Tae Jae Lee

Field emission from well-aligned zinc oxide nanowires grown at low temperature

Field electron emission from vertically well-aligned zinc oxide (ZnO) nanowires, which were grown by the vapor deposition method at a low temperature of 550 °C, was investigated. The high-purity ZnO nanowires showed a single crystalline wurtzite structure. The turn-on voltage for the ZnO nanowires was found to be about 6.0 V/μm at current density of 0.1 μA/cm2. The emission current density from the ZnO nanowires reached 1 mA/cm2 at a bias field of 11.0 V/μm, which could give sufficient brightness as a field emitter in a flat panel display. Therefore, the well-aligned ZnO nanowires grown at such low temperature can promise the application of a glass-sealed flat panel display in a near future.

Synthesis of aligned carbon nanotubes using thermal chemical vapor deposition

Aligned carbon nanotubes have been synthesized on transition metal-coated silicon substrates with C2H2 using thermal chemical vapor deposition. It was found that nanotubes can be mostly vertically aligned on a large area of plain Si substrates when the density of metal domains reaches a certain value. Pretreatment of Co–Ni alloy by HF dipping and etching with NH3 gas prior to the synthesis is crucial for vertical alignment. Steric hindrance between nanotubes at an initial stage of growth forces nanotubes to align vertically. Nanotubes are grown by a catalyst-cap growth mechanism. Applications to field emission displays are demonstrated with emission patterns.

Structural study of nitrogen-doping effects in bamboo-shaped multiwalled carbon nanotubes

We have investigated nitrogen doping effects on the structure and crystallinity of bamboo-shaped multiwalled carbon nanotubes (BS-MWNTs) by means of x-ray photoemission spectroscopy (XPS) and transmission electron microscopy. By controlling the NH3/C2H2 flow ratio during the chemical vapor deposition, the nitrogen concentrations of 0.4% to 2.4% were obtained. According to the XPS measurements, the increasing nitrogen concentration gave rise to an increase of the N-sp3 C bonds as well as the deterioration of the crystallinity of the BS-MWNTs. Besides, the N-sp3 C bonds were found to prevail over the N-sp2 C bonds above 5% nitrogen concentration. At higher nitrogen concentrations, the BS-MWNTs showed shorter compartment distances, presumably due to the suppressed surface diffusion of carbon on the catalyst particles.

Synthesis of uniformly distributed carbon nanotubes on a large area of Si substrates by thermal chemical vapor deposition

We have synthesized carbon nanotubes by thermal chemical vapor deposition of C2H2 on transition metal-coated silicon substrates. Multiwalled carbon nanotubes are uniformly synthesized on a large area of the plain Si substrates, different from previously reported porous Si substrates. It is observed that surface modification of transition metals deposited on substrates by either etching with dipping in a HF solution and/or NH3 pretreatment is a crucial step for the nanotube growth prior to the reaction of C2H2 gas. We will demonstrate that the diameters of carbon nanotubes can be controlled by applying the different transition metals.

Carbon nanofibers grown on sodalime glass at 500°C using thermal chemical vapor deposition

Abstract Carbon nanofibers are grown homogeneously on a large area of nickel-deposited sodalime glass substrate by thermal chemical vapor deposition of acetylene at 500°C. The diameters of carbon nanofibers are uniformly distributed in the range between 50 and 60 nm. Most of the carbon nanofibers are curved or bent in shape, but some fractions are twisted. They consist of defective graphitic sheets with a herringbone morphology. The maximum emission current density from the carbon nanofibers is 0.075 mA / cm 2 at 16 V / μm , which is sufficient for commercializing the carbon-nanofibers-based field emission displays.

Growth mechanism of vertically aligned carbon nanotubes on silicon substrates

Abstract We have synthesized vertically aligned carbon nanotubes by thermal chemical vapor deposition using C 2 H 2 gas on a large area of transition metal-coated Si substrates. It is observed that control of nucleation sites of transition metals deposited on Si substrates by a dipping in a HF solution and/or NH 3 pretreatment is a crucial step for the growth of vertically aligned carbon nanotubes prior to the reaction of C 2 H 2 gas. We show that the transition metals play as nucleation seeds and lead to further cap growth by forming a metal cap at the end of nanotubes.

Metallic conductivity in bamboo-shaped multiwalled carbon nanotubes

Temperature-dependent resistivity measurements were carried out on bamboo-shaped multiwalled carbon nanotubes (MWNTs) grown by thermal chemical vapor deposition. They were deposited on Al2O3/Ti and SiO2/Ti substrates using cobalt and iron catalysts, respectively. As a result, a metallic conductivity, i.e. resistivities with a positive temperature slope, was observed for the MWNTs grown on the SiO2/Ti substrates. The different temperature behaviors of the resistivity for the MWNTs grown on different substrates are discussed in view of the substrate morphology and crystallinity.

Selective growth and field emission of vertically well-aligned carbon nanotubes on hole-patterned silicon substrates

We have achieved selective growth of high-purity carbon nanotubes (CNTs) on iron-deposited hole-patterns by thermal chemical vapor deposition (CVD) of acetylene gas. The vertically well-aligned CNTs were uniformly synthesized with good selectivity on hole-patterned silicon substrates. The CNTs indicated multiwalled and bamboo-like structure. The turn-on gate voltage at the CNT-based triode structure was about 55 V and emission current density was 2.0 μA at the applied gate voltage of 100 V.

Synthesis of single- and double-walled carbon nanotubes by catalytic decomposition of methane

Abstract Single-walled carbon nanotubes (SWNTs) and double-walled carbon nanotubes (DWNTs) are simultaneously synthesized by catalytic decomposition of CH 4 over Fe–Mo/Al 2 O 3 catalyst. High-resolution transmission electron microscopy observation shows that produced carbon materials consist of about 70% SWNTs and about 30% DWNTs. The diameters of SWNTs are in the range of 0.8–1.5 nm while the outer and inner diameters of DWNTs are in the range of 1.75–3.1 and 0.95–2.3 nm, respectively. Raman analysis indicates that the synthesized SWNTs and DWNTs have high-quality graphite structure.

Controlled growth of carbon nanotubes over cobalt nanoparticles by thermal chemical vapor deposition

Controlled growth of carbon nanotubes (CNTs) has been achieved by thermal chemical vapor deposition of acetylene gas over nanometer-sized cobalt particles. The well-aligned CNTs, which have a uniform diameter and high purity, are synthesized over cobalt nanoparticles distributed on substrates of large area. The alignment, density, and diameter of the CNTs are easily controlled by adjusting the density of the cobalt nanoparticles. Moreover, growth rate, density, diameter, and crystallinity of CNTs grown over the cobalt nanoparticles are also well controlled by the growth temperature. Our results demonstrate that the controlled growth of CNTs can be effectively realized by adjusting cobalt nanoparticles and growth temperature.