Research

Subtle Facial Expression Recognition Becomes Practicable (2009.4.5)

In the Steven Spielberg movie A.I., robots not only resemble humans in looks but read their facial expressions, reacting accordingly. With the new technology developed by Professor Daijin Kim (Department of Computer Science and Engineering) and his group, the movie may soon become reality. Automatic expression analysis is difficult because people’s faces vary so much. Existing software could effectively recognize only extreme expressions, which is not particularly helpful in real-life situations. But the system developed by Professor Kim’s team recognizes even slight facial movements. The technology, which automatically reads on people joy, anger, sadness and surprise, the 4 primary facial expressions, uses motion magnification which converts subtle expressions into exaggerated expressions. Professor Kim’s group programmed software to identify 27 facial feature points, such as the eight points around the mouth and three around the nostrils, and to track how each point moved in a sequence of images for each facial expression. The system then exaggerates the movements in each sequence, generating extreme facial expressions which existing software can identify. In tests in which 80 subtle expressions of neutral, happy or angry feelings were exaggerated in this way, the system judged them correctly 88% of the time. The team has used the technique to allow a robot with a human-like face to read and mimic a person’s subtle expressions. They also hope to install the software on digital cameras and camera phones, says Professor Kim, so the devices can do things like automatically take a picture when the subject smiles. When applied to human sensing, the technique could also be used in areas including biometric information system, smart home control, rehabilitation and health services, human machine interaction, and secretarial services. The primary goal is to apply and utilize the technology to advance the quality of life for the elderly and disabled. The research is jointly conducted with Professor Takeo Kanade’s group of the Robotics Institute at Carnegie Mellon University, with the financial support of the Korean Ministry of Education, Science and Technology. Professor Daejin Kim Department of Computer Science and Engineering Tel: +82-54-279-2249 Fax: +82-54-279-2299 E-mail: dkim@postech.ac.kr

Thermosensory Neurons Control Lifespan of Ectotherms (2009.4.21)

Astudy revealed that thermosensory neurons of ectotherms affect lifespan at warm temperature by changing the activity of a steroid-signaling pathway that regulates longevity. Professor Seung-Jae Lee (Department of Life Science, School of Interdisciplinary Bioscience and Bioengineering, Division of IT Convergence Engineering) and his team, in collaboration with University of California, San Francisco, discovered that thermosensory neurons of C. elegans (Caenorhabditis elegans), an ectotherm, actively regulates the temperature dependence of lifespan. Many ectotherms, including C. elegans, have shorter lifespan at high temperature than at low temperature. This is generally attributed to the effect of temperature on metabolic rates: at 25°C, the “rate of living” is accelerated because of the increased rate of chemical reactions including growth and digestion, and the C. elegans ages more quickly and has a shorter lifespan than at the temperature of 15°C. Professor Lee found through his research that without the thermosensory neurons, the lifespan of the C. elegans is much shorter, and that the thermosensory neurons influence lifespan at high temperature by changing the activities of the steroid-signaling pathway. The research group concluded that this thermosensory system allows C. elegans to reduce the effect that warm temperature would otherwise have on processes that affect aging, something that warm-blooded animals do by controlling temperature itself. Professor Lee explained that the thermosensory system of C. elegans “controls the aging to progress at a normal rate even at a high temperature,” and that the study indicates that ectotherms regulate their lifespan by controlling their physical response to high temperature. Professor Cynthia Kenyon of University of California, San Francisco, who collaborated in the research, evaluated the results to be “an outstanding achievement that would rewrite the chapter on ectotherms in high school biology textbooks.” Professor Lee’s research is to be supported by the Korean Ministry of Education, Science and Technology’s World Class University program, continuing to use the C. elegans to identify genes that regulate the aging process, and their operational processes. The results of the study were published in Current Biology Volume 19, Number 9. Professor Seung-Jae Lee Department of Life Science School of Interdisciplinary Bioscience and Bioengineering Division of IT Convergence Engineering Tel: +82-54-279-2351 Fax: +82-54-279-2199 E-mail: seungjaelee@postech.ac.kr

Fluorescent Sensor for Mercury Detection Developed (2009.4.3)

