Masayo Horikawa

Nanomanipulation of Single Nanoparticle Using a Carbon Nanotube Probe in a Scanning Electron Microscope

A novel method is presented for manipulation of a single nanoparticle using a carbon nanotube probe in a scanning electron microscope chamber. Nanomanipulation was achieved without dependence on the conductivity of the nanoparticle. In order to demonstrate the effectiveness of this technique, insulative Fe2O3 and conductive Fe3O4 were arranged on a nanogap junction for measurement of the electrical conductivity. The result clearly showed the difference between the resistance of the two iron oxides. It is considered that this technique could be a milestone in the measurement of the physical properties of nanomaterials.

Dependence of Electric Properties of a Nanogap Junction on Electrode Material

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).

Non-volatile Resistance Switching using Single-Wall Carbon Nanotube Encapsulating Fullerene Molecules

A resistance switching effect was found for a nanogap junction that has electrodes composed of single-wall carbon nanotubes (SWCNTs) that encapsulate fullerenes. A clear negative differential resistance effect and repeated on–off cycles were observed in the current–voltage characteristics of the nanogap junctions. The results suggest that the resistance switch effect is due to gap size changes that result in the migration of fullerene molecules. This implied that the electrode areas of a resistance switch could be miniaturized to true nanoscale size.

Influence of electrode size on resistance switching effect in nanogap junctions

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.

Non-Volatile Resistance Switching Using Silicon Nanogap Junction

We have investigated the resistance switching effect of a silicon nanogap structure when pulse bias voltages are applied. Silicon nanogap junctions were prepared by applying large-bias voltages across a Si wire and their electrical properties were measured in a vacuum chamber. The measured current–voltage characteristics exhibited a clear negative differential resistance effect and repeated on-off cycles with a large on-off ratio of over 103. The results suggest that resistance switching effects can be generated in a nanogap junction that is composed of a covalently bonded material such as silicon.

Self-Aligned Formation of Sub 1 nm Gaps Utilizing Electromigration during Metal Deposition

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.

Non-volatile high-speed resistance switching nanogap junction memory

Different voltage pulses were applied to Au nanogap junction to study the resistance switching characteristics. Consistent switching from a low to high resistance state was accomplished even at 20 ns pulse. Instead of setting current compliance for the reverse switching, we introduced a series resistance to the nanogap junction to limit the tunneling current and effectively performed the switching. The parasitic capacitance is shown to affect the programming speed. Upon reducing the capacitance, ns regime switching speed is achieved which indicates the potentiality of nanogap junction for high-speed random access memory.

Field Effect of Self-Assembled Organic Multilayer in Nanogap Electrode; Current Oscillation Behaviour at Room Temperature

We have measured the field effect of the conductance of self-assembled multilayers of insulating alkylchain molecules in 20-nm-gap electrodes. We observed the field-induced currents at specific gate bias voltages and current oscillation behavior against gate bias voltage at room temperature. In addition, similar device characters were obtained with a high yield of more than 70%.

Resistance switch using metal nanogap electrodes in air

Resistance switching in nanogap electrodes, the electrodes of which are made of platinum and gold, was investigated in air. The “off-to-on” transition in air was achieved by voltage sweeping enforced with a current-compliance operation that suppresses the overcurrent just after the change in tunneling resistance. It was also found that the applied voltages for the “on-to-off” resistance transition could be suppressed in air. These results imply that resistance switching is caused in air, and moreover, that the switching voltage is affected by the surroundings.

Physical Model for High-to-Low Resistive Switching of Gold Nanogap Junction

Electric properties of resistive switching in gold nanogap junction were investigated to discuss a physical model of high-to-low resistive switching of the junction. The threshold voltages during the switching are in proportion to logarithm of resistances immediately before the switching. This result indicates that the threshold voltages are depended on electric field, the critical value of which is estimated at about 1.8 V/nm. This implies that the high-to-low resistive switching can be explained by field-induced-migration model.