Geunbae Lim

Pohang University of Science and Technology, Geijitsu, Gyeongsangbuk-do, South Korea

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Publications (103)210.97 Total impact

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    ABSTRACT: The application of nanomaterials for biosensors and fuel cells is becoming more common, but it requires an understanding of the relationship between the structure and electrochemical characteristics of the materials at the nanoscale. Herein, we report the development of scanning electrochemical microscopy–atomic force microscopy (SECM–AFM) nanoprobes for collecting spatially resolved data regarding the electrochemical activity of nanomaterials such as carbon nanotube (CNT) networks. The fabrication of the nanoprobe begins with the integration of CNT-bundle wire into a conventional AFM probe followed by the deposition of an insulating layer and cutting of the probe end. In addition, a protrusive insulating tip is integrated at the end of the insulated CNT-bundle wire to maintain a constant distance between the nanoelectrode and the substrate; this yields an L-shaped nanoprobe. The resultant nanoprobes produced well-fitted maps of faradaic current data with less than 300 nm spatial resolution and topographical images of CNT networks owing to the small effective distance (on the order of tens of nanometers) between the electrode and substrate. Electrochemical imaging using the L-shaped nanoprobe revealed that the electrochemical activity of the CNT network is not homogeneous and provided further understanding of the relationship between the topography and electrochemical characteristics of CNT networks.
    The Analyst 03/2015; DOI:10.1039/C4AN02139H · 3.91 Impact Factor
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    ABSTRACT: Ion concentration polarization (ICP) is a distinctive electrochemical phenomenon that occurs near an ion-exchange membrane with an applied DC electric field, generating a significant concentration gradient in back and forth on the membrane. To date, however, there have been only a few attempts to introduce unconventional materials for ion transport in micro-nanofluidic systems. Here, we describe the development of a novel ICP system using an entangled single-wall carbon nanotube (SWNT) film as an ion-selective membrane instead of a Nafion membrane, for investigating the detailed relationship between electrical properties, i.e., ionic conductance through nanojunctions, and nonlinear electrokinetic behavior.
    Japanese Journal of Applied Physics 03/2015; 54(3):035102. DOI:10.7567/JJAP.54.035102 · 1.06 Impact Factor
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    ABSTRACT: Optical-resolution photoacoustic microscopy (OR-PAM) is a novel label-free microscopic imaging tool to provide in vivo optical absorbing contrasts. Specially, it is crucial to equip a real-time imaging capability without sacrificing high signal-to-noise ratios (SNRs) for identifying and tracking specific diseases in OR-PAM. Herein we demonstrate a 2-axis water-proofing MEMS scanner made of flexible PDMS. This flexible scanner results in a wide scanning range (9 × 4 mm(2) in a transverse plane) and a fast imaging speed (5 B-scan images per second). Further, the MEMS scanner is fabricated in a compact footprint with a size of 15 × 15 × 15 mm(3). More importantly, the scanning ability in water makes the MEMS scanner possible to confocally and simultaneously reflect both ultrasound and laser, and consequently we can maintain high SNRs. The lateral and axial resolutions of the OR-PAM system are 3.6 and 27.7 μm, respectively. We have successfully monitored the flow of carbon particles in vitro with a volumetric display frame rate of 0.14 Hz. Finally, we have successfully obtained in vivo PA images of microvasculatures in a mouse ear. It is expected that our compact and fast OR-PAM system can be significantly useful in both preclinical and clinical applications.
    Scientific Reports 01/2015; 5:7932. DOI:10.1038/srep07932 · 5.08 Impact Factor
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    Seong Kyung, Geunbae Lim, Seong J Cho
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    ABSTRACT: Electrospinning of polyurethane (PU) is widely used to fabricate breathable fabrics for applications in outdoor sportswear, and can be used to produce a highly porous structure, which is an essential property of a breathable fabric. To increase the breathability of the fabric, herein we describe the use of a metal mesh as the ground electrode in place of the conventional planar electrode during electrospinning. This electrode geometry results in an electric field that leads to the electrospun fibers being predominantly stacked over the metal wires of the mesh, with fewer fibers over the holes, resulting in larger pores in the membrane. The average pore size and thickness of the membrane were compared with those of a membrane fabricated using a conventional planar electrode. A quantitative analysis performed according to the Korean Industrial Standards (KS) indicated improved breathability.
