Showing papers by "Mildred S. Dresselhaus published in 2008"

High-Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys

Abstract: The dimensionless thermoelectric figure of merit (ZT) in bismuth antimony telluride (BiSbTe) bulk alloys has remained around 1 for more than 50 years. We show that a peak ZT of 1.4 at 100°C can be achieved in a p-type nanocrystalline BiSbTe bulk alloy. These nanocrystalline bulk materials were made by hot pressing nanopowders that were ball-milled from crystalline ingots under inert conditions. Electrical transport measurements, coupled with microstructure studies and modeling, show that the ZT improvement is the result of low thermal conductivity caused by the increased phonon scattering by grain boundaries and defects. More importantly, ZT is about 1.2 at room temperature and 0.8 at 250°C, which makes these materials useful for cooling and power generation. Cooling devices that use these materials have produced high-temperature differences of 86°, 106°, and 119°C with hot-side temperatures set at 50°, 100°, and 150°C, respectively. This discovery sets the stage for use of a new nanocomposite approach in developing high-performance low-cost bulk thermoelectric materials.

Carbon nanotubes : advanced topics in the synthesis, structure, properties and applications

Abstract: Ultrafast Spectroscopy of Carbon Nanotubes.- Rayleigh Scattering Spectroscopy.- New Techniques for Carbon-Nanotube Study and Characterization.- High Magnetic Field Phenomena in Carbon Nanotubes.- Carbon-Nanotube Optoelectronics.- Electrical Transport in Single-Wall Carbon Nanotubes.- Double-Wall Carbon Nanotubes.- Doped Carbon Nanotubes: Synthesis, Characterization and Applications.- Electrochemistry of Carbon Nanotubes.- Single-Wall Carbon Nanohorns and Nanocones.- to the Important and Exciting Aspects of Carbon-Nanotube Science and Technology.- Inorganic Nanotubes and Fullerene-Like Structures (IF).- Electron and Phonon Properties of Graphene: Their Relationship with Carbon Nanotubes.- Potential Applications of Carbon Nanotubes.- Carbon-Nanotube Metrology.- Carbon Nanotube Synthesis and Organization.- Mechanical Properties, Thermal Stability and Heat Transport in Carbon Nanotubes.- Quasiparticle and Excitonic Effects in the Optical Response of Nanotubes and Nanoribbons.- Role of the Aharonov-Bohm Phase in the Optical Properties of Carbon Nanotubes.- Excitonic States and Resonance Raman Spectroscopy of Single-Wall Carbon Nanotubes.- Photoluminescence: Science and Applications.

Enhanced thermoelectric figure-of-merit in nanostructured p-type silicon germanium bulk alloys.

Abstract: A dimensionless thermoelectric figure-of-merit (ZT) of 0.95 in p-type nanostructured bulk silicon germanium (SiGe) alloys is achieved, which is about 90% higher than what is currently used in space flight missions, and 50% higher than the reported record in p-type SiGe alloys. These nanostructured bulk materials were made by using a direct current-induced hot press of mechanically alloyed nanopowders that were initially synthesized by ball milling of commercial grade Si and Ge chunks with boron powder. The enhancement of ZT is due to a large reduction of thermal conductivity caused by the increased phonon scattering at the grain boundaries of the nanostructures combined with an increased power factor at high temperatures.

Enhanced thermoelectric figure of merit in nanostructured n-type silicon germanium bulk alloy

Abstract: The dimensionless thermoelectric figure of merit (ZT) of the n-type silicon germanium (SiGe) bulk alloy at high temperature has remained at about one for a few decades. Here we report that by using a nanostructure approach, a peak ZT of about 1.3 at 900 °C in an n-type nanostructured SiGe bulk alloy has been achieved. The enhancement of ZT comes mainly from a significant reduction in the thermal conductivity caused by the enhanced phonon scattering off the increased density of nanograin boundaries. The enhanced ZT will make such materials attractive in many applications such as solar, thermal, and waste heat conversion into electricity.

Transferring and Identification of Single- and Few-Layer Graphene on Arbitrary Substrates

Abstract: The transferring and identification of single- and few- layer graphene sheets from SiO2/Si substrates to other types of substrates is presented. Features across large areas (∼cm2) having single and few-layer graphene flakes, obtained by the microcleaving of highly oriented pyrolytic graphite (HOPG), can be transferred reliably. This method enables the fast localization of graphene sheets on substrates on which optical microscopy does not allow direct and fast visualization of the thin graphene sheets. No major morphological deformations, corrugations, or defects are induced on the graphene films when transferred to the target surface. Moreover, the differentiation between single and bilayer graphene via the G′ (∼2700 cm−1) Raman peak is demonstrated on various substrates. This approach opens up possibilities for the fabrication of graphene devices on a substrate material other than SiO2/Si.

