Excellent field emission properties of short conical carbon nanotubes prepared by microwave plasma enhanced CVD process

Randomly oriented short and low density conical carbon nanotubes ( CNTs) were prepared on Si substrates by tubular microwave plasma enhanced chemical vapor deposition process at relatively low temperature ( 350-550 degrees C) by judiciously controlling the microwave power and growth time in C2H2+NH3 gas composition and Fe catalyst. Both length as well as density of the CNTs increased with increasing microwave power. CNTs consisted of regular conical compartments stacked in such a way that their outer diameter remained constant. Majority of the nanotubes had a sharp conical tip ( 5 - 20 nm) while its other side was either open or had a cone/ pear-shaped catalyst particle. The CNTs were highly crystalline and had many open edges on the outer surface, particularly near the joints of the two compartments. These films showed excellent field emission characteristics. The best emission was observed for a medium density film with the lowest turn-on and threshold fields of 1.0 and 2.10 V/mu m, respectively. It is suggested that not only CNT tip but open edges on the body also act as active emission sites in the randomly oriented geometry of such periodic structures