Making carbon nanotube electron sources of defined lengths and with closed caps.

A method is reported to make an electron source consisting of an individual multi-walled carbon nanotube (MWNT) mounted on a tungsten support tip, and cut to length using localized electron beam irradiation in a scanning electron microscope. The apex of the MWNT was transformed into a closed cap with at least one fullerene-like layer via an annealing process involving simultaneous heating and the extraction of an emission current of ∼ 1 mA. The electron emission occurred at localized emission sites. The electron emission showed Fowler-Nordheim behavior, was highly stable with time, and exhibited a low energy spread. The structure of the caps of two MWNTs was studied with transmission electron microscopy before and after the cap closure.