Transport in molecular transistors: Symmetry effects and nonlinearities

We report first-principles calculations of the current-voltage and current-gate-field characteristics of model molecular transistors to explore the factors that control current amplification and other properties. We show that both the position and amplitude of resonant peaks are modified by the use of substituents that affect the symmetry and dipole moments of the molecules, and allow a linear versus nonlinear Stark effect. In addition, strong nonlinearities arise at large source-drain currents.