Glass-Fabric Reinforced Ag Nanowire/Siloxane Composite Heater Substrate: Sub-10 nm Metal@Metal Oxide Nanosheet for Sensitive Flexible Sensing Platform.

The development of flexible chemiresistors is imperative for real-time monitoring of air quality and/or human physical conditions without space constraints. However, critical challenges such as poor sensing characteristics, vulnerability under toxic chemicals, and weak reliability hinder their practical use. In this work, for the first time, an ultrasensitive flexible sensing platform is reported by assembling Pt loaded thin-layered (≈10 nm) SnO2 nanosheets (Pt-SnO2 NSs) based 2D sensing layers on Ag nanowires embedded glass-fabric reinforced vinyl-phenyl siloxane hybrid composite substrate (AgNW-GFRVPH film) as a heater. The thermally stable AgNW-GFRVPH film based heater is fabricated by free radical polymerization of vinyl groups in vinyl-phenyl oligosiloxane and phenyltris(dimethylvinylsiloxy)silane with Ag NW and glass-fabric, showing outstanding heat generation (≈200 °C), high dimensional stability (13 ppm °C-1), and good thermal stability (≈350 °C). The Pt-SnO2 NSs, which are synthesized by calcination of Sn precursor coated graphene oxide (GO) sheets and subsequent Pt functionalization, exhibit high mechanical flexibility and superior response (Rair/Rgas = 4.84) to 1 ppm level dimethyl sulfide. Taking these advantages, GO-templated oxide NSs combined with a highly stable AgNW-GFRVPH film heater exhibits the best dimethyl sulfide sensing performance compared to state-of-the-art flexible chemiresistors, enabling them as a superior flexible gas sensing platform.

Quantum Dot/Siloxane Composite Film Exceptionally Stable against Oxidation under Heat and Moisture.

We report on the fabrication of a siloxane-encapsulated quantum dot (QD) film (QD-silox film), which exhibits stable emission intensity for over 1 month even at elevated temperature and humidity. QD-silox films are solidified via free radical addition reaction between oligosiloxane resin and ligand molecules on QDs. We prepare the QD-oligosiloxane resin by sol-gel condensation reaction of silane precursors with QDs blended in the precursor solution, forgoing ligand-exchange of QDs. The resulting QD-oligosiloxane resin remains optically clear after 40 days of storage, in contrast to other QD-containing resins which turn turbid and ultimately form sediments. QDs also disperse uniformly in the QD-silox film, whose photoluminescence (PL) quantum yield (QY) remains nearly unaltered under harsh conditions; for example, 85 °C/5% relative humidity (RH), 85 °C/85% RH, strongly acidic, and strongly basic environments for 40 days. The QD-silox film appears to remain equally emissive even after being immersed into boiling water (100 °C). Interestingly, the PL QY of the QD-silox film noticeably increases when the film is exposed to a moist environment, which opens a new, facile avenue to curing dimmed QD-containing films. Given its excellent stability, we envision that the QD-silox film is best suited in display applications, particularly as a PL-type down-conversion layer.

Moisture Barrier Composites Made of Non-Oxidized Graphene Flakes.

Graphene flakes (GFs) with minimized defects and oxidation ratios are incorporated into polyethylene (PE) to enhance the moisture barrier. GFs produced involving solvothermal intercalation show extremely low oxidation rates (3.17%), and are noncovalently functionalized in situ, inducing strong hydrophobicity. The fabricated composite possesses the best moisture barrier performance reported for a polymer-graphene composite.

Stretchable and transparent electrodes using hybrid structures of graphene-metal nanotrough networks with high performances and ultimate uniformity.

Transparent electrodes that can maintain their electrical and optical properties stably against large mechanical deformations are essential in numerous applications of flexible and wearable electronics. In this paper, we report a comprehensive analysis of the electrical, optical, and mechanical properties of hybrid nanostructures based on graphene and metal nanotrough networks as stretchable and transparent electrodes. Compared to the single material of graphene or the nanotrough, the formation of this hybrid can improve the uniformity of sheet resistance significantly, that is, a very low sheet resistance (1 Ω/sq) with a standard deviation of less than ±0.1 Ω/sq, high transparency (91% in the visible light regime), and superb stretchability (80% in tensile strain). The successful demonstration of skin-attachable, flexible, and transparent arrays of oxide semiconductor transistors fabricated using hybrid electrodes suggests substantial promise for the next generation of electronic devices.

