Kaist

Fabrication of Shape-conformable Batteries Based o..

(from left: Dr. Bok Yeop Ahn, Dr. Chanhoon Kim, Professor Il-Doo Kim and Professor Jennifer A. Lewis)   Flexible, wireless electronic devices are rapidly emerging and have reached the level of commercialization; nevertheless, most of battery shapes are limited to either spherical and/or rectangular structures, which results in inefficient space use. Professor Il-Doo Kim’s team from the Department of Materials Science at KAIST has successfully developed technology to significantly enhance the variability of battery design through collaboration research with Professor Jennifer A. Lewis and her team from the School of Engineering and Applied Sciences at Harvard University.   Most of the battery shapes today are optimized for coin cell and/or pouch cells. Since the battery as an energy storage device occupies most of the space in microelectronic devices with different designs, new technology to freely change the shape of the battery is required.    The KAIST-Harvard research collaboration team has successfully manufactured various kinds of battery shapes, such as ring-type, H, and U shape, using 3D printing technology. And through the research collaboration with Dr. Youngmin Choi at the Korea Research Institute of Chemical Technology (KRICT), 3D-printed batteries were applied to small-scale wearable electronic devices (wearable light sensor rings).    The research group has adopted environmentally friendly aqueous Zn-ion batteries to make customized battery packs. This system, which uses Zn2＋ instead of Li＋ as charge carriers, is much safer compared with the conventional lithium rechargeable batteries that use highly inflammable organic electrolytes. Moreover, the processing conditions of lithium-ion batteries are very complicated because organic solvents can ignite upon exposure to moisture and oxygen.   As the aqueous Zn-ion batteries adopted by the research team are stable upon contact with atmospheric moisture and oxygen, they can be fabricated in the ambient air condition, and have advantages in packaging since packaged plastic does not dissolve in water even when plastic packaging is applied using a 3D printer.    To fabricate a stable cathode that can be modulated in various forms and allows high charge-discharge, the research team fabricated a carbon fiber current collector using electrospinning process and uniformly coated electrochemically active polyaniline conductive polymer on the surface of carbon fiber for a current collector-active layer integrated cathode. The cathode, based on conductive polyaniline consisting of a 3D structure, exhibits very fast charging speeds (50％ of the charge in two minutes) and can be fabricated without the detachment of active cathode materials, so various battery forms with high mechanical stability can be manufactured.    Prof. Kim said, “Zn-ion batteries employing aqueous electrolytes have the advantage of fabrication under ambient conditions, so it is easy to fabricate the customized battery packs using 3D printing.”    “3D-printed batteries can be easily applied for niche applications such as wearable, personalized, miniaturized micro-robots, and implantable medical devices or microelectronic storage devices with unique designs,” added Professor Lewis.   With Dr. Chanhoon Kim in the Department of Materials Science and Engineering at KAIST and Dr. Bok Yeop Ahn School of Engineering and Applied Sciences at Harvard University participating as equally contributing first authors, this work was published in the December issue of ACS Nano.   This work was financially supported by the Global Research Laboratory (NRF-2015K1A1A2029679) and Wearable Platform Materials Technology Center (2016R1A5A1009926).   Figure 1.Fabrication of shape-conformable batteries based on 3D-printing technology and the application of polyaniline carbon nanofiber cathodes and wearable electronic devices     Figure 2.Fabricated shape-conformable batteries based on a 3D-printing method     Meanwhile, Professor Il-Doo Kim was recently appointed as an Associate Editor of ACS Nano, a highly renowned journal in the field of nanoscience.    Professor Kim said, “It is my great honor to be an Associate Editor of the highly renowned journal ACS Nano, which has an impact factor reaching 13.709 with 134,596 citations as of 2017. Through the editorial activities in the fields of energy, I will dedicate myself to improving the prominence of KAIST and expanding the scope of Korea’s science and technology. I will also contribute to carrying out more international collaborations with world-leading research groups.”    (Associate Editor of ACS Nano Professor Il-Doo Kim)

Sound-based Touch Input Technology for Smart Table..