Professor Kyo Han Ahn (Department of Chemistry) and his team, in a joint research with a Yonsei University group, have developed a new probe for mercury that can be used for imaging organs in living organisms. Mercury is a highly toxic and widespread pollutant, but the existing fluorescence probes for it mostly detected only its inorganic forms. The element is commonly found in organic forms which are much more toxic than inorganic mercury as their lipophilicity allows them to cross biological membranes. Consequently, new ways of detecting organic mercury species, particularly in organisms, was of crucial importance. The research team has developed a structurally simple probe which reacts with both organic and inorganic mercury to give a fluorescent product. The probe was used to monitor methylmercury species in mammalian cells and zebrafish organs incubated with methylmercury. While existing probes for inorganic mercury used heteroatomic ligands, Professor Ahn’s group took a different approach, making a conceptual advance: inspiration was taken from the field of oxymercuration reactions, in which alkenes, unsaturated hydrocarbons, when met with mercury, react to water. Professor Ahn said the probe could be vital to the study of organic mercury poisoning in living species. “Now we have a molecular probe that can be used for studying and tracing the notoriously toxic methylmercury in living species. By using the probe, we may study the distribution and fate of methylmercury in organisms,” he explained. Professor Ahn also added that the next step was to develop a probe that is more discriminating. “One of the most challenging issues in mercury sensing is to discriminate methylmercury from inorganic mercury,” said Professor Ahn. “We do not have such a molecular probe yet but we are working on it.” The findings were published in the Royal Society of Chemistry’s Chemical Communications, and highlighted in Chemical Biology and other magazines. Professor Kyo Han Ahn Department of Chemistry Tel: +82-54-279-2105 Fax: +82-54-279-3399 E-mail: ahn@postech.ac.kr

Novel Brush Polymers Developed as Biomaterials (2009.4.23)

Professor Moonhor Ree (Department of Chemistry) and his research group, in a joint research with a Dongguk University Medical School team (Professor Heesoo Kim), successfully synthesized new brush polymers. The newly developed brush polymers are suitable for use in biomedical applications including medical devices and biosensors that require biocompatibility and the reduced possibility of post-operative infection. Because cell membranes play an important role protecting the material inside the cell and maintaining the structure and function of the cell, studies on polymer materials that mimic membranes have received significant attention. In the study, Professor Ree’s team reported new brush polymers with various numbers of bristle ends incorporating phosphorylcholine (PC) moieties, which in thin films have self-assembled nanostructures and cell-membrane-mimicking surfaces. The bristles in the novel brush polymers have a strong tendency to self-assemble, allowing for the polymers to have three phases depending on the temperature regime. Further, the brush polymers in thin films demonstrate to have a molecular multibilayer structure up to 55°C with stable PC-rich surfaces that successfully mimic cell membrane surfaces. The brush polymers exhibit excellent water wettability and water sorption while retaining the remarkable molecular multi-bilayer structure, thus having hydrophilic surfaces. Also, the novel multibilayer structured films repel fibrinogen molecules and blood platelets from their surfaces, at the same time having bactericidal effects. Moreover, the brush polymer films are found to provide comfortable surface environments for the successful anchoring and growth of human cells (for example, HEp-2 cells), and to exhibit excellent biocompatibility in mice. The structural analysis of the thin films was carried out by using grazing-incidence X-ray scattering (GIXS) with synchrotron radiation sources at the Pohang Light Source on POSTECH campus. The self-assembly structures, properties, and biocompatibility of the newly developed brush polymers are expected to contribute to the development of side effect-free coating materials for artificial organs or medical devices. The study achievements were introduced in the April 4 online edition of Advanced Functional Materials (2009, vol. 19, 1631-1644). Professor Moonhor Ree Department of Chemistry Tel: +82-54-279-2120 Fax: +82-54-279-3399 E-mail: ree@postech.ac.kr

New Clues Found on Mechanism for Chronic Neurological Disorder (2009.3.19)