Bulk production of a new form of sp(2) carbon: crystalline graphene nanoribbons.

Abstract: We report the use of chemical vapor deposition (CVD) for the bulk production (grams per day) of long, thin, and highly crystalline graphene ribbons (<20−30 μm in length) exhibiting widths of 20−300 nm and small thicknesses (2−40 layers). These layers usually exhibit perfect ABAB... stacking as in graphite crystals. The structure of the ribbons has been carefully characterized by several techniques and the electronic transport and gas adsorption properties have been measured. With this material available to researchers, it should be possible to develop new applications and physicochemical phenomena associated with layered graphene.

The Hydrogen Fuel Alternative

Abstract: The cleanliness of hydrogen and the efficiency of fuel cells taken together offer an appealing alternative to fossil fuels Implementing hydrogen-powered fuel cells on a significant scale, however, requires major advances in hydrogen production, storage, and use Splitting water renewably offers the most plentiful and climate-friendly source of hydrogen and can be achieved through electrolytic, photochemical, or biological means Whereas presently available hydride compounds cannot easily satisfy the competing requirements for on-board storage of hydrogen for transportation, nanoscience offers promising new approaches to this challenge Fuel cells offer potentially efficient production of electricity for transportation and grid distribution, if cost and performance challenges of components can be overcome Hydrogen offers a variety of routes for achieving a transition to a mix of renewable fuels

Extreme‐Performance Rubber Nanocomposites for Probing and Excavating Deep Oil Resources Using Multi‐Walled Carbon Nanotubes

Abstract: The scarcity of oil resources is going to become one of the main factors threatening the stability of the global economy. To avoid an energy crisis in the future, it is essential to increase oil extraction in much deeper wells, experiencing higher temperatures and pressures. Exploring these deeper areas will demand novel and robust materials. Rubber sealants, or O-rings, are especially key components in enabling the probing and production of oil in deeper wells, so that higher temperature and pressure reservoirs are reached. In this account, it is demonstrated that carbon nanotubes homogeneously and randomly dispersed in rubber matrices, are able to generate durable sealants that operate satisfactorily at extremely high temperatures and pressures (e.g., 260 °C and 239 MPa). The key issues in these novel composites are: i) the nanotube surface-control and reactivity, ii) the used of multi-walled carbon nanotubes (MWNTs)-embedded in fluorinated rubber, and iii) the formation of a cellulation structure. This rubber nanocomposite with a cellulation structure and having extreme performance leads to a balanced pressure resistance, sealing ability, thermal resistance, and durability, which can contribute to doubling the current average global oil recovery efficiency.

The Reinforcing Effect of Combined Carbon Nanotubes and Acetylene Blacks on the Positive Electrode of Lithium‐Ion Batteries

Abstract: Here, we demonstrate the preparation of high-performance positive electrodes for lithium-ion batteries by adding small amounts of both carbon nanotubes and acetylene blacks to LiCoO2-based active materials. The merits of using carbon nanotubes together with acetylene blacks as cathode fillers include not only the enhancement of the electrical and the thermal properties of the electrode but also the enhancement of the density of the electrode and the shortening of the electrolyte absorption time. We envisage that the use of carbon nanotubes as multifunctional fillers will increase in both cathode and anode materials for lithium-ion secondary batteries. Since the development of lithium-ion batteries in 1990, they have had an enormous influence on our lives. 2] At present, portable electronic devices and hybrid vehicles have evergrowing requirements for safe and high-performance lithiumion batteries. Therefore, new types of the nanostructure electrode materials or fillers including carbon nanotubes have been examined to improve the electrochemical performance of lithium-ion batteries (e.g. , large capacity, high rate capability and long life cycle), as well as for developing new end-use products (e.g. , cosmetics). In commercial lithium-ion batteries, up to 100 tons per year of highly pure crystalline carbon nanotubes are incorporated as effective fillers in anode materials, in which the resilience and the electrical properties of carbon nanotubes are believed to play an important role in extending the life cycle of the batteries. Similarly, several studies have examined the capability of carbon nanotubes to enhance the electrical conductivity of cathode materials in relation to that of conventionally used carbon blacks as lithium metal oxides, which have low electrical conductivity, experience structural deterioration or capacity degradation during charging and discharging cycles. However, there appears to be a critical question regarding the complete replacement of acetylene blacks by carbon nanotubes in cathodes owing to the capability of acetylene blacks to store a significant amount of electrolyte in their primary structure in addition to enhancing the conductivity. Also, previous studies have emphasized the electrical conductivity of the cathode as the only advantage of the incorporated carbon nanotubes, even though homogeneously distributed carbon nanotubes appear to give rise to additional functions. In this study, we examine the advantages of adding a hybridtype filler, consisting of acetylene blacks and high-purity crystalline thick multiwalled carbon nanotubes, to a LiCoO2-based cathode as compared to a cathode with added acetylene blacks or carbon nanotubes, from the viewpoint of their electrical and thermal properties and electrolyte adsorption capabilities as well as their electrochemical performance. Consequently, we demonstrate that optimally combined carbon nanotubes within a cathode act as electrical, thermal and structure-linking segments and provide suitably created pores, thereby decreasing the electrolyte absorption time. The prepared electrode consisted of three different morphological components: micrometer-sized LiCoO2 particles, long carbon nanotubes and nanometer-sized acetylene blacks. The technical reason for selecting LiCoO2 (Figure 1 c) as an active

