Vacuum gap dependence of field electron emission properties of large area multi-walled carbon nanotube films

Field electron emission properties of aligned multi-walled carbon nanotube films were studied with variation of the vacuum gap d between anode and cathode. With d varying in the range of 0.4-2 mm, the emission current-gap voltage characteristics and the corresponding Fowler-Nordheim (FN) plots show distinct nonlinearity and regular changes with electrode separation. Three field enhancement factors may be derived from the three linear sections of a FN plot. Their variation with gap d results in different behaviours; significantly a drop of five times in the field enhancement factor is observed. The physical process responsible for our findings is suggested to be the space charge effect and both theoretical and experimental evidence is provided to support our arguments. The implication of our findings in technical applications is also discussed.