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Heavy doping is inevitable for utilizing single-walled carbon nanotubes for wiring. However, the electrical conductivity of their films is currently as low as one tenth of the films made from typical metal pastes. Herein we report on metal-comparable electrical conductivity from single-walled carbon nanotube network films. We use ionic liquids and crown ether complexes for p-type and n-type doping, respectively. The encapsulation of counterions into carbon nanotubes promotes the conductivities in the range of 7000 S cm-1 , approximately ten times larger than those of undoped films.

This chapter introduces recent studies on thermoelectric materials and devices based on single-walled carbon nanotubes. Due to requirements in energy sources for Internet of things (IoT), thermoelectric properties of single-walled carbon nanotubes have widely been studied for thermal energy harvesting at about room temperature. Although the current thermoelectric figure of merit is still low, experimental designs for the thermoelectric properties enhancement and their theoretical backgrounds have recently been explored. In this chapter, the basic and also unique characteristics of single-walled carbon nanotubes (SWCNTs) as thermoelectric conversion materials and recent research trends are summarized. The essential attractiveness and issues as regards the thermoelectric materials are discussed.

The upcoming IoT society requires portable energy harvesters including thermoelectric generators around room temperature. Here we show a prototype, lightweight thermoelectric generator based on doped single-walled carbon nanotubes. The generator is fabricated by the standard printing and cut-and-paste techniques. The 12 cm-scale generator with a commercial DC-DC converter exhibits thermoelectric outputs high enough to drive small devices such as a light-emitting diode (LED). We believe such demonstration facilitates the studies not only of further improvements in the thermoelectric properties of carbon nanotube materials but also of the novel design for thermoelectric generators on the basis of thermal engineering.

Aqueous surfactant dispersion is the most typical starting step to functionalize materials consisting of carbon nanotubes, but the effects of surfactants on the electronic properties are still unclear. Here we report how the functional groups of surfactants affect the electronic properties of carbon nanotube films. Using spectroscopic and thermoelectric characterization, we demonstrate that anionic and non-ionic surfactants contribute to the formation of p-type and n-type carbon nanotubes, respectively. Additionally, p-type doping with oxygen adsorption is found to compete with surfactants' doping. These findings are useful for designing the srarting carbon nanotube materials exhibiting desirable electronic properties.

The ratio of metallic and semiconducting carbon nanotubes (CNTs) populations is a crucial factor for elucidating their structure-property relationship. Visible-to-near infrared absorption spectroscopy with dispersion is mostly used for the estimation of the metal-to-semiconductor ratio. Due to spectral cofactors including background absorption, however, it is still uneasy to obtain correct population ratios. Herein the detection of pure semiconducting fractions is performed by using the Fourier transform infrared (FTIR) spectroscopy. The working principle is based on the fact that highly pure semiconducting CNTs show no far-infrared (FIR) plasmon resonance while unsorted films containing metallic CNTs have a strong FIR band. This off-on characteristics enables the evaluation of metallic-to-semiconducting CNTs population. Several limitations in this technique are discussed for avoiding significant artifacts. This FTIR-based, solid-state endpoint detection will lead to the straightforward investigation of purity-to-transport properties relationship in solid-state CNT thin films.

In this paper, we demonstrate the electrochemical n-type doping of single-walled carbon nanotubes with supramolecular electrolytes. Quantitative electrochemical charging allows the tunable conversion of p-type SWNT films to their n-type forms. Their conversion and the stability of the products are found to rely on supramolecular cation structures.

We propose a transfer technique of single-wall carbon nanotube (SWCNT) films to be applied to a wide variety of flexible electronic devices. SWCNT films were first heat-treated in advance to remove solvents and dispersants in the films on high heat-resistant Cu substrates and were then poly (methyl methacrylate)-supported transferred onto low heat-resistant polyethylene terephthalate and paper substrates. We characterized the flexibility and electrical properties of the films transferred by this technique. The transferred films showed high flexibility and high electrical conductivity (8.4 × 104 ∼ 1.1 × 105 S/m). X-ray photoelectron spectroscopy measurements revealed that the transferred films were free from Fe residues. Although a slight amount of the Cu residues existed, it was found that the residues did not contributed to the electrical conduction. By using this technique, we enabled to fabricate highly flexible and highly electrical conductive SWCNT films on flexible substrates with low heat resistance, which provide further potential for the application of these films to flexible electronic devices.