(from left: MS candidate Anish Byanjankar, Research Assistant Professor Hyosu Kim and Professor Insik Shin)   Time passes so quickly, especially in the morning. Your hands are so busy brushing your teeth and checking the weather on your smartphone. You might wish that your mirror could turn into a touch screen and free up your hands. That wish can be achieved very soon. A KAIST team has developed a smartphone-based touch sound localization technology to facilitate ubiquitous interactions, turning objects like furniture and mirrors into touch input tools.   This technology analyzes touch sounds generated from a user’s touch on a surface and identifies the location of the touch input. For instance, users can turn surrounding tables or walls into virtual keyboards and write lengthy e-mails much more conveniently by using only the built-in microphone on their smartphones or tablets. Moreover, family members can enjoy a virtual chessboard or enjoy board games on their dining tables.   Additionally, traditional smart devices such as smart TVs or mirrors, which only provide simple screen display functions, can play a smarter role by adding touch input function support (see the image below).    Figure 1.Examples of using touch input technology: By using only smartphone, you can use surrounding objects as a touch screen anytime and anywhere. The most important aspect of enabling the sound-based touch input method is to identify the location of touch inputs in a precise manner (within about 1cm error). However, it is challenging to meet these requirements, mainly because this technology can be used in diverse and dynamically changing environments. Users may use objects like desks, walls, or mirrors as touch input tools and the surrounding environments (e.g. location of nearby objects or ambient noise level) can be varied. These environmental changes can affect the characteristics of touch sounds.   To address this challenge, Professor Insik Shin from the School of Computing and his team focused on analyzing the fundamental properties of touch sounds, especially how they are transmitted through solid surfaces.   On solid surfaces, sound experiences a dispersion phenomenon that makes different frequency components travel at different speeds. Based on this phenomenon, the team observed that the arrival time difference (TDoA) between frequency components increases in proportion to the sound transmission distance, and this linear relationship is not affected by the variations of surround environments.   Based on these observations, Research Assistant Professor Hyosu Kim proposed a novel sound-based touch input technology that records touch sounds transmitted through solid surfaces, then conducts a simple calibration process to identify the relationship between TDoA and the sound transmission distance, finally achieving accurate touch input localization.    The accuracy of the proposed system was then measured. The average localization error was lower than about 0.4 cm on a 17-inch touch screen. Particularly, it provided a measurement error of less than 1cm, even with a variety of objects such as wooden desks, glass mirrors, and acrylic boards and when the position of nearby objects and noise levels changed dynamically. Experiments with practical users have also shown positive responses to all measurement factors, including user experience and accuracy.    Professor Shin said, “This is novel touch interface technology that allows a touch input system just by installing three to four microphones, so it can easily turn nearby objects into touch screens.”   The proposed system was presented at ACM SenSys, a top-tier conference in the field of mobile computing and sensing, and was selected as a best paper runner-up in November 2018.     (The demonstration video of the sound-based touch input technology)

Crystal size of organic semiconductors can be cont..

