Effects of TiC synthesis conditions on microstructures and electron emission properties of TiC-coated carbon nanotubes

Abstract

Recently, direct growth of carbon nanotubes (CNTs) on a sharp metal tip was attempted to reduce the emission sites for the use of CNT-based emitter as a micro-focused x-ray generation source with high resolution [1]. In a practical aspect, however, the CNT-emitter has to overcome change for structures and performance degradation due to high driving voltage and long time operating [2]. This work aims to enhance the electron-emission property of CNT-based field emitters as well as the long-term stability by coating a thin titanium carbide (TiC) layer on CNTs. The CNTs are directly grown on a nano-sized (< 500 nm in diameter) tungsten-tip by inductively coupled plasma-chemical vapor deposition (ICP-CVD). Detailed conditions used for CNT growth has been described elsewhere [3]. Ti films with thickness of 5 ~ 30 nm are deposited on CNTs by DC magnetron sputtering. The synthesis of TiC is performed at 700 oC in the ICP-CVD reactor by flowing acetylene (C2H2) gas with the gas mixing ratio of He:C2H2 = 500:125 and controlling the reaction time (3 ~ 10 min). Figure 1 shows the Raman spectra measured from the as-grown CNTs without TiC as well as the TiC-coated CNTs. The insets in figure 1 display their SEM images. It is observed that by TiC-coating the aspect ratio (i.e., the ratio of length to diameter in CNTs) is somewhat decreased, but the crystal quality is improved (i.e., the Raman peak ratio of ID/IG is decreased). Figure 2 indicates that the driving voltage necessary for approaching the same level of emission current is significantly reduced by TiC-coating. This may be attributed to the relatively low work function (~ 3 eV) of TiC, which leads to the decrease of effective work function for field-emission. In addition, the long-term stability of emission current is noticeably improved by TiC-coating. It is conjectured that TiC, which has high melting point, hardness and oxidation resistance, may inhibit degradation of CNT-emitter. It is suggested that the field-emission characteristics of TiC-coated CNT-emitters and their long-term stability behavior would be desirable as a micro-focused electron source for a practical x-ray system with high resolution.