纳米非晶碳膜表面氢吸附对其场致电子发射特性的影响

Effect of Hydrogen Chemisorption on the Field Emission Property of Nano-Amorphous Carbon Film

采用微波等离子体化学气相沉积系统在陶瓷衬底上制备了具有sp2键价结构的纳米非晶碳膜。用Raman谱、XPS谱、SEM和XRD等手段分析了薄膜结构。样品经过多次场发射测试后I-U曲线趋于稳定,然后用氢等离子体处理使碳膜表面重新吸附氢。可以发现氢吸附后场发射电流增加,相应的F-N直线的斜率减小。根据F-N理论,F-N直线的斜率正比于φ3/2β,其中是表面功函数,β是取决于形貌的场增强因子,由SEM观察可知氢吸附后样品的形貌没有变化,F-N直线的斜率减小意味着功函数的降低,即氢吸附可降低sp2碳膜表面的功函数从而提高了场发射电流。为了验证以上推论,采用密度泛函理论计算了氢原子和氢离子吸附对sp2碳表面功函数的影响。作为近似,用石墨(0001)面来代表sp2非晶碳,计算了氢原子和氢离子在石墨(0001)表面不同的位置以1/2覆盖度化学吸附后石墨(0001)表面的真空能级、费米能级和表面功函数。计算结果显示氢原子和氢离子吸附后石墨(0001)表面的功函数可降低0.6和2.5 eV左右。这和从实验中得到的结论基本一致。

The nano-structure sp^2-bonded amorphous carbon film was deposited on Titanium coated ceramic substrate by microwave plasma chemical vapor deposition system. The surface morphology and the nano-structure of the film were examined using Raman, XPS, SEM and XRD. It had been observed that the field emission I-U curves of as-grown carbon film became reproducible after several voltage cycles, the subsequent hydrogen plasma treatment yielded hydrogen terminated surfaces again, the field emission current was improved and the slope of corresponding F-N plot decreased due to hydrogen adsorption. According to F-N theory, the slope of the F-N plot is proportional to Ф^（3/2）β, where Ф is the surface work function of the film and i3 is field enhancement factor determined by geometrical shapes of the film, which was not changed due to hydrogen adsorption observed by SEM, the decrease of the slope of the F-N plot meant the lowering of the work function of the film. Namely hydrogen chemisorption on sp^2-bonded carbon film lowered its work function and then improved the field emission current. For confirmation of the above assumptions, theoretical calculation based on DFT-GGA was performed to investigate the effect of hydrogen atom/ion chemisorption on the work function of sp^2-bonded carbon film. Graphite （0001） surface was adopted as an approximation of the deposited sp^2-bonded carbon film, the vacuum level, Fermi level and work function of graphite （0001） surface with different hydrogen atom/ion chemisorption sites with 1/2 coverage were calculated and the calculation results revealed that hydrogen atom and hydrogen ion chemisorption on the surface leads to a lowering of work function of up to about 0.6eV and 2.5 eV respectively, which were in agreement with experimental results.