Field emission properties of carbon nanotubes

We have investigated the field emission properties of nanotube thin films d eposited by a plasma enhanced chemical vapor deposition process from 2% CH4 in H-2 atmosphere. Depending on the deposition of the metallic catalyst [F e(NO3)(3) in an ethanol solution or sputtered Ni] the nanotube films showed a nested or continuous dense distribution of tubes. The films consisted of multiwalled nanotubes (MWNTs) with diameters ranging from 40 down to 5 nm, with a large fraction of the tubes having open ends. The nanotube thin fil m emitters showed a turn-on field of less than 2 V mu m(-1) for an emission current of 1 nA. An emission site density of 10 000 emitters per cm(-2) is achieved at fields around 4 V mu m(-1). The emission spots, observed on a phosphorous screen, show various irregular structures, which we attribute t o open ended tubes. A combined measurement of the field emitted electron en ergy distribution (FEED) and the current-voltage characteristic allowed us to determine the work function at the field emission site. In the case of t he MWNT thin films and are discharge grown MWNTs we found work function val ues around 5 eV, which agree well with the global work function of 4.85 eV we determined by photoelectron spectroscopy. From the shape of the FEED pea ks we can conclude that the held emission originates from continuum states at the Fermi energy, indicating the metallic character of the emission site . In the case of single-walled nanotubes we found significantly lower work function values of around 3.7 eV compared to those of MWNTs. We attribute t his to a size dependent electrostatic effect of the image potential, which lowers the work function for small (<5 nm) structures. (C) 2000 American Va cuum Society. [S0734-211X(00)05702-4].