The precise control of carbon nanotube structures plays a crucial role in understanding their intrinsic transport as well as in utilizing them for energy harvesting applications. In this paper, we elucidate that slight differences in the purity and diameter distribution of semiconducting single-walled carbon nanotubes (sc-SWCNTs) lead to the significant modulation of thermoelectric transport in their networks. Conducting polymers examined here enable the sorting of the sc-SWCNTs with desired purity and diameter distribution, as well as fixed solid state morphology. Particularly, the approximately tenfold enhancement of thermoelectric power factors is achieved by improving sc-SWCNT purity from 94% to 99% and increasing mean diameters from 1.0 to 1.2 nm. This work provides a rational design for boosting the thermoelectric properties of sc-SWCNT networks.

Simultaneous controlling of the threshold voltage of both p- and n-type transistors, comprising complementary integrated circuits, is required to develop low-voltage and low-power flexible electronics. In this study, we report tuning the threshold voltage of carbon nanotube thin-film transistors with organic and metal-ion complex salts as dopants, and using device passivation to secure air-stability. Chemical doping affords simple yet precise control of the dopant level and enables the threshold voltages to be finely tuned. Complementary inverters were fabricated on a plastic substrate. Operation at a low supply voltage of 0.5 V was achieved with fairly high gain and noise margins.

This study investigates the hydride-mediated electron transfer doping of single-walled carbon nanotubes using absorption spectroscopy and thermoelectric measurements. Specific solvent basicity is found to be important for the efficient n-type doping of carbon nanotubes. This progress is an essential requirement for the future development of electronic and energy devices.

This paper presents a rational method for doping π-electronic materials including carbon nanotube (CNT) films, and its application in the fabrication of thermoelectric power generators. Particularly, n-type doping developed here enables the unprecedented stabilization of n-type CNTs, which is beneficial for the development of tough and flexible thermoelectric generators. At the same time, the present methodology with structure-specific CNTs is used for the investigation of thermoelectric transport in one-dimensional semiconductors

The structure of carbon nanotubes and their electronic interaction with a matrix are important for extracting the unprecedented electronic properties, which have yet to be explored. Here we investigate the dispersibility of single-walled carbon nanotubes (SWNTs) in ionic liquid-derived polymers (PILs), revealed by cross-sectional transmission electron microscopy, infrared optical spectroscopy, and Raman spectroscopy. Surprisingly, SWNTs studied here are highly dispersed, at least down to 7.5 nm-fibres, in a trimethylammonium-suspended PILs. Based on this discovery, we found that the well-dispersed and almost fully dispersed SWNTs in PILs are responsible for the enhanced thermoelectric properties, a future energy harvesting technique.

Noncovalent interactions play a critical, but not yet understood, role in enhancing the thermoelectric properties of carbon nanotube composites. We herein report the dispersion of synthetic MoS2 flakes and their self-assembly with single-walled carbon nanotubes via in-plane and multipoint S-π interactions. The hybridization of MoS2 and carbon nanotubes leads to improved electrical and thermoelectric transport, and resulted in a tenfold enhancement of the thermoelectric figure-of-merit of nanotube-based films. The present technique could contribute to advancing the flexible energy materials that harvest abundant waste heat.

The effects of polymer structures on the thermoelectric properties of polymer-wrapped semiconducting carbon nanotubes have yet to be clarified for elucidating intrinsic transport properties. We systematically investigate thickness dependence of thermoelectric transport in thin films containing networks of conjugated polymer-wrapped semiconducting carbon nanotubes. Well-controlled doping experiments suggest that the doping homogeneity and then in-plane electrical conductivity significantly depend on film thickness and polymer species. This understanding leads to achieving thermoelectric power factors as high as 412 μW m−1 K−2 in thin carbon nanotube films. This work presents a standard platform for investigating the thermoelectric properties of nanotubes., GRAPHICAL ABSTRACT

One-dimensional coordination polymers (1D-CPs) tend either to dissociate into constitutive ligands and metals readily in solution, or to aggregate randomly and amorphously, which prevents them from widespread application. In the present research, 1D-CPs comprising bridging dipyrrin ligands and divalent metal ions (Zn2+, Ni2+, and Cu2+) are synthesized. A liquid/liquid interfacial reaction gives rise to single crystals suitable for X-ray diffraction analysis: A dichloromethane solution of the ligand is layered with aqueous metal(ii) acetate, such that the coordination reaction proceeds at the liquid/liquid interface. Isolated single fibers of the zinc coordination polymer may be exfoliated from the single crystal or bulk solid upon ultrasonication. Atomic force microscopy (AFM) detects the isolated fibers with lengths of more than several μm. The exfoliated 1D-CP wires feature good processability, realizing a conjugate with single-wall carbon nanotubes (SWCNTs), and a thin film on a transparent SnO2 electrode. The processed materials show electric conversion ability: For example, the modified SnO2 electrode serves as a photoanode for a photoelectric conversion system. The designability and tunability of the present 1D-CPs is demonstrated by a ligand modification, affording a luminescent property and an extension of the photoelectric conversion response to longer wavelengths.

