Field emission from nano-cluster carbon films

Abstract

Abstract 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 sp 3 /sp 2 bonding, like nanocrystalline diamond and nanocluster graphitic films emit at lower fields with a higher emission site density than single-phase films. The sp 2 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 sp 2 cluster size embedded in a sp 3 matrix until the sp 3 matrix disappears completely and the film transforms into nanocrystalline graphite. To distinguish the effects of the sp 2 cluster size from other material parameters, a series of different carbon films were annealed post-deposition and the sp 2 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 sp 2 phase and this effect dominates the effects of the other parameters, such as chemical composition, surface termination, sp 3 content or conductivity. The optimum size of the sp 2 was of the order of 1 nm for all systems tested. We believe that field emission occurs form the localised conducting, predominantly sp 2 bonded regions, which provideds the large field enhancement required for effective emission.