Mechanism of Efficient Field Emission from Carbon Nanotubes
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Mat. Res. Soc. Symp. Proc. Vol. 558 0 2000 Materials Research Society
emitters are easily deposited over large areas, without the need for complex fabrication processes. Field emission has been reported from other forms of carbon, however at lower current densities (0.3 mA/crn2 for graphite [17], 30mA/cm2 for diamond [18,19 and references therein]). 2 While higher current densities can be deduced if localized emission is assumed (e.g., IOOA/cm , neither are spots hot emission the localized and measured, are not areas reference [18]) the actual understood nor reproducible. EXPERIMENTS Our emitters are SWNT films deposited on Si substrates. They are grown in a laser ablation system, as described previously.[20] Briefly, a graphite target containing I atomic percent of Ni/Co catalyst is placed in a tube furnace at 1,1 50'C under a constant flow of argon gas. The target is ablated by the primary beam (),=1,064 nm) of a pulsed Nd:YAG laser. The 0 resultant material is a free-standing mat, deposited downstream on cold surfaces (200-300 C). of measurements) and Raman SEM TEM, (by The raw material contains about 70% in volume SWNTs with an average tube diameter of 1.3-1.6 nm in 10-30 nm bundles. Figure la is a highresolution transmission electron micrograph of the SWNTs. Emitters are made of purified SWNTs by dispersing the nanotubes in a solvent, filtering, and re-depositing on a Si substrate. Figure lb shows a scanning electron micrograph of such a SWNT emitter. The emitter contains a high density of nanotubes on the surface. Each of the curved lines in the micrograph represents a SWNT bundle. The field emission measurements were carried out at room temperature in a vacuum chamber with a 10-8 Torr base pressure. Images of the emission from individual nanotubes were obtained by suspending a phosphor screen 500pm above the emitter at a potential of 700V. A CCD camera on a telescope recorded the images from outside the vacuum chamber. The exposure was 30ms with a 13pm pixel size. The experimental procedures for current density measurements were described in detail elsewhere.[14,15] Briefly, a voltage up to 2 kV was applied to a hemispherical molybdenum anode probe (radius of curvature R;%z 250 pm) which is 10-300 pm above the emitter surface. We used a translation stage in the vacuum chamber to control the distance between the anode and the cathode. A hemispherical probe approach is a better measurement technique than a parallel plate geometry because it avoids uncertainties caused by field enhancements at the edge of a planar anode or emitter, and it also allows easy measurements at many points on the sample. Large area measurements (5 mm diameter) show good agreement with the probe measurements, and probe measurements of multiple spots show voltages within a factor of two at a given current and anode-emitter distance. The emission current-voltage (I-V) characteristics were measured as a function of the anode-cathode distance (Z). At each distance, the anode voltage was raised from zero until the current density reached -0
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