric properties. The nanocomposite materials include two or more components, with at least one of the components forming nanosized structures within the composite material. The components are chosen such that thermal conductivity of the composite is decreased without substantially diminishing the composite’s electrical conductivity. Suitable component materials exhibit similar electronic band structures. For example, a band-edge gap between at least one of a conduction band or a valence band of one component material and a corresponding band of the other component material at interfaces between the components can be less than about 5kBT, wherein kB is the Boltzman constant and T is an average temperature of said nanocomposite composition.

Phonon-assisted excitonic recombination channels observed in DNA-wrapped carbon nanotubes using photoluminescence spectroscopy.

Abstract: By using a sample of DNA-wrapped single-wall carbon nanotubes strongly enriched in the $(6,5)$ nanotube, photoluminescence emissions observed at special excitation energy values were identified with specific mechanisms of phonon-assisted excitonic absorption and recombination processes associated with $(6,5)$ nanotubes, including one-phonon, two-phonon, and some continuous-luminescence processes. Such detailed processes are not separately identified in three-dimensional semiconducting materials. A general theoretical framework is presented to interpret the experimentally observed phonon-assisted processes in terms of excitonic states.

High Energy-Density Capacitor Based on Ammonium Salt Type Ionic Liquids and Their Mixing Effect by Propylene Carbonate

Abstract: Novel ionic liquids comprised of a quaternary ammonium salt type cation have been applied to an electrolyte for high-performance electric double-layer capacitors (EDLCs). The novel ionic liquids [IL-B; N, N-diethyl-N-methyl(2-methoxyethyl)ammonium tetrafluoroborate (DEME-BF 4 ) and IL-T; N, N-diethyl-N-methyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide (DEME-TFSI)] are promising candidates for EDLC electrolytes in terms of high decomposition voltage (wide voltage window), nonflammability, easy handling, nonvolatility, and low production costs. Notably, the wide voltage window indicates that IL-B and IL-T are more advantageous in energy density than typical propylene carbonate-based electrolytes (i.e., TEA-BF 4 /PC) and a conventional imidazolium type ionic liquid (i.e., 1-ethyl-3-methylimidazolium tetrafluoroborate, EMI-BF 4 ). The effectiveness of IL-B and IL-T on the application to EDLC electrolytes has been confirmed by using KOH-activated mesophase pitch-based carbon fibers (MPCFs) as an electrode material. The combination of IL-T (IL-B) and KOH-activated MPCFs has provided 56 F/g (51 F/g) of high specific capacitance at maximum (1 mA/cm 2 discharge current density, 3.5 V charging voltage), which is equivalent to 224 F/g (204 F/g) in a conventional three-compartment measuring system. In addition, the specific capacitance of both ionic liquids has increased proportional to the increase in the applied voltage from 2.5 to 3.5 V, in contrast to the decline observed for TEA-BF 4 /PC at 3.5 V. Furthermore, the mixture of the IL-B exhibiting high viscosity with propylene carbonate (1 M of IL-B in PC) has been found to provide an excellent capacitance behavior comparable to that observed for the pure IL-B. This indicates that the mixture has great potential for application to EDLC electrolytes, similar to pure IL-B and IL-T.

Progressive and invasive functionalization of carbon nanotube sidewalls by diluted nitric acid under supercritical conditions

Abstract: Distinctive direct sidewall functionalization of multi-wall carbon nanotubes (MWCNTs) has been carried out using dilute nitric acid (HNO3) under supercritical water (SCW) conditions. The functionalization proceeded invasively from the outer to the inner graphitic layers of the MWCNTs as the reaction progressed. The resulting nanotube-derived material was comprised of a functionalized amorphous carbon sheath and the remaining inner nanotube covered with the sheath. The functional groups induced in the sheath region included alcoholic hydroxyl groups, which endowed the nanotube-derived material with hygroscopicity. The reaction pathways for the functional group formation involved hydration and dehydration processes as well as HNO3 oxidation in the SCW medium. The results of this work have demonstrated the effectiveness of the SCW regime on the direct chemical modification of CNT sidewalls.

