Showing papers by "Mildred S. Dresselhaus published in 1998"

Raman intensity of single-wall carbon nanotubes

Abstract: Using nonresonant bond-polarization theory, the Raman intensity of a single-wall carbon nanotube is calculated as a function of the polarization of light and the chirality of the carbon nanotube. The force-constant tensor for calculating phonon dispersion relations in the nanotubes is scaled from those for two-dimensional graphite. The calculated Raman spectra do not depend much on the chirality, while their frequencies clearly depend on the nanotube diameter. The polarization and sample orientation dependence of the Raman intensity shows that the symmetry of the Raman modes can be obtained by varying the direction of the nanotube axis, keeping the polarization vectors of the light fixed.

Bismuth quantum-wire arrays fabricated by a vacuum melting and pressure injection process

Abstract: Ultrafine bismuth nanowire arrays were synthesized by injecting its liquid melt into nanochannels of a porous anodic alumina template. A large area (1 cm × 1.5 cm) of parallel wires with diameters as small as 13 nm, lengths of 30–50 μm, and packing density as high as 7.1 × 1010 cm−2 has been fabricated. X-ray diffraction patterns revealed these nanowires, embedded in the insulating matrix, to be essentially single crystalline and highly oriented. The optical absorption spectra of the nanowire arrays indicate that these bismuth nanowires undergo a semimetal-to-semiconductor transition due to two-dimensional quantum confinement effects.

Process for fabricating an array of nanowires

Abstract: An array of nanowires having a relativley constant diameter and techniques and apparatus for fabrication thereof are described. In one embodiment, a technique for melting a material under vacuum and followed by pressure injection of the molten material into the pores of a porous substrate produces continuous nanowires. In another embodiment, a technique to systematically change the channel diameter and channel packing density of an anodic alumina substrate includes the steps of anodizing an aluminum substrate with an electrolyte to provide an anodic aluminum oxide film having a pore with a wall surface composition which is different than aluminum oxide and etching the pore wall surface with an acid to affect at least one of the surface properties of the pore wall and the pore wall composition.

Carrier pocket engineering to design superior thermoelectric materials using GaAs/AlAs superlattices

Abstract: A large enhancement in the thermoelectric figure of merit for the whole superlattice, Z3DT, is predicted for short-period GaAs/AlAs superlattices relative to bulk GaAs. Various superlattice parameters (superlattice growth direction, superlattice period, and layer thicknesses) are explored to optimize Z3DT, including quantum well states formed from carrier pockets at various high symmetry points in the Brillouin zone. The highest room-temperature Z3DT obtained in the present calculation is 0.41 at the optimum carrier concentration for either (001)- or (111)-oriented GaAs (20 A)/AlAs (20 A) superlattices, which is about 50 times greater than the corresponding ZT for bulk GaAs.

Carrier Pocket Engineering to Design Superior Thermoelectric Materials Using GaAs/AlAs Superlattices

Abstract: A large enhancement in the thermoelectric figure of merit for the whole superlattice, Z 3D T, is predicted for short period GaAs/AlAs superlattices relative to bulk GaAs. Various superlattice parameters (superlattice growth direction, superlattice period and layer thicknesses) are explored to optimize Z 3D T, including quantum wells formed at various high symmetry points in the Brillouin zone. The highest room temperature Z 3D T obtained in the present calculation is 0.41 at the optimum carrier concentration for either (001) or (111) oriented GaAs(20 A)/AIAs(20 A) superlattices, which is about 50 times greater than the corresponding ZT for bulk GaAs obtained using the same basic model.