(from left: MS cadidate Jeong-Chan Lee, Professor Steve Park and PhD candidate Jin-Oh Kim)   A KAIST research team led by Professor Steve Park from the Department of Materials Science and Engineering   Recently, solution-processable organic semiconductors are being highlighted for their potential application in printed electronics, becoming a feasible technique to fabricate large-area flexible thin film at a low cost. The field-effect mobility of small-molecule organic semiconductors is dependent on the crystallinity, crystal orientation, and crystal size. A variety of solution-based coating techniques, such as ink-jet printing, dip-coating, and solution shearing have been developed to control the crystallinity and crystal orientation, but a method for developing techniques to increase the crystal size of organic semiconductors is still needed.        To overcome this issue, the research team developed an inorganic polymer micropillar-based solution shearing system to increase the crystal size of an organic semiconductor with pillar size. Using this technique, the crystallization process of organic semiconductors can be controlled precisely, and therefore large-area organic semiconductor thin film with controlled crystallinity can be fabricated.      A variety of solution-based coating techniques cannot control the fluid-flow of solutions appropriately, so the solvent evaporates randomly onto the substrate, which has difficulty in the fabrication of organic semiconductor thin film with a large crystal size.   The research team integrated inorganic polymer microstructures into the solution shearing blade to solve this issue. The inorganic polymer can easily be microstructured via conventional molding techniques, has high mechanical durability, and organic solvent resistance. Using the inorganic polymer-based microstructure blade, the research team controlled the size of small molecule organic semiconductors by tuning the shape and dimensions of the microstructure. The microstructures in the blade induce the sharp curvature regions in the meniscus line that formed between the shearing blade and the substrate, and therefore nucleation and crystal growth can be regulated. Hence, the research team fabricated organic semiconductor thin-film with large crystals, which increases the field-effect mobility.        The research team also demonstrated a solution shearing process on a curved surface by using a flexible inorganic polymer-based shearing blade, which expands the applicability of solution shearing.         Professor Park said, “Our new solution shearing system can control the crystallization process precisely during solvent evaporation.” He added, “This technique adds another key parameter that can be utilized to tune the property of thin films and opens up a wide variety of new applications.   The results of this work entitled “Inorganic Polymer Micropillar-Based Solution Shearing of Large-Area Organic Semiconductor Thin Films with Pillar-Size-Dependent Crystal Size” was published in the July 2018 issue of Advanced Materials as a cover article.   Figure 1. Cover article of the July 2018 Issue of Advanced Materials Figure 2. Chemical structure of inorganic polymer (AHPCS) and the fabrication process of a microstructured AHPCS shearing blade.   Figure 3.The increasing trend of organic semiconductor crystal size with increasing the microstructure dimension.

Ultrathin Digital Camera Inspired by Xenos Peckii ..

(Professor Ki-Hun Jeong from the Department of Bio and Brain Engineering)     The visual system of Xenos peckii, an endoparasite of paper wasps, demonstrates distinct benefits for high sensitivity and high resolution, differing from the compound eyes of most insects. Taking their unique features, a KAIST team developed an ultrathin digital camera that emulates the unique eyes of Xenos peckii.      The ultrathin digital camera offers a wide field of view and high resolution in a slimmer body compared to existing imaging systems. It is expected to support various applications, such as monitoring equipment, medical imaging devices, and mobile imaging systems.      Professor Ki-Hun Jeong from the Department of Bio and Brain Engineering and his team are known for mimicking biological visual organs. The team’s past research includes an LED lens based on the abdominal segments of fireflies and biologically inspired anti-reflective structures.      Recently, the demand for ultrathin digital cameras has increased, due to the miniaturization of electronic and optical devices. However, most camera modules use multiple lenses along the optical axis to compensate for optical aberrations, resulting in a larger volume as well as a thicker total track length of digital cameras. Resolution and sensitivity would be compromised if these modules were to be simply reduced in size and thickness.     To address this issue, the team have developed micro-optical components, inspired from the visual system of Xenos peckii, and combined them with a CMOS (complementary metal oxide semiconductor) image sensor to achieve an ultrathin digital camera.    This new camera, measuring less than 2mm in thickness, emulates the eyes of Xenos peckii by using dozens of microprism arrays and microlens arrays. A microprism and microlens pair form a channel and the light-absorbing medium between the channels reduces optical crosstalk. Each channel captures the partial image at slightly different orientation, and the retrieved partial images are combined into a single image, thereby ensuring a wide field of view and high resolution.    Professor Jeong said, “We have proposed a novel method of fabricating an ultrathin camera. As the first insect-inspired, ultrathin camera that integrates a microcamera on a conventional CMOS image sensor array, our study will have a significant impact in optics and related fields.”   This research, led by PhD candidates Dongmin Keum and Kyung-Won Jang, was published in Light: Science & Applications on October 24, 2018.    Figure 1. Natural Xenos peckii eye and the biological inspiration for the ultrathin digital camera (Light: Science & Applications 2018)  Figure 2. Optical images captured by the bioinspired ultrathin digital camera (Light: Science & Applications 2018)

KAIST Develops Technology to Control Near-Field Th..

