Showing papers by "Mildred S. Dresselhaus published in 2007"
TL;DR: In this review, experimental results for the D, D' and G' bands obtained with different laser lines, and in samples with different crystallite sizes and different types of defects are presented and discussed.
Abstract: Raman spectroscopy has historically played an important role in the structural characterization of graphitic materials, in particular providing valuable information about defects, stacking of the graphene layers and the finite sizes of the crystallites parallel and perpendicular to the hexagonal axis Here we review the defect-induced Raman spectra of graphitic materials from both experimental and theoretical standpoints and we present recent Raman results on nanographites and graphenes The disorder-induced D and D′ Raman features, as well as the G′-band (the overtone of the D-band which is always observed in defect-free samples), are discussed in terms of the double-resonance (DR) Raman process, involving phonons within the interior of the 1st Brillouin zone of graphite and defects In this review, experimental results for the D, D′ and G′ bands obtained with different laser lines, and in samples with different crystallite sizes and different types of defects are presented and discussed We also present recent advances that made possible the development of Raman scattering as a tool for very accurate structural analysis of nano-graphite, with the establishment of an empirical formula for the in- and out-of-plane crystalline size and even fancier Raman-based information, such as for the atomic structure at graphite edges, and the identification of single versus multi-graphene layers Once established, this knowledge provides a powerful machinery to understand newer forms of sp2 carbon materials, such as the recently developed pitch-based graphitic foams Results for the calculated Raman intensity of the disorder-induced D-band in graphitic materials as a function of both the excitation laser energy (Elaser) and the in-plane size (La) of nano-graphites are presented and compared with experimental results The status of this research area is assessed, and opportunities for future work are identified
3,601 citations
TL;DR: In this article, the ability to achieve a simultaneous increase in the power factor and a decrease in the thermal conductivity of the same nanocomposite sample and for transport in the same direction is discussed.
Abstract: Many of the recent advances in enhancing the thermoelectric figure of merit are linked to nanoscale phenomena found both in bulk samples containing nanoscale constituents and in nanoscale samples themselves. Prior theoretical and experimental proof-of-principle studies on quantum-well superlattice and quantum-wire samples have now evolved into studies on bulk samples containing nanostructured constituents prepared by chemical or physical approaches. In this Review, nanostructural composites are shown to exhibit nanostructures and properties that show promise for thermoelectric applications, thus bringing together low-dimensional and bulk materials for thermoelectric applications. Particular emphasis is given in this Review to the ability to achieve 1) a simultaneous increase in the power factor and a decrease in the thermal conductivity in the same nanocomposite sample and for transport in the same direction and 2) lower values of the thermal conductivity in these nanocomposites as compared to alloy samples of the same chemical composition. The outlook for future research directions for nanocomposite thermoelectric materials is also discussed.
3,562 citations
TL;DR: In this paper, a review summarizes the effects that nanotechnology can have on the main properties of metal hydrides and highlights the main competing behaviours between the system requirements, the necessary trade-offs, and the research priorities necessary to obtain hydride storage materials for practical automotive applications.
Abstract: Hydrogen is considered a good energy carrier candidate for future automotive applications that could be part of a carbon-free cycle. Metal hydrides are often preferred over pressurized gas and other hydrogen storage methods because of their gravimetric and volumetric storage capacities and safe operating pressures. In addition to the hydrogen storage capacity, other properties that have often been disregarded must now be addressed before hydrogen storage in metal hydrides becomes feasible. The slow hydriding/dehydriding kinetics, high release temperature, low storage efficiency due to the high enthalpy of formation, and thermal management during the hydriding reaction remain important difficulties in meeting the objectives set by the Department of Energy (DOE) for hydrogen storage systems. Nanotechnology offers new ways of addressing those issues by taking advantage of the distinctive chemical and physical properties observed in nanostructures. Nanostructured materials significantly improve the reaction kinetics, reduce the enthalpy of formation, and lower the hydrogen absorption and release temperatures through destabilization of the metal hydride and multiple catalytic effects in the system. But nanostructures can also lead to poor cyclability, degradation of the sorption properties, and a significant reduction of the thermal conductivity that could make metal hydrides impractical for hydrogen storage. This review summarizes the effects that nanotechnology can have on the main properties of metal hydrides and highlights the main competing behaviours between the system requirements, the necessary trade-offs, and the research priorities necessary to obtain hydride storage materials for practical automotive applications. Copyright © 2007 John Wiley & Sons, Ltd.