Intensity of the resonance Raman excitation spectra of single-wall carbon nanotubes

Abstract: The electron-phonon matrix elements are calculated for the radial breathing mode (RBM) and the $G$-band $A$ symmetry mode of single-wall carbon nanotubes. The RBM intensity decreases with increasing nanotube diameter and chiral angle. The RBM intensity at van Hove singular $k$ points is larger outside the two-dimensional Brillouin zone around the $K$ point than inside the Brillouin zone. For the $G$ band $A$ symmetry mode, the matrix element shows that all semiconducting nanotubes have nonzero LO mode intensity, and the LO mode generally has a larger intensity than the TO mode, while the ratio of the intensity of the LO mode to that of the TO mode decreases with increasing chiral angle. In particular, zigzag nanotubes have zero intensity for the TO mode, and armchair nanotubes have zero intensity for the LO mode. Using the matrix elements thus obtained, the resonance Raman excitation profiles are calculated for nanotube samples under different broadening factor $\ensuremath{\gamma}$ regimes. For semiconducting nanotubes, the excitation profiles for the RBM are consistent with experiments. For metallic nanotubes, a quantum interference effect in the Raman intensity is found for both the RBM and LO modes. For the RBM and LO modes, different kinds of excitation profiles are discussed for nanotube samples in the large and small $\ensuremath{\gamma}$ regimes by considering the electron-phonon matrix element and the trigonal warping effect. For nanotube samples in the large $\ensuremath{\gamma}$ regime, a shift in the energy of the peak in the RBM intensity relative to the corresponding peak in the joint density of states is found.

Synthesis and characterization of single-wall-carbon-nanotube-doped emeraldine salt and base polyaniline nanocomposites

Abstract: Nanocomposites of polyaniline (PANI) and single-wall carbon nanotubes (SWNTs) were prepared and characterized via resonance Raman and electronic absorption spectroscopy (ultraviolet–visible/near-infrared). The chemical synthesis of PANI was performed in the presence of SWNTs in concentrations ranging from 10 to 50 wt % (SWNT/PANI). The obtained materials were hydrophilic, homogeneous composite compounds. The chemical interaction between PANI (in the conducting emeraldine salt form and in the insulating emeraldine base form) and metallic and semiconducting nanotubes was investigated. The emeraldine salt form of the polymer was significantly stabilized in the composite in comparison with plain PANI. A selective electronic interaction process between PANI and metallic SWNTs was found. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 815–822, 2005

Photoexcited electron relaxation processes in single-wall carbon nanotubes

Abstract: The relaxation processes for photoexcited electrons in single-wall carbon nanotubes (SWNTs) are discussed in connection with recent photoluminescence excitation (PLE) spectroscopy experiments. The electron-phonon $(e\text{\ensuremath{-}}\mathrm{ph})$ interaction of SWNTs is calculated for each phonon mode as a function of photoexcited electron energy. Because of the cylindrical surface of a SWNT, the twisting and radial breathing phonon modes are responsible for the $e\text{\ensuremath{-}}\mathrm{ph}$ interaction in the lower phonon energy region. Optic phonon modes are also responsible for the relaxation of photoexcited electrons. Compared with graphite, the relaxation of electrons is much faster in SWNTs because of their cylindrical shape. We identify and explain some aspects of both photoluminescent and Raman processes appearing in PLE plots for SWNTs.

Metal-doped semiconductor nanoparticles and methods of synthesis thereof

Abstract: The present invention generally relates to binary or higher order semiconductor nanoparticles doped with a metallic element, and thermoelectric compositions incorporating such nanoparticles. In one aspect, the present invention provides a thermoelectric composition comprising a plurality of nanoparticles each of which includes an alloy matrix formed of a Group IV element and Group VI element and a metallic dopant distributed within the matrix.

