Resistance and tunneling spectra of aligned multiwalled carbon nanotube arrays

The resistance and tunneling spectra of samples formed by depositing silver electrodes at the two ends of aligned, template-grown, carbon nanotube arrays were measured in the temperature range 0.67-440 K. Two types of samples were fabricated, one with small oxide tunnel junctions separating the carbon nanotubes from the metal electrodes, the other with a significant Al2O3 tunnel barrier. The measurements indicate the presence of three regimes for dI/dV(V). For T > 220 K, dI/dV(V) and the zero-bias conductivity show a broad minimum and an activation temperature dependence suggesting semiconductor behavior. In the temperature range 10 <T < 140 K, the zero-bias conductivity shows a square-root temperature dependence. For T <2 K, a very steep rise in the zero-bias tunneling resistance is observed with a strong simultaneous suppression of the tunneling conductivity near the Fermi energy. Coulomb blockade is suggested as a plausible explanation of the observed behavior. (C) 2000 American Institute of Physics. [S0021-8979(00)09722-X].