Selective optical property modification of double-walled carbon nanotubes by fluorination.

Abstract: We found that by fluorination of double-walled carbon nanotubes (DWNTs), it is possible to suppress only the Raman radial breathing mode and absorption peaks from the outer (large diameter) tubes of DWNTs. In contrast, Raman signals from the inner shells showed no difference from the pristine DWNTs. The stability of the inner shells of fluorinated DWNTs was also confirmed from the photoluminescence (PL) map and the optical absorption spectra, which only showed the signals from the inner shells of DWNTs, with no distinct change in the optical properties of the inner tubes after fluorination. Our results indicate that once fluorinated, there exists only a weak, if not none, interaction between the inner tube and the outer fluorinated tube, proving that fluorination can be used to suppress the optical properties of carbon nanotubes without interfering the properties of inner tubes. The present finding can be important in electronic and sensor applications, keeping the inner tube from having unwanted contact ...

Synthesis and isolation of molybdenum atomic wires.

Abstract: One of the main challenges in nanoscience and nanotechnology consists in the production and isolation of metallic atomic-scale nanowires (Benzryadin, C. N.; Lau, A.; Tinkham, M. Q. Nature 2000, 404, 971-974; Zach, M. P.; Ng, K. H.; Penner, R. M. Science 2000, 290, 2120-2123; Nilius, N.; Wallis, T. M.; Ho, W. Science 2002, 297, 1853-1856.). Here we report a unique and controllable way of isolating individual atomic molybdenum (Mo) chains by their encapsulation inside double-walled carbon nanotubes, exhibiting inner diameters ranging from 0.6 to 0.8 nm. We have found that these individual atomic chains form spontaneously within the hollow core of tubes in the absence of any reducing agent. We believe that these atomic-scale nanowires could now be studied without suffering oxidation, so that their physical and chemical properties are elucidated.

Curvature-induced optical phonon frequency shift in metallic carbon nanotubes

Abstract: The quantum corrections to the frequencies of the point longitudinal-optical LO- and transverse-optical TO-phonon modes in carbon nanotubes are investigated theoretically. The frequency shift and broadening of the TO-phonon mode strongly depend on the curvature effect due to a special electron-phonon coupling in carbon nanotubes, which is shown by the Fermi energy dependence of the frequency shift for different nanotube chiralities. It is also shown that the TO mode near the point decouples from electrons due to local gauge symmetry and that a phonon mixing between LO and TO modes is absent due to time-reversal symmetry. Some comparisons between theory and experiment are presented.

Simple synthesis of multiwalled carbon nanotubes from natural resources.

Abstract: The mass production of carbon nanotubes (CNTs) at low cost remains an important technological challenge if applications of CNTs are to be exploited further. Here, we report an extremely efficient method for growing pure and highly crystalline CNTs using natural resources: garnet powder as a catalyst, and city gas as a carbon source. The chemically reduced iron particles on the surface of the sand granules catalyze the decomposition of city gas, leading effectively to a high growth rate of CNTs. This novel method is very promising for providing lowcost, high-quality industrially available CNT products for developing large-scale applications in the near future. To produce CNTs in bulk quantities, a well-developed ironbased catalytic chemical vapor deposition (CCVD) method has been commonly used, as established by one of us after obtaining ultrathin hollow CNTs grown on nanosized iron particles. 2] However, for the production of CNTs, the catalyst is considered as one of the most expensive components. Therefore, to find an alternative cheap and effective catalyst for producing highquality and high-quantity CNTs remains a challenge for the bulk production of this key material in nanotechnology. Recently, Su and Chen reported the preparation of CNTs grown on lava rocks. However, those CNTs did not exhibit a well-ordered structure. In addition, the lava is not easy to handle in the CCVD process. Interestingly, one of us reported early on that the origin of the nanosized iron catalyst which promoted the production of a large amount of fibrous carbon material was the emery paper that was used for polishing the aluminum or graphite substrate used in the CCVD chamber. Later, it was found that brown emery paper is made of fine garnet powder. From this perspective, we report an alternative, environmentally friendly and efficient method for growing multiwalled carbon nanotubes (MWCNTs) using purely natural resources: stone garnet sand as a catalyst and support, and city gas based natural gas as the carbon source. Garnet powder consists of SiO2, FeO, Fe2O3, and Al2O3, among other things (Figure 1a) and is inexpensive ($1.4kg ). The size of the garnet sand particles is around 200 mm (see inset in Figure 1a). We

