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Modifying the angle of “twist” between layers in a bilayer graphene enhances its optical response While conventional, single-layer graphene attracts tremendous attention in material science, double-layer graphene has its own appeal due to its peculiarities. In a new study, scientists from Ajou University, Korea report and explore the cause of a remarkable 60-fold enhanced optical response in a “twisted” bilayer graphene for an optimum orientation of the layers, paving the way for nonlinear optic applications for efficient ultrafast signal processing and developing novel quantum light sources. By changing the angle between the layers of a “twisted” double-layer graphene, scientists observed an enhancement in its optical response that depended strongly on the rotation angle. The property can help create quantum light sources with novel applications. Photo courtesy: ShutterstockFew materials today enjoy the kind of popularity and attention in material science that graphene does. Given its unusual properties, graphene has found a multitude of applications including drug delivery, electronics, energy storage, and ultra-sensitive sensor technology. Graphene is essentially a single layer of carbon atoms arranged hexagonally, giving it the moniker “monolayer” graphene (MLG). However, graphene can also exist in multiple layers; “bi-layer” graphene in particular is known for its unique peculiarities. Scientists from Ajou University, Korea recently explored the optical properties of a peculiar structure called “twisted bilayer graphene” (tBLG), in which two MLGs are stacked on one another at different orientations and discovered that the rotation angle between the layers play a crucial role in enhancing how photons (light particles) interact with the structure to produce higher-energy photons, a phenomenon called “nonlinear optical response.” “Recently, there have been exciting reports about tBLG due to its unique inter-layer interaction absent in MLG. This got me excited about studying a nonlinear optical property of tBLG called “third-harmonic generation” (THG), in which input photons generate output photons three times their energy,” explains Prof. Dong-Il Yeom who led the study, which was published in Light: Science & Applications. The scientists excited the sample with pulsed, infrared laser light, inducing enhanced THG signals whose intensity varied with the “twist angle” and became as high as 60 times for an angle of 12°! Raman measurements and structure calculations revealed an interesting peculiarity in the electronic states called “van Hove Singularity,” which depends on the twist angle, and causes the enhanced THG signal when its energy gap coincides with the energy of three photons of the incident light.Prof. Yeom suggests potential novel applications of the phenomenon. “I think our work could be potentially useful in creating a quantum entangled light source based on nonlinear optics, with applications in quantum sensing or quantum communication for defense and security purposes,” he speculates. Graphene with a ‘twist’ thus holds many promises for a brighter future!ReferenceAuthors:Seongju Ha1, Nam Hun Park1,2, Hyeonkyeong Kim1, Jiseon Shin3, Jungseok Choi1, Sungmin Park1, Ji-Yun Moon1, Kwanbyung Chae1, Jeil Jung3,4, Jae-Hyun Lee1,5, Youngdong Yoo6, Ji-Yong Park1,7, Kwang Jun Ahn1 and Dong-Il Yeom1,7Title of original paper:Enhanced third-harmonic generation by manipulating the twist angle of bilayer graphene Journal:Light: Science & Applications DOI:10.1038/s41377-020-00459-5 Affiliations:1Department of Energy Systems Research, Ajou University 2Korea Research Institute of Standards and Science3Department of Physics, University of Seoul4Department of Smart Cities, University of Seoul5Department of Materials Science and Engineering, Ajou University6Department of Chemistry, Ajou University7Department of Physics, Ajou University *Corresponding author’s email: diyeom@ajou.ac.kr About Ajou UniversityFounded in 1973, Ajou University has quickly grown to become one of the top universities in the Republic of Korea. With over 15,000 students and 50 research centers in diverse fields, Ajou University partakes in the largest national research and graduate education project funded by the Korean Ministry of Education. In line with its recently reformed vision, Ajou University’s goal is to change society by connecting minds and carrying out high-impact research to improve the welfare of people in and outside Korea. Website: https://www.ajou.ac.kr/en/index.do About the authorDong-Il Yeom is a Professor of the Department of Physics and Department of Energy Systems Research at Ajou University. He received his Ph.D. in 2005 in Physics (Optics) from Korea Advanced Institute of Science and Technology (KAIST). Before joining Ajou University in 2008, he worked as a research staff at the University of Sydney for two years. His group is developing fiber lasers and related optical systems for military applications. He also studies the optical properties of 2-dimensional materials and their application to photonic and optoelectronic devices.