558 citations
TL;DR: The first demonstration of the synthesis of single-walled carbon nanotubes using gold nanoparticle catalysts with thermal chemical vapor deposition is presented, characterized by atomic force microscopy and resonant Raman spectroscopy.
Abstract: We present the first demonstration of the synthesis of single-walled carbon nanotubes using gold nanoparticle catalysts with thermal chemical vapor deposition. Nanotubes were successfully grown using monodispersed gold nanoparticles prepared through a block copolymer templating technique. The nanotubes are characterized by atomic force microscopy and resonant Raman spectroscopy and are found to have a diameter range between 1.0 and 2.0 nm.
260 citations
TL;DR: The 10-year history of Raman scattering in single walled carbon nanotubes (SWNTs) is reviewed and future prospects for the field are discussed in this article.
Abstract: Over the last 10 years, carbon nanotubes have offered a unique system for the study of Raman spectra in one-dimensional systems, and at the same time Raman spectroscopy has provided a widely used and powerful tool for the characterization of single walled carbon nanotubes (SWNTs). The 10 year history of Raman scattering in SWNTs is reviewed here and future prospects for the field are discussed.
250 citations
TL;DR: The goal of this chapter is to review the importance of excitons to single-wall carbon nanotube (SWNT) optics to provide a summary of the most important work in the field, as well as to identify open questions.
Abstract: The goal of this chapter is to review the importance of excitons to single-wall carbon nanotube (SWNT) optics. We have developed the presentation for both researchers in the SWNT field who want to learn more about the unusual aspects of SWNT exciton photophysics and researchers more knowledgeable about the physics of excitons, but not about SWNT physics. Excitons in SWNTs are special because graphite has two energy bands at the Fermi energy related to time-reversal symmetry and because SWNTs are actually one dimensional. This review discusses both theoretical and experimental points of view, thus aiming to provide a summary of the most important work in the field, as well as to identify open questions.
221 citations
TL;DR: In this paper, a novel nitrogen-rich carbon material, silk carbon, was prepared from Bombyx mori silk fibroins by simple heat-treatment under inert atmosphere, and the activation of the silk carbons with steam and potassium hydroxide provided nitrogen-containing activated carbons (ACs) with different poresize distribution.
214 citations
TL;DR: In this article, the Bethe-Salpeter equation was solved within tight-binding models of single-wall carbon nanotubes and the screening effect of the electrons was treated within the random phase and static screened approximations, and the exciton wave functions along the tube axis and circumference were discussed as a function of n,m.
Abstract: We have studied the exciton properties of single-wall carbon nanotubes by solving the Bethe-Salpeter equation within tight-binding models. The screening effect of the $\ensuremath{\pi}$ electrons in carbon nanotubes is treated within the random phase and static screened approximations. The exciton wave functions along the tube axis and circumference are discussed as a function of $(n,m)$. A $2n+m=\mathrm{const}$ family behavior is found in the exciton wave function length, excitation energy, binding energy, and environmental shift. This family behavior is understood in terms of the trigonal warping effect around the $K$ point of a graphene layer and curvature effects. The large family spread in the excitation energy of the Kataura plot is found to come from the single-particle energy.
205 citations
TL;DR: A big picture view of Raman scattering in carbon nanotubes is presented in this paper, starting from its early history and the discovery of the unique Raman spectra of carbon nanogase, and following on to the discovery at the single nanotube level and a detailed understanding of the scattering mechanism in terms of the excitonic picture.
114 citations
Patent•
03 Dec 2007TL;DR: In this article, the properties of nanostructured thermoelectric materials with high ZT values are discussed, and various compositions and methods relating to aspects of these materials (e.g., modulation doping) are further disclosed.
Abstract: Thermoelectric materials with high figures of merit, ZT values, are disclosed. In many instances, such materials include nano-sized domains (e.g., nanocrystalline), which are hypothesized to help increase the ZT value of the material (e.g., by increasing phonon scattering due to interfaces at grain boundaries or grain/inclusion boundaries). The ZT value of such materials can be greater than about 1, 1.2, 1.4, 1.5, 1.8, 2 and even higher. Such materials can be manufactured from a thermoelectric starting material by generating nanoparticles therefrom, or mechanically alloyed nanoparticles from elements which can be subsequently consolidated (e.g., via direct current induced hot press) into a new bulk material. Non-limiting examples of starting materials include bismuth, lead, and/or silicon-based materials, which can be alloyed, elemental, and/or doped. Various compositions and methods relating to aspects of nanostructured thermoelectric materials (e.g., modulation doping) are further disclosed.
