High performance nanocarbon field emitter

Abstract

By taking advantage of the superior material properties and high aspect ratio geometry of nanocarbon materials, many researchers have been able to devote their efforts toward the development of nanocarbon emitters, which are capable of providing low turn-on voltage, high emission current density and long-term emission stability. Electron beams of field emitter are widely employed in various applications, including X-ray photography and computed tomography (CT) scans in hospitals, fault detection of semiconductor devices, airport security, and terahertz generation. Moreover, electron beam generators are progressively becoming more miniaturized, light-weighted, and low-powered so that they can be applied to diverse areas. For a high performance field emitter, new concept of field emitter is needed to overcome the previous research. In this thesis, four novel approaches are developed in terms of fabricating the multidimensional field emitters. 1) CNT point emitter with graphene interfacial layer: we report graphene as an interfacial layer between the metal and CNTs to improve the interfacial contact. The interfacial graphene layer results in a dramatic decrease of the electrical contact resistance by an order of 2 and an increase of the interfacial thermal conductivity by 16%. Such a high improvement in the electrical and thermal interface leads to superior field emission performance with a very low turn-on field of 1.49 V μm−1 at 10 μA cm−2 and a threshold field of 2.00 V μm−1 at 10 mA cm−2, as well as the maximum current of 16 mA (current density of 2300 A cm−2). 2) CNT line emitter using clamping process: we report a robust and scalable method to fabricate high-performance carbon nanotube (CNT) line emitters by using a macroscopic mechanical clamping process. The process utilizes a handheld, metallic tongs (which also serves as an electrode for field emission) to pick up vertically aligned, super-grown CNTs from a substrate. By applying mechanical compression to a CNT forest in the uniaxial direction, the hemicylindrical shape of CNT structure strongly held with the tongs electrode is fabricated. With robust mechanical, thermal, and electrical contact characteristics created by mechanical clamping of CNTs, the CNT line emitter shows superior field emission performance with an emission current of 43 mA, a current density of 2700 mAcm-2, and stable operation over 10 hours. Furthermore, an extremely high emission current of 103 mA is achieved by clamping multiple CNT forests in a single tongs, showing prompt applicability of the present emitter for high-power electron beam sources. 3) Holey graphene film as a planar field emitter: we introduce an emitter in the shape of a hole, or hole emitter. An array of holes in graphene film can serve as a planar emitter, in which each circular hole is an emitter. Because of the nature of the sidewall emission through the edge of graphene film, the present planar emitter does not need any cumbersome process of leveling off emitter height and contriving to obtain the desirable inter-distance between emitters. Therefore, the screening effect and the field emission uniformity considered as obstacles in developing high efficiency planar emitters are easily resolved by an optimal design of the circular hole array. Moreover, the graphene film emitter is so flexible that it can be wrapped around a cylindrically shaped anode. This flexibility allows the present emitter to be used as a high power electron beam source because of the focusing of the emitted electron beams rendered by the concavely curved surface. 4) Carbonized cork emitter for multi-dimensional field emission: to broaden the range of application of electron beams, low power field emitters are needed that are miniature and light. We introduce here carbonized cork as a material for field emitters. The light natural cork becomes graphitic honeycomb upon carbonization, with the honeycomb cell walls 100-200 nm thick and the aspect ratio larger than 100, providing an ideal structure for the field electron emission. Compared to nanocarbon field emitters, the cork emitter produces a high current density and long-term stability with a low turn-on field. The nature of cork material makes it quite simple to fabricate the emitter. Furthermore, any desired shape of emitter tailored for the final application can easily be prepared for point, line, or planar emission.