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14. Native point defects and low $p$-doping efficiency in $Mg_2 (Si,Sn)$ solid solutions: A hybrid-density functional study

Author

Ryu, Byungki, Choi, Eun-Ae, Park, Sungjin, Chung, Jaywan, de Boor, Johannes, Ziolkowski, Pawel, Müller, Eckhard, and Park, SuDong

Subjects
Condensed Matter - Materials Science
Abstract

We perform hybrid-density functional calculations to investigate the charged defect formation energy of native point defects in $Mg_2 Si$, $Mg_2 Sn$, and their solid solutions. The band gap correction by hybrid-density functional is found to be critical to determine the charged defect density in these materials. For $Mg_2 Si$, $Mg$ interstitials are dominant and provide unintentional $n$-type conductivity. Additionally, as the $Mg$ vacancies can dominate in $Mg$-poor $Mg_2 Sn$, $p$-type conductivity is possible for $Mg_2 Sn$. However, the existence of low formation energy defects such as $Mg_{Sn}^{1+}$ and $I_{Mg}^{2+}$ in $Mg_2 Sn$ and their diffusion can cause severe charge compensation of hole carriers resulting in low $p$-type doping efficiency and thermal degradation. Our results indicate that, in addition to the extrinsic doping strategy, alloying of $Mg_2 Si$ with $Mg_2 Sn$ under $Mg$-poor conditions would be necessary to enhance the $p$-type conductivity with less charge compensation., Comment: Main: 17 pages (including title, abstract, main, references, figure captions. 4 figures). This manuscript is accepted for publication in JALCOM. The article will be published as Gold Open Access

Published
2020
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15. Counterintuitive example on relation between ZT and thermoelectric efficiency

Author

Ryu, Byungki, Chung, Jaywan, Choi, Eun-Ae, Ziolkowski, Pawel, Müller, Eckhard, and Park, SuDong

Subjects
Physics - Applied Physics, Condensed Matter - Materials Science
Abstract

The thermoelectric figure of merit ZT, which is defined using electrical conductivity, Seebeck coefficient, thermal conductivity, and absolute temperature T, has been widely used as a simple estimator of the conversion efficiency of a thermoelectric heat engine. When material properties are constant or slowly varying with T, a higher ZT ensures a higher maximum conversion efficiency of thermoelectric materials. However, as material properties can vary strongly with T, efficiency predictions based on ZT can be inaccurate, especially for wide-temperature applications. Moreover, although ZT values continue to increase, there has been no investigation of the relationship between ZT and the efficiency in the higher ZT regime. In this paper, we report a counterintuitive situation by comparing two materials: although one material has a higher ZT value over the whole operational temperature range, its maximum conversion efficiency is smaller than that of the other. This indicates that, for material comparisons, the evaluation of exact efficiencies as opposed to a simple comparison of the ZTs is necessary in certain cases., Comment: 12 pages, 2 tables, 2 figures

Published
2020
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20. Hybrid-functional and quasi-particle calculations of band structures of Mg2Si, Mg2Ge, and Mg2Sn

Author

Ryu, Byungki, Park, Sungjin, Choi, Eun-Ae, de Boor, Johannes, Ziolkowski, Pawel, Chung, Jaywan, and Park, SuDong

Subjects
Condensed Matter - Materials Science, Physics - Applied Physics, Physics - Computational Physics
Abstract

We perform hybrid functional and quasi-particle band structure calculations with spin-orbit interaction to investigate the band structures of Mg2Si, Mg2Ge, and Mg2Sn. For all Mg2X materials, where X = Si, Ge, and Sn, the characteristics of band edge states, i.e., band and valley degeneracies, and orbital characters, are found to be conserved, independent of the computational schemes such as density functional generalized gradient approximation, hybrid functionals, or quasi-particle calculations. However, the magnitude of the calculated band gap varies significantly with the computational schemes. Within density-functional calculations, the one-particle band gaps of Mg2Si, Mg2Ge, and Mg2Sn are 0.191, 0.090, and -0.346 eV, respectively, and thus severely underestimated compared to the experimental gaps, due to the band gap error in the density functional theory and the significant relativistic effect on the low-energy band structures. By employing hybrid-functional calculations with a 35% fraction of the exact Hartree-Fock exchange energy (HSE-35%), we overcame the negative band gap issue in Mg2Sn. Finally, in quasi-particle calculations on top of the HSE-35% Hamiltonians, we obtained band gaps of 0.835, 0.759, and 0.244 eV for Mg2Si, Mg2Ge, and Mg2Sn, respectively, consistent with the experimental band gaps of 0.77, 0.74, and 0.36 eV, respectively., Comment: 23 pages, including 84 references, 5 tables, 3 figures

