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Prof. Yoo Young-dong discovers new mixed-dimensional heterostructures

Prof. Yoo Young-dong’s team has developed new mixed-dimensional in-plane heterostructures, generating hopes for the invention of a new core material for flexible and transparent electric and energy semiconductors. The team, led by Prof. Yoo (Dept. of Chemistry, pictured) has found a new method for synthesizing mixed-dimensional in-plane heterostructures, and published their findings under the title, “Mixed-Dimensional In-Plane Heterostructures from 1D Mo6Te6 and 2D MoTe2 Synthesized by Te-Flux-Controlled Chemical Vapor Deposition” in Small, an international nanoscience journal. Kim Hyeon-kyeong, currently in the combined postgraduate program at Ajou University, is listed as the first author on this paper that went on to be featured on the journal’s November 26 issue. Chalcogen compounds, used in two-dimensional semiconductors, promise utility as materials for next-generation electric and energy semiconductors for wearable devices and rollable displays as the rollable and transparent compounds possess superior electric and optic properties. However, the significant contact resistance that arises when these compounds and metallic electrodes are combined to create two-dimensional semiconductors has consistently limited compound performance. Prof. Yoo’s team used the chemical vapor deposition method (i.e., depositing films on semiconductors and metals through the vaporization of precursors with heat or plasma) to control the dimensions of the synthetic materials to be created, as the method using is suited to controlling the flux of precursors. As a result, the researchers were able to synthesize mixed-dimensional metal-semiconductor heterostructures. The metallic molybdenum telluride (Mo6Te6) on the first dimension is combined horizontally with the semiconductor-like molybdenum ditelluride (MoTe2) on the second dimension to create these novel structures. The team’s discovery provides new clues to the solution for minimizing contact resistance. A precursor refers to a material that transforms into a desired substance through chemical reactions. The team, moreover, has demonstrated the specific mechanism for synthesizing low-dimensional substances through flux control. When the amount of tellurium (Te) supplied per unit of time remains small, one-dimensional Mo6Te6 is synthesized with molybdenum (Mo) and Te atoms matched in an equal ratio. When the amount of Te supply per unit of time increases, two-dimensional MoTe2 is obtained, with double the amount of Te atoms bonding with Mo atoms. This mechanism can help synthesize not only mixed-dimensional materials, but also either one-dimensional or two-dimensional materials. The diverse low-dimensional materials so synthesized can be applied to electronics, optoelectronics, and catalysts. Prof. Yoo remarked: “The synthesis mechanism we have discovered is simple and expandable, and is therefore expected to help create much more diverse mixed-dimensional heterostructures. I expect these new synthetic structures to become core materials for making flexible and transparent next-generation electronic and energy semiconductors.” The study has been conducted with support from the Ministry of Science and ICT and the National Research Foundation of Korea’s New Researcher Support Programs.

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Team of researchers from Ajou and Konkuk Universities develop sequential polymer doping technology

A team of researchers from Ajou and Konkuk Universities has created a new polymer doping strategy based on their discovery of dynamic and thermodynamic factors affecting polymer doping. Prof. Kim Jong-hyun (Dept. of Applied Chemistry and Biological Engineering and Dept. of Molecular Science and Technology, pictured left) and Prof. Seo Hyung-tak (Dept. of Materials Science and Engineering, pictured right), both from Ajou, teamed up with Konkuk researchers led by Prof. Kim Bong-gi (Dept. of Organic and Nano System Engineering) to conduct the groundbreaking study, which has gone on to appear on the front cover of the December 10 issue of Advanced Materials (IF = 27.398). Professors Kim Jong-hyun, Seo Hyung-tak, and Kim Bong-gi participated as corresponding authors, and Yoon Sang-eun, currently in the combined postgraduate program in the Department of Molecular Science and Technology at Ajou University, was listed as the first author. The title of the study is “Improvement of Electrical Conductivity in Conjugated Polymers through Cascade Doping with Small-Molecular Dopants.” Active research on conjugated polymers for organic semiconductors has led to leaps in the performance of electric charge mobility, but researchers have struggled to achieve the desired high level of conductivity due to the difficulty of creating electric charges through doping that equally affects conductivity. The efficiency of molecular doping is known to be proportional to the energy offset between the highest occupied molecular orbital (HOMO) of the electron-supplying conjugated polymer and the lowest unoccupied molecular orbital (LUMO) of the electron-receiving dopant. In their latest study, the Ajou and Konkuk researchers demonstrated that sequential use of dopants with different LUMOs increases doping efficiency, and were able to achieve the world’s highest level of conductivity for molecular doping—above 600 S/cm—with their strategy. The team has also demonstrated that the low doping efficiency associated with the use of dopants with high energy offsets from conjugated polymers stems from thermodynamics. They discovered that doping efficiency can be dramatically increased when doping is first attempted using a molecular dopant with a low energy offset and superior doping dynamics, followed by the use of a second dopant with a high energy offset. The researchers confirmed that this cascade effect is attributed to the decrease in activation energy needed for molecular doping. Prof. Kim Jong-hyun explained: “Our study provides in-depth insights into the energy orbitals of various molecular dopants, the dynamic and thermodynamic equilibria, and the conductivity of finally doped polymers, as well as an effective polymer doping strategy. Using this kind of strategy can significantly improve the conductivity of organic conductors that has hit an impasse as of late.” The team expects that development of new materials alongside their cascade doping strategy can lead to the discovery of highly flexible and highly elastic organic conductors capable of replacing conventional metal and oxide-film inorganic conductors. <Courtesy of WILEY VCH>

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