Control of anisotropic conduction of carbon nanotube sheets and their use as planar-type thermoelectric conversion materials.

The large anisotropic thermal conduction of a carbon nanotube (CNT) sheet that originates from the in-plane orientation of one-dimensional CNTs is disadvantageous for thermoelectric conversion using the Seebeck effect since the temperature gradient is difficult to maintain in the current flow direction. To control the orientation of the CNTs, polymer particles are introduced as orientation aligners upon sheet formation by vacuum filtration. The thermal conductivities in the in-plane direction decrease as the number of polymer particles in the sheet increases, while that in the through-plane direction increases. Consequently, a greater temperature gradient is observed for the anisotropy-controlled CNT sheet as compared to that detected for the CNT sheet without anisotropy control when a part of the sheet is heated, which results in a higher power density for the planar-type thermoelectric device. These findings are quite useful for the development of flexible and wearable thermoelectric batteries using CNT sheets.

Effects of Fe cations in ruthenium-complex multilayers fabricated by a layer-by-layer method.

Molecular multilayers were fabricated using a Ru complex containing Fe cations on an indium tin oxide surface to control the properties of the Ru-complex multilayers such as the multilayer orientation and the electron transport. The Ru-complex multilayer films containing Fe cations were thicker than those containing Zr cations, which have been used previously. The electron transport properties of the multilayers containing Fe cations were evaluated. Solid-state sandwich cell measurements showed that the Ru-complex multilayer films containing Fe cations exhibited increased electron transport with a lower transport coefficient ß of 0.01 Å(-1), whereas those that contain Zr cations have ß âˆ¼ 0.07 Å(-1). Thus, Fe cations are effective in obtaining thicker Ru-complex layers with increased electron transport abilities.

Dynamic pattern formation of liquid crystals using binary self-assembled monolayers on an ITO surface under DC voltage.

There have been numerous studies of liquid crystal (LC) convection using sandwich-type LC cells under AC voltage. In contrast to previous LC convection studies under AC voltage, we propose the use of a binary self-assembled monolayer (SAM) with a redox-active Ru complex and insulating octadecyl phosphonic acid (C18) molecules on an indium tin oxide (ITO) surface as the electrode of sandwich-type LC cells under DC bias voltage. This is because the functionalized molecules immobilized on the ITO surface are expected to control the LC orientation and electrical conduction of LC cells, under an exact DC bias voltage. We successfully achieved LC pattern formation using ITO electrodes with binary SAMs in LC cells. Moreover, we confirmed that the LC pattern size was increased by increasing the coverage of the Ru complex in binary SAMs. We consider that a combination of three factors, electrical conduction change, controlling of LC orientation in the initial stage and redox-activity of the Ru-complex, is the reason for LC convection although we cannot fully explain the distribution of these three factors. We believe that our LC pattern formation is promising for new type devices e.g., artificial compound eyes using the LC device technology.

[A case of advanced gastric cancer treated with curative resection after preoperative combined chemotherapy with docetaxel, cisplatin, and S-1].

A 67-year-old woman with epigastralgia was admitted to our hospital and was diagnosed with type 3 advanced gastric cancer with lymph node metastases.The clinical diagnosis was Stage III A(cT3, N2, M0).Since curative surgery was not feasible, we administered preoperative combination chemotherapy with docetaxel, cisplatin(CDDP), and S-1.After 3 courses of chemotherapy, the lymph nodes became undetectable on computed tomography(CT).Distal gastrectomy was performed with curative intent, and the final diagnosis was Stage IIA(ypT3, N0, M0).There has been no recurrence for 1 year and 4 months after the operation.

Improving the Long-Term Stability of PbTe-Based Thermoelectric Modules: From Nanostructures to Packaged Module Architecture.

