Enhancing Efficiency and Stability of Tin Halide Perovskite Light-Emitting Diodes via Engineered Alkali/Multivalent Metal Salts.

Sn-based perovskite light-emitting diodes (PeLEDs) have emerged as promising alternatives to Pb-based PeLEDs with their rapid increase in performance owing to the various research studies on inhibiting Sn oxidation. However, the absence of defect passivation strategies for Sn-based perovskite LEDs necessitates further research in this field. We performed systematic studies to investigate the design rules for defect passivation agents for Sn-based perovskites by incorporating alkali/multivalent metal salts with various cations and anions. From the computational and experimental analyses, sodium trifluoromethanesulfonate (NaTFMS) was found to be the most effective passivation agent for PEA2SnI4 films among the explored candidate agents owing to favorable reaction energetics to passivate iodide Frenkel defects. Consequently, the incorporation of NaTFMS facilitates the formation of uniform films with relatively large crystals and reduced Sn4+. The NaTFMS-containing PEA2SnI4 PeLEDs demonstrate an improved luminance of 138.9 cd/m2 and external quantum efficiency (EQE) of 0.39% with an improved half-lifetime of more than threefold. This work provides important insight into the design of defect passivation agents for Sn-based perovskites.

Three-Dimensional Touch Device with Two Terminals.

A crossbar array is an essential element that determines the operating position and simplifies the structure of devices. However, in the crossbar array, wiring numerous electrodes to address many positions poses significant challenges. In this study, a method is proposed that utilizes only two electrodes to determine multiple positions. The method significantly simplifies the wiring and device fabrication process. Instead of defining the node location of the crossbar, it is experimentally demonstrated that the x-y-z coordinates can be determined from i) the resistance change as a function of distance, ii) the resistance variation influenced by the electrode composition, and iii) capacitance fluctuation resulting from changes in the dielectric thickness. By employing two-terminal transparent electrodes, a fully functional 3D touch device is successfully fabricated, introducing a groundbreaking approach to simplify input device architectures.

Ultrathin Metal Film on Graphene for Percolation-Threshold-Limited Thermal Emissivity Control.

Translucent Au/graphene hybrid films are shown to be effective in reducing thermal emission from the underlying surfaces when the deposition thickness of Au is close to the percolation threshold. The critical Au deposition thickness for an abrupt change in emissivity is reduced from 15 nm (Si substrate) to a percolation-threshold-limited thickness of 8.5 nm (graphene/Si substrate) because of the chemical inertness of graphene leading to the deposited Au atoms forming a thin, crystalline layer. The effect of the graphene layer on the optical properties of the hybrid film is highlighted by a drastic increase in infrared absorptivity, whereas the visible absorptivity is marginally affected by the presence of a graphene layer. The level of thermal emission from the Au/graphene hybrid films with the percolation-threshold-limited Au thickness is stable even with high background temperatures of up to 300 °C and mechanical strains of ≈4%. As an example of a thermal management application, an anti-counterfeiting device is demonstrated; thermal-camouflage-masked text fabricated with an Au/graphene hybrid film is discernible only using a thermographic camera. Ultrathin metal film assisted by a graphene layer will provide a facile platform for thermal management with semi-transparency, flexibility, and transferability to arbitrary surfaces.

Super-Bright Green Perovskite Light-Emitting Diodes Using Ionic Liquid Additives.

Halide perovskites have potential for use in next-generation low-cost, high-efficiency, and highly color-pure light-emitting diodes (LED) that can be used in various applications, such as flat and flexible displays and solid-state lighting. However, they still lag behind other mature technologies, such as organic LEDs and inorganic LEDs, in terms of performance, particularly brightness. This lag is partly due to the insulating nature of the long-chain organic ligands used to control the perovskite-film morphology. Herein, a 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid (IL) is incorporated as a potential additive with CsPbBr3 perovskite precursors, which results in a super-bright green perovskite light emitting diode (PeLED) achieving a peak luminance of 3.28 × 105  cd m-2 only at a bias voltage of 6 V, with a peak external quantum efficiency of 13.75%. This achievement is the outcome of multirole support from IL that simultaneously enables superior control over the perovskite-film morphology, passivates defects, modifies the band energy levels, and prevents ion migration. Hence, this work demonstrates IL as a novel alternative additive with the potential to outperform conventional long-chain ligands in high-performance PeLED device fabrication.

