• Applied Microscopy
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• 제50권
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• pp.27.1-27.6
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• 2020

Transition metal dichalcogenides (TMD), which is composed of a transition metal atom and chalcogen ion atoms, usually form vacancies based on the knock-on threshold of each atom. In particular, when electron beam is irradiated on a monolayer TMD such as MoS₂ and WS₂, S vacancies are formed preferentially, and they are aligned linearly to constitute line defects. And then, a hole is formed at the point where the successively formed line defects collide, and metal clusters are also formed at the edge of the hole. This study reports a process in which the line defects formed in a monolayer WS₂ sheet expends into holes. Here, the process in which the W cluster, which always occurs at the edge of the formed hole, goes through a uniform intermediate phase is explained based on the line defects and the formation behavior of the hole. Further investigation confirms the atomic structure of the intermediate phase using annular dark field scanning transition electron microscopy (ADF-STEM) and image simulation.

• Journal of the Korean Physical Society
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• 제81권
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• pp.133-138
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• 2022

Using the first principles calculations, we investigated the strain dependent magnetic properties of the 1T-VSe₂ monolayer (up to ± 3%). We obtained a metallic band structure, and this feature was preserved under both compressive and tensile strain. The pristine system had a magnetic moment of 0.9 µ_B/unit cell and decreased to 0.68 µ_B/unit cell under - 3% compressive strain whereas it was increased to 1.03 µ_B/unit cell under + 3% tensile strain. The 1T-VSe₂ monolayer had an in-plane magnetic anisotropy with a value of - 0.48 meV/cell. The in-plane anisotropy features were maintained in both compressive and tensile strains. The orbital resolved magnetic anisotropy indicated that the V atom contributed to the perpendicular magnetic anisotropy while the Se atom had an in-plane anisotropy. We found that the Se dominated the anisotropy. We also calculated the temperature dependent Curie temperature (T_C). The pristine structure had a T_C of 260 K, and the strain effect enhanced the T_C. Particularly, the compressive strain affected further the exchange parameter resulting in substantial enhancement of the Curie temperature where a T_C of 570 K was achieved at - 3% strain. Our finding regarding the strained VSe₂ could help for further investigation in spintronics and straintronics applications.

• Current Optics and Photonics
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• 제4권4호
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• pp.353-360
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• 2020

Adopting an elaborately designed reflective objective consisting of four mirrors, we have developed a rotating-polarizer-type microspot spectroscopic ellipsometer (SE) with an ultra-small spot size. The diameter of the focused beam, whether evaluated using a direct-image method or a knife-edge method, is less than 8.4 ㎛. After proper correction for the polarizing effect of the mirrors in the reflective objective, we unambiguously determine the dispersion of the complex refractive index and the thickness of monolayer MoS₂ using the measured microspot-spectroellipsometric data. The measured ellipsometric spectra are sensitive enough to identify small variations in thickness of MoS₂ flakes, which ranged from 0.48 nm to 0.67 nm.

• Journal of Adhesion and Interface
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• 제22권2호
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• pp.57-62
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• 2021

Recently, the atomically thin two-dimensional transition-metal dichalcogenides (TMDs) have received considerable attention for the application to next-generation semiconducting devices, owing to their remarkable properties including high carrier mobility. However, while a technique for growing graphene is well matured enough to achieve a wafer-scale single crystalline monolayer film, the large-area growth of high quality TMD monolayer is still a challenging issue for industrial application. In order to enlarge the size of single crystalline MoS₂ monolayer, here, we systematically investigated the effect of process parameters in molten-salt-assisted chemical vapor deposition method. As a result, with optimized process parameters, we found that single crystalline monolayer MoS₂ can be grown as large as 420 ㎛.

• Proceeding of EDISON Challenge
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• 제4회(2015년)
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• pp.367-370
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• 2015

Monolayer transition metal dicalcogenide (TMDC) materials are currently attracting extensive attention due to their distinctive electronic, transport, and optical properties. For example, monolayer $MoS_2$ exhibits a direct band gap in the visible frequency range, which makes it an attractive candidate for the photocatalytic water splitting. For the photoelectrochemical water splitting, the appropriate band edge positions that overlap with the water redox potential are necessary. Similarly, appropriate band level alignments will be crucial for the light emitting diode and photovoltaic applications utlizing heterojunctions between two TMDC materials. Carrying out first-principles calculations, we here investigate how the band edges of $MoS_2$ can be adjusted by surface ligand functionalization. This study will provide useful information for the realization of ligand-based band engineering of monolayer $MoS_2$ for various electronic, energy, and bio device applications.