Anisotropic Thermal Conductivity of A Si/Ge Superlattice

Abstract: Experimental evidence for a significant thermal conductivity reduction have been reported in recent years for GaAs/AlAs, Si/Ge, and Bi2 Te3 /Sb2 Te3 superlattices. In this work, we present preliminary experimental results on the reduction of the in-plane and cross-plane thermal conductivity for a symmetric Si/Ge superlattice. A differential 2-wire 3ω method is developed to perform the anisotropic thermal conductivity measurements. In this technique, a patterned heater with a width much larger than the film thickness yields the cross-plane thermal conductivity of the film. The in-plane thin film thermal conductivity is inferred from the temperature rise of a narrow width heater that can create more heat spreading in the in-plane direction of the thin film. A differential method to measure the temperature drop across the film is employed in order to increase the accuracy of the measurement.

Nanotechnology in Carbon Materials

Abstract: This article reviews carbon materials from the standpoint of nanotechnology. The unusual features of carbon materials stem from the ability of carbon to form materials with vastly different structures and hence with vastly different proper ties. The major focus of this review is on the ability of carbon to form zero dimensional quantum dots of subnanometer dimensions in the form of fullerenes, and one-dimensional quantum wires in the form of carbon nanotubes. The struc ture and properties of fullerenes and carbon nanotubes are reviewed in this article in the context of nanotechnology.

Approaches Toward Chemically Prepared Multiple Quantum Well Structures

Abstract: An enhanced thermoelectric figure of merit, ZT, has been predicted for Bi2 Te3 in the form of 2-dimensional quantum wells. A new approach to making multiple quantum well (MQW) structures for thermoelectric applications utilizing a chemical intercalation technique is proposed and investigated. It is proposed that by starting from Li intercalated Bi2 Te3 and Bi2 Se3, the layers of these materials can be separated by chemical means. The layers of Bi2 Te3 or Bi2 Se3 can then be restacked, by self-assembly, forming a non-periodic array of quantum wells. These chemically prepared MQWs are characterized by X-ray diffraction, SEM (scanning electron microscopy) and TEM (transmission electron microscopy) at various stages in the sample preparation to assess the degree to which the actual samples match the proposal. Experimental measurements of the Seebeck coefficient ( S) and the electrical conductivity (σ) were performed over a range of temperatures for the initial bulk materials. It is found that some of the steps in the proposed fabrication have been achieved but still much improvement is needed before any practical thermoelectric 2D-system can be provided.

The AAAS Celebrates Its 150th

Abstract: The American Association for the Advancement of Science (AAAS), which has become the preeminent professional society worldwide in interdisciplinary sciences, technology and public policy, was founded 150 years ago. The President of AAAS presents several vignettes on the venerable history of AAAS and their influence on the Association today are presented.

Investigation of the Mechanism of the Enhanced Z3DT in PbTe Based Superlattices

Abstract: Various possible mechanisms for the recently discovered enhanced Seebeck coefficient S in PbTe/Te superlattices relative to the corresponding PbTe bulk are investigated. Among the various mechanisms which can account for the enhanced S, the energy dependent τ model (τ ∼ ɛr) seems the most plausible. Here the effective scattering parameter r is preferably increased due to the extra scattering by the periodic Te layers introduced in the superlattice. Other transport properties including the longitudinal magnetoresistance are also discussed.

Matrices de nanofils

Abstract: L'invention porte sur une matrice de nanofils dont le diametre est relativement constant, et sur des techniques et un appareil de fabrication de ceux-ci. Selon une realisation, une technique consiste a faire fondre un materiau sous vide et a injecter sous pression le materiau fondu dans les pores d'un substrat poreux de facon a former des nanofils continus. Selon une autre realisation, une technique de modification systematique du diametre du canal et de la densite de tassement dans le canal d'un substrat d'alumine anodique consiste a anodiser un substrat d'aluminium avec un electrolyte de facon a obtenir un film d'oxyde d'aluminium anodique dont les pores sont remplis d'une composition de surface differente de celle de l'oxyde d'aluminium, et a attaquer la surface de la paroi poreuse avec un acide de facon a modifier au moins une des proprietes de surface de la paroi poreuse et de sa composition.