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.

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Hybrid nanostructures consisting of tungsten oxide nanorods with mushroom-shaped carbon caps were grown on electrochemically etched tungsten tips by thermal chemical vapor deposition with methane and argon. These nanorods grow along the radial direction and are very straight and smooth. Electron microscopy revealed a dominant diameter and length of approximately 50 nm and approximately 0.6 microm, respectively. High-resolution transmission electron microscopy (HRTEM), and electron energy loss spectroscopy (EELS) revealed the presence of crystalline monoclinic W18O49 in the nanorods, and the cap was entirely amorphous carbon. A plausible growth mechanism involves the reduction of tungsten oxide WO3, present on the tungsten surface, by methane at 900 degrees C.

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