Field‐Emission Characteristics of Individual Carbon Nanotubes with a Conical Tip: The Validity of the Fowler–Nordheim Theory and Maximum Emission Current

Abstract

Carbon nanotubes (CNTs) are widely considered as one of the most promising candidates for building field emission (FE) devices. These devices include X-ray tubes, scanning X-ray sources, and flat panel displays. In addition to its high aspect ratio, good electrical conduction, and superb chemical stability, the tip structure of the CNT is a crucial factor in determining its excellent performance as a field emitter. In particular, for a cappedCNT, the closed apex with each carbon atom bound to three neighbor atoms via covalent bonds shows much higher FE stability than that of an opened end, and capped CNTs have therefore become the focus of scientific and industrial interest for their potential uses as, for example, high-brightness point-electron sources. However, even for a capped CNT with a closed termination built by strong C–C bonds, the emission can be unstable. Its cap structure can be damaged at extremely strong local field levels and high temperatures resulting from current heating, and so these factors define a upper limit for the FE current from a CNT. While technically this maximum FE current (Imax) is a very important parameter, this quantity has not been investigated by directly correlating it to the CNT cap structure. The CNT emitter was found to deviate from Fowler–Nordheim (F–N) behavior in many experiments, but some recent results show that the F–N theory might still give a good description for capped CNTs. So a question arises: to what extent does the conventional F–N theory applies for an extremely curved CNT cap surface? Recently we demonstrated an in situ cap–engineering technique inside a transmission electron microscope (TEM) for fabricating conical tips with controllable size on a single CNT. This cone-shaped CNT, in fact, provides a good experimental object for us to investigate the two fundamental FE issues raised above: what is the applicability of the F–N theory, and what is themaximum stable FE current that can be extracted from a single CNT?