Yoon Huh

Temperature effect on the growth of carbon nanotubes using thermal chemical vapor deposition

Abstract Vertically aligned carbon nanotubes (CNTs) are grown on iron-deposited silicon oxide substrates by thermal chemical vapor deposition (CVD) of acetylene gas at the temperature range 750–950°C. As the growth temperature increases from 750°C to 950°C, the growth rate increases by four times and the average diameter also increases from 30 nm to 130 nm while the density decreases by a factor of about two. The relative amount of crystalline graphitic sheets increases progressively with the growth temperature and a higher degree of crystalline perfection can be achieved at 950°C. This result demonstrates that the growth rate, diameter, density, and crystallinity of CNT can be controlled with the growth temperature.

Low-temperature growth of carbon nanotubes by thermal chemical vapor deposition using Pd, Cr, and Pt as co-catalyst

Abstract Palladium (Pd), chromium (Cr), and platinum (Pt) are used as co-catalysts to decrease the growth temperature of carbon nanotubes to 500–550°C. Pd is found to be the most efficient co-catalyst for the growth of carbon nanotubes on cobalt-nickel catalytic particles deposited on a silicon oxide substrate by thermal chemical vapor deposition using C 2 H 2 . High-resolution transmission electron microscopy reveals the bamboo-shaped carbon nanotubes grown at 500°C using Pd, while the curled carbon nanofibers are grown at 550°C using Cr.

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.

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.

Low temperature growth of vertically aligned carbon nanotubes by thermal chemical vapor deposition

Abstract Vertically well-aligned carbon nanotubes (CNTs) are grown on Fe-deposited silicon oxide substrate at 550°C by thermal chemical vapor deposition of C 2 H 2 gas. We employed two-stage heating technique that the reactants heated at 850°C in the first zone flow into the second zone maintained at 550°C for CNT growth. The CNTs have bamboo structure, closed tip, and defective graphite sheets.

Aligned silver nanowire-based transparent electrodes for engineering polarisation-selective optoelectronics.

We herein report on a remarkably simple, fast, and economic way of fabricating homogeneous and well oriented silver nanowires (AgNWs) that exhibit strong in-plane electrical and optical anisotropies. Using a small quantity of AgNW suspension, the horizontal-dip (H-dip) coating method was applied, in which highly oriented AgNWs were deposited unidirectionally along the direction of coating over centimetre-scale lengths very rapidly. In applying the H-dip-coating method, we adjusted the shear strain rate of the capillary flow in the Landau-Levich meniscus of the AgNW suspension, which induced a high degree of uniaxial orientational ordering (0.37–0.43) of the AgNWs, comparable with the ordering seen in archetypal nematic liquid crystal (LC) materials. These AgNWs could be used to fabricate not only transparent electrodes, but also LC-alignment electrodes for LC devices and/or polarising electrodes for organic photovoltaic devices, having the potential to revolutionise the architectures of a number of polarisation-selective opto-electronic devices for use in printed/organic electronics.

Investigating the origin of S-shaped photocurrent-voltage characteristics of polymer:fullerene bulk-heterojunction organic solar cells

We herein investigated the origin of the S-shape behaviour exhibited near the compensation voltage in the photocurrent density-voltage (Jph-V) characteristics of organic solar cells. P3HT:PCBM bulk-heterojunction solar cells annealed at a too high temperature show the S-shaped Jph-V characteristics. Optical microscopy images revealed the interface degradation. Utilizing a drift-diffusion model, we found that the S-shape behavior cannot be seen by a sole effect of charge accumulation or imbalance of charge carrier mobilities. Rather, the effect of both combined resulting in strong bimolecular recombination causes a reduction in photocurrent near the compensation voltage, producing the S-shape feature.

Power generating reflective-type liquid crystal displays using a reflective polariser and a polymer solar cell

We herein report the results of a study of a power generating reflective-type liquid crystal display (LCD), composed of a 90° twisted nematic (TN) LC cell attached to the top of a light-absorbing polymer solar cell (PSC), i.e., a Solar-LCD. The PSC consisted of a polymer bulk-heterojunction photovoltaic (PV) layer of poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] and [6,6]-phenyl C71 butyric acid methyl ester (PCDTBT:PCBM70), and showed a high power conversion efficiency of about 5%. In order to improve the visibility of the Solar-LCD, between the TN-LC and the PV cells we inserted a reflective polariser of a giant birefringent optical (GBO) film. The reflectivity from the Solar-LCD was observed to be considerably increased by more than 13–15% under illumination by visible light. The Solar-LCD also exhibited a significantly improved contrast ratio of more than 17–19. We believe there is a clear case for using such Solar-LCDs in new power-generating reflective-type displays; taken as a whole these results also demonstrate the possibility of their application in a number of energy-harvesting opto-electrical display devices.

A Detailed Investigation of the Growth Conditions of Gallium Nitride Nanorods by Hydride Vapor Phase Epitaxy

We studied GaN nanorods grown by hydride vapour phase epitaxy processes to identify optimal growth conditions that yield nanorods appropriate for use in nanodevices. The growth temperature was varied over the range 625–670 °C, and the morphology of the samples changed with increasing growth temperature. GaN nanorods formed at growth temperatures of 645 °C on a Si(111) substrate. At a fixed growth temperature of 645 °C, the HCl:NH3 gas flow ratio was adjusted from 1:37 to 1:41. GaN nanorods with a small diameter of 26 nm formed at a HCl:NH3 ratio of 1:38. Individual GaN nanorods clearly grew along the axial direction, perpendicular to the substrate. Cathodoluminescence measurements at room temperature revealed a red shift as the acceleration energy was increased to 15 keV, possibly associated with the internal electric field.

Synthesis of multiwalled carbon nanotubes using C14H10O7W catalyst

High-purity carbon nanotubes were synthesized using a novel catalyst: tungsten-containing complex (C14H10O7W). The carbon nanotubes have a multiwalled structure that is hollow on the inside and a clean surface without carbonaceous particles. The carbon nanotubes have average diameters in the range of 20–60nm and lengths of 30μm. Transmission electron microscopy analysis reveals that the inner graphite sheets of these nanotubes are highly crystallized, but the outer graphite sheets are defective. The microstructures of the carbon nanotubes produced from C14H10O7W, tungsten hexacarbonyl [W(CO)6], and iron pentacarbonyl [Fe(CO)5] were analyzed using high-resolution transmission electron microscopy. We demonstrate that the tungsten-based catalyst can effectively be used to produce the carbon nanotubes.