-
- The Thinnest Photodetector in the World
- The Thinnest Photodetector in the World - Graphene-based device could accelerate the development of 2D photoelectronics - The Center for Integrated Nanostructure Physics, within the Institute for Basic Science (IBS) has developed the world's thinnest photodetector, that is a device that converts light into an electric current. With a thickness of just 1.3 nanometers - 10 times smaller than the current standard silicon diodes - this device could be used in the Internet of Things, smart devices, wearable electronics and photoelectronics. This 2D technology, published on Nature Communications, uses molybdenum disulfide (MoS2) sandwiched in graphene. Graphene is a fantastic material: It's conductive, thin (just one-atom thick), transparent and flexible. However, since it does not behave as a semiconductor, its application in the electronics industry is limited. Therefore, in order to increase graphene's usability, IBS scientists sandwiched a layer of the 2D semiconductor MoS2 between two graphene sheets and put it over a silicon base. They initially thought the resulting device was too thin to generate an electric current but, unexpectedly, it did. "A device with one-layer of MoS2 is too thin to generate a conventional p-n junction, where positive (p) charges and negative (n) charges are separated and can create an internal electric field. However, when we shine light on it, we observed high photocurrent. It was surprising! Since it cannot be a classical p-n junction, we thought to investigate it further," explains YU Woo Jong, first author of this study. To understand what they found, the researchers compared devices with one and seven layers of MoS2 and tested how well they behave as a photodetector, that is, how they are able to convert light into an electric current. They found that the device with one-layer MoS2 absorbs less light than the device with seven layers, but it has higher photoresponsitivity. "Usually the photocurrent is proportional to the photoabsorbance, that is, if the device absorbs more light, it should generate more electricity, but in this case, even if the one-layer MoS2 device has smaller absorbance than the seven-layer MoS2, it produces seven times more photocurrent," describes Yu. ▲ (top) Devices with one-layer and seven-layer MoS2 were built on top of a silicon base and compared. Dielectric constants responsible for the difference in electrostatic potentials are shown in parenthesis. (bottom) The device with one-layer MoS2 (inside the violet box) showed better performance in converting light to electric current than the seven-layer device (inside the pink box). Why is the thinner device working better than the thicker one? The research team proposed a mechanism to explain why this is the case. They recognized that the photocurrent generation could not be explained with classical electromagnetism, but could be with quantum physics. When light hits the device, some electrons from the MoS2 layer jump into an excited state and their flow through the device produces an electric current. However in order to pass the boundary between MoS2 and graphene, the electrons need to overcome an energy barrier (via quantum tunnelling), and this is where the one-layer MoS2 device has an advantage over the thicker one. The monolayer is thinner and therefore more sensitive to the surrounding environment: The bottom SiO2 layer increases the energy barrier, while the air on top reduces it, thus electrons in the monolayer device have a higher probability to tunnel from the MoS2 layer to the top graphene (GrT). The energy barrier at the GrT/MoS2 junction is lower than the one at the GrB/MoS2, so the excited electrons transfer preferentially to the GrT layer and create an electric current. Conversely, in the multi-layer MoS2 device, the energy barriers between GrT/MoS2 and GrB/MoS2 are symmetric, therefore the electrons have the same probability to go either side and thus reduce the generated current. Imagine a group of people in a valley surrounded by two mountains. The group wants to get to the other side of the mountains, but without making too much effort. In one case ( the seven-layers MoS2 device), both mountains have the same height so whichever mountain is crossed, the effort will be the same. Therefore half the group crosses one mountain and the other half the second mountain. In the second case (analogue to the one-layer MoS2 device), one mountain is taller than the other, so the majority of the group decide to cross the smaller mountain. However, because we are considering quantum physics instead of classical electromagnetism, they do not need to climb the mountain