(from left clockwise: Professor Seung Seob Lee, Professor Bong Jae Lee, PhD Mikyung Lim and PhD candidate Jaeman Song)   A KAIST research team succeeded in measuring and controlling the near-field thermal radiation between metallo-dielectric (MD) multilayer structures.   Their thermal radiation control technology can be applied to next-generation semiconductor packaging, thermophotovoltaic cells and thermal management systems. It also has the potential to be applied to a sustainable energy source for IoT sensors.   In the nanoscale gaps, thermal radiation between objects increases greatly with closer distances. The amount of heat transfer in this scale was found to be from 1,000 to 10,000 times greater than the blackbody radiation heat transfer, which was once considered the theoretical maximum for the rate of thermal radiation. This phenomenon is called near-field thermal radiation. With recent developments in nanotechnology, research into near-field thermal radiation between various materials has been actively carried out.   Surface polariton coupling generated from nanostructures has been of particular interest because it enhances the amount of near-field thermal radiation between two objects, and allows the spectral control of near-field thermal radiation. This advantage has motivated much of the recent theoretical research on the application of near-field thermal radiation using nanostructures, such as thin films, multilayer nanostructures, and nanowires. Nevertheless, thus far, most of the studies have focused on measuring near-field thermal radiation between isotropic materials.    A joint team led by Professor Bong Jae Lee and Professor Seung Seob Lee from the Department of Mechanical Engineering succeeded in measuring near-field thermal radiation according to the vacuum distance between MD multilayer nanostructures by using a custom MEMS (Micro-Electro-Mechanical Systems)-device-integrated platform with three-axis nanopositioner.    MD multilayer nanostructures refer to structures in which metal and dielectric layers with regular thickness alternate. The MD single-layer pair is referred to as a unit cell, and the ratio of the thickness occupied by the metal layer in the unit cell is called the fill factor.    By measuring the near-field thermal radiation with a varying number of unit cells and the fill factor of the multilayer nanostructures, the team demonstrated that the surface plasmon polariton coupling enhances near-field thermal radiation greatly, and allows spectral control over the heat transfer.    Professor B. J. Lee said, “The isotropic materials that have so far been studied experimentally had limited spectral control over the near-field thermal radiation. Our near-field thermal radiation control technology using multilayer nanostructures is expected to become the first step toward developing various near-field thermal radiation applications.”      This research, led by PhD Mikyung Lim and PhD candidate Jaeman Song, was published in Nature Communications on October 16.        Figure 1. Experimental setup for measuring near-field thermal radiation between MD multilayers       Figure 2. Investigation of manipulated near-field heat flux by modifying the surface conditions with MD multilayers

From Concept to Reality： Changing Color of Light U..

(from left: Professor Bumki Min, PhD candidate Jaehyeon Son and PhD Kanghee Lee)   A KAIST team developed an optical technique to change the color (frequency) of light using a spatiotemporal boundary. The research focuses on realizing a spatiotemporal boundary with a much higher degree of freedom than the results of previous studies by fabricating a thin metal structure on a semiconductor surface. Such a spatiotemporal boundary is expected to be applicable to an ultra-thin film type optical device capable of changing the color of light.   The optical frequency conversion device plays a key role in precision measurement and communication technology, and the device has been developed mainly based on optical nonlinearity.    If the intensity of light is very strong, the optical medium responds nonlinearly so the nonlinear optical phenomena, such as frequency doubling or frequency mixing, can be observed. Such optical nonlinear phenomena are realized usually by the interaction between a high-intensity laser and a nonlinear medium.   As an alternative method frequency conversion is observed by temporally modifying the optical properties of the medium through which light travels using an external stimulus. Since frequency conversion in this way can be observed even in weak light, such a technique could be particularly useful in communication technology.   However, rapid optical property modification of the medium by an external stimulus and subsequent light frequency conversion techniques have been researched only in the pertubative regime, and it has been difficult to realize these theoretical results in practical applications.   To realize such a conceptual idea, Professor Bumki Min from the Department of Mechanical Engineering and his team collaborated with Professor Wonju Jeon from the Department of Mechanical Engineering and Professor Fabian Rotermund from the Department of Physics. They developed an artificial optical material (metamaterial) by arranging a metal microstructure that mimics an atomic structure and succeeded in creating a spatiotemporal boundary by changing the optical property of the artificial material abruptly.   While previous studies only slightly modified the refractive index of the medium, this study provided a spatiotemporal boundary as a platform for freely designing and changing the spectral properties of the medium. Using this, the research team developed a device that can control the frequency of light to a large degree.    The research team said a spatiotemporal boundary, which was only conceptually considered in previous research and realized in the pertubative regime, was developed as a step that can be realized and applied.   Professor Min said, “The frequency conversion of light becomes designable and predictable, so our research could be applied in many optical applications. This research will present a new direction for time-variant media research projects in the field of optics.”   This research, led by PhD Kanghee Lee and PhD candidate Jaehyeon Son, was published online in Nature Photonics on October 8, 2018.    This work was supported by the National Research Foundation of Korea (NRF) through the government of Korea. The work was also supported by the Center for Advanced Meta-Materials (CAMM) funded by the Korea Government (MSIP) as the Global Frontier Project (NRF-2014M3A6B3063709).    Figure 1. The frequency conversion process of light using a spatiotemporal boundary.   Figure 2. The complex amplitude of light at the converted frequency with the variation of a spatiotemporal boundary.

