11 results on '"J Baek"'
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2. Multimodal Encapsulation to Selectively Permeate Hydrogen and Engineer Channel Conduction for p-Type SnO x Thin-Film Transistor Applications.
- Author
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Lee DH, Zhang Y, Chang SJ, Park H, Kim CS, Baek J, Park J, No K, Song HW, Park H, and Lee S
- Abstract
It has been challenging to synthesize p-type SnO
x (1 < x < 2) and engineer the electrical properties such as carrier density and mobility due to the narrow processing window and the localized oxygen 2p orbitals near the valence band. Herein, we report on the multifunctional encapsulation of p-SnOx to limit the surface adsorption of oxygen and selectively permeate hydrogen into the p-SnOx channel for thin-film transistor (TFT) applications. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurements identified that ultrathin SiO2 as a multifunctional encapsulation layer effectively suppressed the oxygen adsorption on the back channel surface of p-SnOx and selectively diffused hydrogen across the entire thickness of the channel. Encapsulated p-SnOx -based TFTs demonstrated much enhanced channel conductance modulation in response to the gate bias applied, featuring higher on-state current and lower off-state current (on/off ratio > 103 ), field effect mobility of 3.41 cm2 /(V s), and threshold voltages of ∼5-10 V. The fabricated devices show minimal deviations as small as ±6% in the TFT performance parameters, which demonstrates good reproducibility of the fabrication process. The relevance between the TFT performance and the effects of hydrogen permeation is discussed in regard to the intrinsic and extrinsic doping mechanisms. Density functional theory calculations reveal that hydrogen-related impurity complexes are in charge of the enhanced channel conductance with gate biases, which further supports the selective permeation of hydrogen through a thin SiO2 encapsulation.- Published
- 2022
- Full Text
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3. Hollow Porous N and Co Dual-Doped Silicon@Carbon Nanocube Derived by ZnCo-Bimetallic Metal-Organic Framework toward Advanced Lithium-Ion Battery Anodes.
- Author
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Kim H, Baek J, Son DK, Ruby Raj M, and Lee G
- Abstract
Silicon (Si) has been recognized as a promising alternative to graphite anode materials for advanced lithium-ion batteries (LIBs) owing to its superior theoretical capacity and low discharge voltage. However, Si-based anodes undergo structural pulverization during cycling due to the large volume expansion (ca. 300-400%) and continuous formation of an unstable solid electrolyte interphase (SEI), resulting in fast capacity fading. To address this challenge, a series of different amounts of silicon nanoparticles (Si NPs)-encapsulated hollow porous N-doped/Co-incorporated carbon nanocubes (denoted as p-CoNC@Si X , where X = 50, 80, and 100) as anode materials for LIBs are reported in this paper. These hollow nanocubic materials were derived by facile annealing of different contents of Si NPs-encapsulated Zn/Co-bimetallic zeolitic imidazolate frameworks (ZIF@Si) as self-sacrificial templates. Owing to the advantages of well-defined hollow framework clusters and highly conductive hollow carbon frameworks, the hollow porous p-CoNC@Si X significantly improved the electronic conductivity and Li
+ diffusion coefficient by an order of magnitude higher than that of Si NPs. The as-prepared p-CoNC@Si80 with 80 wt % Si NPs delivered a continuously increasing specific capacity of 1008 mAh g-1 at 500 mA g-1 over 500 cycles, excellent reversible capacity (∼1361 mAh g-1 at 0.1 A g-1 ), and superior rate capability (∼603 mAh g-1 at 3 A g-1 ) along with an unprecedented long-life cyclic stability of ∼1218 mAh g-1 at 1 A g-1 over 1000 cycles caused by low volume expansion (9.92%) and suppressed SEI side reactions. These findings provide new insights into the development of highly reversible Si-based anode materials for advanced LIBs.- Published
- 2022
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4. Effect of Fluoroalkylsilane Surface Functionalization on Boron Combustion.
