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Dependence of field emission properties of carbon nanotube films on their graphitization

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3 Author(s)
Ting, Jyh-Hua ; National Nano Device Laboratories, No. 26, Prosperity Road I, Science-Based Industrial Park, Hsinchu 300, Taiwan, Republic of China ; Tsung-Lung Li ; Hong, Ying-Chi

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The purpose of this study is to investigate the effect of graphitization of carbon nanotube (CNT) films on their field emission efficiencies. CNT films were prepared by microwave plasma-enhanced chemical vapor deposition (MPCVD) method. Transmission electron microscopy images reveal the center hollowness and multiwalled structure of a CNT. The tip-growth mechanism of the CNTs prepared by MPCVD is confirmed by the nickel particles enclosed at the tips of the CNTs. The intensity ratio of IG/(ID+IG) under the Raman spectrum was defined to characterize the degree of graphitization of the CNTs. At any methane flow ratios [CH4/(H2+CH4)], the CNT graphitization increases with the microwave power. Also, the CNT graphitization increases with the CH4 flow ratio and begins to drop at the flow ratio of 15%. It is mainly attributed to the relative concentration of carbon radicals in the hydrocarbon-based plasma. Enhancement of the graphitization of CNT emitter array leads to the decrease of the threshold field and the increase of the field-controlled current density consistently. This consistency suggests that higher concentration of sp2 bonding enhances conductivity. Therefore, the field emission efficiency of the CNTs increases with the CNT graphitization. At the microwave power of 1200 W and the methane flow ratio of 15%, the graphitization intensity ratio is 0.637. The optimal threshold field and field-controlled current density at the field of 3 Vm are 1.38 Vm and 8.934 mA/cm2, respectively.

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Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures  (Volume:24 ,  Issue: 4 )