Field emission microscopy of adsorption and desorption of residual gas molecules on a carbon nanotube tip

Field emission of electrons from multiwall carbon nanotubes has been investigated by field emission microscopy (FEM) in ultra-high vacuum. A carbon nanotube, at the end of which at least six pentagons exist to make a closed cap, gives an FEM pattern consisting of bright pentagonal rings if the nanotube surface is clean. Adsorption of residual gas molecules is observed as bright spots in the FEM pattern, giving rise to an abrupt increase in the emission current. Adsorbed molecules seem to reside preferentially on the pentagonal sites where the strong electric field is concentrated. A heat cleaning of the emitter at about 1300 K allows the molecules to desorb, and the nanotube emitter recovers its original clean surface. It has been revealed that the adsorption and desorption of gas molecules are responsible for stepwise fluctuation of the emission current.