research
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A Hole in One for Holographic Display
(Professor YongKeun Park)
Researchers have designed an ultrathin display that can project dynamic, multi-coloured, 3D holographic images, according to a study published in Nature Communications.
The system’s critical component is a thin film of titanium filled with tiny holes that precisely correspond with each pixel in a liquid crystal display (LCD) panel. This film acts as a ‘photon sieve’ – each pinhole diffracts light emerging from them widely, resulting in a high-definition 3D image observable from a wide angle.
The entire system is very small: they used a 1.8-inch off-the-shelf LCD panel with a resolution of 1024 x 768. The titanium film, attached to the back of the panel, is a mere 300 nanometres thick.
“Our approach suggests that holographic displays could be projected from thin devices, like a cell phone,” says Professor YongKeun Park, a physicist at KAIST who led the research. The team demonstrated their approach by producing a hologram of a moving, tri-coloured cube.
Specifically, the images are made by pointing differently coloured laser beams made of parallel light rays at the small LCD panel. The photon sieve has a hole for each pixel in the LCD panel. The holes are precisely positioned to correspond to the pixel’s active area. The pinholes diffract the light emerging from them, producing 3D images.
Previous studies from Professor Park’s group have used optical diffusors for the same purpose, but the size of the device was bulky and difficult to be operated, and it took a long period of time to calibrate. In the present work, on the other hand, the group tailored their photon sieve to demonstrate a simple, compact and scalable method for 3D holographic display. This technique can be readily applied to existing LCD displays.
Applications for holograms have been limited by cumbersome techniques, high computation requirements, and poor image quality. Improving current techniques could lead to a wide variety of applications, including 3D cinema viewing without the need for glasses, watching holographic videos on television and smart phone screens.
Figure 1. The actual 3D holographic display, and an electron microscope image of the non-periodic pinholes.
Figure 2. Three-dimensional dynamic color hologram operating at 60 Hz
2019.04.18 View 32972 -
Unravelling Inherent Electrocatalysis to Improve the Performance of Hydrogen Fuel Cells
(Figure 1. Electrode structure for the precise evaluation of the metal nanoparticles’ electrochemical catalytic characteristics at a high temperature.)
A KAIST team presented an ideal electrode design to enhance the performance of high-temperature fuel cells. The new analytical platform with advanced nanoscale patterning method quantitatively revealed the electrochemical value of metal nanoparticles dispersed on the oxide electrode, thus leading to electrode design directions that can be used in a variety of eco-friendly energy technologies.
The team, working under Professor WooChul Jung and Professor Sang Ouk Kim at the Department of Materials Science and Engineering, described an accurate analysis of the reactivity of oxide electrodes boosted by metal nanoparticles, where all particles participate in the reaction. They identified how the metal catalysts activate hydrogen electro-oxidation on the ceria-based electrode surface and quantify how rapidly the reaction rate increases with the proper choice of metals.
Metal nanoparticles with diameters of 10 nanometers or less have become a key component in high-performance heterogeneous catalysts, primarily serving as a catalytic activator. Recent experimental and theoretical findings suggest that the optimization of the chemical nature at the metal and support interfaces is essential for performance improvement.
However, the high cost associated with cell fabrication and operation as well as poorer stability of metal nanoparticles at high temperatures have been a long-standing challenge. To solve this problem, the team utilized a globally recognized metal nano patterning technology that uses block copolymer self-assembled nano templates and succeeded in uniformly synthesizing metal particles 10 nanometers in size on the surface of oxide fuel cell electrodes. They also developed a technology to accurately analyze the catalyst characteristics of single particles at high temperatures and maximize the performance of a fuel cell with minimal catalyst use.
The research team confirmed that platinum, which is a commonly used metal catalyst, could boost fuel cell performance by as much as 21 times even at an amount of 300 nanograms, which only costs about 0.015 KRW.
The team quantitatively identified and compared the characteristics of widely used metal catalysts other than platinum, such as palladium, gold, and cobalt, and also elucidated the precise principle of catalyst performance through theoretical analysis.
