Electron-emission properties of titanium carbide-coated carbon nanotubes grown on a nano-sized tungsten tip

Abstract

Recently, the x-ray tube using a carbon nanotubes (CNTs)-based cold cathode has been reported to yield better resolved x-ray images than that using the conventional thermionic emitters. 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, but this manner brings about reduction of emission current level. Furthermore, when the CNTs grown on a micro-tip shaped substrate are used as emitter, the degradation of CNTs at the emitter-summit due to prolonged emission of electrons is a crucial problem to be overcome. Titanium carbide (TiC) is known to have good electrical and thermal conductivities, high melting point and hardness, and a low work function (~ 3eV). This work aims to enhance the electron-emission properties and the stability of field emitters based on CNTs by coating the thin TiC layer. The CNTs are directly grown on a nano-sized (< 500nm) W-tip with Ni catalyst and Al/Ni/TiN buffer by using an inductively coupled plasma-chemical vapor deposition system. Prior to TiC-coating, CNTs are annealed at 700 oC in He atmosphere and Ti films are deposited on CNTs by RF magnetron sputtering. The synthesis of TiC is performed at 700 oC by introducing the C2H2 gas. Morphologies and microstructures of TiC-coated CNTs are monitored by SEM and TEM. Raman spectroscopy is also used to analyze the carbon structure and the crystal quality of CNTs. Effects of TiC-coating on electron-emission properties of the CNTs-based field emitters are examined by characterizing the threshold voltage for electron-mission, the maximum emission current, and the long-term stability.