The field emission properties of a single closed multiwalled carbon nanotub
e tip were investigated before and after doping with potassium. The thresho
ld voltage was significantly reduced from 143 V to 76 V, but the emitter de
graded in about 4 min. The current-voltage measurements fit the Fowler-Nord
heim equation well up to about 100 nA of emission current. Additionally the
work function of the multiwalled carbon nanotube tip could be estimated as
being about 5.3 eV.