Seung Hee Lee

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

Are you Seung Hee Lee?

Claim your profile

Publications (318)571.26 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A nitrogen-doped carbon layer coated thermally exfoliated graphene (NC-TEG) is prepared by in-situ polymerization of p-phenylene diamine (PD) with thermally exfoliated graphene (TEG) and subsequent high temperature pyrolysis (600 °C for 1 h in argon gas environment). Fourier transfer infrared spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy confirm the formation of poly-(p-phenylene diamine) layers on the TEG surfaces with a nitrogen doping level of ~6.1%. Physisorption analysis indicates that NC-TEG not only has the enlarged surface area, but also forms hierarchical three dimensional structures with several micro and meso-pores compared to pristine TEG. Due to the synergic effect of nitrogen atoms in the carbon structures and augmented surface area, the capacitance measured from cyclic voltammetry and galvanic charge–discharge increases to 282.5 F g−1 from the 95.1 F g−1 of TEG. Moreover, the PD monomer acts as a reversible faradaic agent. The capacitive performance of the NC-TEG electrode is investigated in different mixed electrolytes. The specific capacitance is significantly increased to 635.6 F g−1 in a mixed electrolyte of 0.025 M PD and 2 M KOH. After 10,000 cycles, the capacitive retention shows remarkable stability as high as 87.4%.
    Carbon 04/2015; 85. DOI:10.1016/j.carbon.2014.12.069 · 6.16 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The formation of optically isotropic liquid crystal (LC) media has been investigated by doping the star-shaped LC molecular surfactants (SiLC) into the rod-shaped twin LC host molecules (DiLC). The experimental phase diagram was constructed on the basis of differential scanning calorimetry (DSC) and then theoretical calculation was conducted through a combined Flory-Huggins (FH)/Maier-Saupe-McMillan (MSM)/phase field (PF) model to account for the experimental results. Phase diagram of the SiLC/DiLC mixtures revealed the broad coexistence regions such as smectic A + crystal (SmA1 + Cr2), liquid + crystal (L1 + Cr2), and liquid + nematic (L1 + N2) at the intermediate composition along with narrow single phase crystal (Cr1, Cr2), smectic (SmA1), and nematic (N2) regions. The morphologies and structures of these coexistence regions were further confirmed by polarized optical microscopy (POM) and wide-angle x-ray diffraction (WAXD). At the 80/20 SiLC/DiLC composition, the optical anisotropy was induced under an alternating current (AC) electric field above its isotropization temperature. The formation of optically isotropic LC medium in mixtures of the SiLC molecular surfactants and nematic LC host may allow us to develop new electro-optical devices.
    Soft Matter 03/2015; DOI:10.1039/C5SM00005J · 4.15 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We investigated the orientational and electro-optical properties of a nematic liquid crystal (LC) aligned with a directly spinnable carbon nanotube (CNT) web functioning both as an electrode and as an alignment layer. The LC molecules were uniformly oriented along the drawing direction of the CNT web and the spatially averaged birefringence was comparable to a rubbed polyimide sample. The CNT web sample also showed smaller residual DC and hysteresis compared to the polyimide sample.
    Liquid Crystals 01/2015; DOI:10.1080/02678292.2014.986233 · 2.35 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Reported herein are the simultaneous phase and anchoring transitions of the chiral azodye-doped liquid crystal (LC) mixture. The transitions are influenced by photostimulation and are stable against light and thermal treatments. The photochromic trans state of the azobenzene molecule used in the study is mesogenic with a chiral nature. The photoinstigated cis form of the molecule has a much-enhanced dipole moment with a broken chromophore symmetry. Due to the high dipole–dipole interaction, the cis-state molecules stack together and form nano-sized aggregates. Subsequently, the cis aggregates get phase-separated, resulting in the chiral- to achiral-phase transition of the host LC. The phase-separated cis aggregates diffuse to the interface between indium-tin-oxide and LC and modify the surface, causing planar to homeotropic anchoring transition of the host LC. The stability and irreversibility of the transition, and the fact that there is no need for pre-treatments for LC alignment, provide a fascinating opportunity for LC device applications.
