Interfacial Triboelectricity Lights Up Phosphor-Polymer Elastic Composites: Unraveling the Mechanism of Mechanoluminescence in Zinc Sulfide Microparticle-Embedded Polydimethylsiloxane Films.

Composites comprising copper-doped zinc sulfide phosphor microparticles embedded in polydimethylsiloxane (ZnS:Cu-PDMS) have received significant attention over the past decade because of their bright and durable mechanoluminescence (ML); however, the underlying mechanism of this unique ML remains unclear. This study reports empirical and theoretical findings that confirm this ML is an electroluminescence (EL) of the ZnS:Cu phosphor induced by the triboelectricity generated at the ZnS:Cu microparticle-PDMS matrix interface. ZnS:Cu microparticles that exhibit bright ML are coated with alumina, an oxide with strong positive triboelectric properties; the contact separation between this oxide coating and PDMS, a polymer with strong negative triboelectric properties, produces sufficient interfacial triboelectricity to induce EL in ZnS:Cu microparticles. The ML of ZnS:Cu-PDMS composites varies on changing the coating material, exhibiting an intensity that is proportional to the amount of interfacial triboelectricity generated in the system. Finally, based on these findings, a mechanism that explains the ML of phosphor-polymer elastic composites (interfacial triboelectric field-driven alternating-current EL model) is proposed in this study. It is believed that understanding this mechanism will enable the development of new materials (beyond ZnS:Cu-PDMS systems) with bright and durable ML.

The Enhanced Hydrogen Storage Capacity of Carbon Fibers: The Effect of Hollow Porous Structure and Surface Modification.

In this study, highly porous carbon fiber was prepared for hydrogen storage. Porous carbon fiber (PCF) and activated porous carbon fiber (APCF) were derived by carbonization and chemical activation after selectively removing polyvinyl alcohol from a bi-component fiber composed of polyvinyl alcohol and polyacrylonitrile (PAN). The chemical activation created more pores on the surface of the PCF, and consequently, highly porous APCF was obtained with an improved BET surface area (3058 m2 g-1) and micropore volume (1.18 cm3 g-1) compare to those of the carbon fiber, which was prepared by calcination of monocomponent PAN. APCF was revealed to be very efficient for hydrogen storage, its hydrogen capacity of 5.14 wt% at 77 K and 10 MPa. Such hydrogen storage capacity is much higher than that of activated carbon fibers reported previously. To further enhance hydrogen storage capacity, catalytic Pd nanoparticles were deposited on the surface of the APCF. The Pd-deposited APCF exhibits a high hydrogen storage capacity of 5.45 wt% at 77 K and 10 MPa. The results demonstrate the potential of Pd-deposited APCF for efficient hydrogen storage.

Highly twisted supercoils for superelastic multi-functional fibres.

Highly deformable and electrically conductive fibres with multiple functionalities may be useful for diverse applications. Here we report on a supercoil structure (i.e. coiling of a coil) of fibres fabricated by inserting a giant twist into spandex-core fibres wrapped in a carbon nanotube sheath. The resulting supercoiled fibres show a highly ordered and compact structure along the fibre direction, which can sustain up to 1,500% elastic deformation. The supercoiled fibre exhibits an increase in resistance of 4.2% for stretching of 1,000% when overcoated by a passivation layer. Moreover, by incorporating pseudocapacitive-active materials, we demonstrate the existence of superelastic supercapacitors with high linear and areal capacitance values of 21.7 mF cm-1 and 92.1 mF cm-2, respectively, that can be reversibly stretched by 1,000% without significant capacitance loss. The supercoiled fibre can also function as an electrothermal artificial muscle, contracting 4.2% (percentage of loaded fibre length) when 0.45 V mm-1 is applied.

Colorimetric hydrogen gas sensor based on PdO/metal oxides hybrid nanoparticles.