Korean scientists have identified the structure of a protein linked to epilepsy and other brain function abnormalities, which they say could present new opportunities in treatment and drug development. In a study published by peer-review European Molecular Biology Organization (EMBO) Journal, a team of researchers led Kim Kyung-jin of Pohang University of Science and Technology (POSTECH) presented the crystal structure of the succinic semialdehyde dehydrogenase (SSADH) protein. SSADH deficiency has been associated with epilepsy and other neurological conditions, and understanding the structure of the protein provides an important clue in revealing the mechanism of how they develop, Kim said. One out of every 200 people suffers from epilepsy, and the over-expression of gamma-aminobutyric acid (GABA), the most abundant inhibitory neurotransmitter in the brain, has been cited among the main causes for epileptic seizures. In healthy individuals, GABA is converted to succinic semialdehyde (SSA) by GABA transaminase (GABA-T), and SSA is then converted to succinic acid by the SSADH protein. Thus, the deficiency of SSADH, which results in an up-regulation of GABA levels in the brain, has been pointed to as the cause for nerve cells in the brain becoming overly active, but there has been little research on what causes SSADH functions to deteriorate. Using the 6C1 beam-line at POSTECH's accelerator laboratory to unveil SSADH's crystal structure, Kim found that SSADH loses its ability to interact with SSA when it becomes oxidized by reactive oxygen. "The over-expression of GABA has been linked to epilepsy and other conditions such as speech disorders and mental retardation, and identifying a link between GABA levels and reactive oxygen is a significant achievement,'' Kim said. "We believe that our findings could give us the core technology to achieve breakthroughs in drug development.''

Genes for Mad Cow Disease Spotted (2009.3.26)

Mad cow disease made headlines in the early 2000s when 4.4 million cattle in the United Kingdom alone were killed as a precaution against it. Now, there may be a way to not only trace the disease in its incubation period but also medically treat it. Professor Daehee Hwang (School of Interdisciplinary Bioscience and Bioengineering) and his group, in collaboration with the Institute for Systems Biology (ISB), the McLaughlin Research Institute, the University of California, San Francisco, the Allen Brain Institute, and the European Bioinformatics Institute, have spotted a set of genes that go haywire in mice infected with a form of mad cow disease. The research group infected 5 different kinds of mice with 2 types of prion extracted from sheep and cattle, and took 30 million measurements from the brains of the infected mice, and used high-powered statistical and computer models to separate signal from noise. They were able to narrow it down to 7,400 genes - about one-third of the mouse genome - that were affected by the misfolded prions, which was again narrowed down to 333 “core” genes that were perturbed. Researchers looked at the genes at 10 different time points, and noted which genes were altered for weeks before symptoms showed up. Some of these genes make proteins that are secreted into the blood, which could make for a relatively practical new diagnostic test, they said. The system developed through this study can be applied to the study of other intractable diseases including cancers, degenerative brain diseases, autoimmune diseases, and infections, making possible the discrimination of the core genes in the development and progress of the diseases, Professor Hwang commented. The findings were published online in the March 24 online issue of Molecular Systems Biology. Professor Daehee Hwang School of Interdisciplinary Bioscience and Bioengineering Tel: +82-54-279-2393 Fax: +82-54-279-8409 E-mail: dhhwang@postech.ac.kr

Microcantilevers with Nanochannels (2008.4.15)

