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Dive into the research topics where Chulmin Choi is active.

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Featured researches published by Chulmin Choi.


Nano Letters | 2013

Tailoring n‑ZnO/p-Si Branched Nanowire Heterostructures for Selective Photoelectrochemical Water Oxidation or Reduction

Ke Sun; Yi Jing; Chulmin Choi; Huisu Jeong; Yuchun Zhou; Kristian N. Madsen; Perry Naughton; Sungho Jin; Gun Young Jung; Deli Wang

We report the fabrication of three-dimensional (3D) branched nanowire (NW) heterostructures, consisting of periodically ordered vertical Si NW trunks and ZnO NW branches, and their application for solar water splitting. The branched NW photoelectrodes show orders of magnitudes higher photocurrent compared to the bare Si NW electrodes. More interestingly, selective photoelectrochemical cathodic or anodic behavior resulting in either solar water oxidation or reduction was achieved by tuning the doping concentration of the p-type Si NW core. Specifically, n-ZnO/p-Si branched NW array electrodes with lightly doped core show broadband absorption from UV to near IR region and photocathodic water reduction, while n-ZnO/p(+)-Si branched NW arrays show photoanodic water oxidation with photoresponse only to UV light. The photoelectrochemical stability for over 24 h under constant light illumination and fixed biasing potential was achieved by coating the branched NW array with thin layers of TiO2 and Pt. These studies not only reveal the promise of 3D branched NW photoelectrodes for high efficiency solar energy harvesting and conversion to clean chemical fuels, but also developing understanding enabling rational design of high efficiency robust photocathodes and photoanodes from low-cost and earth-abundant materials allowing practical applications in clean renewable energy.


ACS Nano | 2013

3D Branched Nanowire Photoelectrochemical Electrodes for Efficient Solar Water Splitting

Ke Sun; Yi Jing; Chulmin Choi; Huisu Jeong; Gun Young Jung; Sungho Jin; Deli Wang

We report the systematic study of 3D ZnO/Si branched nanowire (b-NW) photoelectrodes and their application in solar water splitting. We focus our study on the correlation between the electrode design and structures (including Si NW doping, dimension of the trunk Si and branch ZnO NWs, and b-NW pitch size) and their photoelectrochemical (PEC) performances (efficiency and stability) under neutral conditions. Specifically, we show that for b-NW electrodes with lightly doped p-Si NW core, larger ZnO NW branches and longer Si NW cores give a higher photocathodic current, while for b-NWs with heavily doped p-Si NW trunks smaller ZnO NWs and shorter Si NWs provide a higher photoanodic current. Interestingly, the photocurrent turn-on potential decreases with longer p-Si NW trunks and larger ZnO NW branches resulting in a significant photocathodic turn-on potential shift of ~600 mV for the optimized ZnO/p-Si b-NWs compared to that of the bare p-Si NWs. A photocathode energy conversion efficiency of greater than 2% at -1 V versus Pt counter electrode and in neutral solution is achieved for the optimized ZnO/p-Si b-NW electrodes. The PEC performances or incident photon-to-current efficiency are further improved using Si NW cores with smaller pitch size. The photoelectrode stability is dramatically improved by coating a thin TiO2 protection layer using atomic-layer deposition method. These results provide very useful guidelines in designing photoelectrodes for selective solar water oxidation/reduction and overall spontaneous solar fuel generation using low cost earth-abundant materials for practical clean solar fuel production.


Acta Biomaterialia | 2011

Hydrophobic nanopillars initiate mesenchymal stem cell aggregation and osteo-differentiation

Karla S. Brammer; Chulmin Choi; Christine J. Frandsen; Seunghan Oh; Sungho Jin

Surface engineering approaches that alter the physical topography of a substrate could be used as an effective tool and as an alternative to biochemical means of directing stem cell interactions and their subsequent differentiation. In this paper we compare hydrophobic micro- vs. nanopillar type fabrication techniques for probing mesenchymal stem cell (MSC) interaction with the surface physical environment. The roles played by the topography of the nanopillar in particular influenced MSC growth and allowed for regulatory control of the stem cell fate. The nanopillar induced large 3-D cell aggregates to form on the surface which had up-regulated osteogenic specific matrix components. The ability to control MSC differentiation, using only the topographical factors, has a profound effect on both MSC biology and tissue engineering. This study aims to highlight the importance of the physical material carrier in stem cell based tissue engineering schemes.


Langmuir | 2011

Extreme superomniphobicity of multiwalled 8 nm TiO2 nanotubes.

