Paper GR-TuP2
Enhanced Field Emission from Carbon Nanosheets by Thin Film Coatings

Tuesday, October 21, 2008, 6:30 pm, Room Hall D

Carbon nanosheets (CNS) are a promising two-dimensional carbon allotrope for high current field emission cathodes and are grown by radio frequency (RF) plasma-enhanced chemical vapor deposition from a C₂H₂/H₂ gas blend at substrate temperatures of ~ 600 °C. The resulting film consists of vertically oriented, honeycomb sp² carbon arrays terminating in single graphene sheets that serve as field emission cathodes. Conditioned films have previously been shown to provide high emission current density (~ 2 mA/mm²), and stable lifetime (less than 5% variation over >200 h). The Fowler-Nordheim equation predicts that the field emission current density is a strong function of the local effective work function, e.g., a reduction of 1 eV in work function results in an increase in emission current of up to two orders of magnitude. Since the work function of graphite, carbon nanotubes, and amorphous carbon is relatively high, 4 - 5 eV, selective thin film coatings can substantially lower the effective work function at active emission sites. Mo₂C and NbC films of ~1 nm have been grown on CNS by e-beam physical vapor deposition (PVD) in very high vacuum and subsequently characterized by Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) in UHV. Comparison of the as-grown CNS to coated CNS show large increases in current density; for example, a 2 nm coating of Cr₂O₃ /CNS gave an enhanced emission of a factor of 135. The emission shows a strong film thickness sensitivity that may be a consequence of band bending in the surface potential barrier. Coatings of ThO₂, Cr₂O₃ and NbO_x on CNS have also been similarly grown and demonstrate similar improvements in current density.