Fluctuation in Interface and Electronic Structure of Single-Molecule Junctions Investigated by Current versus Bias Voltage Characteristics

Abstract

Structural and electronic detail at the metal-molecule interface has a significant impact on the charge transport across the molecular junctions, but its precise understanding and control still remain elusive. On the single-molecule scale, the metal-molecule interface structures and relevant charge transport properties are subject to fluctuation, which contain the fundamental science of single-molecule transport and implication for manipulability of the transport properties in electronic devices. Here, we present a comprehensive approach to investigate the fluctuation in the metal-molecule interface in single-molecule junctions, based on current-voltage ( I- V) measurements in combination with first-principles simulation. Contrary to conventional molecular conductance studies, this I- V approach provides a correlated statistical description of both the degree of electronic coupling across the metal-molecule interface and the molecular orbital energy level. This statistical approach was employed to study fluctuation in single-molecule junctions of 1,4-butanediamine (DAB), pyrazine (PY), 4,4'-bipyridine (BPY), and fullerene (C). We demonstrate that molecular-dependent fluctuation of σ-, π-, and π-plane-type interfaces can be captured by analyzing the molecular orbital (MO) energy level under mechanical perturbation. While the MO level of DAB with the σ-type interface shows weak distance dependence and fluctuation, the MO level of PY, BPY, and C features unique distance dependence and molecular-dependent fluctuation against the mechanical perturbation. The MO level of PY and BPY with the σ+π-type interface increases with the increase in the stretch distance. In contrast, the MO level of C with the π-plane-type interface decreases with the increase in the stretching perturbation. This study provides an approach to resolve the structural and electronic fluctuation in the single-molecule junctions and insight into the molecular-dependent fluctuation in the junctions.

Electronic Properties of Single Atom and Molecule Junctions

“…DAB shows a single distribution, while BPY shows two distinct distributions, which is consistent with the 1D conductance histogram. The 2D conductance histogram indicates that the conductance decreases with the stretch distance, and the conductance slope is more shallow for DAB than BPY …”

“…The I–V curve analysis reveals that the fluctuation of Γ is larger than that of ϵ , and that Γ almost linearly depends on the log G values where G is the conductance. The larger fluctuation of Γ indicates that the fluctuation on the electronic coupling is the main origin of the conductance fluctuation for the single‐molecule junctions . The electronic coupling depends on the metal‐molecule interface structure, and thus, the conductance fluctuation is caused by the fluctuation of the interface structure.…”

“…The σ‐type N lonepair ‐Au bond is directional and has small structural degree of freedom, giving rise to small fluctuation of ϵ against the structural and mechanical perturbation. The interplay between the σ‐ and π‐type bond under the mechanical perturbation is the origin of the nonlinear ϵ ‐log G distribution of BPY single‐molecule junctions …”

“…The 2D conductance histogram indicates that the conductance decreases with the stretch 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 distance, and the conductance slope is more shallow for DAB than BPY. [50] In order to understand the origin of the conductance fluctuation, the electronic structure of the single-molecule junction has been investigated by analysing the I-V curves. Figure 13 shows distribution of I-V curves for the singlemolecule junctions of DAB and BPY.…”

“…The interplay between the sand p-type bond under the mechanical perturbation is the origin of the nonlinear e-logG distribution of BPY single-molecule junctions. [50] The thermopower has been successfully measured for the single-molecule junctions. [16,[56][57][58][59] Due to their discrete energy levels, large thermopower can be expected.…”