While many nanotube field emission properties can be computed to a good approximation by assuming the nanotube is a sharp graphite emitter, the reality of implementing nanotube field emitters in devices is more complex. The field emission current depends exponentially on nanotube diameter, nanotube height, geometrical placement, and nanotube orientation. These parameters are not well controlled in typical devices. Consequently, only a small fraction of a percent of available nanotubes contribute meaningful current, and more importantly, these nanotubes are so rare that they are not imaged in typical scanning electron microscope (SEM) and transmission electron microscope surveys of the emissive material. Many device properties are driven by the behavior at the individual nanotube level, and there is no reason to believe that the "average" nanotubes behave similar to the field-emitting minority. A detailed understanding of field emission behavior at the level of an individual emitter is important for producing good device designs.
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