(Figure 2. Comparison of the electrochemical catalytic characteristics for various 10nm metal nanoparticles (platinum, palladium, cobalt, gold) at a high temperature.)
Professor Jung said, "We have broken the conventional methods of increasing the amount of catalyst which have deemed inefficient and expensive. Our results suggest a clear idea for high performance fuel cells using very small amounts of nanoparticles. This technology can be applied to many different industrial fields, advancing the commercialization of eco-friendly energy technologies such as fuel cells that generate electricity and electrolytic cells that produce hydrogen from water.”
The research has been published as the cover article of Nature Nanotechnology in the March issue. This research was carried out with support from the Nano-Material Technology Development Program through the National Research Foundation of Korea.
2019.03.28 View 30461 -
True-meaning Wearable Displays: Self-powered, Washable and Wearable
(Video: The washing process of wearing display module) When we think about clothes, they are usually formed with textiles and have to be both wearable and washable for daily use; however, smart clothing has had a problem with its power sources and moisture permeability, which causes the devices to malfunction. This problem has now been overcome by a KAIST research team, who developed a textile-based wearable display module technology that is washable and does not require an external power source.
To ease out the problem of external power sources and enhance the practicability of wearable displays, Professor Kyung Cheol Choi from the School of Electrical Engineering and his team fabricated their wearing display modules on real textiles that integrated polymer solar cells (PSCs) with organic light emitting diodes (OLEDs).
PSCs have been one of the most promising candidates for a next-generation power source, especially for wearable and optoelectronic applications because they can provide stable power without an external power source, while OLEDs can be driven with milliwatts. However, the problem was that they are both very vulnerable to external moisture and oxygen. The encapsulation barrier is essential for their reliability. The conventional encapsulation barrier is sufficient for normal environments; however, it loses its characteristics in aqueous environments, such as water. It limits the commercialization of wearing displays that must operate even on rainy days or after washing.
To tackle this issue, the team employed a washable encapsulation barrier that can protect the device without losing its characteristics after washing through atomic layer deposition (ALD) and spin coating. With this encapsulation technology, the team confirmed that textile-based wearing display modules including PSCs, OLEDs, and the proposed encapsulation barrier exhibited little change in characteristics even after 20 washings with 10-minute cycles. Moreover, the encapsulated device operated stably with a low curvature radius of 3mm and boasted high reliability.
Finally, it exhibited no deterioration in properties over 30 days even after being subjected to both bending stress and washing. Since it uses a less stressful textile, compared to conventional wearable electronic devices that use traditional plastic substrates, this technology can accelerate the commercialization of wearing electronic devices. Importantly, this wearable electronic device in daily life can save energy through a self-powered system.
Professor Choi said, “I could say that this research realized a truly washable wearable electronic module in the sense that it uses daily wearable textiles instead of the plastic used in conventional wearable electronic devices. Saving energy with PSCs, it can be self-powered, using nature-friendly solar energy, and washed. I believe that it has paved the way for a ‘true-meaning wearable display’ that can be formed on textile, beyond the attachable form of wearable technology.”
This research, in collaboration with Professor Seok Ho Cho from Chonnam National University and led by Eun Gyo Jeong, was published in Energy and Environmental Science (10.1039/c8ee03271h) on January 18, 2019.
Figure 1. Schematic and photo of a washable wearing display module
Figure 2. Cover page of Energy and Environmental Science
2019.03.21 View 31465 -
Wafer-Scale Multilayer Fabrication of Silk Fibroin-Based Microelectronics
A KAIST research team developed a novel fabrication method for the multilayer processing of silk-based microelectronics. This technology for creating a biodegradable silk fibroin film allows microfabrication with polymer or metal structures manufactured from photolithography. It can be a key technology in the implementation of silk fibroin-based biodegradable electronic devices or localized drug delivery through silk fibroin patterns.