Published
2019
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28. Thermoelectric power factor of Bi-Sb-Te and Bi-Te-Se alloys and doping strategy: First-principles study

Author

Ryu, Byungki, Chung, Jaywan, Choi, Eun-Ae, Kim, Bong-Seo, and Park, Su-Dong

Subjects
Condensed Matter - Materials Science
Abstract

By performing first principles calculations combined with Boltzmann transport equations, we calculate the thermoelectric power factor (PF) of Bi-Sb-Te and Bi-Te-Se ternary alloys as a function of alloy composition ratio, carrier concentration, and temperature. The point defect formation energy calculations also perform to search potential n-type dopant candidates in ternaries., Comment: 18 pages, 9 figures

Published
2017
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29. Neuropeptide FF Promotes Neuronal Survival and Enhances Synaptic Protein Expression Following Ischemic Injury.

Author

Choi, In-Ae, Yun, Ji Hee, Lee, Jongmin, and Choi, Dong-Hee

Subjects
*NEUROPLASTICITY, *RECOMBINANT proteins, *ISCHEMIC stroke, *SIRTUINS, *LACTATE dehydrogenase, *PEROXISOME proliferator-activated receptors
Abstract

This study explores the neuroprotective effects of neuropeptide FF (NPFF, FLFQPQRFamide) in the context of ischemic injury. Based on transcriptomic analysis in stroke models treated with 5-Aza-dC and task-specific training, we identified significant gene expression changes, particularly involving NPFF. To further explore NPFF's role in promoting neuronal recovery, recombinant NPFF protein (rNPFF) was used in primary mixed cortical cultures subjected to oxygen-glucose deprivation and reoxygenation. Our results demonstrated that rNPFF significantly reduced lactate dehydrogenase release, indicating decreased cellular damage. It also significantly increased the expression of TUJ1 and MAP2, markers of neuronal survival and dendritic integrity. Additionally, rNPFF significantly upregulated key synaptic proteins, including GAP43, PSD95, and synaptophysin, which are essential for synaptic repair and plasticity. Post-injury rNPFF treatment led to a significant upregulation of pro-brain-derived neurotrophic factor (BDNF) and mature BDNF, which play critical roles in neuronal survival, growth, and synaptic plasticity. Moreover, rNPFF activated the protein kinase Cε isoform, Sirtuin 1, and peroxisome proliferator-activated receptor gamma pathways, which are crucial for regulating cellular stress responses, synaptic plasticity, and energy homeostasis, further promoting neuronal survival and recovery. These findings suggest that rNPFF may play a pivotal role in enhancing neuronal survival and synaptic plasticity after ischemic injury, highlighting its potential as a therapeutic target for stroke recovery. [ABSTRACT FROM AUTHOR]

Published
2024
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42. Governing deceleration : the natures, times, and spaces of ecotourism in South Korea

Author

Choi, Myung-Ae and Lorimer, Jamie

Subjects
338.4, Ecotourism--Korea (South), Environmental policy--Korea (South), Human geography--Korea (South), Environmental protection--Korea (South), Korea (South)--Environmental conditions
Abstract

This thesis explores the governmentalities of ecotourism in South Korea in relation to the specific historical-political experience of accelerated modernisation, focussing on three selected analytical themes of nature, time, and space. It develops a theoretical framework that combines Foucauldian governmentality analysis with concepts and insights related to nature, time and space developed in more-than-human and relational geographies and cognate social sciences. Drawing on three cases of tidal flat tourism, countryside walking, and whale tourism, it first examines the assemblages and technologies of ecotourism governance. It argues that ecotourism in South Korea is characterised by a decentralised mode of governance involving an array of political actors. This mode relies less on sovereign power and more on disciplinary and biopolitical techniques. Second, it examines the ways in which political technologies relating to nature, time, and space are engaged in the governmentalities of South Korean ecotourism. The analysis centres on: understandings of nature enacted through the discourse of saengmyeong [life] and therapeutic experiences; a discourse of slowness enacted through a paradoxical temporal organisation of accelerated slowness; and the multiple spatial relations entangled in the geographical-historical connections of South Korean modernisation. Together, these political technologies are deployed to create an ecotourism subject who cares about the self and the environment, which differs from the prevalent South Korean positions of the disciplined worker and the practical user of nature. This thesis argues that ecotourism in South Korea serves as a new biopolitical intervention to conduct the conduct of its human participants in ways that differ from those established through accelerated modernisation. By offering one of the first social science accounts of ecotourism in South Korea, it provides novel concepts and practices for the analysis of ecotourism. These differ from the mainstream approaches that deploy a political economy framework and focus largely on examples drawn from Sub-Saharan Africa, Latin America and Southeast Asia.

