Field emission signature of pentagons at carbon nanotube caps

Localization of emitting states and their tunneling probabilities cause the nanotube cap geometry to have decisive impact on field emission patterns and currents. We show how different arrangements of pentagon rings at the tip can create specific field emission features, utilizing a method based on first principles calculations. The results give an explanation for different field emission patterns observed in experiments, and provide a feasible way to distinguish different cap structures from experimental results. A set of general rules is deduced to infer the tip configuration through the experimental field emission patterns. The calculations agree very well with our experimental results, and are of fundamental interest in characterization and design of carbon nanotube emitters and probes.