Cheol Jin Lee

Korea University, Sŏul, Seoul, South Korea

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Publications (175)417.41 Total impact

  • Hyun Pyo Hong · Joon Hyub Kim · Cheol Jin Lee · Nam Ki Min
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    ABSTRACT: We present the impedancemetric operation of ammonia gas sensors based on drop-deposited, 99% semiconductor-enriched single-walled carbon nanotube networks. Impedance spectroscopic data for these devices exhibit a complex impedance over a range of frequencies (0.5 Hz to 300 kHz) and are well fit with a proposed equivalent electrical circuit model. The effect of NH3 on the high-frequency arc resistance is described by a linear law at all NH3 concentrations, indicating the possibility of utilizing the impedance spectra as the main sensing signal. The impedancemetric operation mode demonstrates a sensitivity of 3.70%/ppm at 3.6–41.4 NH3 concentration, which is equivalent to an improvement in sensitivity of 2.1 times compared to conventional direct current (dc) measurement. Furthermore, impedancemetric sensing is much less susceptible than conventional dc to noise problems in the very low frequency or dc due to ionic contamination or dissociated NH3 molecules.
    No preview · Article · Dec 2015 · Sensors and Actuators B Chemical
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    ABSTRACT: We report novel two-dimensional (2D) shaped carbon nanotube (CNT) field emitters using triangular-shaped CNT film and its field emission properties. Using the 2D shaped CNT field emitters, we achieved excellent field emission performance with an extremely high emission current of 22 mA (equivalent to an emission current density >105 A/cm2) and excellent emission stability at 1 mA for 20 h. We also discuss the field emission behavior of the 2D shaped CNT field emitter in detail.
    No preview · Article · Aug 2015 · Carbon
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    ABSTRACT: Point-type carbon nanotube (CNT) field emitters have attracted much interest because they can promise a focused electron beam spot with a high emission current. The point-type CNT field emitters can be used for diverse applications such as x-ray tubes, electron beam sources and microwave sources. Here, we report fabrication and emission performance of various point-type field emitters made by CNTs.
    No preview · Conference Paper · Jul 2015
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    ABSTRACT: We demonstrated high performance field emission of carbon nanotube (CNT) field emitters fabricated on a graphite rod. After the thermal annealing at 900 °C in vacuum ambient, the emitter showed a high emission current of 11.3 mA, corresponding to an emission current density of 5.8 A/cm2. The CNT emitter indicated a low turn-on field of 1.9 V/μm and stable emission stability at a current of 1 mA. In addition, it also showed a small and uniform emission pattern at the phosphor coated ITO glass. The CNT field emitter can be a good candidate for X-ray source applications.
    Full-text · Conference Paper · Jul 2015
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    ABSTRACT: We have proposed a new approach to fabricating 2D triangular-shaped CNT point emitters using freestanding CNT films, and have investigated the field emission properties as a function of the emitter tip angle. The turn-on electric field and the maximum emission current were increased as the tip angle of the CNT point emitter increased, while the size of a electron beam spot was reduced. The CNT point emitter with a wide tip angle of 150˚ demonstrated an extremely high emission current of up to 22.4 mA and a corresponding to an emission current density of over 10 5 A/cm2 with excellent long-term emission stability over 20 h. We also investigated electron beam shapes of the triangular-shape field emitters according to the tip angle, the emission area and the emitter-anode gap. The triangular-shape field emitters exhibit round shapes of the electron beams regardless of the tip angles when the emission area is confined near the tip apex, even though the triangular-shape does not have a rotational symmetry. The outstanding emission performance has been attributed to the 2D shape of the CNT point emitter because it concentrates the electric field at the tip and distributes the mechanical stress effectively.
