Direct growth of carbon nanotubes on a micro-sized cobalt tip and characterization of electron-emission properties

Abstract

In high-resolution x-ray tube application, carbon nanotubes (CNTs) offer several advantages such as excellent electron emission, low threshold voltage, micro-focusing, and enhanced current stability. To realize a micro-focused x-ray tube using a CNTs-based cold cathode, the emittance of electron beam should be decreased as small as possible. For this reason, substrates having a conical shape with a few hundreds of micrometer in diameter have usually been used by coating catalyst metals. This manner, however, may cause several inherent problems, such as poor adhesion between CNTs and substrates, large contact resistance, and technical difficulty of growing CNTs directly on such tip-type substrates. In this study, we suggest for the first time a novel method for direct growth of CNTs on various micro-tip materials, such as nickel (Ni), cobalt (Co), and iron (Fe), without using any other catalyst layers and present experimental results regarding to their electron emission properties. CNTs are grown by inductively coupled plasma-chemical vapor deposition (ICP-CVD). Sharpening of metal-tips is done by electro-chemically etching the metal-wires with diameter of 250 ?m in a KCl solution by applying dc voltage between the metal-wire and the platinum (Pt) electrode. The metal-tips are exposed to NH3 plasma prior to growth of CNTs. For all the grown CNTs, Raman spectroscopy, field emission SEM, and high-resolution TEM are used to analyze their structural properties. Electron-emission currents have been measured using a compactly designed field-emission measurement system. The long-term stability of emission currents are discussed in terms of the micro-tip metals used.