Cheol Jin Lee

Korea University, Sŏul, Seoul, South Korea

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Publications (152)376.9 Total impact

  • [Show abstract] [Hide abstract]
    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. · 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. · 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
  • AIP Advances 08/2014; 4(8):087120. · 1.59 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. · 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.
    Carbon Leters. 07/2014; 15(3):214-217.
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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; 4:077110. · 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
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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; · 12.03 Impact Factor
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    ABSTRACT: Boron nitride nanotubes (BNNTs) are considered as a promising cold electron emission material owing to their negative electron affinity. BNNT field emitters show excellent oxidation endurance 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. The thermally annealed BNNTs exhibit a high maximum emission current density of 8.39 mA/cm2 and show very robust emission stability. The BNNTs can be a promising emitter material for field emission devices under harsh oxygen environments.
    Applied Physics Letters 04/2014; 104(16):163102-163102-5. · 3.52 Impact Factor
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    ABSTRACT: This paper presents a facile electrochemical exfoliation method to mass produce device quality MoS2 semiconducting nanosheets. During the exfoliation process, anionic SO42− and OH− ions intercalate into a bulk MoS2 and they form gaseous SO2 or O2 gas bubbles which produce a separating force for MoS2 nanosheets exfoliation. Monolayer or few layers of MoS2 nanosheets were obtained on a SiO2/Si substrate. N-type field effect transistors (n-FETs) were fabricated using the exfoliated MoS2 nanosheets and these n-FETs showed excellent device characteristics. The measured on/off current ratio was around 103, the field-effect mobility on SiO2 gate dielectrics was 2 cm2/(V s).
    Materials Letters 04/2014; 121:31–35. · 2.27 Impact Factor
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    ABSTRACT: The p-type InP:Be/Mn/InMnP:Be triple epilayers were prepared using MBE to increase Tc (>300 K) by preventing MnO2. After milling 1–3 nm of epilayers thickness from the top surface, the transmission electron microscopy (TEM) and X-ray diffraction (XRD) revealed no MnO2 and precipitates, and TEM and XRD results coincide with results of ferromagnetism. The enhanced ferromagnetic transition at >300 K corresponds to InMnP:Be. The increased ferromagnetic coupling without MnO2 is considered to originate from the increased p–d hybridation. These results demonstrate that InP-based ferromagnetic semiconductor layers having enhanced ferromagnetism can be formed by above process.
    Current Applied Physics 04/2014; 14(4):558–562. · 2.03 Impact Factor
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    ABSTRACT: Carbon nanotube (CNT) emitters with small emission area were fabricated on graphite rods using CNT films. By introducing the edge polishing process, the field emission performance of the CNT emitter was much improved, which showed a very high emission current of 6.34 mA (1.6 A/cm2) under an applied electric field of 5.3 V/μm. It also indicates good long-term emission stability, which reveals no degradation in the emission current for 20 h. The emission patterns demonstrate uniform and well-focused electron beam spots. The enhanced field emission performance is mainly attributed to the suppressed edge emission after the edge polishing process.
    Applied Physics Letters 01/2014; 104(4):043104-043104-5. · 3.52 Impact Factor
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    ABSTRACT: The p-type InP:Zn epilayers were prepared by using metal-organic chemical vapor deposition, and Mn was subsequently deposited onto the epilayer by using molecular beam epitaxy. The p-type InMnP:Zn epilayers were annealed at relatively low temperatures of 200–350 °C and contained no secondary phases such as InMn, MnP, and MnO2, as verified by x-ray diffraction. However, minute presence of MnO2 was confirmed using transmission electron microscopy, which agreed with the magnetic properties measured by using a superconducting quantum interference device (SQUID). From the SQUID measurements, consistent and systematic ferromagnetic properties with clear ferromagnetic hysteresis loops were observed. The Curie temperature, T C , which persisted up to ∼ 180 K, was recorded depending on the Mn concentrations and annealing temperature. These results indicate that the ferromagnetic semiconductor InMnP:Zn can be fabricated at a very low annealing temperature without forming ferromagnetic precipitates except for MnO2.
    Journal- Korean Physical Society 12/2013; 63(11). · 0.43 Impact Factor
  • Young Ran Park, Na Liu, Cheol Jin Lee