Silk fibroins are biocompatible, biodegradable, transparent, and flexible, which makes them excellent candidates for implantable biomedical devices, and they have also been used as biodegradable films and functional microstructures in biomedical applications. However, conventional microfabrication processes require strong etching solutions and solvents to modify the structure of silk fibroins.
To prevent the silk fibroin from being damaged during the process, Professor Hyunjoo J. Lee from the School of Electrical Engineering and her team came up with a novel process, named aluminum hard mask on silk fibroin (AMoS), which is capable of micropatterning multiple layers composed of both fibroin and inorganic materials, such as metal and dielectrics with high-precision microscale alignment. The AMoS process can make silk fibroin patterns on devices, or make patterns on silk fibroin thin films with other materials by using photolithography, which is a core technology in the current microfabrication process.
The team successfully cultured primary neurons on the processed silk fibroin micro-patterns, and confirmed that silk fibroin has excellent biocompatibility before and after the fabrication process and that it also can be applied to implanted biological devices.
Through this technology, the team realized the multilayer micropatterning of fibroin films on a silk fibroin substrate and fabricated a biodegradable microelectric circuit consisting of resistors and silk fibroin dielectric capacitors in a silicon wafer with large areas.
They also used this technology to position the micro-pattern of the silk fibroin thin film closer to the flexible polymer-based brain electrode, and confirmed the dye molecules mounted on the silk fibroin were transferred successfully from the micropatterns.
Professor Lee said, “This technology facilitates wafer-scale, large-area processing of sensitive materials. We expect it to be applied to a wide range of biomedical devices in the future. Using the silk fibroin with micro-patterned brain electrodes can open up many new possibilities in research on brain circuits by mounting drugs that restrict or promote brain cell activities.”
This research, in collaboration with Dr. Nakwon Choi from KIST and led by PhD candidate Geon Kook, was published in ACS AMI (10.1021/acsami.8b13170) on January 16, 2019.
Figure 1. The cover page of ACS AMI
Figure 2. Fibroin microstructures and metal patterns on a fibroin produced by using the AMoS mask.
Figure 3. Biocompatibility assessment of the AMoS Process. Top: Schematics image of a) fibroin-coated silicon b) fibroin-pattered silicon and c) gold-patterned fibroin. Bottom: Representative confocal microscopy images of live (green) and dead (red) primary cortical neurons cultured on the substrates.
2019.03.15 View 22649 -
1g-Ultrasound System for the Brain Stimulation of a Freely-moving Mouse
A KAIST research team developed a light-weight capacitive micromachined ultrasonic transducer (CMUT) and succeeded in the ultrasound brain stimulation of a freely-moving mouse. With this lightweight and compact system, researchers can conduct a versatile set of in vivo experiments.
Conventional methods for stimulating a specific brain region, such as deep brain stimulation (DBS) and optogenetics technology, are highly invasive because they have to insert probes into a target brain, which makes them difficult to use for clinical application. While transcranial magnetic stimulation (TMS) and transcranial electrical stimulation (TES) are noninvasive, they have a wide range of stimulation and problems with in-depth stimulation, which makes them problematic for target-specific treatment.
Therefore, noninvasive and focused ultrasound stimulation technology is gaining a great deal of attention as a next-generation brain stimulation alternative. Since it is delivered noninvasively, it can be applied safely in humans as well as animal experiments. Focused ultrasound stimulation is more advantageous than conventional methods in terms of providing both local and deep stimulation.
Animal behavior experiments are essential for brain stimulation research; however, ultrasonic brain stimulation technology is currently in the early stages of development. So far, only research outcomes with fixed anesthetized mice have been studied because of the heavy ultrasonic device.
Professor Hyunjoo J. Lee from the School of Electrical Engineering and her team reported a technology that can provide ultrasound stimulation to the brain of a freely-moving mouse through a microminiaturized ultrasound device.
