Field Emission from Individual Free-Standing Carbon Nanotubes

Abstract

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 <100> 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 β 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/µm for a single straight tubular VACNT and 4–8 V/µm 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 β, than the long tubular CNTs (with higher aspect ratio).