    No preview · Conference Paper · Jun 2015
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    ABSTRACT: Carbon nanotubes (CNTs) have been considered as a good field emission material due to their unique morphology and excellent properties. CNTs field emitters with a small size have attracted much attention for miniature x-ray source applications. Especially the point-type CNT field emitters can be a powerful candidate for x-ray sources and electron beam sources because it is suitable to get a focused electron beam from the CNT emitters with a few focal lenses. In this work, we fabricated a point-type CNT emitters, in which CNTs were attached on the graphite rod using graphite adhesive material between the CNTs and the graphite rod. Thermal annealing at 900 ˚C in vacuum ambient was applied to encourage crystallinity of CNTs and reduce unwanted residual organic material at the CNT emitters. The CNT field emitters showed much improved field emission properties after high temperature thermal annealing, such as the low turn-on electric field of 1.3 V/um and the threshold electric field of 1.8 V/um. A high emission current of over 10 mA was achieved from the CNT field emitter with a diameter of 0.5 mm. Moreover, the CNT field emitter indicated quite good emission stability after high temperature thermal annealing.
    No preview · Conference Paper · Jun 2015
  • Chang-Soo Park · Cheol Jin Lee · Eun Kyu Kim
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    ABSTRACT: We report a highly stable p-type doping for single walled carbon nanotubes using an electrochemical method. The Raman spectroscopy showed the upshift of the G-band when the applied potential increased. Furthermore, the carbon core level shifted as much as 0.14 eV in binding energy of XPS measurement, which is an evidence of p-type doping with a Fermi level change. The highly doped SWCNTs at an applied potential of 1.5 V during the electrochemical doping process showed long time stability, as long as 28 days.
    No preview · Article · May 2015 · Physical Chemistry Chemical Physics
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    ABSTRACT: A multi-walled carbon nanotube (MWNT) was physically cured with oxygen plasma treatment, and the as-prepared oxygenated MWNT (OMWNT) was incorporated into TiO2 nanopowders to prepare a spray coatable OMWNT-TiO2 composite suspension. The composite layer was directly formed on a fluorinated tin oxide surface by spray coating and served as a photoanode of a photoelectrochemical cell (PEC). The cell performance was optimized in terms of the plasma treatment time and compared with a conventional PEC, showing 37% increased energy conversion efficiency. The efficiency improvement confirmed by the electrochemical impedance spectra was related to the reduced charge-transfer resistance and efficient electron transport through the OMWNT network.
    No preview · Article · Apr 2015 · The Journal of Physical Chemistry C
  • Yu Zhao · Jihoon Choi · Paul Kim · Weidong Fei · Cheol Jin Lee
