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Abstract Among the three primary colors, blue emission in organic light-emitting diodes (OLEDs) are highly important but very difficult to develop. OLEDs have already been commercialized; however, blue OLEDs have the problem of requiring a high applied voltage due to the high-energy of blue emission. Herein, an ultralow voltage turn-on at 1.47 V for blue emission with a peak wavelength at 462 nm (2.68 eV) is demonstrated in an OLED device with a typical blue-fluorescent emitter that is widely utilized in a commercial display. This OLED reaches 100 cd/m2, which is equivalent to the luminance of a typical commercial display, at 1.97 V. Blue emission from the OLED is achieved by the selective excitation of the low-energy triplet states at a low applied voltage by using the charge transfer (CT) state as a precursor and triplet-triplet annihilation, which forms one emissive singlet from two triplet excitons.

Abstract Owing to the emerging trend of non‐volatile memory and data‐centric computing, the demand for more functional materials and efficient device architecture at the nanoscale is becoming stringent. To date, 2D ferroelectrics are cultivated as channel materials in field‐effect transistors for their retentive and switchable dipoles and flexibility to be compacted into diverse structures and integration for intensive production. This study demonstrates the in‐plane (IP) ferroelectric memory effect of a 100 nm channel‐length 2D ferroelectric semiconductor α‐In2Se3 stamped onto nanogap electrodes on Si/SiO2 under a lateral electric field. As α‐In2Se3 forms the bottom contact of the nanogap electrodes, a large memory window of 13 V at drain voltage between ±6.5 V and the on/off ratio reaching 103 can be explained by controlled IP polarization. Furthermore, the memory effect is modulated by the bottom gate voltage of the Si substrate due to the intercorrelation between IP and out‐of‐plane (OOP) polarization. The non‐volatile memory characteristics including stable retention lasting 17 h, and endurance over 1200 cycles suggest a wide range of memory applications utilizing the lateral bottom contact structure.

A near-infrared to visible photon upconversion device was previously shown to increase external quantum efficiency by up to 2.3%. Here, the upconversion mechanism is studied by time-resolved spectroscopy, revealing that 67% of incident photons are utilized.

ABSTRACT: Numerical simulations of the liquid flow scattering from rotary atomizers are performed using an incompressible smoothed particle hydrodynamics (SPH) method. The influence of grooves at the edges of the atomizers on the formation of ligaments and droplets is investigated changing the numbers and shapes of the grooves. As a result, it is found that small droplets are likely to be generated when the number of grooves is large and the depth of grooves is deep. It is also found that the grooves work more effectively in bell-cup atomizers than in disk type atomizers. Keywords: Smoothed particle hydrodynamics (SPH), Rotary atomizer, Grooves, Droplet

A numerical simulation is performed to find out a key vortical structure in the laminar-turbulent transition. A low-speed streak is generated inside a laminar boundary layer using an isolated cuboid roughness, aimed at providing an environment unstable to outer disturbances. Then, a short duration jet is issued into the boundary layer. When the jet velocity is low, some vortices appear in the boundary layer, but the transition of the boundary layer does not take place. However, when the jet velocity exceeds a certain threshold, two vortices newly appear above the elongated legs of a V-shaped vortex and only one of them is stretched and survives. After that, vortices are generated one after another around the survived one. By comparing the decayed and the survived vortices, it is found that the difference in their heights is the key characteristic which leads to the transition. Keywords: Turbulent onset, Key vortical structure, Laminar-turbulent transition, Boundary layer, Streaky structures, Short duration jet, Numerical simulation

An experimental study is performed to investigate the flow structures near the edges of rotating disks with different edge shapes. By changing the rotational speed, the Reynolds number is changed from 277 to 356. It is found that the fluid motion near the sharp edge differs depending on whether a rotating shaft is at the disk surface. A flow developing on the flat side of the disk surface changes its direction toward the curved outer surface on the shaft side, and the flow goes straight outward, leaving the disk on the flat side. The existence of the supporting shaft increases the radial velocity of the flow while decreasing the azimuthal velocity. The effects of the edge shape of the disk on the flow fields are also investigated by changing the shape of the disk edges. Rounded and chamfered edges have no noticeable effect on the azimuthal velocity on the curved outer surface, whereas changing the edge shape enhanced the velocity in the disk-thickness direction. In the FFT analyses of the azimuthal velocity measured at the edge of the curved outer surface when the disk edge is rounded, an increase in power across a range of frequencies is observed. Only in the chamfered edge disk case, a peak in the spectrum of the velocity that corresponds to a wavenumber which appears in the transitional boundary layers is observed.