Fabrication of high-density In(3)Sb(1)Te(2) phase change nanoarray on glass-fabric reinforced flexible substrate.

Mushroom-shaped phase change memory (PCM) consisting of a Cr/In(3)Sb(1)Te(2) (IST)/TiN (bottom electrode) nanoarray was fabricated via block copolymer lithography and single-step dry etching with a gas mixture of Ar/Cl(2). The process was performed on a high performance transparent glass-fabric reinforced composite film (GFR Hybrimer) suitable for use as a novel substrate for flexible devices. The use of GFR Hybrimer with low thermal expansion and flat surfaces enabled successful nanoscale patterning of functional phase change materials on flexible substrates. Block copolymer lithography employing asymmetrical block copolymer blends with hexagonal cylindrical self-assembled morphologies resulted in the creation of hexagonal nanoscale PCM cell arrays with an areal density of approximately 176 Gb/in(2).

Direct stamping of silver nanoparticles toward residue-free thick electrode.

Direct stamping of functional materials has been developed for cost-effective and process-effective manufacturing of nano/micro patterns. However, there remain several challenging issues like the perfect removal of the residual layer and realization of high aspect ratio. We have demonstrated facile fabrication of flexible strain sensors that have microscale thick interdigitated capacitors with no residual layer by a simple direct stamping with silver nanoparticles (AgNPs). Polyurethane (PU) prepolymer was utilized as an adhesive layer to transfer AgNPs more efficiently during the separation step of the flexible stamp from directly stamped AgNPs. Scanning electron microscopy images and energy dispersive x-ray spectroscopy analysis revealed residue-free transfer of microscale thick interdigitated electrodes onto two different flexible substrates (elastomeric and brittle) for the application to highly sensitive strain sensors.

Photopatternable Poly(dimethylsiloxane) (PDMS) for an Intrinsically Stretchable Organic Electrochemical Transistor.

Patterning elastomers is an essential process for the application of elastomers to stretchable bioelectric devices. In general, replication of a mold and laser ablation are used for patterning elastomers. However, these methods are inefficient and time consuming due to complex patterning procedures and a heat-induced curing mechanism. In this work, we developed a photopatternable elastomer called thiol-ene cross-linked poly(dimethylsiloxane) (TC-PDMS). TC-PDMS showed high-resolution patternability (∼100 µm) through a direct patterning process. It also had high stretchability (∼140%) and low Young's modulus (∼2.9 MPa) similar to conventional PDMS. To demonstrate its practicability in stretchable bioelectric devices, TC-PDMS was applied to a passivation layer of an intrinsically stretchable organic electrochemical transistor (OECT), which showed a low leakage current (∼20 µA) and a high transconductance (0.432 mS) at high strain (60%). The stretchable OECT was able to record electrocardiographic (ECG) signals from human skin, and the measured ECG signals exhibited a high signal-to-noise ratio of 12.2 dB.

Stretchable Printed Circuit Board Based on Leak-Free Liquid Metal Interconnection and Local Strain Control.

In order to realize a transition from conventional to stretchable electronics, it is necessary to make a universal stretchable circuit board in which passive/active components can be robustly integrated. We developed a stretchable printed circuit board (s-PCB) platform that enables easy and reliable integration of various electronic components by utilizing a modulus-gradient polymeric substrate, liquid metal amalgam (LMA) circuit traces, and Ag nanowire (AgNW) contact pads. Due to the LMA-AgNW biphasic structure of interconnection, the LMA is hermetically sealed by a homogeneous interface, realizing complete leak-free characteristics. Furthermore, integration reliability is successfully achieved by local strain control of the stretchable substrate with a selective glass fiber reinforcement (GFR). A strain localization derived by GFR makes almost 50,000% of strain difference within the board, and the amount of deformation applied to the constituent elements can be engineered. We finally demonstrated that the proposed integrated platform can be utilized as a universal s-PCB capable of integrating rigid/conventional electronic components and soft material-based functional elements with negligible signal distortion under various mechanical deformations.

