Masaaki Fujimori

Conductivity measurements of individual poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) nanowires on nanoelectrodes using manipulation with an atomic force microscope

We have prepared conducting polymer nanowires of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) with diameters under 10 nm by a molecular combing method, and have measured the conductivity of the individual PEDOT nanowires on platinum nanoelectrodes using manipulation with an atomic force microscope (AFM). The temperature dependence of the conductance was explained well by a quasi-one-dimensional variable range hopping model. The conductivity of two single nanowires was determined to be 0.6 and 0.09S∕cm, which is of the same order as that of PEDOT/PSS films. After all the nanowires crossed over the nanoelectrodes were cut off with AFM manipulation, the current was drastically decreased down to the background level. These results directly indicate that the conductivity was derived from the PEDOT nanowires on the nanoelectrodes.

Self-aligned fabrication process of electrode for organic thin-film transistors on flexible substrate using photosensitive self-assembled monolayers

A new long-wavelength-photosensitive self-assembled-monolayer (SAM) was developed, and a process fabricating alignment-free printable electrodes for flexible organic thin-film transistors (OTFTs) with the SAM was proposed. The SAM was degraded and its wetability was changed from hydrophobic to hydrophilic by irradiating it with UV light (>350 nm). The irradiated areas of SAM became more hydrophilic with tetramethylammonium hydroxide (TMAH) solution, which is a commercially available photoresist developer. Using the SAM and a back substrate exposure technique with TMAH rinsing, we can induce the self-alignment of solution-processed source/drain electrodes with respect to a gate electrode. A 20-µm-gap pattern of PEDOT was successfully formed by spin-coating on a plastic substrate.

Injection of molecules onto hydrogen-terminated Si(100) surfaces via a pulse valve

Using scanning tunneling microscopy and spectroscopy, we tested a pulse-injection method for placing molecules onto hydrogen-terminated Si(100) surfaces. The target molecules were multiwalled carbon-nanotube and π-conjugated poly(3-hexylthiophene) molecules dispersed or dissolved into a hexane or chloroform solvent. The results suggest that pulse injection can be used for fixing a variety of molecules in organic solvents onto H-terminated Si(100) surfaces while keeping most of the surface intact.

Pulse Injection of Carbon Nanotubes onto a H-terminated Si(100) Surface

We have used a pulse-injection method to fix multi-walled carbon nanotubes (MWNTs) onto hydrogen-terminated Si(100) surfaces. Using scanning-tunneling-microscopy (STM), we first tested several kinds of solvents for organic molecules which were pulse-injected onto the H-terminated Si(100) surfaces. Most of the solvent molecules of hexane and chloroform were desorbed from the surfaces after annealing the substrates, which indicates that they are suitable for fixing molecules onto the surface. Then, we fixed MWNTs which were dispersed in hexane and pulse-injected onto the H-terminated surface. An isolated MWNT was observed by STM and was investigated by scanning tunneling spectroscopy (STS). The STS spectra revealed a metallic feature of the particular MWNT.

Current-driven switching of exchange biased spin-valve giant magnetoresistive nanopillars using a conducting nanoprobe

An array of exchange biased spin-valve giant-magnetoresistance nanopillars was fabricated and the current I dependence of the resistance R was investigated using an electrically conducting atomic-force microscope (AFM) probe contact at room temperature. We observed current induced switching in a MnIr∕CoFe∕Cu∕CoFe∕NiFe nanopillar using the AFM probe contact. Current-driven switching using nanoprobe contact is a powerful method for developing nonvolatile and rewritable magnetic memory with high density.

Fabrication of four-probe fine electrodes on an atomically smooth Si(100)-2 × 1-H surface

Four-probe fine electrodes were fabricated on a Si(100)-2 × 1-H surface using conventional lithographic and scanning-probe nano-fabrication techniques followed by standard cleaning and hydrogenation processes of the surface. It was confirmed by scanning tunnelling microscopy that a Si(100)-2 × 1-H surface is formed in the area except for the fine electrodes. Tunnelling spectroscopy of the electrode surface and electrical conduction measurements of nanoscale wires were performed for evaluating the fabricated electrodes. The results showed that the fabricated fine electrodes are capable of measuring the electrical properties of nanoscale surface objects.

Electronic Structures of Individual Poly(3-hexylthiophene) Nanowires on Hydrogen-Terminated Si(100) Surfaces

Electronic structures of individual conducting polymers, poly(3-hexylthiophene)s (P3HTs), fixed on a hydrogen-terminated Si(100) surface have been examined by scanning tunneling microscopy/spectroscopy and first-principles calculations within the density-functional approach. The calculations reveal that the electronic structure of the polymer is only weakly influenced by the substrate, which ensures that the fixed polymers maintain a conduction property similar to that of isolated polymers. The current–voltage curves for the substrate-molecule-tip junction show rectification characteristics, indicating the carrier doping in the fixed polymers.

Current-driven magnetization reversal in exchange-biased spin-valve nanopillars

We have investigated the current-driven magnetization reversal of exchange-biased spin-valve giant magnetoresistive nanopillars with a magnetically pinned ferromagnetic layer. Current-driven magnetization reversal of a ferromagnetic layer with a smaller MV (M: magnetization, V: volume) value is found to take place even when the layer is pinned by the exchange bias induced by an antiferromagnet. The critical current density Jc of the spin-valve nanopillar with a MnIr layer adjacent to the current-driven free layer is of the same order as that of a Co/Cu/Co nanopillar (∼107A∕cm2).

Electrical Conductivity Measurement of DNA Double-Stranded Chains by "One-by-One" Cutting Method Using Atomic Force Microscopy

We measured the electrical current-voltage characteristics of λDNA in vacuum using fine electrodes with a gap of about 200 nm. It was found that the electrical resistance of λDNA molecules between the fine electrodes had a large variation: from 7.8 MΩ to values larger than 1 TΩ. This was consistent with the controversial results given in previous reports. The temperature dependence of conductivity was explained well by the Arrhenius equation. In addition, the conductivity of a single molecule of double-stranded λDNA was measured by a “one-by-one” cutting method using atomic force microscopy (AFM). The conductivity was determined to be 3×10 1 S/cm, which is about the same order as that of semiconductors.

Electronic structure of a polymer nanowire on H-terminated Si(100)

We measured current–voltage (I–V) characteristics of individual conducting polymers, poly(3-hexylthiophene)s (P3HTs), fixed on hydrogen-terminated Si(100) using scanning tunneling microscopy (STM)∕spectroscopy. The I–V curves reveal rectification characteristics that are attributed to the shift of the energy level of the valence-band maximum of P3HT under bias. The current suppression at positive substrate bias results from the effect of differential charging of the polymer between the opposite bias polarities, indicating the possibility for doping of polymer nanowires by using a STM tip as a gate electrode.