Electrostatically driven energy shift of molecular orbitals of benzene and nicotine in carbon nanotube gaps

The requirement for controllable frontier orbital energy shift in single-molecule devices based on electronic (tunneling) transport yielded several rules for device design that lean on molecular level pinning to the electrochemical potential of nano-electrodes. We previously found that the pinning (designated as the strong pinning) was the consequence of the bias-induced molecular charge accumulation related to the hybridization of the highest occupied molecular orbital (HOMO) with one of the electrodes. However, in the wide bias range, only "partial" pinning (designated as the weak pinning) happens. In this work, we address the bias-induced shift of molecular orbitals in a weak pinning regime, where no hybridization or covalent bonds with electrodes exist. We found using density functional theory coupled with non-equilibrium Green's functions that the energy shift of frontier molecular orbitals of benzene and nicotine, placed between H-terminated (3, 3) CNTs, in weak pinning regime, is driven only by the electrostatic potential energy of an empty gap. For nicotine, whose HOMO and LUMO (lowest unoccupied molecular orbital) are located on different sides of the gap center, we show that the HOMO-LUMO energy gap changes with bias. We developed a theoretical model of a dielectric in a gap to depict this behavior. Application-wise, we expect that the weak pinning effect would be observable in novel single-molecule sensors based on electronic transport and molecular rectifying as long as the system exhibits a non-resonant behavior, and could serve for molecular gap tuning in single-molecule readout such as DNA, RNA and protein sequencing, or harmful single-molecule detection in gas phase