Controlling the Performance of a Three-Terminal Molecular Transistor: Conformational versus Conventional Gating

The effect of conformational changes in the gate arm of a three-terminal device is investigated. In the ground state, the gate (triphenyl) arm is nonplanar, where the middle phenyl ring is approximately 30° out-of-plane relative to other two rings. At this geometry, the calculated tunnel current (Id) as a function of external bias (Vds) across the two D−A substituted arms exhibits a typical insulator-semiconductor behavior. Similar Id−Vds characteristics is calculated when planarity of the triphenyl arm is restored. However, a significant increase, by more than an order of magnitude, and a distinct variation in the current are predicted in its operational mode (Vds > 1.5 V) when additional nonplanarity is induced in the triphenyl chain. Analysis of the results suggest that, unlike in “voltage” gating, neither the HOMO−LUMO gap nor the dipole moment of the system undergo significant changes due to pure conformational gating, as observed in this study. Instead, the observed conformational gating affects the current via localization/delocalization of the electronic wave function in the conduction channel. Furthermore, the tunneling current corresponding to conformational gating in two different directions appears to exhibit oscillatory nature with a phase factor of π/2 in the presence of the gate field. The current modulation is found to reach its maximum only under exclusive effect of voltage or conformational gating and diminishes when both of them are present.