Press releases on Research & Outcome
▣ Prof. Jung Ho Je: Improved Understanding of “Vortex Rings” Found in Typhoons and Nebulae


lae. The POSTECH research team of Prof. Jung Ho Je and Dr. Ji San Lee in the department of materials science and engineering announced on the 6th that it discovered the principle by imaging the formation of vortex rings at the moment of the drop impact using an ultrafast X-ray microscope. 
 The research outcome has been published in the latest issue of Nature Communications, an academic journal affiliated with Nature. The team vividly captured how small vortices are momentarily formed onto the walls of a water drop when it falls on the surface of liquid through a X-ray microscope. 
 If the Ohnesorge number is small enough, the researchers found, vortices were formed from the energy transfer by capillary waves. The research team suggested the Ohnesorge number as a new standard to predict the formation of vortices. The Ohnesorge number is a constant in hydrodynamics that relates the viscous forces to inertial and surface tension forces. 
 The POSTECH team also found out for the first time that vortex rings can be formed in succession and revealed the dynamics, angular velocities and detailed spiral shapes of vortex rings. 
 Prof. Je said, “Applying this principle, we will be hopefully able to better understand natural disasters and improve the prediction accuracy, and also raise process efficiency in the industries using fluid transport materials.”


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Origin and dynamics of vortex rings in drop splashing 
- Nature Communications 6, 8187 (2015) (IF : 10.742)
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▣ Prof. Moon-Ho Jo: Increased Possibility of the Development of Advanced Two-Dimensional Optical Devices (Controlling rotation angles between atom layers)


A POSTECH research team led by Prof. Moon-Ho Jo in the department of materials science and engineering (also in the division of advanced materials science) discovered a new optical feature by grafting different two-dimensional materials and increased the possibility of developing advanced two-dimensional optical devices. 
 Prof. Jo’s research team adjusted the relative orientation between MoS2 and WS2 and materialized light absorption and emission in MoS2/WS2 monolayer stacks. By having succeeded in controlling optical features of two-dimensional materials, the research team opened up the possibility of developing advanced optical devices including two-dimensional illuminators, laser machines, and   photo detectors. 
 Atomic two-dimensional materials have multiple atom layers, and when they are mono-layered, different physical properties are observed. The best example is graphene that is a single layer of a carbon atom. 
 Such two-dimensional materials are expected to become next-generation semiconductors thanks to their excellent physical and chemical properties. MoS2 and WS2 are also layered compounds that could be torn off into single layers, and prospective candidates for the next-generation semiconductor that can replace graphene. 
 Two-dimensional materials have their own orientation. When two atom layers are stacked and their orientations are tuned to each other, their interactive reponses strengthen and increase the applicability of the materials as electronic or optical devices. Especially, such materials despite nano-sized thickness can absorb over 10 percent of light and considerably increase the performance of optical devices. The researchers succeeded in creating a new process and having the rotation angles in MoS2/WS2 monolayer stacks be tuned to each other. 
 The monolayer stacks formed by the team produce a high level of electron momentum and the efficiency of electron transfer. To bring about this result, the researchers adopted an unconventional method to make the structure of the stacks. 
 Conventionally, mono-layered materials used to be stripped through the mechanical method and physically transcribed to make a stacked structure. However, relative orientation between monolayers were misfit and interlayer reponses were too weak and limited. 
 The POSTECH team unconventionally adopted the large-area chemical vapor deposition*. It made sure that the rotational angle between atom layers was zero and orientations were matched. 
   *Chemical Vapor Deposition is a method to synthesize a material onto a chosen substrate using chemical reactions between vapors produced when materials are heated at high temperatures. 
 Prof. Jo said, “We demonstrated for the first time that tuning orientations between two atom layers can control the properties of light absorption and emission at the new wavelengths. Therefore, a variety of photophysical phenomena are expected to be better researched.” He added, “Since such materials absorb and emit visible light that can be detected by human eyes, the performance of two-dimensional optical devices could be drastically improved.” 
 The research outcome was published in the June 23rd online issue of Nature Communications (IF 11.470). This research was jointly conducted with Prof. Choi Hyungyong’s research team in the school of electrical and electronic engineering at Yonsei University and Prof. Ji Hoon Shim’s team of POSTECH department of chemistry. 


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Interlayer orientation-dependent light absorption and emission in monolayer semiconductor stacks 
- Nature Communications 6, 7372 (2015) (IF : 10.742)
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▣ Prof. Tae-Woo Lee: New Technology for the Commercialization of Flexible AMOLED Displays (Adding flexibility to glass encapsulation process)


 A POSTECH research team has developed a technology that will enable the commercialization of flexible AMOLED displays that are known as dream displays thanks to their flexibility and collapsibility and attracted attention from the related industries. 
 The novel technology can not only protect flexible AMOLED displays that are vulnerable to water and oxygen but also produce crucial flexibility, overcoming the glass capsulation process that is not available for flexible displays and allowing mass production with reduced cost. 
 Prof. Tae-Woo Lee and Ph.D. student Min-Ho Park of POSTECH Materials Science and Engineering developed flexible encapsulation method (Flex Lami-capsulation) that effectively protects organic matter and is available for flexible displays, and published it in Advanced Materials, a prestigious academic journal in materials science. 
 Prof. Lee’s team maintained the reliability of the conventional glass encapsulation and the properties of organic electronic devices, and at the same time found out a new way to increase flexibility that can be directly applied to the roll-to-roll process for mass production. 
 The novel technology could possibly be used for a wide range of flexible electronics that require high flexibility and collapsibility including solar cells, memory devices and lighting fixtures as well as next-generation displays. 
 Prof. Tae-Woo Lee who led the research said, “It still maintains the strengths of glass encapsulation and secures flexibility. Since low-cost mass production could be available, the new technology will be able to advance the commercialization of flexible displays and lighting.”  

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Flexible Lamination Encapsulation 
- Advanced Materials 27, 4308 (2015) (IF : 15.409)
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▣ Prof. Jang Sik Lee: New Technology for Chitosan-Based Resistive Switching Memory


A domestic research team discovered a new way to produce memory devices using chitosan that could be earned from crab shells. Since the material is eco-friendly and biocompatible, it could likely be applied to future memory devices for video pills, artificial muscles and artificial organs.

Prof. Jang Sik Lee of POSTECH Materials Science and Engineering announced on the 12th that his team had developed a biocompatible memory device using chitosan that was extracted from the shells of crustacean including crabs and shrimps.

Chitosan is formed by deacetylating chitin that is the main component of crabs’ or shrimps’s shells. It does no harm to human body, and naturally decomposes. Since the material is a byproduct from processing seafood, the production cost is low.

Prof. Lee’s team used chitosan and materialized resistive switching memory that is regarded as a next-generation memory. Resistive switching memory draws on the phenomenon that resistance changes depending on voltage and it is capable of controlling the level of resistance.

The newly developed memory consists of platinum (Pt), chitosan, and silver (Ag). Platinum, silver and chitosan were respectively used as its lower electrode, upper electrode and resistant-switching material. Chitosan-based resistive switching memory displayed low operating voltage and excellent information storage ability, and it maintained high performance even after repeatedly storing and erasing information on it.

Prof. Lee said, “We aimed at making a memory that is safe and eco-friendly, and we could develop a biocompatible memory based on chitosan from crab shells.”

The research outcome was published in the December 16th online issue of ACS NANO, which is an international academic journal in nano science. 

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Resistive Switching Memory Based on Bioinspired Natural Solid Polymer Electrolytes 
- ACS NANO 9, 419 (2015) (IF : 12.033)
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