    01/2015; 16(1):1-6. DOI:10.1080/15980316.2014.1002545
  • The Journal of Physical Chemistry C 01/2015; 119(1):766-774. DOI:10.1021/jp5099574 · 4.84 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Developments of metal oxide nanostructures for simultaneous improvements in light harvesting and charge collection can lead to a significant technical progress in various applications such as photoelectrodes for photoelectrochemical cells and various types of solar cells. Here we present an array of three-dimensional titanium dioxide (TiO2) nanohelixes infiltrated with TiO2 nanoparticles as a multifunctional photoanode for dye sensitized solar cells (DSSCs). The unique geometry and the near-single crystallinity of the vertically aligned TiO2 nanohelix array results simultaneously in strong light scattering and enhanced carrier transport and collection, while maintaining a comparable surface area accessible for dye molecules by the infiltrated TiO2 nanoparticles. Consequently, despite a similar to 40% reduction in dye loading, the overall photon conversion efficiency of the DSSC with the nanoparticle-infiltrated nanohelix-array photoanode is enhanced by 6.2% in comparison with the DSSC with the conventional nanoparticle photoanode.
    Solar Energy Materials and Solar Cells 01/2015; 132:47–55. DOI:10.1016/j.solmat.2014.08.007 · 5.03 Impact Factor
  • Hankook Kwanghak Hoeji 12/2014; 25(6):301-310. DOI:10.3807/KJOP.2014.25.6.301
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: An efficient approach for the preparation of 9-anthracene carboxylic acid (ACA) modified reduced graphene oxide (rGO) was demonstrated in this study. ACA was used as a surface-modifying agent and underwent a reversible redox reaction. The benzene ring of the ACA anion was attached to the rGO surface via π-π interactions, and the carboxylate anions helped to disperse the hybrid materials in water due to hydrogen bonding. Therefore, water-dispersible, ACA-modified rGO (ACA-rGO) improved the wettability and capacitance performance in aqueous electrolyte solutions. The morphology of the ACA-rGO was studied using transmission electron microscopy and atomic force microscopy image analysis. The dispersion characteristics of the exfoliated materials were investigated using UV-vis spectroscopy analysis. The chemical states and natures of the samples were investigated using Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy (XPS). The appearance of a new peak at 288.7 eV in the XPS of ACA-rGO confirmed the successful surface modification of rGO using ACA. Raman spectra were studied to compare the electronic structure and defect concentrations in the ACA-rGO with respect to GO. The low intensity and shifted D- and G-bands indicated non-covalent functionalization of rGO with ACA anions. Electrochemical performances of ACA-rGO, rGO,and GO were evaluated in 1 M aqueous Na2SO4 electrolyte. The capacitance performance was investigated through galvanometric charge-discharge with ACA-rGO, rGO, and GO in an operating voltage of -1 to 1V. The range of specific capacitance in the three-electrode system was 610 to 115 F g-1 at a current density range of 0.8 to 20 A g-1. In addition, the capacitance performance of ACA-rGO was studied in 1 M Na2SO4 electrolyte using two-electrode systems. The cell capacitance, energy density, and power density at a current density of 0.2 A g-1 of the asymmetric assembly with multiwall carbon nanotubes were 77 F g-1, 41.3 Wh kg-1, and 200 W kg-1, respectively
    RSC Advances 12/2014; 5(9). DOI:10.1039/C4RA12356E · 3.71 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We proposed wideband antireflective circular polarizer for realizing a true black state in all viewing directions in organic light-emitting-diode displays (OLEDs). Present commercialized wideband circular polarizer consisted of a half wave and a quarter wave plates having the refractive index parameter (Nz) of 1.5 in both films exhibits light leakage in the oblique viewing directions, deteriorating image quality of a black state. We evaluated Nzs of both films and proposed a new wideband antireflective circular polarizer with a perfect dark state in all viewing directions with Nz = 0.5 in both plates, which will greatly improve image quality of OLEDs.