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    ABSTRACT: A large-scale synthesis of water-assisted single-walled carbon nanotubes (SWCNTs) was investigated over Fe-Mo/MgO catalysts by catalytic chemical vapor deposition of ethylene. Introduction of water vapor into a reactor induced super-bundle SWCNTs (SB-SWCNTs) and dramatically improved the product yield of SWCNTs from 40 to 206 wt%. By adding water vapor, the average diameter of the SB-SWCNTs was increased from 1.5 to 3.0 nm and distribution of the diameter became wider. The Raman peak intensity ratio (IG/ID) of the SWCNTs, which indicates the crystallinity and defect degree of SWCNTs, showed an almost constant value of 8 regardless of water vapor concentration. The possible growth mechanism of SB-SWCNTs was discussed.
    No preview · Article · Mar 2015 · RSC Advances
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    ABSTRACT: We report the ferromagnetic graphene field effect transistor with a band gap. The Mn doped graphene has a coercive field (Hc) of 188 Oe and a remanent magnetization of 102 emu/cm3 at 10 K. The temperature dependent conductivity indicates that the Mn doped graphene has a band gap of 165 meV. A fabricated graphene FET revealed the p-type semiconducting behavior, and the field effect mobility was determined to be approximately 2543 cm2V-1s-1 at room temperature.
    Full-text · Article · Mar 2015
  • Na Liu · Ki Nam Yun · Hyun-Yong Yu · Joon Hyung Shim · Cheol Jin Lee
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    ABSTRACT: Single-walled carbon nanotubes (SWCNTs) are promising materials as active channels for flexible transistors owing to their excellent electrical and mechanical properties. However, flexible SWCNT transistors have never been realized on paper substrates, which are widely used, inexpensive, and recyclable. In this study, we fabricated SWCNT thin-film transistors on photo paper substrates. The devices exhibited a high on/off current ratio of more than 106 and a field-effect mobility of approximately 3 cm2/V·s. The proof-of-concept demonstration indicates that SWCNT transistors on flexible paper substrates could be applied as low-cost and recyclable flexible electronics.
    No preview · Article · Mar 2015 · Applied Physics Letters
  • Yu Zhao · Chang Soo Park · Wei Dong Fei · Cheol Jin Lee
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    ABSTRACT: We demonstrated the generation of a bandgap in the bilayer graphene synthesized by plasma-enhanced chemical vapor deposition. By adjusting the growth time, the defect density and nano-crystallite size of bilayer graphene were easily controlled, affecting the bandgap of bilayer graphene and the field effect mobility of bilayer graphene field effect transistor (FET). The defect density increased with increasing growth time, whereas the nano-crystallite size decreased. The semiconducting behavior of bilayer graphene was observed by measuring the temperature-dependent conductivity. Defects generated by plasma radiation induce broken symmetry in graphene, thus opening a bandgap. The bandgap energies in the bilayer graphene are 90, 156, and 187 meV for growth times of 5, 10, and 30 min, respectively. The back-gate bilayer graphene FET presented the p-type semiconducting behavior and the field effect mobility of approximately 1000 cm2 V−1 s−1 when the bandgap energy was 156 meV.
    No preview · Article · Dec 2014 · Materials Letters
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    ABSTRACT: We demonstrate a very efficient synthesis of vertically-aligned ultra-long carbon nanofibers (CNFs) with sharp tip ends using thermal chemical vapor deposition. Millimeter-scale CNFs with a diameter of less than 50 nm are readily grown on palladium thin film deposited Al2O3 substrate, which activate the conical stacking of graphitic platelets. The field emission performance of the as-grown CNFs is better than that of previous CNFs due to their extremely high aspect ratio and sharp tip angle. The CNF array gives the turn-on electric field of 0.9 V/μm, the maximum emission current density of 6.3 mA/cm2 at 2 V/μm, and the field enhancement factor of 2585.
    No preview · Article · Nov 2014 · Carbon
  • Yenan Song · Pei Zhao · Cheol Jin Lee · Miao Wang · Hongtao Wang