Nanostructured lead telluride PbTe is among the best-performing thermoelectric materials, for both p- and n-types, for intermediate temperature applications. However, the fabrication of power-generating modules based on nanostructured PbTe still faces challenges related to the stability of the materials, especially nanoprecipitates, and the bonding of electric contacts. In this study, in situ high-temperature transmission electron microscopy observation confirmed the stability of nanoprecipitates in p-type Pb0.973Na0.02Ge0.007Te up to at least ∼786 K. Then, a new architecture for a packaged module was developed for improving durability, preventing unwanted interaction between thermoelectric materials and electrodes, and for reducing thermal stress-induced crack formation. Finite element method simulations of thermal stresses and power generation characteristics were utilized to optimize the new module architecture. Legs of nanostructured p-type Pb0.973Na0.02Ge0.007Te (maximum zT ∼ 2.2 at 795 K) and nanostructured n-type Pb0.98Ga0.02Te (maximum zT ∼ 1.5 at 748 K) were stacked with flexible Fe-foil diffusion barrier layers and Ag-foil-interconnecting electrodes forming stable interfaces between electrodes and PbTe in the packaged module. For the bare module, a maximum conversion efficiency of ∼6.8% was obtained for a temperature difference of ∼480 K. Only ∼3% reduction in output power and efficiency was found after long-term operation of the bare module for ∼740 h (∼31 days) at a hot-side temperature of ∼673 K, demonstrating good long-term stability.

Achieving Compatible p/n-Type Half-Heusler Compositions in Valence Balanced/Unbalanced Mg1-xVxNiSb.

In thermoelectric and other inorganic materials research, the significance of half-Heusler (HH) compositions following the 18-electron rule has drawn interest in developing and exploiting the potential of intermetallic compounds. For the fabrication of thermoelectric modules, in addition to high-performance materials, having both p- and n-type materials with compatible thermal expansion coefficients is a prerequisite for module development. In this work, the p-type to n-type transition of valence balanced/unbalanced HH composition of Mg1-xVxNiSb was demonstrated by changing the Mg:V chemical ratio. The Seebeck coefficient and power factor of Ti-doped Mg0.57V0.33Ti0.1NiSb are -130 µV K-1 and 0.4 mW m-1 K-2 at 400 K, respectively. In addition, the reduced lattice thermal conductivity (κL < 2.5 W m-1 K-1 at 300 K) of n-type compositions was reported to be much smaller than κL of conventional HH materials. As high thermal conductivity has long been an issue for HH materials, the synthesis of p- and n-type Mg1-xVxNiSb compositions with low lattice thermal conductivity is a promising strategy for producing high-performance HH compounds. Achieving both p- and n-type materials from similar parent composition enabled us to fabricate a thermoelectric module with maximum output power Pmax ∼ 63 mW with a temperature difference of 390 K. This finding supports the benefit of exploring the huge compositional space of valence balanced/unbalanced quaternary HH compositions for further development of thermoelectric devices.

Thermoelectric efficiency of organometallic complex wires via quantum resonance effect and long-range electric transport property.

Superior long-range electric transport has been observed in several organometallic wires. Here, we discuss the role of the metal center in the electric transport and examine the possibility of high thermoelectric figure of merit (ZT) by controlling the quantum resonance effects. We examined a few metal center (and metal-free) terpyridine-based complexes by first-principles calculations and clarified the role of the metals in determining the transport properties. Quasi-resonant tunneling is mediated by organic compounds, and narrow overlapping resonance states are formed when d-electron metal centers are incorporated. Distinct length (L) and temperature (T) dependencies of thermopower from semiconductor materials or organic molecular junctions are presented in terms of atomistic calculations of ZT with and without considering the phonon thermal conductance. We present an alternative approach to obtain high ZT for molecular junctions by quantum effect.

Photoresponse enhancement by mixing of an alcohol-soluble C60 derivative into a ruthenium complex monolayer.

