基于气体吸附的碳纳米管场发射低压氢传感技术

Adsorption based carbon nanotube field emission low pressure hydrogen sensing technique

本文主要研究了一种新型基于碳纳米管(CNT)场发射原理和气体吸附的低压氢传感技术。CNT通过化学气相沉积(CVD)直接生长在镍合金基底。部分样品有氢传感性能,即小电流恒压场发射在氢分压下逐步增加,而电流增长速率随着氢压强的增大而加快;另一部分样品则没有传感效应。拉曼测试表明有传感特性的样品晶体性好,D峰/G峰比值R值小于1,而无传感效应的R值大于1。第一性原理模拟研究表明:氢在碳纳米管表面发生解离化学吸附时,使碳纳米管的有效功函数降低,从而使CNT具有氢传感效应;而在晶体性差的CNT表面,氢原子优先吸附在缺陷位,功函数不发生变化。加热测试显示:传感样品温度在500-600℃时脉冲发射电流的变化与氢传感测试时持续场发射下电流变化大致相同,证明场发射焦耳加热是促进氢的解离吸附、进而使电流增长的一个重要因素。

In this paper, we introduce an innovative new type of adsorption based low pressure carbon nanotube （CNT） field emission hydrogen sensing technique. CNTs are grown directly on nickel alloy substrate by chemical vapor deposition （CVD）. Part of CNT emitters exhibit the hydrogen sensing property, i.e., the field emission current increases in hydrogen ambient, and the current rising rate climbs with the increase of hydrogen pressure. Another part of the sample doesn＇t exhibit the sensing effect. Raman spectra shows that the sensing emitters possess high crystallinity with the ratio R of D peak/G peak less than 1, while R is greater than 1 for the no-sensing emitter. The first principles simulation shows that the dissociative chemisorption of hydrogen on carbon nanotubes results in the reduction of the effective work function, leading to the hydrogen sensing effect. However, for CNTs of poor crystallinity, the hydrogen atoms are apt to adsorb on the defect sites, which will not change the work function significantly. Heating test shows that under 500-600℃the emission behavior in pulse mode is about the same as that in continuous emission mode for hydrogen sensing, indicating that the field emission Joule heating is a key factor to promote the dissociative hydrogen adsorption and the emission current climb.