New Anisotropic Conductive Film for Ultra-Fine Pit..

(Professor Paik(right) and PhD Candidate Yoon) Higher resolution display electronic devices increasingly needs ultra-fine pitch assemblies. On that account, display driver interconnection technology has become a major challenge for upscaling display electronics. Researchers have moved to one step closer to realizing ultra-fine resolution for displays with a novel thermoplastic anchoring polymer layer structure. This new structure can significantly improve the ultra-fine pitch interconnection by effectively suppressing the movement of conductive particles. This film is expected to be applied to various mobile devices, large-sized OLED panels, and VR, among others. A research team under Professor Kyung-Wook Paik in the Department of Materials developed an anchoring polymer layer structure that can effectively suppress the movement of conductive particles during the bonding process of the anisotropic conductive films (ACFs). The new structure will significantly improve the conductive particle capture rate, addressing electrical short problems in the ultra-fine pitch assembly process. During the ultra-fine pitch bonding process, the conductive particles of conventional ACFs agglomerate between bumps and cause electrical short circuits. To overcome the electrical shortage problem caused by the free movement of conductive particles, higher tensile strength anchoring polymer layers incorporated with conductive particles were introduced into the ACFs to effectively prevent conductive particle movement. The team used nylon to produce a single layer film with well-distributed and incorporated conductive particles. The higher tensile strength of nylon completely suppressed the movement of conductive particles, raising the capture rate of conductive particles from 33％ of the conventional ACFs to 90％. The nylon films showed no short circuit problem during the Chip on Glass assembly. Even more, they obtained excellent electrical conductivity, high reliability, and low cost ACFs during the ultra-fine pitch applications. Professor Paik believes this new type of ACFs can further be applied not only to VR, 4K and 8K UHD display products, but also to large-size OLED panels and mobile devices. His team completed a prototype of the film supported by the ‘H&S High-Tech,’ a domestic company and the ‘Innopolis Foundation.’ The study, whose first author is PhD candidate Dal-Jin Yoon, is described in the October issue of IEEE TCPMT.  Figure 1: Schematic process of APL structure fabrication.               Figure 2: Proto-type production of APL ACFs.

Enhanced Video Quality despite Poor Network Condit..