The team studied miniaturization and ultra-lightweight CMUTs through microelectromechanical systems (MEMS) technology and designed a device suitable for behavior experiments. The device weighing less than 1g (around 0.05% of the mouse’s weight) has the center frequency, size, focal length, and ultrasonic intensity to fit a mouse’s dimensions.
To evaluate the performance of the ultrasonic device, the team stimulated the motor cortex of the mouse brain and observed the movement reaction of its forefoot. They also measured the electromyography (EMG) of the trapezius.
As a result, the team confirmed that their ultrasonic device can deliver ultrasound to a depth of 3-4mm in the mouse brain and stimulate an area of the mouse brain that represents 25% of its total size.
Based on this research, the team is investigating the effects of ultrasound on sleep by stimulating the brain of sleeping mice.
Professor Lee said, “Going beyond experimenting on fixed anesthetized mice, this research succeeded in the brain stimulation of a freely-moving mouse. We are planning to study mice with diseases, such as Parkinson’s disease, dementia, depression, and epilepsy. I believe that this basic research can contribute to treating human brain-related diseases through ultrasound brain stimulation.
This research, led by Masters candidates Hyunggug Kim and Seongyeon Kim, was published in Brain Stimulation (10.1016/j.brs.2018.11.007) on November 17, 2018.
Figure 1. The miniature transducer for the transcranial ultrasound of a freely-moving mouse
Figure 2. Its structure and simulated 2D beam profile in the axial ad radial directions
2019.03.13 View 9540 -
Blue-enriched White Light to Wake You Up in the Morning
(from left: Professor Hyun Jung Chung, Professor Hyeon-Jeong Suk, Taesu Kim and Professor Kyungah Choi)
Here is a good news for those of who have difficulty with morning alertness. A KAIST research team proposed that a blue-enriched LED light can effectively help people overcome morning drowsiness. This study will provide the basis for major changes in future lighting strategies and thereby help create better indoor environments.
Considerable research has been devoted to unmasking circadian rhythms. The 2017 Nobel Prize in Physiology or Medicine went to Jeffrey C. Hall, Michael Rosbash, and Michael W. Young for unveiling the molecular mechanisms that control circadian rhythms. In particular, the relationship between light and its physiological effects has been investigated since the discovery of a novel, third type of photoreceptor in the human retina in the early 2000s. Rods and cones regulate visual effects, while the third type, photosensitive retinal ganglion cells, regulate a large variety of biological and behavioral processes including melatonin and cortisol secretion, alertness, and functional magnetic resonance imaging (fMRI).
Initial studies on light sources have shown that blue monochromatic, fully saturated lights are effective for stimulating physiological responses, but the relative effectiveness of commercially available white light sources is less well understood. Moreover, the research was more focused on the negative effects of blue light; for instance, when people are exposed to blue light at night, they have trouble achieving deep sleep because the light restrains melatonin secretion.
However, Professor Hyeon-Jeong Suk and Professor Kyungah Choi from the Department of Industrial Design and their team argue that the effects of blue-enriched morning light on physiological responses are time dependent, and that it has positive effects on melatonin levels and the subjective perception of alertness, mood, and visual comfort compared with warm white light.
The team conducted an experiment with 15 university students. They investigated whether an hour of morning light exposure with different chromaticity would affect their physiological and subjective responses differently. The decline of melatonin levels was significantly greater after the exposure to blue-enriched white light in comparison with warm white light.
Professor Suk said, “Light takes a huge part of our lives since we spend most of our time indoors. Light is one of the most powerful tools to affect changes in how we perceive and experience the environment around us.”
Professor Choi added, “When we investigate all of the psychological and physiological effects of light, we see there is much more to light than just efficient quantities. I believe that human-centric lighting strategies could be applied to a variety of environments, including residential areas, learning environments, and working spaces to improve our everyday lives.”
This research was collaborated with Professor Hyun Jung Chung from the Graduate School of Nanoscience and Technology and was published in Scientific Reports (10.1038/s41598-018-36791-5) on January 23, 2019.