Hyunsu Kim; Kunbae Noh; Chulmin Choi; Jirapon Khamwannah; Diana Villwock; Sungho Jin

We report unprecedented superomniphobic characteristics of nanotube-structured TiO(2) surface fabricated by electrochemical etching and hydrothermal synthesis process, with the wettability contact angles for water and oil both being ∼174° or higher. A tangled forest of ∼8-nm-diameter, multiwalled nanotubes of TiO(2) was produced on the microtextured Ti surface, with the overall nanotube length controlled to 150 nm by adjusting the processing time. Wettability measurements indicate that the nanotube surface is extremely nonwettable to both water and oil. The contact angle of the 8 nm TiO(2) nanotube surface after perfluorosilane coating is extremely high (178°) for water droplets indicating superhydrophobic properties. The contact angle for oil, measured using a glycerol droplet, is also very high, about 174°, indicating superoleophobic characteristics. These dual nonwetting properties, superomniphobic characteristics, are in sharp contrast to the as-made TiO(2) nanotubes which exhibit superhydrophilic properties with a contact angle of essentially ∼0°. Such an extreme superomniphobic material made by a simple and versatile method can be useful for a variety of technical applications. It is interesting to note that all three properties can be obtained with identical nanotube structures. A nanometer-scaled structure introduced by hydrothermally grown TiO(2) nanotubes is an effective air trapping nanostructure in enhancing the amphiphobic (superomniphobic) wettability.


Acta Biomaterialia | 2011

Comparative cell behavior on carbon-coated TiO2 nanotube surfaces for osteoblasts vs. osteo-progenitor cells.

Karla S. Brammer; Chulmin Choi; Christine J. Frandsen; Seunghan Oh; Gary Johnston; Sungho Jin

Surface engineering approaches that alter the topological chemistry of a substrate could be used as an effective tool for directing cell interactions and their subsequent function. It is well known that the physical environment of nanotopography has positive effects on cell behavior, yet direct comparisons of nanotopographic surface chemistry have not been fully explored. Here we compare TiO(2) nanotubes with carbon-coated TiO(2) nanotubes, probing osteogenic cell behavior, including osteoblast (bone cells) and mesenchymal stem cell (MSC) (osteo-progenitor cells) interactions with the different surface chemistries (TiO(2) vs. carbon). The roles played by the material surface chemistry of the nanotubes did not have an effect on the adhesion, growth or morphology, but had a major influence on the alkaline phosphatase (ALP) activity of osteoblast cells, with the original TiO(2) chemistry having higher ALP levels. In addition, the different chemistries caused different levels of osteogenic differentiation in MSCs; however, it was the carbon-coated TiO(2) nanotubes that had the greater advantage, with higher levels of osteo-differentiation. It was observed in this study that: (a) chemistry plays a role in cell functionality, such as ALP activity and osteogenic protein gene expression (PCR); (b) different cell types may have different chemical preferences for optimal function. The ability to optimize cell behavior using surface chemistry factors has a profound effect on both orthopedic and tissue engineering in general. This study aims to highlight the importance of the chemistry of the carrier material in osteogenic tissue engineering schemes.


Nano Letters | 2009

Antibiofouling, sustained antibiotic release by Si nanowire templates.

Karla S. Brammer; Chulmin Choi; Seunghan Oh; Christine J. Cobb; Laura Connelly; Mariana Loya; Seong Deok Kong; Sungho Jin

Loading or filling nanostructures with antibiotics can be one of the relevant approaches for obtaining a controlled drug release rate. Vertically aligned silicon nanowire (SiNW) arrays with 10-40 nm diameter wires having 1-3 microm in length obtained by the electroless etching (EE) technique are used in this study as novel nanostructures for mediating drug delivery. Here we report controlled antibiotic activity and sustained bioavailability from SiNW arrays and also show microstructural manipulations for a tunable release rate. As well, we have demonstrated biodegradability of SiNWs in phosphate buffer saline (PBS) solution. Strikingly suppressed cell and protein adhesion was observed on our SiNW surface, which indicates a reduced probability for biofouling and drug release impediments. Such antibiotic release from the nanowire-structured surface can provide more reliable antibiotic protection at a targeted implantation or biosensor site.


ACS Nano | 2009

50 nm DNA Nanoarrays Generated from Uniform Oligonucleotide Films

Hyunwoo Noh; Albert M. Hung; Chulmin Choi; Ju Hun Lee; Jin-Yeol Kim; Sungho Jin; Jennifer N. Cha

One of the most challenging but potentially rewarding goals in nanoscience is the ability to direct the assembly of nanoscale materials into functional architectures with high yields, minimal steps, and inexpensive procedures. Despite their unique physical properties, the inherent difficulties of engineering wafer-level arrays of useful devices from nanoscale materials in a cost-effective manner have provided serious roadblocks toward technological impact. To address nanoscale features while still maintaining low fabrication costs, we demonstrate here an inexpensive printing method that enables repeated patterning of large-area arrays of nanoscale materials. DNA strands were patterned over 4 mm areas with 50 nm resolution by a soft-lithographic subtraction printing process, and DNA hybridization was used to direct the assembly of sub-20 nm materials to create highly ordered two-dimensional nanoparticle arrays. The entire printing and assembly process was accomplished in as few as three fabrication steps and required only a single lithographically templated silicon master that could be used repeatedly. The low-cost procedures developed to generate nanoscale DNA patterns can be easily extended toward roll-to-roll assembly of nanoscale materials with sub-50 nm resolution and fidelity.


