Microscopic analysis of performance variations in carbon nanotube field emission cathodes: Implications for device optimization

Abstract

Carbon nanotubes (CNTs) are considered very promising for the realization of low-cost field emission electron sources. However, despite intensive research and development efforts, the fabrication of reliable CNT cathodes for high current density (>100 mA/cm 2 ) applications remains a formidable challenge. In this study we use scanning anode field emission microscopy (SAFEM) to investigate the microscopic origins of macroscopic emission performance variations in chemical vapor deposition (CVD) grown CNT planar field emission cathodes. The field enhancement distributions are determined and the field emission properties of individual emission sites on the cathodes are probed. Contact I(V) measurements are carried out to estimate the resistance of individual emitters. The degradation behavior of individual sites is also studied and can be related with the macroscopic cathode performances. Scanning (SEM) and transmission electron microscopy (TEM) provide additional information on the contact and structural properties of the cathodes. Our results indicate that the sample macroscopic performances depend strongly on the individual emitter field emission properties in terms of maximum current before degradation and contact resistance.

Active vacuum brazing of CNT films to metal substrates for superior electron field emission performance

“…The power dissipated at the base of the nanotube at a mere 1 μ A can reach several hundreds of W cm −2 . Resistive heating can lead to substrate melting and explosive damage of CNT bundles as was experimentally observed [ 30 ]. The slight increase in nanotube graphitization during brazing may also have contributed to improve the emission current by reducing the intrinsic resistance of the nanotubes.…”

“…In spite of this demonstration, several challenges remain and limit the widespread use of CNTs as cold electron sources. The maximum current that can be drawn per emitter and the contact resistance between the CNTs and the substrate were identified as the most crucial parameters affecting macroscopic emission behavior [ 30 ]. It is possible to reduce the contact resistance by employing metallization layers between the nanotube growth catalyst and the Si substrate and by carrying out post-treatments on the emitters [ 30 ].…”

“…Only 0.1 μ A was drawn from the as-grown sample at this field while 30 μ A was obtained at 1 V μ m −1 . Individual CNT emitters typically provide maximum 10–100 μ A [ 30 ] and can be pushed to yield up to 120 μ A when annealed in vacuum [ 21 ]. We thus conclude on the basis of the measured current that only a limited number of high field-enhancement emitters, randomly distributed over the cathode area contribute to the measured currents in figure 7 (b)).…”

Electric Field Enhancing Artifacts as Precursors for Vacuum High-Voltage Breakdown

“…Under specific conditions, carbon diffusion through or along the surface of metallic nanoparticles results in the growth of carbonaceous fibers. CNTs may be harvested from the so-called vapor-solid-solid (VSS) growth mechanism under the conditions of RF-PECVD, see Figures 3-6 and [10][11][12][13][14] and the literature review [15]. Filament-like hybrid structures may comprise carbon fibers and metallic nanoparticles, typically grafted near their apexes.…”

Enhanced Field Emission Properties from CNT Arrays Synthesized on Inconel Superalloy

“…Since the reports made by Heer et al and Rinzer et al, on the field emission from films and individual multiwalled carbon nanotubes (MWCNTs), respectively, various experimental studies on the field emission of MWCNTs grown using various techniques on different substrates have been pursued. − Reliability and reproducibility are the biggest aspects needing improvement when using CNTs in field emission devices; electrical breakdown and current degradation are the two major limiting factors of the reliability of CNT based field emitters. The electrical breakdown is a sudden discharge caused by an avalanche of charged particles above a certain threshold field.…”