67 citations
TL;DR: The ability to grow carbon nanotubes directly on monoliths of these materials to afford novel carbon aerogel-carbon nanotube composites is demonstrated and the doped iron species are reduced during carbonization to form metallic iron and iron carbide nanoparticles.
Abstract: We present the synthesis and characterization of Fe-doped carbon aerogels (CAs) and demonstrate the ability to grow carbon nanotubes directly on monoliths of these materials to afford novel carbon aerogel−carbon nanotube composites. Preparation of the Fe-doped CAs begins with the sol−gel polymerization of the potassium salt of 2,4-dihydroxybenzoic acid with formaldehyde, affording K+-doped gels that can then be converted to Fe2+- or Fe3+-doped gels through an ion exchange process, dried with supercritical CO2, and subsequently carbonized under an inert atmosphere. Analysis of the Fe-doped CAs by TEM, XRD, and XPS revealed that the doped iron species are reduced during carbonization to form metallic iron and iron carbide nanoparticles. The sizes and chemical composition of the reduced Fe species were related to pyrolysis temperature as well as the type of iron salt used in the ion exchange process. Raman spectroscopy and XRD analysis further reveal that, despite the presence of the Fe species, the CA frame...
TL;DR: Resonance Raman spectra of individual strained ultralong single-wall carbon nanotubes (SWNTs) are studied and the Raman intensity change reflects the effect of these strains on the SWNT electronic band structure.
Abstract: Resonance Raman spectra of individual strained ultralong single-wall carbon nanotubes (SWNTs) are studied. Torsional and uniaxial strains are introduced by atomic force microscopy manipulation. Torsional strain strongly affects the Raman spectra, inducing a large downshift in the E2 symmetry mode in the G+ band, but a slight upshift for the rest of the G modes and also an upshift in the radial breathing mode (RBM). Whereas uniaxial strain has no effect on the frequency of either the E2 symmetry mode in the G+ band or the RBM, it downshifts the rest of the G modes. The Raman intensity change reflects the effect of these strains on the SWNT electronic band structure.
TL;DR: In this paper, a systematic investigation of the Raman spectra variation of 15 individual carbon nanotubes partially suspended on trench-contained substrates was conducted with low laser power to exclude possible heating effects.
Abstract: The electrical and optical properties of single-walled carbon nanotubes (SWNTs) have been shown to be sensitive to their environment. Therefore it is very important to understand and exploit the environmental effect on the properties of nanotubes, especially for individual SWNTs. We report herein a systematic investigation of the Raman spectra variation of 15 individual SWNTs partially suspended on trench-contained substrates. Our experiments are conducted with low laser power to exclude possible heating effects. Most SWNTs show enhanced Raman signals for their suspended segment compared with their segment sitting on the SiO2 substrate, with several exceptions exhibiting either similar Raman intensity or a reverse result. Apart from this distinct intensity contrast, moderate radial breathing mode (RBM) frequency variations are observed for some nanotubes, which can be attributed to nanotube−substrate interactions. By analyzing the behaviors of the RBM full width at half-maximum (fwhm) and the intensity ra...
Patent•
06 Jul 2007TL;DR: In this article, a tunable transmissive grating comprises a transmissive dispersive element (1), a reflective element (2) and an angle t formed between the two elements.
Abstract: A tunable transmissive grating comprises a transmissive dispersive element (1), a reflective element (2) and an angle t formed between the two elements. A first optical path is formed according to the angle t, wherein light dispersing from the dispersive element is directed onto the reflective element and reflects therefrom. At least one element is rotatable about a rotational center (6) to cause a second optical path and thereby tune the wavelength of the light reflecting from the reflective element. Both elements can be rotatable together around a common rotational center point (6) according to certain embodiments, and/or each element can be independently rotated around a rotational axis associated only with that element. According to some embodiments, the relative angle t formed between the elements is held constant; however, in other embodiments t can vary. A control system can be used to operate the device.
TL;DR: In this article, a systematic resonance Raman study has been carried out on DNA-wrapped single walled carbon nanotubes (SWCNTs) of three different average lengths using seven different values of laser excitation energy Elaser.
Abstract: A systematic resonance Raman study has been carried out on DNA-wrapped single walled carbon nanotubes (SWCNTs) of three different average lengths ⟨Ltube⟩ using seven different values of laser excitation energy Elaser. The dependence of the intensity ratio of the D-band and G-band Raman features (ID∕IG) on ⟨Ltube⟩ indicates that nanotube length can be used as an important structural parameter for Raman characterization. By systematically varying Elaser, the ratio ID∕IG is found to be much stronger for metallic than for semiconducting SWCNTs but appears to have the same functional dependence on Elaser and ⟨Ltube⟩ or crystallite size as does nanographite.
