Y. Fujimoto

Growth of aligned carbon nanotubes by plasma-enhanced chemical vapor deposition: Optimization of growth parameters

Direct-current plasma-enhanced chemical vapor deposition (CVD) with mixtures of acetylene and ammonia was optimized to synthesize aligned carbon nanotubes (CNTs) on Co- or Ni-covered W wires with regard to wire temperature, wire diameter, gas pressure, and sample bias. A phase diagram of CNT growth was established experimentally in this optimization process. It was revealed by transmission electron microscopy that Co-catalyzed CNTs encapsulated a Co carbide nanoparticle at their tip, disagreeing with a previous report that Co particles were located at the base of CNTs CVD grown on Co-covered Si substrates [C. Bower et al., Appl. Phys. Lett. 77, 2767 (2000)]. This leads to the conclusion that the formation mechanism of aligned CNTs depends significantly on the catalyst support material as well as the catalyst material itself. Since the sample bias strongly affected the morphology of CNTs, the selective supply of positive ions to CNT tips was possibly responsible for the alignment of growing CNTs.

Field electron emission from sputter-induced carbon nanofibers grown at room temperature

Graphite, carbon-coated silicon, and carbon-coated nickel surfaces were bombarded with obliquely incident Ar+ ions at room temperature. The sputtered surfaces were covered with conical protrusions, ∼2.5×105mm−2 or higher in numerical density, and partially aligned single carbon nanofibers (CNFs), ∼20nm in diameter and 0.3‐2μm in length, grew on the tips. They were characterized by the amorphous nature and the boundaryless structure between the CNF and the conical base. The field electron emission measurements for the CNFs thus grown on the carbon-coated silicon substrate showed the threshold field of 1.8V∕μm with a current density of 1μA∕cm2, and the field enhancement factor was estimated to be 1951 from the Fowler-Nordheim plot assuming the work function of 4.6eV for graphite. The morphological structure of CNFs grown on conical bases was thought to be effective to reduce the screening effect due to sufficient distance between adjacent CNFs. Thus, the sputter-induced CNFs were concluded to be quite promi...

FORMATION OF CARBON NANOTUBES AND THEIR FILLING WITH METALLIC FIBERS ON ION-EMITTING FIELD ANODES

Detailed descriptions are given of the formation of carbon nanotubes and their filling with metallic fibers developing on field anodes in the so-called “anode activation” process. Unlike the multishell carbon nanotubes produced in arc plasmas, these carbon nanotubes are not perfectly graphitized. The metallic fibers grown inside the nanotubes contain chromium as their major component, but their structure does not conform to any of the known compounds of Cr nor to elemental Cr. The growth morphology and lateral dimension of metallic fibers are governed by those of their host nanotubes, thus falling into the category of “template-mediated” crystal growth. Evidence is also presented that the metallic fibers epitaxially promote the graphitization of the carbon nanotubes.

Morphological and structural features of Cu seed cones

Morphologies and structures of surface cones formed on polycrystalline Cu targets seeded with Mo were surveyed by means of high‐resolution electron microscopy. The sputtering of the target with a focused Ar+‐ion beam generated Cu cones having diversified configurations and crystalline states, obviously reflecting an intricate mechanism of cone evolution. Some of the cones observed were characterized by an amorphouslike structure, commonly grown at the apex areas, and Mo was never detected from coned areas. These findings strongly suggest that the role of Mo seeds was to nucleate cones, and that the direct supply of sputtered Cu atoms to the cone nucleation sites interplayed with the ion‐etching process so as to develop the nucleated cones.

Structural and compositional analyses of cones formed on ion-sputtered GaAs surfaces

Abstract The structural and compositional features of “dual structured” cones formed on monocrystalline GaAs surfaces exposed to 3 keV Ar+ and Xe+ ions were determined by means of transmission electron microscopy and energy-dispersive X-ray analysis. The cones were found to be comprised of monocrystalline and amorphous phases, with the latter covering the former. The amorphous phase was Ga-rich, compared with the monocrystalline phase, and was too thick to explain its formation in terms of the ion-induced disordering of semiconductor surfaces. The growth of such a thick amorphous layer may be attributed to the interplay between an enhanced supply of sputtered atoms onto evolving cone tips and a preferential removal of redeposited As atoms from the tips.

Field electron emission from carbon nanotubes grown by plasma-enhanced chemical vapor deposition

Densely distributed, aligned carbon nanotubes were grown on Co-covered W wires by dc-plasma-enhanced chemical vapor deposition with a mixture of acetylene and ammonia as precursors. Each nanotube was capped with a Co carbide particle. The diameter of the nanotubes ranged from 20 to 130 nm, and their average length was about 5 μm. Their field emission characteristics, measured in a modified ultra-high vacuum scanning electron microscope, were analyzed by a statistical field emission model developed for aligned carbon nanotube film cathodes. An indirect method was thus established to obtain the average electrical parameters of the film using only a limited amount of experimental data.

fcc needle crystals of molybdenum grown from Mo(CO)6

It is shown that fcc needle crystals vapor‐grown from Mo(CO)6 through a cathodic growth process are fcc polymorphs of Mo. Electron diffraction studies coupled with Auger spectroscopy revealed that the needles consist of randomly oriented single microcrystals, whose lattice constant, determined by electron diffraction, is 4.14 – 4.21 A. This is greater than the theoretically expected value. It is believed that this expanded fcc lattice is attributable primarily to the growth of the needles in a high electric field.

Structural and geometrical features of copper seed cones observed by high‐resolution electron microscopy

Structural and geometrical properties of cones developed on Cu surfaces bombarded with obliquely incident Ar+ ions while simultaneously supplying ‘‘seeds’’ by sputtering were determined in atomic detail, by means of high‐resolution electron microscopy. The seed materials employed were Mo, W, Ta, Ti, and Pt, and sputtered seed atoms were found to coat the upper cone slopes which received high seed fluxes, independently of seed species. The seed films thus formed were monocrystalline for Mo, W, and Pt, and amorphous or semiamorphous for Ta and Ti. Geometrically, the cones induced by Mo, W, Ta, and Ti were conelike, but those formed on the Pt‐seeded surfaces were tentlike rather than conelike. For the conelike geometries, the tip of seed layer was grown so as to shield the substrate part of the cone from the ion beam. Perhaps, seed atoms must be more resistant to sputtering than target atoms, for seed cones to be conelike.

TEM observation of Ag microcones

Abstract Transmission electron microscopy of ion-induced microcones of Ag revealed that their tip and shank regions were entirely different in their crystalline state. The tip region exhibited a less developed crystallinity, being composed of crystallites and platelets of Ag, whereas the shank region was usually monocrystalline. It is supposed that the tips of the evolving cones were at too high a temperature and pressure to maintain the original bulk structure.

Palladium nanoparticles grown by hydrogen and deuterium ion bombardment

Palladium particles with nm dimensions are shown to grow on polycrystalline Pd interacting with low-energy hydrogen or deuterium ions. The Pd particles thus formed are mostly spherical in geometry, dispersing in the amorphous matrix. To interpret this unique mode of particle growth, we hypothesize that the particles are solidified from liquid droplets. Hydrogen or deuterium ion bombardment of Pd is thus supposed to involve an exothermic process, which may play a vital role in nucleating Pd particles in the matrix and developing the nucleated particles.