Figure 1. Changes in melatonin secretion during day and night time
2019.03.06 View 11762 -
New Catalyst for Synthesizing Chiral Molecules Selectively
(from left: Dr. Yoonsu Park and Professor Sukbok Chang from the Department of Chemistry)
Molecules in nature often have “twin” molecules that look identical. In particular, the twin molecules that look like mirror images to each other are called enantiomers. However, even though they have the same type and number of elements, these twin molecules exhibit completely different properties.
Professor Sukbok Chang and Dr. Yoonsu Park from the Department of Chemistry developed a new catalyst capable of selectively synthesizing only one of the two enantiomers. Using this catalyst, the have succeeded in manufacturing the chiral lactam, an essential ingredient in pharmaceuticals, from a hydrocarbon compound.
Enantiomerism or chirality is considered very important for drug development. Biomaterials, such as DNAs and proteins also have chiral properties, but they exhibit different physiological activities depending on the types of drugs. One type of the enantiomer could be useful while the other is toxic. Hence, the technology for selective synthesizing (i.e. asymmetric synthesis) is required, but it is still regarded as a great challenge faced by modern chemistry to date.
The researchers solved this problem by developing a new catalyst. Earlier they presented their research on developing an iridium catalyst that converts hydrocarbons into high value γ-lactam compounds, and published it in Science in March 2018. However, the developed catalyst still had a limitation that both types of enantiomers are obtained without selectivity.
In this study, they found that among dozens of other catalyst candidates, iridium catalysts with chiral diamine scaffolds were able to select the correct enantiomer with a selectivity of 99% or more. This novel catalyst can be used to synthesize the various chiral γ-lactam as required. A left-handed γ-lactam and a right-handed γ-lactam can be produced using a left-handed iridium catalyst and a right-handed iridium catalyst, respectively.
They analyzed the reason for the high selectivity through computational chemistry simulations. They identified that temporal hydrogen bonding occurred between the chiral diamine catalysts and the hydrocarbon compound during the reaction. As a result of the hydrogen bonding, the formation of the left-handed lactam was boosted.
With their new catalyst, they also succeeded in synthesizing chiral lactam compounds with different structures. By using inexpensive and readily available feedstock hydrocarbons, the researchers produced a group of chiral lactams in different shapes. As their chirality and diverse structures enable lactams to function as an active compound in the body for antibiotic, anti-inflammatory, or anti-tumoral functions, this study may facilitate the development of potential drugs in a more efficient and cheaper way.
Professor Chang said, “We hope that our research on selectively producing core units of effective drugs will lead to developing new drugs that demonstrate fewer side-effects and higher efficacy. There are also economic advantages of this research because it uses hydrocarbon compounds, which can be abundantly found in nature, to produce high-value raw materials.
This research was published in Nature Catalysis(10.1038/s41929-019-0230-x) on February 19, 2019.
Figure 1. Asymmetric formation of chiral γ-lactam
Figure 2. Outline of research outcome
2019.03.05 View 7585 -
KAIST Develops Analog Memristive Synapses for Neuromorphic Chips
(Professor Sung-Yool Choi from the School of Electrical Engineering)
A KAIST research team developed a technology that makes a transition of the operation mode of flexible memristors to synaptic analog switching by reducing the size of the formed filament. Through this technology, memristors can extend their role to memristive synapses for neuromorphic chips, which will lead to developing soft neuromorphic intelligent systems.
Brain-inspired neuromorphic chips have been gaining a great deal of attention for reducing the power consumption and integrating data processing, compared to conventional semiconductor chips. Similarly, memristors are known to be the most suitable candidate for making a crossbar array which is the most efficient architecture for realizing hardware-based artificial neural network (ANN) inside a neuromorphic chip.
A hardware-based ANN consists of a neuron circuit and synapse elements, the connecting pieces. In the neuromorphic system, the synaptic weight, which represents the connection strength between neurons, should be stored and updated as the type of analog data at each synapse.
