A study on electrical and thermal transportation properies of Nano-patterned CNT and bismuth film

학위논문 (석사)-- 서울대학교 대학원 : 재료공학부(하이브리드 재료), 2012. 8. 김기범.본 논문에서는 나노스케일의 공정기술이 열전소자에 사용되는 재료에 적용되었을 때 열전성능과 전기적 성능을 밝혀보고자 하였다. 나노스케일의 공정기술이 열전성능을 높일 수 있다는 가능성이 발표된 후 나노메쉬 (Nanomesh) 구조와 나노와이어 (Nanowire) 구조가 잠재적으로 열전소자에 쓰일 수 있는 재료들에 적용이 되었다. 본 논문에서는 나노메쉬와 나노와이어 구조가 각각 CNT film과 bismuth에 적용을 시켜 전기적, 열적 transport 특성에 관하여 분석한 결과를 고찰하였다. 첫 번째로, 나노메쉬 패턴을 CNT film에 삽입한 nanomesh CNT film과 나노메쉬 패턴을 삽입하지 않은 pristine CNT film의 전기적, 열적 광학적 특성을 비교하였다. Metallic carbon nanotube는 Seebeck coefficient가 0에 가까워야 함에도 불구하고 nanotube film은 상당히 높은 Seebeck coefficient를 가지고 있다. 본 실험에서는 anodized aluminum oxide membrane을 etching hard mask로 이용하여 nanomesh pattern을 CNT film에 삽입하였다. 조밀한 나노스케일의 hole들이 patterning 되어 삽입된 CNT film의 thermoelectric power가 30% (29 to 39 V/K) 가량 증가된 것이 관찰되었다. 이러한 현상을 Nano-patterning에 의한 electron localization 에 의한 것으로 설명하였다. 두 번째로, 표면의 거칠기가 낮은 bismuth film을 증착하기 위해 pulsed laser deposition (PLD) system의 증착조건이 최적화 되었다. Bismuth film의 전기적 특성과 표면의 거칠기를 PLD system의 Working pressure와 target to substrate (TS) distance, 그리고 기판의 온도를 조절해 가며 관찰 하였다. 각 증착 parameter 들의 영역은 working pressure의 경우 10 mTorr 에서 base pressure (10-6 Torr), TS distance의 경우 4cm ~ 7cm, 그리고 기판의 온도는 22 °C 에서 200 °C까지 조절하였다. Bismuth film의 electrical resistivity와 표면의 거칠기는 working pressure, TS distance, 그리고 기판의 온도에 따라 달라지는 경향을 보였으며, 최적화 된 조건에서 증착 된 bismuth film의 RMS surface roughness value는 1.57nm 이고 electrical resistivity는 300µΩcm 이다. 이러한 좋은 표면을 가진 필름을 바탕으로 하여 최종적으로 bismuth nanowire를 top-down fabrication 방식인 e-beam lithography를 통해 제작하였고 bismuth nanowire의 온도에 따른 전기적 성질을 bismuth thin film과 bulk bismuth와 비교해 보았다.ABSTRACT i TABLE OF CONTENTS iv LIST OF FIGURES vi CHAPTER 1. Thermoelectric １ 1. 1. Introduction ２ 1. 2. Figure of merit ５ 1. 3. Nano-scale strategies ８ 1. 5. References １２ CHAPTER 2. Nanomesh CNT film １３ 2.1. Introduction １４ 2.2. Experiments １７ 2.3. Results and discussion ２１ 2.4. Conclusion ２７ 2.5. References ２８ CHAPTER 3. Bismuth film and nanowire ３０ 3.1. Introduction ３１ 3.2. Experiments ３４ 3.3. Results and discussion ３５ 3.6. References ４６ CHAPTER 4. Bismuth nanowire ４７ 4.1. Experiments ４８ 4.2. Result and discussion ５９ Abstract (in Korean) ６１Maste

Disorder-dependent Li diffusion in Li6PS5Cl\mathrm{Li_6PS_5Cl} investigated by machine learning potential