A wind tunnel experiment was performed to investigate the requirements for providing a suitable environment for easily creating a localized turbulent region in a laminar boundary layer. A combination of a short-duration jet and suctions was used to prepare a potentially unstable region upstream, and another jet was ejected downstream against the region at various timings and different relative spanwise locations. Based on the results, the combination of the potentially unstable region and the short-duration jet promoted the transition to turbulence only under limited conditions, whereas in most cases, it worked negatively. However, the turbulent spot generation was enhanced when the downstream jet was used at the timing and location that enlarged the low-velocity area created upstream. Moreover, the locally disturbed region generated by the combination of the potentially unstable region and the short-duration jet did not directly grow into a turbulent spot; rather, the turbulent spot grew in the region following the disturbed region.

In this study, the receptivity of a flat-plate boundary layer was studied by introducing a thin sheet-type disturbance. An airfoil-shaped device was used to generate a thin disturbance without velocity deficit in which a steady jet was ejected from its trailing edge to the downstream. Despite the absence of strong disturbances in the freestream outside the boundary layer, streaky structures similar to an ordinary bypass transition were generated. They meandered slowly in the spanwise direction where their downstream parts were oscillating in a delayed fashion. Turbulent spots were formed in the further downstream region. Consequently, the energy growth of the low frequency band in the velocity fluctuation spectrum was found to originate from this meandering motion of the streaks, whereas the growth of the middle- and high-frequency bands was attributed to the appearances of the turbulent spots.

A wind tunnel experiment was performed to investigate the suitable conditions in which a localized turbulent region can easily be generated in a flat-plate boundary layer. An artificial disturbance of zero-mass flux was introduced upstream using a combination of a short-duration jet and suctions to prepare a potentially unstable environment. The disturbed region by itself decayed downstream. Another jet was then ejected downstream at several different timings and two different spanwise locations relative to the passage of the locally disturbed region to promote transition to turbulence. Although the jet was too weak to trigger the turbulence transition by itself, an isolated turbulent region, the so-called turbulent spot, was generated when ejected against the disturbed region. The optimum conditions were found when the jet was ejected between the high- and low-speed areas of the convecting unstable region.

Behaviors of liquid films scattering from a disk-type or cup-type rotary atomizer are studied using computations based on the three-dimensional Smoothed Particle Hydrodynamics (SPH) method. To reduce computational costs while maintaining a high spatial resolution, the computational domain is limited to a fan-shaped region near the edge of the atomizers using a periodic boundary condition in the circumferential direction. Steady inflow is considered as the inlet condition. In both disk- and cup-type cases, the liquid film leaving the atomizer edge becomes wavy in the circumferential direction and forms elongated ligaments, which break up into droplets. When the atomizer is equipped with grooves on its outer edge, the process of ligament formation and droplet break up is strongly affected by the number and shape of the grooves, particularly for the cup-type atomizer, in which the centrifugal force works more effectively. Two ligament formation patterns are observed: one ligament from each groove and a pairing of ligaments from two neighboring grooves. Droplets of uniform sizes are likely to be generated when the former pattern appears. The results suggest that droplets of uniform sizes can be obtained by choosing the appropriate shape and number of grooves.