Sensing with MXenes: Progress and Prospects.

Various fields of study consider MXene a revolutionary 2D material. Particularly in the field of sensors, the metal-like high electrical conductivity and large surface area of MXenes are desirable characteristics as an alternative sensor material that can transcend the boundaries of existing sensor technology. This critical review provides a comprehensive overview of recent advances in MXene-based sensor technology and a roadmap for commercializing MXene-based sensors. The existing sensors are systematically categorized as chemical, biological, and physical sensors. Each category is then classified into various subcategories depending on the electrical, electrochemical, structural, or optical sensing mechanism, which are the four fundamental working mechanisms of sensors. Representative structural and electrical approaches for boosting the performance of each category are presented. Finally, factors that hinder commercializing MXene-based sensors are discussed, and several breakthroughs in realizing commercially available MXene-based sensors are suggested. This review provides broad insights pertaining to previous and existing MXene-based sensor technology and perspectives on the future generation of low-cost, high-performance, and multimodal sensors for soft-electronics applications.

Electroplated Silver-Nickel Core-Shell Nanowire Network Electrodes for Highly Efficient Perovskite Nanoparticle Light-Emitting Diodes.

The low sheet resistance and high optical transparency of silver nanowires (AgNWs) make them a promising candidate for use as the flexible transparent electrode of light-emitting diodes (LEDs). In a perovskite LED (PeLED), however, the AgNW electrode can react with the overlying perovskite material by redox reactions, which limit the electroluminescence efficiency of the PeLED by causing the degradation of and generating defect states in the perovskite material. In this study, we prepared Ag-Ni core-shell NW electrodes using the solution-electroplating technique to realize highly efficient PeLEDs based on colloidal formamidinium lead bromide (FAPbBr3) nanoparticles (NPs). Solvated Ni ions from the NiSO4 source were deposited onto the surface of AgNW networks in three steps: (i) cathodic cleaning, (ii) adsorption of the Ni-ion complex onto the AgNW surface, and (iii) uniform electrodeposition of Ni. An ultrathin (∼3.5 nm) Ni layer was uniformly deposited onto the AgNW surface, which exhibited a sheet resistance of 16.7 Ω/sq and an optical transmittance of 90.2%. The Ag-Ni core-shell NWs not only increased the work function of the AgNW electrode, which facilitated hole injection into the emitting layer, but also suppressed the redox reaction between Ag and FAPbBr3 NPs, which prevented the degradation of the emitting layer and the generation of defect states in it. The resulting PeLEDs based on FAPbBr3 NPs with the Ag-Ni core-shell NWs showed high current efficiency of 44.01 cd/A, power efficiency of 35.45 lm/W, and external quantum efficiency of 9.67%.

Wearable and Semitransparent Pressure-Sensitive Light-Emitting Sensor Based on Electrochemiluminescence.

Tactile sensors are being researched as a key technology for developing an electronic skin and a wearable display, which have recently been attracting much attention. However, to develop a next-generation wearable tactile sensor, it is necessary to implement an interactive display that responds immediately to external stimuli. Herein, a wearable and semitransparent pressure-sensitive light-emitting sensor (PLS) based on electrochemiluminescence (ECL) is successfully implemented with visual alarm functions to prevent damage to the human body from external stimuli. The PLS is fabricated with a very simple structure using the ECL gel as the light-emitting layer and a carbon nanotube embedded polydimethylsiloxane as the electrode. The ECL light-emitting layer using a redox reaction is advantageous for the fabrication of next-generation wearable devices due to the advantages of a simple structure and the use of electrodes without work function limitation. The PLS can display various external stimuli immediately and operate at a high luminance, making it safe to use as a wearable sensor. Therefore, the PLS using ECL can be a simple and meaningful solution for next-generation wearable tactile sensors.

Residual Image Suppression Through Annealing Process of Amorphous Indium Gallium Zinc Oxide Thin Film Transistor for Plastic Organic Light-Emitting Diode Display.

