Effect of geometrical parameters on the field-emission properties of single-walled carbon nanotube ropes
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Aligned single-walled carbon nanotube (SWNT) ropes show excellent field-emission performance due to their high aspect ratio and sound alignment. In this study, the effect of geometrical parameters, such as cathode-anode distance and the height of SWNT ropes on the field-emission properties of SWNT ropes was investigated. It was found that the cathode-anode distance influences the emission properties, such as the turn-on field Fto and threshold field Fthr, of SWNT ropes, and the turn-on and threshold fields are marginally decreased at relatively larger gaps between the SWNT emitter tip and the anode plane. It was also found that the emission properties of SWNT ropes are improved by increasing the rope length, at least in the present experimental range. A possible two-step field amplification model was proposed to explain this length effect of SWNT ropes. The estimated results show that the local field at the tip of SWNT ropes, which causes a certain emission-current density, seems not to change with the cathode-anode distance, and the effective emission area of the SWNT rope is much smaller than the apparent cross-sectional area of the SWNT rope. The results obtained suggest that it is possible to optimize the performance of SWNT rope-based cold cathode by adjusting geometrical parameters of SWNT rope emitters. I. INTRODUCTION
II. EXPERIMENTAL
Many studies show that carbon nanotubes have great advantages, such as large and controllable current density, narrow electron-energy distribution, normal working temperature, and low operational voltage, as electron sources.1,2 Thus, carbon nanotubes are expected to be used as excellent field emitters in commercial fieldemission devices such as field-emission display panels,2 electron guns,3 and x-ray tubes.4 Theoretical simulation predicts that the geometrical parameters of carbon nanotubes influence the surface local field around them, so the emission performance of carbon nanotubes, to a certain extent, is determined by their geometrical structure.5 By optimizing the geometrical parameters of carbon nanotubes, better emission performance may be obtained in the fabrication of carbon nanotube-based field-emission devices. In this article, single-walled carbon nanotube (SWNT) ropes were used for the measurements of their fieldemission performance. The variations in their emission properties were analyzed by changing the geometrical parameters of SWNT ropes. The results obtained in this work may provide useful information for the fabrication of high-performance SWNT rope-based field-emission devices.
Macroscopically long ropes of aligned SWNTs used in our work were synthesized by the hydrogen and argon arc discharge method.6 Long ropes of SWNT bundles with rope lengths up to 200 mm could be isolated in the product. The collected SWNT ropes were lightweight and freestanding. Scanning electron microscopy (SEM) observations showed that these ropes were typically 50–100 m in diameter and were made up of numerous tightly packed, soundly aligned, long and straight SWNT bundles (Fi
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