    Sensors and Materials 01/2015; 27(1):77-85. · 0.46 Impact Factor
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    ABSTRACT: We report a novel passive frequency-trimming method for microelectromechanical system (MEMS) resonators using carbon nanotube (CNT) rope synthesis on the side of the MEMS resonator. The method has a number of advantages over conventional methods, including low cost, reduced processing time, and greater applicability. First, the method requires only an ac voltage source and a dispersed CNT suspension to synthesize CNT ropes using dielectrophoresis. The method can be implemented in <;3 min at room temperature and atmospheric pressure. The resonant frequency of the MEMS resonator can be shifted by 0.5%-24%. In addition, the method can restore the original frequency by cutting the CNT ropes by passing a current through them, so that trimming can be carried out repeatedly by connecting and disconnecting the CNT ropes until the desired frequency is achieved.
    Journal of Microelectromechanical Systems 12/2014; 23(6):1383-1388. DOI:10.1109/JMEMS.2014.2313624 · 1.92 Impact Factor
  • Jungwoo Sung, Geunbae Lim
    11/2014; 23(6):388-391. DOI:10.5369/JSST.2014.23.6.388
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    Seong J. Cho, Taechang An, Geunbae Lim
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    ABSTRACT: An ideal black material absorbs light perfectly over all wavelengths and is totally nonreflective. Material and structural design are crucial to the management of reflectivity. Here, we report a three-dimensionally designed (3D) silicon structure consisting of silicon pillars. To our knowledge, this 3D hierarchical surface has the lowest specular reflectance among silicon-based materials reported to date.
    Chemical Communications 10/2014; DOI:10.1039/C4CC07341J · 6.72 Impact Factor
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    Hyoryung Nam, Taechang An, Geunbae Lim
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    ABSTRACT: The extracellular matrix provides mechanical support and affects cell behaviour. Nanoscale structures have been shown to have functions similar to the extracellular matrix. In this study, we fabricated nanoprotrusion structures with polyaniline as cell culture plates using a simple method and determined the effects of these nanoprotrusion structures on cells.
    Nanoscale Research Letters 10/2014; 9(1):566. DOI:10.1186/1556-276X-9-566 · 2.52 Impact Factor
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    ABSTRACT: Hydrophobic surface properties are sought after in many areas of research, engineering, and consumer product development. Traditionally, hydrophobic surfaces are produced by using various types of coatings. However, introduction of foreign material onto the surface is often undesirable as it changes surface chemistry and cannot provide a long lasting solution (i.e. reapplication is needed). Therefore, surface modification by transforming the base material itself can be preferable in many applications. Femtosecond laser ablation is one of the methods that can be used to create structures on the surface that will exhibit hydrophobic behavior. The goal of the presented research was to create micro and nano-scale patterns that will exhibit hydrophobic properties with no additional post treatment. As a result, dual scale patterned structures were created on the surface of steel aluminum and tungsten carbide samples. Ablation was performed in the open air with no subsequent treatment. Resultant surfaces appeared to be strongly hydrophobic or even superhydrophobic with contact angle values of 140 degrees and higher. In conclusion, the nature of surface hydrophobicity proved to be highly dependent on surface morphology as the base materials used are intrinsically hydrophilic. It was also proven that the hydrophobicity inducing structures could be manufactured using femtosecond laser machining in a single step with no subsequent post treatment.
