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Vincent H Crespi

Publications and source records attributed to Vincent H Crespi.

9 recordsLinked to original sources
PubMed2006

Universal behavior of nearly free electron states in carbon nanotubes.

The nearly free electron state of a carbon nanotube drops rapidly in energy relative to the other conduction bands under alkali doping. A natural (and previously proposed) explanation for this rapid downshift is hybridization with the potassium states. However, we show that the downshift occurs even when the extra electrons are compensated by a uniform positive background, wherein there can be no hybridization, since there are no alkali atoms. Instead, the motion of the nearly free band arises from a universal electrostatic mechanism, which applies for any type of positive countercharge independent of tube diaf/meter and helicity. The nearly free electron state, being weakly bound to the tube wall, is extraordinarily labile and deforms onto the countercharge, whereas the remaining pi* conduction band states are held to the surface of the carbon sheet by the strong carbon potential.

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PubMed2006

Microstructured optical fibers as high-pressure microfluidic reactors.

Deposition of semiconductors and metals from chemical precursors onto planar substrates is a well-developed science and technology for microelectronics. Optical fibers are an established platform for both communications technology and fundamental research in photonics. Here, we describe a hybrid technology that integrates key aspects of both engineering disciplines, demonstrating the fabrication of tubes, solid nanowires, coaxial heterojunctions, and longitudinally patterned structures composed of metals, single-crystal semiconductors, and polycrystalline elemental or compound semiconductors within microstructured silica optical fibers. Because the optical fibers are constructed and the functional materials are chemically deposited in distinct and independent steps, the full design flexibilities of both platforms can now be exploited simultaneously for fiber-integrated optoelectronic materials and devices.

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PubMed2004

Theory of carbon nanocones: mechanical chiral inversion of a micron-scale three-dimensional object.

Graphene cones have two degenerate configurations: their original shape and its inverse. When the apex is depressed by an external probe, the simulated mechanical response is highly nonlinear, with a broad constant-force mode appearing after a short initial Hooke's law regime. For chiral cones, the final state is an atomically exact chiral invert of the original system. If the local reflection symmetry of the graphene sheet is broken by the chemisorption of just five hydrogen atoms to the apex, then the maximal yield strength of the cone increases by approximately 40%. The high symmetry of the conical geometry can concentrate micron-scale mechanical work with atomic precision, providing a way to activate specific chemical bonds.

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PubMed2004

Catalytic nanomotors: autonomous movement of striped nanorods.

Rod-shaped particles, 370 nm in diameter and consisting of 1 microm long Pt and Au segments, move autonomously in aqueous hydrogen peroxide solutions by catalyzing the formation of oxygen at the Pt end. In 2-3% hydrogen peroxide solution, these rods move predominantly along their axis in the direction of the Pt end at speeds of up to 10 body lengths per second. The dimensions of the rods and their speeds are similar to those of multiflagellar bacteria. The force along the rod axis, which is on the order of 10(-14) N, is generated by the oxygen concentration gradient, which in turn produces an interfacial tension force that balances the drag force at steady state. By solving the convection-diffusion equation in the frame of the moving rod, it was found that the interfacial tension force scales approximately as SR(2)gamma/muDL, where S is the area-normalized oxygen evolution rate, gamma is the liquid-vapor interfacial tension, R is the rod radius, mu is the viscosity, D is the diffusion coefficient of oxygen, and L is the length of the rod. Experiments in ethanol-water solutions confirmed that the velocity depends linearly with the product Sgamma, and scaling experiments showed a strong dependence of the velocity on R and L. The direction of motion implies that the gold surface is hydrophobic under the conditions of the experiment. Tapping-mode AFM images of rods in air-saturated water show soft features that are not apparent in images acquired in air. These features are postulated to be nanobubbles, which if present in hydrogen peroxide solutions, would account for the observed direction of motion.

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PubMed2004

Nanotube-substrate interactions: distinguishing carbon nanotubes by the helical angle.

We investigate the interaction of a carbon nanotube with a graphite substrate, using an interlayer potential that explicitly treats the registry dependence of the interaction. The carbon-carbon bond lengths in nanotubes differ slightly from those in flat graphite, so that the naively commensurate angular orientations for the tube with respect to the substrate lattice are destroyed. The interaction of a one-dimensional tube with a two-dimensional substrate then leads to an unusual registry phenomenon not visible in standard layer-on-layer growth: the system develops favorable orientations which clearly are incommensurate.

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PubMed2003

Chemically doped double-walled carbon nanotubes: cylindrical molecular capacitors.

A double-walled carbon nanotube is used to study the radial charge distribution on the positive inner electrode of a cylindrical molecular capacitor. The outer electrode is a shell of bromine anions. Resonant Raman scattering from phonons on each carbon shell reveals the radial charge distribution. A self-consistent tight-binding model confirms the observed molecular Faraday cage effect, i.e., most of the charge resides on the outer wall, even when this wall was originally semiconducting and the inner wall was metallic.

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PubMed2002

Abrupt topological transitions in the hysteresis curves of ferromagnetic metalattices.

When a metal is confined to the interstices of an inert colloidal crystal, the intrinsic order parameter(s) of electronic and magnetic phenomena within the metal interact with the structural order parameter of the surrounding (and confining) colloidal crystal. If the magnetic stiffness length is comparable to the colloidal lattice constant, the interplay of competing interactions stabilizes multiple topologically distinct magnetic phases separated by sharp transitions in the hysteresis curves. The colloidal confinement also induces substantial coercivity in metals that are perfectly soft in the bulk.

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PubMed2002

Theory of B(2)O and BeB(2) nanotubes: new semiconductors and metals in one dimension.

We describe two new boron-based nanotubes: B(2)O and BeB(2). Both are isoelectronic to graphite, have reasonable curvature energies, and have already been made in their bulk planar forms. The lowest energy allotrope of planar single-layer B(2)O is a semiconductor with a moderate band gap. The local density approximation band gap of the corresponding (3,0) B(2)O nanotube [similar in size to (9,0) carbon nanotube tube] is direct and around 1.6 eV, within a range inaccessible to previous C or BN nanotubes. Single-layer BeB(2) has a fascinating structure: the Be atoms rest above the boron hexagonal faces, nearly coplanar to the boron sheet. The unusual K-point pi-pi(*) Fermi-level degeneracy of graphite survives, while a new nearly pointlike Fermi surface appears at the M point. As a result, BeB(2) nanotubes are uniformly metallic.

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