Field emission of electrons from carbon nanotubes: Emission mechanisms, current stability, and the current saturation effect.

摘要

Single-walled nanotubes (SWNTs) are emerging as suitable electron sources for field emission devices due to several interesting properties. These include field emission at anomalously low applied voltages, stable emission in non-ideal vacuum environments, and intrinsic saturation of emission current. This dissertation determines the underlying mechanisms responsible for these three properties and characterizes the field emission behavior of clean SWNTs.; The field emission behavior of SWNTs has been investigated over a wide range temperatures and electric fields. Utilizing field emission microscopy, electron energy distribution (EED) measurements, and current-voltage-temperature measurements, several behavioral regimes of field emission have been identified. In the first regime, adsorbate surface states dominate the field emission behavior at room temperature, even under ultrahigh vacuum conditions. These adsorbate states correlate with the presence of water. They enhance the tunneling probability, thus explaining field emission at anomalously low voltages. The second behavioral regime, that of clean SWNTs, is obtained by desorbing the adsorbates above 900 K. Field emission microscope images of clean SWNTs show fine structure related to electronic states of the nanotube caps. Current-temperature and EED measurements indicate that SWNTs emit electrons through surface states in the caps. At high fields and temperatures where current from an individual SWNT exceeds ∼2 μA, rings form around field emission microscope patterns and the patterns spin. Simultaneous pattern rearrangement and current degradation indicate the removal of carbon atoms at the nanotube end-cap.; SWNTs were found to exhibit long-term field emission stability in environments that destroy metal emitters in hours. Protrusion growth and current runaway were not observed, but nanotubes do show susceptibility to oxidation damage. Environmental stability results from geometry, strong carbon bonding, and the lack of protrusion growth.; Carbon nanotubes exhibit intrinsic current saturation that was found result from adsorbates states. Current saturation occurs at emission currents above 100 nA per nanotube and coincides with rapid current fluctuations and distinctive changes in the field emission microscope patterns. Current saturation results from the displacement of adsorbates from configurations of tunneling enhancement as electric field and current are increased. At high fields, adsorbates can be completely removed from the nanotube.