(from left: Jaehong Kim, Youngmok Jung, Hyunho Yeo, Professor Dongsu Han and Professor Jinwoo Shin)   Professor Jinwoo Shin and Professor Dongsu Han from the School of Electrical Engineering developed neural adaptive content-aware internet video delivery. This technology is a novel method that combines adaptive streaming over HTTP, the video transmission system adopted by YouTube and Netflix, with a deep learning model.   This technology is expected to create an internet environment where users can enjoy watching 4K and AV/VR videos with high-quality and high-definition (HD) videos even with weak internet connections.    Thanks to video streaming services, internet video has experienced remarkable growth; nevertheless, users often suffer from low video quality due to unfavorable network conditions. Currently, existing adaptive streaming systems adjust the quality of the video in real time, accommodating the continuously changing internet bandwidth. Various algorithms are being researched for adaptive streaming systems, but there is an inherent limitation; that is, high-quality videos cannot be streamed in poor network environments regardless of which algorithm is used.    By incorporating super-resolution in adaptive streaming, the team overcame the limit of existing content distribution networks, of which their quality relies too much on the bandwidth. In the conventional method, the server that provides the video splits a video into certain lengths of time in advance. But the novel system introduced by the team allows the downloading of neural network segments. To facilitate this method, the video server needs to provide deep neural networks for each video segment as well as sizes of Deep Neural Networks (DNN) according to the specifications of the user’s computing capacity.    The largest neural network size is two megabytes, which is considerably smaller than video. When downloading the neural network from the user’s video player, it is split into several segments. Even its partial download is sufficient for a slightly comprised super-resolution.    While playing the video, the system converts the low quality video to a high-quality version by employing super-resolution based on deep convolution neural networks (CNN). The entire process is done in real time, and users can enjoy the high-definition video.     Even with a 17％ smaller bandwidth, the system can provide the Quality of Experience equivalent to the latest adaptive streaming service. At a given internet bandwidth, it can provide 43％ higher average QoE than the latest service.    Using a deep learning method allows this system to achieve a higher level of compression than the existing video compression methods. Their technology was recognized as a next-generation internet video system that applies super-resolution based on a deep convolution neural network to online videos.    Professor Han said, “So far, it has only been implemented on desktops, but we will further develop applications that work in mobile devices as well. This technology has been applied to the same video transmission systems used by streaming channels such as YouTube and Netflix, and thus shows good signs for practicability.”    This research, led by Hyunho Yeo, Youngmok Jung and Jaehong Kim, was presented at the 13th UNSENIX OSDI conference on October 10 2018 and completed for filing international patent application.      Figure 1. Image quality before (left) and after (right) the technology application       Figure 2. The technology Concept     Figure 3. A transition from low-quality to high quality video after video transmission from the video server

A Molecular Sensor for In-Situ Analysis of Complex..

A KAIST research group presented a molecular sensor with a microbead format for the rapid in-situ detection of harmful molecules in biological fluids or foods in a collaboration with a Korea Institute of Materials Science (KIMS) research group. As the sensor is designed to selectively concentrate charged small molecules and amplify the Raman signal, no time-consuming pretreatment of samples is required. Raman spectra are commonly known as molecular fingerprints. However, their low intensity has restricted their use in molecular detection, especially for low concentrations. Raman signals can be dramatically amplified by locating the molecules on the surface of metal nanostructures where the electromagnetic field is strongly localized. However, it is still challenging to use Raman signals for the detection of small molecules dissolved in complex biological fluids. Adhesive proteins irreversibly adsorb on the metal surface, which prevents the access of small target molecules onto the metal surface. Therefore, it was a prerequisite to purify the samples before analysis. However, it takes a long time and is expensive. A joint team from Professor Shin-Hyun Kim’s group in KAIST and Dr. Dong-Ho Kim’s group in KIMS has addressed the issue by encapsulating agglomerates of gold nanoparticles using a hydrogel. The hydrogel has three-dimensional network structures so that molecules smaller than the mesh are selectively permeable. Therefore, the hydrogel can exclude relatively large proteins, while allowing the infusion of small molecules. Therefore, the surface of gold nanoparticles remains intact against proteins, which accommodates small molecules. In particular, the charged hydrogel enables the concentration of oppositely-charged small molecules. That is, the purification is autonomously done by the materials, removing the need for time-consuming pretreatment. As a result, the Raman signal of small molecules can be selectively amplified in the absence of adhesive proteins. Using the molecular sensors, the research team demonstrated the direct detection of fipronil sulfone dissolved in an egg without sample pretreatment. Recently, insecticide-contaminated eggs have spread in Europe, South Korea, and other countries, threatening health and causing social chaos. Fipronil is one of the most commonly used insecticides for veterinary medicine to combat fleas. The fipronil is absorbed through the chicken skin, from which a metabolite, fipronil sulfone, accumulates in the eggs. As the fipronil sulfone carries partial negative charges, it can be concentrated using positively-charged microgels while excluding adhesive proteins in eggs, such as ovalbumin, ovoglobulin, and ovomucoid. Therefore, the Raman spectrum of fipronil sulfone can be directly measured. The limit of direct detection of fipronil sulfone dissolved in an egg was measured at 0.05 ppm. Professor Kim said, “The molecular sensors can be used not only for the direct detection of harmful molecules in foods but also for residual drugs or biomarkers in blood or urine.” Dr. Dong-Ho Kim said, “It will be possible to save time and cost as no sample treatment is required.” This research was led by graduate student Dong Jae Kim and an article entitled “SERS-Active Charged Microgels for Size- and Charge-Selective Molecular Analysis of Complex Biological Samples” was published on October 4, 2018 in Small and featured on the inside cover of the journal. Figure 1. Schematic illustrating the concentration of charged small molecules and the exclusion of large adhesive proteins using a charged hydrogel microbead containing an agglomerate of gold nanoparticles. The Raman signal of the small molecules is selectively amplified by the agglomerate. Figure 2. Confocal laser scanning microscope images showing the concentration of oppositely charged molecules, where negatively-charged microgels are denoted by red circles and positively-charged microgels are denoted by blue circles. Green fluorescence originates from negatively-charged dye molecules and red fluorescence originates from positively-charged dye molecules. Figure 3. Raman spectra measured from fipronil sulfone-spiked eggs, where the concentrations of fipronil sulfone are denoted; 0 ppm indicates no fipronil sulfone in the egg. The characteristic peaks of fipronil sulfone are denoted by the dotted lines.