Published
2016

43. Role of NADPH Oxidases in Stroke Recovery.

Author

Choi, Dong-Hee, Choi, In-Ae, and Lee, Jongmin

Subjects
STROKE, NADPH oxidase, REACTIVE oxygen species, BRAIN damage, CAUSES of death
Abstract

Stroke is one of the most significant causes of death and long-term disability globally. Overproduction of reactive oxygen species by NADPH oxidase (NOX) plays an important role in exacerbating oxidative stress and causing neuronal damage after a stroke. There is growing evidence that NOX inhibition prevents ischemic injury and that the role of NOX in brain damage or recovery depends on specific post-stroke phases. In addition to studies on post-stroke neuroprotection by NOX inhibition, recent reports have also demonstrated the role of NOX in stroke recovery, a critical process for brain adaptation and functional reorganization after a stroke. Therefore, in this review, we investigated the role of NOX in stroke recovery with the aim of integrating preclinical findings into potential therapeutic strategies to improve stroke recovery. [ABSTRACT FROM AUTHOR]

Published
2024
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44. Advances in an In Vitro Tuberculosis Infection Model Using Human Lung Organoids for Host-Directed Therapies.

Author

Kim, Seung-Yeon, Choi, Ji-Ae, Choi, Seri, Kim, Kee K., Song, Chang-Hwa, and Kim, Eun-Mi

Subjects
*BRACHYDANIO, *GUINEA pigs, *LUNGS, *TUBERCULOSIS, *MYCOBACTERIUM tuberculosis, *ORGANOIDS, *ANIMAL culture
Abstract

The emergence of drug-resistant Mycobacterium tuberculosis (M.tb) has led to the development of novel anti-tuberculosis (anti-TB) drugs. Common methods for testing the efficacy of new drugs, including two-dimensional cell culture models or animal models, have several limitations. Therefore, an appropriate model representative of the human organism is required. Here, we developed an M.tb infection model using human lung organoids (hLOs) and demonstrated that M.tb H37Rv can infect lung epithelial cells and human macrophages (hMφs) in hLOs. This novel M.tb infection model can be cultured long-term and split several times while maintaining a similar number of M.tb H37Rv inside the hLOs. Anti-TB drugs reduced the intracellular survival of M.tb in hLOs. Notably, M.tb growth in hLOs was effectively suppressed at each passage by rifampicin and bedaquiline. Furthermore, a reduction in inflammatory cytokine production and intracellular survival of M.tb were observed upon knockdown of MFN2 and HERPUD1 (host-directed therapeutic targets for TB) in our M.tb H37Rv-infected hLO model. Thus, the incorporation of hMφs and M.tb into hLOs provides a powerful strategy for generating an M.tb infection model. This model can effectively reflect host-pathogen interactions and be utilized to test the efficacy of anti-TB drugs and host-directed therapies. Author summary: Establishment of M.tb infection model is imperative to develop new anti-TB drugs based on the pathogenesis of TB. Various animal models, including mice, rats, guinea pigs, non-human primates, rabbits, cattle, and zebrafish, are commonly used in TB research to mimic TB symptoms and study immune responses to M.tb infection. In vitro models, such as agent-based models allow examination of host-pathogen interactions, early granuloma formation and drug screening, providing cellular-level insights. However, these models may not fully represent human immunopathology owing to differences in immune cell distributions. Lung organoids mimic human lung dynamics and functions, providing crucial insights into immune responses to TB. In this study, an M.tb infection model developed using hLOs demonstrated infection of lung epithelial cells and human macrophages, reflecting host-pathogen interactions. This model is attractive for evaluating the efficacy of anti-TB drugs and host-directed therapies. [ABSTRACT FROM AUTHOR]

Published
2024
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