    Applied Surface Science 09/2014; 313:411-417. DOI:10.1016/j.apsusc.2014.05.224 · 2.54 Impact Factor
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    ABSTRACT: Possible mechanisms of overlimiting current in unsupported electrolytes, exceeding diffusion limitation, have been intensely studied for their fundamental significance and applications to desalination, separations, sensing, and energy storage. In bulk membrane systems, the primary physical mechanism is electro-convection, driven by electro-osmotic instability on the membrane surface. It has recently been predicted that confinement by charged surfaces in microchannels or porous media favors two new mechanisms, electro-osmotic flow (EOF) and surface conduction (SC), driven by large electric fields in the depleted region acting on the electric double layers on the sidewalls. Here, we provide the first direct evidence for the transition from SC to EOF above a critical channel height, using in situ particle tracking and current-voltage measurements in a micro/nanofluidic device. The dependence of the over-limiting conductance on channel depth (d) is consistent with theoretical predictions, scaling as d^-1 for SC and d^4/5 for EOF with a transition around d=8um. This complete picture of surface-driven over-limiting current can guide engineering applications of ion concentration polarization phenomena in microfluidics and porous media.
    Physical Review Letters 09/2014; 114(11). · 7.73 Impact Factor
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    ABSTRACT: A stretchable humidity sensor which has mechanical stability has remained a challenging issue in stretchable electronics. Here, we report a novel stretchable nanostructured polyaniline humidity sensor. The sensor was fabricated by simple fabrication methods such as a pre-stretching process and self-assembly texturing process. The periodically wrinkled structure supported by microstructured elastomeric substrates provides an excellent stretchability. The micro-textured substrate was introduced for solving the crack problem by the Poisson effect as a stretchable sensor. The sensor maintains its humidity sensitivity well at different elongations. To the best of our knowledge, this is the first report of a stretchable humidity sensor.
    RSC Advances 08/2014; 4(75):39767-39770. DOI:10.1039/c4ra04938a · 3.71 Impact Factor
  • Sungchan Yun, Geunbae Lim
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    ABSTRACT: Non-axisymmetric drops can significantly alter impact dynamics via rebound suppression when compared to axisymmetric drops. In this study, we focus on ellipsoidal drop impact on a non-wetting surface and investigate the effects of the geometric aspect ratio ($\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}{AR}$) and the Weber number (${\mathit{We}}$) on the dynamics and outcomes of impacts, both experimentally and numerically. Non-axisymmetric spreading features are characterized by scrutinizing the maximal extensions along the $x$-axis ($D_{mx}$) and $y$-axis ($D_{my}$) with respect to ${AR}$ and ${\mathit{We}}$. The ratio of the maximal extensions depends strongly on ${AR}$, following our scaling relation $D_{mx}/D_{my} \sim {AR}^{1/2}$. Experimental and numerical studies show that increasing ${AR}$ induces a high degree of axis switching during retraction, thereby resulting in the prevention of drop rebound, where axis switching denotes alternate expansion and contraction along the principal axes. We determine the transition between rebound and deposition (rebound suppression) over the ${AR}$ and ${\mathit{We}}$ domains and discuss the transition based on a non-axial distribution of the kinetic energy. The understanding of ellipsoidal drop impacts will potentially provide applications to surface patterning, cleaning, and cooling.
    Journal of Fluid Mechanics 08/2014; 752:266-281. DOI:10.1017/jfm.2014.332 · 2.29 Impact Factor
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    ABSTRACT: Nanofabrication technologies have been a strong advocator for new scientific fundamentals that have never been described by traditional theory, and have played a seed role in ground-breaking nano-engineering applications. In this study, we fabricated ultra-high-aspect (∼10(6) with O(100) nm nanochannel opening and O(100) mm length) orthogonal nanochannel array using only polymeric materials. Vertically aligned nanochannel arrays in parallel can be stacked to form a dense nano-structure. Due to the flexibility and stretchability of the material, one can tune the size and shape of the nanochannel using elongation and even roll the stack array to form a radial-uniformly distributed nanochannel array. The roll can be cut at discretionary lengths for incorporation with a micro/nanofluidic device. As examples, we demonstrated ion concentration polarization with the device for Ohmic-limiting/overlimiting current-voltage characteristics and preconcentrated charged species. The density of the nanochannel array was lower than conventional nanoporous membranes, such as anodic aluminum oxide membranes (AAO). However, accurate controllability over the nanochannel array dimensions enabled multiplexed one microstructure-on-one nanostructure interfacing for valuable biological/biomedical microelectromechanical system (BioMEMS) platforms, such as nano-electroporation.