Chirality-dependent frequency shift of radial breathing mode in metallic carbon nanotubes

Abstract: A phonon frequency shift of the radial breathing mode for metallic single wall carbon nanotubes is predicted as a function of Fermi energy. Armchair nanotubes do not show any frequency shift while zigzag nanotubes exhibit phonon softening, but this softening is not associated with the broadening. This chirality dependence originates from a curvature-induced energy gap and a special electron-phonon coupling mechanism for radial breathing modes. Because of the particle-hole symmetry, only the off-site deformation potential contributes to the frequency shift. On the other hand, the on-site potential contributes to the Raman intensity, and the radial breathing mode intensity is stronger than that of the G band. The relationship between the chirality dependence of the frequency shift of the radial breathing mode and the point optical-phonon frequency shift is discussed.

Double-wall carbon nanotubes doped with different Br2 doping levels: a resonance Raman study.

Abstract: This report focuses on the effects of different Br2 doping levels on the radial breathing modes of “double-wall carbon nanotube (DWNT) buckypaper”. The resonance Raman profile of the Br2 bands are shown for different DWNT configurations with different Br2 doping levels. Near the maximum intensity of the resonance Raman profile, mainly the Br2 molecules adsorbed on the DWNT surface contribute strongly to the observed ωBr−Br Raman signal.

Diameter-selective separation of double-walled carbon nanotubes

Abstract: Here, we report a simple and effective way of separating double-walled carbon nanotubes as a function of their diameter using individually dispersed nanotube solutions with the aid of long and random single-stranded DNA. The subtle pH change in nanotube solutions gives rise to the preferential coagulation of large-diameter tubes and allows the easy preparation of small-diameter tubes. The stronger van der Waals forces between large-diameter tubes, combined with the decreased solubility of DNA in water at low pH, lead to the preferential agglomeration of large-diameter tubes.

CdSe quantum dot-decorated double walled carbon nanotubes: The effect of chemical moieties

Abstract: We report that fluorine atoms on the outer tubes of double walled carbon nanotubes (DWNTs) are more effective for nucleating and growing CdSe nanoparticles than oxygen-containing functional groups. The CdSe particles with an average size of 5–7 nm grow through infiltration into an interstitial space created by four to five thin bundled DWNTs. We envisage that DWNTs will replace single and multiwalled carbon nanotubes in a wide range of applications because chemical moieties could be introduced selectively on the outer tubes while the optical and physical properties of the inner tubes remain almost unchanged.

Raman study on electrochemical lithium insertion into multiwalled carbon nanotubes

Abstract: We have carried out in situ Raman studies during the electrochemical insertion of lithium ions (Li+) into pristine and thermally treated multiwalled carbon nanotubes (MWNTs). We found an improved structural integrity as well as the removal of defects in the thermally treated tubes. The different Li+ insertion behaviors above 0.5 V in as-grown and thermally treated tubes could be explained by the presence of defects on the outer surface of the tubes. No change of Raman spectra from 2.8 to 0.8 V is characterized by the coverage of Li+ on the outer surface of tubes, whereas the upshift of G band and the absence of a separated G band below 0.75 V indicate the formation of diluted graphite intercalation below the stage-2 phase (LiC12). Copyright © 2008 John Wiley & Sons, Ltd.

Sputter deposition of multilayer thermoelectric films: An approach to the fabrication of two-dimensional quantum wells

Abstract: The relative efficiency of a thermoelectric material is measured in terms of a dimensionless figure of merit, ZT. Though all known thermoelectric materials are believed to have ZT≤1, recent theoretical results predict that thermoelectric devices fabricated as two‐dimensional quantum wells (2D QWs) or one‐dimensional (1D) quantum wires could have ZT≥3. Multilayers with the dimensions of 2D QWs have been synthesized by alternately sputtering Bi0.9Sb0.1 and PbTe0.8Se0.2 onto a moving single‐crystal sapphire substrate from dual magnetrons. These materials have been used to test the thermoelectric quantum‐well concept and gain insight into relevant transport mechanisms. If successful, this research could lead to thermoelectric devices that have efficiencies close to that of an ideal Carnot engine. Ultimately, such devices could be used to replace conventional heat engines and mechanical refrigeration systems.