However, most memristors have digital characteristics suitable for nonvolatile memory. These characteristics put a limitation on the analog operation of the memristors, which makes it difficult to apply them to synaptic devices.
Professor Sung-Yool Choi from the School of Electrical Engineering and his team fabricated a flexible polymer memristor on a plastic substrate, and found that changing the size of the conductive metal filaments formed inside the device on the scale of metal atoms can make a transition of the memristor behavior from digital to analog.
Using this phenomenon, the team developed flexible memristor-based electronic synapses, which can continuously and linearly update synaptic weight, and operate under mechanical deformations such as bending.
The team confirmed that the ANN based on these memristor synapses can effectively classify person’s facial images even when they were damaged. This research demonstrated the possibility of a neuromorphic chip that can efficiently recognize faces, numbers, and objects.
Professor Choi said, “We found the principles underlying the transition from digital to analog operation of the memristors. I believe that this research paves the way for applying various memristors to either digital memory or electronic synapses, and will accelerate the development of a high-performing neuromorphic chip.”
In a joint research project with Professor Sung Gap Im (KAIST) and Professor V. P. Dravid (Northwestern University), this study was led by Dr. Byung Chul Jang (Samsung Electronics), Dr. Sungkyu Kim (Northwestern University) and Dr. Sang Yoon Yang (KAIST), and was published online in Nano Letters (10.1021/acs.nanolett.8b04023) on January 4, 2019.
Figure 1. a) Schematic illustration of a flexible pV3D3 memristor-based electronic synapse array. b) Cross-sectional TEM image of the flexible pV3D3 memristor
2019.02.28 View 9605 -
Novel Material Properties of Hybrid Perovskite Nanostructures for Next-generation Non-linear Electronic Devices
(from left: Juho Lee, Dr. Muhammad Ejaz Khan and Professor Yong-Hoon Kim)
A KAIST research team reported a novel non-linear device with the founding property coming from perovskite nanowires. They showed that hybrid perovskite-derived, inorganic-framework nanowires can acquire semi-metallicity, and proposed negative differential resistance (NDR) devices with excellent NDR characteristics that resulted from a novel quantum-hybridization NDR mechanism, implying the potential of perovskite nanowires to be realized in next-generation electronic devices.
Organic-inorganic hybrid halide perovskites have recently emerged as prominent candidates for photonic applications due to their excellent optoelectronic properties as well as their low cost and facile synthesis processes. Prominent progresses have been already made for devices including solar cells, light-emitting diodes, lasers and photodetectors.
However, research on electronic devices based on hybrid halide perovskites has not been actively pursued compared with their photonic device counterparts.
Professor Yong-Hoon Kim from the School of Electrical Engineering and his team took a closer look at low-dimensional organic-inorganic halide perovskite materials, which have enhanced quantum confinement effects, and particularly focused on the recently synthesized trimethylsulfonium (TMS) lead triiodide (CH3)3SPbI3.
Using supercomputer simulations, the team first showed that stripping the (CH3)3S or TMS organic ligands from the TMS PbI3 perovskite nanowires results in semi-metallic PbI3 columns, which contradicts the conventional assumption of the semiconducting or insulating characteristics of the inorganic perovskite framework.
Utilizing the semi-metallic PbI3 inorganic framework as the electrode, the team designed a tunneling junction device from perovskite nanowires and found that they exhibit excellent nonlinear negative differential resistance (NDR) behavior. The NDR property is a key to realizing next-generation, ultra-low-power, and multivalued non-linear devices. Furthermore, the team found that this NDR originates from a novel mechanism that involves the quantum-mechanical hybridization between channel and electrode states.
Professor Kim said, “This research demonstrates the potential of quantum mechanics-based computer simulations to lead developments in advanced nanomaterials and nanodevices. In particular, this research proposes a new direction in the development of a quantum mechanical tunneling device, which was the topic for which the Nobel Laureate in Physics in 1973 was awarded to Dr. Leo Esaki.
