Characterizing field emission from individual carbon nanotubes at small distances

Abstract

This article demonstrates the characterization of field emission from individual carbon nanotubes (CNTs) attached to a tungsten tip, when the separation distance s between the anode and tip of the CNT (cathode) is less than 15μm. The separation distance is adjusted with a nanopositioning stage after establishing a datum by detecting the anode surface with the CNT tip. Our separation distance s differs by the height h of the CNT from the distance d that is often measured between the planar anode and the planar substrate of an emitting cathode. Consequently, the electric field at the tip of the CNT is modeled by F=λV∕s, where λ is our field amplification factor, rather than by F=γV∕d, where γ is the more conventional field enhancement factor. Twenty-four sets of current-voltage I(V) data were measured from an individual multiwall CNT at separation distances s between 1.4 and 13.5μm. A nonlinear curve-fitting algorithm extracted Fowler-Nordheim (FN) parameters from each set of I(V) data, rather than conventional extraction from the FN plots. The turn-on voltage Vto (to emit 1nA) as a function of the separation distance followed an empirical power relation Vto=asb, and the field amplification factor λ empirically followed the relation λ=λ∞s∕(s+h)+1. This experimental characterization is an improvement over other techniques since the gap is controlled more precisely over a larger range, the electric field at the CNT tip is not disturbed by other CNTs, and the anode is flat to within a few nanometers.