Kyu Hong Kyung

Optical phenomena and antifrosting property on biomimetics slippery fluid-infused antireflective films via layer-by-layer comparison with superhydrophobic and antireflective films

Sophisticated material interfaces generated by natural life forms such as lotus leaves and Nepenthes pitcher plants have exceptional abilities to resolve challenges in wide areas of industry and medicine. The nano- and microstructures inspired by these natural materials can repel various liquids and form self-cleaning coatings. In particular, slippery liquid-infused surfaces are receiving remarkable interest as transparent, nonfouling, and antifrosting synthetic surfaces for solar cells and optical devices. Here we focus on the transparency of lubricant-infused texture on antireflective films fabricated by layer-by-layer self-assembly that decrease light scattering, which is important to maintain device properties. A slippery fluid-infused antireflective film composed of chitin nanofibers less than 50 nm in diameter prevented light scattering at the long-wavelength side by Rayleigh scattering to achieve 97.2% transmittance. Moreover, films composed of the same materials demonstrated three different morphologies: superhydrophilicity with antireflection, superhydrophobicity, and omniphobicity, mimicking the biological structures of moth eyes, lotus leaves, and pitcher plants, respectively. The effect of thermal changes on the ability of each film to prevent frost formation was investigated. The slippery fluid-infused antireflective film showed effective antifrosting behavior.

Biocompatible Slippery Fluid-Infused Films Composed of Chitosan and Alginate via Layer-by-Layer Self-Assembly and Their Antithrombogenicity

Antifouling super-repellent surfaces inspired by Nepenthes, the pitcher plant, were designed and named slippery liquid-infused porous surfaces (SLIPS). These surfaces repel various simple and complex liquids including water and blood by maintaining a low sliding angle. Previous studies have reported the development of fluorinated SLIPS that are not biocompatible. Here, we fabricated fluid-infused films composed of biodegradable materials and a biocompatible lubricant liquid. The film was constructed using a combination of electrostatic interactions between chitosan and alginate and hydrogen-bonding between alginate and polyvinylpyrrolidone (PVPON) via the layer-by-layer self-assembly method. After chitosan and alginate were cross-linked, the PVPON was removed by increasing the pH to generate porosity from the deconstruction of the hydrogen-bonding. The porous underlayer was hydrophobized and covered by biocompatible almond oil. Blood easily flowed over this biodegradable and biocompatible SLIPS without leaving stains on the surface, and the material is environmentally durable, has a high transmittance of about 90%, and is antithrombogenic. The results of this study suggest that this SLIPS may facilitate the creation of nonfouling medical devices through a low-cost, eco-friendly, and simple process.

Controllable Broadband Optical Transparency and Wettability Switching of Temperature-Activated Solid/Liquid-Infused Nanofibrous Membranes

Inspired by biointerfaces, such as the surfaces of lotus leaves and pitcher plants, researchers have developed innovative strategies for controlling surface wettability and transparency. In particular, great success has been achieved in obtaining low adhesion and high transmittance via the introduction of a liquid layer to form liquid-infused surfaces. Furthermore, smart surfaces that can change their surface properties according to external stimuli have recently attracted substantial interest. As some of the best-performing smart surface materials, slippery liquid-infused porous surfaces (SLIPSs), which are super-repellent, demonstrate the successful achievement of switchable adhesion and tunable transparency that can be controlled by a graded mechanical stimulus. However, despite considerable efforts, producing temperature-responsive, super-repellent surfaces at ambient temperature and pressure remains difficult because of the use of nonreactive lubricant oil as a building block in previously investigated repellent surfaces. Therefore, the present study focused on developing multifunctional materials that dynamically adapt to temperature changes. Here, we demonstrate temperature-activated solidifiable/liquid paraffin-infused porous surfaces (TA-SLIPSs) whose transparency and control of water droplet movement at room temperature can be simultaneously controlled. The solidification of the paraffin changes the surface morphology and the size of the light-transmission inhibitor in the lubricant layer; as a result, the control over the droplet movement and the light transmittance at different temperatures is dependent on the solidifiable/liquid paraffin mixing ratio. Further study of such temperature-responsive, multifunctional systems would be valuable for antifouling applications and the development of surfaces with tunable optical transparency for innovative medical applications, intelligent windows, and other devices.

