Effects of boron-doping and heat-treatment on the electrical resistivity of carbon nanohorn-aggregates

Boron-doping of carbon nanohorn-aggregates was carried out by using arc-vaporization of a boron-containing composite carbon rod in a balanced gas of Ar with 5% of O2. Transmission and scanning electron microscopy revealed that the obtained sample powder consisted of high purity nanohorn-aggregates. The temperature dependence of the electrical resistivity of a pellet formed from the sample indicated a feature based on the three-dimensional variable-range hopping (3D-VRH) conduction. From the analyses of 3D-VRH, it was found that the electronic density of states near the Fermi-level, N(0), was enhanced due to the boron-doping, reaching an enhancement factor of ∼1.7 × 103 compared with the N(0) of the un-doped nanohorns. Moreover, the magnitude of the electrical resistivity decreased as the boron concentration increased. The electrical resistivity of the un-doped nanohorns also decreased with heat-treatment, but the N(0) value did not change significantly. These facts suggest that the change in N(0) should be associated with the boron atoms that are substitutionally doped in the sp2-bonded carbon network.