    Optics Express 12/2014; 22 Suppl 7(25):A1725-30. DOI:10.1364/OE.22.0A1725 · 3.53 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Seamless stitching of graphene domains on polished copper (111) is proved clearly not only at atomic scale by scanning tunnelling microscopy (STM) and transmission electron micoscopy (TEM), but also at the macroscale by optical microscopy after UV-treatment. Using this concept of seamless stitching, we are able to synthesis 6 cm × 3 cm monocrystalline graphene without grain boundaries on polished copper (111) foil, which is only limited by the chamber size. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Advanced Materials 12/2014; 27(8). DOI:10.1002/adma.201404541 · 15.41 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report a HC(NH2)2PbI3 perovskite solar cell based on TiO2 nanohelices. Well-aligned helical TiO2 arrays with different pitch (p) and radius (r), helix-1 (p/2 = 118 nm, r = 42 nm), helix-2 (p/2 = 353 nm, r = 88 nm) and helix-3 (p/2 = 468 nm, r = 122 nm), were grown on FTO glass by oblique-angle electron beam evaporation. HC(NH2)2PbI3 perovskite was deposited on the TiO2 nanohelices by two-step dipping method. Helix-1 showed higher short-circuit current density (JSC), while helix-3 exhibited slightly higher open-circuit voltage (VOC). HC(NH2)2PbI3 perovskite combined with helix-1 demonstrated average power conversion efficiency of 12.03±0.07% due to higher JSC compared to helix-2 and helix-3. High JSC from helix-1 was attributed to enhanced light scattering efficiency and higher absorbed photon-to-current conversion efficiency. Moreover, helix-1 showed fast electron diffusion despite longest pathway structure, which was attributed to higher charge injection efficiency due to larger contact area between perovskite and TiO2. We propose here that fine tuning of interface between perovskite and electron injecting oxide is one of crucial factors for achieving high performance perovskite solar cell.
    11/2014; DOI:10.1039/C4TA04988H
  • [Show abstract] [Hide abstract]
    ABSTRACT: The facile fabrication of polypyrrole (PPy) foams via vapor phase polymerization using microcellular polystyrene, prepared by unidirectional freeze-drying, as a template was demonstrated. Remarkably, the PPy foams maintained the porous morphology of the template, which exhibited a bamboo-like honeycomb channel structure. In addition, the PPy foams were carbonized at 800 degrees C with iodine doping and the carbon products showed similar morphologies to that of the precursor. The carbon foams were characterized by Raman spectroscopy and XRD analysis and were confirmed to be amorphous carbon with a nitrogen content of about 7%.
    Synthetic Metals 10/2014; 196:33–37. DOI:10.1016/j.synthmet.2014.07.017 · 2.22 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: In general, the super in-plane switching (S-IPS) liquid crystal (LC) mode is widely used for LCD applications because of its wide-viewing properties. However, it can also diminish the aperture ratio in active areas due to the zigzag pattern. In this paper, we proposed an IPS LC mode that has wide-viewing properties even if the stripe-patterned electrodes, which can provide a high transmittance property, are applied using the double-exposed UV alignment method to the upper/lower domain of the active area. The initial alignment direction of the upper/lower domain was optimized with simple experiments and we achieved both the superior dark level in the initial mode and wide-viewing properties in the applied voltage mode. As a result, we obtained an increased aperture ratio of about 10% and a higher optical transmittance of 15% compared to the conventional S-IPS LC cell because of the high aperture ratio without any optical loss of the initial dark state and viewing angle property.
    Journal of Modern Optics 09/2014; 62(3):195-199. DOI:10.1080/09500340.2014.967732 · 1.17 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We propose a film patterned retarder (FPR) for stereoscopic three-dimensional display with polarization glasses using ink-jet printing method. Conventional FPR process requires coating of photo-alignment and then UV exposure using wire-grid mask, which is very expensive and difficult. The proposed novel fabrication method utilizes a plastic substrate made of polyether sulfone and an alignment layer, poly (4, 4’ - (9, 9 -fluorenyl) diphenylene cyclobutanyltetracarboximide) (9FDA/CBDA) in which the former and the latter aligns reactive mesogen along and perpendicular to the rubbing direction, respectively. The ink-jet printing of 9FDA/CBDA line by line allows fabricating the cost effective FPR which can be widely applied for 3D display applications.
