Yasuhisa Naitoh

Simple and Scalable Gel-Based Separation of Metallic and Semiconducting Carbon Nanotubes

We report a rapid and scalable method for the separation of metallic and semiconducting single-wall carbon nanotubes (SWCNTs); the separation is performed by the selective adsorption of semiconducting SWCNTs on agarose gel. The most effective separation was realized by a simple procedure in which a piece of gel containing SWCNTs and sodium dodecyl sulfate was frozen, thawed, and squeezed. This process affords a solution containing 70% pure metallic SWCNTs and leaves a gel containing 95% pure semiconducting SWCNTs. Field-effect transistors constructed from the separated semiconducting SWCNTs have been demonstrated to function without any electrical breakdown.

Morphological Change and Mobility Enhancement in PEDOT:PSS by Adding Co-solvents

Adding ethylene glycol (EG) to a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solution improves the crystallinity of the PEDOT and the ordering of the PEDOT nanocrystals in solid films. The carrier-mobility enhancement is confirmed by using ion-gel transistors combined with in situ UV-vis-NIR spectroscopy.

Recent Progress on PEDOT-Based Thermoelectric Materials

The thermoelectric properties of poly(3,4-ethylenedioxythiophene) (PEDOT)-based materials have attracted attention recently because of their remarkable electrical conductivity, power factor, and figure of merit. In this review, we summarize recent efforts toward improving the thermoelectric properties of PEDOT-based materials. We also discuss thermoelectric measurement techniques and several unsolved problems with the PEDOT system such as the effect of water absorption from the air and the anisotropic thermoelectric properties. In the last part, we describe our work on improving the power output of thermoelectric modules by using PEDOT, and we outline the potential applications of polymer thermoelectric generators.

Polymer thermoelectric modules screen-printed on paper

We report organic thermoelectric modules screen-printed on paper by using conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and silver paste. Our large-area devices provided sufficient power to illuminate light-emitting diodes. This is the first example of thermoelectric modules containing conducting polymers being used to power practical devices. The stability of this proof-of-concept module was tested at 100 °C for over 100 h without any encapsulation. We showed that the decrease in device performance was caused not by the deterioration of the materials but by degradation of the interface between the conducting polymers and silver paste. These results suggest that organic thermoelectric modules could be used to harvest heat energy at low temperature, although the stability of the interface must be improved.

Thermoelectric power enhancement of PEDOT:PSS in high-humidity conditions

We report an increase in the thermoelectric power factor of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) from 23 ± 5 to 225 ± 130 µW/(m·K2) in high-humidity conditions. This enhancement was caused mainly by an increase in the apparent Seebeck coefficient, which could be related to morphological change after water absorption or electrochemical reaction of PEDOT in air. Our results demonstrate a positive effect of water in the PEDOT:PSS system and indicate the need for well-controlled measurement conditions, particularly humidity, in evaluating the performance of conducting organic materials.

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.

Electrochromic carbon electrodes: Controllable visible color changes in metallic single-wall carbon nanotubes

Electrochromism is the phenomenon of reversible changes in the optical properties of a material during electrochemical redox processes. [ 1 ] Typical electrochromic devices rely on transparent electrodes such as indium tin oxide (ITO) glasses to apply a potential to electrochromic materials. However, it is desirable to develop a system without using ITO due to emerging competing demands on rare metal resources. Here, we demonstrate that electrochromic devices can be made using metallic singlewall carbon nanotubes (SWCNTs). The color changes can be produced by metallic SWCNTs without ITO layers. The metallic SWCNTs act as both electrochromic components and electrodes, indicating a route to all carbon nanotube electrochromic devices. We demonstrate very clear, stable and reversible color changes in metallic SWCNTs with diameters of 0.84, 1.0, and 1.4 nm, exhibiting yellow, magenta, and blue-green colors, respectively. The color changes are reversible and repetitive, and a relatively good coloration effi ciency (1.9 ± 0.2) × 10 2 cm 2 C − 1

Performance Enhancement of Thin-Film Transistors by Using High-Purity Semiconducting Single-Wall Carbon Nanotubes

Thin-film transistors (TFTs) using a random network of semiconductor-enriched single-wall carbon nanotubes (SWCNTs) were fabricated on a SiO2/Si substrate. Semiconductor-enriched SWCNTs were extracted from a pristine sample by centrifugation using agarose gel. Prior to depositing the SWCNTs, the substrate surface was modified by self-assembly of a monolayer of aminosilanes to produce an ideal two-dimensional network structure. As a result, all the TFTs fabricated on the substrate had on/off current ratios higher than 104 without electrical breakdown, while TFTs fabricated using pristine SWCNTs had a broad distribution of on/off ratios from 101 to 104. This improvement in transfer characteristics demonstrates a major advantage of using semiconductor-enriched SWCNTs.

A simple fabrication method of nanogap electrodes for top-contacted geometry: application to porphyrin nanorods and a DNA network

We have developed a fabrication method for nanogap electrodes without employing photo- or electron-beam lithography to measure the electrical characteristics of nanostructured molecules. This angle-controlled shadow-masking method enables us to construct nanogap electrodes without a wet process after the molecules are positioned on the substrate. The proposed method makes it possible to measure electrical characteristics without structurally deforming or denaturing the molecules due either to the step edge of an electrode or to the organic solvents used in the wet process. The results demonstrate that a gap length between the electrodes of less than 100 nm can be fabricated reproducibly. We have measured the electrical characteristics of lambda DNA (λ-DNA) networks and molecular nanorods made of porphyrin-derivative molecules (TPPS: 5,10,15,20-tetraphenyl-21H,23H-porphyrine tetrasulfonic acid) in which J-aggregates are formed inside. Experimental findings reveal that the electrical conductivity of λ-DNA decreased under a vacuum condition, whereas that of TPPS nanorods decreased under oxygen and nitrogen gas-purged conditions.

A nano tester: A new technique for nanoscale electrical characterization by point-contact current-imaging atomic force microscopy

A new atomic force microscopy (AFM) technique, called point-contact current-imaging AFM (PCI-AFM)-which combines tapping mode (for mapping topographic image) and point-contact operation (for measuring current?voltage characteristics)-has been developed. This new AFM technique can simultaneously map high-resolution topographic image and measure spatially resolved I?V characteristics of materials (placed on an insulative substrate and connected to a gold electrode) on the nanoscale. The high performance of the PCI-AFM system was evaluated in experiments on single-walled carbon nanotubes (SWNTs).