Effects of plasma power and plasma sheath on field emission properties of carbon nanotubes

With a nickel catalyst, carbon nanotubes (CNTs) were prepared by microwave plasma-enhanced chemical vapor deposition (MPCVD). Transmission electron microscopy (TEM) images reveal the center hollowness and multiwall structure of CNTs. The tip-growth mechanism of the CNTs prepared by MPCVD is confirmed by the Ni particles enclosed at the tips of the CNTs. The degree of CNT graphitization increases with plasma power up to about 1000 W and then reaches the upper limit. This is attributed to an insufficient carbon feedstock for CNT growth. The field emission (FE) efficiency of the CNTs increases with plasma power. For plasma powers not greater than 1000 W, the increase in graphitization degree and the decrease in the number of defects in a CNT emitter array enhance FE performance. The defects produced by the large split catalysts remaining in the CNTs at low plasma powers (700 and 800 W) also result in a low FE efficiency. Although the CNTs grown at moderate to high plasma powers (1000 to 1200W) have similar graphitization degrees, a low plasma power results in split catalysts and hence defects, as evidenced by TEM observation. These defects accumulate electrons, block electronic transport, and hence reduce the FE efficiency of the CNTs. The effect of kinked CNTs produced by the a plasma sheath on FE efficiency is negligible.