We have synthesized new colorimetric hydrogen-sensing materials, PdO/metal oxide hybrid nanoparticles, in which palladium oxide was loaded upon surface of substrate materials via an acid-base reaction between a H2PdCl4 solution and substrate materials, ZnO, MgO, TiO2, and SiO2 respectively at 25 °C. The colorimetric hydrogen gas sensing properties of all the samples, PdO/ZnO, PdO/MgO, PdO/TiO2 and PdO/SiO2, were characterized and compared in order to investigate how hydrogen gas sensitivity would be affected by surface property of substrate materials. It was confirmed that the amount of the loaded PdO, which was thought to be closely related with the colorimetric hydrogen sensitivity, was quite different according to the substrate materials and was increased with increasing of the basicity of substrate materials (ZnO > MgO > TiO2 > SiO2). Consequently, among the PdO/metal oxide hybrid nanoparticles, the largest amount of PdO was observed to be loaded on ZnO substrate nanoparticles due to its highest basicity. The best colorimetric hydrogen gas sensing properties (color difference, ΔE = 71.57) was observed in PdO/ZnO hybrid nanoparticles, showing the most prominent color change from brown to black, when the sample was exposed to hydrogen gas of 4 vol% balanced with nitrogen for 2 min.

Manipulation of Structural Colors in Liquid-Crystal Helical Structures Deformed by Surface Controls.

Structural colors from cholesteric liquid crystals (CLCs) are manipulated by changing the only surface anchoring energy of an alignment layer. This behavior comes from the fact that weak surface energy of the perfluoropolymer induces the tilting of the cholesteric helix. Such deformed CLC structures with durability are successfully demonstrated without any external field applications and additional solidification processes. In addition, electrical tunings of structural colors from the deformed CLCs occur at very low operating voltages, compared to those of conventional CLC structures. On the basis of easy and simple fabrication, high durability, electrical tunability at low operating voltages, and the unique optical characteristics, the new deformed CLC structure could lead to extension in applications of CLCs, including multifunctional sensors, displays, and lasers.

Ultrahigh photosensitivity of the polar surfaces of single crystalline ZnO nanoplates.

Single crystalline ZnO nanoplatelet structures were synthesized via a hydrothermal process on the surface of GaN microparticles. Growth of ZnO seeded on the GaN surface promoted faster growth along the directions within the basal plane of the ZnO crystal structure, resulting in the formation of 2-dimensional nanoplates with a thickness less than a few tens of nanometers at most. Electrical conduction across an individual nanoplate was measured and found to be extremely sensitive to UV illumination and the surrounding atmospheric environment. Such electrical behaviors of the nanoplates were attributed to the dominance of the polar (0001) surfaces and the adsorption and desorption of the ambient gas molecules on these surfaces. Their coupling with conduction electrons near the surface is the critical factor responsible for the highly sensitive electrical properties of the nanoplate. Virtually the entire volume of the nanoplates is under the influence of the surface adsorbed molecules, which changes the electrical properties of the nanoplates extensively, depending on their environmental conditions. Combining the very high photocurrent to dark current ratio and the high effective resistance of the ZnO nanoplates reported in the present study, ultrasensitive photo-devices operating at very low power can be expected with the use of 2-dimensional nanoplates.

Transient SHG Imaging on Ultrafast Carrier Dynamics of MoS2 Nanosheets.

Understanding the collaborative behaviors of the excitons and phonons that result from light-matter interactions is important for interpreting and optimizing the underlying fundamental physics at work in devices made from atomically thin materials. In this study, the generation of exciton-coupled phonon vibration from molybdenum disulfide (MoS2 ) nanosheets in a pre-excitonic resonance condition is reported. A strong rise-to-decay profile for the transient second-harmonic generation (TSHG) of the probe pulse is achieved by applying substantial (20%) beam polarization normal to the nanosheet plane, and tuning the wavelength of the pump beam to the absorption of the A-exciton. The time-dependent TSHG signals clearly exhibit acoustic phonon generation at vibration modes below 10 cm-1 (close to the Γ point) after the photoinduced energy is transferred from exciton to phonon in a nonradiative fashion. Interestingly, by observing the TSHG signal oscillation period from MoS2 samples of varying thicknesses, the speed of the supersonic waves generated in the out-of-plane direction (Mach 8.6) is generated. Additionally, TSHG microscopy reveals critical information about the phase and amplitude of the acoustic phonons from different edge chiralities (armchair and zigzag) of the MoS2 monolayers. This suggests that the technique could be used more broadly to study ultrafast physics and chemistry in low-dimensional materials and their hybrids with ultrahigh fidelity.