Journal Article•
TL;DR: In this paper, a systematic resonance Raman study has been carried out on DNA-wrapped single walled carbon nanotubes (SWCNTs) of three different average lengths using seven different values of laser excitation energy Elaser.
Abstract: A systematic resonance Raman study has been carried out on DNA-wrapped single walled carbon nanotubes (SWCNTs) of three different average lengths ⟨Ltube⟩ using seven different values of laser excitation energy Elaser. The dependence of the intensity ratio of the D-band and G-band Raman features (ID∕IG) on ⟨Ltube⟩ indicates that nanotube length can be used as an important structural parameter for Raman characterization. By systematically varying Elaser, the ratio ID∕IG is found to be much stronger for metallic than for semiconducting SWCNTs but appears to have the same functional dependence on Elaser and ⟨Ltube⟩ or crystallite size as does nanographite.
TL;DR: In this article, the ability to achieve a simultaneous increase in the power factor and a decrease in the thermal conductivity of the same nanocomposite sample and for transport in the same direction is discussed.
Abstract: Many of the recent advances in enhancing the thermoelectric figure of merit are linked to nanoscale phenomena found both in bulk samples containing nanoscale constituents and in nanoscale samples themselves. Prior theoretical and experimental proof-of-principle studies on quantum-well superlattice and quantum-wire samples have now evolved into studies on bulk samples containing nanostructured constituents prepared by chemical or physical approaches. In this Review, nanostructural composites are shown to exhibit nanostructures and properties that show promise for thermoelectric applications, thus bringing together low-dimensional and bulk materials for thermoelectric applications. Particular emphasis is given in this Review to the ability to achieve 1) a simultaneous increase in the power factor and a decrease in the thermal conductivity in the same nanocomposite sample and for transport in the same direction and 2) lower values of the thermal conductivity in these nanocomposites as compared to alloy samples of the same chemical composition. The outlook for future research directions for nanocomposite thermoelectric materials is also discussed.
01 Jan 2007
TL;DR: Carbon nanotubes have been a model system for studying physics and for developing new measurement tools as discussed by the authors, and the field is now mature formaking the transition from nanoscience to nanotechnology, as we show here.
Abstract: The 21 contributions of this book cover advanced topics on the application,synthesis, structure and properties of carbon nanotubes and related materials. Thesetopics are linked to one another in this introductory contribution. By describing thebook structure and highlighting advances in the field since the publication of the firstvolume (Carbon Nanotubes: Synthesis, Structure, Properties and Applications,edited by M. S. Dresselhaus. G. Dresselhaus and P. Avouris, Vol. 80, Topicsin Applied Physics Series, Springer, Berlin, 2000), it seems clear that thecarbon nanotube field is on the verge of approaching a “phase-transitioncritical point”. Carbon nanotubes have been a model system for studyingphysics and for developing new measurement tools. The field is now mature formaking the transition from nanoscience to nanotechnology, as we show here.
TL;DR: In this article, the concept of using self-assembled and force-engineered nanostructures to enhance the thermoelectric figure of merit relative to bulk homogeneous and composite materials is presented in general terms.
Abstract: The concept of using “self-assembled” and “force-engineered” nanostructures to enhance the thermoelectric figure of merit relative to bulk homogeneous and composite materials is presented in general terms. Specific application is made to the Si-Ge system for use in power generation at high temperature. The scientific advantages of the nanocomposite approach for the simultaneous increase in the power factor and decrease of the thermal conductivity are emphasized along with the practical advantages of having bulk samples for property measurements and a straightforward path to scale-up materials synthesis and integration of nanostructured materials into thermoelectric cooling and power generation devices.
TL;DR: In this article, axial strain is introduced into individual single wall carbon nanotubes (SWCNTs) suspended from a trench-containing Si∕SiO2 substrate by employing the van der Waals interaction between the SWCNT and the substrate.
Abstract: Axial strain is introduced into individual single wall carbon nanotubes (SWCNTs) suspended from a trench-containing Si∕SiO2 substrate by employing the van der Waals interaction between the SWCNT and the substrate. Resonance Raman spectroscopy is used to characterize the strain, and up to 3% axial strain is observed. It is also found that a significant friction between the SWCNT and the substrate, on the order of 10pN∕nm, governs the localization and propagation of the strain in the SWCNTs sitting on the substrate. This method can be applied to introduce strain into materials sitting on a substrate, such as a graphene sheet.