until they reach the top (as they would need to do with classical physics), but they can pass through a tunnel. Although electron tunneling and walking a tunnel in a mountain are very different of course, the idea is that electric current is generated by the flow of electrons, and the thinner device can generate more current because more electrons flow towards the same direction. ▲ Mechanism to explain why the device with one-layer MoS2 generates more photocurrent than the seven-layer MoS2 one. (top) In the one-layer device MoS2 (right), the electron (red circle) has a higher probability to tunnel from the MoS2 layer to the GrT because the energy barrier (white arch) is smaller in that junction. In the seven-layers MoS2 device (left) instead, the energy barrier between MoS2/GrT and MoS2/GrB is the same so electrons do not have a preferred direction flow. More energy is generated in the one-layer MoS2 device because more electrons flow in the same direction. (bottom) Imagine that people want to cross a mountain without too much effort. If the mountains have different height (right), more people choose to climb (or better, to tunnel) the small mountain, while if the mountains have the same height (left), they do not have a preferred route. (Graphics modified from Freepiks) Actually, when light is absorbed by the device and MoS2 electrons jump into an excited state, they leave the so-called holes behind. Holes behave like positive mobile charges and are essentially positions left empty by electrons that absorbed enough energy to jump to a higher energy status. Another problem of the thicker device is that electrons and holes move too slowly through the junctions between graphene and MoS2, leading to their undesired recombination within the MoS2 layer. For these reasons, up to 65% of photons absorbed by the thinner device are used to generate a current. Instead, the same measurement (quantum efficiency) is only 7% for the seven-layer MoS2 apparatus. "This device is transparent, flexible and requires less power than the current 3D silicon semiconductors. If future research is successful, it will accelerate the development of 2D photoelectric devices," explains the professor. Letizia Diamante - References Woo Jong Yu, Quoc An Vu, Hyemin Oh, Hong Gi Nam, Hailong Zhou, Soonyoung Cha, Joo-Youn Kim, Alexandra Carvalho, Munseok Jeong, Hyunyong Choi, Antonio H. Castro-Neto, Young Hee Lee, and Xiangfeng Duan. Unusually efficient photocurrent extraction in monolayer van der Waals heterostructure by tunneling through discretized barriers. Nature Communications (2016). DOI: 10.1038/ncomms13278
-
- 작성일 2017-06-22
- 조회수 2400
-
- BME students were selected as 2017 URP researchers
- This year, 11 undergraduate research teams of SKKU were selected as 2017 URP which is managed by Ministry of Education and Korea Foundation for the Advancement of Science and Creativity. As this result, SKKU ranked first 4times straight. In particular, professor SEO Min A(BME) has been selected for this program 4times straight. Total 6 BME undergraduate students(Lee Yu Kyung(15), Lee Hyun Kyu(15), Lee Jee Kyeong(16), Lee Jeong Min(16), Lee Ju Yeong(16), Hwang On Yu(17)) were selected for this program, and government will support 7~9million won of research support fund to each team for 1 year.
-
- 작성일 2017-06-19
- 조회수 2271
-
- 2017 ISMRM - CHOI Won Min(Integrated M.S.-Ph.D) get the award
- Won Min CHOI(CNIR) was awarded in the 2017 ISMRM 25th Annual Meeting & Exhibition held in Hawaii, US from Apr. 22 to Apr. 27. The ISMRM Magma Cum Laude Award / CHOI Won Min(Integrated M.D-Ph.D / academic advisor: KIM Sung Gi Presentation title: Effect of Osmolality on Dyanmic Glucose Enhanced(DGE) MRI "Congratulations." -
-
- 작성일 2017-05-25
- 조회수 2244
-
- Prof. Ahn Tae kyu team published new paper about perovskite solar cells by deep level transient spectroscopy
- We report the presence of defects in CH3NH3PbI3, which is one of the main factors that deteriorates the performance of perovskite solar cells. Although the efficiency of the perovskite solar cells has been improved by curing defects using various methods, deeply trapped defects in the perovskite layer have not been systematically studied, and their function is still unclear. The comparison and analysis of defects in differently prepared perovskite solar cells reveals that both solar cells have two kinds of deep level defects (E1 and E2). In the one-pot solution processed solar cell, the defect state E1 is dominant, while E2 is the major defect in the solar cell prepared using the cuboid method. Since the energy level of E1 is higher than that of E2, the cuboid solar cell shows higher open-circuit voltage and efficiency.