    Current Applied Physics 11/2013; 13(9):2026-2032. · 2.03 Impact Factor
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    ABSTRACT: We have grown graphene directly on alumina (Al2O3) substrates without catalysts using conventional thermal chemical vapor deposition. By choosing Al2O3 as a growth substrate, the polycrystallinity of graphene was enhanced to form nanometer-size dome-like grains, which ensured a statistically homogeneous electrical property of graphene over a large area. As-grown bilayer, the nanographene (nGr) film showed a sheet resistance of 3 kΩ −1 with a standard deviation of 2.3% over 15 mm × 15 mm. Top- and bottom-gate nGr thin film transistors (TFTs) fabricated directly on the Al2O3 substrate exhibited field-effect mobilities of 89 and 41 cm2 V−1 s−1, respectively. Moreover, the grown nGr could be easily detached from the Al2O3 substrate due to weak adhesion between the nGr and Al2O3, which has abundant fixed charges. Dry-transfer of the grown nGr from the Al2O3 substrate was realized via spin-coating a polyimide (PI) or poly(4-vinylphenol) film and subsequently detaching the film together with the nGr film. The recycled substrates provided the nGr films with reproducibility. The nGr devices on a 3 μm-thick PI film were stable upon bending with a bending diameter of down to 6 mm.
    J. Mater. Chem. C. 09/2013; 1(39).
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    ABSTRACT: We report for the first time synthesis of high-density arrays of vertically well-aligned cobalt monosilicide (CoSi) nanowires (NWs) in a large area via a solid-state reaction. The vertical arrays of 1-μm-long Si NWs were first grown on a p-type (1 0 0) Si substrate by the aqueous electroless etching (AEE) method, and a 40-nm-thick Co layer was conformally deposited using a thermal atomic layer deposition system as revealed by SEM and transmission electron microscope analyses. The rapid thermal annealing process was carried out at various temperatures ranging from 700 to 1000 °C; the X-ray diffraction analysis confirmed that the polycrystalline CoSi NW arrays were formed at temperatures above 900 °C. The required high driving force for this silicide formation can be attributed to the significant amounts of oxygen-related contaminants at the defect sites of the highly rough surfaces of AEE-grown Si NWs. To demonstrate practical applications, field emitters and Schottky diodes were fabricated using the vertically aligned CoSi NW arrays. The field emission measurements showed a turn-on field of 10.9 V/μm and a field enhancement factor of 328, indicating the feasibility of vertically aligned CoSi NW arrays as promising field emitters. For the Schottky diodes, the measured Schottky barrier height was 0.52 eV and the estimated ideality factor obtained from the I-V characteristic curves was 2.28.
    IEEE Transactions on Nanotechnology 09/2013; 12(5):704-711. · 1.62 Impact Factor
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    ABSTRACT: Graphite nanoplatelet (GNP) that consists of several tens of layers of graphene sheets is a promising candidate for electron field emission. The GNP emitter shows good field emission properties with a high emission current and a robust long-term stability because of the sharp edges, the high aspect ratio, and the stacked graphene sheets. Most of the electrons are emitted from the sharp edges of GNPs. The GNP emitters fabricated by the screen printing method reveal the turn-on electric field of 1.77 V/μm, the threshold electric field of 4.47 V/μm, the maximum emission current density of 39 mA/cm2, and the good emission stability for 20 h with little degradation rate of the emission current. The GNP field emitters can be a good candidate for several field emission applications such as flat lamps, field emission displays, and x-ray sources.
    Applied Physics Letters 08/2013; 103(7). · 3.52 Impact Factor

Publication Stats

3k Citations
376.90 Total Impact Points

Institutions

  • 2005–2013
    • Korea University
      • • Department of Electrical Engineering
      • • Department of Physics
      Sŏul, Seoul, South Korea
  • 2010
    • Max Planck Institute for Solid State Research
      Stuttgart, Baden-Württemberg, Germany
    • Lovely Professional University
      Phagwāra, Punjab, India
    • Inje University
      • College of Medicine
      Kimhae, South Gyeongsang, South Korea
  • 2008
    • Chonbuk National University
      • Department of Physics
      Seoul, Seoul, South Korea
  • 2006–2007
    • Cheongju University
      Sŏul, Seoul, South Korea
    • Sungkyunkwan University
      • Department of Physics
      Sŏul, Seoul, South Korea
  • 2002–2006
    • Hanyang University
      • Department of Bio-Nano Technology
      Sŏul, Seoul, South Korea
    • Inha University
      • Department of Materials Science and Engineering
      Seoul, Seoul, South Korea
    • Chosun University
      • Department of Earth Science Education
      Gwangju, Gwangju, South Korea
  • 1998–2002
    • Kunsan National University
      • Department of Electrical Engineering
      Gunzan, North Jeolla, South Korea