The biphenyl molecule as a model transistor.

We study transport and charge control in a gated 4,4'-biphenyl diradical molecular transistor using self-consistent density-functional calculations. We track both electron-like and hole-like conduction and relate it to the field dependence of current-carrying pi-derived states. Owing to the coupling between the two benzene rings, the pi-states become segregated into extended, current-carrying states and localized states. Under application of the source/drain field, along the axis of the molecule, the localized pi-states become split, while the extended states become polarized and screen the field. The localized states act like isolated islands within the molecule--while they make a substantial contribution to the density of states, they make only a small contribution to transport.