Tomoji Kawai

Photoconductive coaxial nanotubes of molecularly connected electron donor and acceptor layers.

Controlled self-assembly of a trinitrofluorenone-appended gemini-shaped amphiphilic hexabenzocoronene selectively formed nanotubes or microfibers with different photochemical properties. In these nanotubes, which are 16 nanometers in diameter and several micrometers long, a molecular layer of electron-accepting trinitrofluorenone laminates an electron-donating graphitic layer of π-stacked hexabenzocoronene. The coaxial nanotubular structure allows photochemical generation of spatially separated charge carriers and a quick photoconductive response with a large on/off ratio greater than 104. In sharp contrast, the microfibers consist of a charge-transfer complex between the hexabenzocoronene and trinitrofluorenone parts and exhibit almost no photocurrent generation.

Identifying single nucleotides by tunnelling current

A major goal in medical research is to develop a DNA sequencing technique that is capable of reading an entire human genome at low cost. Recently, it was proposed that DNA sequencing could be performed by measuring the electron transport properties of the individual nucleotides in a DNA molecule. Here, we report electrical detection of single nucleotides using two configurable nanoelectrodes and show that electron transport through single nucleotides occurs by tunnelling. We also demonstrate statistical identification of the nucleotides based on their electrical conductivity, thereby providing an experimental basis for a DNA sequencing technology based on measurements of electron transport.

Resistive switching multistate nonvolatile memory effects in a single cobalt oxide nanowire.

A multistate nonvolatile memory operated at sublithographic scale has been strongly desired since other nonvolatile memories have confronted the fundamental size limits owing to their working principles. Resistive switching (RS) in metal-oxide-metal junctions, so-called ReRAM, is promising for next generation high-density nonvolatile memory. Self-assembled oxide nanowire-based RS offers an attractive solution not only to reduce the device size beyond the limitation of current lithographic length scales but also to extract the underlying nanoscale RS mechanisms. Here we demonstrate the multistate bipolar RS of a single Co(3)O(4) nanowire (10 nm scale) with the endurance up to 10(8). In addition, we succeeded to extract a voltage-induced nanoscale RS mechanism rather than current-induced RS. These findings would open up opportunities to explore not only for the intrinsic nanoscale RS mechanisms with the ultimate size limit but also for next generation multistate three-dimensional ReRAM.

Partial sequencing of a single DNA molecule with a scanning tunnelling microscope

The scanning tunnelling microscope is capable of the real-space imaging and spectroscopy of molecules on an atomic scale. Numerous attempts have been made to use the scanning tunnelling microscope to sequence single DNA molecules, but difficulties in preparing samples of long-chain DNA molecules on surfaces, and problems in reproducing results have limited these experiments. Here, we report single-molecule DNA sequencing with a scanning tunnelling microscope by using an oblique pulse-injection method to deposit the molecules onto a copper surface. First, we show that guanine bases have a distinct electronic state that allows them to be distinguished from the other nucleic acid bases. Then, by comparing data on M13mp18, a single-stranded phage DNA, with a known base sequence, the electronic fingerprint of guanine bases in the DNA molecule is identified. These results show that it is possible to sequence individual guanine bases in real long-chain DNA molecules with high-resolution scanning tunnelling microscope imaging and spectroscopy.

Nonvolatile bipolar resistive memory switching in single crystalline NiO heterostructured nanowires.

We have demonstrated the nonvolatile bipolar resistive memory switching in single crystalline NiO heterostructured nanowires for the first time. The self-assembled NiO nanowires are expected to open up opportunities to explore not only the detailed nanoscale mechanisms in NiO resistive memory switching but also next-generation nanoscale nonvolatile memory devices with the potential for high-density device integration and improved memory characteristics.

Resistive-switching memory effects of NiO nanowire/metal junctions.

We have demonstrated the construction of highly stable resistive switching (RS) junctions with a metal/NiO nanowire/metal structure and used them to elucidate the crucial role of redox events in the nanoscale bipolar RS. The presented approaches utilizing oxide nanowire/metal junctions offer an important system and platform for investigating nanoscale RS mechanisms of various oxide materials.

Intrinsic Mechanisms of Memristive Switching

Resistive switching (RS) memory effect in metal-oxide-metal junctions is a fascinating phenomenon toward next-generation universal nonvolatile memories. However the lack of understanding the electrical nature of RS has held back the applications. Here we demonstrate the electrical nature of bipolar RS in cobalt oxides, such as the conduction mechanism and the switching location, by utilizing a planar single oxide nanowire device. Experiments utilizing field effect devices and multiprobe measurements have shown that the nanoscale RS in cobalt oxides originates from redox events near the cathode with p-type conduction paths, which is in contrast with the prevailing oxygen vacancy based model.

Unipolar resistive switching characteristics of room temperature grown SnO2 thin films

The resistive switching characteristics of room temperature grown SnO2 films were investigated by fabricating the metal-oxide-metal sandwich structures. The unipolar operation was found in all devices. Experiments, including the size and material dependencies of the top electrodes and the three terminal device structures, demonstrated the rupture and formation of conducting filaments near the anode. The Ohmic behavior was observed in both on- and off-states when using Au and Ti top electrodes, whereas the Schottky behavior was only found in the off-state for Pt. The analysis on the transport properties indicates the presence of insulative crystalline SnO2 near the anode in the off-state.

Single-molecule sensing electrode embedded in-plane nanopore

Electrode-embedded nanopore is considered as a promising device structure for label-free single-molecule sequencing, the principle of which is based on nucleotide identification via transverse electron tunnelling current flowing through a DNA translocating through the pore. Yet, fabrication of a molecular-scale electrode-nanopore detector has been a formidable task that requires atomic-level alignment of a few nanometer sized pore and an electrode gap. Here, we report single-molecule detection using a nucleotide-sized sensing electrode embedded in-plane nanopore. We developed a self-alignment technique to form a nanopore-nanoelectrode solid-state device consisting of a sub-nanometer scale electrode gap in a 15u2005nm-sized SiO2 pore. We demonstrate single-molecule counting of nucleotide-sized metal-encapsulated fullerenes in a liquid using the electrode-integrated nanopore sensor. We also performed electrical identification of nucleobases in a DNA oligomer, thereby suggesting the potential use of this synthetic electrode-in-nanopore as a platform for electrical DNA sequencing.

Scaling Effect on Unipolar and Bipolar Resistive Switching of Metal Oxides

Electrically driven resistance change in metal oxides opens up an interdisciplinary research field for next-generation non-volatile memory. Resistive switching exhibits an electrical polarity dependent “bipolar-switching” and a polarity independent “unipolar-switching”, however tailoring the electrical polarity has been a challenging issue. Here we demonstrate a scaling effect on the emergence of the electrical polarity by examining the resistive switching behaviors of Pt/oxide/Pt junctions over 8 orders of magnitudes in the areas. We show that the emergence of two electrical polarities can be categorised as a diagram of an electric field and a cell area. This trend is qualitatively common for various oxides including NiOx, CoOx, and TiO2-x. We reveal the intrinsic difference between unipolar switching and bipolar switching on the area dependence, which causes a diversity of an electrical polarity for various resistive switching devices with different geometries. This will provide a foundation for tailoring resistive switching behaviors of metal oxides.