F. Okuyama

Carbon nanotubes as electron source in an x-ray tube

Field emitters comprised of aligned carbon nanotubes are shown to be promising as a primary electron source in an x-ray tube working in a nonultrahigh vacuum ambience. At a pressure of 2×10−7 Torr, the nanotube emitters continue to emit electrons for more than 1 h, and yield better resolved x-ray images than do thermionic emitters, independently of whether the sample is biological or nonbiological. The near-uniformity in energy distribution of electrons emitted from carbon nanotubes might be related to the improved image quality in the field-emission mode.

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.

A miniature x-ray tube

A miniature x-ray tube is described. The tube is made of Kovar, inside which a grounded target is located close to a field-electron emitter consisting of aligned carbon nanofibers, which continues to work for around 100 h in the 10−6 Pa region unless arcing is induced between the electrodes. The resolution of the contact x-ray images provided by the tube would be impossible using the existing techniques of conventional x-ray radiography, whether the sample is biological or nonbiological.

Modeling the electron field emission from carbon nanotube films.

A theoretical framework for the electron field emission from carbon nanotubes (CNTs) is discussed. Using the tunneling theory, the influence of the detailed electron energy dispersion is proven to be of little importance for the electron field emission. By means of numerical computations in a simplified model, the influence of the environment on the local field on a CNT is discussed for an aligned CNT film. In a simple triangular model for the potential energy barrier at the tube end, a tunneling probability was obtained. A statistical model was developed for the structural and functional parameters of aligned CNT films. Practical CNT films of excellent alignment, obtained directly on a tungsten wire by plasma-enhanced chemical vapor deposition, were analyzed by this statistical model. Their distribution in the enhancement factors was thus deduced. An indirect method to get the average electrical parameters of the film using only a limited amount of experimental data was thus established.

Super-miniature x-ray tube

A transmission x-ray tube super-miniature in size is described. The x-ray tube is 5mm in diameter, and comprised of a built-in electron-emitter assembly and a grounded planar target. The key component of the emitter assembly is a Kovar pipe 2mm in diameter, inside which carbon nanofibers aligned on an electro-polished molybdenum tip are loaded to serve as the electron emitter. This type of electron emitter is highly robust in non-ultrahigh vacuum, continuing to field emit electrons for 100h or longer at pressures in the 10−5Pa region. This x-ray tube provides clear x-ray images.

Modeling of the electron field emission from carbon nanotubes

Using a tunneling approach for the field emission from a single carbon nanotube, expressions for the emission current as a function of the anode voltage and of the emitted electron energy spectrum are obtained. The low dimensionality of the electronic system of a carbon nanotube is taken into account. The extraction field on the nanotube’s tip is evaluated using numerical computations. For nanotubes of practical interest, having large enough diameters, it is demonstrated that the influence of the detailed form of the electron energy dispersion relations is not of major importance. This influence could be generally embedded in a numerical factor entering the expression of the emission current. The influence of the various tube parameters on the characteristics is also identified and analyzed. An approximate formula for use in practical analysis in field emission is deduced and its validity for different nanotube sizes is verified.

Tungsten needles produced by decomposition of hexacarbonyltungsten

Tungsten needles grown from hexacarbonyltungsten vapors in a glow‐discharge condition are described. The needle growth occurs on a tungsten substrate in a temperature range from nearly room temperature to 1500 °K, but needles produced below and above ∼1100 °K are quite different in morphology and crystalline state. The products at lower temperatures are tungsten particles agglomerated in dendritelike shapes while those at higher temperatures possibly are whiskers.

New field-emission x-ray radiography system

A new field-emission x-ray radiography system based on our design is described. The key component of the system is a triode-type x-ray source with a built-in nanostructured electron source. The electron source is comprised of palladium-induced carbon nanofibers, which continue to field-emit electrons for more than 10 h at 2×10−7 Torr with a fluctuation of ±8%. Feedback control of the potential of the electron-extracting electrode, or the gate, reduces the current fluctuation to ±0.5%, but this current regulation does little to improve the image resolution. Our system provides sharp x-ray images of both biological and nonbiological samples.

Direct evidence for In-crystallite growth on sputter-induced InP cones

High-resolution transmission electron microscopy proved that the cone evolution on Ar+-sputtered InP(100) entails the growth of In crystallites on the cone surface, obviously due to a preferential loss of P atoms. The In crystallites grew on the cone shank, as well as the cone tip, in a definite orientation formulated as InP(011) ∥ In(010) with InP[001] ∥ In[101]. The cones themselves were solely composed of InP, but involved the polycrystalline phase surrounding the original monocrystalline phase. Such a structural duality of InP cones may indicate that the target surface was in a quasi-liquid state during sputtering.

Growth of metallic whisker crystals incorporated with field electron emission

Metallic whisker crystals were found to grow on the electron‐emitting region of a pointed field cathode when it was operated in a vacuum containing metal carbonyl vapors. Metal ions are produced by collisions of carbonyl molecules with field electrons and the rapid growth of the whiskers is controlled by the supply of metal ions to the electron‐emitting area. The crystals are basically monocrystalline, and the evidence obtained indicates that they do not contain an axial screw dislocation emerging at their growing tip.