Solid-state electrolytes with argyrodite structures, such as $\mathrm{Li_6PS_5Cl}$, have attracted considerable attention due to their superior safety compared to liquid electrolytes and higher ionic conductivity than other solid electrolytes. Although experimental efforts have been made to enhance conductivity by controlling the degree of disorder, the underlying diffusion mechanism is not yet fully understood. Moreover, existing theoretical analyses based on ab initio MD simulations have limitations in addressing various types of disorder at room temperature. In this study, we directly investigate Li-ion diffusion in $\mathrm{Li_6PS_5Cl}$ at 300 K using large-scale, long-term MD simulations empowered by machine learning potentials (MLPs). To ensure the convergence of conductivity values within an error range of 10%, we employ a 25 ns simulation using a $5\times5\times5$ supercell containing 6500 atoms. The computed Li-ion conductivity, activation energies, and equilibrium site occupancies align well with experimental observations. Notably, Li-ion conductivity peaks when Cl ions occupy 25% of the 4c sites, rather than at 50% where the disorder is maximized. This phenomenon is explained by the interplay between inter-cage and intra-cage jumps. By elucidating the key factors affecting Li-ion diffusion in $\mathrm{Li_6PS_5Cl}$, this work paves the way for optimizing ionic conductivity in the argyrodite family.Comment: 34 pages, 6 figure

Atomic Scale Study on Growth and Heteroepitaxy of ZnO Monolayer on Graphene

Atomically thin semiconducting oxide on graphene carries a unique combination of wide band gap, high charge carrier mobility, and optical transparency, which can be widely applied for optoelectronics. However, study on the epitaxial formation and properties of oxide monolayer on graphene remains unexplored due to hydrophobic graphene surface and limits of conventional bulk deposition technique. Here, we report atomic scale study of heteroepitaxial growth and relationship of a single-atom-thick ZnO layer on graphene using atomic layer deposition. We demonstrate atom-by-atom growth of zinc and oxygen at the preferential zigzag edge of a ZnO monolayer on graphene through in situ observation. We experimentally determine that the thinnest ZnO monolayer has a wide band gap (up to 4.0 eV), due to quantum confinement and graphene-like structure, and high optical transparency. This study can lead to a new class of atomically thin two-dimensional heterostructures of semiconducting oxides formed by highly controlled epitaxial growth.ope

Blood pressure and dementia risk by physical frailty in the elderly: a nationwide cohort study

Background Midlife hypertension has been recognized as a modifiable risk factor for dementia, but association between blood pressure (BP) in late life and dementia has been inconclusive. In addition, few studies have investigated effects of BP control on dementia incidence in the frail elderly. Thus, this study aimed to investigate the association of BP and dementia incidence with concomitant consideration of physical frailty in the young elderly population. Methods Using the Korean National Health Information Database, we identified 804,024 subjects without history of dementia at age 66. Dementia diagnosis was defined with prescription records of anti-dementia drugs and dementia-related diagnostic codes. Physical frailty was measured using the Timed Up and Go test. Association of BP and dementia incidence with concomitant consideration of physical frailty was investigated using Cox hazards analyses. Results The risks of Alzheimers and vascular dementia increased from systolic BP ≥ 160 and 130–139mmHg, respectively; a significant association of dementia incidence with low BP was not observed. In the analyses stratified by the physical frailty status, low BP was not associated with increased risks of dementia within the groups both with and without physical frailty. Conclusions High BP was associated with increased risks of dementia, especially for vascular dementia, while low BP was not associated with increased risks of any type of dementia in young elderly people, even in those with physical frailty. This study suggests the need for tight BP control in young elderly people, irrespective of frailty status, to prevent dementia and supports the current clinical guidelines of hypertension treatment

Simulation Method for the Physical Deformation of a Three-Dimensional Soft Body in Augmented Reality-Based External Ventricular Drainage

