A metallic carbon nanotube can be made into a semiconductor and vice versa via the Aharonov-Bohm effect. Semiconductor nanotubes would allow for building nanoscale computers.

The Aharonov-Bohm effect is a quantum phenomenon in which the wavefunction of an electron acquires a phase shift as it follows a trajectory that encloses a magnetic flux (such as the path round the surface of a cylinder in a magnetic field). This phase shift alters the way the electron trajectories interfere, and therefore affects the electronic properties of the material.