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.

The electronic structure of molecular junctions has a significant impact on their transport properties. Despite the decisive role of the electronic structure, a complete characterization of the electronic structure remains a challenge. This is because there is no straightforward way of measuring electron spectroscopy for an individual molecule trapped in a nanoscale gap between two metal electrodes. Herein, a comprehensive approach to obtain a detailed description of the electronic structure in single-molecule junctions based on the analysis of current-voltage (I-V) and thermoelectric characteristics is described. It is shown that the electronic structure of the prototypical C single-molecule junction can be resolved by analyzing complementary results of the I-V and thermoelectric measurement. This combined approach confirmed that the C single-molecule junction was highly conductive with molecular electronic conductances of 0.033 and 0.003 G and a molecular Seebeck coefficient of -12 μV K . In addition, we revealed that charge transport was mediated by a LUMO whose energy level was located 0.5≈0.6 eV above the Fermi level of the Au electrode.

The relationship between the current through an electronic device and the voltage across its terminals is a current–voltage characteristic (I–V) that determine basic device performance. Currently, I–V measurement on a single-molecule scale can be performed using break junction technique, where a single molecule junction can be prepared by trapping a single molecule into a nanogap between metal electrodes. The single-molecule I–Vs provide not only the device performance, but also reflect information on energy dispersion of the electronic state and the electron-molecular vibration coupling in the junction. This mini review focuses on recent representative studies on I–Vs of the single molecule junctions that cover investigation on the single-molecule diode property, the molecular vibration, and the electronic structure as a form of transmission probability, and electronic density of states, including the spin state of the single-molecule junctions. In addition, thermoelectronic measurements based on I–Vs and identification of the charged carriers (i.e., electrons or holes) are presented. The analysis in the single-molecule I–Vs provides fundamental and essential information for a better understanding of the single-molecule science, and puts the single molecule junction to more practical use in molecular devices.

We have investigated the electric conductance and atomic structure of single molecular junctions of pyrazine (Py), 4,4'-bipyridine (BiPy), fullerene (C), and 1,4-diaminobutane (DAB). The single molecular junctions were fabricated by breaking Au contacts between an Au tip and the Au electrode surface in the presence of the target molecules (breaking process) or approaching the Au tip to the Au electrode surface covered by the molecules (making process). In the making process, no major conductance state was observed in the conductance histogram for DAB, while single conductance states were observed for the π-conjugated molecules of Py (10 mG, G∼ 77.45 μΩ), BiPy (3 mG), and C (5 mG). In contrast to the making process, two major conductance states were observed for Py (1 mG and 0.3 mG), BiPy (0.6 mG and 0.2 mG), and C (30 mG and 3 mG) in the breaking process. The observed conductance behavior could be explained by the characters of the anchoring unit of molecules. In the making process, anchoring of molecules cannot be established by the point contact between the Au electrode and the localized lone pair of the N atom of DAB, Py, and BiPY, while molecules anchor on the Au electrode by using the plane-like contact between the Au electrode and the delocalized π electrons in the aromatic ring of Py, BiPY, and C, which explains the difference in the observed conductance behavior between DAB and the π conjugated molecules of Py, BiPY, and C in the making process. In the breaking process, the delocalized π-electrons of Py, BiPY, and C can make a contact with the Au electrode surface and the lone pair on the N atom of DAB as well as Py and BiPY can bind to Au atoms, and therefore, single molecular junctions were formed for all systems. The present results indicate that the π-plane is important for making metal-molecule contacts and for the formation of the single molecular junction in the making process.

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.