In recent years, several researchers have reported the occurrence of reversible resistance switching effects in simple metal nanogap junctions. A large negative resistance is observed in the I-V characteristics of such a junction when high-bias voltages are applied. This phenomenon is characteristic behaviour on the nanometre scale; it only occurs for gap widths slightly under 13 nm. Furthermore, such a junction exhibits a non-volatile resistance hysteresis when the bias voltage is reduced very rapidly from a high level to around 0 V, and when the bias voltage is reduced slowly. This non-volatile resistance change occurs as a result of changes in the gap width between the metal electrodes, brought about by the applied bias voltage.

We developed a procedure for the fabrication of sub 1 nm gap Au electrodes via electromigration. Self-aligned nanogap formation was achieved by applying a bias voltage, which causes electromigration during metal evaporation. We also demonstrated the application of this method for the formation of nanogaps as small as 1 nm in width, and we found that the gap size can be controlled by changing the magnitude of the applied voltage. On the basis of the electric conductance and surface-enhanced Raman scattering (SERS) measurements, the fabricated gap size was estimated to be nearly equal to the molecular length of 1,4-benzenedithiol (BDT). Compared with existing electromigration methods, the new method provides two advantages: the process currents are clearly suppressed and parallel or large area production is possible. This simple method for the fabrication of a sub 1 nm gap electrode is useful for single-molecule-sized electronics and opens the door to future research on integrated sub 1 nm sized nanogap devices.

Oblique deposition was used to fabricate two metal electrodes separated by a gap of less than 10 nm on a SiO2 substrate. By sweeping voltage between these electrodes, a negative resistance change of several digits was observed in vacuum. In this work, electrodes made of Au, Pd, Pt, and Ta were fabricated, and their electric properties were measured in vacuum. The negative resistance was observed for all of the four metals. The result of the measurements clearly shows the correlation between the voltage at the minimum resistance and the melting point. Also, the calculated temperature rise shows a correlation with the melting point. These facts support the effect that the thermal change of the electrode metal has a considerable effect on the electric properties of the nanogap switch (NGS).

The size dependence of the resistance switching effect in nanogap junctions was investigated to determine the nature of the local structural changes responsible for the effect. The maximum current, during resistance switching, decreased with the total emission area across the nanogap to an average of 146 μA at a linewidth of 45 nm. This implies that the resistance switching effect stems from changes in the gap width at multiple local sites on the metal surface.

We designed and synthesized a new quadrivial anchoring unit 4-TEB, to construct a stable single-molecule junction with gold electrodes, which should have equivalent conducting electron pathways between two electrodes. The conductances of single-molecule junctions comprising 4-TEB and its bidirectional counterpart 2-TEB were determined to be 2.7 × 10 −4 G 0 (2e 2 /h) and 5.0 × 10 −5 G 0 , respectively, by using scanning tunneling microscope break junction (STM-BJ) techniques. The single 4-TEB molecule junction had higher stability and conductivity compared to those of the single 2-TEB molecule junction. Although the number of electron pathways from/to the electrode to/from the molecule was additive using the equivalent multianchoring, the conductance of the singlemolecule junction was not additive. From first-principles electronic transport calculations, the mechanism for the new quadrivial 4-TEB single-molecule junction involved an overlap resonance effect to the HOMO conducting orbital, giving rise to tunneling. Using fixed nanogap electrodes, we constructed stable molecular junctions of 4-TEB and observed symmetric peaks in the derivative of the conductance−voltage (G−V) curves, which were assigned to electron transport through the HOMO on the basis of theoretical calculations.