High-current field emission of point-type carbon nanotube emitters on Ni-coated metal wires

The fabrication and field emission characteristics are reported for point-type carbon nanotube (CNT) emitters formed by transferring a CNT film onto a Ni-coated Cu wire with a diameter of 1.24 mm. A Ni layer plays a role in enhancing the adhesion of CNTs to the substrate and improving their field emission characteristics. On firing at 400 °C, CNTs appear to directly bonded to a Ni layer. With a Ni layer introduced, a turn-on electric field of CNT emitters decreases from 1.73 to 0.81 V/μm by firing. The CNT film on the Ni-coated wire produces a high emission current density of 667 mA/cm² at quite a low electric field of 2.87 V/μm. This CNT film shows no degradation of emission current over 40 h for a current density of 60 mA/cm² at electric field of 6.7 V/μm. X-ray imaging of a printed circuit board with fine features is demonstrated by using our point-type CNT emitters.     

Purification of single-walled carbon nanotubes using a fixed bed reactor packed with zirconia beads

Single-walled carbon nanotube (SWCNT) soot produced by arc discharge was purified through gas and liquid phase oxidations. In the gas-phase oxidation, zirconia beads with different diameters of 1, 5, and 10 mm were packed together with raw SWCNT soot inside a vertical quartz tube to enhance air flow uniformity and an exposed surface area of the raw soot during thermal oxidation in air. A decrease of the bead sizes led to such a stronger oxidation of carbonaceous impurities that ∼10 wt.% higher weight loss was then achieved with the 1 mm beads than without them. A subsequent HNO₃ treatment and the second thermal oxidation were engaged to improve further the purity of SWCNTs. Thermogravimetric (TG) analysis, scanning electron microscopy, high resolution transmission electron microscopy, and Raman spectroscopy were used to characterize the samples. The derivative TG (DTG) curves were deconvoluted to quantitatively determine the SWCNT purity of the samples. Our final purified samples showed a yield of ∼26%, a metal impurity of ∼7% and a SWCNT purity of ∼83% as calculated from the deconvoluted DTG curves.     

Spectroscopic studies and electrical properties of transparent conductive films fabricated by using surfactant-stabilized single-walled carbon nanotube suspensions

This study evaluates the effect of anionic and cationic surfactants on the dispersion of purified SWCNTs in water in terms of dispersibility and on electrical conductivity of TCFs and electronic band structures of SWCNTs. The dispersibility of surfactants in an aqueous SWCNT suspension is assessed with the amount of SWCNTs dispersed, the content of surfactants required to suspend SWCNTs, and the long-term stability of dispersion. Sodium dodecylbenzene sulfonate (SDBS) shows better dispersibility and electrical conductivity of SWCNTs than sodium dodecyl sulfate, sodium cholate, and cetyltrimethyl ammonium bromide. Electronic band structures of SWCNTs vary with surfactants and nitric acid treatment, investigated by using UV–Vis–NIR and Raman spectroscopy. Metallic and semiconducting SWCNTs and surfactants make electrostatic charge interactions between them, which occur in different manners according to the electronic types of tubes and the natures of surfactants. TCFs are fabricated by using the SWCNT suspension dispersed with SDBS, which reveal a low percolation threshold with the two dimensional percolation behavior. The highest ratio of dc to optical conductivity (σ_dc/σ_op) is observed to be ∼23.1, corresponding to sheet resistance of 69 Ω/sq at the 550-nm optical transmission of 80%, upon nitric acid treatment of the SWCNT films.