For the evaluation of the residual image suppression, the amorphous indium-gallium-zinc-oxide thin film transistor was manufactured with electric field shield metal on silicon oxide multi-buffer layer, without the need for a silicon crystallization process through the excimer laser process, and is advantageous for the manufacture of large-scale plastic organic light-emitting display. We conducted a study on the propensity to suppress a residual image according to the temperature of the annealing process in amorphous indium gallium zinc oxide. The evaluation divided by the ambient process temperature conditions to measure the change and restoration tendency of the gray current by the black/white current of thin film transistors, and for precise measurement of the current change intervals, the current was analyzed in 0.004 seconds per point. Through the study, residual image of amorphous Indium Gallium Zinc Oxide transistor was found to be suppressed as the temperature of the annealing crystallization increased from 250°C to 325°C, and there was no improvement effect on the 325°C or higher. The trend of threshold voltage shift of thin film transistors according to the two process temperature conditions, 250°C and 325°C, was analyzed by Two sample T analysis method, and the analysis confirmed that the trend of current deterioration is different through p-value 0.007.

Residual Image Reduction Using Electric Field Shield Metal in Plastic Organic Light-Emitting Diode Display.

A plastic organic light-emitting diode display is a device that emits light in an organic layer in proportion to the amount of current applied from a thin film transistor, which constitutes a pixel. However, it was confirmed that the residual image was shown by the operation of the thin film transistor. To suppress residual image, the effect of electric field was studied in operation of a-IGZO thin film transistor. The a-IGZO thin film transistor, in which a polyimide film was used as a substrate, was applied as a driving thin film transistor for pixel circuits in a plastic organic light-emitting diode display, and the effect of the electric field behavior inside the film on residual images was studied. Residual images were strongly connection with the electric field distribution characteristics inside the polyimide substrate, and they were reduced by introducing an electric field shield metal layer in the a-IGZO thin film transistor. The correlation between residual image generation and the operation of the a-IGZO thin film transistor was further explained through technology computer-aided design simulation (Silvaco Group Inc.).

Minimizing Residual Images of Amorphous Indium Gallium Zinc Oxide Thin-Film Transistor-Based Flexible Organic Light-Emitting Diode Displays by Controlling Oxygen Partial Pressure.

Plastic organic light emitting diode displays suffer from residual image, which is closely connected with the hysteresis of the driving thin-film transistor in the pixels. Therefore, in researching paper, we manufactured an OLED display comprise a polyimide substrate and an amorphous indium gallium zinc oxide thin film transistor active layer. Paper proposed a solution for reducing hysteresis through oxygen partial pressure control and evaluated it using hysteresis analysis. The results showed that hysteresis is strongly dependent on the threshold voltage is settled by the oxygen partial pressure while active layer deposition of the TFT. Moreover, hysteresis decreases with increasing temperature.

High-Performance Green Light-Emitting Diodes Based on MAPbBr3 with π-Conjugated Ligand.

The morphology, crystal size, and trap density of perovskite films significantly affect the luminescent properties of perovskite light-emitting diodes (PeLEDs). Recently, numerous studies have been conducted on ligands that surround the surface of perovskite crystals and passivate the trap sites to improve the performance of PeLEDs. In this study, a 4-aminobenzonitrile (ABN) ligand improved the performance of methylammonium lead bromide (MAPbBr3)-based PeLEDs by reducing the MAPbBr3 crystal size to the nanoscale and reducing the trap density. Moreover, the properties of PeLEDs with ABN were further improved using a surface-modified hole-transport layer (HTL) with a hydrophilic polymer. Finally, a bright green PeLED was fabricated, which exhibited the maximum luminance of 3350 cd/m2 with an external quantum efficiency of 8.85%. Therefore, it is believed that the use of proper ligands for the perovskite layer and the optimization of the charge-transport layer have great potential for the development of high-performance PeLEDs.

Near-field sub-diffraction photolithography with an elastomeric photomask.

Photolithography is the prevalent microfabrication technology. It needs to meet resolution and yield demands at a cost that makes it economically viable. However, conventional far-field photolithography has reached the diffraction limit, which imposes complex optics and short-wavelength beam source to achieve high resolution at the expense of cost efficiency. Here, we present a cost-effective near-field optical printing approach that uses metal patterns embedded in a flexible elastomer photomask with mechanical robustness. This technique generates sub-diffraction patterns that are smaller than 1/10th of the wavelength of the incoming light. It can be integrated into existing hardware and standard mercury lamp, and used for a variety of surfaces, such as curved, rough and defect surfaces. This method offers a higher resolution than common light-based printing systems, while enabling parallel-writing. We anticipate that it will be widely used in academic and industrial productions.

