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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.

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–1and 0.4 mW m–1K–2at 400 K, respectively. In addition, the reduced lattice thermal conductivity (κL< 2.5 W m–1K–1at 300 K) of n-type compositions was reported to be much smaller than κLof 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.

Recently, the electrical conductivity of conducting polymers has been significantly improved. Unfortunately, there are nearly no reports on the carrier mobility and carrier density measurements at this stage because traditional measurement approaches are difficult to apply in highly doped disordered systems. In this study, we developed a method to extract the carrier mobility in a highly doped disordered system using a solid state photoinduced charge transfer reaction. Combining the number of the charges transferred from photobase generators to the conducting polymers, the carrier mobility can be extracted from the conductivity change. This approach was successfully applied to conducting polymers with different conductivities. Potentially, this method could also be used to measure the carrier mobility of other disordered systems such as p-type and n-type carbon nanotube films.

Flexible thermoelectric (TE) devices have been of rapidly growing interest for long-lasting and maintenance-free wearable power source that makes use of the temperature difference between human skin and ambient environment. Despite the high TE performance, conventional inorganic TE semiconductors, such as Bi2Te3, skutterudites, are restricted for this application due to their non-flexibility structure and non-scalable manufacturing techniques. In this paper, we report large-area free-standing TE foil with several centimeters in size through a scalable, cost-effective and solution-based approach for flexible thermoelectric devices. The foil is made by self-assembling two-dimensional hybrid superlattices of TiS2layers and hexylamine molecules. Through a Lewis base-acid reaction, electrons are transferred from the hexylamine molecules into the TiS2layers, making the material n-type and generating a large power factor of 0.23mW/mK2, comparable to the best p-type conducting polymers. The flexible foil shows superior bending deformation tolerance, even better than the conducting polymers. A power output of 32µWcm−2for a 15-µm-thick foil were obtained under a temperature difference of 20K at ambient temperature, high enough to drive the wearable nanowatt-to-microwatt level circuit chips or devices.

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.

The conventional thermoelectric figure of merit and power factor is not sufficient as a measure of thin film quality of thermoelectric materials, where the power conversion efficiency depends on the film dimensions. By considering the film size, the effective thermoelectric figure of merit and effective Seebeck coefficient is introduced to guarantee that the maximum energy conversion efficiency increases as the effective thermoelectric figure of merit increases. Similarly, the effective power factor is defined. By introducing typical material properties for Bi2Te3and PEDOT, we study the thickness dependence of the effective figure of merit and the effective power factor.

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.

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.

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.

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–Vmeasurements using PEDOT:PSS to eliminate the risk of pinhole contributions to the mechanism and have found small β 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 EF, making resonant tunneling possible. Our observations may support Tuccito et al.’s proposed mechanism.

A fundamental understanding of the conduction mechanisms in single-wall carbon nanotube (SWCNT) networks is crucial for their use in thin-film transistors and conducting films. However, the uncontrollable mixture state of metallic and semiconducting SWCNTs has always been an obstacle in this regard. In the present study, we revealed that the conduction mechanisms in nanotube networks formed by high-purity metallic and semiconducting SWCNTs are completely different. Quantum transport was observed in macroscopic networks of pure metallic SWCNTs. However, for semiconducting SWCNT networks, Coulomb-gap-type conduction was observed, due to Coulomb interactions between localized electrons. Crossovers among a weakly localized state and strongly localized states with and without Coulomb interactions were observed for transport electrons by varying the relative content of metallic and semiconducting SWCNTs. It was found that hopping barriers, which always exist in normal SWCNT networks and are serious obstacles to achieving high conductivity, were not present in pure metallic SWCNT networks.

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 ? were about 0.45 to 0.61 Å?1measured 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 tipSAMmetal junction.

We investigated an effective pretreatment method for a polycrystalline gold film surface on which an ordered self-assembled monolayer (SAM) could form. The wetting properties of the SAM surface determined by the hysteresis between advancing and receding contact angles against water and hexadecane revealed that the ordered SAM was formed on the gold surface pretreated with a combination of UV/ozone exposure and ethanol rinsing prior to SAM formation. The change in optical properties of the gold film surface was observed by surface plasmon spectroscopy (SPS), indicating that pure ethanol rinsing makes the gold surface smooth by removing small gold grains oxidized by UV/ozone exposure.

We investigate the origin of fabricated nanogap width variations using the electromigration method during metal deposition. This method also facilitates improved control over the nanogap width. A large suppression in the variation is achieved by sample annealing at 373 K during the application of bias voltages for electromigration, which indicates that the variation is caused by structural changes. This electromigration method during metal deposition for the fabrication of an accurate 1-nm gap electrode is useful for single-molecule-sized electronics. Furthermore, it opens the door for future research on integrated sub-1-nm-sized nanogap devices.