- Author
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Baek J, Jiang Y, Demko AR, Jimenez-Thomas AR, Vallez L, Ka D, Xia Y, and Zheng X
- Abstract
Boron has been regarded as a promising high-energy fuel due to its high volumetric and gravimetric heating values. However, it remains challenging for boron to attain its theoretical heat of combustion because of the existence of its native boron oxide layer and its high melting and boiling temperatures that delay ignition and inhibit complete combustion. Boron combustion is known to be enhanced by physically adding fluorine-containing chemicals, such as fluoropolymer or metal fluorides, to remove surface boron oxides. Herein, we chemically functionalize the surface of boron particles with three different fluoroalkylsilanes: FPTS-B (F3-B), FOTS-B (F13-B), and FDTS-B (F17-B). We evaluated the ignition and combustion properties of those three functionalized boron particles as well as pristine ones. The boron particles functionalized with the longest fluorocarbon chain (F17) exhibit the most powerful energetic performance, the highest heat of combustion, and the strongest BO
2 emission among all samples. These results suggest that the surface functionalization with fluoroalkylsilanes is an efficient strategy to enhance boron ignition and combustion.- Published
- 2022
- Full Text
- View/download PDF
5. Enhancing Mechanical and Combustion Performance of Boron/Polymer Composites via Boron Particle Functionalization.
- Author
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Jiang Y, Dincer Yilmaz NE, Barker KP, Baek J, Xia Y, and Zheng X
- Abstract
High-speed air-breathing propulsion systems, such as solid fuel ramjets (SFRJ), are important for space exploration and national security. The development of SFRJ requires high-performance solid fuels with excellent mechanical and combustion properties. One of the current solid fuel candidates is composed of high-energy particles (e.g., boron (B)) and polymeric binder (e.g., hydroxyl-terminated polybutadiene (HTPB)). However, the opposite polarities of the boron surface and HTPB lead to poor B particle dispersion and distribution within HTPB. Herein, we demonstrate that the surface functionalization of B particles with nonpolar oleoyl chloride greatly improves the dispersion and distribution of B particles within HTPB. The improved particle dispersion is quantitatively visualized through X-ray computed tomography imaging, and the particle/matrix interaction is evaluated by dynamic mechanical analysis. The surface-functionalized B particles can be uniformly dispersed up to 40 wt % in HTPB, the highest mass loading reported to date. The surface-functionalized B (40 wt %)/HTPB composite exhibits a 63.3% higher Young's modulus, 87.5% higher tensile strength, 16.2% higher toughness, and 16.8% higher heat of combustion than pristine B (40 wt %)/HTPB. The surface functionalization of B particles provides an effective strategy for improving the efficacy and safety of B/HTPB solid fuels for future high-speed air-breathing vehicles.
- Published
- 2021
- Full Text
- View/download PDF
6. Significantly Enhanced Thermoelectric Performance of Graphene through Atomic-Scale Defect Engineering via Mobile Hot-Wire Chemical Vapor Deposition Systems.
- Author
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Choi M, Novak TG, Byen J, Lee H, Baek J, Hong S, Kim K, Song J, Shin H, and Jeon S
- Abstract
Over the years, numerous studies have attempted to develop two-dimensional (2D) materials for improving both the applicability and performance of thermoelectric devices. Among the 2D materials, graphene is one of the promising candidates for thermoelectric materials owing to its extraordinary electrical properties, flexibility, and nontoxicity. However, graphene synthesized through traditional methods suffers from a low Seebeck coefficient and high thermal conductivity, resulting in an extremely low thermoelectric figure of merit (ZT). Here, we present an atomic-scale defect engineering strategy to improve the thermoelectric properties of graphene using embedded high-angle tilt boundary (HATB) domains in graphene films. These HATB domains serve as both energy filtering sites to filter out lower-energy charge carriers and scattering sites for phonons. Compared to the conventionally grown chemical vapor deposited graphene, the graphene with HATB domains shows an improved Seebeck coefficient (50.1 vs 21.1 μV K
-1 ) and reduced thermal conductivity (382 vs 952 W m-1 K-1 ), resulting in a ZT value that is ∼7 times greater at 350 K. This defect engineering strategy is promising not only for graphene-based materials but also for 2D materials, in general, where further research and optimization could overcome the limitations of conventional bulk thermoelectric materials in energy-harvesting systems.- Published
- 2021
- Full Text
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7. Facile Fabrication of High-Definition Hierarchical Wrinkle Structures for Investigating the Geometry-Sensitive Fate Commitment of Human Neural Stem Cells.