We have investigated the effect of mixing an alcohol-soluble C60 derivative into a self-assembled monolayer (SAM), which consisted of a redox-active Ru-complex with multipoint anchoring groups, on an indium tin oxide surface. Angle-resolved X-ray photoelectron spectroscopy of the mixed SAM revealed that the C60 derivative was well incorporated into the redox-active Ru-complex SAM. In addition, some of the C60 derivatives were present on the mixed SAM surface. In the presence of a sacrificial reagent, the action spectra of the mixed molecular layer exhibited a broad spectral response due to the presence of the C60 derivative, whereas a sharp response was observed for the monocomponent Ru-complex SAM. We propose that an efficient charge separation arising from the combination of the C60 derivative and the Ru-complex enhanced the spectral response of the mixed SAM.

Humidity control in a closed system utilizing conducting polymers.

In this study, we demonstrate that conducting polymers could be ideal materials for continuously managing humidity in a wide range of enclosed spaces. We demonstrate a simple battery-driven humidity control unit to manage the humidity in a closed environment and studied humidity-responsive nanocapsules using Zn-coordinated lipid nanovesicles. This study not only promises new applications for conducting polymers but also provides an easy approach for fabricating chambers with a controlled environment, which are often used by physicists, chemists, and biologists.

Surface potential switching by metal ion complexation/decomplexation using bipyridinethiolate monolayers on gold.

Surface potential switching on gold(111) surfaces is induced by complexation/decomplexation reactions of a bipyridine (BP) derivative and palladium(II) chloride, as observed by Kelvin probe force microscopy (KFM). On the basis of the theoretical predictions, a 4-(5-phenylethynyl-2,2'-bipyridine-5'-yl-ethynyl)benzenethiol (PhBP) derivative was synthesized and used as an active monolayer to catch transition metal ions. By using the microcontact printing (CP) technique, micron-size patterned PhBP monolayers, which act as effective hosts to coordinate palladium(II) chloride, were prepared on gold(111) surfaces. The KFM signal decreases by complexation of the Pd(II) chloride in PhBP monolayers and is recovered by removal of Pd ions using an ethylenediamine solution, as confirmed by X-ray photoelectron spectroscopy. This process is reversible, indicating that the surface potential switching is realized by complexation/decomplexation of Pd(II). A CP PhBP monolayer, when it detects the target palladium ion, shows sensitivity for the picomolar level detection judged from surface potential changes in KFM measurements. The dipole moment estimated by the surface potentials is much smaller than the calculated value, indicating that mechanisms for the reduction of the surface dipole moment exist in real monolayers prepared by the CP method.

Photoinduced Dedoping of Conducting Polymers: An Approach to Precise Control of the Carrier Concentration and Understanding Transport Properties.

Exploring the various applications of conjugated polymers requires systematic studies of their physical properties as a function of the doping density, which, consequently, calls for precise control of their doping density. In this study, we report a novel solid-state photoinduced charge-transfer reaction that dedopes highly conductive polyelectrolyte complexes such as poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate). Varying the UV-irradiation time of this material allows the carrier density inside the film to be precisely controlled over more than 3 orders of magnitude. We extract the carrier density, carrier mobility, and Seebeck coefficient at different doping levels to obtain a clear image of carrier-transport mechanisms. This approach not only leads to a better understanding of the physical properties of the conducting polymer but also is useful for developing applications requiring patterned, large-area conducting polymers.

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.

Potential Tuning of Nanoarchitectures Based on Phthalocyanine Nanopillars: Construction of Effective Photocurrent Generation Systems.

Nanopillars composed of a photoresponsive phthalocyanine derivative have been conveniently fabricated using a continuous silane coupling reaction on a substrate. The chemical potentials of phthalocyanine nanopillars (PNs) are precisely controlled by changing the number of phthalocyanine derivatives on the substrate. In addition, photocurrent generation efficiencies have been strongly influenced by the number of phthalocyanine derivatives. High photocurrent conversion cells in a solid state have been obtained by the combination of PNs and a fullerene derivative.

