Field emission from Mo2C coated carbon nanosheets

Abstract

Carbon nanosheets have recently evolved into useful edge emitters with high emission current densities, low threshold electric fields, and long lifetimes. In addition to further improvement in these characteristics, good stability and repeatability are also essential for these materials to be suitable for high vacuum applications such as microwave tubes and flat panel displays. Since the work function of graphite, carbon nanotubes, and amorphous carbon is relatively high, 4.6–4.8eV, selective thin film coatings may offer significant advantages. Carbides are a good film choice for their corrosive resistance, chemical stability, and substantially lower work function. Approximately 3 ML (monolayer) (∼1nm) of molybdenum were deposited on carbon nanosheets by physical vapor deposition and the carbide (Mo2C) formed by heating to >200°C at 1×10−8Torr. The carbide stoichiometry was confirmed in situ by the characteristic Auger triple peak at 272eV. A stoichiometric Mo2C calibration sample was used to acquire the Auger electron spectroscopy asymmetric ratio of 0.7 and this was used to determine the carbide growth as a function of temperature (from room temperature to 1000°C). Field emission currents of up to 400μA were compared with uncoated CNS at a given electric field. The Mo2C∕CNS cathodes were shown to have greater than a factor of 100 increase in current and greater than 2V∕μm decrease in threshold. The Fowler-Nordheim plots were exceptionally linear and quite repeatable (correlation coefficient R2=0.999+). Using the slope and vertical intercept, an emission area for the 0.07cm2 Mo2C∕CNS dot sample was determined to be ∼3×10−9cm2 and the field enhancement factor was found to be β∼530.