D.T. Shaw

Field emission of different oriented carbon nanotubes

Field emission data from aligned high-density carbon nanotubes (CNTs) with orientations parallel, 45°, and perpendicular to the substrate have been obtained. The large-area uniformly distributed CNTs were synthesized on smooth nickel substrates via dc plasma-assisted hot filament chemical vapor deposition. CNTs with diameters in the range of 100–200 nm were employed in this study. The different orientations were obtained by changing the angle between the substrate and the electrical field direction. The growth mechanism for the alignment and orientation control of CNTs has been discussed. The CNTs oriented parallel to the substrate have a lower onset applied field than those oriented perpendicular to the substrate. This result indicates that electrons can emit from the body of the CNT, which means that the CNT can be used as a linear emitter. The small radius of the tube wall and the existence of defects are suggested as the reasons for the emission of electrons from the body of the tubes.

Deposition of Superconducting Y-Ba-Cu-O Films at 400°C Without Post-annealing

Superconducting thin films of Y1Ba2Cu3O7−x were fabricated using the process of plasma‐assisted laser deposition. The substrate temperature was as low as 400 °C and high‐temperature post‐annealing in an O2 atmosphere was not necessary. The as‐deposited films have a Tc of ∼85 K, and are oriented mostly with the c axis perpendicular to the substrate surface. The measured Jc at 80 K was 105 A/cm2.

Nanostructure Science and Technology

Nanostructure science and technology now forms a common thread that runs through all physical and materials sciences and is emerging in industrial applications as nanotechnology. The breadth of the subject material is demonstrated by the fact that it covers and intertwines many of the traditional areas of physics, chemistry, biology, and medicine. Within each main topic in this field there can be many subfields. For example, the electrical properties of nanostructured materials is a topic that can cover electron transport in semiconductor quantum dots, self-assembled molecular nanostructures, carbon nanotubes, chemically tailored hybrid magnetic-semiconductor nanostructures, colloidal quantum dots, nanostructured superconductors, nanocrystalline electronic junctions, etc. Obviously, no one book can cope with such a diversity of subject matter. The nanostructured material system is, however, of increasing significance in our technology-dominated economy and this suggests the need for a series of books to cover recent developments. The scope of the series is designed to cover as much of the subject matter as possible – from physics and chemistry to biology and medicine, and from basic science to applications. At present, the most significant subject areas are concentrated in basic science and mainly within physics and chemistry, but as time goes by more importance will inevitably be given to subjects in applied science and will also include biology and medicine. The series will naturally accommodate this flow of developments in the sciences and technology of nanostructures and maintain its topicality by virtue of its broad emphasis. It is important that emerging areas in the biological and medical sciences, for example, not be ignored as, despite their diversity, developments in this field are often interlinked. The series will maintain the required cohesiveness from a judicious mix of edited volumes and monographs that while covering subfields in depth will also contain more general and interdisciplinary texts. Thus the series is planned to cover in a coherent fashion the developments in basic research from the distinct viewpoints of physics, chemistry, biology, and materials science and also the engineering technologies emerging from this research. Each volume will also reflect this flow from science to technology. As time goes by, the earlier series volumes will then serve as reference texts to subsequent volumes.

Generation of high‐energy atomic beams in laser‐superconducting target interactions

High‐energy atomic beams with Mach numbers as high as 5 were observed in excimer laser‐superconducting target interactions. The velocity distributions of the Y, Ba, Cu, and O atoms and ions could be described very well by a supersonic expansion‐type mechanism similar to a molecule beam. The physics of the atomic beam formation process is discussed.

Field emission from aligned high-density graphitic nanofibers

Field emission data from aligned graphitic nanofibers have been obtained. The aligned nanofibers are 50–100 nm in diameter and 6–10 μm in length, with a density of 109–1010/cm2. The fibers were grown on polycrystalline nickel substrate by plasma-assisted hot filament chemical vapor deposition using a gas mixture of nitrogen and acetylene. The onset of emission current in microampere level was detected at about 1.8 V/μm with an emission area of 1 mm2. The Fowler–Nordheim model was used to analyze the data obtained. The field emission current required for flat panel display can be easily achieved at 2.5 V/μm.

Role of the oxygen atomic beam in low-temperature growth of superconducting films by laser deposition

An oxygen jet placed near the target during plasma‐assisted laser deposition produces a strong atomic oxygen beam with kinetic energies of 5.6 eV, simultaneous with the laser‐induced atomic beams of Ba, Cu, and Y from the target. All atomic beams can be well characterized by a supersonic expansion mechanism. The behavior of the velocity distributions was studied as a function of the distance from the target and laser energy fluence. A target‐substrate separation of 7 cm was found to be optimum in terms of producing the best as‐deposited films. At that distance, the velocity distributions of all atomic beams become nearly the same.

Enhancement of persistent currents in Bi2Sr2CaCu2O8 tapes with splayed columnar defects induced with 0.8 GeV protons

Composite tapes of the superconductor Bi2Sr2CaCu2O8 on silver were irradiated with energetic light ions (0.8 GeV protons), creating extended splayed tracks ∼7 nm in diameter via fission of Bi nuclei. Magnetic hysteresis indicates large enhancements of persistent currents J, especially at high fields and temperatures, and substantial expansion of the irreversible regime. The technique may be suitable for large scale applications due to the long range (∼half meter) of fast protons.

Hydrogen storage in aligned carbon nanotubes

Aligned carbon nanotubes (CNTs) with diameters of 50–100 nm, synthesized by plasma-assisted hot filament chemical vapor deposition, were employed for hydrogen adsorption experiments in their as-prepared and pretreated states. Quadruple mass spectroscopy and thermogravimetric analysis show a hydrogen storage capacity of 5–7 wt% was achieved reproducibly at room temperature under modest pressure (10 atm) for the as-prepared samples. Pretreatments, which include heating the samples to 300 °C and removing of the catalyst tips, can increase the hydrogen storage capacity up to 13 wt% and decrease the pressure required for storage. The weight gains were measured after the samples moved out of the hydrogen environment. The release of the adsorbed hydrogen can be achieved by heating the samples up to 300 °C.

As‐deposited Y‐Ba‐Cu‐O superconducting films on silicon at 400 °C

Y‐Ba‐Cu‐O thin films have been grown on silicon at a substrate temperature of 400 °C by plasma‐assisted laser deposition technique. These films were superconducting in an as‐deposited state. Films deposited directly on silicon and films with a MgO buffer layer differed in their superconducting properties. Films with a MgO layer showed higher critical temperatures (70 K) and higher critical currents (3×103 A/cm2 at 31 K) than films deposited directly on Si. Depth profiling by Auger and x‐ray photoelectron spectroscopy has been employed to study the diffusion and structural variation near the interface.

Spectroscopic study of plasma-assisted laser deposition of Y-Ba-Cu-O

Photoluminescence spectroscopy of the plasma plume during plasma‐assisted laser deposition of YBa2Cu3O7−x was performed. The spectrum in the range of 355–900 nm was measured at several spatial positions using an optical multichannel analyzer. It was found that both atomic and molecular components were present. Rich emission lines of O, O+, Y, Y+, Ba, Ba+, and Cu were observed, from which the plasma temperature could be estimated. Cu+ emission was conspicuously absent. The mechanism of the plasma process and its relevance to thin‐film formation is discussed.