The invention of the scanning tunnel microscope (STM) opened the door to nanoworld, enabling humankind to actually touch and feel the individual atoms at the surface of a material. It has inspired a series of inventions such as the atomic force microscope (AFM), the lateral force microscope (LFM), the magnetic force microscope (MFM), etc. Recently, the scanning probe microscope (SPM), which encompasses all these inventions, has evolved into the millipede technology which has a potential to make competing data storage technologies obsolete. A common denominator of these exciting developments is a microscopic structure fabricated by MEMS techniques: the microcantilever. Microcantilevers are not confined to imaging microscopy. They can also be found in the nanopatterning efforts using near-field scanning optical microscopy (NSOM), scanning electrochemical microscopy (SECM), dip-pen nanolithography (DPN), as well as in microsensors. Microcalorimeters offering ultra-high sensitivity have been demonstrated, and ultra-low (ppb) concentrations of toxic gases in air detected. Moreover, surface stresses were measured for the self-assembly of alkanethiols on gold, DNA hybridization or receptor-ligand binding. By using a nanoscale resonator, a mass sensitivity of a few femtograms was reported. Microcantilevers certainly have fundamental advantages as microsensors. Thousands of microcantilevers can be prepared in a single wafer, so they are well-suited for miniaturization. The detection mechanisms are very simple - just measuring the bending deformation or the shift of the resonance frequency of a beam. Chemical or biological stimuli are directly converted to mechanical responses, resulting in a high degree of selectivity and efficiency. We have devised novel microcantilevers with a radically different material and structure, as well as fabrication method. Figures 1(a) through (c) show SEM photographs of the proposed microcantilever array at different magnifications. The overall shape of the microcantilever array can be seen in Figure 1(a). All cantilevers have the same thickness (2μm) and length (50μm), but different widths ranging from 10μm to 50 μm. Figure 1(b) is the magnified image of the edge of one microcantilever in Figure 1(a). Straight nanochannels are clearly visible at this magnification. Further magnification in Figure 1(c) unveils the hexagonal arrangement of nanochannels on the surface of the microcantilever. This photograph was taken from the bottom surface to prove the presence of nanochannels all the way through the thickness of the microcantilever. These nanochannels with extremely high aspect ratio are impossible to obtain by conventional lithographic techniques. The centerpiece of this novel development is the wellknown nanomaterial, anodic aluminum oxide (AAO). It typically has parallel channels of tens of nanometers in diameter up to hundreds of microns in length. It has been commonly used as molds for nanomaterials such as nanotubes and nanowires, taking advantage of these high-aspect-ratio nanochannels. Moreover, replicas of these nanochannels can also be applied in many important fields such as nanoimprint lithography, superhydrophobic surface, and photonic crystals. Recently, AAO technology has experienced a major technological leap by adopting photolithography. This combination will result in various MEMS/NEMS structures with nanochannels in the years to come. To our knowledge, microcantilevers have been made exclusively of isotropic materials until now. Since the operation of a microcantilever relies on the bending of the beam, Young’s modulus is an important material property. A microcantilever made of an isotropic material has a fixed Young’s modulus, so cantilever design is rather limited. In contrast, our microcantilevers possess tunable nanochannels. Since the nanochannels are arranged in one direction, the AAO is not isotropic, and the Young’s modulus is by no means a unique material property. In fact, our microcantilever has multiple Young’s moduli depending upon the direction, and, best of all, the Young’s moduli can be controlled by varying the dimensions of the nanochannels. This is an extremely important advantage in the design of microcantilevers, which has the potential to greatly expand their applicability. The mere presence of nanochannels is beneficial to some applications. Microcantilevers are frequently operated in a vibration mode which has a high sensitivity. The resistance from the surrounding fluid consumes much energy, and eventually causes damping of the vibration of a microcantilever. It becomes a serious problem when microcantilevers are used as remote sensors with microbattery power sources. With the microcantilevers developed here, gas molecules freely move through the nanochannels, so that the resistance is reduced greatly. The deflection of a cantilever stems from the change in surface energy. Therefore, a large surface area is generally advantageous in most sensor applications. Cantilevers based on AAO can provide surface areas several orders of magnitude larger than conventional silicon cantilevers with flat surfaces. The fabrication method for the microcantilevers proposed here is quite different from that of conventional silicon counterparts. The proposed method consists of (1) AAO fabrication by anodization, (2) patterning on the AAO layer by photolithography, (3) fabrication of the beam structures by anisotropic etching of the AAO layer, and (4) removal of the substrate below the AAO layer by electrochemical etching. The fabrication process of AAO microcantilevers is outlined with the corresponding SEM images in Figure 2. The first step is to form an AAO layer on top of an aluminum layer. The particular sample shown in Figure 2(a) has hexagonally ordered nanochannels 30nm in diameter. The channel diameter, the channel-tochannel distance, and the thickness of the AAO layer can be controlled by changing the anodization conditions such as reaction time, applied voltage, temperature, etc. Patterns of microcantilevers are made in the second step by conventional photolithography. A thin layer of aluminum was deposited on the surface of the AAO as a transfer layer, followed by the spin coating of a photoresist layer. The transfer layer is necessary to provide a smooth surface, which generates sharper boundaries. Patterns for the microcantilevers were formed on the photoresist layer using a photomask, and transferred to the transfer layer by removing the