Numerical simulations are carried out aimed at finding a key flow structure which leads to a laminar-turbulent transition of a boundary layer with streaky structures. In the preliminary computation, an array of cuboids is used to form streaky structures inside a boundary layer. Then, a disturbance is introduced into the boundary layer by ejecting a short-duration jet from a hole in the wall into a low-speed region in the streaky structures. Although the boundary layer returns to a laminar state when the jet velocity is set to 18% of the uniform flow velocity, it eventually turns into a turbulent state downstream in the 20% case. The differences are investigated in detail in terms of the vortical structures. As a result, only in the stronger jet case, a flat spanwise vortex is generated beside one leg of a hairpin vortex and it merges with the streamwise vortex nearby forming an inclined streamwise vortex. On the other hand, the flat spanwise vortex disappears without being connected to the streamwise vortex in the weaker jet case. The inclined streamwise vortex is stretched by the mean velocity gradient of the boundary layer growing into a strong vortex, and new vortex structures are generated around the inclined one, which leads to turbulence. Therefore, the results suggest that formation of the inclined vortex is the key to transition of the boundary layer.

Active layers for organic photovoltaic devices (OPVs) were prepared by hand drawing with paint marker pens containing solutions of the materials. Although the pen-coated organic films were visually non-uniform with quite high surface roughness, OPV devices using these films exhibited similar or slightly better performances than those using spin-coated films. As such, the pen-coating technique represents an easily accessible, inexpensive, and highly material-efficient method for fabricating OPVs.

Three-dimensional numerical simulation of the flow around a leading edge of a flat plate when vortices with their axes normal to the flat plate surface is carried out to investigate the leading-edge receptivity to the vortical disturbances. It is shown that the vertical vortices outside the boundary layer are not titled and deformed, contradicting to what was reported in previous studies. It is revealed that the streamwise structures, which are dominant in the boundary layer, are formed due to the velocity field induced by the wall-normal vortices outside the boundary layer. It also is found that neighboring pair of vortices side-by-side in the spanwise direction are connected to each other very close to the wall.

The streamwise interfaces of an isolated turbulent region in a flat plate boundary layer are investigated by performing a wind tunnel experiment. A bimorph-type piezoceramic actuator is used to generate a trapezoidal turbulent region whose spanwise width is larger than those of a typical turbulent spot. The experimental results show that the traveling speeds of the leading and trailing ends of the turbulent region are almost same as those of the turbulent spots with an arrow-head shape. This result indicate that the spanwise width have little influence to the streamwise growth of an isolated turbulent region. It is also found that the traveling speed of the trailing end of the turbulent region exceeds that of local flow velocity in the near-wall region, which is η < 1.3, and vice versa. This result shows that a relaminarization is in progress in the near-wall region, where the turbulent fluid is moving across the trailing end turning laminar. The critical wall-normal distance of η = 1.3 corresponds to about 10 in the wall unit (y+), suggesting that the relaminarization process is taking place in the viscous sublayer.

The interface characteristics in a millimeter-scale channel are investigated in order to find a technique to speed up fluid mixing. The channel is fabricated on an acrylic-resin plate, where a cavity is attached in the downstream region of a T-shaped conduit. The mixing effect is evaluated using a blue dye and a colorless liquid, which are alternately injected into the channel by two syringe pumps. The important factors for highly efficient mixing under the combination of alternate inflow and cavity are investigated. Two-dimensional and three-dimensional numerical simulations are also performed, and the results are compared those of the experiment. It is confirmed that the fractal dimension and interface length inside a cavity can be used as indicators to predict the mixing level in the downstream region, although they are not perfect. The importance of focusing on both the stretching rate of the interface and the concentration gradient where the stretching of the interface occurs is presented.

An attempt to control an unsteady flow separation from a flat plate surface using a jet ejection is carried out. The unsteady flow is artificially generated by periodically moving a curved ceiling up and down at a constant frequency. The controlling system consists of a jet ejecting device on the upstream side and tufts attached to the surface on the downstream side. The movements of the tufts are monitored by a USB camera to watch for a flow separation. Once a separation is detected, a jet is ejected into the boundary layer through a slot in the plate. The jet ejection causes the separation region to shift downstream and become smaller. In addition, it is revealed that the control effect can be enhanced by stirring the fluid, generating turbulence, before it is ejected into the boundary layer.