This research, led by Dr. Muhammad Ejaz Khan and PhD candidate Juho Lee, was published online in Advanced Functional Materials (10.1002/adfm.201807620) on January 7, 2019.
Figure. The draft version of the cover page of 'Advanced Functional Materials'
2019.02.22 View 7348 -
New LSB with Theoretical Capacity over 90%
(Professor Hee-Tak Kim and Hyunwon Chu)
A KAIST research team has developed a lithium sulfur battery (LSB) that realizes 92% of the theoretical capacity and an areal capacity of 4mAh/cm2.
LSBs are gaining a great deal of attention as an alternative for lithium ion batteries (LIBs) because they have a theoretical energy density up to six to seven times higher than that of LIBs, and can be manufactured in a more cost-effective way.
However, LSBs face the obstacle of having a capacity reaching its theoretical maximum because they are prone to uncontrolled growth of lithium sulfide on the electrodes, which leads to blocking electron transfer.
To address the problem of electrode passivation, researchers introduced additional conductive agent into the electrode; however, it drastically lowered the energy density of LSBs, making it difficult to exceed 70% of the theoretical capacity.
Professor Hee-Tak Kim from the Department of Chemical and Biomolecular Engineering and his team replaced the lithium salt anions used in conventional LSB electrolytes with anions with a high donor number. The team successfully induced the three-dimensional growth of lithium sulfide on electrode surfaces and efficiently delayed the electrode passivation.
Based on this electrolyte design, the research team achieved 92% of the theoretical capacity with their high-capacity sulfur electrode (4mAh/cm2), which is equivalent to that of conventional LIB cathode. Furthermore, they were able to form a stable passivation film on the surface of the lithium anode and exhibited stable operation over 100 cycles.
This technology, which can be flexibly used with various types of sulfur electrodes, can mark a new milestone in the battery industry.
Professor Kim said, “We proposed a new physiochemical principle to overcome the limitations of conventional LSBs. I believe our achievement of obtaining 90% of the LBSs’ theoretical capacity without any capacity loss after 100 cycles will become a new milestone.”
This research, first-authored by Hyunwon Chu and Hyungjun Noh, was published in Nature Communications on January 14, 2019. It was also selected in the editor’s highlight for its outstanding achievements.
Figure 1. Lithium sulfur growth and its deposition mechanism for different sulfide growth behaviors
Figure 2. Capacity and cycle life characteristics of the LSBs
2019.02.11 View 8109 -
KAIST Develops Core Technology for Ultra-small 3D Image Sensor
(from left: Dr. Jong-Bum Yo, PhD candidate Seong-Hwan Kimand Professor Hyo-Hoon Park)
A KAIST research team developed a silicon optical phased array (OPA) chip, which can be a core component for three-dimensional image sensors. This research was co-led by PhD candidate Seong-Hwan Kim and Dr. Jong-Bum You from the National Nanofab Center (NNFC).
A 3D image sensor adds distance information to a two-dimensional image, such as a photo, to recognize it as a 3D image. It plays a vital role in various electronics including autonomous vehicles, drones, robots, and facial recognition systems, which require accurate measurement of the distance from objects.
Many automobile and drone companies are focusing on developing 3D image sensor systems, based on mechanical light detection and ranging (LiDAR) systems. However, it can only get as small as the size of a fist and has a high possibility of malfunctioning because it employs a mechanical method for laser beam-steering.
OPAs have gained a great attention as a key component to implement solid-state LiDAR because it can control the light direction electronically without moving parts. Silicon-based OPAs are small, durable, and can be mass-produced through conventional Si-CMOS processes.
However, in the development of OPAs, a big issue has been raised about how to achieve wide beam-steering in transversal and longitudinal directions. In the transversal direction, a wide beam-steering has been implemented, relatively easily, through a thermo-optic or electro-optic control of the phase shifters integrated with a 1D array. But the longitudinal beam-steering has been remaining as a technical challenge since only a narrow steering was possible with the same 1D array by changing the wavelengths of light, which is hard to implement in semiconductor processes.