    Optics Express 09/2014; 22(19). DOI:10.1364/OE.22.022661 · 3.53 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The main objective of this study is to investigate the effect of dielectrophoresis and electrophoresis forces on charge and zeta potential values for colloidal carbon nanotubes in liquid crystal medium, which are investigated in homogeneously aligned in-plane field driven cell. It has been detected that carbon nanotubes suspended in liquid crystal medium oppose an applied electric field and behave like diaelectric material. The dielectrophoresis force, electrophoresis force, electrophoretic velocity, zeta potential, charges on carbon nanotubes, Dukhin number, and surface conductance of colloidal carbon nanotubes in liquid crystal medium have been evaluated. The significant effect of dielectrophoretic force is found to be present in colloidal system and the value of surface conductance of suspending particles was of the order of 10(-17)S.
    Soft Materials 07/2014; 12(3):284-289. DOI:10.1080/1539445X.2014.901227 · 1.74 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We demonstrate distinct entanglement of single-walled carbon nanotube (SWCNT) and multiwalled carbon nanotube (MWCNT) clusters in nematic liquid crystal medium using scanning electron microscopy technique and the entanglement influence on electric field-induced stretching phenomena of the said clusters in the same medium under optical microscopy investigation. The observed stretching threshold field for MWCNT clusters is found to be higher than the SWCNT counterpart caused by the interplay between attractive field-induced dipolar interaction of intercarbon nanotube (CNT) bundles and the distinct degree of entanglement of neighboring CNT bundles. Subsequently observed different tensile elasticity modulus results for different CNT kinds also confirm different CNT bundle entanglement and attractive dipolar interaction between adjacent CNT bundles in CNT clusters are responsible for distinct stretching threshold field behavior.
    Physical Review E 07/2014; 90(1-1):012508. DOI:10.1103/PhysRevE.90.012508 · 2.33 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Conventional film patterned retarder (FPR) production requires a photo-alignment layer and a UV exposure process through a patterned wire-grid photo-mask, increasing the cost as well as limiting the resolution of FPR. We proposed a novel method for the fabrication of FPR without using the alignment layer and the photo-mask. Reactive mesogen (RM) was coated on a base film, and then the substrate with 2-domain interdigitated electrodes was contacted over the RM layer. The in-plane electric field reoriented the randomly orientated RM molecules to the field direction, generating the slow axes in each domain. Then, the UV light was exposed to the film, fixing the slow axes of the polymerized RM with orthogonal orientation between neighboring domains. Finally, an incident linearly polarized light gave rise to giving oppositely handed circular polarizations of light after passing the film.
    Optics Express 06/2014; 22(12):15315-15319. DOI:10.1364/OE.22.015315 · 3.53 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We propose a novel approach to achieve a homogeneous alignment of nematic liquid crystal (LC). Two bare substrates are rubbed in antiparallel direction and a mixture of LC and reactive mesogen (RM) is filled into the cell using capillary injection, and then UV is exposed after confirming the clear homogenous alignment. The cell shows an excellent dark state and stable electro-optic on/off switching in the IPS cell, which proves polymerized RM on surfaces plays role of alignment layer. Proposed fabrication method has several advantages such as simple process and low cost for IPS cell because alignment layer as well as the coating process is not required.
    06/2014; 45(1). DOI:10.1002/j.2168-0159.2014.tb00374.x
  • [Show abstract] [Hide abstract]
    ABSTRACT: In this paper, we investigated effects of stabilizing blue phase liquid crystal using only reactive chiral dopant, without any other monomers and photoinitiators. First, photo-inactive chiral dopants with high helical twist power (~166μm−1) synthesized by us are doped into host nematic LC to induce blue phase and then, the photo-reactive chiral dopant is added to stabilize double twist cylinder. After UV irradiation, BP temperature range is clearly extended compared with pure BP mixture and hysteresis in V-T curve is also dramatically decreased.
