Controlling Electronic States of Few-walled Carbon Nanotube Yarn via Joule-annealing and p-type Doping Towards Large Thermoelectric Power Factor.

Flexible, light-weight and robust thermoelectric (TE) materials have attracted much attention to convert waste heat from low-grade heat sources, such as human body, to electricity. Carbon nanotube (CNT) yarn is one of the potential TE materials owing to its narrow band-gap energy, high charge carrier mobility, and excellent mechanical property, which is conducive for flexible and wearable devices. Herein, we propose a way to improve the power factor of CNT yarns fabricated from few-walled carbon nanotubes (FWCNTs) by two-step method; Joule-annealing in the vacuum followed by doping with p-type dopants, 2,3,5,6-tetrafluo-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). Numerical calculations and experimental results explain that Joule-annealing and doping modulate the electronic states (Fermi energy level) of FWCNTs, resulting in extremely large thermoelectric power factor of 2250 µW m-1 K-2 at a measurement temperature of 423 K. Joule-annealing removes amorphous carbon on the surface of the CNT yarn, which facilitates doping in the subsequent step, and leads to higher Seebeck coefficient due to the transformation from (semi) metallic to semiconductor behavior. Doping also significantly increases the electrical conductivity due to the effective charge transfers between CNT yarn and F4TCNQ upon the removal of amorphous carbon after Joule-annealing.

Liquid-like dielectric response is an origin of long polaron lifetime exceeding 10 µs in lead bromide perovskites.

Lead halide perovskites are promising materials for optoelectronic applications because of their exceptional performances in carrier lifetime and diffusion length; however, the microscopic origins of their unique characteristics remain elusive. The organic-inorganic hybrid perovskites show unique dielectric functions, i.e., ferroelectric-like phonon responses in the 0.1-10 THz region and liquid-like rotational relaxation in the 1-100 GHz range. To reveal the role of the dielectric responses is of primal importance because the dielectric screening is a key to understanding the optoelectronic properties governed by polarons in the perovskites. Here, we conducted comparative studies of broadband dielectric spectroscopy on both all-inorganic CsPbBr3 and organic-inorganic hybrid (CH3NH3)PbBr3 single crystals to uncover the origin of the liquid-like dielectric relaxation in the 1-100 GHz range. We confirmed the absence of the dielectric response in the range of 106-1010 Hz in CsPbBr3, which was clearly present in the hybrid (CH3NH3)PbBr3. This suggests that the response is almost purely due to the rotational motions of the organic dipoles in the hybrid perovskites. We evaluated the lifetimes of the polarons using surface-free transient photoluminescence. The lifetime in CsPbBr3 was up to 1.6 µs, while the lifetime in (CH3NH3)PbBr3 was 18 µs. The lifetime in the hybrid (CH3NH3)PbBr3 was significantly longer than in CsPbBr3, also confirmed by transient infrared spectroscopy. We concluded that the liquid-like dielectric response inhibits polaron recombination due to the efficient separation of opposite charges by the additional dynamic disorder.

Synthesis of Chiral γ,γ-Disubstituted γ-Butenolides via Direct Vinylogous Aldol Reaction of Substituted Furanone Derivatives with Aldehydes.

An organocatalyzed method for synthesizing chiral γ,γ-disubstituted γ-butenolides via direct vinylogous aldol reactions of γ-substituted ß,γ-butenolides with aldehydes is reported. This reaction is catalyzed by a squaramide-sulfonamide organocatalyst to afford a range of anti-aldol adducts possessing vicinal quaternary and tertiary stereocenters with high to excellent enantioselectivities (reaching 95% ee). This is the first report of a successful stereoselective direct vinylogous aldol reaction of aldehydes with γ-substituted ß,γ-butenolides.

Bond Dissociation Triggering Molecular Disorder in Amorphous H2O.

We developed a system to deposit H2O molecules onto ultrathin silicon nitride substrates in situ using time-resolved transmission electron diffraction apparatus and performed ultrafast time-resolved electron diffraction measurements in the noncrystalline (amorphous) H2O under near-ultraviolet photoexcitation. The observed dynamics directly represent O-H bond dissociation via multiphoton absorption and charge transfer, which trigger ionization and intermolecular disorder in the amorphous H2O. Our results illustrate the intriguing nature of light-matter and matter-matter interactions in H2O molecules.

Squaramide-Sulfonamide Organocatalyst for Asymmetric Direct Vinylogous Aldol Reactions.

Asymmetric direct vinylogous aldol reactions of furan-2(5H)-one with aldehydes in the presence of a catalytic amount of novel squaramide-sulfonamide organocatalyst resulted in the corresponding addition products with high to excellent enantioselectivities. This is the first successful report illustrating an example of highly stereoselective reactions using a squaramide-sulfonamide organocatalyst.