If a light wavelength is changed, characteristics of element devices consisting the OPA can vary, which makes it difficult to control the light direction with reliability as well as to integrate a wavelength-tunable laser on a silicon-based chip. Therefore, it is essential to devise a new structure that can easily adjust the radiated light in both transversal and longitudinal directions.
By integrating tunable radiator, instead of tunable laser in a conventional OPA, Professor Hyo-Hoon Park from the School of Electrical Engineering and his team developed an ultra-small, low-power OPA chip that facilitates a wide 2D beam-steering with a monochromatic light source.
This OPA structure allows the minimizing of the 3D image sensors, as small as a dragonfly’s eye.
According to the team, the OPA can function as a 3D image sensor and also as a wireless transmitter sending the image data to a desired direction, enabling high-quality image data to be freely communicated between electronic devices.
Kim said, “It’s not an easy task to integrate a tunable light source in the OPA structures of previous works. We hope our research proposing a tunable radiator makes a big step towards commercializing OPAs.”
Dr. You added, “We will be able to support application researches of 3D image sensors, especially for facial recognition with smartphones and augmented reality services. We will try to prepare a processing platform in NNFC that provides core technologies of the 3D image sensor fabrication.”
This research was published in Optics Letters on January 15.
Figure 1.The manufactured OPA chip
Figure 2. Schematic feature showing an application of the OPA to a 3D image sensor
2019.02.08 View 7080 -
Stretchable Multi-functional Fiber for Energy Harvesting and Strain Sensing
(from left: Professor Steve Park, Jeongjae Ryu and Professor Seungbum Hong)
Fiber-based electronics are expected to play a vital role in next-generation wearable electronics. Woven into textiles, they can provide higher durability, comfort, and integrated multi-functionality. A KAIST team has developed a stretchable multi-functional fiber (SMF) that can harvest energy and detect strain, which can be applied to future wearable electronics.
With wearable electronics, health and physical conditions can be assessed by analyzing biological signals from the human body, such as pulse and muscle movements. Fibers are highly suitable for future wearable electronics because they can be easily integrated into textiles, which are designed to be conformable to curvilinear surfaces and comfortable to wear. Moreover, their weave structures offer support that makes them resistant to fatigue. Many research groups have developed fiber-based strain sensors to sense external biological signals. However, their sensitivities were relatively low.
The applicability of wearable devices is currently limited by their power source, as the size, weight, and lifetime of the battery lessens their versatility. Harvesting mechanical energy from the human body is a promising solution to overcome such limitations by utilizing various types of motions like bending, stretching, and pressing. However, previously reported, fiber-based energy harvesters were not stretchable and could not fully harvest the available mechanical energy.
Professor Seungbum Hong and Professor Steve Park from the Department of Materials Science and Engineering and their team fabricated a stretchable fiber by using a ferroelectric layer composed of P(VDF-TrFE)/PDMS sandwiched between stretchable electrodes composed of a composite of multi-walled carbon nanotubes (MWCNT) and poly 3,4-ethylenedioxythiophene polystyrenesulfonate (PEDOT:PSS).
Cracks formed in MWCNT/PEDOT:PSS layer help the fiber show high sensitivity compared to the previously reported fiber strain sensors. Furthermore, the new fiber can harvest mechanical energy under various mechanical stimuli such as stretching, tapping, and injecting water into the fiber using the piezoelectric effect of the P(VDF-TrFE)/PDMS layer.
Professor Hong said, “This new fiber has various functionalities and makes the device simple and compact. It is a core technology for developing wearable devices with energy harvesting and strain sensing capabilities.”
This article, led by PhD candidate Jeongjae Ryu, was published in the January 2019 issue of Nano Energy.
Figure 1.Schematic illustration of an SMF fiber and its piezoelectric voltage output and response to strain.
Figure 2. Photographs of a stretchable multi-functional fiber being stretched by 100%, bent, and twisted.
2019.01.31 View 8896