- Author
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Baek J, Jung WB, Cho Y, Lee E, Yun GT, Cho SY, Jung HT, and Im SG
- Subjects
- Anisotropy, Cell Culture Techniques, Cell Differentiation drug effects, Cell Differentiation genetics, Cell Lineage drug effects, Cell Proliferation drug effects, Focal Adhesion Kinase 1 genetics, Gene Expression Regulation, Developmental drug effects, Humans, Neural Stem Cells metabolism, Stem Cell Transplantation, Nanostructures chemistry, Nerve Regeneration, Neural Stem Cells drug effects, Neurogenesis drug effects
- Abstract
As neural stem cells (NSCs) interact with biophysical cues from their niche during development, it is important to understand the biomolecular mechanism of how the NSCs process these biophysical cues to regulate their behaviors. In particular, anisotropic geometric cues in micro-/nanoscale have been utilized to investigate the biophysical effect of the structure on NSCs behaviors. Here, a series of new nanoscale anisotropic wrinkle structures with the a range of wavelength scales (from 50 nm to 37 μm) was developed to demonstrate the effect of the anisotropic nanostructure on the fate commitment of NSCs. Intriguingly, two distinct characteristic length scales promoted the neurogenesis. Each wavelength scale showed a striking variation in terms of dependency on the directionality of the structures, suggesting the existence of at least two different ways in the processing of anisotropic geometries for neurogenesis. Furthermore, the combined effect of the two distinctive length scales was observed by employing hierarchical multiscale wrinkle structures with two characteristic neurogenesis-promoting wavelengths. Taken together, the wrinkle structure system developed in this study can serve as an effective platform to advance the understanding of how cells sense anisotropic geometries for their specific cellular behaviors. Furthermore, this could provide clues for improving nerve regeneration system of stem cell therapies.
- Published
- 2019
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8. Influence of Branched Alkyl Ester-Labeled Side Chains on Specific Chain Arrangement and Charge-Transport Properties of Diketopyrrolopyrrole-Based Conjugated Polymers.
- Author
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Kim HJ, Pei M, Ko JS, Ma MH, Park GE, Baek J, Yang H, Cho MJ, and Choi DH
- Abstract
A series of diketopyrrolopyrrole (DPP)-based copolymers, with DPP and bithiophene (BT) as the electron-acceptor and donor backbone units, respectively, are synthesized with branched alkyl side chains that are either directly coupled to the N-positions of DPP or separated by an alkyl ester group. The ester moieties in the side chains induce specific cohesive molecular interactions between these side chains, as compared to the alkyl-only side chains with weak van der Waals interactions. Structure analysis of the DPPBT-based copolymers demonstrated that the introduction of a proper alkyl ester spacer to the branched alkyl chains can shorten the π-π stacking distance between the DPPBT backbones down to 3.61 Å and promote the development of two-dimensionally extended domains. DPPBT-based copolymers, including different branched alkyl ester-labeled side chains, are spun-cast on polymer-treated SiO
2 dielectrics from dilute chloroform solutions for organic thin-film transistors. A DPPBT-based copolymer with properly engineered side chains (i.e., 2-decyltetradecyl ester-labeled side chains) shows the highest hole mobility of 2.30 cm2 V-1 s-1 and an on/off current ratio of above 106 .- Published
- 2018
- Full Text
- View/download PDF
9. Distinct Mechanosensing of Human Neural Stem Cells on Extremely Limited Anisotropic Cellular Contact.
- Author
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Baek J, Cho SY, Kang H, Ahn H, Jung WB, Cho Y, Lee E, Cho SW, Jung HT, and Im SG
- Subjects
- Anisotropy, Cell Cycle Proteins, Cell Line, Humans, Neurogenesis, Nuclear Proteins metabolism, Transcription Factors metabolism, rho GTP-Binding Proteins metabolism, Cytoskeleton metabolism, Focal Adhesions metabolism, Mechanotransduction, Cellular, Nanostructures, Neural Stem Cells cytology, Neural Stem Cells metabolism
- Abstract
Human neural stem cells (hNSCs) can alter their fate choice in response to the biophysical cues provided during development. In particular, it has been reported that the differentiation of neural stem cells (NSCs) is enhanced by anisotropic contact, which facilitates focal adhesion (FA) formation and cytoskeletal organization. However, a biomolecular mechanism governing how the cells process the biophysical cues from these anisotropic geometries to their fate commitment is still poorly understood due to the limited availability of geometrical diversities (contact width above 50 nm) applicable to cell studies. Here, we firstly demonstrate that the biomolecular mechanism for enhanced neurogenesis on an anisotropic nanostructure is critically dependent on the resolution of a contact feature. We observed a totally different cellular response to anisotropic geometries by first utilizing a high-resolution nanogroove (HRN) structure with an extremely narrow contact width (15 nm). The width scale is sufficiently low to suppress the integrin clustering and enable us to elucidate how the contact area influences the neurogenesis of hNSCs at an aligned state. Both the HRN and control nanogroove (CN) pattern with a contact width of 1 μm induced the spontaneous topographic alignment of hNSCs. However, intriguingly, the focal adhesion (FA) formation and cytoskeletal reorganization were substantially limited on the HRN, although the cells on the CN showed enhanced FA formation compared with flat surfaces. In particular, the hNSCs on the HRN surface exhibited a strikingly lower fraction of nuclear yes-associated protein (YAP) than on the CN surface, which was turned out to be regulated by Rho GTPase in the same way as the cells sense the mechanical properties of the environment. Considering the previously reported role of YAP on neurogenesis, our finding newly substantiates that YAP and Rho GTPase also can be transducers of hNSCs to process topographical alternation to fate decision. Furthermore, this study with the unprecedented high-resolution nanostructure suggests a novel geometry sensing model where the functional crosstalk between YAP signaling and Rho GTPase integrally regulate the fate commitment of the hNSCs.