Conductive probe AFM measurements of conjugated molecular wires.

The electrical conduction of self-assembled monolayers (SAMs) made from conjugated molecules was measured using conductive probe atomic force microscopy (CP-AFM), with a focus on the molecular structural effect on conduction. First, the electrical conduction of SAMs made from phenylene oligomer SAMs was measured. The resistances through the monolayers increased exponentially with an increase in molecular length and the decay constants of transconductance beta were about 0.45 to 0.61 A(-1) measured at lower bias region. We further investigated the influence of applied load on the resistances. The resistances through terphenyl SAMs increased with an increase in the applied load up to 14 nN. Second, using an insertion technique into insulating alkanethiol SAMs, the electrical conduction of single conjugated terphenyl methanethiol and oligo(para-phenylenevinylene) (OPV) molecules embedded into insulating alkanethiol SAMs were measured. Electrical currents through these single molecules of OPVs were estimated to be larger than those through single terphenyl molecules, suggesting that the OPV structure can increase the electrical conduction of single molecules. Third, apparent negative differential resistance (NDR) was observed at higher bias measurements of SAMs. The appearance of NDR might be related to roughness of SAM surface, because apparent NDR was often observed on rough surfaces. In any case, the tip-molecule contact condition strongly affected carrier transport through metal tip/SAM/metal junction.

STM-based molecular detection of "catch-and-release" of protons for bipyridine bound to phenylene-ethynylene thiol.

The protonation/deprotonation response of a novel bipyridine containing (phenylene-ethynylene) thiol adsorbed to a Au surface was investigated with scanning tunneling microscopy (STM), showing reversible changes in the average heights (approximately 50 spots) and the height distribution arising from protonation/deprotonation.

Experimental Studies on the Anisotropic Thermoelectric Properties of Conducting Polymer Films.

We reported general methods for studying the thermoelectric properties of a polymer film in both the in-plane and through-plane directions. The bench-mark PEDOT/PSS films have highly anisotropic carrier transport properties and thermal conductivity. The anisotropic carrier transport properties can be explained by the lamellar structure of the PEDOT/PSS films where the PEDOT nanocrystals could be isolated by the insulating PSS in the through-plane direction. The anisotropic thermal conductivity was mainly attributed to the lattice contribution from PSS because the polymer chain is oriented along the substrate.

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.

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.

Molecular nanostamp based on one-dimensional porphyrin polymers.

Surface design with unique functional molecules by a convenient one-pot treatment is an attractive project for the creation of smart molecular devices. We have employed a silane coupling reaction of porphyrin derivatives that form one-dimensional polymer wires on substrates. Our simple one-pot treatment of a substrate with porphyrin has successfully achieved the construction of nanoscale bamboo shoot structures. The nanoscale bamboo shoots on the substrates were characterized by atomic force microscopy (AFM), UV-vis spectra, and X-ray diffraction (XRD) measurements. The uneven and rigid nanoscale structure has been used as a stamp for constructing bamboo shoot structures of fullerene.

Long-range electron transport of ruthenium-centered multilayer films via a stepping-stone mechanism.

We studied electron transport of Ru complex multilayer films, whose structure resembles redox-active complex films known in the literature to have long-range electron transport abilities. Hydrogen bond formation in terms of pH control was used to induce spontaneous growth of a Ru complex multilayer. We made a cross-check between electrochemical measurements and I-V measurements using PEDOT:PSS to eliminate the risk of pinhole contributions to the mechanism and have found small ß values of 0.012-0.021 Å(-1). Our Ru complex layers exhibit long-range electron transport but with low conductance. On the basis of the results of our theoretical-experimental collaboration, we propose a modified tunneling mechanism named the "stepping-stone mechanism", where the alignment of site potentials forms a narrow band around E(F), making resonant tunneling possible. Our observations may support Tuccito et al.'s proposed mechanism.