exposed area using aluminum etchant. Since the aluminum etchant also dissolves alumina, the channels in the underlying AAO layer were slightly widened during this step. The SEM image in Figure 2(b) shows the microcantilever patterns formed on the AAO. The brighter area is the area with the exposed AAO, though the channels are not visible at this magnification. Note that the microcantilevers were intentionally fabricated with different widths for later measurements. After the pattern transfer, the sample was immersed in a phosphoric acid solution to remove the AAO in the exposed area. The acid solution penetrated into the nanochannels of the exposed area, while the area blocked by the photoresist was protected from the acid solution. As a result, the AAO in the exposed area was selectively etched away. The SEM image in Figure 2(c) shows the structures after AAO etching. Although the AAO layer is rather thick, 10μm, the microcantilever patterns of the AAO were successfully formed with vertical sidewalls which are hard to obtain with the wet etching of silicon. The presence of nanochannels in the AAO made the anisotropic etching of the complex patterns possible. Note that the photoresist layer and the transfer layer still exist on top of the AAO structures. Suspended cantilevers are fabricated by removing the aluminum under the patterns without damaging the AAO. Electrochemical etching was chosen for this. Suspended microcantilvers are clearly observed in the SEM image of Figure 2(d). The conventional fabrication method for suspended microcantilevers requires a much more complex procedure including deposition, patterning, and etching. Low pressure chemical vapor deposition (LPCVD), plasma enhanced CVD (PECVD) as well as reactive ion etching (RIE) are frequently used in the fabrication of silicon microcantilevers. Our method greatly simplifies the fabrication process through electrochemical etching. The huge surface area of an AAO microcantilever offers an enormous number of binding sites. The adsorption of chemical or biological molecules on the binding sites of a cantilever shifts the resonance frequency. The relation between the mass of adsorbed materials and the shift of resonance frequency is given as , where f1 and f0 are the resonance frequencies after and before mass loading. The more molecules adsorbed on the surface of a cantilever, the larger the shift of the resonance frequency, as obvious from this equation. Figure 3 shows the shifts of the observed resonance frequencies after injecting saturated dodecanthiol on two different microcantilevers. Both cantilevers have the same thickness(2μm), length(250μm) and width(35μm), but the AAO microcantilever has vertically parallel pores with 50nm diameter while the silicon cantilever has a flat surface. The red line in this figure is the observed frequency shift of the AAO microcantilever as a function of time, and the black one is those of the Si cantilever. The shift of resonance frequency of the AAO microcantilever (68Hz) is about 6 times as large as that of the Si cantilever (11Hz) at the saturated condition. This large frequency shift is partly due to the large surface area of the AAO cantilever, and partly due to the different physical properties such as smaller Young’s modulus. Therefore, the AAO microcantilever is much more sensitive than the Si cantilever of identical overall dimensions. Moreover, the presence of nanochannels provides not only a large surface area, but also selectivity among different analytes based on a sieving mechanism. Thus, the AAO microcantilever seems to be a promising candidate for biosensors. The fabrication method proposed here offers more freedom than conventional methods. For example, AAO microcantilevers can be fabricated with thicknesses up to hundreds of micrometers. Figure 4(a) shows ultra-thick AAO microcantilevers of 100μm in thickness. The spring constant of a microcantilever increases in proportion to the cube of the thickness of the beam. Therefore, such thick microcantilevers are favorable for applications which require very stiff beams, i.e., they can operate at much higher frequencies. Note that these structures have vertical sidewalls which are hard to obtain with conventional microcantilever fabrication methods. The fabrication process of AAO microcantilevers also guarantees structural versatility at a larger scale. Such versatility is demonstrated in Figures 4(b) and 4(c) with rectangular and triangular loop microcantilevers. The former is routinely adopted in the microcalorimetric detection, while the latter is found in most AFMs. The nanochannels of an AAO cantilever can be made with only one end open. A sensing material with a special affinity to a specific analyte can be deposited inside the bottom of the nanochannels. A strand of DNA may attach to the sensing material without the interference of other DNAs, which is hard to achieve with microcantilevers with flat surfaces. The enormous difference in the surface area between the upper and the lower surfaces may create other interesting applications. In summary, we have reported microcantilevers which have a radically different material and structure, as well as fabrication method. They are made of alumina, and have hexagonally-arranged parallel channels tens of nanometers in diameter. They can provide a surface area several orders of magnitude larger than those of conventional microcantilevers. Precise control of the dimensions of the nanochannels is made possible through the versatile fabrication method, resulting in fine tuning of the physical properties. This fact gives us an additional handle for the design of microsensors. These revolutionary microcantilevers are made possible by combining photolithography with nanotemplates. This technique should greatly expand our ability to fabricate other 2-D structures with nanochannels, and has great potential to be the general fabrication method of various NEMS devices in the years to come. Professor Kun-Hong Lee Department of Chemical Engineering Tel: +82-54-279-2271 Fax: +82-54-279-8298 E-mail: ce20047@postech.ac.kr Professor Sangmin Jeon Department of Chemical Engineering Tel: +82-54-279-2329 Fax: +82-54-279-5528 E-mail:jeons@postech.ac.kr Professor Hyun Chul Park Department of Mechanical Engineering Tel: +82-54-279-2167 Fax: +82-54-279-5899 E-mail: hcpark@postech.ac.kr Professor Woonbong Hwang Department of Mechanical Engineering Tel: +82-54-279-2174 Fax: +82-54-279-5899 E-mail: whwang@postech.ac.kr