- Published
- 2018
- Full Text
- View/download PDF
10. Improving the Stability of High-Performance Multilayer MoS 2 Field-Effect Transistors.
- Author
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Liu N, Baek J, Kim SM, Hong S, Hong YK, Kim YS, Kim HS, Kim S, and Park J
- Abstract
In this study, we propose a method for improving the stability of multilayer MoS
2 field-effect transistors (FETs) by O2 plasma treatment and Al2 O3 passivation while sustaining the high performance of bulk MoS2 FET. The MoS2 FETs were exposed to O2 plasma for 30 s before Al2 O3 encapsulation to achieve a relatively small hysteresis and high electrical performance. A MoOx layer formed during the plasma treatment was found between MoS2 and the top passivation layer. The MoOx interlayer prevents the generation of excess electron carriers in the channel, owing to Al2 O3 passivation, thereby minimizing the shift in the threshold voltage (Vth ) and increase of the off-current leakage. However, prolonged exposure of the MoS2 surface to O2 plasma (90 and 120 s) was found to introduce excess oxygen into the MoOx interlayer, leading to more pronounced hysteresis and a high off-current. The stable MoS2 FETs were also subjected to gate-bias stress tests under different conditions. The MoS2 transistors exhibited negligible decline in performance under positive bias stress, positive bias illumination stress, and negative bias stress, but large negative shifts in Vth were observed under negative bias illumination stress, which is attributed to the presence of sulfur vacancies. This simple approach can be applied to other transition metal dichalcogenide materials to understand their FET properties and reliability, and the resulting high-performance hysteresis-free MoS2 transistors are expected to open up new opportunities for the development of sophisticated electronic applications.- Published
- 2017
- Full Text
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11. Effects of dispersion conditions of single-walled carbon nanotubes on the electrical characteristics of thin film network transistors.
- Author
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Barman SN, LeMieux MC, Baek J, Rivera R, and Bao Z
- Subjects
- Colloids chemistry, Electric Conductivity, Equipment Design, Equipment Failure Analysis, Materials Testing, Membranes, Artificial, Nanotechnology instrumentation, Nanotubes, Carbon chemistry, Transistors, Electronic
- Abstract
To facilitate solution deposition of single-walled carbon nanotubes (SWNTs) for integration into electronic devices they need to be purified and dispersed into solutions. The vigorous sonication process for preparing these dispersions leads to large variations in the length and defect density of SWNTs, affecting the resulting electronic properties. Understanding the effects of solution processing steps can have important implications in the design of SWNT films for electronic applications. Here, we alter the SWNTs by varying the sonication time, followed by deposition of sub-monolayer SWNT network films onto functionalized substrates. The corresponding electrical performance characteristics of the resulting field effect transistors (FETs) are correlated with SWNT network sorting and morphology. As sonication exposure increases, the SWNTs shorten, which not only affects electrical current by increasing the number of junctions but also presumably leads to more defects. The off current of the resulting transistors initially increased with sonication exposure, presumably due to less efficient sorting of semiconducting SWNTs as the defect density increases. After extended sonication, the on and off current decreased because of increased bundling and fewer percolation pathways. The final transistor properties are influenced by the nanotube solution concentration, density, alignment, and the selectivity of surface sorting of the SWNT networks. These results show that in addition to chirality, careful consideration of SWNT dispersion conditions that affect SWNT length, bundle diameter, and defect density is critical for optimal SWNT-FET performance and potentially other SWNT-based electronic devices.
- Published
- 2010
- Full Text
- View/download PDF
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