-
- 작성일 2017-05-22
- 조회수 2189
-
- SKKU Int’l Students Experience Seoul and Korean Culture
- The Office of International Student Services (OISS) held the “SKKU International Student Culture Experience and Exchange Program of the Humanities Social Sciences Campus (HSSC) and Natural Sciences Campus (NSC) - 2017 Sungkyunwan Tour and Seoul Castle Road Walk” as a student activity on the 13th (SAT) of May. More than 50 international students participated in this event. Especially, 30 students from the NSC enjoyed the Seoul castle walk and HSS Campus tour as it was their first visit to the HSSC. After the walking tour, all of the participants enjoyed Korean traditional food (bibimbab, seafood & potato pancake) for lunch. The OISS plans to provide various events for international students at both the Humanities and Social Sciences and Natural Sciences Campus to experience Korea’s unique culture. This event was the start of cultural programs for 2017 and a tour of the NSC and Suwon Hwasung Fortress will be held next time.
-
- 작성일 2017-05-16
- 조회수 2248
-
- Research on New Electro-active Energy Materials(Department of Energy Science)
- Electro-Active Energy Material Lab. of the Department of Energy Science studies various materials related to energy conversion, magnetism, catalyst and electronics. We are targeting the first discovery and global-top performance in electrides as new electro-active materials, focusing on thermoelectric energy conversion materials low-dimensional transition metal dichalogenides from fundamental to applied researches. Followings are the main research topics: 1. Electride Electrides are ionic compounds in which electrons act as anions occupying interstitial real space of crystal structures, not orbitals. Because the electrides shows a low work function and high electron concentration, the materials are of interest for electronics and catalysts. We design and synthesize a new electrides by theoretical and experimental researches for industrial applications. 2. Thermoelectric materials Thermoelectric materials can convert electricity to heat, and vice versa. Utilizing the Seebeck and Peltier effect, thermoelectric refrigeration and power generation are possible, enabling efficient energy consumption. We are developing a world-record high performance in state-of-the-art materials by studying fundamental material physics and material process. 3. Low-dimensional materials We study the low-dimensional transition metal dichalcogenides utilizing a high quality single crystal and epitaxial thin film fabricated in our laboratory. Beyond graphene, we explore a new two-dimensional crystal structure distinct from conventional layered structures.
-
- 작성일 2017-04-18
- 조회수 2167
-
- Prof. Hyun Jung SHIN Developed a New Catalyst which can Produce Clean Energy Sources
- In joint research with Dr. Chang Deuck BAE, a team led by Prof. Hyun Jung SHIN proposed a new catalyst producing hydrogen from water. In principle, electrolysis of water into oxygen and hydrogen can offer a clean, renewable energy resource. The cost-effective and efficient splitting of water is a critical issue in technologies and economies with energy delivery systems that use hydrogen, enabling zero emission of greenhouse gases. A key challenge in the electrocatalyst design is the performance-cost trade-off of using the platinum-group metals as a cathode for the hydrogen evolution reaction (HER). Two different approaches have constituted the major branches of science in this domain of research. One approach is to use less or optimized amounts of precious elements, such as nanoparticulate Pt on carbon supports. The other approach is to find new alloy compositions based on Pt. In exploring new alloy composition, computational simulations are beneficial in the development of previously unknown single-phase systems. Although more complex surface structures involving platinum have exhibited experimental improvements, efficient electrocatalysts consisting of nonprecious elements have recently been more actively studied. Transition metal chalcogenide–based systems, such as MoS2, provide many possibilities for HER because they have unique anisotropic surface/transport properties and provide surfaces with the desired binding energy/site for H+. However, a material equivalent to platinum in terms of the onset potential, the Tafel slope, and the long-term stability has not yet been developed. The activation processes on the catalyst surfaces were believed to be the essential mechanisms for HER, and thus, surface energies matter in the development of HER catalysts with high efficacy. This would be true as for the case of monometallic Pt, and seeking platinum-like surfaces seems rational among cost-effective elements. Recent research activities have been heading in this direction accordingly. However, when nonmetallic catalysts are involved, the charge transfer resistance rather than surface reactions themselves would play a significant