    No preview · Conference Paper · Sep 2014
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    ABSTRACT: Boron nitride nanotubes (BNNTs) consist of equal numbers of alternating boron and nitrogen atoms with nanoscale tubular structures. BNNTs possess excellent mechanical properties, high thermal conductivity, high oxidation resistance, and high negative electron affinity (NEA). The presence of a high NEA is especially valuable for field emission because it reduces the surface potential barrier height for electron tunneling. Due to these unique features, BNNT can be considered as a useful field emission material. Nevertheless, only a few groups have reported field emission properties of BNNTs until now. It is well known that BNNTs are stable at high temperature in air ambient. This means that the thermal stability of BNNTs may be much better than that of carbon nanotubes (CNTs). In general, CNTs start degrading at 450-500 °C in air ambient while BNNTs can endure much higher temperature in air ambient. In this study, we have investigated the field emission properties of BNNT emitters to confirm the oxidation resistance of BNNT emitters. The thermal annealed BNNTs exhibit a high maximum emission current density of 8.39 mA/cm2. BNNT field emitters show excellent oxidation resistance after high temperature thermal annealing of 600 °C in air ambient. There is no damage to the BNNTs after thermal annealing at a temperature of 600 °C and also no degradation of field emission properties. It means that BNNT field emitters can be a possible candidate for field emission applications, especially for use at high temperature in an oxygen environment.
    No preview · Conference Paper · Aug 2014
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    ABSTRACT: A ferromagnetic ordering with a Curie temperature of 50 K of fifteen layer of InGaMnAs/GaAs multi quantum wells (MQWs) structure grown on high resistivity (100) p-type GaAs substrates by molecular beam epitaxy (MBE) was found. It is likely that the ferromagnetic exchange coupling of sample with Curie temperature of 50 K is hole-mediated resulting in Mn substituting In or Ga sites. Temperature and excitation power dependent PL emission spectra of InGaMnAs MQWs sample grown at temperature of 170 C show that an activation energy of Mn ion on the first quantum confinement level in InGaAs quantum well is 36 meV and impurity Mn is partly ionized. It is found that the activation energy of 36 meV of Mn ion in the QW is lower than the activation energy of 110 meV for a substitutional Mn impurity in GaAs. These measurements provide strong evidence that an impurity band existing in the bandgap due to substitutional Mn ions and it is the location of the Fermi level within the impurity band that determines Curie temperature.
    No preview · Article · Aug 2014 · Current Applied Physics
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    ABSTRACT: We have doped manganese-oxide onto graphene by an electrochemical method. Graphene showed a clear ferromagnetic semiconductor behavior after doping of manganese-oxide. The manganese-oxide doped graphene has a coercive field (Hc) of 232 Oe at 10 K, and has the Curie temperature of 270 K from the temperature-dependent resistivity using transport measurement system. The ferromagnetism of manganese-oxide doped graphene attributes to the double-exchange from the coexistence of Mn3+ and Mn4+ on the surface of graphene. In addition, the semiconducting behavior is caused by the formation of manganese-oxide on graphene. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
    Preview · Article · Aug 2014 · AIP Advances
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    ABSTRACT: Flexible carbon nanotube (CNT) field emitters are fabricated using CNT films on polyethylene terephthalate films. The flexible CNT emitters, which are made using double-walled CNTs, show high emission performance and also indicate stable field emission properties under several bending conditions. The flexible CNT emitters have a low turn-on field of about 0.82 V/μm and a high emission current density of about 2.0 mA/cm2 at an electric field of 1.6 V/μm. During stability tests, the flexible CNT emitters initially degrade over the first 4 h but exhibit no further significant degradation over the next 16 h testing while being continually bent. A flexible lamp made using the flexible CNT emitter displays uniform and bright emission patterns in a convex mode.
    No preview · Article · Jul 2014 · Applied Physics Letters
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    ABSTRACT: A filtration-taping method was demonstrated to fabricate carbon nanotube (CNT) emitters. This method shows many good features, including high mechanical adhesion, good electrical contact, low temperature, organic-free, low cost, large size, and suitability for various CNT materials and substrates. These good features promise an advanced field emission performance with a turn-on field of 0.88 V/mm at a current density of 0.1 mA/cm2, a threshold field of 1.98 V/mm at a current density of 1 mA/cm2, and a good stability of over 20 h. The filtration-taping technique is an effective way to realize low-cost, large-size, and high-performance CNT emitters.
    No preview · Article · Jul 2014 · Carbon letters
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    ABSTRACT: The carbon nanotube (CNT) field emitters have been fabricated by attaching a CNT film on a graphite rod using graphite adhesive material. The CNT field emitters showed much improved field emission properties due to increasing crystallinity and decreasing defects in CNTs after the high temperature thermal annealing at 900 °C in vacuum ambient. The CNT field emitters showed the low turn-on electric field of 1.15 V/μm, the low threshold electric field of 1.62 V/μm, and the high emission current of 5.9 mA which corresponds to a current density of 8.5 A/cm2. In addition, the CNT field emitters indicated the enhanced field emission properties due to the multi-stage effect when the length of the graphite rod increases. The CNT field emitter showed good field emission stability after the high temperature thermal annealing. The CNT field emitter revealed a focused electron beam spot without any focusing electrodes and also showed good field emission repeatability.
    Full-text · Article · Jul 2014 · AIP Advances

Publication Stats

5k Citations
417.41 Total Impact Points

Institutions

  • 2005-2015
    • Korea University
      • Department of Electrical Engineering
      Sŏul, Seoul, South Korea
  • 2010
    • Inje University
      • College of Medicine
      Kimhae, South Gyeongsang, South Korea
  • 2006-2007
    • Cheongju University
      Sŏul, Seoul, South Korea
  • 2002-2007
    • Hanyang University
      • Department of Bio-Nano Technology
      Sŏul, Seoul, South Korea
  • 1998-2002
    • Kunsan National University
      • Department of Electrical Engineering
      Gunzan, North Jeolla, South Korea