Tailoring the field emission property of nitrogen-doped carbon nanotubes by controlling the graphitic/pyridinic substitution

Nitrogen-doped carbon nanotubes (CNTs) are known to be better field emitters than the pristine ones. But the field emissions (FE) property closely depends on what kinds of nitrogen moieties are substituted in the CNT matrix. While graphitic N-substituents (gN) give rise to additional sub-levels in the unoccupied states near the Fermi level, pyridinic N-substituents (pN) destroy the existing sub-levels. Here, we show that the FE property of N-doped CNTs can be tailored by controlling the gN/pN ratio therein. Vertically aligned N-doped CNTs were grown by chemical vapor deposition of camphor in the presence of ferrocene catalyst, using dimethylformamide (DMF) as a nitrogen source. A step-wise increase of DMF concentration (0–45 wt.%) in camphor caused a systematic rise of N-doping (0.8–5.2 at.%) in the resulting CNTs. The gN- and pN-dopings were identified by XPS analysis, and gN/pN ratio was found to increase from 1.7 to 3.5 in a regular manner. The FE measurements of as-grown N-doped CNTs exhibited a corresponding decrease of turn-on field from 1.8 to 0.6 V/μm and threshold field from 4.2 to 2.0 V/μm. This study thus presents the first experimental demonstration of FE enhancement by controlling the gN/pN ratio.