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.

Mechanical Reliability Assessment of a Flexible Package Fabricated Using Laser-Assisted Bonding.

The aim of this study was to develop a flexible package technology using laser-assisted bonding (LAB) technology and an anisotropic solder paste (ASP) material ultimately to reduce the bonding temperature and enhance the flexibility and reliability of flexible devices. The heat transfer phenomena during the LAB process, mechanical deformation, and the flexibility of a flexible package were analyzed by experimental and numerical simulation methods. The flexible package was fabricated with a silicon chip and a polyimide (PI) substrate. When the laser beam was irradiated onto the flexible package, the temperatures of the solder increased very rapidly to 220 °C, high enough to melt the ASP solder, within 2.4 s. After the completion of irradiation, the temperature of the flexible package decreased quickly. It was found that the solder powder in ASP was completely melted and formed stable interconnections between the silicon chip and the copper pads, without thermal damage to the PI substrate. After the LAB process, the flexible package showed warpage of 80 µm, which was very small compared to the size of the flexible package. The stress of each component in the flexible package generated during the LAB process was also found to be very low. The flexible device was bent up to 7 mm without failure, and the flexibility can be improved further by reducing the thickness of the silicon chip. The bonding strength and environmental reliability tests also showed the excellent mechanical endurance of the flexible package.

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.

Thermochemical Mechanism of the Epoxy-Glutamic Acid Reaction with Sn-3.0 Ag-0.5 Cu Solder Powder for Electrical Joining.

An epoxy-based solder paste (ESP) is a promising alternative to conventional solder pastes to improve the reliability of fine-pitch electrical joining because the epoxy encapsulates the solder joint. However, development of an appropriate epoxy formulation and investigation of its reaction mechanism with solder powder is challenging. In this study, we demonstrate a newly designed ESP consisting of diglycidyl ether of bisphenol F (DGEBF) resin, Sn-3.0 Ag-0.5 Cu (SAC305) solder powder, and L-glutamic acid (Glu), which is a proteinogenic amino acid for biosynthesis of proteins in living systems. The mechanism of the thermochemical reaction was explored and tentatively proposed, which reveals that the products of the reaction between SAC305 and Glu function as catalysts for the etherification of epoxides and alcohols produced by chemical bonding between DGEBF and Glu, consequently leading to highly crosslinked polymeric networks and an enhancement of impact resistance. Our findings provide further insight into the mechanism of the reaction between various formulations comprising an epoxy, amino acid, and solder powder, and their potential use as ESPs for electrical joining.

Transparent Urethane-Siloxane Hybrid Materials for Flexible Cover Windows with Ceramic-Like Strength, yet Polymer-Like Modulus.

Any transition toward an era of flexible electronics will have to overcome the mechanical limitations of materials. Specifically, the attainment of both strength and flexibility, which are generally mutually exclusive, is required including glass-like wear resistance, plastic-like compliance, and a high level of strain. Here, we fabricate a urethane-methacrylate-siloxane hybrid (UMSH) material. It is found that UMSH, with molecule-level hybridization of urethane linkage and methacrylate-siloxane conetworks, demonstrates ceramic-like high strength (574 MPa), yet polymer-like low modulus (8.42 GPa), and even high strain (6.3%) at fracture with excellent optical transparency. This combination of high strength, flexibility, and optical transparency indicates that this is a suitable material for glass substitution and can be used as a transparent flexible cover window for foldable display.

High-Performance and Simply-Synthesized Ladder-Like Structured Methacrylate Siloxane Hybrid Material for Flexible Hard Coating.

A high performance ladder-like structured methacrylate siloxane hybrid material (LMSH) was fabricated via simple hydrolytic sol⁻gel reaction, followed by free-radical polymerization. A structurally ordered siloxane backbone, the ladder-like structure, which is an essential factor for high performance, could be achieved by a short period of sol⁻gel reaction in only 4 h. This results in superior optical (Transmittance > 90% at 550 nm), thermal (T5 wt % decomposition > 400 ℃ ), mechanical properties(elastic recovery = 0.86, hardness = 0.6 GPa) compared to the random- and even commercialized cage-structured silsesquioxane, which also has ordered structure. It was investigated that the fabricated ladder-like structured MSH showed the highest overall density of organic/inorganic co-networks that are originated from highly ordered siloxane network, along with high conversion rate of polymerizable methacrylate groups. Our findings suggest a potential of the ladder-like structured MSH as a powerful alternative for the methacrylate polysilsesquioxane, which can be applied to thermally stable and flexible optical coatings, even with an easier and simpler preparation process.

Flexible Hard Coating: Glass-Like Wear Resistant, Yet Plastic-Like Compliant, Transparent Protective Coating for Foldable Displays.

A flexible hard coating for foldable displays is realized by the highly cross-linked siloxane hybrid using structure-property relationships in organic-inorganic hybridization. Glass-like wear resistance, plastic-like flexibility, and highly elastic resilience are demonstrated together with outstanding optical transparency. It provides a framework for the application of siloxane hybrids in protective hard coatings with high scratch resistance and flexibility for foldable displays.

Bioinspired Transparent Laminated Composite Film for Flexible Green Optoelectronics.

Herein, we report a new version of a bioinspired chitin nanofiber (ChNF) transparent laminated composite film (HCLaminate) made of siloxane hybrid materials (hybrimers) reinforced with ChNFs, which mimics the nanofiber-matrix structure of hierarchical biocomposites. Our HCLaminate is produced via vacuum bag compressing and subsequent UV-curing of the matrix resin-impregnated ChNF transparent paper (ChNF paper). It is worthwhile to note that this new type of ChNF-based transparent substrate film retains the strengths of the original ChNF paper and compensates for ChNF paper's drawbacks as a flexible transparent substrate. As a result, compared with high-performance synthetic plastic films, such as poly(ethylene terephthalate), poly(ether sulfone), poly(ethylene naphthalate), and polyimide, our HCLaminate is characterized to exhibit extremely smooth surface topography, outstanding optical clarity, high elastic modulus, high dimensional stability, etc. To prove our HCLaminate as a substrate film, we use it to fabricate flexible perovskite solar cells and a touch-screen panel. As far as we know, this work is the first to demonstrate flexible optoelectronics, such as flexible perovskite solar cells and a touch-screen panel, actually fabricated on a composite film made of ChNF. Given its desirable macroscopic properties, we envision our HCLaminate being utilized as a transparent substrate film for flexible green optoelectronics.