Brightness of carbon nanotube electron sources

The virtual source sizes of individual multiwalled carbon nanotube electron emitters were investigated with a point projection microscope. The average radius of the virtual source size was found to be 2.6 nm, which does not correspond to the standard model of a field emitter. Instead, a model based on a flattened cap or an open cap seems to provide a more realistic description. The broadening effect of Coulomb interactions on the virtual source was calculated. The reduced angular current density was measured at the maximum current at which stable emission was obtained and arrived at an average of 30 nA sr(-1) V-1. The reduced brightness values obtained for two emitters were (2.5+/-1)x10(9) and (1.3+/-0.5)x10(9) A m(-2) sr(-1) V-1, respectively. These values are an order of magnitude larger than the values of state-of-the-art commercial sources. (C) 2004 American Institute of Physics.