• Journal of Magnetics
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• 제12권2호
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• pp.59-63
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• 2007

Ferromagnet/Semiconductor heterostructures have attracted much attention because of their potential applications in spintronic devices. We investigated the electronic structures and magnetism of monolayer Fe on $CuGaSe_2(001)$ by using the all-electron full-potential linearized augmented plane-wave method within a generalized gradient approximation. We considered the monolayer Fe deposited on both the CuGa atoms terminated (CuGa-Term) and the Se atom terminated (Se-Term) surfaces of $CuGaSe_2(001)$. The calculated magnetic moment of the Fe atom on the CuGa-Term was about $2.90\;{{\mu}_B}$. Those of the Fe atoms on the Se-Term were in the range of $2.85-2.98\;{{\mu}_B}$. The different magnetic behaviors of the Fe atoms on two different surfaces were discussed using the calculated layer-projected density of states.

• Proceeding of EDISON Challenge
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• 제3회(2014년)
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• pp.511-513
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• 2014

$MoS_2$ monolayer에 Au 원자를 흡착시켰을 때 가장 안정한 위치를 찾아 내기위한 연구를 수행하였다. 이를 위하여 $MoS_2$ $1{\times}1$ unit cell 위의 on-top, bridge, hollow 위치에 Au 원자를 놓고 DFT 기반 제일원리 계산을 통하여 최적화된 구조에서의 에너지를 계산, 비교하였다. 그 결과 S 원자 위에 Au 원자가 흡착 되었을 때 가장 안정한 구조를 이루는 것을 알 수 있었다.

• Proceeding of EDISON Challenge
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• 제6회(2017년)
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• pp.417-423
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• 2017

Monolayer $MoS_2$의 metastable phase인 T' phase가 stable한 H phase보다 안정해지는 조건을 모색하기 위해 substitutional doping을 했을 때와 strain을 걸어주었을 때의 에너지 차이를 DFT 방법으로 계산하였다. Doping을 했을 때와 strain이 있을 때 T' phase와 H phase의 에너지 차이가 감소함을 확인하였으나 H phase보다 T' phase가 안정해지는 조건을 찾지는 못하였다. 하지만 이 방법을 기존의 alkali adsorption 방법과 병행하여 기존 방법의 단점을 보완할 수 있을 것을 기대해 볼 수 있다. 또한 전자구조 분석 중 얻은 dopant의 주기와 족에 따른 경향성은 다른 TMD 물질의 phase engineering을 design할 때 universal한 design rule로서 응용할 수 있음을 기대해 볼 수 있다.

• Proceeding of EDISON Challenge
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• 제4회(2015년)
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• pp.373-377
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• 2015

이 연구에서는 원자 궤도 함수 기반 DFT 전자구조 계산을 이용해서 최근 각광받고 있는 이차원 물질인 MoS2 monolayer의 S 자리와 Mo 자리에 각각 전자가가 다른 원자를 치환하였을 때의 도핑 특성을 Density of States (DOSs)와 밴드구조 등의 전자구조를 통해 분석해 보았다. S자리에 Cl, Si, I, B, C, Mo 자리에는 Hf, Ta, 그리고 Re을 치환해 보았으며 계산 결과 S자리에 Cl을 치환했을 때 가장 얕은 acceptor level (VBM으로부터 0.08 eV)이 형성되었으며, Mo자리에 Re를 치환했을 때에는 resonant state를 형성하였다. 또한 Mo자리에 Ta를 치환했을 ? 가장 얕은 donor level (CBM으로부터 0.02 eV)가 형성되었다.

• Journal of the Korean Crystal Growth and Crystal Technology
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• 제31권4호
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• pp.154-158
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• 2021

The optical transmittance of Mn-doped SnO₂ monolayer film increased gradually from 80.9 to 85.4 % at 550 nm wavelengths upon increasing the O₂/Ar+O₂ concentration rate from 0 to 7.9 % and the band gap energy changed from 3.0 to 3.6 eV. The resistivity tended to decrease from 3.21 Ω·cm to 0.03 Ω·cm, reaching a minimum at 2.7 %, and then gradually increased from 0.03 to 52.0 Ω·cm at higher O₂/Ar+O₂ gas concentration ratio. Based on XPS spectra analysis, the Sn 3d_5/2 peak of Mn-doped SnO₂ single layer shifted slightly from 486.40 to 486.58 and O_1s peak also shifted from 530.20 to 530.33 eV with increase the O₂/Ar+O₂ concentration ratio. Therefore, the XPS spectra results indicate that a multiphase with SnO and SnO₂ coexisted in the sputtered Mn-doped SnO₂ monolayer film.