Mussel-Inspired Defect Engineering Enhances the Me..

Researchers demonstrated the mussel-inspired reinforcement of graphene fibers for the improvement of different material properties. A research group under Professor Sang Ouk Kim applied polydopamine as an effective infiltrate binder to achieve high mechanical and electrical properties for graphene-based liquid crystalline fibers. This bio-inspired defect engineering is clearly distinguishable from previous attempts with insulating binders and proposes great potential for versatile applications of flexible and wearable devices as well as low-cost structural materials. The two-step defect engineering addresses the intrinsic limitation of graphene fibers arising from the folding and wrinkling of graphene layers during the fiber-spinning process. Bio-inspired graphene-based fiber holds great promise for a wide range of applications, including flexible electronics, multifunctional textiles, and wearable sensors. In 2009, the research group discovered graphene oxide liquid crystals in aqueous media while introducing an effective purification process to remove ionic impurities. Graphene fibers, typically wet-spun from aqueous graphene oxide liquid crystal dispersion, are expected to demonstrate superior thermal and electrical conductivities as well as outstanding mechanical performance. Nonetheless, owing to the inherent formation of defects and voids caused by bending and wrinkling the graphene oxide layer within graphene fibers, their mechanical strength and electrical/thermal conductivities are still far below the desired ideal values. Accordingly, finding an efficient method for constructing the densely packed graphene fibers with strong interlayer interaction is a principal challenge. Professor Kim's team focused on the adhesion properties of dopamine, a polymer developed with the inspiration of the natural mussel, to solve the problem. This functional polymer, which is studied in various fields, can increase the adhesion between the graphene layers and prevent structural defects. Professor Kim’s research group succeeded in fabricating high-strength graphene liquid crystalline fibers with controlled structural defects. They also fabricated fibers with improved electrical conductivity through the post-carbonization process of polydopamine. Based on the theory that dopamine with subsequent high temperature annealing has a similar structure with that of graphene, the team optimized dopamine polymerization conditions and solved the inherent defect control problems of existing graphene fibers. They also confirmed that the physical properties of dopamine are improved in terms of electrical conductivity due to the influence of nitrogen in dopamine molecules, without damaging the conductivity, which is the fundamental limit of conventional polymers. Professor Kim, who led the research, said, "Despite its technological potential, carbon fiber using graphene liquid crystals still has limits in terms of its structural limitations." This technology will be applied to composite fiber fabrication and various wearable textile-based application devices." This work, in which Dr. In-Ho Kim participated as first author was selected as a front cover paper of Advanced Materials on October 4. This research was supported by the National Creative Research Initiative (CRI) Center for Multi-Dimensional Directed Nanoscale Assembly and the Nanomaterial Technology Development Program through the National Research Foundation of Korea funded by the Ministry of Science and ICT.     Figure 1. Cross-section SEM image of pure graphene fiber (left) and that of graphene fiber after       two-stage defect control using polydopamine (middle and right).