Violet Light-Emitting Diodes Based on p-CuI Thin Film/n-MgZnO Quantum Dot Heterojunction.

As the lighting technology evolves, the need for violet light-emitting diodes (LEDs) is growing for high color rendering index lighting. The present technology for violet LEDs is based on the high-cost GaN materials and metal-organic chemical vapor deposition process; therefore, there have recently been intensive studies on developing low-cost alternative materials and processes. In this study, for the first time, we demonstrated violet LEDs based on low-cost materials and processes using a p-CuI thin film/n-MgZnO quantum dot (QD) heterojunction. The p-CuI thin film layer was prepared by an iodination process of Cu films, and the n-MgZnO layer was deposited by spin-coating presynthesized n-MgZnO QDs. To maximize the performance of the violet LED, an optimizing process was performed for each layer of p- and n-type materials. The optimized LED with 1 × 1 mm2-area pixel fabricated using the p-CuI thin film at the iodination temperature of 15 °C and the n-MgZnO QDs at the Mg alloying concentration of 2.7 at. % exhibited the strongest violet emissions at 6 V.

Paper-like flexible optically isotropic liquid crystal film for tunable diffractive devices.

We have demonstrated a paper-like diffractive film in which nano-structured liquid crystal droplets are embedded in elastomeric monomer incorporated polymer matrix by polymerization induced phase-separation. The film with voltage-tunable phase grating exhibits an optically isotropic phase with high transparency and an effective chromatic diffraction for an incident white light with sub-millisecond switching time. In addition, the proposed diffractive film is exhibiting excellent chemical stability against organic and inorganic solvents. In this paper, the diffraction properties of test films depending on incident polarization direction, wavelength, and spatial dispersion are characterized. Easy processing and optically isotropic nature of the film imparts potential applications to flexible electro-optic devices that can be widely implemented in wearable photonics.

Air-like plasmonics with ultralow-refractive-index silica aerogels.

The coupling of the surface plasmon near-field into the sensing medium is key to the sensitivity of surface plasmon-based sensing devices. A low-index dielectric is necessary for the sensing medium to support a highly-penetrating surface plasmon evanescent field that extends well into the dielectric medium. The air-like refractive index, n, of an aerogel substrate provides another dimension for ultralow-index plasmonic devices. In this paper, we experimentally observed an angular surface plasmon resonance dip at 74° with the ultralow-index aerogel substrate, as was expected from theory. We also demonstrated the comparatively high-sensitivity surface plasmon resonance wavelength, λ, while the change in Δλ/Δn with different substrates was studied in detail. A 740 nm-period metal grating was imprinted on aerogel (n = 1.08) and polydimethylsiloxane (PDMS; n = 1.4) substrates. The ultraviolet-visible-near-infrared spectra were observed in the reflection mode on the grating, resulting in sensitivities of 740.2 and 655.9 nm/RIU for the aerogel and PDMS substrates, respectively. Numerical simulations were performed to understand the near-field of the surface plasmon, which demonstrated resonances well correlated with the experimentally observed results. The near-field due to excitation of the surface plasmon polaritons is observed to be more confined and to penetrate deeper into the sensing medium when a low-index substrate is used.

Flexible multi-wavelength photodetector based on porous silicon nanowires.

Porous silicon nanowires (PSi NWs) responding to red, green and blue lights simultaneously are reported in this study. NWs with a diameter of 120 nm were prepared from p-type Si wafer by metal-catalyst-assisted etching. The NWs produced in this manner had an outer porous region consisting of Si nanocrystals small enough to ensure the quantum confinement effect. Hydrazine treatment passivated the porous region by reduction of the surface oxides and the formation of Si-H bonds. The energy band structure of the porous region was a direct transition type, while the emission/absorption energies corresponded to red (1.94 eV), green (2.24 and 2.44 eV) and blue (2.98 eV) light photons. A device structure prepared by suspending 10 PSi NWs between two Al electrodes by dielectrophoresis generated photocurrents upon exposure to red, green and blue lights. The rising and decay times were both 30 µs and the ratio of photocurrent to dark current was 92.0, 59.8 and 24.4 for the blue, green and red lights, respectively. The device could be bent down to the bending radius of 5 mm without any significant deterioration of the photoresponse performances. Further, the device survived repeated bending and unbending over 10 000 cycles with this bending radius, demonstrating its application potential as a flexible device with multiple-wavelength photosensing capabilities.