A facile method of synthesizing size-controlled hollow cyanoacrylate nanoparticles for transparent superhydrophobic/oleophobic surfaces

Hollow nanoparticles have broad technological implications in a wide range of applications. Particularly, they have attracted great attention as functional coatings in applications such as optical devices, which have an optical transparency derived from a low refractive index. However, creating a facile and versatile method that can accurately control the hollow nanoparticle size has proven extremely challenging. Herein, we report a simple, instantly complete, one-pot method, designated the supersaturated gas-cored instant polymerization (SGCIP) method, to synthesize size-controlled hollow cyanoacrylate nanoparticles (HCNPs). The SGCIP method uses supersaturated gas created by mixing two solvents (water and acetone) and the instant polymerization of cyanoacrylate, whereby it demonstrates facile control of the particle diameters ranging from 13 to 1830 nm reproducibly by simply changing the solvent ratio. Moreover, a unique phase transition (from network to particle formation) is observed during the adjustment of the solvent ratio. As a one-concept application, transparent superhydrophobic/oleophobic coatings are achieved by self-assembly of the HCNPs and silanization. The successful synthesis of such fascinating materials may also provide new insights into the design and development of functional hollow nanoparticles for various applications.

Self-Assembled nano-heterostructural thin film for optical lens

The layer-by-layer (LBL) self-assembly method enables the deposition of functional nanoscale multilayer thin films on intricate-shape substrates. The present problem with optical devices is the formation of a uniform coating of a nanoscale heterostructural thin film on both faces of the lens. In this paper, we introduce self-assembled heterostructural optical films with higher- and lower-refractive index layers, which were fabricated using a stable water base titanium complex [titanium(IV) bis(ammonium lactato) dihydroxide (TALH)] and cationic and anionic polyelectrolytes [poly(diallyldimethylammonium chloride) (PDDA)] and [poly(acrylic acid) (PAA)] on a lens via the LBL method. The antireflectance characteristics of (PDDA/TALH)20/(PDDA/PAA)20 showed a maximum transmittance of 99.2% with (PDDA/TALH)20 (ca. 1.75, 80 nm) and (PDDA/PAA)20 (ca. 1.48, 87 nm) on both faces of the lens. These results demonstrate that we can control the refractive index and film thickness of self-assembled nano-heterostructural films on optical lenses.

Control of Structure and Film Thickness Using Spray Layer-by-Layer Method: Application to Double-Layer Anti-Reflection Film

The recently developed practice of spraying solutions onto a substrate to fabricate thin films via layer-by layer (LBL) method has been further investigated and extended. We successfully fabricated double-layer anti-reflection (AR) thin films with high- and low-refractive-index layers by the spray layer-by-layer (spray-LBL) method. For the deposition of a high-refractive-index layer, layers of poly(diallyldimethylammonium chloride) (PDDA) and titanium(IV) bis(ammoniumlactato) dihydroxide (TALH) were alternatively assembled. The average thickness of (PDDA/TALH) was determined to be 7 nm and the refractive index was n=1.76 at 550 nm. Poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) layers were assembled on the high-refractive-index layer for the deposition of the low-refractive-index layer. The average thickness of (PAH/PAA) was determined to be 14 nm and the refractive index was n=1.48 at 550 nm. This AR thin film showed the maximum transmittance (94.5%) and the minimum reflection (0.5%) at approximately 550 nm in wavelength.

Investigation of TiO2 Thin Film Growth by Layer-by-Layer Self-Assembly for Application to Optical Devices

Recently, optical thin films fabricated using a water-based process have been strongly demanded. We fabricated TiO2 thin films consisting of poly(diallyl dimethyl ammonium chloride) (PDDA) and titanium(IV) bis(ammonium lactate) dihydroxide (TALH) for optical devices fabricated using layer-by-layer self-assembly. We report the effects of the pH and concentration of a solution, immersion time, and the amount of NaCl added to a solution on the thickness, morphology, surface roughness, and transmittance of fabricated thin films. The thickness, surface morphology, and transparency of (PDDA/TALH) thin films were determined by ellipsometry, field-emission scanning microscopy (FE-SEM), atomic force microscopy (AFM), and ultraviolet–visible (UV–vis) spectrometry. It was found that the thickness and surface morphology of (PDDA/TALH) multilayer films can be controlled by adjusting the TiO2 particle size of TALH solution. TiO2 particle size was controlled by adjusting the pH of TALH solution and the concentration of PDDA solution, and by adding NaCl to PDDA solution. It was found that we can increase deposition speed while maintaining optical quality by suppressing the surface roughness within 10 nm. These experimental results showed that the fabrication speed of thin films can be markedly increased, by approximately 6-fold.

