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11/28/2016 HYU News > Academics Important News

Title

Unified Model of a Minute World

Professor Cho Jun-hyeong (Department of Physics)

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http://www.hanyang.ac.kr/surl/xr4B

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Professor Cho Jun-hyeong of Department of Physics is interested in the study of low-dimension nanomaterial of one and two dimensional nanostructures formed on the surface of solid matters. Working as an editorial staff of Scientific Reports, a sister magazine of Nature, Cho is the member of the Korean Physical Society, American Physical Society, and the Korean Vaccum Society. Cho's paper, completed with a second editor, Lee Se-ho (Physics, Doctoral program), 'Dimensionality and Valency Dependent Quantum Growth of Metallic Nanostructures: A Unified Perspective', suggests a unitary, simple model that explains the preferred length and thickness of nanowires and nanofilms made by various kinds of metals, by using diameter of the nanostructure and the phenomenon called Friedel Oscillations.
 
Cho embodies himself in the field of nanostructures.
(Photo courtesy of Cho)

The atoms of a solid mass are arranged in a periodical manner. However, there is a phenomenon which breaks this periodicity, called crystallographic defect. For example, if an atom is not present where it should be situated, it is called point defect. In addition, planar defect occurs when many atoms do not exist in a surface form. Nanowires that are covered in Cho’s paper have point defect from a certain place of their infinite length. On the other hand, nanofilms have planar defect from some amount of their infinite width. When defects of a solid mass occur, the electrons of solid matter and the defects interact together, forming a density wave named Friedel Oscillations. Friedel Oscillations are a similar to water waves made when a rock is thrown on the surface of a calm lake.

 
In the study, Cho discovered that nanowires are energetically stable at the length that matches the wavelength of Friedel Oscillations. The period of Friedel Oscillations is determined by the composition and diameter of the nanostructure. Cho found that the preferred length of the nanowire and thickness of nanofilm, called magic length and magic thickness, differentiates depending on the diameter of the nanowire and its metal component. Cho found out that as the diameter of nanowire extended, the period where magic length occurs differs in length in accordance with the type of metal. The period of alkali metals and group IB metals (copper, silver, gold) increased as the diameter of nanowire elongated. In the case of transition metals and groups IIIA to VA metals, the period decreased.
 
The structure of nanomaterials (left) and the magic length of nanowires composed of diverse metals (right).
(Photo courtesy of Cho)

Cho confirmed the structural stability of nanowires by changing their diameters. When the diameter of a nanowire is more than 10Å [Å: angstrom, unit of length equal to 10−10 m], it can be called a nanoisland. If the diameter of the nanowire becomes infinitely large, it will become a nanofilm. “In this study, we found that when the diameter of the nanowire is increased, the vibration period becomes the same as that of the nanofilm, also being saturated,” Cho said. This means that when the diameter of the nanowire becomes larger, the magic length equals the magic thickness of the nanofilm. The reason for this saturation of the oscillation period is because the Friedel Oscillations are the same in the case of the above two systems.

 
There was a need for a comprehensive theory that encompasses studies on nanowires and nanofilms that have been ensuing for the past 30 years, because there was a lack of unified understanding about different magic lengths, and the thickness of nanowires and nanofilms from diverse substances. “I believe that finding new puzzle pieces has a lot of meaning but putting those piled pieces together into a big picture is also very significant,” Cho emphasized. “This research may spur motivation for other research on new nanostructures, since it explained a preferred length and thickness in a uniform approach when low-dimensional nanostructures are formed,” he added.
 
Currently, Cho is handling a joint study with University of Science and Technology of China (USTC) and Zhengzhou University's research teams, as well as continuing theoretical research on different nanostructures. The research plan of Cho’s laboratory is to proceed with a study which combines surface, nano, and topology fields. Not only has Cho achieved great accomplishments in the field of nanostructures, but he is concerned about his students who would lead the scientific domain in the future. “I am trying to offer students a lot of experiences, such as encouraging them to attend academic conferences. I also try to converse with them, because science can advance in that way- through involvement and communication,” he said. Cho thinks what professors, schools, and the government should aim to create suitable atmospheric and foundational provisions for science students for them to focus on their work.
 


Jang Soo-hyun        luxkari@hanyang.ac.kr
 
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