TL;DR: Carbon aerogel (CA) spheres were fabricated by inverse suspension polymerization of an emulsified resorcinol-formaldehyde aqueous solution with a basic surfactant used as catalyst, and then drying by gradually heating the resultant sol-gel spheres at ambient pressure as mentioned in this paper.
Abstract: Carbon aerogel (CA) spheres were fabricated by inverse suspension polymerization of an emulsified resorcinol–formaldehyde aqueous solution with a basic surfactant used as catalyst, and then drying by gradually heating the resultant sol–gel spheres at ambient pressure. The fabrication conditions, nanostructures of the resultant CA spheres, and their electrochemical properties were investigated. By controlling the time at which stirring started, the stirring speed, and the recipe of the emulsion solution, the diameter of the CA spheres can be controlled from 70 to 1000 μm. The internal carbon nanoparticles of the spheres are of a size ranging from 20 to 40 nm, and are interconnected into a three-dimensional network. The highest surface area and mesopore volume of the spherical CAs thus prepared reach 626 m2/g and 0.69 cm3/g, respectively. These CAs can be used as an electrode in supercapacitors, and the specific capacitance is as high as 197 F/g. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007
TL;DR: The role of the hydrogen economy in the future of energy has been discussed in this paper, with a focus on scalability, usability, cost, and life cycle footprint on the environment.
Abstract: Since the publication of the 2003 report on Basic Energy Needs for the Hydrogen Economy, many important advances in hydrogen research have occurred, a cadre of enthusiastic researchers has entered the field with great interest shown by students, and private industry has made significant commitment to this technology and investment in its development worldwide. Concurrently, other energy technologies have made major strides forward. These technologies must be evaluated for their scalability, usability, cost and life cycle footprint on the environment. This overview discusses these topics and looks toward the role for the hydrogen economy into our energy future.
Patent•
03 Dec 2007
TL;DR: The authors concerne des materiaux thermoelectriques avec des facteurs de merite eleves, des valeurs ZT elevees, which sont hypothetiques pour augmenting the valeur ZT du materiau (par exemple, par l'augmentation de la diffusion des phonons en raison d'interfaces au niveau de limites de grains or de limite grains/inclusions).
Abstract: L'invention concerne des materiaux thermoelectriques avec des facteurs de merite eleves, des valeurs ZT elevees. Dans de nombreux exemples, de tels materiaux comprennent des domaines nanodimensionnes (par exemple, nanocristallins), qui sont hypothetiques pour aider a augmenter la valeur ZT du materiau (par exemple, par l'augmentation de la diffusion des phonons en raison d'interfaces au niveau de limites de grains ou de limites grains/inclusions). La valeur ZT de tels materiaux peut etre superieure a environ, 1, 1,2, 1,4, 1,5, 1,8, 2 et meme superieure. De tels materiaux peuvent etre fabriques a partir d'un materiau de depart thermoelectrique par la generation de nanoparticules a partir de celui-ci, ou de nanoparticules a alliage mecanique a partir d'elements qui peuvent etre ulterieurement consolides (par exemple, par l'intermediaire d'une presse a chaud induite par courant continu) en un nouveau materiau massif. Des exemples non limitatifs de materiaux de depart comprennent des materiaux a base de bismuth, de plomb et/ou de silicium, qui peuvent etre allies, elementaires et/ou dopes. L'invention concerne en outre diverses compositions et des procedes apparentes aux aspects de materiaux thermoelectriques nanostructures (par exemple, dopage par modulation).
TL;DR: In this paper, the structural behavior and electrical properties of bismuth nanobelts and nanoparticles were studied using an integrated TEM-STM system, and it was shown that clean Bi nanostructures free of oxides can be produced by in-situ high temperature electro-migration and Joule annealing processes occurring within the electron microscope.
Abstract: Nanostructured thermoelectric materials have attracted lots of interest in recent years, due to their enhanced performance determined by their thermoelectric dimensionless figure of merit. However, because of equipment limitations, not much work has been done on combining simultaneous transport measurements and structural characterization on individual nanostructured thermoelectric materials. With an integrated TEM-STM system, we studied the structural behavior and electrical properties of bismuth (Bi) nanobelts and nanoparticles. Results showed that clean Bi nanostructures free of oxides can be produced by in-situ high temperature electro-migration and Joule annealing processes occurring within the electron microscope. Preliminary electrical measurements indicate a conductivity of two orders of magnitude lower for Bi nanoparticles than that for bulk Bi. Such in-situ studies are highly advantageous for studying the semimetal-semiconductor transition and how this transition could enhance thermoelectric properties.