Individual carbon nanotube (CNT) field emission characteristics present a number of advantages for potential applications in electron microscopy and electron beam lithography. Mechanical and electrical reliability of individual CNT cathodes, however, remains a challenge and thus device integration of these cathodes has been limited. In this work, we present an investigation into the reliability issues concerning individual CNT field emission cathodes. We also introduce and analyze the reliability of a novel individual CNT cathode. The cathode structure is composed of a multi-walled carbon nanotube (MWNT) attached by Joule heating to a nickel-coated Si microstructure. The junction of the CNT and the Si microstructure is mechanically and electrically robust to withstand the strong electric field conditions that are typical for field emission devices. An optimal Ni film coating of 25 nm on the Si microstructure is required for mechanical and electrical stability. Experimental current-voltage data for the new cathode structure definitively demonstrates carbon nanotube field emission. Additionally, we demonstrate that our new nanofabrication method is capable of producing sophisticated cathode structures that were previously not realizable, such as one consisting of two parallel MWNTs, with highly controlled CNT lengths with 40 nm accuracy and nanotube-to-nanotube separations of less than 10 µm.

Carbon nanotube pillar arrays (CPAs) for cold field emission applications were grown directly on polished 70∕30at.% NiCr alloy surfaces patterned by photolithography. A carbon nanotube (CNT) pillar is a localized, vertically aligned, and well-ordered group of multiwalled CNTs resulting from van der Waals forces within high-density CNT growth. The edge effect, in which the applied electric field is enhanced along the edge of each pillar, is primarily responsible for the excellent emission properties of CPAs. We achieved efficient emission with turn-on fields as low as 0.9V∕μm and stable current densities as high as 10mA∕cm2 at an applied macroscopic field of 5.7V∕μm. We investigated the effects of pillar aspect ratio, density, and spacing on CPA field emission and quantified the edge effect with respect to pillar aspect ratio through modeling. We also investigated the field emission stability and found substantial improvement with CPAs compared to continuous and patterned CNT films.

We report the effect of cathode structure on the field emission properties of individual carbon nanotubes. Experimental field emission data are obtained for two well-defined cathode structures: a multi-walled carbon nanotube (MWNT) attached to an etched Ni metal wire and a MWNT attached to a flat Ni-coated Si microstructure. We observed different macroscopic turn-on fields of 1.6 and 2.5 V µm−1, respectively, for the aforementioned experimental structures. This effect is investigated by detailed finite element analysis. We demonstrate that the geometry of the cathode structures significantly affects the microscopic tip field, leading to different turn-on voltages and field distributions for such individual MWNT emitters. Simulations show that changing the support geometry from a hemispherically capped shank to a cylindrical shank produces an increase in the macroscopic threshold field of 0.91 V µm−1. This effect is further investigated by varying the support radius from 0.5 to 30 µm for a cylindrically shaped support structure. The results show that such a variation in the radius of the support structure produces an increase in the macroscopic turn on field from 0.72 to 5.89 V µm−1. We also report quantitative evidence for the nonlinear relationship between the field enhancement factor as a function of support structure radius for nanostructures of three different aspect ratios.

We introduce an innovative geometry carbon nanotube (CNT) field emitter array capable of achieving stable and high current densities. Arrays of toroid CNT pillars were grown directly on bulk metal alloy substrates and on patterned metal catalyst on silicon substrates. Compared to a solid CNT pillar array (CPA), this toroid CPA (tCPA) provides a larger edge area for achieving a higher stable current density of 50 mA/cm2 at an applied dc field of less than 8 V/μm. Electrostatic simulation data confirming the field enhancement at the inner and outer edges of the tCPA are also presented.

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One area which has generated much interest for potential use of CNT cold field emitters involves devices operating in the terahertz (THz) regime of the electromagnetic spectrum. The potential applications in astronomy, biology, medical imaging, and communications continue to motivate research in THz technology [1,2]. Viable THz sources such as backward wave oscillators (BWOs), gyrotrons, and free electron lasers (FELs) require current densities ranging from tens of milliamps to several amps. Presently, available sources rely on thermionic emitters, and thus they are prone to slow switching speeds and high power consumption. In addition, thermionic electron emitters also tend to degrade due to chemical contamination and/or sputter erosion leading to emission instability. Carbon nanotubes (CNTs) are highly attractive materials for use as cold field emitters because they have exceptional intrinsic properties such as high aspect ratio, good chemical stability, high mechanical strength, and thermal conductivity [3,4]. These attributes enable CNT cold field emitters (CNTFE) to exhibit long lifetime and high current densities at low turn-on fields [3,5].Recently, our laboratory has developed a cold field emitter consisting of CNT pillar arrays (CPAs) [6]. A CNT pillar can be described as a uniform, highly dense, vertically aligned, compact bundle of CNTs as shown in Fig. 1a [7]. These CNT pillar structures provide higher current densities at lower turn-on fields. The exceptional field emission properties of CNT pillars is directly related to the electric field enhancement along the edge of the pillar known as the edge effect [8], as displayed in Fig 2. These structures have provided current densities as high as 10 mA/cm 2 at turn-on fields below 2 V/μm [6].Here we report further improvement of these CPA cathodes for achieving even higher current densities. Toward this end, we explore a novel geometry of patterned CNT emitter pillar array with increased edge area, as seen in Fig 1c. This new geometry differs from the CNT pillar in that it has an open-center, similar to the structure of a toroid. The toroid CNT pillar with an inner radius of 5 μm and an outer radius of 15 μm increases the total emitting edge area, while minimizing electric field screening by eliminating the CNTs in the middle of each pillar [9]. This leads to a greater total current from the cathode due to an increased number of sites that emit.Two cathodes of different geometries were fabricated on patterned metal catalyst on silicon substrates using a conventional fabrication process in order to compare their field emission characteristics. One substrate was patterned with CPAs and the other with toroid CNT pillar arrays (tCPAs). Two-minute growth for both substrates resulted in high-aspect ratio CNTs approximately 5 to 10 μm in height as shown in Figs 1b and d. Field emission data were collected using a diode configuration at 10 -8 Torr with a fixed separation of 100 μm between the anode and the top of the CNT array. The turn-on field, defined as the f...

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