Quantum Conductance Steps in Solutions of Multiwalled Carbon Nanotubes

We have prepared solutions of multiwalled carbon nanotubes in Aroclor 1254, a mixture of polychlorinated biphenyls. The solutions are stable at room temperature. Transport measurements were performed using a scanning-tunneling probe on a sample prepared by spin coating the solution on gold substrates. Conductance steps were clearly seen. A histogram of a high number of traces shows maximum peaks at integer values of the conductance quantum $G_0=2e^2/h$, demonstrating ballistic transport at room temperature along the carbon nanotube over distances longer than $1.4\mu \mathrm{m}$.

Figure 1

A sequence of measurements of conductance versus time showing steps at $2G_0$, where $G_0$ is the conductance quantum $G=2e^2/h$. Two steps of different lengths out of a sequence of more than fifty are shown. The histogram shows the persistence of the quantized value along the sequence, with a clearly defined peak at $2G_0$. The bias voltage between tip and substrate was $V_p=10\mathrm{m}\mathrm{V}$.

Figure 2

In some of the sequences of conductance, three steps appeared at even integer values of the conductance quantum $G_0$. The conductance is plotted versus distance between tip and contact point with the sample. Two peaks are clearly seen in the histogram. A third peak is blurred by the noise. The bias voltage between tip and substrate was $V_p=25\mathrm{m}\mathrm{V}$.