Cheol Jin Lee

Korea University, Sŏul, Seoul, South Korea

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Publications (161)405.29 Total impact

  • [Show abstract] [Hide abstract]
    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.
    Sensors and Actuators B Chemical 12/2015; 220. DOI:10.1016/j.snb.2015.05.014 · 3.84 Impact Factor
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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.
    Carbon 08/2015; 89:404-410. DOI:10.1016/j.carbon.2015.03.041 · 6.16 Impact Factor
  • 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.
    Physical Chemistry Chemical Physics 05/2015; DOI:10.1039/C5CP01667C · 4.20 Impact Factor
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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.
    The Journal of Physical Chemistry C 04/2015; 119(17):150408102141009. DOI:10.1021/acs.jpcc.5b00788 · 4.84 Impact Factor
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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.
    RSC Advances 03/2015; 5(39). DOI:10.1039/C5RA03000E · 3.71 Impact Factor
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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.
    Applied Physics Letters 03/2015; 106(10):103106. DOI:10.1063/1.4914400 · 3.52 Impact Factor
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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.
    03/2015; 3(17). DOI:10.1039/C5TC00051C
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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.
    Materials Letters 12/2014; 136:103–106. DOI:10.1016/j.matlet.2014.08.028 · 2.27 Impact Factor
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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.
    Carbon 11/2014; 79:149–155. DOI:10.1016/j.carbon.2014.07.054 · 6.16 Impact Factor
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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.
    The 14th International Meeting on Information Display (IMID2014); 08/2014
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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.
    AIP Advances 08/2014; 4(8):087120. DOI:10.1063/1.4893240 · 1.59 Impact Factor
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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.
    Current Applied Physics 08/2014; 14(8):1063-1066. DOI:10.1016/j.cap.2014.05.017 · 2.03 Impact Factor
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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.
    Applied Physics Letters 07/2014; 105(3):033110-033110-5. DOI:10.1063/1.4891328 · 3.52 Impact Factor
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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.
    07/2014; 15(3):214-217. DOI:10.5714/CL.2014.15.3.214
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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.
    AIP Advances 07/2014; 4(7):077110. DOI:10.1063/1.4889896 · 1.59 Impact Factor
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    ABSTRACT: We have demonstrated the fabrication of point-typed field emitters by using carbon nanotube films on graphite rods and investigated their field emission properties. The field emitter with edge polishing showed larger emission current and better long-term emission stability compared to the emitters without edge polishing. After the edge polishing process, the highest emission current of the field emitter was increased from 3.20 mA (corresponding to an emission current density of 0.4 A/cm2) under an applied electric field of 4.2 V/μm to 6.34 mA (corresponding to an emission current density of 1.6 A/cm2) under an applied electric field of 5.3 V/μm. The field emitter with edge polishing indicated a nearly negligible degradation of emission current in 20 h. We consider that the high field emission performance of the field emitter with edge polishing is caused by suppressed edge emission, resulting in higher emission current, lower field enhancement factor, and higher emission stability compared to the field emitter without edge polishing. Simulation results also strongly support that the edge emission could be effectively reduced owing to the suppressed electric field at the edge polished field emitter. The emission pattern of the field emitter showed a small and uniform spot of approximately 2 mm diameter after edge polishing treatment. We suggest that the point-typed field emitters with edge polishing can be a possible candidate for X-ray source or electron beam source applications.
    27th International Vacuum Nanoelectronics Conference (IVNC2014); 07/2014
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    ABSTRACT: We have demonstrated a point-typed CNT field emitter with high emission current and large current density. The CNT field emitter was fabricated by attaching CNT films on graphite rods. The turn-on electric field of the CNT field emitter is 1.4 V/μm, at an emission current density of 0.1 μA/cm2. The threshold electric fields corresponding to an emission current density of 1 mA/cm2 is 2.0 V/ μm. A very high emission current of 4.6 mA (corresponding to an emission current density of 6.5 A/cm2) was achieved under an electric field of 4 V/μm without any arcing during the measurement. We consider that the CNT field emitter could be a good candidate for x-ray source application.
    The World Conference on Carbon (Carbon2014); 07/2014
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    ABSTRACT: We have demonstrated the fabrication of a CNT emitter by attaching a CNT film on the graphite rod and investigated their field emission properties. The CNT emitters with edge polishing indicated a very high emission performance, which presents the high emission current and good long-term emission stability when compared to the CNT emitters without edge polishing. The turn-on electric field of the CNT emitters was 1.7 V/μm at an emission current density of 0.1 μA/cm2. The maximum emission current of the CNT emitter was increased from 2.1 mA (corresponding to an emission current density of 0.3 A/cm2) to 4.1 mA (corresponding to an emission current density of 1.0 A/cm2) after the edge polishing process. This result is attributed to the suppressed edge emission at the edge polished CNT emitter. The straight line of the F-N plot indicated that electron emission follows the quantum mechanical tunneling phenomenon and the field enhancement factor β of the CNT emitter with edge polishing treatment was 1285. The CNT emitter with edge polishing indicated a nearly negligible degradation of emission current in 20 h. We consider that the high field emission performance of the CNT emitter with edge polishing is caused by suppressed edge emission, resulting in higher emission current, lower field enhancement factor, and higher emission stability compared to the CNT emitter without edge polishing. We also suggest that the CNT emitters with edge polishing can be a possible candidate for a focused electron beam spot for an X-ray source or electron beam source applications.
    The World Conference on Carbon (Carbon2014); 07/2014
  • 2014 27th International Vacuum Nanoelectronics Conference (IVNC); 07/2014
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    ABSTRACT: Molybdenum disulfide (MoS2) is an extremely intriguing material because of its unique electrical and optical properties. The preparation of large-area and high-quality MoS2 nanosheets is an important step in a wide range of applications. This study demonstrates that monolayer and few-layer MoS2 nanosheets can be obtained from electrochemical exfoliation of bulk MoS2 crystals. The lateral size of the exfoliated MoS2 nanosheets is in the 5-50 µm range, which is much larger than that of chemically or liquid-phase exfoliated MoS2 nanosheets. The MoS2 nanosheets undergo low levels of oxidation during the electrochemical exfoliation. In addition, microscopic and spectroscopic characterizations indicate that the exfoliated MoS2 nanosheets are of high quality and have an intrinsic structure. A back-gate field-effect transistor was fabricated using an exfoliated monolayer MoS2 nanosheet. The on/off current ratio is over 10(6), and the field-effect mobility is approximately 1.2 cm(2)∙V(-1)∙s(-1); these values are comparable to the results for micromechanically exfoliated MoS2 nanosheets. The electrochemical exfoliation method is simple and scalable, and it can be applied to exfoliate other transition metal dichalcogenides.
    ACS Nano 06/2014; 8(7). DOI:10.1021/nn5016242 · 12.03 Impact Factor

Publication Stats

3k Citations
405.29 Total Impact Points

Institutions

  • 2005–2015
    • Korea University
      • Department of Electrical Engineering
      Sŏul, Seoul, South Korea
  • 2010
    • Max Planck Institute for Solid State Research
      Stuttgart, Baden-Württemberg, Germany
    • 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