We examine the effect of UV/O3 oxidation on the thermoelectric properties of semiconducting carbon nanotube films. The oxidative UV/O3 treatment leads to the introduction of epoxy and carbonyl groups and a significant increase in the thermoelectric power factor up to 140 μW m−1 K−2. This power factor is three times larger than that of chemically-doped films because of the enhanced Seebeck coefficient. Characterization with Raman and mid-IR absorption/extinction spectroscopy reveals that the UV/ozone treatment results in simultaneous charge carrier doping and defect formation. This simple way of enhancing thermoelectric properties is suitable for the production of large-area, flexible thermoelectric devices based on semiconducting carbon nanotubes.

Water-dispersed organic base nanoparticles are utilized for the highly stable n-type doping of single-walled carbon nanotubes in aqueous dispersion. Long-term stability is often a critical challenge in the application of n-type organic conductors. The present n-type organic materials exhibit almost no degradation in the thermoelectric properties over months, in air.

The thermally-triggered n-type doping of single-walled carbon nanotubes is demonstrated using 1,1'-bis(diphenylphosphino)ferrocene, a novel n-type dopant. Through a simple thermal vacuum process, the phosphine compounds are moderately encapsulated inside single-walled carbon nanotubes. The encapsulation into SWNTs is carefully characterized using Raman/X-ray spectroscopy and transmission electron microscopy. This easy-to-handle doping with air-stable precursors for n-type SWNTs enables the large-scale fabrication of thermoelectric materials showing an excellent power factor exceeding approximately 240 μW mK(-2) .

The self-assembly of Prussian blue nanoparticles and single-walled carbon nanotubes is demonstrated by using analytical TEM. The nanotube composites containing compositionally tunable Prussian blue...

We present a rational route to stable carbon nanotubes, where supramolecular chemistry is important for stabilizing ionic bonding between charged nanotubes and counter ions. Developed SWNT materials, which are stable in air, at 100 ?C, could readily be implemented to thermoelectric power generators.

Single-walled carbon nanotubes are promising candidates for light-weight and flexible energy materials. Recently, the thermoelectric properties of single-walled carbon nanotubes have been dramatically improved by ionic liquid addition; however, controlling factors remain unsolved. Here the thermoelectric properties of single-walled carbon nanotubes enhanced by electrolytes are investigated. Complementary characterization with absorption, Raman, and X-ray photoelectron spectroscopy reveals that shallow hole doping plays a partial role in the enhanced electrical conductivity. The molecular factors controlling the thermoelectric properties of carbon nanotubes are systematically investigated in terms of the ionic functionalities of ionic liquids. It is revealed that appropriate ionic liquids show a synergistic enhancement in conductivity and the Seebeck coefficient. The discovery of significantly precise doping enables the generation of thermoelectric power factor exceeding 460 µW m–1 K–2.

Thermoelectrics is a challenging issue for modern and future energy conversion and recovery technology. Carbon nanotubes are promising active thermoelectic materials owing to their narrow bandgap energy and high charge carrier mobility, and they can be integrated into flexible thermoelectrics that can recover any waste heat. We here report air-stable n-type single walled carbon nanotubes with a variety of weak electron donors in the range of HOMO level between ca. -4.4 eV and ca. -5.6 eV, in which partial uphill electron injection from the dopant to the conduction band of single walled carbon nanotubes is dominant. We display flexible films of the doped single walled carbon nanotubes possessing significantly large thermoelectric effect, which is applicable to flexible ambient thermoelectric modules.

Flexible polymer composite materials are advantageous for future thermoelectrics. In this paper we report the controlled dispersion of single-walled carbon nanotubes into an elastic polymer matrix for efficient thermoelectric power generation capability. Optimized rubber composites with a 30 wt % loading level of single-walled carbon nanotubes showed a 15 µW m−1 K−2 thermoelectric power factor, comparable to those of non-elastic carbon nanotube sheets, and larger than those of flexible composite films based on carbon nanotubes.