Phonon-assisted exciton relaxation dynamics for a (6,5)-enriched DNA-wrapped single-walled carbon nanotube sample

Abstract: A series of nondegenerate pump-probe measurements were carried out on a DNA-wrapped single-walled carbon nanotube (SWNT) sample that is enriched with the (6,5) species. The pump pulse excites SWNTs at $\ensuremath{\sim}1.567\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, which corresponds to an energy of two $D$-band phonons above the excitonic band edge of the (6,5) SWNT. The dynamics of different channels of exciton relaxation for the (6,5) SWNT is analyzed in terms of the decay in the time-resolved differential transmission spectra. By systematically varying the values of the probe energy, ${E}_{\text{probe}}$, to be in and out of resonance with the minority $(n,m)$ SWNT species in the sample at different pump fluence levels, an intermediate decay component associated with a hot phonon-absorption process is studied in detail. When the values of ${E}_{\text{probe}}$ were further tuned to off-resonance positions, photo-induced absorption processes could be investigated.

Addressing Grand Energy Challenges through Advanced Materials

Abstract: The following article is based on the plenary presentation given by Mildred S. Dresselhaus of the Massachusetts Institute of Technology on November 29, 2004, at the Materials Research Society Fall Meeting in Boston. Advanced materials offer new promise for addressing some of the grand societal challenges of our future, including that of global energy. This article will review opportunities that have opened up at the nanoscale, with materials of reduced dimensionality and enhanced surface-to-volume ratio. Some examples of research accomplishments and opportunities at the nanoscale will be described, with special attention given to the potential for advanced materials and nanoscience to have an impact on the grand challenges related to a sustainable energy supply for the 21st century and beyond.

Semiconducting Carbon Nanotubes

Abstract: The use of Raman spectroscopy to reveal the remarkable structure and properties of carbon nanotubes is briefly reviewed. Particular emphasis is given to the fact that a nanotube can be semiconducting or metallic depending on its diameter dt and chirality θ, and how Raman spectroscopy at the single nanotube level reveals such information. Some of the implications of the unusual properties of carbon nanotubes are summarized. It is shown how the vibrational spectra of one tiny tube, only about 1 nm in diameter, can be observed experimentally. Raman spectroscopy normally measures vibrational frequencies. What is unique about carbon nanotubes is that for this one‐dimensional system, resonance Raman spectroscopy (RRS) also determines, in addition, the geometrical structure of the resonant nanotube, that is its diameter and chirality. Some of the recent advances in single nanotube spectroscopy are briefly discussed. The use of RRS for characterizing carbon nanotube samples is also discussed. The connection betwe...

Resonance Raman spectroscopy characterization of single-wall carbon nanotube separation by their metallicity and diameter.

Abstract: Several techniques were recently reported for the bulk separation of metallic (M) and semiconducting (S) single wall carbon nanotubes (SWNTs), using optical absorption and resonance Raman spectroscopy (RRS) as a proof of the separation. In the present work, we develop a method for the quantitative evaluation of the M to S separation ratio, and also for the SWNT diameter selectivity of the separation process, based on RRS. The relative changes in the integrated intensities of the radial-breathing mode (RBM) features, with respect to the starting material, yield the diameter probability distribution functions for M and S SWNTs in the separated fractions, accounting for the different resonance conditions of individual SWNTs, while the diameter distribution of the starting material is obtained following the fitting procedure developed by Kuzmany and coworkers. Features other than the RBM are generally less effective for characterization of the separation process for SWNTs.

Methods for synthesis of semiconductor nanocrystals and thermoelectric compositions

Abstract: The present invention provides methods for synthesis of IV–VI nanostructures, and thermoelectric compositions formed of such structures. In one aspect, the method includes forming a solution of a Group IV reagent, a Group VI reagent and a surfactant. A reducing agent can be added to the solution, and the resultant solution can be maintained at an elevated temperature, e.g., in a range of about 20° C. to about 360° C., for a duration sufficient for generating nanoparticles as binary alloys of the IV–VI elements.

Carbon Nanotubes and Bismuth Nanowires

Abstract: 3.

Structural Variations in Boron-Doped Single Wall Carbon Nanotubes

Abstract: Extended abstract of a paper presented at Microscopy and Microanalysis 2005 in Honolulu, Hawaii, USA, July 31--August 4, 2005

Carbon nanotube photo-physics

Abstract: A review is given of how resonance Raman spectroscopy (RRS) and photoluminescence (PL) can be used to reveal unique information about nanostructures, 1nm in diameter, thus providing new techniques for probing the electronic and vibrational properties of nanostructures with particular regard to nanosensing applications. Special attention is given to recent advances made in this field and to perspectives about future research directions.

