Kunbae Noh

University of California, San Diego, San Diego, California, United States

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Publications (23)48.55 Total impact

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    ABSTRACT: Graphene has received appreciable attention for its potential applications in flexible conducting film due to its exceptional optical, mechanical, and electrical properties. However increasing transmittance of graphene without sacrificing the electrical conductivity has been difficult. The fabrication of optically highly transparent (≈98%) graphene layer with a reasonable electrical conductivity is demonstrated here by nanopatterning and doping. Anodized aluminium oxide nanomask prepared by facile and simple self-assembly technique is utilized to produce an essentially hexagonally nanopatterned graphene. The electrical resistance of the graphene increases significantly by a factor of ≈15 by removal of substantial graphene regions via nanopatterning into hexagonal array pores. However, the use of chemical doping on the nanopatterned graphene almost completely recovers the lost electrical conductivity, thus leading to a desirably much more optically transparent conductor having ≈6.9 times reduced light blockage by graphene material without much loss of electrical conductivity. It is likely that the availability of large number of edges created in the nanopatterned graphene provides ideal sites for chemical dopant attachment, leading to a significant reduction of the sheet resistance. The results indicate that the nanopatterned graphene approach can be a promising route for simultaneously tuning the optical and electrical properties of graphene to make it more light-transmissible and suitable as a flexible transparent conductor. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    No preview · Article · Mar 2015 · Small
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    ABSTRACT: We demonstrate a successful fabrication of Nanopatterned Graphene (NPG) using a PS-b-P4VP polymer, which was never used previously for the graphene patterning. The NPG exhibits homogeneous mesh structures with an average neck width of ∼19 nm. Electronic characterization of single and few layers NPG FETs (field effect transistors) were performed at room temperature. We found that the sub-20 nm neck width creates a quantum confinement in NPG, which has led to a bandgap opening of ∼0.08 eV. This work also demonstrates that BCP (block co-polymer) lithography is a pathway for low-cost, high throughput large-scale production of NPG with critical dimensions down to the nanometer regime.
    Preview · Article · Apr 2014 · Materials Research Letters
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    ABSTRACT: Nanostructured surface geometries have been the focus of a multitude of recent biomaterial research, and exciting findings have been published. However, only a few publications have directly compared nanostructures of various surface chemistries. The work herein directly compares the response of human osteoblast cells to surfaces of identical nanotube geometries with two well-known orthopedic biomaterials: titanium oxide (TiO2) and tantalum (Ta). The results reveal that the Ta surface chemistry on the nanotube architecture enhances alkaline phosphatase activity, and promotes a ~ 30% faster rate of matrix mineralization and bone-nodule formation when compared to results on bare TiO2 nanotubes. This study implies that unique combinations of surface chemistry and nanostructure may influence cell behavior due to distinctive physico-chemical properties. These findings are of paramount importance to the orthopedics field for understanding cell behavior in response to subtle alterations in nanostructure and surface chemistry, and will enable further insight into the complex manipulation of biomaterial surfaces. With increased focus in the field of orthopedic materials research on nanostructured surfaces, this study emphasizes the need for careful and systematic review of variations in surface chemistry in concurrence with nanotopographical changes.
    No preview · Article · Apr 2014
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    ABSTRACT: Various approaches have been studied to engineer the implant surface to enhance bone in-growth properties, particularly using micro- and nano-topography. In this study, the behavior of osteoblast (bone) cells was analyzed in response to a titanium oxide (TiO2) nanotube-coated commercial zirconia femoral knee implant consisting of a combined surface structure of a micro-roughened surface with the nanotube coating. The osteoblast cells demonstrated high degrees of adhesion and integration into the surface of the nanotube-coated implant material, indicating preferential cell behavior on this surface when compared to the bare implant. The results of this brief study provide sufficient evidence to encourage future studies. The development of such hierarchical micro- and nano-topographical features, as demonstrated in this work, can provide insightful designs for advanced bone-inducing material coatings on ceramic orthopedic implant surfaces.
    No preview · Article · Jul 2013
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    ABSTRACT: Titanium oxide (TiO2) nanotube arrays were prepared by anodization of Ti/Au/Ti trilayer thin film DC sputtered onto forged and cast Co-28Cr-6Mo alloy substrate at 400°C. Two different types of deposited film structures (Ti/Au/Ti trilayer and Ti monolayer), and two deposition temperatures (room temperature and 400°C) were compared in this work. The concentrations of ammonium fluoride (NH4F) and H2O in glycerol electrolyte were varied to study their effect on the formation of TiO2 nanotube arrays on a forged and cast Co-28Cr-6Mo alloy. The results show that Ti/Au/Ti trilayer thin film and elevated temperature sputtered films are favorable for the formation of well-ordered nanotube arrays. The optimized electrolyte concentration for the growth of TiO2 nanotube arrays on forged and cast Co-28Cr-6Mo alloy was obtained. This work contains meaningful results for the application of a TiO2 nanotube coating to a CoCr alloy implant for potential next-generation orthopedic implant surface coatings with improved osseointegrative capabilities.
