Field emission from nano-cluster carbon films

Field emission has been reported to occur at much lower fields in carbon based thin film systems than from any other material systems. The emission has been shown to depend on the various material parameters, but whichever carbon based system is used, it is found that emission occurs at localised sites rather than uniformly over the entire surface. Carbon films with mixed sp3 sp2 bonding, like nanocrystalline diamond and nanocluster graphitic films emit at lower fields with a higher emission site density than single-phase films. The sp2 cluster size in any carbon film can be altered during deposition, but it is easier to control nanocluster size by post-deposition annealing. Annealing increases the sp2 cluster size embedded in a sp3 matrix until the sp3 matrix disappears completely and the film transforms into nanocrystalline graphite. To distinguish the effects of the sp2 cluster size from other material parameters, a series of different carbon films were annealed post-deposition and the sp2 cluster size was measured using visible Raman. Field emission was then measured at a vacuum of 10 8 mbar on all films using a parallel plate configuration. It was found that the field emission for all films tested depended upon the clustering of the sp2 phase and this effect dominates the effects of the other parameters, such as chemical composition, surface termination, sp3 content or conductivity. The optimum size of the sp2 was of the order of 1 nm for all systems tested. We believe that field emission occurs form the localised conducting, predominantly sp2 bonded regions, which provideds the large field enhancement required for effective emission. 2001 Elsevier Science B.V. All rights reserved.