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Wei, Siyu
Ph.D. in Interdisciplinary Materials Science, December 2009

Research Information

Ph.D. Dissertation
Field Emitters and Supercapacitors Based on Carbon Nanotube Films

Ph.D. Committee
Weng Poo Kang, Electrical Engineering
Jim Davidson, Electrical Engineering
Deyu Li, Mechanical Engineering
Alvin Strauss, Mechanical Engineering
Norman Tolk, Physics


Abstract.  Carbon nanotubes (CNTs) have many potential electronic applications due to their extraordinary material properties. The extremely small diameter and high aspect ratio of CNTs, along with their excellent electron transport property and thermal stability are ideal for vacuum field emission (VFE). The high surface area to volume ratio, high electrical conductivity, and chemical stability of CNTs, are ideal for energy storage, especially supercapacitors. At comparable volumes, supercapacitors demonstrate energy density hundred times that of dielectric capacitors, and power densities a hundred times that of batteries.

This research is focused on the synthesis of CNTs using thermal chemical vapor deposition (CVD) at atmospheric pressure with Pd, Ni, and Co as catalysts, and characterization of the CNTs for field emission applications. In this work, a systematic study was performed on the catalyst processing parameters and morphologies of as-grown CNT arrays. By controlling catalyst pretreatment parameters, high-density catalyst nanoparticles with uniform size have been produced. The result indicates that Pd is a more effective catalyst than conventional catalysts such as Co and Ni, and the corresponding CNT cathodes demonstrated comparable field emission behavior (turn-on field of 2.5V/um and field enhancement factor of 7800).

Secondly, synthesis of vertically aligned CNT arrays under atmospheric pressure has been achieved by thermal CVD using cobalt (Co) and nickel (Ni) as catalysts. These well-aligned nanotubes demonstrate lower turn-on field (~ 1.2V/um) than randomly oriented CNTs. Vertical alignment of CNTs is also a critical advantage in construction of vacuum field emission triodes. The synthesis process we have developed is of very low cost thus commercially promising.

Finally, an innovative approach has been developed to fabricate CNT/transition-metal-oxide (TMO) nanocomposite thin film for supercapacitor electrodes. Particular effort has been invested into manganese dioxide (MnO2) due to its excellent pseudocapacitance, low cost, non-toxicity and readily availability. This novel approach of using nano-structured CNTs architectures provides a high surface area of attachment for MnO2 nano-particles to maximize the charge efficiency and the power density and to reduce the series resistance. In this newly developed system, the charge transfer between CNTs and MnO2 nanoparticles is very efficient due to the exceptional electronic conductivity of CNTs. The direct growth of CNTs on conductive Si substrate helps to reduce contact resistance and ESR is significantly reduced and power is enhanced. A high capacitance of 30000uF (270F/g) has been observed on CNT/ MnO2 composite electrode which is over 400 times that of MnO2-free CNT sample. And a record-breaking charging/discharging current of 1.92mA, or 17.32 A/g, has been achieved. This supercapacitor utilizes less corrosive 0.1M KCl aqueous solution as the electrolyte and is more environment-friendly than its existing counterparts. The fabrication of the CNT/MnO2 nano-architectures confirmed that the vertically aligned CNTs fabricated by this method give rise to better electrode platform configuration for further integration or loading of the as-grown vertically aligned CNTs with other transitional metal oxides to enhance the supercapacitive performance.

Selected Publications

Supercapacitive behavior of CVD carbon nanotubes grown on Ti coated Si wafer. Wei, S; Kang, WP; Davidson, JL; Huang, JH, DIAMOND AND RELATED MATERIALS, 17, 906-911 , (2008)

Vertically aligned carbon nanotube field emission devices fabricated by furnace thermal chemical vapor deposition at atmospheric pressure. Wei, S; Kang, WP; Davidson, JL; Choi, BK; Huang, JH, JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B, 24, 1190-1196 , (2006)

Device characterization of carbon nanotubes field emitters in diode and triode configurations. Wong, YM; Kang, WP; Davidson, JL; Hofmeister, W; Wei, S; Huang, JH, DIAMOND AND RELATED MATERIALS, 14, 697-703 , (2005)

Effects of deposition and synthesis parameters on size, density, structure, and field emission properties of Pd-catalyzed carbon nanotubes synthesized by thermal chemical vapor deposition. Wei, S; Kang, WP; Hofmeister, WH; Davidson, JL; Wong, YM; Huang, JH, JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B, 23, 793-799 , (2005)

Transistor characteristics of thermal chemical vapor deposition carbon nanotubes field emission triode. Wong, YM; Kang, WP; Davidson, JL; Hofmeister, W; Wei, S; Huang, JH, JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B, 23, 868-873 , (2005)

Carbon nanotubes field emission devices grown by thermal CVD with palladium as catalysts. Wong, YM; Wei, S; Kang, WP; Davidson, JL; Hofmeister, W; Huang, JH; Cui, Y, DIAMOND AND RELATED MATERIALS, 13, 2105-2112 , (2004)

Vanderbilt University