J.P. Zhang

Controlled production of aligned-nanotube bundles

Carbon nanotubes might be usefully employed in nanometre-scale engineering and electronics. Electrical conductivity measurements on the bulk material, on individual multi-walled and single-walled nanotubes and on bundles of single-walled nanotubes have revealed that they may behave as metallic, insulating or semiconducting nanowires, depending on the method of production—which controls the degree of graphitization, the helicity and the diameter. Measurements of Youngs modulus show that single nanotubes are stiffer than commercial carbon fibres. Methods commonly used to generate nanotubes—carbon-arc discharge techniques, catalytic pyrolysis of hydrocarbons and condensed-phase electrolysis—generally suffer from the drawbacks that polyhedral particles are also formed and that the dimensions of the nanotubes are highly variable. Here we describe a method for generating aligned carbon nanotubes by pyrolysis of 2-amino-4,6-dichloro-s-triazine over thin films of a cobalt catalyst patterned on a silica substrate by laser etching. The use of a patterned catalyst apparently encourages the formation of aligned nanotubes. The method offers control over length (up to about 50 μm) and fairly uniform diameters (30–50 nm), as well as producing nanotubes in high yield, uncontaminated by polyhedral particles.

Current rectification in a single GaN nanowire with a well-defined p-n junction

This letter discusses Mg incorporation in GaN nanowires with diameters ∼35 nm, fabricated by vapor–liquid–solid synthesis in p-type nanowires. Turning on the Mg doping halfway through the synthesis produced nanowires with p–n junctions that showed excellent rectification properties down to 2.6 K. The nanowires are shown to possess good-quality, crystalline, hexagonal GaN inner cores surrounded by an amorphous GaN outer layer. Most wires grow such that the crystalline c axis is normal to the long axis of the nanowire. The temperature dependence of the current–voltage characteristics is consistent with electron tunneling through a voltage-dependent barrier.

Electron irradiation damage in oxides

Abstract Structural changes occurring at or near surfaces during irradiation in the electron microscope have been reported in a broad range of oxide systems, but only relatively recently have they been investigated primarily in terms of radiation damage. The present study seeks to categorize some recent results as well as results given in the literature by the type of structural modification observed in order to gain a better understanding of the damage mechanism, whether it be ballistic, electronic, or thermal in nature. A specific example is given comparing TiO2, a material known to undergo surface ionization damage in the form of desorption induced by electronic transitions (or DIET), and NiO, a material whose DIET characteristics are not well established. Electron irradiation damage is found to depend not only on the nature of the system, for example the valency of the cationic species, but also on the crystal orientation and surface preparation. The behavior of these materials indicates a complex radiation damage scheme which cannot be explained solely by electronic, thermal or ballistic damage considerations.

Interface ferromagnetism in (110)-oriented La 0.7 Sr 0.3 MnO 3 / SrTiO 3 ultrathin superlattices

We explore manganite interface magnetism in epitaxially grown La0.7Sr0.3MnO3(LSMO)/SrTiO3 ultrathin superlattices (SL) along (110) orientation. we show that robust ferromagnetism persists down to four monolayers LSMO(MLs) (~1.1 nm in thickness), of which 50% Mn is at the interface state. Above eight MLs, the magnetic moment is nearly saturated to the theoretical value of 3.7u_B, with an estimated interface moment of 3.2u_B. In comparison to (100)-oriented SLs which were previously shown to have a spin canted ground state, (110)-oriented SLs exhibit stronger low-dimensional ferromagnetism and better metallicity, suggesting a ferromagnetic interface spin state well suited for all-oxide spintronic devices. The underlining mechanism is qualitatively discussed.

Order-disorder in YBa2Cu3O7

Abstract We summarize here results of a fairly detailed study by high-resolution electron microscopy and electron diffraction of the high-temperature superconductor YBa 2 Cu 3 O 7−δ . Data from the perfect structure for (100), (110), and (001) zones indicate that the material has an oxygen-deficient perovskite structure as previously reported from X-ray and neutron diffraction studies. In some cases planar defects along (001) directions were observed. A more important observation is the existence of a phase with disorder of the Y and Ba ion positions and of an additional amorphous phase. The perfectly ordered (defect-free) phase grows from the disordered material, and the planar defects are probably due to mechanical deformation during preparation of the material. These defects probably are not related to the superconducting properties, as recently suggested. The conclusions are consistent with the commonly encountered problems in the preparation of superconductors in high yields.

Encapsulation, diffusion and diet in the electron microscope

Abstract Observations of a large number of different oxides in the electron microscope clearly indicate that a crucial role is played by the presence or absence of encapsulating layers, either amorphous carbon, graphitic carbon or damaged metallic regions produced by DIET of the oxygen, and point defect diffusion in the subsurface region. In the presence of an encapsulation layer DIET slows by a factor which is related to the point defect migration through this layer.

Submicron GaMn quasicrystals in ferromagnetic GaAs

GaMn icosahedral particles with quasicrystalline order have been found in Mn implanted and rapidly annealed GaAs by means of selected-area electron diffraction, high-resolution, and dark-field electron microscopy. The orientation relationship between the submicron quasicrystals with the icosahedral (2/m 35) symmetry and the face-centered cubic (fcc) GaAs matrix is determined to be: i5 (the fivefold inversion axis in (2/m 35))//[110]GaAs, i3 (the threefold inversion axis)//[111]GaAs, and i2 (the twofold axis)//[121]GaAs. The statistics of these structural studies, combined with magnetic force microscopy, indicate that the submicron quasicrystals are ferromagnetic.

Atomic imaging of surfaces in plan view

We report experimental results imaging the surface diffraction spots in the plan view geometry from the Si(111) surface and the Ir(001) surface. High quality images have been obtained using conventional large aperture high resolution electron microscopy (HREM), a smaller aperture to exclude the bulk diffraction spots and with highly tilted crystals. The experimental data indicates that there should be no major problems in obtaining atomic scale surface information in plan view.

Superconductors and nonsuperconductors in Nd2-χCeχCuO4

Abstract Results are presented on the chemical composition and structure of the n-type superconductors Nd 2-χ Ce χ CuO 4 . Electron diffraction and high resolution electron microscopy indicate that there are three slightly different structures, two of which are modulated versions of the first. By matching both the images and diffraction patterns it is shown that the modulations are due to small displacements of the cations, possibly a charge density wave, not simply ordering of the Ce or oxygen vacancies, although oxygen stoichiometry may be the driving force for the modulations. Both types of phases occur in materials which are superconducting, and those which are not. Microanalysis indicates that the Ce content of the superconducting specimens after reduction is slightly smaller at 0.16 rather than 0.19, and the cation stoichiometry is not detectably different between the modulation and unmodulated materials.

Is there an electron wind

Abstract The question of whether there is any momentum transfer to a specimen during electron diffraction, as dictated by Newtons third law, is discussed. It is pointed out that there must be momentum and energy transfer during “elastic” scattering. Even though what is normally considered as “elastic” scattering is in fact inelastic, electron density systems are insensitive to the changes in energy. Therefore the energy changes during diffraction do not prevent coherent interference phenomena such as electron holography and high-resolution electron microscopy. The momentum transfer, in effect, introduces an electron wind which can, mechanically, alter the specimen; the wind is equivalent to a storm with gusts of up to 300 km/h. Experimental evidence in support of this is presented.