Patternable and Widely Colour-Tunable Elastomer-Based Electroluminescent Devices.

We demonstrate wide colour tunability of polydimethylsiloxane-based alternating-current-driven electroluminescent devices with intrinsically stretchable characteristics achieved by simply modulating the electrical frequency. By employing both a screen-printed emitting layer and frequency-dependent colour tuning of ZnS:Cu-based phosphors, we demonstrate various coloured patterned images in a single device. We also show enhanced colour-tuning performance by mixing multi-colour phosphors, which results in a broad range of available coordinates in colour space. We believe that our demonstrated method could be used for manipulating broader colour expression as well as in various applications involving stretchable devices.

Coherent Raman Imaging of Live Muscle Sarcomeres Assisted by SFG Microscopy.

In this study, we used spectrally focused coherent anti-Stokes Raman scattering (spCARS) microscopy assisted by sum-frequency generation (SFG) to monitor the variations in the structural morphology and molecular vibrations of a live muscle of Caenorhabditis elegans. The subunits of the muscle sarcomeres, such as the M-line, myosin, dense body, and α-actinin, were alternatively observed using spCARS microscopy for different sample orientations, with the guidance of a myosin positional marker captured by SFG microscopy. Interestingly enough, the beam polarization dependence of the spCARS contrasts for two parallel subunits (dense body and myosin) showed a ~90° phase difference. The chemically sensitive spCARS spectra induced by the time-varying overlap of two pulses allowed (after a robust subtraction of the non-resonant background using a modified Kramers-Krönig transformation method) high-fidelity detection of various genetically modified muscle sarcomeres tuned to the C-H vibration (2800-3100 cm-1). Conversely, SFG image mapping assisted by phase-retrieved spCARS spectra also facilitated label-free monitoring of the changes in the muscle content of C. elegans that are associated with aging, based on the hypothesis that the C-H vibrational modes could serve as qualitative chemical markers sensitive to the amount and/or structural modulation of the muscle.

Electrically tunable birefringence of a polymer composite with long-range orientational ordering of liquid crystals.

We report an optical film with electrically tunable birefringence in which the liquid crystals (LCs), mixed with the host polymer, form long-range ordering. The film was prepared through polymerization without phase separation between the LCs and polymers. Driving voltage below 30 V for full switching of birefringence is achieved in a 6 µm-thick film. Electro-optical investigations for the film suggest that the long-range ordering of the LCs mixed in the film caused by polymerization lead to rotations of the LCs as well as optical anisotropy in the film. These films with electrically tunable birefringence could have applications as flexible light modulators and phase retardation films for 2D-3D image switching.

Color manipulation of mechanoluminescence from stress-activated composite films.

The prospective application of luminescence to imaging devices is shown using a combination of color-tunable and patternable mechanoluminescent materials. A white light source is demonstrated by using an alternative color tuning method, induced under high vibration conditions. As the implementation is fairly straightforward, it is expected that the present results will find a number of potential uses in current industrial applications.

Electroluminescent devices with function of electro-optic shutter.

