Koji Asaka

Nanowelding of a multiwalled carbon nanotube to metal surface and its electron field emission properties

Multiwalled carbon nanotubes (MWNTs) were manipulated inside a high-resolution transmission electron microscope combined with a nano manipulator, and a single MWNT emitter freestanding on a platinum surface was fabricated by nanowelding. The electron field emission properties of the single MWNT were in situ measured at various gap distances in a range from 27to442nm with simultaneous imaging by transmission electron microscopy. Field enhancement factors varying with the gap distance and the thermally enhanced electron emission were observed.

Buckling of C60 whiskers

The authors demonstrated the mechanics of materials for crystalline whiskers composed of C60 molecules; compressive deformation of the whiskers was observed by in situ transmission electron microscopy with simultaneous force measurement by means of an optical cantilever method, as used in atomic force microscopy. In response to compression along the long axis, the whiskers bent first elastically, then buckled. A whisker with 160nm diameter fractured brittlely at a strain of 0.08. According to Euler’s formula, Young’s modulus of the whisker was estimated to be 32–54GPa, which is 160%–650% of that of C60 bulk crystals.

Martensitic transformation in nanometer-sized particles of Fe–Ni alloys

Abstract We studied the martensitic transformation in nanometer(nm)-sized particles of Fe–Ni alloys with the composition range from 15 to 30 at.% Ni by means of transmission electron microscopy and electron diffraction. Here are reported results obtained mainly for nm-sized particles of an Fe-25.0 at.% Ni alloy. The forward transformation temperatures were depressed to a fair extent, compared to those in bulk, while the reverse transformation temperatures were less affected by size. It appeared that both the austenite and martensite particles were faceted by certain crystallographic planes. The temperature parameter in the Debye–Waller factor measured for the austenite particles was found to be about one order larger than those for Fe and Ni in bulk. These results suggested that the observed size-dependence of transformation temperatures is accounted for in terms of the surface free energy difference between the fcc and bcc nm-sized particles.

Graphitization of amorphous carbon on a multiwall carbon nanotube surface by catalyst-free heating

Structural changes in amorphous carbon coating the surfaces of multiwall carbon nanotubes (MWNTs) under applying an electric current were investigated by in situ transmission electron microscopy with simultaneous measurements of the bias voltage and electric current. Joule heating transformed amorphous carbon on the surfaces of individual MWNTs suspended between gold electrodes into graphite layers even without a metal catalyst through a phase of glasslike carbon. The MWNTs after the formation of ordered surface layers sustained a high current with a density of up to 3.1 × 108 A/cm2.

Light-emitting filaments composed of nanometer-sized carbon hollow capsules

Filaments composed of multilayered carbon hollow capsules less than 10nm in diameter have been synthesized from crystalline whiskers of fullerene C60 of submicrometer in diameter by preliminary heating at 1373K and successive impression of current. The structural, electrical and optical properties of the filaments were studied by in situ transmission electron microscopy combined with the functions of scanning probe microscopy. We found that the filaments easily emit visible light at a maximum wavelength of 700-730 nm by the application of a few volts.

Evolution of Field Electron Emission Pattern from Multilayered Graphene Induced by Structural Change of Edge

Structures and field emission (FE) properties of exfoliated multilayered graphene have been investigated by in situ transmission electron microscopy (TEM) and field emission microscopy (FEM). TEM revealed that edges of multilayered graphene, which were open before FE measurement, became closed after FE under high emission current, say over a few tens µA. Corresponding to this change in the edge structure, FEM images evolved from a striped pattern for an open edge to a dim one for a closed edge. The structural change under the high current condition is presumably induced by Joule heating of the graphene tip.

The toughness of multi-wall carbon nanocapsules

A single multi-wall carbon nanocapsule (NC) was compressed by piezomanipulation inside a transmission electron microscope and then allowed to recover. The atomic structural dynamics were observed in situ with simultaneous measurement of the sub-nanonewton forces acting on the NC. The NC completely bent under a load of 4.6 nN exerted by a gold nanotip and recovered to its original shape when the nanotip was retracted. The observations showed that the NC possesses a high toughness for compression.

Transmission electron microscopy and electron diffraction studies on martensitic transformations in nanometre-sized particles of Au-Cd alloys of near-equiatomic compositions

Abstract Martensitic transformations in nanometre-sized particles of Au-Cd alloys of near-equiatomic compositions were studied by means of transmission electron microscopy and electron diffraction. The alloy particles with an average size of several nanometres were prepared by high-vacuum electron-beam deposition. In the as-deposited state the particles were crystalline and possessed a B2-type structure at room temperature, similar to that of the β2 phase in the corresponding bulk. The β2−phase particles with an average composition of Au-44.5at.∼Cd were transformed at 76 K into an orthorhombic structure similar to that of the γ2 phase in the bulk, while those with an average composition of Au-50.2at.∼ Cd were transformed at 60K into a trigonal structure similar to that of the ζ2 phase in bulk. The temperature parameter in the Debye-Waller factor measured at room temperature for the β2−phase particles was roughly four times that for Au in the bulk. It was thus found that the M s temperatures for the β2 ζ γ‘2 and β2 → ζ2’ transformations in the nanometre-sized particles were lowered far below room temperature despite the increasing surface area and the enhanced lattice softening. Their reverse transformations, however, took place on heating to room temperature. It was thus seen that the lowering of the transformation temperatures was due to the decrease in the thermodynamic equilibrium temperatures T 0 between the β2 and γ2 and between the β, and ζ′2 phases. The decrease in T 0 indicated that the free energies of the β2, γ2 and ζ2 phases of the alloys in the small-particle state are definitely different from those in the bulk state.

Structure of nanometer-sized Au–Cd alloy particles near equiatomic compositions at room temperature

Abstract Nanometer (nm)-sized Au–Cd alloy particles with a composition of Au–44.6at.% Cd on average were prepared by a high vacuum electron beam deposition method, and their structure at room temperature was studied by lattice imaging and Fourier transform techniques in transmission electron microscopy, and electron diffraction. The Au–Cd alloy particles exhibited polyhedral shapes with facets, and possessed basically the B 2 type structure similar to that of the β 2 phase Au–Cd alloys near equiatomic compositions in bulk. It was thus seen that the high temperature β 2 phase becomes stable in the nm-sized particles at room temperature. It was found, however, in some β 2 phase particles that modulated regions were formed a few nm in size with periods two and six times as large as the {110} planar spacing of the B 2 lattice. The Fourier transform revealed that the modulated regions with twofold and six-fold periods have, respectively, definite crystal structures of the β 2 ′ and γ 2 ′ martensites in bulk. The modulated regions thus seemed to be the nuclei of these martensites. These martensite nuclei were coherent with each other and with the surrounding B 2 matrix lattice. No clear interfaces between these martensites and the matrix were present, other than the common close-packed planes with which these martensites and the matrix are connected.

In Situ High-Resolution Transmission Electron Microscopy of Elastic Deformation and Fracture of Nanometer-Sized Fullerene C60 Whiskers

We deformed crystalline whiskers composed of fullerene C60 molecules inside a high-resolution transmission electron microscope. The diameters of the whiskers ranged from 200 to 1000 nm and the length extended to the submillimeter scale. We selected one whisker with a body-centered tetragonal structure with a typical diameter, i.e., 540 nm, for analyses of deformation. It was found that the whisker arched elastically and the radius of curvature decreased to 20 µm until fracture, showing that the whiskers fracture strain was 0.013. Fracture occurred along {100} of the whisker, perpendicular to its longer growth axis.