Siloxane Hybrid Material-Encapsulated Highly Robust Flexible µLEDs for Biocompatible Lighting Applications.

Flexible micro-light-emitting diodes (f-µLEDs) have been regarded as an attractive light source for the next-generation human-machine interfaces, thanks to their noticeable optoelectronic performances. However, when it comes to their practical utilizations fulfilling industrial standards, there have been unsolved reliability and durability issues of the f-µLEDs, despite previous developments in the high-performance f-µLEDs for various applications. Herein, highly robust flexible µLEDs (f-HµLEDs) with 20 × 20 arrays, which are realized by a siloxane-based organic-inorganic hybrid material (SHM), are reported. The f-HµLEDs are created by combining the f-µLED fabrication process with SHM synthesis procedures (i.e., sol-gel reaction and successive photocuring). The outstanding mechanical, thermal, and environmental stabilities of our f-HµLEDs are confirmed by a host of experimental and theoretical examinations, including a bending fatigue test (105 bending/unbending cycles), a lifetime accelerated stress test (85 °C and 85% relative humidity), and finite element method simulations. Eventually, to demonstrate the potential of our f-HµLEDs for practical applications of flexible displays and/or biomedical devices, their white light emission due to quantum dot-based color conversion of blue light emitted by GaN-based f-HµLEDs is demonstrated, and the biocompatibility of our f-HµLEDs is confirmed via cytotoxicity and cell proliferation tests with muscle, bone, and neuron cell lines. As far as we can tell, this work is the first demonstration of the flexible µLED encapsulation platform based on the SHM, which proved its mechanical, thermal, and environmental stabilities and biocompatibility, enabling us to envisage biomedical and/or flexible display applications using our f-HµLEDs.

Conformable microneedle pH sensors via the integration of two different siloxane polymers for mapping peripheral artery disease.

Flexible microneedles are important tools that allow access to the inside of biological tissue from the outside without surgery. However, it had been hard to realize microneedle sensor arrays on flexible substrates because of the difficulty of attaining a needle with a high Young's modulus for a selected area on a thin or soft substrate. In this work, we developed a microneedle sensor on a hybrid substrate based on high Young's modulus epoxy siloxane for the microneedles and low Young's modulus polydimethylsiloxane for the conformable substrate. Polyaniline was deposited on the microneedle for pH sensing. The mechanical durability of the device was assessed by insertion into pig skin 1000 times. Last, the flexible microneedle pH sensors showed their utility for monitoring pH distribution in rats in a peripheral artery diseases model.

Extremely Stable Luminescent Crosslinked Perovskite Nanoparticles under Harsh Environments over 1.5 Years.

Organic-inorganic hybrid perovskite nanoparticles (NPs) are a very strong candidate emitter that can meet the high luminescence efficiency and high color standard of Rec.2020. However, the instability of perovskite NPs is the most critical unsolved problem that limits their practical application. Here, an extremely stable crosslinked perovskite NP (CPN) is reported that maintains high photoluminescence quantum yield for 1.5 years (>600 d) in air and in harsher liquid environments (e.g., in water, acid, or base solutions, and in various polar solvents), and for more than 100 d under 85 °C and 85% relative humidity without additional encapsulation. Unsaturated hydrocarbons in both the acid and base ligands of NPs are chemically crosslinked with a methacrylate-functionalized matrix, which prevents decomposition of the perovskite crystals. Counterintuitively, water vapor permeating through the crosslinked matrix chemically passivates surface defects in the NPs and reduces nonradiative recombination. Green-emitting and white-emitting flexible large-area displays are demonstrated, which are stable for >400 d in air and in water. The high stability of the CPN in water enables biocompatible cell proliferation which is usually impossible when toxic Pb elements are present. The stable materials design strategies provide a breakthrough toward commercialization of perovskite NPs in displays and bio-related applications.

Low-Temperature and High-Quality Growth of Bi2O2Se Layered Semiconductors via Cracking Metal-Organic Chemical Vapor Deposition.

