Field Emission from Individual Free-Standing Carbon Nanotubes

The field emission (FE) characteristics of individual free-standing vertically aligned carbon nanotubes (VACNTs) grown by inductively coupled plasma chemical vapor deposition (ICP-CVD) were studied. The processes comprised electron beam lithography (EBL) with various exposure periods, the deposition of nickel metal followed by lift-off, and the growth of carbon nanotubes by ICP-CVD on a $\langle 100\rangle$ p-type silicon substrate. Straight tubular and stubby conical VACNFs were formed by varying the size of the graphite electrode that supports the silicon substrate. Current--voltage ($I$--$V$) curve characteristics for the tubular shape of isolated carbon nanotubes with different diameters and lengths were studied. The lowest turn-on voltage was shown to be about 24.5 V for the highest aspect ratio of a single vertically-aligned CNT. The highest field enhancement factor $\beta$ determined from fitting the FN equation was about 110. In addition, the enhancement factor was proportional to the aspect ratio of the CNTs. The turn-on field at an emission current of 1 nA was 8--12 V/\mbox{$\mu$m} for a single straight tubular VACNT and 4--8 V/\mbox{$\mu$m} for a stubby conical VACNF. The stubby conical CNFs (with smaller radii of curvature of their tips) had a lower turn-on field, but a slightly lower $\beta$, than the long tubular CNTs (with higher aspect ratio).