Graphene Nanoribbon Spin-Valve Device (2008.6.15)

Spin-valve devices are a key component of a magnetoresistive random access memory. Mr. Woo Youn Kim and Professor Kwang Soo Kim of Department of Chemistry of POSTECH predicted supermagnetoresistance in a graphene nanoribbon device, the article of which has appeared in Nature Nanotech (3, 408-412, 2008). The reported graphene nanoribbon spin-valve device shows extremely large magnetoresistance (ten thousand times larger than that of conventional devices), which promises high speed access, and good sensitivity. The striking enhancement originates from the peculiar symmetry of band structures of graphene nanoribbon in addition to the spin symmetry. The characteristic symmetry of the graphene band structure plays the role of a spin filter in perfectly transmitting the spin current in the case of symmetric band alignments between both ends of the nanoribbon and in completely forbidding the spin current in the case of orthogonal symmetric alignments. This phenomenon is highly contrasted to conventional spin-valves of giant magnetoresistance (GMR) or tunneling magnetoresistance (TMR) which utilizes only the spin symmetry and antisymmetry. The discovery of the GMR phenomenon was awarded the Nobel Prize in Physics last year. The present supermagnetoresistance ideally approaches to the infinite. Thus, the predicted new physics would open a new pathway much beyond the current limit of spin-valve devices. Professor Kwang Soo Kim Department of Chemistry Tel: +82-54-279-2110 Fax: +82-54-279-8137 E-mail: kim@postech.ac.kr

Digital Non-Volatile Polymer Memory Device Developed:An Efficient, Low-Cost Means of Permanent Data Storage (2008.4.15)