Owing to the emerging trend of non-volatile memory and data-centric computing, the demand for more functional materials and efficient device architecture at the nanoscale is becoming stringent. To date, 2D ferroelectrics are cultivated as channel materials in field-effect transistors for their retentive and switchable dipoles and flexibility to be compacted into diverse structures and integration for intensive production. This study demonstrates the in-plane (IP) ferroelectric memory effect of a 100 nm channel-length 2D ferroelectric semiconductor α-ln2Se3 stamped onto nanogap electrodes on Si/SiO2 under a lateral electric field. As α-ln2Se3 forms the bottom contact of the nanogap electrodes, a large memory window of 13 V at drain voltage between ±6.5 V and the on/off ratio reaching 103 can be explained by controlled IP polarization. Furthermore, the memory effect is modulated by the bottom gate voltage of the Si substrate due to the intercorrelation between IP and out-of-plane (OOP) polarization. The non-volatile memory characteristics including stable retention lasting 17 h, and endurance over 1200 cycles suggest a wide range of memory applications utilizing the lateral bottom contact structure. [ABSTRACT FROM AUTHOR]

The laminar-turbulent transition of a rotating-disk flow dominated by global instability is studied by solving the full Navier-Stokes equations in direct numerical simulations. A flow field in the 2π/32 region is computed using a periodic boundary condition. The flow field is disturbed in two ways. In the first case, a disturbance is introduced at the Reynolds number, Re ≈ 600, while in the second case, a disturbance is introduced at Re ≈ 650. In both cases, wall-normal short-duration suction and blowing are used to disturb the flow field. When a disturbance is added upstream at Re ≈ 600, the wavenumber 64 component becomes dominant when the flow reaches a steady state, whereas when a disturbance is added downstream at Re ≈ 650, the wavenumber 96 component becomes prominent. The transition points are different between the two cases. In addition, in both cases, the distances between neighboring spiral vortices are quite the same when measured at the locations where the turbulence begins. [ABSTRACT FROM AUTHOR]

Recent research on cellular circadian rhythms suggests that the coupling of transcription-translation feedback loops and intracellular redox oscillations is essential for robust circadian timekeeping. For clarification of the molecular mechanism underlying the circadian rhythm, methods that allow for the dynamic and simultaneous detection of transcription/translation and redox oscillations in living cells are needed. Herein, we report that the cyanobacterial circadian redox rhythm can be electrochemically detected based on extracellular electron transfer (EET), a process in which intracellular electrons are exchanged with an extracellular electrode. As the EETbased method is non-destructive, concurrent detection with transcription/translation rhythm using bioluminescent reporter strains becomes possible. An EET pathway that electrochemically connected the intracellular region of cyanobacterial cells with an extracellular electrode was constructed via a newly synthesized electron mediator with cell membrane permeability. In the presence of the mediator, the open circuit potential of the culture medium exhibited temperature-compensated rhythm with approximately 24 h periodicity. Importantly, such circadian rhythm of the open circuit potential was not observed in the absence of the electron mediator, indicating that the EET process conveys the dynamic information regarding the intracellular redox state to the extracellular electrode. These findings represent the first direct demonstration of the intracellular circadian redox rhythm of cyanobacterial cells. [ABSTRACT FROM AUTHOR]

The flapping motion of a permeable square-shaped flag is studied using an incompressible smoothed particle hydrodynamics method together with a newly proposed permeable flag model. Wind tunnel experiments are also performed, and the results are compared. The present model is successful in simulating two characteristic states depending on the uniform velocity, namely the ‘stretched-straight state’ and the ‘flapping state.’ Both the oscillation periods and the tail-end trajectories quantitatively agree with the experiments. However, the bistable region, which appears in the experiments, is not seen in the computational results. Except for the high-permeability case, the flags oscillate two dimensionally and no flow separation is observed irrespective of the permeability conditions. This might be due the flexibility of the flag in which the flag can change its shape in response to the low fluid pressure acting on the surface. A pair of longitudinal vortices rotating in opposite directions is generated at both spanwise ends of the flapping flag. Their strengths vary in accordance with the instantaneous lift force. When the flag is impermeable, these longitudinal vortices are connected to the spanwise vortex shed from the trailing edge and deform into a V-shaped vortex downstream. Conversely, the longitudinal vortices generated from a permeable flag simply move downstream without reconnecting. In addition, it is found that fluid permeation takes place at the location where the work by the fluid on the flag is concentrated. [ABSTRACT FROM AUTHOR]