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 C² radicals by adding Ar enables catalyst-free growth of nanowalls at a high rate up to about 1 μm/min and reduces the deposition temperature (T_D) down to around 650 °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 s p₂ cluster size and crystallinity, which is responsible for increasing delocalization of defect states associated with π bonding and, hence, quasimetallic character.