    Nanoscale 07/2014; 6(16). DOI:10.1039/c4nr00350k · 6.74 Impact Factor
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    ABSTRACT: MEMS gyroscopes have favorable characteristics, including small size, high throughput, and low cost. The performance of MEMS gyroscopes depends on the displacement sensitivity of the capacitors. In this paper, we describe the fabrication of 300-?m-thick gyroscopes that can provide high displacement sensitivity and are robust to fabrication tolerances, i.e. deep reactive ion etch (DRIE) rate uniformity. When thick structures are perforated using DRIE to achieve high-aspect-ratio features, footing is commonly observed. However, we describe a fabrication method that circumvents problems associated with footing and side-wall etching, so that the gyroscopes can have uniform dimensions and small variations across the wafer. Using a post-fabrication translation approach, the position of capacitors is modified following DRIE, and the gap in the gyroscopes can be reduced to 3??m, which leads to an aspect ratio of 100. Using this method, we fabricated MEMS gyroscopes that can overcome the DRIE aspect ratio limit and have capacitors with higher sensitivities than those of other gyroscopes, which typically employ substrates that are less than 100??m thick. The gyroscope had a resonant frequency of 9.91?kHz, a quality factor of 2500 and a sensitivity of 23?mV/[deg/s].
    Journal of Micromechanics and Microengineering 06/2014; 24(7):075013. DOI:10.1088/0960-1317/24/7/075013 · 1.73 Impact Factor
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    ABSTRACT: Synchrotron hard X-ray irradiation can be utilized in lithography processes to manufacture precise structures. Due to the difficulty of precise X-ray mask fabrication in hard X-ray lithography, this X-ray process has been used mainly to fabricate precise microstructures. In this study, a technology is proposed for fabricating novel multi-scale patterns that include submicron-scale structures using hard X-rays. The required X-ray masks for submicron-sized patterning are fabricated by a simple UV lithography process and sidewall metal deposition. Above all, thanks to the high penetration capability of hard X-rays with sub-nanometer wavelengths, it is possible to employ multiple masks to fabricate a variety of patterns. By combining each sub-micron X-ray mask with typical micro-sized X-ray masks, a unique X-ray lithography is performed, and various multi-scale structures are fabricated. The usefulness of the proposed technology is demonstrated by the realization of these structures. (C) 2014 Published by Elsevier B.V.
    Current Applied Physics 05/2014; 14(5). DOI:10.1016/j.cap.2014.03.013 · 2.03 Impact Factor
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    ABSTRACT: Conventional machining technologies such as a milling process have limitations in accuracy to fabricate microstructures. Deep X-ray lithography using the synchrotron radiation is a promising micromachining process with an excellent accuracy, whereas there are difficulties in the fabrication of multi-layered structures. Therefore, it is mainly used for fabricating simple mono-layered microstructures with a high aspect ratio. In this study, a novel technology for fabricating multi-layered microstructures is proposed by combining two processes. In advance, an X-ray resist material is cut and machined into various shapes and heights by the micro milling process. Subsequent X-ray irradiation process facilitates the fabrication of multi-layered microstructures. The proposed technology can overcome the limitation of the pattern accuracy in conventional milling process and the difficulty of the multi-layered machining in x-ray process. The usefulness of the proposed technology is demonstrated in this study by applying the technique in the realization of various multi-layered microstructures.
