Crown ether salt-doped ladder-type conducting polymers for air-stable n-type thermoelectric materials.

Thermoelectric energy harvesters based on p- and n-type organic semiconductors are in high demand, while the air stability of n-type devices has long been a challenge. Here, we demonstrate that supramolecular salt-functionalized n-doped ladder-type conducting polymers exhibit excellent stability in the presence of dry air.

Outstanding Robust Photo- and Thermo-Electric Applications with Stabilized n-Doped Carbon Nanotubes by Parylene Coating.

Stabilization techniques for n-doped carbon nanotubes (CNTs) are essential for the practical use of CNT devices. However, none of the reported n-dopants have sufficient robustness in a practical environment. Herein, we report a highly stable technique for fabricating n-doped CNT films. We elucidate the mechanism by which air stability can be achieved by completely covering CNTs with n-dopants to prevent oxidation; consequently, the stability is lost when exposed to scratches or moisture. Therefore, we introduce parylene as a protective layer for n-doped CNTs and achieve air stability for more than 365 d. Moreover, we demonstrate outstanding robust thermo-electric power generation from strong acids, alkalis, and alcohols, which cannot be realized with conventional air-stable n-dopants. The proposed stabilization technique is versatile and can be applied to various n-dopants. Thus, it is expected to be a key technology in the practical application of CNT devices.

Air-stable and efficient electron doping of monolayer MoS2 by salt-crown ether treatment.

To maximize the potential of transition-metal dichalcogenides (TMDCs) in device applications, the development of a sophisticated technique for stable and highly efficient carrier doping is critical. Here, we report the efficient n-type doping of monolayer MoS2 using KOH/benzo-18-crown-6, resulting in a doped TMDC that is air-stable. MoS2 field-effect transistors show an increase in on-current of three orders of magnitude and degenerate the n-type behaviour with high air-stability for ∼1 month as the dopant concentration increases. Transport measurements indicate a high electron density of 3.4 × 1013 cm-2 and metallic-type temperature dependence for highly doped MoS2. First-principles calculations support electron doping via surface charge transfer from the K/benzo-18-crown-6 complex to monolayer MoS2. Patterned doping is demonstrated to improve the contact resistance in MoS2-based devices.

Ionic Dopant-Encapsulating Single-Walled Carbon Nanotube Films with Metal-Like Electrical Conductivity.

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.

Enhanced Thermoelectric Properties of Boron-Substituted Single-Walled Carbon Nanotube Films.

Atomic doping is the most fundamental approach to modulating the transport properties of carbon nanotubes. In this paper, we demonstrate the enhanced thermoelectric properties of boron-substituted single-walled carbon nanotube (B-SWCNT) films. The developed two-step synthesis of large quantities of B-SWCNTs readily enables the measurements of thermoelectricity of bulk B-SWCNT films. Complementary structural characterization implies the unique configuration of boron atoms at the doping sites of SWCNTs, successfully enabling carrier doping to SWCNTs. The developed boron substitution, in combination with chemical doping, is found to substantially improve the thermoelectric properties.

Surfactant-driven Amphoteric Doping of Carbon Nanotubes.

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.

Thickness-dependent thermoelectric power factor of polymer-functionalized semiconducting carbon nanotube thin films.

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.

Crystallinity-Dependent Thermoelectric Properties of a Two-Dimensional Coordination Polymer: Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2.

The evaluation of thermoelectric properties has recently become a standard method for revealing the electronic properties of conducting polymers. Herein we report on the thermoelectric properties of a two-dimensional coordination polymer pellets. The pellets of Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2, which has recently been developed, show n-type thermoelectric transport, dependent on crystallinity. The present results provide systematic feedback to the guideline for high-performance molecular thermoelectric materials.

Solvent basicity promotes the hydride-mediated electron transfer doping of carbon nanotubes.

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.

Synergistic Impacts of Electrolyte Adsorption on the Thermoelectric Properties of Single-Walled Carbon Nanotubes.

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 .

Water-Processable, Air-Stable Organic Nanoparticle-Carbon Nanotube Nanocomposites Exhibiting n-Type Thermoelectric Properties.

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.

Air-tolerant Fabrication and Enhanced Thermoelectric Performance of n-Type Single-walled Carbon Nanotubes Encapsulating 1,1'-Bis(diphenylphosphino)ferrocene.

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) .

Bis(dipyrrinato)metal(ii) coordination polymers: crystallization, exfoliation into single wires, and electric conversion ability.

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.

Dispersion of Synthetic MoS2 Flakes and Their Spontaneous Adsorption on Single-Walled Carbon Nanotubes.

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.

Systematic conversion of single walled carbon nanotubes into n-type thermoelectric materials by molecular dopants.

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.

Dual Transient Bleaching of Au/PbS Hybrid Core/Shell Nanoparticles.

We examined the optical response of hybrid Au/PbS core/shell nanoparticles (NPs) using transient absorption spectroscopy. Finite-difference time-domain (FDTD) calculations and transient absorption measurements show that Au/PbS NPs have unique two extinction peaks: the peak at the longer wavelength (∼700 nm) is originated from the plasmon, and that at the shorter wavelength (550 nm) is from the local maximum of the refractive index of PbS. The transient absorption dynamics of Au/PbS NPs excited at 400 nm have clear oscillation behavior, which is assigned to the breathing mode of whole particle. We observed a weak excitation-wavelength dependence of the plasmon band. The time constant of electron-phonon coupling of Au/PbS NPs was obtained by changing the excitation intensity. We show that spectral properties of Au/PbS NPs are strongly altered by the hybrid formations, while their dynamics differ only minimally compared with those of Au NPs.

Oligomerization of cadmium chalcogenide nanocrystals into CdTe-containing superlattice chains.

Hierarchically-assembled binary cadmium chalcogenide (CdX, X = S, Se, Te) oligomers were engineered. CdTe precursor injections into a solution containing seeded nanocrystals provided longer oligomers possessing alternating and phase-separated CdX and CdTe constituents in the same wire, like an alternating copolymer.

In situ photopolymerization of pyrrole in mesoporous TiO2.

We examined mesoporous TiO(2) as a photosensitizer and template for creating hybrid TiO(2)-polypyrrole materials. Optical excitation of mesoporous TiO(2) was used to generate the electronic potential necessary for the oxidation and polymerization of the pyrrole monomer. The photopolymerization process was monitored by the quartz crystal microbalance, nitrogen sorption, and thermogravimetric techniques. In situ generation of polypyrrole was observed to be self-limiting after approximately 20-30% filling of the mesoporous TiO(2) network. In situ generation of a complementary phase by means of charge transfer from an active host phase represents an alternative means of assembling hybrid inorganic-organic materials with potential applications ranging from electrocatalysis to photovoltaics.