KAIST Presents a Technology for Reducing the Drag ..

(from left: Professor Yeunwoo Cho and PhD Jaeho Chung)   Professor Yeunwoo Cho and his team from the Department of Mechanical Engineering developed new technology that reduces the drag force of a moving body in a still fluid by using the supercavitation phenomenon.    When a body moves in air, the frictional drag is lower than that of the same body moving in water. Therefore, the body that moves in water can reduce the drag significantly when it is completely enveloped in a gaseous cavity.    The team used compressed air to create so-called supercavitation, which is a phenomenon created by completely enveloping a body in a single large gaseous cavity. The drag force exerted on the body is then measured.    As a result, the team confirmed that the drag force for a moving body enveloped in air is about 25％ of the drag force for a moving body without envelopment.   These results can be applied for developing high-speed underwater vehicles and the development of air-lubricated, high-speed vessels.    The team expects that the results can be applied for developing high-speed underwater vehicles and the development of air lubrication for a ship’s hull.    This research, led by PhD Jaeho Chung, was published in the Journal of Fluid Mechanics as a cover article on November 10, 2018.    Figure 1. The cover article of the Journal of Fluid Mechanics Vol. 854

Highly Scalable Process to Obtain Stable 2D Nanosh..

(Professor Do Hyun Kim and his team)   A KAIST team developed technology that allows the mass production of two-dimensional (2D) nanomaterial dispersion by utilizing the characteristic shearing force of hydraulic power.    The 2D nanosheet dispersion can be directly applied to solution-based processes to manufacture devices for electronics as well as energy storage and conversion. It is expected to be used in these devices with improved performance.    There have been numerous researches on the mass production of various 2D nanomaterial because they show outstanding physical and chemical characteristics when they are truly 2D.   With strong mechanical force or chemical reaction only, each existing exfoliation method has its limitation to make 2D material when the scale of manufacturing increases. They also face the issues of high cost and long process time.   Moreover, 2D nanosheets by the exfoliation have the tendency of agglomeration due to the surface energy. Usually, organic solvent or surfactant is required to obtain high yield and concentration of 2D material by minimizing agglomeration.   After several years of research, Professor Do Hyun Kim in the Department of Chemical and Biomolecular Engineering and his team verified that optimized shearing in their reactor provided the highest efficiency for the exfoliation of nanomaterial. For the increased reactor capacity, they selected a flow and a dispersive agent to develop a high-speed, mass-production process to get 2D nanosheets by physical exfoliation with an aqueous solution.   The team proposed a flow reactor based on Taylor-Couette flow, which has the advantage of high shear rate and mixing efficiency even under large reactor capacity.    In this research, Professor Young-Kyu Han at Dongguk University-Seoul carried out the Ab initio calculation to select the dispersive agent. According to his calculation, an ionic liquid can stabilize and disperse 2D nanomaterial even in a small concentration. This calculation could maximize the exfoliating efficiency.    Professor Bong Gill Choi at Kangwon National University carried out the evaluation of device made of resulting dispersion. The team used a membrane filtration process to make a flexible and highly conductive film of 2D material. The film was then applied to produce an electrode for the supercapacitor device with very high capacity per volume. They also confirmed its stability in their supercapacitor device.    Additionally, they applied dispersive nanomaterials including graphene, molybdenum disulfide (MoS₂), and boron nitride (BN) to inkjet printer ink and realized micrometer-thick nanomaterial patterns on A4 paper. The graphene ink showed no loss of electrical property after printing without additional heat treatment.    Professor Kim said, “This new technology for the high-speed mass production of nanomaterials can easily be applied to various 2D nanomaterials. It will accelerate the production of highly efficient devices for optoelectronics, biosensors, and energy storage/conversion units with low cost.”    This research, led by Dr. Jae-Min Jeong, was published in Advanced Functional Materials on August 12.    Figure 1. The cover page of Advanced Functional Materials