    03/2014; 31(3). DOI:10.7736/KSPE.2014.31.3.269
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    ABSTRACT: Microelectrodes are widely used for monitoring neural activities in various neurobiological studies. The size of the neural electrode is an important factor in determining the signal-to-noise ratio (SNR) of recorded neural signals and, thereby, the recording sensitivity. Here, it is demonstrated that commercial tungsten microelectrodes can be modified with carbon nanotubes (CNTs), resulting in a highly sensitive recording ability. The impedance with the respect to surface area of the CNT-modified electrodes (CNEs) is much less than that of tungsten microelectrodes because of their large electrochemical surface area (ESA). In addition, the noise level of neural signals recorded by CNEs is significantly less. Thus, the SNR is greater than that obtained using tungsten microelectrodes. Importantly, when applied in a mouse brain invivo, the CNEs can detect action potentials five times more efficiently than tungsten microelectrodes. This technique provides a significant advance in the recording of neural signals, especially in brain regions with sparse neuronal densities.
    02/2014; 3(2). DOI:10.1002/adhm.201300183
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    ABSTRACT: Microbes have been used extensively in various fields of researches and industries but has not been used widely for microfluidic biosensor applications because it is difficult to immobilize properly to a small space. Therefore, we developed a microbial immobilization method for microfluidic devices using single-walled nanotubes and dielectrophoretic force. Single-walled nanotubes and Escherichia coli were aligned between two cantilever electrodes by a positive dielectrophoretic force resulting in a film of single-walled nanotubes with attached Escherichia coli. The optimal condition of film formation without a cell lysis was investigated. Diameter of single-walled nanotubes and electric field (intensity and duration of application) had an effect on the cell viability. On the other hand, the cell concentration of the suspension did not affect the cell viability. Paraoxon was detected using single-walled nanotubes film with attached Escherichia coli that expressed organophosphorus hydrolase. This film which is suspended from the substrate showed faster response time than sensors that are not suspended from the substrate.
    01/2014; 23(1). DOI:10.5369/JSST.2014.23.1.35
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    ABSTRACT: Recently, tremendous application utilizing electrospun nanofibers have been actively reported due to its several advantages, such as high surface to volume ratio, simple fabrication and high-throughput manufacturing. In this paper, we developed highly sensitive and consistent nanofiber humidity sensor by electrospinning. The humidity sensor was fabricated by rapid electrospinning (~2 sec) /PVP/LiCl mixed solution on the micro-interdigitated electrode. In order to evaluate the humidity sensing performances, we measured current response using DC bias voltage under various relative humidity levels. The results show fast response / recovery time and marginal hysteresis as well as long-term stability. In addition, with the aid of micro-interdigitated electrode, we can reduce a total resistance of the sensor and increase the total reaction area of nanofibers across the electrodes resulting in high sensitivity and enhanced current level. Therefore, we expect that the electrospun nanofiber array for humidity sensor can be feasible and promising for diverse humidity sensing application.
    01/2014; 23(1). DOI:10.5369/JSST.2014.23.1.42
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    ABSTRACT: We present a droplet-manipulation method using opto-thermal flows in oils. The flows are originated from Marangoni and buoyancy effects due to temperature gradient, generated by the adsorption of light on an amorphous silicon thin film. Using this method, we can transport, merge and mix droplets in an extremely simple system. Since the temperature rise during the operation is small, this method can be used for biological applications without the damage on cell viability.
    01/2014; 31(1). DOI:10.7736/KSPE.2014.31.1.91

Publication Stats

525 Citations
210.97 Total Impact Points

Institutions

  • 2004–2015
    • Pohang University of Science and Technology
      • Department of Mechanical Engineering
      Geijitsu, Gyeongsangbuk-do, South Korea
    • SAIT Polytechnic
      Calgary, Alberta, Canada
  • 2012
    • Massachusetts Institute of Technology
      • Department of Electrical Engineering and Computer Science
      Cambridge, Massachusetts, United States
  • 2003–2007
    • Kyungpook National University
      • Department of Electronic Engineering
      Daikyū, Daegu, South Korea
    • Samsung Advanced Institute of Technology
      Usan-ri, Gyeonggi-do, South Korea