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A team of researchers from multiple organizations, led by Prof. Kim Ju-min (Departments of Chemical Engineering and Energy Systems Research) and his team, has developed a microscale flow mixing technique capable of synthesizing more even nanoparticles. The researchers expect their latest discovery to have a wide range of applications, including medical diagnostics, as well as preprocessing of specimens and reaction-based processes in microfluidics. Prof. Kim’s team worked with two other teams, led by Prof. Lee Chang-soo of Chungnam National University and Dr. Lee Sung-sik of ETH Zürich, respectively, to produce a microscale reactor. Their findings were published in a paper entitled, “Gear-shaped micromixer for synthesis of silica particles utilizing inertio-elastic flow instability,” featured on the cover of the February 7 issue of Lab on a Chip. Profs. Kim and Lee and Dr. Lee participated as corresponding co-authors. Hong Sun-ok, a recent graduate of Ajou University with a doctorate in energy systems research and now affiliated with Lotte Chemical, was listed as first author. Microfluidics is increasingly used in a number of applied fields, most notably for engineering point-of-care diagnostic devices that provide fast diagnosis. Efficient mixing of fluids is a crucial prerequisite for the preprocessing and core reactions of specimens in microfluidics. It is also the key performance requirement of micromixers. Given the near impossibility of generating turbulence in micromixers, the inefficient expansion-based mixing method has been preferred thus far. Manual mixing, which relies on conventional fluid dynamics, requires fluid channels with complex designs and complex manufacturing processes. Moreover, as layer-based mixing requires normal-state flows of fluid, the mixed output is prone to sedimentation along the fluid surface. Prof. Kim and the researchers discovered that flow instability, associated with diluted polymeric solutions, increases greatly in serpentine microscale channels that repeatedly contract and expand. They decided to apply this discovery to the development of a new micromixer with significantly greater efficiency. The inertio-elastic micromixer that came out of this discovery offers greater mixing efficiency at a wider range of flow rates. In other words, it is capable of more efficiently mixing a wider variety of fluid materials with greater simplicity. The researchers expect that their new model can be more easily applied to large-capacity micromixers. The researchers indeed applied their new micromixer to the synthesization of silica nanoparticles and confirmed that it was capable of producing particles of a more uniform size distribution (as measured in terms of distribution of particles of different sizes in a given specimen). They also demonstrated that their mixer was able to produce nanoparticles over a long span of time without the fluid channel becoming blocked—a problem common with conventional mixers. Prof. Kim explained: “Our study demonstrates that it is possible to design a micromixer capable of maximizing flow instability of viscoelastic fluids like polymer solutions. Our study is also significant because it demonstrates our technique by applying it to the synthesis of actual nanoparticles with a uniform size distribution.” This research has been made possible in part with support from the National Research Foundation of Korea’s Advanced Center of Excellence and personal research grants.# Captions* Upper left: The serpentine channel developed by Prof. Kim’s team, and a conceptual diagram of how it increases inertio-elastic flow instability to facilitate mixing.* Lower left: Particles mixed in a Newtonian fluid (a) show size distribution with a wide variance and nonspherical shapes (b). Particles synthesized in a dilute polymer solution with the help of inertio-elastic flow instability (c) boast uniform sizes and spherical shapes (d).* Right: Cover of the February 2021 issue of Lab on a Chip featuring Prof. Kim’s study.[Source: https://doi.org/10.1039/D0LC00834F]
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- 작성자OIA
- 작성일2021-03-19
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A team of researchers from Ajou and Seoul National universities has discovered a new way to control the pace of cellular aging. The key anti-aging solution found consists in the reconfiguration of telomeres, often understood as “timers” of cell division, which promises to unlock the secret of aging. The findings of the latest research were published under the title, “Telomeres reforged with non-telomeric sequences in mouse embryonic stem cells” in the February 17 issue of Nature Communications, a prestigious publication on natural sciences, with Prof. Park Dae-chan (Departments of Biological Sciences and Molecular Science and Technology) from Ajou University and Prof. Lee Jun-ho from Seoul National University listed as corresponding authors. Kim Da-eun, currently studying molecular science and technology at Ajou’s graduate school, was also listed as a co-author. The cells that make up the human body continue to divide and multiply to sustain the natural functions of life and growth. At some point in time, however, cells stop dividing, while some genetic information is lost as cells repeat division. Telomeres are protective sheaths found at the tips of chromosomes that serve as a kind of shield against the loss of genetic information during cell division, as telomeres are lost instead of vital genetic information. As telomeres shorten to a certain length or below, cells stop dividing and begin aging. Cells that continue to divide more actively than others, such as reproductive cells, stem cells, and cancer cells, utilize an enzyme known as telomerase to maintain the telomeric lengths and continue to divide almost infinitely. In other words, the continued healthy functioning of cells relies crucially on regulating the lengths of telomeres and the number of times cells divide. An accurate understanding of how