Showing papers by "Mildred S. Dresselhaus published in 1996"

Edge state in graphene ribbons: Nanometer size effect and edge shape dependence.

Abstract: Finite graphite systems having a zigzag edge exhibit a special edge state. The corresponding energy bands are almost flat at the Fermi level and thereby give a sharp peak in the density of states. The charge density in the edge state is strongly localized on the zigzag edge sites. No such localized state appears in graphite systems having an armchair edge. By utilizing the graphene ribbon model, we discuss the effect of the system size and edge shape on the special edge state. By varying the width of the graphene ribbons, we find that the nanometer size effect is crucial for determining the relative importance of the edge state. We also have extended the graphene ribbon to have edges of a general shape, which is defined as a mixture of zigzag and armchair sites. Examining the relative importance of the edge state for graphene ribbons with general edges, we find that a non-negligible edge state survives even in graphene ribbons with less developed zigzag edges. We demonstrate that such an edge shape with three or four zigzag sites per sequence is sufficient to show an edge state, when the system size is on a nanometer scale. The special characteristics of the edge state play a large role in determining the density of states near the Fermi level for graphite networks on a nanometer scale.

Experimental study of the effect of quantum-well structures on the thermoelectric figure of merit

Abstract: Recently, it has been shown theoretically that it may be possible to increase the thermoelectric figure of merit ($Z$) of certain materials by preparing them in the form of two-dimensional quantum-well structures. Using $\mathrm{PbTe}/{\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Eu}}_{x}\mathrm{Te}$ multiple-quantum-well structures grown by molecular-beam epitaxy, we have performed thermoelectric and other transport measurements as a function of quantum-well thickness and doping. Our results are found to be consistent with theoretical predictions and indicate that an increase in $Z$ over bulk values may be possible through quantum confinement effects using quantum-well structures.

Effect of Quantum-Well Structures on the Thermoelectric Figure of Merit in the Si/Si1-xGex System

Abstract: The Si/Si1-xGex quantum well system is attractive for high temperature thermoelectric applications and for demonstration of proof-of-principle for enhanced thermoelectric figure of merit Z, since the interfaces and carrier densities can be well controlled in this system. We report theoretical calculations for Z in this system, based on which Si/Si1-xGex quantum-well structures were grown by molecular-beam epitaxy. Thermoelectric and other transport measurements were made, indicating that an increase in Z over bulk values is possible through quantum confinement effects in the Si/Si1-xGex quantum-well structures.

Tunneling conductance of connected carbon nanotubes

Abstract: A three-dimensional structure of two carbon nanotubes (CN) joined by a connecting region containing a pentagon and heptagon pair is given by the use of a projection method. The connecting joint is uniquely determined for the given two chiral vectors of CN by a vector which defines a three-dimensional dihedral angle. The tunneling conductance is calculated for a metal-metal CN junction and a metal-semiconducting CN junction. The calculated results clearly show that these junctions work as the smallest semiconductor devices.

Activated Carbon Aerogels

Abstract: Activated carbon aerogels were obtained from the CO2 activation of carbon aerogels. The adsorption isotherms of nitrogen on activated carbon aerogels at 77 K were measured and analyzed by the high-resolution αS plot to evaluate their porosities. The αS plot showed an upward deviation from linearity below αS = 0.5, suggesting that the presence of micropores becomes more predominant with the extent of the activation. Activation increased noticeably the pore volume and the surface area (the maximum value: 2600 m2·g-1) without change of the basic network structure of primary particles. Activated carbon aerogels had a bimodal pore size distribution of uniform micropores and mesopores.

Experimental study of the effect of quantum-well structures on the thermoelectric figure of merit

Abstract: Experimental investigations have been performed on the thermoelectric properties on the multiple-quantum-well structures of PbTe/Pb/sub 1-x/Eu/sub x/Te grown by molecular beam epitaxy. Our results are found to be consistent with theoretical predictions and indicate that an increase in Z over bulk values may be possible through quantum confinement effects using quantum-well structures.

Experimental study of the effect of quantum-well structures on the thermoelectric figure of merit.

Abstract: Experimental investigations have been performed on the thermoelectric properties on the multiple-quantum-well structures of PbTe/Pb/sub 1-x/Eu/sub x/Te grown by molecular beam epitaxy. Our results are found to be consistent with theoretical predictions and indicate that an increase in Z over bulk values may be possible through quantum confinement effects using quantum-well structures.

Carbon connections promise nanoelectronics

Abstract: As the semiconductor industry inches towards integrated circuit technology based on silicon chips with features just 200 nm wide, researchers are trying to build carbon-based electronic devices that are hundreds of times smaller These devices would be based on the remarkable electronic properties of carbon nanotubes and the junctions between dissimilar nanotubes

Physics of carbon nanotubes

Abstract: The fundamental relations governing the geometry of carbon nanotubes are reviewed, and explicit examples are presented. A framework is given for the symmetry properties of carbon nanotubes for both symmorphic and non-symmorphic tubules which have screw-axis symmetry. The implications of symmetry on the vibrational and electronic structure of 1D carbon nanotube systems are considered. The corresponding properties of double-wall nanotubes and arrays of nanotubes are also discussed.

Applications of Carbon Nanostructures

Abstract: Several unusual optical properties exhibited by fullerenes show promise for applications. The nonlinear properties of optical absorption in the excited triplet state give rise to optical limiting devices and photorefractive devices while photo induced charge transfer between polymer and C 60 constituents in C 60 -polymer composites gives rise to photoconductivity applications photodiodes and photovoltaic devices. One promising application for fullerenes is as an optical limiter because of the large magnitude of the nonlinear third-order electric susceptibility and nonlinear optical response, which in turn arise from basic optical properties associated with the interplay between allowed and forbidden transitions and between singlet and triplet state transitions. Optical limiters are used to protect materials from damage by intense incident light pulses via a saturation of the transmitted light intensity with increasing exposure time and/or incident light intensity.