    06/2014; 45(1). DOI:10.1002/j.2168-0159.2014.tb00368.x
  • [Show abstract] [Hide abstract]
    ABSTRACT: In this paper, we propose a color transparent liquid crystal (LC) mode that can control the properties of the color gamut and transparency in a single panel. To achieve high transmittance in the transparent LC mode, a reactive mesogen (RM) with embedded color dichroic dyes was applied instead of a color filter. Basically, the LC mode applied a 3-terminal electrode structure to switch between the transparent LC mode and the conventional color LC mode. Depending on the direction of the applied voltage, we can operate both the color mode and the transparent mode in a single panel, and modulate the transparency and color purity of the cell through appropriate voltage control. In the experiments, we confirmed that the transmittance and the color gamut of the cell were 39.4% and 2% in the transparent LC mode and 14.9% and 34% in the color LC mode, respectively. Modulation of the color gamut and transparency between each LC mode are also demonstrated in the paper.
    Optics Express 05/2014; 22(10):12505-12512. DOI:10.1364/OE.22.012505 · 3.53 Impact Factor

Publication Stats

2k Citations
571.26 Total Impact Points

Institutions

  • 2014–2015
    • Pohang University of Science and Technology
      • Department of Materials Science and Engineering
      Geijitsu, Gyeongsangbuk-do, South Korea
    • Kangbuk Samsung Hospital
      Sŏul, Seoul, South Korea
    • Kyungpook National University Hospital
      Sŏul, Seoul, South Korea
  • 2002–2015
    • Chonbuk National University
      • • Department of Polymer Nano Science & Technology
      • • Division of Advanced Materials Engineering
      Tsiuentcheou, Jeollabuk-do, South Korea
  • 2013
    • Southern Illinois University Carbondale
      Illinois, United States
  • 2006–2013
    • Sungkyunkwan University
      • • Institute of Basic Science
      • • Department of Physics
      Sŏul, Seoul, South Korea
  • 2003–2013
    • Pusan National University
      Tsau-liang-hai, Busan, South Korea
  • 2011
    • Ewha Womans University
      Sŏul, Seoul, South Korea
  • 2008–2011
    • University of Texas at Austin
      • School of Nursing
      Texas City, TX, United States
    • Korea Photonics Technology Institute
      Sŏul, Seoul, South Korea
  • 2005–2011
    • Gwangju Institute of Science and Technology
      • Department of Life Sciences
      Gwangju, Gwangju, South Korea
    • Seoul National University Hospital
      • • Department of Radiation Oncology
      • • Department of Internal Medicine
      Sŏul, Seoul, South Korea
  • 2010
    • Catholic Sangji College
      Antō, North Gyeongsang, South Korea
  • 2009
    • Catholic University of Daegu
      • Department of Ophthalmology
      Hayang, North Gyeongsang, South Korea
  • 2008–2009
    • Inha University Hospital
      Sinhyeon, South Gyeongsang, South Korea
  • 2005–2009
    • Seoul National University
      • • College of Veterinary Medicine
      • • Department of Agricultural Biotechnology
      Sŏul, Seoul, South Korea
  • 2003–2008
    • Hongik University
      • Department of Chemical Engineering
      Sŏul, Seoul, South Korea
  • 2004
    • CHA University
      • College of Medicine
      Sŏul, Seoul, South Korea
  • 2001
    • Yonsei University
      Sŏul, Seoul, South Korea
  • 2000–2001
    • Hong-ik university
      United States
    • Ulsan University Hospital
      Urusan, Ulsan, South Korea
  • 1998
    • Samsung Advanced Institute of Technology
      Usan-ri, Gyeonggi Province, South Korea
  • 1996
    • Kent State University
      • Liquid Crystal Institute
      Kent, OH, United States