A Band Structure Phase Diagram Calculation of 2D BiSb Films

Abstract: Ever since the birth of thermoelectrics, it has been well known that semiconductors (materials with a relative small bandgap) give the best thermoelectric performance. From quantum mechanics, it is also well known that low dimension quantum confinement leads to changes in the band alignment of a material. Thus, a semimetallic material can be made semiconducting by using low dimensionality quantum confinement effects. BiSb alloys have been of particular interest for thermoelectric application in the temperature range of 70K to 100K. In bulk form, BiSb alloys can either be semimetal or semiconductor, depending on the alloy composition. Moreover, semimetallic BiSb alloys can be made semiconducting by using the low dimensionality quantum confinement concept. With these two previous concepts in mind, it is valuable to further explore the dependence of the band alignment for different alloy concentrations and different confinement conditions for BiSb alloys. Following the study of the effect of the Sb concentration and of the wire diameter on the semimetallic or semiconducting phase of BiSb alloy nanowires, we now examine the corresponding effect of the Sb concentration and the film thickness on the properties of BiSb alloy films. A band structure phase diagram is calculated, giving the details on the dependence of the relative band edge position on the film thickness and the Sb concentration. This phase diagram gives a first hand guideline for choosing the film thickness and the Sb concentration to better improve the thermoelectric performance of BiSb alloy films.

History and structure in carbon nanotube

Abstract: Carbon nanotubes, which consist of rolled graphene layers built from sp 2 carbon units have attracted the attention of scientists, not only from a fundamental point of view but also from technoLogical grounds. Numerous physicochemical properties of carbon nanotubes, such as their electronic, mechanical, optical and chemical characteristics, arise from the quantum confinement of states in the circumferential direction of the cylinders. In this paper, a historical review and basic structure of carbon nanotubes are presented briefly.

Thermoelectric Modeling of Si-Si1-xGex Ordered Nanowire Composites

Abstract: Thermoelectrics have always been attractive for power generation and cooling because of power reliability and environmentally friendly issues. However, this concept remains non-competitive due to the limitation in the efficiency of available thermoelectric materials and device designs [1]. In the 1990s, Hicks and Dresselhaus predicted the possibility of a dramatic enhancement in thermoelectric performance based on the special behavior of low dimensional materials [2, 3]. This enhancement is in part due to the increase in quantum confinement effects, the increase in electronic density of states at specified energies, and the increase in the phonon interface scattering for low dimensional structures. Nanowires and core-shell nanowires can be considered to be model systems to illustrate representative behavior in low dimensional thermoelectric materials. It is expected that a system made out of nanowires or core-shell nanowires would have a higher thermoelectric performance than its bulk counterpart due to an increase in the number of interfaces. The interfaces that are introduced must be such that phonons are scattered more strongly than are electrons. Theoretical studies have been carried out to better understand the transport properties of Si-Si1−xGex ordered nanowire composites. The composite is modeled as having Si wires embedded in a Si1−xGex host matrix. Thus, core-shell Si/Si1−xGex nanowires can be considered as a building block of the composite. The effect of the wire diameter and the shell alloy composition on ZT is presented.

Effects of strain on the resonant Raman profile of metallic

Abstract: In this paper we report the effects of strain on the electronic properties of single wall carbon nanotubes and its consequence on the resonant Raman cross section. A quantum interference effect has been predicted for the radial breathing mode spectra for metallic tubes. For metallic tubes, the lower and upper components ofEii resulting from the trigonal warping effect are affected differently and for low chiral angle they cross for some strain value. Near (at) the crossing point, the resonant Raman spectra profile exhibits a maximum (minimum) value due to a quantum interference in the Raman cross section. This Raman cross section interference effect was observed in Raman experiments carried out on isolated SWNTs. The Raman experiment performed on an isolated strained metallic SWNT supports our modeling predictions.