Semitransparent polymer-based solar cells via simple wet lamination process with TiO2 layer using automatic spray layer-by-layer method

Semitransparent polymer-based solar cells are renewable, lightweight and low-cost energy sources and are attracting increasing attention in terms of both pure scientific research and commercial applications. The carrier transport layer in most polymer solar cells is composed of a TiO2 layer, and the thickness and homogeneity of this layer strongly influence the cells power conversion efficiency. However, despite considerable development efforts, it is difficult to produce precisely-controlled TiO2 layers at room temperature with low fabrication times and without using materials that are toxic to humans. Here, we demonstrate semitransparent polymer-based solar cells fabricated via a simple wet lamination process with a TiO2 layer produced by an automatic spray layer-by-layer (spray-LBL) self-assembly method for improved solar cells. The spray-LBL method produced nano-ordered precursor films, composed of the non-toxic materials poly(diallyldimethylammonium chloride) (PDDA) and titanium(IV) bis(ammonium lactato) dihydroxide (TALH), from water solutions with high uniformity in a short fabrication time, and these films were then annealed to transform them into TiO2 layers at different temperatures. After TiO2 layer preparation, the polymer solar cells were fabricated via a lamination process. The resulting cells were semitransparent and generated electricity by using the incident light from both the front and back sides. These semitransparent polymer-based solar cells produced power conversion efficiencies of more than 2% by varying the TiO2 layer thickness via control of the spray-LBL process and the annealing temperature.

Layer-by-layer self-assembled thin films of chitin fibers and heparin with anti-thrombus characteristics

Anti-thrombus coatings using natural polymers with high biocompatibility were successfully fabricated by a Layer-by-Layer (LbL) method. The cationic and anionic components were chitin nanofibers and heparin, respectively. LbL multilayers were successfully fabricated by depositing different numbers of bilayers. Analyzing the damping waves of Quartz Crystal Microbalance (QCM) measurements showed that the film hardened as the number of bilayers deposited increased. Mass change measurements via QCM, scanning electron microscopy, atomic force microscopy, ellipsometry, and X-ray photoelectron spectroscopy were also carried out. The density of the chitin nanofiber layers increased as the number of bilayers increased. Simultaneously, the film stability in solution improved. The fabricated 30-bilayer film showed a fibrous structure on its surface and no structural disorder, even after 48 hours immersion in phosphate buffered saline (PBS), so high stability in PBS was demonstrated. The anti-thrombus properties of the film were also investigated by measuring the amount of fibrinogen adsorbed using QCM. This film not only inhibits fibrinogen adsorption, but those fibrinogen which do adsorb detach relatively easily (compared with fibrinogen adsorbed onto an uncoated QCM substrate). Furthermore, while the uncoated glass substrate adsorbed a large quantity of blood on its surface, the 30-bilayer coated glass substrate successfully inhibited blood adsorption and blood solidification.

Nanoscale texture control of polyelectrolyte multilayer using spray layer-by-layer method

Weak polyelectrolyte multilayer thin films deposited by sequential spraying of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) solutions are described. Using the spray layer-by-layer (spray-LBL) method, nanoscale texture structures were fabricated considering several factors such as the concentration of spray solution, spray quantity, and the flow rate of spray solution. It was also found that the formation of nanoscale texture structures was dependent on all three factors. Then, their surface morphologies were characterized. The surface morphologies of the fabricated films were observed by field-emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). The FE-SEM and AFM images showed that using the spray-LBL method, the surface morphology can be controlled with nanometer-order accuracy. As a result, the speed of fabricating thin films by the spray-LBL method was markedly increased compared with that by the dipping LBL method.