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An extremely low operating electric field has been achieved on zinc oxide (ZnO) nanowire field emitters grown on carbon cloth. Thermal vaporization and condensation was used to grow the nanowires from a mixture source of ZnO and graphite powders in a tube furnace. An emission current density of 1mA∕cm2 was obtained at an operating electric field of 0.7V∕μm. Such low field results from an extremely high field enhancement factor of 4.11×104 due to a combined effect of the high intrinsic aspect ratio of ZnO nanowires and the woven geometry of carbon cloth.

Field emission from carbon nanotubes grown on carbon cloth has been studied. An extremely low electric field of less than 0.4V∕μm is required to reach an emission current density of 1mA∕cm2. This ultralow operating electric field of carbon nanotubes grown on carbon cloth is mainly due to a very high field enhancement factor of 1.882×104, which is the result of geometrical configuration of the carbon nanotubes and the substrate. In addition to the field enhancement, the highly disordered microstructure of carbon nanotubes grown on carbon cloth plays an important role to field emission. This unexpected result indicates that the roughness of the substrates on which carbon nanotubes grow is very important. This result also brings us significantly closer to practical applications such as highly efficient lamps, field emission displays, micro vacuum electron sources, etc.

The length and the spacing of carbon nanotube (CNT) films are varied independently to investigate their effect on the field-emission characteristics of the vertically aligned CNT films grown by plasma-enhanced hot filament chemical vapor deposition using pulsed-current electrochemically deposited catalyst particles. It is shown that, in general, the macroscopic electric field Emac,1, defined as the electric field when the emission current density reaches 1 mA/cm2, can be reduced by increasing the length and the spacing of CNTs. However, for the very-high-density CNT films, the increase of length increases Emac,1 slightly, whereas for the very short CNT films, the increase of spacing does not effectively reduce Emac,1.

Epitaxial superconducting thin films Tl2Ba2CuO6+δ (Tl-2201) with a continuous range of doping levels have been synthesized. The films were made by rf magnetron sputtering and two-step postdeposition annealing, and their doping levels were controlled by varying the oxygen content through annealing in flowing argon. By annealing in flowing argon, the as-grown Tl-2201 thin films were changed continuously from the initially overdoped state, through the optimally doped state and then to the underdoped state. The normal state resistivity versus temperature curves of all the underdoped Tl-2201 thin films are convex, whereas those of the overdoped Tl-2201 thin films are all concave. Transmission electron microscopy bright-field images and selected area diffraction patterns of the 58 K underdoped Tl-2201 thin film showed neither twinning bands nor spot splitting associated with an orthorhombic crystal lattice. Therefore, it is concluded that underdoped Tl-2201 thin films are also tetragonal just like all the overd...

Epitaxial films of Tl2Ba2CuO6+δ (Tl‐2201) have been prepared on SrTiO3 by sputtering and postdeposition annealing. The films were deposited onto (100)SrTiO3 single crystal substrates by rf magnetron sputtering and then annealed using a two‐step process. The annealed films typically exhibit transition temperatures of about 11 K. Subsequent reannealing to optimize the oxygen doping in the film results in an increase in the transition temperature, and optimally reannealed films exhibit transition temperatures of about 83 K. The crystal structure of these films has been studied by x‐ray diffraction and transmission electron microscopy. These results indicate that both the annealed (Tc=11 K) and optimally reannealed samples (Tc=83 K) have a tetragonal crystal structure. This structural information is crucial in evaluating recent measurements on these films to determine the electron pairing symmetry (d wave or s wave) in high‐Tc superconductors.

In situ synthesis has been developed for the first time to fabricate the Tl‐based silver‐sheathed superconducting tapes. Tapes of Tl0.5Pb0.5Sr1.6Ba0.4Ca0.8Y0.2Cu2Oy (Tl‐1212) made by the in situ synthesis method under optimal oxygen partial pressure were found to have transport Jc of 7000 A/cm2 in the absence of external magnetic field. In low magnetic field (

Simple geometric considerations show that to maximize the transport Jc for a given type of bulk superconductor, it is only necessary to have the individual grains in the sample oriented with maximal area of close contact with their immediate neighbors. With this guideline, a 24‐meter silver‐sheathed thallium‐containing superconductor of the 1223‐type without long‐range granular alignment has been made. It is flexible, uniform, and exhibits a transport Jc of 104 A/cm2 at 77 K in the absence of external magnetic field.

Perovskite-type polycrystalline La1/3Sr2/3FeO3 particles with different sizes (80-2000 nm) were prepared using a simple sol-gel technique. In samples of nanoparticles with a diameter of less than 300 nm, weak ferromagnetism was observed at room temperature, which was attributed to the lattice distortion. The magnetic and specific heat measurements suggest that the charge ordering state was largely suppressed due to the lowering of the particle size, but the charge ordering temperature remained unaffected. (C) 2007 American Institute of Physics.

Well-aligned Ca3Co2O6 nanorods (∼300nm in length and ∼40nm in diameter) on Si substrates are prepared by pulsed laser deposition. The steplike magnetization feature as identified in bulk Ca3Co2O6 can be retained in the nanorods in spite of the enhanced distortion of the magnetically ordered spin chains due to the finite size effect. The out-of-plane magnetization value is slightly larger than the in-plane value, which shows the small magnetic anisotropy. An additional distinct transition at 54K associated with the spin frustration is revealed, different from the bulk samples.

Atomic-scale microstructure changes of carbon nanotubes under high bias∕high current conditions were studied by in situ high-resolution transmission electron microscopy. We found that high bias voltage caused significant structure changes, such as crystallization and elimination of amorphous coating on the surface of nanotube walls, removal of nanotube walls, and formation of atomic-scale kinks. These structural changes are attributed to high temperatures induced by high bias resistive heating on the nanotubes. These structural changes cause dramatic electronic property changes of the nanotubes correspondingly. The results provide an efficient route to tailor the electronic properties of nanotubes by appropriate structural modifications.

Recently, we have reported an extremely strong field emission from carbon nanotubes grown on carbon cloth, with the field-enhancement factor of up to 18 800. In this paper, we study the origins of this effect, by investigating field emission from individual carbon nanotubes, in a transmission electron microscope equipped with a piezo manipulator. Microscopic analysis reveals a multistage structure of some of the nanotubes, characterized by an order of magnitude smaller nanotubes branching off the tips of bigger nanotubes (or carbon fibers). The multistage structure causes a macroscopic enhancement of the electric field, which can match that of a single macroscopically long nanotube with length equal to the combined length of all stages, and the tip radius equal to that of the thinnest nanotube in the structure. This not only explains the observed giant field enhancement, but also provides important clues for the design of nanotube emitters for electronic applications.