    Full-text · Article · Apr 2013
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    ABSTRACT: Highly ordered TiO2 nanotube arrays with large diameter of 680–750 nm have been prepared by high voltage anodization in an electrolyte containing ethylene glycol at room temperature. To effectively suppress dielectric breakdown due to high voltage, pre-anodized TiO2 film was formed prior to the main anodizing process. Vertically aligned, large sized TiO2 nanotubes with double-wall structure have been demonstrated by SEM in detail under various anodizing voltages up to 225 V. The interface between the inner and outer walls in the double-wall configuration is porous. Surface topography of the large diameter TiO2 nanotube array is substantially improved and effective control of the growth of large diameter TiO2 nanotube array is achieved. Interestingly, the hemispherical barrier layer located at the bottom of TiO2 nanotubes formed in this work has crinkles analogous to the morphology of the brain cortex. These structures are potentially useful for orthopedic implants, storage of biological agents for controlled release, and solar cell applications.
    No preview · Article · Jan 2013 · Materials Science and Engineering C
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    ABSTRACT: We have developed patterned media via ion implantation using Au nano mask approach for local control of coercivity of magnetically hard [Co/Pd]n multilayer film. Au nano-islands produced through a di-block copolymer templated technique is used as the mask for ion implantation. The sputter deposited [Co 0.3 nm/Pd 0.8 nm]8/Pd 3 nm/Ta 3 nm multilayer film having vertical magnetic anisotropy is coated with a diblock copolymer layer, two phase decomposed into vertically pored nanostructure, then chemically processed to nucleate gold nanoislands corresponding to the diblock copolymer nanostructures. Subsequent nitrogen (N) ion implantation, using these Au islands as implantation-blocking masks, allows a patterned penetration of implanted ions into unmasked portion of the [Co/Pd]n multilayer film, thus creating invisible but magnetically isolated bit island geometry while maintaining the overall flat configuration of the patterned media.
    No preview · Article · Nov 2012 · IEEE Transactions on Magnetics
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    ABSTRACT: Patterned media (PM) for magnetic information storage have received increased attention in recent years as the primary candidate for 1 Terabit/in2 or higher recording density for computer hard disk drives. A PM consists of a periodic array of well-defined magnetic islands, each of which can store one bit of data. In the simplest scheme, the structures could be magnetic pillars and dots with a single easy axis of magnetization. The direction of detected magnetization by the read/write head is interpreted as a binary signal 1 or 0. Some of the main technical issues in the PM include the difficulty in fabricating small nanoisland arrays in a periodic fashion over large areas, reliability/reproducibility of magnetic bit characteristics, as well as wear and head flyability issues associated with the media surface roughness, and processing cost. This article deals with a recent investigation of various fabrication approaches, nanostructural features, and magnetic properties for the bit PM.
    No preview · Article · Oct 2012 · JOM: the journal of the Minerals, Metals & Materials Society
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    ABSTRACT: In this paper, we review recent advances in nanotemplate fabrication using anodized aluminum oxide (AAO). In addition to self-ordered AAO nanoarrays, guided AAO self-assembly is of great interest since it can offer highly ordered, vertically aligned nanoporous templates which are suitable for various materials synthesis and alignment of nanosized structures. Moreover, structural modification of AAO nanoarrays by controlling fabrication process parameters are reviewed which can be applicable for advanced micro-and nanosystems. In this aspect, potential applications using AAO will be revealed in the aspects of self-ordered AAO, guided self-assembly of AAO, and biomedical and magnetic applications.
    Preview · Article · Dec 2011 · Nano brief reports and reviews
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    ABSTRACT: Electrochemical formation of tunable nanoscale oxide layers on biomedical metallic surfaces has recently drawn much attention in biomaterials research. In this study, we report on the cellular response to a unique vertically aligned, laterally spaced nanotube nanostructure made of zirconium oxide (ZrO2) fabricated by anodization. The growth, morphology, and functionality of osteoblasts cultured on ZrO2 nanotubes have been investigated. The initial adhesion and spreading was considerably improved on the nanotube surface as compared to a flat zirconium (Zr) surface without a nanostructure. The morphology of the adhered cells on the nanotube surface elicited a highly organized cytoskeleton with crisscross patterned actin, which was lacking on the flat Zr. Increased alkaline phosphatase activity levels and the formation of calcified extracellular matrix implied improved osteoblast functionality and mineralization on the nanotube substrate. This in vitro study suggests that the ZrO2 nanotubes provided an enhanced osteoblast response and demonstrated their apparent role in providing a platform for bone growth.
