Degradation of a CNT-Based Field Emission Cathode due to Ion Sputtering

Abstract

The rate of degradation of carbon nanotubes (CNT) comprising a field emission cathode is calculated. The sputtering of CNT material by incident ions formed through the electron impact ionization of the residual gas molecules is considered as the degradation mechanism. The degradation rate and the relevant lifetime of CNTs have been calculated as a function of the nanotube's geometry, applied voltage, inter-electrode distance and density of the array. It has been found the abrupt dependence of the degradation rate on the applied voltage which is caused by a sharp character of the Fowler-Nordheim dependence. The dependence of the degradation rate on the interelectrode distance is caused by the corresponding dependence of the probability of an ion to reach the CNT surface.

Theory of Carbon Nanotube (CNT)-Based Electron Field Emitters

“…The main issue, determining the applicability of CNTs as emitters in electron field cathode relates to the lifetime of these emitters which is governed by the degradation of emitters during the operation. The following three degradation mechanisms of CNT-based electron field emitters are known: contact failure, thermal decomposition, and ion sputtering [ 74 ]. The degradation mechanism related to contact failure has no universal character and manifests itself when insufficient attention is paid to the contact formation.…”

“…The threshold character of this effect can be used to avoid the thermal decomposition of CNTs by operating at voltages (currents) below the critical value. In contrast to this effect, the ion sputtering mechanism manifests itself at any applied voltage and emission current [ 74 ]; it should be considered separately, as it is done below. The ions forming during the ionization of residual gas molecules by an electron impact are accelerated in an electric field and bombard the emitter surface, which leads to its ion sputtering.…”

“…One should note that the initial approach to the description of the above-mentioned mechanism of the sputtering has been developed in [ 74 , 75 ]. This approach is based on a simplifying supposition neglecting the thermal motion of residual gas molecules whose ionization is a source of formation of ions bombarding the cathode.…”

“…Therewith all the ions formed move along the force lines of the electrical field and participate in the bombardment of the emitter (nanotube). However estimates performed in [ 74 , 75 ] demonstrate that the inclusion of the initial motion of the residual gas molecules results in the only some fraction of ions reaches the nanotube’s surface, while the rest of ions due to the initial motion in radial direction fall on the substrate missing the emitter and do not promoting its degradation. This effect is studied quantitatively below.…”

Field‐Induced Electron Emission from Nanoporous Carbons

“…However, the technological processes for controlled growth of aligned nanotube arrays remain relatively expensive. Furthermore, the problem of their degradation under operational conditions has not yet been solved [3][4][5][6], and all efforts directed to its solution yielded only limited success. These drawbacks expand the focus of emission investigations onto other types of nanocarbon-with relatively smooth surface topography, heterogeneous composition, and (more or less) disordered structure.…”