Improvements in performance and reductions in cost of silicon-based nonvolatile memories, such as flash random access memory (flash-RAM), have rendered floppy discs and many other forms of portable storage obsolete. But for many uses that require data to be written only once, such as archiving and security applications, their cost effectiveness is limited. So-called write-onceread- many (WORM) memories made from low-cost polymer materials could provide a solution. The devices, developed by Professors Moonhor Ree, Ohyun Kim, Su-Moon Park and their research teams, are based on hyperbranched copper phthalocyanine (HCuPc) polymer. The starting material of the HCuPC polymer is copper phthalocyanine (CuPc) which is a common organic semiconductor. The CuPc material, which is usually grown by vacuum deposition techniques, does not usually exhibit switching behaviour that could be used for data storage. But when fabricated from solution to form thin polymer films, the HCuPC reveals electrical switching characteristics. The HCuPc polymer films initially exhibited a high-conductivity state (ON-state). The conduction processes within the device were found to be dominated by hole injection rather than electron injection. The active layer in the devices is short-circuited during the ON-state. Specifically, when a voltage of just over 2.5 V is applied across such an HCuPC film sandwiched between indium-tin-oxide and gold contacts, it switches from the ON-state to low conductivity state (OFF-state). This switching-OFF process of the devices is influenced by the magnitude of the injection current. This switching-OFF process is thought to be governed by the rupture of filaments which takes place when a voltage greater than the turn-off voltage is applied. The conductivity of the OFF-state was found to be more than millions of times smaller than the ON-state, making it easy to read the state of a device. And once switched to the OFF-state the devices remained permanently in that state, even when tested again a full year later. The simplicity of the devices and the fact that the films of which they are based can be fabricated from solution, not only makes them potentially much cheaper than silicon-based memories, it could enable them to store much larger amounts of data more efficiently. The density of the data that can be stored in silicon-based memories can only be improved by making the size of individual memory cells smaller. This is because they can only be fabricated in two-dimensions on the surface of a silicon chip. In contrast, because the HCuPC films are fabricated from solution, it should be possible to build 3-dimensional (3D) arrays of devices by spincoating or dip-coating multiple layers, to achieve very high storage densities. Overall, the HCuPc devices exhibited excellent WORM memory characteristics. These properties open up the possibility of a low-cost mass production of highly dense, and very stable digital non-volatile WORM memory devices. These research results were published in the journal Advanced Materials (volume 20, 1766-1771, 2008), and further highlighted by Nature Asia Materials (June 4, 2008). Professor Moonhor Ree Department of Chemistry Tel: +82-54-279-2120 Fax: +82-54-279-3399 E-mail: ree@postech.edu Professor Su-Moon Park Department of Chemistry Tel: +82-54-279-2102 Fax: +82-54-279-3399 E-mail: smpark@postech.edu Professor Ohyun Kim Department of Electronic and Electrical Engineering Tel: +82-54-279-2215 Fax: +82-54-279-2903 E-mail: ohkim@postech.edu

X-Ray Decreases Surface Tension of Water (2008.5.29)

X-ray imaging shows in real-time and in in-vivo that intensive irradiation of x-rays significantly decreases the surface tension of water in droplets and capillary tubes. X-rays are used in a variety of experimental studies of liquids, in particular, of water. The present synchrotron sources allow intense irradiation, possibly enabling to alter the basic property of surface tension. In spite of its potential impact, this issue has been still unexplored. In 1896 Wilson roughly showed that x-rays ionized the air molecules, and the electrons and ions created cloud nuclei in a cloud chamber. His cloud chamber was used to visualize ionizing radiation and opened up modern quantum physics. The x-ray-induced ionization of matter was studied in quantum physics through photoelectric effect and Compton scattering. In soft-matter physics, it was also known that electrostatic potential changes surface tension, as found by Lippmann in 1873. However, no one has ever observed the radiation effects on the surface tension of water. Professor Jung Ho Je and his research team from Korea, Switzerland, and Taiwan for the first time observed that xray- induced charge build-up leads to a large decrease of the effects of natural surface tension of water in droplets and capillary tubes. The team discovered the x-ray-induced surface tension reduction from direct experimental observations with phase contrast microradiology. The evidence was obtained with three different types of experiments at the 7B2 beamline of the Pohang Light Source (2.5 GeV, 150 mA storage ring in Pohang, Korea). Spatially-coherent synchrotron x-rays in the photon energy range 10-60 keV were used to irradiate water molecules and simultaneously to image the induced effects. They also developed a general physical model to link surface tension and surface ionization by unifying the related equations suggested by Lippmann and Rayleigh. This model successfully described the experimental observations that x-ray photons, by ionizing water molecules and creating charges, change the surface tension of water. The overall results showed a fundamental connection between ionizing radiation and surface tension. The team addressed that “the so far undetected phenomenon is explained by ionization and surface charge creation and can potentially affect many x-ray experimental techniques as well as different natural events.” In particular, this striking phenomenon might be an important issue due to current development of x-ray free-electronlasers with unprecedented brilliance. The research was published in May issue of Physical Review Letters, entitled, “Decreased surface tension of water by hard-x-ray irradiation”. Professor Jung Ho Je Department of Materials Science and Engineering Tel: +82-54-279-2143 Fax: +82-54-279-2992 E-mail:jhje@postech.ac.kr