The polymer-dispersed liquid crystal (PDLC) was used as a dielectric layer of electroluminescent (EL) device to provide multi-function of electroluminescence and electro-optic shutter. A 50 µm-thick PDLC layer was formed between a transparent electrode and a ZnS:Cu phosphor layer. The electro-optic properties of the EL device were not distorted by the introduction of the PDLC layer. The extraction efficiency of luminescence was improved by more than 14% by PDLC layer. The transmittance of the PDLC was also founded not to be degraded significantly by excitation frequency. Therefore, the electroluminescence of the device was ignited by excitation frequency at a given voltage for full transparency of the PDLC. This device has great potential for applications in transparent displays with the function of a privacy window.

Color-temperature tunable white reflector using bichiral liquid crystal films.

We have demonstrated multicolored reflections showing various whitish colors from a bichiral liquid crystal (LC) film. The bichiral LC film was fabricated by using left-handed and right-handed polymeric cholesteric liquid crystal (CLC) films with two different helical pitches and an isotropic polymer film in between. Color temperatures of the multiple reflections are controlled from ~4000 K to ~10000 K by changing linear polarization directions of normally incident and reflected lights. This characteristic can extend practical applications of CLCs to illuminant devices.

Anchoring transitions of transversely polar liquid-crystal molecules on perfluoropolymer surfaces.

We report a strong discontinuous orientational transition (anchoring transition) of liquid-crystal molecules with a large transverse dipole moment. A perfluoropolymer was used as an alignment layer and the transition was observed from planar to homeotropic with decreasing temperature in the nematic phase. Conversely a gradual variation in tilt angle from homeotropic to conical was observed in a liquid crystal with a comparatively smaller transverse dipole moment on the same alignment layer. The experimental results clearly demonstrate the competition between a short-range dipolar force and long-range van der Waals force at the interfacial region. Using discontinuous anchoring transition in the sample, we demonstrate a possible bistable device for memory and light-driven display.

Fabrication of a simultaneous red-green-blue reflector using single-pitched cholesteric liquid crystals.

A cholesteric liquid crystal (CLC) is a self-assembled photonic crystal formed by rodlike molecules, including chiral molecules, that arrange themselves in a helical fashion. The CLC has a single photonic bandgap and an associated one-colour reflection band for circularly polarized light with the same handedness as the CLC helix (selective reflection). These optical characteristics, particularly the circular polarization of the reflected light, are attractive for applications in reflective colour displays without using a backlight, for use as polarizers or colour filters and for mirrorless lasing. Recently, we showed by numerical simulation that simultaneous multicolour reflection is possible by introducing fibonaccian phase defects. Here, we design and fabricate a CLC system consisting of thin isotropic films and of polymeric CLC films, and demonstrate experimentally simultaneous red, green and blue reflections (multiple photonic bandgaps) using the single-pitched polymeric CLC films. The experimental reflection spectra are well simulated by calculations. The presented system can extend applications of CLCs to a wide-band region and could give rise to new photonic devices, in which white or multicolour light is manipulated.

Effects of the polarizability and packing density of transparent oxide films on water vapor permeation.

The tin oxide and silicon oxide films have been deposited on polycarbonate substrates as gas barrier films, using a thermal evaporation and ion beam assisted deposition process. The oxide films deposited by ion beam assisted deposition show a much lower water vapor transmission rate than those by thermal evaporation. The tin oxide films show a similar water vapor transmission rate to the silicon oxide films in thermal evaporation but a lower water vapor transmission rate in IBAD. These results are related to the fact that the permeation of water vapor with a large dipole moment is affected by the chemistry of oxides and the packing density of the oxide films. The permeation mechanism of water vapor through the oxide films is discussed in terms of the chemical interaction with water vapor and the microstructure of the oxide films. The chemical interaction of water vapor with oxide films has been investigated by the refractive index from ellipsometry and the OH group peak from X-ray photoelectron spectroscopy, and the microstructure of the composite oxide films was characterized using atomic force microscopy and a transmission electron microscope. The activation energy for water vapor permeation through the oxide films has also been measured in relation to the permeation mechanism of water vapor. The diffusivity of water vapor for the tin oxide films has been calculated from the time lag plot, and its implications are discussed.