Correlation of field emission and surface microstructure of vertically aligned carbon nanotubes

Two kinds of distinctive field emission behaviors were observed on vertically aligned multiwall carbon nanotube (CNT) films grown by hot filament dc-plasma enhanced chemical vapor deposition. Some samples have stable emission current for more than 60 h (type I), while others degrade substantially in the first 16 h (type II). During the field emission measurement, a brief exposure to air led an abrupt decrease of emission current of all samples. But subsequent reevacuation made type I samples recover the emission current fully, whereas type II samples were damaged permanently reflecting on the irrecoverable emission current. Microstructure studies by transmission electron microscope clearly showed that the stable emission is due to a surface passivation of CNTs by a thin layer of amorphous carbon that prevents CNTs from reacting with ambient gases, e.g., oxygen, during air exposure.