Effects of geometry and doping on the operation of molecular transistors

We report first-principles calculations of current versus gate voltage characteristics of a molecular transistor with a phenyldithiolate molecule as active element. We show that (i) when the molecule is placed in proximity to the gate electrode, current modulation and resonant tunneling can occur at very small gate voltages. This is due to the first-order perturbation of the electronic states induced by the electrostatic potential of the gate in the molecular region. Such perturbation is present even if the molecule does not have an intrinsic dipole moment. (ii) The molecular transistor can be converted from n-type to p-type by the simple co-adsorption of a single oxygen atom placed near the molecule. While the latter finding suggests that the character of molecular transistors can be easily changed by doping the electrode surfaces, it also puts severe constraints on the experimental control of such structures for molecular electronics applications. (C) 2003 American Institute of Physics.