Please use this identifier to cite or link to this item: http://scholarbank.nus.edu.sg/handle/10635/25062
Title: Study of Field Emission Characteristics of Ultrathin Film Coated Carbon Nanotubes Core-Shell Structures
Authors: YU JUN
Keywords: field emission, carbon nanotubes, coating, ultrathin film, metal oxide, tetrahedral amorphous carbon
Issue Date: 24-Sep-2010
Source: YU JUN (2010-09-24). Study of Field Emission Characteristics of Ultrathin Film Coated Carbon Nanotubes Core-Shell Structures. ScholarBank@NUS Repository.
Abstract: The aim of this dissertation was to explore alternative materials to modify CNTs so as to further enhance their FE characteristics and to investigate the enhancement mechanisms of FE for the modified or coated CNTs. To achieve these purposes, the tetrahedral amorphous carbon (ta-C) and metal oxides such as molybdenum oxide and tungsten oxide ultrathin films were coated onto high density vertically-aligned CNT substrates and their FE characteristics were examined. The metal oxide films were deposited by custom-designed metal-organic chemical vapor deposition (MOCVD) technique at varying temperatures. The metal oxides coated CNTs nanostructures obtained at 400 °C exhibited enhanced FE properties. The underlying principles for the enhancement are probably due to the Schottky junction formed at the interface, which leads to lowered electron emission barrier height. In addition, novel cactus-shaped nanostructures were obtained for the 600 °C tungsten oxides coated CNTs and their growth mechanism may be attributed to the dendritic growth. The numerous branches perpendicularly aligned along the main stems may distort the applied electric field and remarkably enlarge the local field of the emission sites, thus explaining the FE enhancement of the composite emitters. The ta-C films were coated with different thicknesses followed by hydrogen plasma treatments with diverse durations to investigate the influence on FE properties. It was found that there was an optimum film thickness demonstrating the best FE performance. Systematic studies showed that with either thinner or thicker films, the effective emission potential barrier and the electron transport would be affected, and surface work function would be changed as well. Further work on modifying the surface of CNTs with hydrogen plasma showed enhanced FE performance due to the positive C-H dipoles generated at the surface and the reduced surface barrier height resulted from the energy band bending caused by the charge transfer between the ta-C and the absorbed water layer on its surface. However, longer duration of hydrogen plasma treatments (> 10 s) would degrade the enhancement by severely damaging the structure of the composite emitters thus making the electron transport within the emitters become difficult. In conclusion, new CNT-based core-shell composite emitters with enhanced FE properties have been successfully fabricated and their enhancement mechanisms have been intensively discussed. The main factors influencing the FE properties of the composite emitters have been determined as well, such as the effective potential emission barrier, field enhancement factor and the electron transport ability.
URI: http://scholarbank.nus.edu.sg/handle/10635/25062
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