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To design ultrabright fluorescent solid dyes, a crystal engineering strategy that enables monomeric emission by blocking intermolecular electronic interactions is required. We introduced propylene moieties to distyrylbenzene (DSB) as bridges between the phenyl rings either side of its C=C bonds. The bridged DSB derivatives formed compact crystals that emit colors similar to those of the same molecules in dilute solution, with high quantum yields. The introduction of flexible seven-membered rings to the DSB core produced moderate distortion and steric hindrance in the DSB π-plane. However, owing to this strategy, it was possible to control the molecular arrangement with almost no decrease in the crystal density, and intermolecular electronic interactions were suppressed. The bridged DSB crystal structure differs from other DSB derivative structures; thus, bridging affords access to novel crystalline systems. This design strategy has important implications in many fields and is more effective than the conventional photofunctional molecular crystal design strategies.

In this study, soft X-ray emission spectroscopy of an aqueous colloidal dispersion of multi-walled carbon nanotubes modified via the plasma process in an aqueous solution was performed for investigating the electronic state of water molecules on the colloidal particles. In the aqueous dispersion, reconstruction of the hydrogen-bonded network was implied by the O 1s spectral changes in the 1b1' and 1b1'' peaks. Furthermore, the O 1s spectral intensity around the 3a1 state was enhanced to an unusually broad energy range in comparison with previous studies. This unusual spectral change might be attributed to the hybridization of the electronic states of oxygen-containing functional groups on the surface of the plasma-modified multi-walled carbon nanotubes and that of the surrounding water molecules. Our observation indicates not only reconstruction of the hydrogen-bonded network in the aqueous dispersion but also a significant interaction of the electronic states between the water molecules and the plasma-modified multi-walled carbon nanotubes.

Interfaces or interfacial layers, such as gas-liquid interfaces, are critical for many physical and chemical reactions and are utilized for designing a wide range of materials. In this study, we propose a plasma-assisted freeze templating (PFT) method for materials processing. It uses a new type of interfacial reaction field, i.e., plasma-ice interface. In PFT, a micro- or nanoscale liquid layer formed on the ice body of a frozen aqueous solution is used as a reaction field in which the solutes are highly enriched and the chemical reactions are initiated by reactive species from the plasma. We demonstrated the synthesis of a self-standing gold nanoparticle (AuNP) film of porous structure by PFT in which a helium cryoplasma jet was irradiated onto a frozen solution of auric ions. This PFT method accomplished a surfactant-free and area-selective synthesis of a AuNP film and was unique in comparison with the conventional chemical synthesis of nanostructured gold materials. Furthermore, simple control of the AuNP film was demonstrated by tuning the thickness of the thin liquid layer. This was done by changing the temperature or concentration of the aqueous solution. PFT was demonstrated as a thermodynamically size-tunable scheme for material design; it exploits the plasma-ice interface and is expected to become a novel technique for a wide range of micro- and nanoengineering applications.

We studied the dynamics of solvated electrons in the early stage of plasma generation in water induced with an intense femtosecond laser pulse. According to the decay kinetics of solvated electrons, fast recombination process of solvated electrons (geminate recombination) occurred with a more prolonged lifetime (500 ps to 1 ns) than that observed in previous pulse photolysis studies (10-100 ps). This unusually longer lifetime is attributed to additional production of solvated electrons due to abundant free electrons generated with the laser-induced plasma, implying significant influence of free electrons on the dynamics of solvated electrons., Comment: 24 pages, 5 figures

Reddish coloration and color diversity among icy bodies in the outer solar system are one of the significant clues for understanding the status and history of the solar system. However, the origin of color distribution remains debatable. Here, we demonstrate reddish coloration that is stable only at cryogenic temperatures in a laboratory experiment. The reddish coloration was produced on methanol- and water-containing ice irradiated with nitrogen-containing cryoplasma at 85 K. The reddish color visually faded and disappeared at 120-150 K as the ice was heated, unlike well-known refractory organic tholins that are stable even when heated to room temperature. This temperature dependence of reddish coloration under cryogenic conditions could provide a new possible explanation for the absence of ultra-red coloration closer to the Sun in the outer solar system. Our result implies that a reddish material specific to cryogenic environments is useful for the investigation of color diversity and formation mechanism of the outer solar system., 31 pages, 13 figures, published in ApJL on March 16, 2020