The protonationdeprotonation 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 ∼50 spots and the height distribution arising from protonationdeprotonation.

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 (~50 spots) and the height distribution arising from protonation/deprotonation.

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.

We succeeded in observing individual single-walled carbon nanotubes (SWNTs) using an atomic force microscope (AFM) in the tapping mode by paying attention to the preparation of both samples and AFM tips. To disentangle the bundles of SWNTs, we added a small amount of amine into N,N-dimethylformamide. To achieve a high resolution in tapping-mode AFM imaging, we used carbon nanotube (CNT) tips whose radii could be reduced. We were able to image individual SWNTs using CNT tips with widths that were half of those imaged using conventional silicon tips. With this improved resolution, we could clearly resolve the two SWNTs lying parallel on a mica substrate.

We investigated binary-component self-assembled monolayers (SAMs) and fabricated small domains of a conjugated molecule in preassembled n-alkanethiol SAM films using an implantation technique. We measured the height of the domains by scanning tunneling microscopy and found that, when the size is smaller than ?5 nm, the observed height of the conjugated molecular domains depends on their lateral size. This size dependence of the domain height indicates that the vertical conductance of the domains should depend on the domain size (number of molecules). When many molecules are interacting, the molecular orbitals are affected, which should cause changes in the electron tunneling and conductance. The degree of the intermolecular interaction was estimated by introducing an effectiveconductance between the molecules using a classical model similar to a resistor network.

We have applied Be as a p-type dopant for the InP layer grown by metalorganic vapor phase epitaxy. Very good control of Be doping has been obtained with bismethylcyclopentadienyl-beryllium. Carrier concentration as high as 2×1018cm-3has been achieved with excellent surface morphology. The photoluminescence peak intensity of the samples was comparable to that of Zn-doped InP. It has been found that Be concentration drops abruptly at the InGaAsP/InP interface in an undoped InGaAsP/Be-doped InP structure, and there is no significant Be diffusion into undoped InGaAsP. This indicates that Be is the most promising p-type dopant alternative to Zn, which has a large diffusion coefficient in the InP layer, for InGaAsP/InP laser diodes.

Growth kinetics of self-assembled monolayers (SAMs) of n-alkanethiols on gold substrates was studied by X-ray photoelectron spectroscopy (XPS). We compared adsorption kinetics for three alkanethiol homologues: butanethiol (C4SH), dodecanethiol (C12SH), and octadecanethiol (C18SH). Results of quantitative analysis by XPS suggested that the adsorption rate of short alkyl chain thiols is higher than that of long ones on gold substrates at the initial growth stage, in contrast to previous reports. For C4SH SAM formation, we have also confirmed the occurrence of a replacement effect from contamination to SAM on gold, based on our observation of a decrease in the amount of carbon and an increase in the amount of gold where as determined from XPS spectra.

Heat-induced phase separation of fluorocarbon-hydrocarbon self-assembled monolayers (SAMs) on a Au(111) surface was studied by scanning tunneling microscopy (STM), atomic force microscopy (AFM) and friction force microscopy (FFM). Phase-separated domains were depressed by 0.3 nm with a lateral dimension of 20-200 nm. The friction forces on the higher parts were smaller than those on the lower domains. The higher parts were assigned to the hydrocarbon chains, because the fluorocarbon chains should have a larger friction force acting on them than the hydrocarbon chains against the silicon nitride tip. Phase separation suggests cleavaging of the S-S bonds of disulfides in the SAMs. XPS measurements clearly showed an increase in the isolated Au-S binding species, suggesting the existence of the thiolate (monomer).

With a direct force modulation technique we measured the regional stiffness of the self-assembled-(SAM) films. A magnetic thin film was deposited at the end of the backside of the AFM cantilever so as to apply forces directly to the tip through the external magnetic field of an electric coil. This technique is able to detect the difference between the phase-separated monolayer films with two similar stiffnesses composed of hydrocarbon and fluorocarbon and identify the components of the film. We obtained the effective spring constant of 4.8 N/m on hydrocarbon SAM and 6.5 N/m on fluorocarbon SAM at the applied force of 1 nN.

We have studied the growth process of 4-mercaptopyridine (4-PySH) self-assembled monolayers (SAMs) on Au(111) substrate, by scanning tunneling microscopy (STM). The three factors which were investigated for the domain formation of 4-PySH were; solvent effect, immersion time dependence and the impurity effect. STM imaging revealed that the solvent effect was very significant in determining the surface morphology of the 4-PySH SAM growth, which was not observed on X-ray photoelectron spectra (XPS). These results indicated that the solvent was an important factor in the domain formation process, especially ethanol which as a typical SAM solvent, assisted the reconstruction of 4-PySH domains on the substrate.