telomeres can best be preserved may be essential to effective cancer treatment and anti-aging strategies with minimal or no risk of side effects. Researchers have discovered a mechanism known as alternative lengthening of telomeres (ALT) in certain cancer cells, which maintains telomeric length without utilizing telomerase enzymes. In human cancer cells, however, researchers have so far found telomeres that utilize the ALT mechanism but whose sequences simply follow the general and repetitive pattern.The researchers on Prof. Park’s team have discovered, for the first time in the world, a recombination of telomeres with unique sequences in a mammalian model. A small number of embryonic stem cells from a mouse with its telomerase inactivated went on to activate the ALT mechanism to recover the normal pace of cell division. The cells divided by this ALT mechanism displayed amplified unique sequences. Prof. Park’s team named the new pattern they discovered the mouse template for ALT (mTALT), arguing that it provided clues as to the possible existence of alternative telomeres in human cancer cells as well.The researchers, moreover, employed a multi-omics analysis to trace how the ALT mechanism actually works, subjecting the mouse embryonic stem cells at diverse stages of growth—cells with normal telomeric lengths, cells in the aging stage, cells after ALT activation, etc.—to analyses of genomes, transcriptomes, single-cell transcriptomes, and proteomes. They were thus able to identify and demonstrate the molecular characteristics of ALT cells.The researchers commented: “Our recent study reveals the possible existence of alternative telomeres in human cancer cells. We have demonstrated that the concept of telomeres extends beyond the repeated sequencing structures found at the tips of chromosomes and telomerase, but may also encompass diverse mechanisms that protect the tips of chromosomes.”The research was made possible with support from the Ministry of Science and ICT and the National Research Foundation of Korea’s support programs for new and experienced researchers.Mechanism of telomeric change and ALT activation due to cell division
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- 작성일2021-03-19
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A team of Ajou University researchers has uncovered a new optical phenomenon in graphene, widely regarded as a material that holds the key to the future. The team’s findings are expected to pave the way for developing new applications, including superspeed optical signal processing using two-dimensional materials, quantum communications, and quantum sensing.Prof. Yeom Dong-il (Departments of Physics and Energy Systems Research, pictured left) and his team have discovered that interlaying two stacks of single-layer graphene can produce considerably more nonlinear optic signals. Their findings were published under the title, “Enhanced third-harmonic generation by manipulating the twist angle of bilayer graphene,” in the January 21 online issue of Light: Science & Applications (IF = 13.714, IF (%) = 2.577).The paper lists two of Ajou’s doctoral students, Ha Seong-ju (pictured middle) and Park Nam-hun (pictured right), as first authors. The credit also goes to Profs. Lee Jae-hyun (Department of Materials Science and Engineering), Yoo Young-dong (Department of Chemistry), Park Ji-yong (Department of Physics), and Ahn Kwang-jun (Department of Energy Systems Research), as well as Prof. Jung Je-il (Department of Physics) from the University of Seoul.Graphene, a two-dimensional material well known for its composition of pure carbon atoms, has been garnering much attention around the world for its high conductivity and transparency as well as flexibility—all the properties desired in next-generation materials for electronic and optical diodes. Stacking layers of single-layer graphene at an angle produces Moiré superlattices. Graphene has recently captured researchers’ attention as the angle at which these layers are stacked can produce dramatically different results, including both a superconductor with zero resistance and a perfect insulator. The Moiré pattern refers to superlattices that emerge with much greater frequency than with the original and denser patterns of nets or fabrics layered together at an angle.Prof. Yeom’s team seized upon the nonlinear optic phenomenon that occurred when strong light was projected onto their twisted bilayer graphene (tBLG). The Moiré superlattices of the tBLG, in particular, gave rise to van Hove singularities (i.e., points at which the state density function of electrons peak-like discontinuities) with the state density of electrons. Moreover, the team discovered that, when the energy gap between the van Hove singularities tripled the energy of light being projected, significant enhancement occurred in the third-harmonic generation (THG) efficiency. Based on this finding, the team was able to increase the nonlinear optic signals of the electrically controlled tBLG by as much as 60 times those of single-layer graphene.Prof. Yeom commented: “With this study, we became the first in the world to demonstrate that the angle at which graphene is stacked can be a new significant variable when it comes to controlling and enhancing the nonlinear optic properties of two-dimensional materials. I expect that our findings will contribute to the development of new superspeed optical signal processing techniques or quantum lighting sources that utilize the nonlinear optic properties of two-dimensional materials.”The study was conducted with support from the National Research Foundation of Korea’s Experienced Researcher Support Program and Korea Energy Technology Evaluation and Planning’s Energy Workforce Development Program. Left: An optical microscope image of twisted bilayer graphene. The different colors represent the areas of graphene layers stacked together at different angles of rotation/twisting. Right: The third-harmonic generation image of bilayer graphene.
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