Field electron emission from a film of carbon nanopearls

Abstract

Layers of non-directional carbon nanotubes (CNTs) are currently used as field emission (FE) cathodes. The main advantage for such cathodes is the low cost fabrication process using conventional film deposition techniques such as screen-printing or imprint. This advantage is unfortunately counter-balanced by the non-uniformity and low density of the field emission sites, a consequence of its mat-like deposition and the tubular geometry of the CNTs. Recently, by a chemical vapour deposition (CVD) technique, strings of nanoballs of carbon are obtained which are called carbon nanopearls. Due to its string-like structure of nanospheres, the deposited layers of the carbon nanopearls naturally present a higher density of field emission sites. As the nanopearls surface is constituted of graphene, as the carbon nanotubes apex, these field emission sites exhibit the same quality for current stability as observed with the carbon nanotubes. It has been demonstrated that a film of conditioned carbon nanopearls exhibits Fowler-Nordheim field emission behaviour, with currents of up to 50 muA readily obtainable under continuous emission in moderate vacuum. Compared to other graphitic nanostructures, in particular the tubular geometry of CNTs, the nanopearls have the advantage of presenting statistically a high density of apex areas with a small radius of curvature (~75 nm) when deposited on a planar surface. Moreover, these spheres are composed of graphitic flakes that are unclosed at the surface and therefore believed to exhibit many dangling bonds with the potential to enhance the field emission current. These two properties give the nanopearls excellent prospects as a cathode material. To investigate the field emission properties, the nanopearls were directly grown on the end of a metallic wire. The field emission was performed in a conventional field emission microscope environment, with the cathode located a few mm away from a flat screen. Systematic analysis of the field emission properties and behaviour of carbon nanopearls based cathodes were done, in particular the energy distribution measurements under different conditions. A comparative analysis with carbon nanotubes will point out the specific quality related to the nanopearl structure