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Professor Chiu Po-Wen’s Research on Structural Defects in Two-dimensional Materials is Published in Nature Communications
Three student research groups working under the guidance of Professor Chiu Po-Wen, Department of Electrical Engineering & Institute of Electronic Engineering, have recently had reports on their innovative research published in the internationally renowned British journal Nature Communications. The group led by Yeh Chao-Hui developed a two-dimensional monolayer WSe2 monocrystalline; the group led by Teng Po-Yuan converted a desktop CD burner into a low-energy laser engraving machine which uses the photochemical reaction of ozone to reduce defects in the high-density lattice points of single-crystal monocrystalline; and the third group led by Lin Yung-Chang used a scanning transmission electron microscope with spherical aberration correction at low voltage to observe the kinetic reactions of lattice point defects, thereby discovering the trefoil structure of the two-dimensional lattice defects. For more details see the latest issue of Nature Communications (DOI: 10.1038/ncomms7736).
Only a few atoms thick, the two-dimensional semiconductor material has a number of advantages over existing electronic and optoelectronic components, and is the new darling of electronic and optoelectronic nano-devices. Chiu points out that the discovery of two-dimensional monolayer graphene could be separated from its three-dimensional base material and has set off a wave of research on two-dimensional material. This has currently been extended from graphene to transition metal dichalcogenides (TMD) with the characteristics of semiconductors. Such two-dimensional material has a thickness of only 1 to 3 atoms, a plane size that can be infinitely extended, and does not have a dangling bond, so that when it contacts the base plate or other material it forms a van der Waals coupling pattern. Moreover, their unique dimensions and crystal structure allow these materials to be made into electronic or optoelectronic components with unique characteristics. For example, transistors made from this material consume very little power, making them highly suitable for wearable devices. Also, the structure of three-dimensional silicon-based transistors has made it possible to overcome the short channel effect. Finally, due to its flexibility and responsiveness, semiconductor and photovoltaic manufacturers can study this type of novel two-dimensional material without investing a lot of resources.
Since TMDs have a thickness of only three atoms, any form of defect structure will be directly reflected in their energy band structure, affecting the transmission of charge carriers and the photoelectric effect. Therefore, understanding the structural defects of TMDs is the first step in controlling their energy band structure and physical properties. In this research one can start by using chemical vapor deposition to grow a TMD such as monocrystalline WSe2. Then one can use photochemical reactions to make the monocrystalline WSe2 produce high-density selenium atom vacancies. While observing these lattice defects, because the heating of the electron beam causes the selenium atoms to move, when the vacancies of the selenium atoms undergo polymerization they appear in the lowest energy form. As a result, the selenium atoms rotate in a triple rotation symmetrical pattern with six rings that form into a new symmetrical structure with eight rings. The really amazing part is that when these defects reach the right density, this new eight-ringed symmetrical structure polymerizes into a symmetrical trefoil pattern.
"Keen observation is the key to new scientific discoveries, and seamless collaboration is the shortcut to success!" exclaims Prof. Chiu. This study was conducted in conjunction with research teams from Finland, Japan, and the National Taiwan University of Science and Technology. All aspects of the research—the growth of the material, the development of the laser-induced electrochemical reaction apparatus, the theoretical computations, the minute observations made with an electron microscope—were carried out using a seamless division of labor, thereby bringing about remarkable results. Such smooth cooperation is built upon a long-term partnership, and keen observation comes from a wealth of knowledge accumulated through long-term study and experience.
Prof. Chiu’s research team in Tokyo celebrating the completion of their research. From right to left: Yeh Chao-Hui, Lin Yung-Chang, Chiu Po-Wen, and Kazutomo Suenaga.

Prof. Chiu’s research team in Tokyo celebrating the completion of their research. From right to left: Yeh Chao-Hui, Lin Yung-Chang, Chiu Po-Wen, and Kazutomo Suenaga.


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