Subband Population in a Single-Wall Carbon Nanotube Diode

We observe current rectification in a molecular diode consisting of a semiconducting single-wall carbon nanotube and an impurity. One-half of the nanotube has no impurity, and it has a current-voltage $\left(I-V\right)$ characteristic of a typical semiconducting nanotube. The other half of the nanotube has the impurity on it, and its $I-V$ characteristic is that of a diode. Current in the nanotube diode is carried by holes transported through the molecule's one-dimensional subbands. At 77 K we observe a stepwise increase in the current through the diode as a function of gate voltage, showing that we can control the number of occupied one-dimensional subbands through electrostatic doping.