Electronic Structure and Transport Properties of Single-Molecule Junctions with Different Sizes of pi-Conjugated System

We investigated the electronic structure and charge transport properties of single-molecule junctions of a series of acene-type molecules (i.e., pyrazine, quinoxaline, and phenazine) with different sizes of the pi-conjugated system using a scanning tunneling microscopy-based break junction method under ambient conditions. A combined statistical analysis of the electric conductance and current versus bias voltage (I-V) characteristics based on the resonant-level model revealed the size dependence of the electronic conductance and its relation to the electronic structure in the single-molecule junctions. Each acene-type molecule sandwiched by Au electrodes showed high (H) and low (L) conductance states associated with two preferential molecular orientations (i.e., tilted and upright orientations, respectively) in the junctions. For the H states, the electronic conductance increased with the increasing size of the pi-conjugated systems, as expected. This was because the molecular orbital level of the junctions (e) approached the Fermi level of the Au electrodes, and the metal-molecule electronic coupling (G) increased with the increasing system size. In the H states, the size dependence of both e and G contributed positively to the conductance change with size. By contrast, for the L states, the conductance decreased with the increasing size. The I-V analysis in combination with the theoretical simulation of the geometry of the junctions suggested that the conductance behavior of the L state was because of the negative contribution of G to the conductance change with size. This originated from the increase in steric hindrance within the junctions and the resultant decrease in the metal-molecule electronic interaction with the increasing system size.

Automated summary

The study investigated the electronic structure and charge transport properties of acene-type molecules with varying sizes of the pi-conjugated system in single-molecule junctions under ambient conditions. It was found that the electronic conductance in the H state increased with size, while in the L state it decreased. This behavior was attributed to the size-dependent electronic structure and metal-molecule electronic interaction within the junctions.