Ternary metal-oxy-chalcogenides are emerging as next-generation layered semiconductors beyond binary metal-chalcogenides (i.e., MoS2). Among ternary metal-oxy-chalcogenides, especially Bi2O2Se has been demonstrated in field-effect transistors and photodetectors, exhibiting ultrahigh performance with robust air stability. The growth method for Bi2O2Se that has been reported so far is a powder sublimation based chemical vapor deposition. The first step for pursuing the practical application of Bi2O2Se as a semiconductor material is developing a gas-phase growth process. Here, we report a cracking metal-organic chemical vapor deposition (c-MOCVD) for the gas-phase growth of Bi2O2Se. The resulting Bi2O2Se films at very low growth temperature (∼300 °C) show single-crystalline quality. By taking advantage of the gas-phase growth, the precise phase control was demonstrated by modulating the partial pressure of each precursor. In addition, c-MOCVD-grown Bi2O2Se exhibits outstanding electrical and optoelectronic performance at room temperature without passivation, including maximum electron mobility of 127 cm2/(V·s) and photoresponsivity of 45134 A/W.

Synthesis of well dispersed uniform sub-4 nm Y2O3:Eu3+ colloidal nanocrystals.

Well dispersed uniform sub-4 nm Y2O3:Eu colloidal nanocrystals have been synthesized through the non-hydrolytic high-temperature thermal decomposition technique. The as-synthesized nanocrystals can be stably dispersed in nonpolar solvents due to the capping organic ligands on their surface. Compared with bulk materials, the nanocrystals exhibited different luminescence features, including the intensity enhancement of the 5D0 --> 7F4 transition observed in the emission spectrum.

Optically Transparent Multiscale Composite Films for Flexible and Wearable Electronics.

One of the key breakthroughs enabling flexible electronics with novel form factors is the deployment of flexible polymer films in place of brittle glass, which is one of the major structural materials for conventional electronic devices. Flexible electronics requires polymer films with the core properties of glass (i.e., dimensional stability and transparency) while retaining the pliability of the polymer, which, however, is fundamentally intractable due to the mutually exclusive nature of these characteristics. An overview of a transparent fiber-reinforced polymer, which is suggested as a potentially viable structural material for emerging flexible/wearable electronics, is provided. This includes material concept and fabrication and a brief review of recent research progress on its applications over the past decade.

Flexible Transparent Crystalline-ITO/Ag Nanowire Hybrid Electrode with High Stability for Organic Optoelectronics.

Metal nanowires (NWs) are promising transparent conducting electrode (TCE) materials because of their excellent optoelectrical performance, intrinsic mechanical flexibility, and large-scale processability. However, the surface roughness, thermal/chemical instability, and limited electrical conductivity associated with empty spaces between metal NWs are problems that are yet to be solved. Here, we report a highly reliable and robust composite TCE/substrate all-in-one platform that consists of crystalline indium tin oxide (c-ITO) top layer and surface-embedded metal NW (c-ITO/AgNW-GFRH) films for flexible optoelectronics. The c-ITO top layer (thickness: 10-30 nm) greatly improves the electrical performance of a AgNW-based electrode, retaining its transparency even after a high-temperature annealing process at 250 °C because of its thermally stable basal substrate (i.e., AgNW-GFRH). By introducing c-ITO thin film, we achieve an extremely smooth surface (Rrms < 1 nm), excellent optoelectrical performance, superior thermal (> 250 °C)/chemical stability (in sulfur-contained solution), and outstanding mechanical flexibility (bending radius = 1 mm). As a demonstration, we fabricate flexible organic devices (organic photovoltaic and organic light-emitting diode) on c-ITO/AgNW-GFRH films that show device performance comparable to that of references ITO/glass substrates and superior mechanical flexibility. With excellent stability and demonstrations, we expect that the c-ITO/AgNW-GFRHs can be used as flexible TCE/substrate films for future thin-film optoelectronics.

Photo-Patternable Quantum Dots/Siloxane Composite with Long-Term Stability for Quantum Dot Color Filters.