    No preview · Article · Dec 2011 · Materials Science and Engineering C
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    ABSTRACT: An atomic force microscope (AFM) system with multiple parallel lithography probes of equal heights on a single cantilever was created in order to improve the throughput of AFM lithography. The multitip probe was fabricated by electron-beam (e-beam) lithography and a dry silicon etching process. Several carbon islands were made on a single cantilever in a straight line by e-beam lithography and were used as an etch mask, whereas the silicon pedestal structure of the multitip probe was fabricated by reactive ion etching (RIE). Finally the carbon islands were sharpened by a RIE process using oxygen gas. The multitip probe was successfully applied to form multidot pattern arrays on a negative resist film coated on silicon by low electric field induced AFM lithography. A pedestal nanopillar structure was utilized as a convenient support feature that enabled better control of multiple nanotip arrays for AFM writing. The authors fabricated such a nanopedestal array with extremely sharp nanoneedle tips.
    No preview · Article · Nov 2011 · Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures
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    ABSTRACT: Porous anodized aluminum oxide (AAO) nanostructures have been extensively investigated as versatile templates for nanodots and nanowires for many applications. Such self-ordered AAO structures are often achieved by so-called two-step anodization. Ordered pore arrangements can be obtained in the second step after removing the AAO layer formed in the first anodizing step, during which hexagonally ordered, concave-pored Al surface is formed so as to serve as vertical pore nucleation sites for the subsequent anodization step. Although such a two-step anodization process has proven useful for bulk Al surface, the relatively large amount of Al material that needs to be used up to obtain a well ordered AAO template is an issue when the starting material is a thin film layer of Al rather than a bulk Al foil. In this paper, we demonstrate successful fabrications of ordered and vertically aligned AAO nanopore patterns directed by a hexagonally patterned poly(styrene-b-4-vinylpyridine) di-block copolymer layer placed on a thin Al film surface. In addition, a successful electrodeposition of Ni nanowires into the AAO nanopores is demonstrated and their magnetization properties are studied.
    No preview · Article · Nov 2011 · Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures
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    ABSTRACT: Co/Pd multilayer media with an antidot structure have been fabricated on hole-patterned Si substrates. Si nanoholes were created by dry etching using a self-assembled diblock copolymer (DBCP) pattern as an etch mask. To enhance dry etching selectivity, thin Cr film was vacuum deposited on DBCP surface at an oblique incident angle. In comparison with the Co/Pd multilayer on a flat Si substrate, the magnetic hysteresis loop of the antidot structured Co/Pd multilayer revealed an enhanced coercive force largely independent of the amount of reactive etching time, while the shape of hysteresis loop was substantially altered as a function of dry etching time. In an optimized condition, high perpendicular magnetic anisotropy was obtained while no significant parallel magnetic anisotropy was seen, which is desirable for perpendicular recording media or pseudobit-patterned media application.
    No preview · Article · Nov 2011 · IEEE Transactions on Magnetics
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    ABSTRACT: Thin films deposited on a flat substrate, if the adhesion is not strong, can be made to ball up and form discrete islands upon heating to elevated temperatures due to the surface energy difference. We have applied this useful process to electron beam lithography (EBL) and nano-imprinting lithography (NIL). By combining the ball-up processes of Ni thin film with e-beam lithography followed by reactive ion etching, Si nano islands and vertical nanopillars as small as 10 nm in diameter have been realized. There is a strong correlation betweentheinitialthicknessof Nimasklayerandfinal Niislanddiameterobtainedafterball-up.Thesmallestislanddiameterisobtained using 5-nm initial Ni layer thickness. Below 3-nm initial layer thickness, the intended ball-up reaction does not occur. With more than 15-nm initial metal film thickness, the Ni layer is broken up into nonspherical and highly irregular structures. Co/Pd multilayer magnetic thin films deposited on prepatterned substrate by nano imprinting lithography with versus without island ball-up process have been investigated for bit patterned media studies. The coercivity of magnetic islands can be enhanced by island diameter reduction using the ball up process.