High-Performance Flexible ZnO Nanorod UV/Gas Dual Sensors Using Ag Nanoparticle Templates.

Flexible zinc oxide (ZnO) nanorod (NR) ultraviolet (UV)/gas dual sensors using silver (Ag) nanoparticle (NP) templates were successfully fabricated on a polyimide substrate with nickel electrodes. Arrays of Ag NPs were used as a template for the growth of ZnO NRs, which could enhance the flexibility and the sensing properties of the devices through the localized surface plasmon resonance (LSPR) effect. The Ag NPs were fabricated by the rapid thermal annealing process of Ag thin films, and ZnO NRs were grown on Ag NPs to maximize the surface area and form networks with rod-to-rod contacts. Because of the LSPR effect by Ag NPs, the UV photoresponse of the ZnO NRs was amplified and the depletion region of ZnO NRs was formed quickly because of the Schottky contact with the Ag NPs. As a consequence, ZnO NR UV/gas dual sensors grown on the Ag NP template with a diameter of 28 nm exhibited the outstanding UV-sensing characteristics with a UV on-off ratio of 3628 and a rising time ( tr) and a decay time ( td) of 3.52 and 0.33 s upon UV exposure, along with excellent NO2-sensing characteristics with a stable gas on-off ratio of 288.5 and a tr and td of 38 and 62 s upon NO2 exposure. Furthermore, the sensors grown on the Ag NP template exhibited good mechanical flexibility and stable sensing properties without significant degradation even after the bending test up to 10 000 cycles at the bending radius of 5 mm.

Effect of ribbon width on electrical transport properties of graphene nanoribbons.

There has been growing interest in developing nanoelectronic devices based on graphene because of its superior electrical properties. In particular, patterning graphene into a nanoribbon can open a bandgap that can be tuned by changing the ribbon width, imparting semiconducting properties. In this study, we report the effect of ribbon width on electrical transport properties of graphene nanoribbons (GNRs). Monolayer graphene sheets and Si nanowires (NWs) were prepared by chemical vapor deposition and a combination of nanosphere lithography and metal-assisted electroless etching from a Si wafer, respectively. Back-gated GNR field-effect transistors were fabricated on a heavily p-doped Si substrate coated with a 300 nm-thick SiO2 layer, by O2 reactive ion etching of graphene sheets using etch masks based on Si NWs aligned on the graphene between the two electrodes by a dielectrophoresis method. This resulted in GNRs with various widths in a highly controllable manner, where the on/off current ratio was inversely proportional to ribbon width. The field-effect mobility decreased with decreasing GNR widths due to carrier scattering at the GNR edges. These results demonstrate the formation of a bandgap in GNRs due to enhanced carrier confinement in the transverse direction and edge effects when the GNR width is reduced.

Effect of Nonionic Surfactant Additive in PEDOT:PSS on PFO Emission Layer in Organic-Inorganic Hybrid Light-Emitting Diode.

Poly(9,9-dioctylfluorene) (PFO) has attracted significant interests owing to its versatility in electronic devices. However, changes in its optical properties caused by its various phases and the formation of oxidation defects limit the application of PFO in light-emitting diodes (LEDs). We investigated the effects of the addition of Triton X-100 (hereinafter shortened as TX) in poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) to induce interlayer diffusion between PEDOT:PSS and PFO to enhance the stability of the PFO phase and suppress its oxidation. Photoluminescence (PL) measurement on PFO/TX-mixed PEDOT:PSS layers revealed that, upon increasing the concentration of TX in the PEDOT:PSS layer, the ß phase of PFO could be suppressed in favor of the glassy phase and the wide PL emission centered at 535 nm caused by ketone defects formed by oxidation was decreased considerably. LEDs were then fabricated using PFO as an emission layer, TX-mixed PEDOT:PSS as hole-transport layer, and zinc oxide (ZnO) nanorods as electron-transport layer. As the TX concentration reached 3 wt %, the devices exhibited dramatic increases in current densities, which were attributed to the enhanced hole injection due to TX addition, along with a shift in the dominant emission wavelength from a green electroluminescence (EL) emission centered at 518 nm to a blue EL emission centered at 448 nm. The addition of TX in PEDOT:PSS induced a better hole injection in the PFO layer, and through interlayer diffusion, stabilized the glassy phase of PFO and limited the formation of oxidation defects.