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

Abstract

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 π-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 π-conjugated systems, as expected. This was because the molecular orbital level of the junctions (ε) approached the Fermi level of the Au electrodes, and the metal–molecule electronic coupling (Γ) increased with the increasing system size. In the H states, the size dependence of both ε and Γ 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 Γ 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.

Manipulating Quantum Interference between σ and π Orbitals in Single-Molecule Junctions via Chemical Substitution and Environmental Control

“…20 Yet few reports of pyrazine conductance are found in the literature and our own measurements show no discernible signal, as we show further below. 21,22 We reason that the absence of a reliable pyrazine conductance signature is due to the reordering of frontier MOs in pyz compared to benzene, as shown in Figure 1A, left column. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of benzene consist of two degenerate MOs that all have π-character.…”

Transport properties of GNR-C60 single-molecule devices

“…Triggered by sophisticated molecular design, understanding the underlying physics of materials at the molecular level, and realizing the rapid miniaturization trend in the semiconductor industry, single-molecule devices with outstanding transport properties have attracted a great deal of attention in recent years. [1][2][3][4][5][6][7][8][9] Obviously, the selection of single-molecule architectures between two electrodes is the basic prerequisite for constructing such devices with excellent performance. Due to their lower energies as well as unique structural and electronical properties, fullerene (C 60 ) is usually considered an ideal candidate for building the basic elements of such molecular-based devices.…”

Regio- and Steric Effects on Single Molecule Conductance of Phenanthrenes

“…35 Although when anchoring groups are involved, measurements have been limited to how an extension of the acenes along the long axis will affect the conductance perpendicular to the path of tunneling electrons as a function of acene length 36 where intermolecular effects and junction geometries play important roles. Another related example by Fujii 37 showed that the conductance of heteroatom acenes (pyrazine, quinoxaline, and phenazine) are not only size dependent but also molecular 38,39 propose a magic ratio rule (MRR) showcasing that the connectivity-driven conductance ratio of graphene-like aromatic molecules is simply the square of the ratio of two "magic integers" whose values depend only on the connectivities to the electrodes. Density functional theory (DFT) calculations results by Palma 41 demonstrate that acene derivatives have near length-independent conductance and the anchoring configuration (steric effect) could dominate the conductance behavior.…”

“…Although when anchoring groups are involved, measurements have been limited to how an extension of the acenes along the long axis will affect the conductance perpendicular to the path of tunneling electrons as a function of acene length where intermolecular effects and junction geometries play important roles. Another related example by Fujii showed that the conductance of heteroatom acenes (pyrazine, quinoxaline, and phenazine) are not only size dependent but also molecular orientation sensitive in the Au–molecule–Au junctions. Still, studies on the regio-effects on the single molecule conductance of PAHs anchored between electrodes are limited to a few examples. − Lambert et al , propose a magic ratio rule (MRR) showcasing that the connectivity-driven conductance ratio of graphene-like aromatic molecules is simply the square of the ratio of two “magic integers” whose values depend only on the connectivities to the electrodes.…”

“…S−S distance (Å) a aMeasured S−S distance after DFT geometry optimization. b Calculated through eq 1 shown below 37.…”