Saturated emission from carbon nanotubes

摘要

We have measured emission currents and energy spectra produced by gated carbon nanotube field emitter arrays. The currents and spectra change after exposing the arrays to water vapor or and hydrogen. At low currents, we find the emission spectra are narrow and increase in intensity at all energies. However the spectral shape is broader than predicted by free electron theory and the peak intensity does not increase as quickly as theory predicts. At high currents the emission completely saturates at energies just below E_F. We interpret this behavior as a sign of field penetration into the nanotubes. The saturated energy range increases after exposing the arrays to hydrogen, indicating that adsorbed hydrogen reduces the electron density within the nanotube. Adsorbed water molecules have the opposite effect, apparently donating charge to the nanotubes. Fowler-Nordheim plots of the emission current sometimes do not produce straight lines, particularly when water vapor is adsorbed. The non-linear behavior does not correlate with the saturation effect.