Objectives Intraoperative navigation reduces the risk of major complications and increases the likelihood of optimal surgical outcomes. This paper presents an augmented reality (AR)-based simulation technique for ventriculostomy that visualizes brain deformations caused by the movements of a surgical instrument in a three-dimensional brain model. This is achieved by utilizing a position-based dynamics (PBD) physical deformation method on a preoperative brain image. Methods An infrared camera-based AR surgical environment aligns the real-world space with a virtual space and tracks the surgical instruments. For a realistic representation and reduced simulation computation load, a hybrid geometric model is employed, which combines a high-resolution mesh model and a multiresolution tetrahedron model. Collision handling is executed when a collision between the brain and surgical instrument is detected. Constraints are used to preserve the properties of the soft body and ensure stable deformation. Results The experiment was conducted once in a phantom environment and once in an actual surgical environment. The tasks of inserting the surgical instrument into the ventricle using only the navigation information presented through the smart glasses and verifying the drainage of cerebrospinal fluid were evaluated. These tasks were successfully completed, as indicated by the drainage, and the deformation simulation speed averaged 18.78 fps. Conclusions This experiment confirmed that the AR-based method for external ventricular drain surgery was beneficial to clinicians

Myotis rufoniger genome sequence and analyses: M-rufoniger's genomic feature and the decreasing effective population size of Myotis bats

Myotis rufoniger is a vesper bat in the genus Myotis. Here we report the whole genome sequence and analyses of the M. rufoniger. We generated 124 Gb of short-read DNA sequences with an estimated genome size of 1.88 Gb at a sequencing depth of 66x fold. The sequences were aligned to M. brandtii bat reference genome at a mapping rate of 96.50% covering 95.71% coding sequence region at 10x coverage. The divergence time of Myotis bat family is estimated to be 11.5 million years, and the divergence time between M. rufoniger and its closest species M. davidii is estimated to be 10.4 million years. We found 1,239 function-altering M. rufoniger specific amino acid sequences from 929 genes compared to other Myotis bat and mammalian genomes. The functional enrichment test of the 929 genes detected amino acid changes in melanin associated DCT, SLC45A2, TYRP1, and OCA2 genes possibly responsible for the M. rufoniger's red fur color and a general coloration in Myotis. N6AMT1 gene, associated with arsenic resistance, showed a high degree of function alteration in M. rufoniger. We further confirmed that the M. rufoniger also has batspecific sequences within FSHB, GHR, IGF1R, TP53, MDM2, SLC45A2, RGS7BP, RHO, OPN1SW, and CNGB3 genes that have already been published to be related to bat's reproduction, lifespan, flight, low vision, and echolocation. Additionally, our demographic history analysis found that the effective population size of Myotis clade has been consistently decreasing since similar to 30k years ago. M. rufoniger's effective population size was the lowest in Myotis bats, confirming its relatively low genetic diversity

Variation block-based genomics method for crop plants

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.Abstract Background In contrast with wild species, cultivated crop genomes consist of reshuffled recombination blocks, which occurred by crossing and selection processes. Accordingly, recombination block-based genomics analysis can be an effective approach for the screening of target loci for agricultural traits. Results We propose the variation block method, which is a three-step process for recombination block detection and comparison. The first step is to detect variations by comparing the short-read DNA sequences of the cultivar to the reference genome of the target crop. Next, sequence blocks with variation patterns are examined and defined. The boundaries between the variation-containing sequence blocks are regarded as recombination sites. All the assumed recombination sites in the cultivar set are used to split the genomes, and the resulting sequence regions are termed variation blocks. Finally, the genomes are compared using the variation blocks. The variation block method identified recurring recombination blocks accurately and successfully represented block-level diversities in the publicly available genomes of 31 soybean and 23 rice accessions. The practicality of this approach was demonstrated by the identification of a putative locus determining soybean hilum color. Conclusions We suggest that the variation block method is an efficient genomics method for the recombination block-level comparison of crop genomes. We expect that this method will facilitate the development of crop genomics by bringing genomics technologies to the field of crop breeding