Acta Biomaterialia | 2010

Plasma-induced nanopillars on bare metal coronary stent surface for enhanced endothelialization.

Mariana Loya; Karla S. Brammer; Chulmin Choi; Li-Han Chen; Sungho Jin

An increased risk of late stent thrombosis associated with polymer carriers on the surface of drug-eluting stents remains one of the challenges in cardiovascular stent technology, which has instigated a renewed interest in the polymer-less, bare metal stent approach. As thrombus formation is most likely augmented by the lack of endothelial cell coverage at the exposed stented site, an improved stent surface that enhances cell coverage is essential for viable polymer-less all metal stents. We demonstrate superior endothelial cell growth, more continuous monolayer formation and overall improved endothelialization with nanopillar arrays created via radio frequency plasma surface texturing on our all metallic stent surface of MP35N stent alloy. It is shown that the nanotextured surface significantly up-regulates primary bovine aortic endothelial cell (BAEC) functionality when compared with unprocessed, smooth MP35N surfaces without a nanopillar topography. The desirable presence of transmembrane tight junctions and highly organized monolayer formation was induced by the presence of the nanopillar surface texture. The nanopillar structure also produced a reduced level of oxidative stress in the BAECs. These findings may contribute to new nanotechnology-based surface design concepts for bare metal stents producing advanced cardiovascular implants which mitigate late stent thrombosis.


Nano Letters | 2012

Magnetically guided nano-micro shaping and slicing of silicon.

Young Oh; Chulmin Choi; Daehoon Hong; Seong Deok Kong; Sungho Jin

Silicon is one of the most important materials for modern electronics, telecom, and photovoltaic (PV) solar cells. With the rapidly expanding use of Si in the global economy, it would be highly desirable to reduce the overall use of Si material, especially to make the PVs more affordable and widely used as a renewable energy source. Here we report the first successful direction-guided, nano/microshaping of silicon, the intended direction of which is dictated by an applied magnetic field. Micrometer thin, massively parallel silicon sheets, very tall Si microneedles, zigzag bent Si nanowires, and tunnel drilling into Si substrates have all been demonstrated. The technique, utilizing narrow array of Au/Fe/Au trilayer etch lines, is particularly effective in producing only micrometer-thick Si sheets by rapid and inexpensive means with only 5 μm level slicing loss of Si material, thus practically eliminating the waste (and also the use) of Si material compared to the ~200 μm kerf loss per slicing and ~200 μm thick wafer in the typical saw-cut Si solar cell preparation. We expect that such nano/microshaping will enable a whole new family of novel Si geometries and exciting applications, including flexible Si circuits and highly antireflective zigzag nanowire coatings.


ACS Nano | 2010

Site-Specific Patterning of Highly Ordered Nanocrystal Superlattices through Biomolecular Surface Confinement

Hyunwoo Noh; Chulmin Choi; Albert M. Hung; Sungho Jin; Jennifer N. Cha

With the increasing demand in recent years for high-performance devices for both energy and health applications, there has been extensive research to direct the assembly of nanoparticles into meso- or macroscale single two- and three-dimensional crystals of arbitrary configuration or orientation. Inorganic nanoparticle arrays can have intriguing physical properties that differ from either individual nanoparticles or bulk materials. For most device applications, it is necessary to fabricate two-dimensional nanoparticle superlattices at programmed sites on a surface. However, it has remained a significant challenge to generate patterned arrays with long-range positional order because most highly ordered close-packed nanocrystal arrays are typically obtained by kinetically driven evaporation processes. In this report, we demonstrate a method to generate patterned nanocrystal superlattices by confining nanoparticles to geometrically defined 2-D DNA sites on a surface and using associative biomolecular interparticle interactions to produce thermodynamically stable arrays of hexagonally packed nanocrystals with significant long-range order observed over 1-2 μm. We also demonstrate the role of chemical and geometrical confinement on particle packing and obtaining long-range order. Finally, we also demonstrate that the formation of DNA-mediated nanocrystal superlattices requires both interparticle DNA hybridization and solvent-less thermal annealing.

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Sungho Jin

University of California

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Kunbae Noh

University of California

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Daehoon Hong

University of California

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Young Oh

University of California

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Cihan Kuru

University of California

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Duyoung Choi

University of California

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Gunwoo Kim

University of California

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Li-Han Chen

University of California

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