Current Density and Distribution of Carbon Nanotubes Anchored on Alumina Templates with Various Pore Diameter

The synthesis of carbon nanotubes (CNTs) on porous alumina using ferrocene as single source precursor in a CVD process was presented. In order to influence the anchoring and number density of CNT emitters, the pore diameter of the alumina templates was varied. The field emission performance of such CNT cathodes was integrally measured in spacer-free diode configuration with luminescent screen, and the current distribution has been derived by image analysis software. As expected, a strong increase of the dc current density has been found for pore diameters below 70 nm. The high current carrying capability of the CNT emitters on templates with 22 nm pores was confirmed by local field emission scanning microscopy (FESM). FESM voltage scans of this CNT sample show four times more emitters on the inlet side, which was exposed to the gas flow in the MOCVD process, than on the outlet side. SEM studies have revealed multiwall CNTs which are longer and more dense on the inlet side. Accordingly, quite different field enhancement factors beta of single emitters have been found for both sides. It is remarkable that the sparsely grown CNTs on the outlet side are favourable for high current densities. In conclusion, less shielding and a better anchoring of the CNTs for matched pore diameters causes the improved current carrying capability of the CNTs on alumina templates