Synthesis and electrical characterization of n-type carbon nanowalls

Nitrogen-incorporated carbon nanowalls are prepared by microwave plasma-enhanced chemical vapor deposition using acetylene and methane. n-type conduction in the nanowalls is confirmed by Hall- and Seebeck-effect measurements. We show that increasing the amount of C-2 radicals by adding Ar enables catalyst-free growth of nanowalls at a high rate up to about 1 mu m/min and reduces the deposition temperature (T-D) down to around 650 degrees C. A substrate pretreatment using diamond powder results in a composite of nanowalls and nanocrystalline diamond films, suggesting that the nanowall growth is limited by gas-phase conditions rather than surface conditions. The low conductivity nanowalls for low T-D exhibit thermal activation in the Arrhenius plot, indicative of semiconducting conduction, while the high conductivity nanowalls for high T-D are almost temperature independent, indicative of quasimetallic conduction. The high conductivity is attributed to a global increase in the sp(2) cluster size and crystallinity, which is responsible for increasing delocalization of defect states associated with pi bonding and, hence, quasimetallic character. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3238276]