role and should be considered. A recent study by Voiry et al. experimentally exhibited that the catalytically inert basal plane of 2H MoS2 can be active by controlling the charge transfer resistance of the system. Beyond identifying platinum-like surfaces, the research team proposes here a novel electrocatalytic concept consisting of two-dimensional (2D) materials in bulk; they refer to this concept as the inorganic bulk layered heterojunction (BLHJ). Our approach is based on a simple fabrication technique for the direct growth of MoS2 on self-supported metallic substrates via sequential gas-phase reactions, which result in the spontaneous formation of dense BLHJ structures via spontaneous sulfidation reactions. These structures contain MoS2 flakes, which are dispersed in the Cu2S matrix (resembling “straw in mud plaster”). The present system not only features distinctive inorganic, dense BLHJ structures, which are difficult to prepare experimentally using any other methods, but also has layered 2D materials as the key anisotropic components that trigger unusual charge transfer processes. The team selected MoS2 as the representative 2D component with the feasibility of anisotropic transport properties. Incorporating the target layered materials into the bulk chalcogenide host with secure contact interfaces between suitable nanoscale junctions in a controlled manner is difficult. They used the sequential gas-phase surface reaction technique for which the reactants, such as Mo or S, are independently delivered into the substrates, so that only the surface-limited reactions occur. Because the substrate metal used for the self-supporting electrode is simultaneously sulfidizable, this growth mode is expected to incorporate a layered system into the bulk chalcogenide host. In the design of the BLHJ, another important consideration is to select two chalcogenide systems that are thermodynamically immiscible at given temperatures. For example, copper as the substrate material and Mo for sulfidation will lead to the desired immiscible phase separation to form an inorganic BLHJ with MoS2 at relatively lower temperatures (<300°C). In addition to the formation of thermodynamically immiscible systems, it is Earth-abundant and cheap. Moreover, the quality of Cu as the electrochemical electrode has been verified in the field of battery, and thus, the possible side effects could be ruled out. This work was published in the online version of Science Advances as of March 31st.
-
- 작성일 2017-04-13
- 조회수 2395
-
- 2017-1 Graduate School Fair finished in success
- 2017-1 Graduate School Fair finished in success SKK General Graduate School held "2017 Graduate School Fair" for students who want to enter graduate school. At Graduate School Fair, 44 departments in Seoul Humanities and Social Sciences Campus and 72 departments/labs in Suwon Natural Sciences Campus participated. At department booth, professors and enrolled seniors individually counselled on admission guide such as department introduction, admission process, scholarship and career.
-
- 작성일 2017-04-11
- 조회수 2362
-
- SKKU is Named No.1 Korea Comprehensive Private University by THE Asia University Rankings 2017
- SKKU has placed 13th in Asia University Rankings 2017 by Times Higher Education, while ranking 1st among Korean comprehensive private universities. SKKU received a total of 60.2 points with the composition of points as; Learning Environment: 56.0, Research: 55.5, Citations: 66.5, International Outlook: 39.6, Industry Income: 88.9. Meanwhile, Seoul National University received 66.1 points, while Korea University and Yonsei University got 52.8 points and 48.8 points respectively. In particular, SKKU received great recognition among universities in Korea in the area of Citations (Research Influence) by having 66.5 points, followed by Ulsan University (60.8), Seoul National University (58.8), Korea University (49.2) and Kyunghee University (48.5). One thing to note from this evaluation is that many Chinese universities received improved results. The editor of THE stated that “as a result of concentrated investment over the past 10 years, many Chinese universities including Fudan University and Shanghai Jiao Tong University have become distinguished. If Chinese universities can secure more outstanding faculties, their research performances, as well as internalization activities, will become more developed.”
-
- 작성일 2017-03-28
- 조회수 2568
-
- 2017 BME Freshmen Orientation(2.16~18)
- 2017 GME OT and Freshmen Orientation were held in 2017. 2. 16(Thu) ~ 2.18(Sat) Prof. Jaeseok Park introduced BME. 2017 Freshmen had an introducing time, also got N-center tour. After brief OT in school, Freshman Orientation was held in Jebu island for 2 nights and 3 days. With variety of games and education, it could be a good chance to be close between freshmen and between seniors and juniors. Congratulations on entering of all BME Freshmen.
-
- 작성일 2017-03-06
- 조회수 2728