    No preview · Article · Oct 2011 · IEEE Transactions on Magnetics
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    ABSTRACT: We have investigated the magnetic M-H loop characteristics of CoCrPt-SiO2 perpendicular recording media as influenced by nonmagnetic ion implantations. We have also developed patterned media via ion implantation using a convenient polymer nanomask approach for local control of coercivity of magnetically hard [Co/Pd]n multilayer film with a [Co 0.3 nm Pd 0.8 nm] 8/Pd 3 nm/Ta 3 nm layer structure. The CoCrPt-SiO2 magnetic layer having perpendicular magnetic anisotropy is sputter deposited on a flat substrate. The [Co/Pd] n multilayer film with vertical magnetic anisotropy is deposited and the regions corresponding to the magnetic recording bit islands are coated with polymer islands using a nanoimprinting technique. Subsequent ion implantation allows patterned penetration of implanted ions into the [Co/Pd]n multilayer film, thus creating magnetically isolated bit island geometry while maintaining the overall flat geometry of the patterned media.
    No preview · Article · Oct 2011 · IEEE Transactions on Magnetics
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    ABSTRACT: 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.
    No preview · Article · Aug 2011 · Langmuir
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    ABSTRACT: An olfactory factory: Using a novel polydimethylsiloxane (PDMS) elastomer-based device, the switchable release of specific gases is demonstrated. Heating elements trigger the gas release by warming the gas-containing capsule (see scheme). The release can be switched on and off by repeated thermal cycles. An X-Y addressable matrix of such capsules results in an odor-releasing system for multiple odors for virtual-reality applications.
    No preview · Article · Jul 2011 · Angewandte Chemie International Edition
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    ABSTRACT: The TiO2 passivation layer on the Ti surface has positive bone-interface qualities including bioactivity and chemical composition that aids in the integration of orthopedic implants, known as osseointegration. New research designs have focused on the topographic structure of the TiO2 oxide layer with a large surface area for optimizing osseointegration properties for improved implant performance. We have utilized a hydrothermal process to further refine the TiO2 structure to an 8 nm diameter nanotube geometry and evaluated its effect on bone cell growth. Interestingly these small diameter TiO2 nanotubes, unlike the anodized TiO2 nanotubes, exhibit mostly multiwalled nanotube configuration with about 2–6 parallel walls along the length of the elongated nanotubes. It has been found that these multiwall nanotubes significantly enhance osteoblast cell response compared to unmodified, control Ti. The nanotube structure provided significantly up-regulated bone forming ability with ≈ 2–3 fold increased alkaline phosphatase (ALP) activity levels, and induced the formation of abundant amounts of bone matrix deposition predominantly consisting of calcium and phosphorous. Biomedical implications of such bone cell and nanotube interface behavior on bone formation and bone bonding of titanium implants are discussed.
    No preview · Article · Mar 2011 · Advanced Engineering Materials
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    ABSTRACT: Anodized aluminum oxide (AAO), well-known hexagonally ordered vertical pore nanostructure, can be altered to form nanotubular AAO arrays potentially having many favorable properties due to its large surface area and unique geometry. We present here a creation of novel nanotubular AAO structure by the hard anodization technique. Because of the guided formation of void channels at triple cell junctions during the course of the controlled anodization process, periodically spaced-apart aluminum oxide nanotubular geometry could be achieved over large areas. Further separation to well-defined individual AAO nanotube arrays was obtained when etched in a mixed CuCl2/HCl solution during Al substrate removal. Nanotubular geometry AAO with periodic and mechanically robust structure can be useful not only for biomedical applications such as to enhance cell adhesion and viability or drug delivery vehicles, but also as a large-surface-area catalyst support or sensor elements.
    No preview · Article · Jan 2011
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    ABSTRACT: Nanotubular materials have many favorable properties for drug delivery. We present here a pioneering study of con-trolled release of a model drug, amoxicillin, from the internal nanopore structure of self-ordered, periodically spaced-apart aluminum oxide with an innovative, nanotubular geometry. This aluminum oxide nanotube geometry has not yet been revealed for biological applications, thus we have selected this oxide nanotube structure and demonstrated its ability as a drug carrier. Controlled, sustained release was achieved for over 5 weeks. The release kinetics from the nanotube layer was thoroughly characterized and it was determined that the amount of drug released was proportional to the square root of time. This type of controlled release and longevity from the nanotube layer has potential for therapeutic surface coatings on medical implants. Furthermore, this type of geometry has many features that are ad-vantageous and biologically relevant for enhancing tissue biointegration.
    Full-text · Article · Jan 2011 · Journal of Biomaterials and Nanobiotechnology