Energy distributions of the beam emitted from gated multi-wall carbon nanotube arrays@footnote 1@ are measured in UHV as a function of gate voltage (emission current), temperature, and gas exposure. The emitters were grown by catalyzed CVD on silicon. At low currents (~1nA), the shape of the energy distribution was similar to that of metal emitters. At moderate current, emission could be detected at energies up to several volts below the Fermi level. The spectral shape at lower energies varied among specimens, after degassing, and after exposure to water. The intensity at the lower energies was reduced after degassing and increased by water adsorption. Operating the emitters removed the water at a rate related to the emission current. At increasing voltage, the fraction of the current emitted at the lower energies increased, while the emission current near the Fermi level saturated. That is, the additional current emitted after increasing the gate voltage occurred only below a specific energy, and that specific energy moved further below the Fermi level at higher voltages. The saturation effect was reduced by elevated temperatures (up to 600C) but still dominated the spectra, indicating that transport through the silicon substrate or at the silicon-nanotube interface was not responsible. Instead, we believe the emission at a specific energy is limited by the ability of the nanotube to replace electrons near the Fermi level, leaving only lower energy electrons available for emission.
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@footnote 1@ D.S.Y Hsu, these proceedings