To develop flexible and tough composite materials with out-of-plane aligned thermally conductive pathways, a novel freeze-casting method was developed using dimethyl sulfoxide (DMSO), carbon nanofibers (CFs), slide-ring polymers (SRs), and cross-linkers. Porous composite materials composed of SR and plasma-surface-modified CFs were obtained. Using DMSO, which has a high melting point (19 °C) and high polymer solubility, the SR cross-linking reaction was performed during freeze-casting. The CFs were treated with plasma before composite preparation for better affinity and dispersibility to the composites. The obtained porous composites contained thermally conductive pathways in the out-of-plane direction and exhibited a low Young's modulus (0.14 MPa). The composite with a thermoplastic polyurethane (TPU) elastomer embedded into its pores exhibited a low Young's modulus (3.62 MPa), high toughness (10.0 MJ/m3), and high thermal conductivity in the out-of-plane direction (0.57 W/mK). The proposed freeze-casting method can be applied for the fabrication of various polymers and fillers and is promising for the development of flexible porous composite elastomers with out-of-plane aligned thermally conductive pathways.

Ice-templating is a powerful technique that exploits self-organized ice structures formed during the freezing of colloidal suspensions as templates to fabricate microstructured materials. However, the active introduction of reactivity to the ice-templating process remains limited. In this study, a Au-TiO2 composite film composed of gold and titanium oxide (TiO2) was fabricated on a frozen aqueous colloidal suspension, using cryoplasma to introduce reactive species to the frozen suspension. Auric ions and TiO2 nanoparticles were segregated on the ice body by one-directional freezing of the aqueous colloidal suspension from the lower side. Meanwhile, a cryoplasma, in which the gas temperature was controlled to be lower than the melting point of water, was irradiated onto the frozen aqueous colloidal suspension to facilitate the chemical reduction of gold ions and the formation of gold bridges between TiO2 nanoparticles. The face of the fabricated Au-TiO2 composite film that had been adjacent to the ice structure during its formation was shown to possess both greater roughness and higher photocatalytic activity for methylene blue degradation when the aqueous colloidal suspension was frozen at a higher temperature during the fabrication of the composite film. The temperature dependence of the roughness and photocatalytic activity indicates a potential of the combination of cryoplasma irradiation and ice-templating processes for structure-controlled fabrication of composite film materials, suggesting that alterations in the structure of the freezing ice front that occur with changes in temperature can be harnessed.

In this article, we discuss the electron density and the temperature of plasmas in solution, which produce an attractive reaction field for processing. The measurement of the electron density and the estimation of the electron temperature of the plasma in NaCl solution were performed using spatiotemporally resolved optical emission spectroscopy. The electron densities were estimated to be of the order of 10^16 cm^-3 from the Stark broadening of the Hβ line. The electron temperatures of the plasma were estimated to be in the range of 0.2–1.4 eV on the basis of the electron-neutral bremsstrahlung. The estimated values were confirmed to be consistent with the experimental values of voltage and current density in the assumption of the Boltzmann equation.

We realized a pseudo-microgravity environment for dusty plasmas in a ground-based experiment, using the field-emitting regime of a surface dielectric barrier discharge in high-pressure carbon dioxide (CO2) including supercritical conditions. Using the high and adjustable density of high-pressure CO2, the balance between gravitational force and buoyancy was controlled. When changing the density of CO2 in the range of 0.234 g/cm3 to 0.668 g/cm3, i.e., smaller and larger than that of the particles (0.5 g/cm3), a particle arrangement in the direction of the gravitational force was formed only when the density of CO2 was in the range of ±0.17 g/cm3 with respect to that of the particles. This experimentally demonstrates that the pseudo-microgravity that emerges due to the buoyancy from the high-pressure CO2 contributes to the particle arrangement in the gravitational direction, and hence, it compensates the gravity-induced anisotropy.

We generated H2O-ice dielectric barrier discharge (DBD), which has H2O ice as a dielectric barrier, at a wide range of cryogenic temperatures down to 6.5 K, and controlled plasma gas temperature as a control parameter. H2O-ice DBD exhibited variations in optical emissions and transition of discharge modes with changing plasma gas temperature. Furthermore, H2O-ice DBD is revealed to be anticipated to have sufficient reactivity as a reaction field on the ice surface in comparison with ultraviolet irradiated chemical reactions such as formation of amino acids. This study is the first report on gas temperature-controlled plasma generation coexistent with H2O ice, focusing on plasma gas temperature as a control parameter.