Yousuk Cho

A high-performance triode-type carbon nanotube field emitter for mass production

A triode-type field emission device based on carbon nanotubes (CNTs) synthesized on an anodic aluminum oxide (AAO) template was fabricated. For the improvement of device performance, in addition to the basic advantages of using AAO to obtain uniform CNTs with strong adhesion, several considerations were taken into account, including highly crystalline CNTs synthesized through thermal chemical vapor deposition (CVD) at 1200 °C, lowering the contact resistance with a Ti buffer layer and reducing the pixel size and gate-to-emitter distance. The triode-type field emitter showed a high current density of 20 mA cm−2 and a low turn-on voltage of 16 V. The very high field enhancement factor of 1.6 × 106 cm−1 confirmed the high efficiency of the triode structure in electron extraction.

A Method to Fabricate Field Emission Tip Arrays by Electrocodeposition of Single-Wall Carbon Nanotubes and Nickel

A new method to fabricate carbon nanotube field emission structure is developed. The emitter structure is a composite of single-wall carbon nanotubes (SWNTs) and Ni, and this is done by codepositing SWNTs and Ni metal via electrolysis. The carbon nanotubes distributed in the Ni matrix show a strong contact with the substrate. To enable good dispersion of the SWNTs in the conventional Ni Watt bath solution which is required for uniform codeposition, several techniques are successfully employed, i.e., oxidation and heat-treatment of the carbon nanotubes, surfactant coating, ultrasonic agitation, etc. A post chemical etching process is also necessary to enhance the emission properties. The emission is uniform, and the current density reached 1.2 mA/cm 2 at 4 V/μm.

Single Wall Carbon Nanotube Films Produced by Arc Discharge

A simple method to deposit carbon nanotube films uniformly on large area substrates using an arc discharge method is reported in this paper. The arc discharge method was modified to deposit carbon nanotube films in situ on the substrates. The substrates were scanned several times over the arcing point for a uniform film thickness. Deposition was carried out under variable dc bias conditions at 600 torr of H2 gas. The thickness uniformity of the single-wall carbon nanotube films as characterized by a four-point probe was within 30% deviation. The morphology and crystal quality of the single-wall carbon nanotube film were also characterized by field emission scanning electron microscopy and Raman spectroscopy.

Hydrogen Sensing Properties of ZnO-SWNTs Composite

The hydrogen gas sensing properties of a zinc oxide nanowire structure were studied. Porous zinc oxide nanowire structures were fabricated by oxidizing zinc deposited on a single-wall carbon nanotube (SWNT) template. This revealed a porous ZnO-SWNT composite due to the porosity in the SWNT film. The gas sensing properties were compared with those of zinc oxide thin films deposited on SiO2/Si substrates in sensitivity and operating temperature. The composite structure showed higher sensitivity and lower operating temperature than the zinc oxide film. It showed a response even at room temperature while the film structure did not.

Electrical and Optical Property of Single-Wall Carbon Nanotubes Films

Thin films of single-wall carbon nanotubes (SWNT) with various thicknesses were fabricated, and their optical andelectrical properties were investigated. The SWNTs of various thicknesses were directly coated in the arc-discharge chamberduring the synthesis and then thermally and chemically purified. The crystalline quality of the SWNTs was improved by thepurification processes as determined by Raman spectroscopy measurements. The resistance of the film is the lowest for thechemically purified SWNTs. The resistance vs. thickness measurements reveal the percolation thickness of the SWNT film tobe ~50nm. Optical absorption coefficient due to Beer-Lambert is estimated to be 7.1×10-2nm-1. The film thickness for 80%transparency is about 32nm, and the sheet resistance is 242Ω/sq. The authors also confirmed the relation between electricalconductance and optical conductance with very good reliability by measuring the resistance and transparency measurements.

Granular Thin Film of Titanium Dioxide for Hydrogen Gas Sensor

Titanium dioxide thin films were fabricated as hydrogen sensors and its sensing properties were tested. The titanium was deposited on a SiO /Si substrate by the DC magnetron sputtering method and was oxidized at an optimized temperature of 850 C in air. The titanium film originally had smooth surface morphology, but the film agglomerated to nano-size grains when the temperature reached oxidation temperature where it formed titanium oxide with a rutile structure. The oxide thin film formed by grains of tens of nanometers size also showed many short cracks and voids between the grains. The response to 1% hydrogen gas was ~2 × 10 at the optimum sensing temperature of 200 C, and ~10 at room temperature. This extremely high sensitivity of the thin film to hydrogen was due partly to the porous structure of the nano- sized sensing particles. Other sensor properties were also examined.

Effect of Surface Morphology and Adhesion Force on the Field Emisson Properties of Carbon Nanotube Based Cathode

The effects of the field emission property in relation to the surface morphology and adhesion force were investigated. The single-wall-nanotube-based cathode was obtained by use of an in-situ arc discharge synthesis method, a screen-printing method and a spray method. The morphologies of the formed emitter layers were very different. The emission stability and uniformity were dramatically improved by employing an in-situ arc discharge synthesis method. In this study, it was confirmed that the current stability and uniformity of the field emission of the cathode depend on the surface morphology and adhesion force of the emitters. The current stability of the field emission device was also studied through an electrical aging process by varying the current and electric field.