Incorporation of quantum dots (QDs) into color filters (CFs) are desired for less energy loss and wider viewing angle compared to a conventional display. However, aggregation and vulnerability to heat, moisture, and chemicals in the photo-patternable matrix are critical issues of the QD-CFs with high QDs concentration. Herein, we fabricated red (10 wt %) and green (20 wt %) QD-CFs using photolithography of QD/siloxane ink containing secondary thiol monomer. Ligand-exchanged QDs were chemically incorporated in methacrylate oligosiloxane resin. QD/siloxane composite showed superior stability under harsh heat and moisture (85 °C/5% RH and 85 °C/85% RH) conditions and chemicals (EtOH, HCl, and NaOH) compared to conventional QD/PR (commercial negative photoresist). QD-CFs (10 µm thick) effectively converted blue light emitted from LED chip into red and green light, and the obtained white PL through QD-CF showed wide color gamut, which was 108% relative to NTSC. From these advantages, QD/siloxane composite will be beneficial as color-conversion photoresists are to be used as color filters in liquid crystal displays, micro light-emitting diodes, and organic light-emitting diodes.

Self-assembled SiO2 photonic crystal infiltrated by Ormosil:Eu(DBM)(3)phen phosphor and its enhanced photoluminescence.

Luminescence films were prepared by infiltration of the tris(dibenzoylmethane) mono(1, 10-phenanthroline) europium incorporated ormosil into colloidal SiO(2) photonic crystal templates. Because a stopband of the template was not overlapped with the PL excitation and emission bands, the stopband did not suppress the PL intensity. The PL intensity of the infiltrated film into the template was about 13.1 times higher than that of the plane film prepared without the template. Three major terms, which are the mass term, the scattering term, and the crystallinity term, were considered as factors that improve the PL intensity. The relative ratio of the effects of the mass term : the scattering term : the crystallinity term was 2.1 : 2.8 : 2.2.

Optical filters fabricated in hybrimer media with soft lithography.

Fabrication and characterization of guided-mode resonance filters made by soft lithography are presented. As these resonant elements are highly sensitive to parametric variations, it is important to develop methods for their reliable fabrication. Thus, we provide a fabrication process that is consistent and simple, employing an elastomeric mold and a UV-curable organic-inorganic hybrid material. Measured spectra show approximately 81% reflectance and approximately 8% transmittance at a resonance wavelength of 1538 nm. The filter's linewidth is approximately 4.5 nm, and the sideband reflectance is approximately 5%. Experimental and theoretical results are in good agreement.

Two-Step-Enhanced Stability of Quantum Dots via Silica and Siloxane Encapsulation for the Long-Term Operation of Light-Emitting Diodes.

Despite innovative optical properties of quantum dots (QDs) for QDs-converted light-emitting diodes (QD-LEDs), the vulnerability of the QDs, against heat and moisture, has been a critical issue for commercialization and long-term use. To overcome the instabilities, we fabricated a thermally and photostable QDs-embedded silica/siloxane (S-QD/siloxane) film by embedding QDs in silica and siloxane encapsulation through a two-step sol-gel reaction. S-QDs were stably dispersed in the oligo-siloxane resin with even a QD concentration of 5 wt % without aggregation. The two-step physical barriers of silica and siloxane acted to decrease the toxicity of QDs and improve the stability against heat and moisture [85 °C/5% relative humidity (RH), 85 °C/85% RH, and 120 °C/5% RH], light (50 and 100 mA), and chemicals (ethanol, HCl, and NaOH). Our S-QD/siloxane film was applied as a color-conversion material on a blue LED chip without additional solidification and encapsulation processes for red and white QD-LEDs, exhibiting a wider color gamut (107% in CIE 1931) compared to NTSC. These enhancements indicate that our S-QD/siloxane film is a suitable material for long-term operation of QD-enhanced films and QD-LEDs in next-generation displays.

Printing of wirelessly rechargeable solid-state supercapacitors for soft, smart contact lenses with continuous operations.

Recent advances in smart contact lenses are essential to the realization of medical applications and vision imaging for augmented reality through wireless communication systems. However, previous research on smart contact lenses has been driven by a wired system or wireless power transfer with temporal and spatial restrictions, which can limit their continuous use and require energy storage devices. Also, the rigidity, heat, and large sizes of conventional batteries are not suitable for the soft, smart contact lens. Here, we describe a human pilot trial of a soft, smart contact lens with a wirelessly rechargeable, solid-state supercapacitor for continuous operation. After printing the supercapacitor, all device components (antenna, rectifier, and light-emitting diode) are fully integrated with stretchable structures for this soft lens without obstructing vision. The good reliability against thermal and electromagnetic radiations and the results of the in vivo tests provide the substantial promise of future smart contact lenses.