Design and Field Emission Test of Carbon Nanotube ... - IEEE Xplore

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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 53, NO. 11, NOVEMBER 2006

Design and Field Emission Test of Carbon Nanotube Pasted Cathodes for Traveling-Wave Tube Applications Hae Jin Kim, Jin Joo Choi, Jae-Hee Han, Jae Hong Park, and Ji-Beom Yoo

Invited Paper

Abstract—This paper presents results of computational and experimental studies of carbon nanotube (CNT) cathodes in gridded focusing guns intended for traveling-wave tube applications. With a curved CNT cathode, a cold beam emission of 11 mA was observed in a diode configuration. Two gridded focusing cathodes pasted with CNTs were then fabricated. When the applied cathode voltage was −0.86 kV, both grid and anode were at ground potential, and the cathode-to-grid distance was 200 µm, the dc emission current from the first gridded CNT cathode was measured to be 4.2 mA. The triode field emission characteristics with two accelerating stages in pulse operation were also examined. With the cathode voltage at −4 kV, a cathode–grid potential difference of 1.38 kV, and a pulsewidth of 20 µs, the measured pulsed current emission was 4.6 mA. The measured current was found to agree with MAGIC2D simulation results. Furthermore, by configuring a double-gridded focusing gun, a significant improvement in the beam focusing data collected through computer modeling was seen. Index Terms—Carbon nanotube (CNT), field emission, gridded focusing gun, traveling-wave tube (TWT).

I. INTRODUCTION

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IELD emission cathodes have been successfully used in prototypes for field emission display, microwave amplifier, and X-ray source [1]–[3]. In recent years, field emission from chemical vapor deposition (CVD)-grown carbon nanotubes (CNTs) has been vigorously studied due to the potential application of CNTs as attractive electron sources [4]–[6]. CNTs have several properties, such as high electrical conductivity, chemical stability, and high aspect ratio, that make them extraordinary materials for field emission. CNTs, in particular, Manuscript received May 25, 2006; revised August 11, 2006. This work was supported by the Korea Science and Engineering Foundation (KOSEF) through the Center for Nanotubes and Nanostructured Composites (CNNC) of Sungkyunkwan University. The review of this paper was arranged by Editor W. Menninger. H. J. Kim and J. J. Choi are with the Department of Radio Science and Engineering, Kwangwoon University, Seoul 139-701, Korea. J.-H. Han is with the Department of Chemical Engineering, University of Illinois, Urbana–Champaign, IL 61801 USA. J. H. Park and J.-B. Yoo are with the Center for Nanotubes and Nanostructured Composites, Sungkyunkwan University, Suwon, Gyeonggi-do 440-746, Korea. Color versions of Figs. 1–5, 7, and 8 are available online at http:// ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2006.884076

are highly stable both physically and chemically. Their superior thermal conductivity and chemical stability make them suitable for high current density emission in low or medium vacuum levels. A traveling-wave tube (TWT) employed with a CNT cathode, which is a representative vacuum microwave amplifier, enables a room-temperature electron gun to operate. However, there are significant technical challenges in generating a high current and a high current density electron beam while simultaneously sustaining exceptional beam emission stability. This paper focuses on the design and field emission test of a CNT cathode in a gridded focusing gun for TWT application. In Section II, a simulation model of a gridded focusing gun is described in detail. We predict beam focusing performance of a gridded focusing gun with a high grid voltage by the use of a two-dimensional particle-in-cell (PIC) code, MAGIC2D [7]. In order to enhance beam focusing in the gridded focusing gun, we also designed a double-gridded focusing gun. The field emission experimental results of the three cases, which include a curved CNT cathode and two gridded focusing guns pasted with CNTs, are described in Section III. In the diode configuration, we were able to measure a dc emission current from the CNT paste on the curved cathode. We also examined the diode and triode field emission characteristics with two gridded CNT guns. The optical transparency of the molybdenum grid mesh is 72%. The cathode–grid and cathode–anode spacings are 200 µm and 4.5 mm, respectively. By varying the grid potential and by measuring the cathode, grid, and anode currents as a function of the voltage difference between the cathode and grid, we were able to examine the triode current emission properties. Some concluding remarks are given in Section IV. II. DESIGN OF GRIDDED FOCUSING GUN Fig. 1(a) and (b) shows a simulation model of the gridded focusing gun and the applied voltage waveforms at all 13 grid electrodes obtained through MAGIC2D simulations. MAGIC2D is an electromagnetic PIC, finite difference, and time-domain computer code developed by ATK Mission Research. The coordinate system used in the simulations is a cylindrically symmetric system (z,r,θ). In the MAGIC2D simulation, the CIRCUIT command is used to apply an external voltage source and maintain a specified voltage between

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KIM et al.: DESIGN AND FIELD EMISSION TEST OF CNT PASTED CATHODES FOR TWT APPLICATIONS

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Fig. 2. (a) Equipotential lines. (b) Electron trajectories of the single-gridded focusing gun.

Fig. 1. (a) Model of a single-gridded focusing gun. (b) Applied voltage waves at 13 grid electrodes.

conductors that are otherwise unconnected. In Fig. 1(b), the voltage waveforms at all 13 grid wires are slightly different from each other, but with little effect on beam focusing. In order to represent the fine structure of CNT tips whose length and diameter is 1 µm and 10 nm, respectively, the mesh size would need to be reduced to approximately 2–5 nm. With only the simulation runtime in consideration, it seems rather unnecessary to simulate the Fowler–Nordheim (F–N) field emission with such a large number of mesh points. Therefore, in this simulation study, a general beam emission model was used to calculate the current transmission ratio and predict the beam focusing in the gridded focusing gun with a high grid voltage [8]. In the beam emission model of MAGIC2D, both current density and beam voltage are provided by the users. The cathode radius is 2.18 mm, and the cathode curvature radius is 9.73 mm. Thirteen grids are modeled with azimuthally symmetrical metal rings. Each metal ring has a dimension of 0.05 by 0.05 mm. They are spaced by 0.13 mm; in which case, the optical transparency becomes 72%, corresponding to the actual transparency of the molybdenum grid mesh. All grid electrodes are positioned 200-µm away from the cathode surface to achieve an electric field of 5 V/µm on the cathode surface with a grid voltage of 1 kV. The grid elements were all kept at the same voltage to minimize defocusing from unbalanced equipotential fields [9]. The cathode, grid, focus electrode, and anode voltages were 0,

1.38, 1.38, and 4 kV, respectively. The applied voltage at all grid electrodes becomes 1.38 kV after 1.2 ns. The electron beam emission turns on at 2 ns when the equipotential fields between the cathode and the grids are at a steady state so that the two “bump” features along the voltage waveforms at the 13th grid shown in Fig. 1(b) would not have a large effect on the focusing. Fig. 2 depicts equipotential lines in the single-gridded focusing gun with a high grid voltage of 1.38 kV and electron trajectories emitted from gridded focusing cathode, where no external magnetic field is applied. It is known that adding a grid to a given diode gun dilates the beam due to the scattering from the ring model grids. As shown in Fig. 2(b), the accelerated electron beams are highly defocused due to the high grid voltage. Conventional thermionic gridded Pierce guns have a low grid voltage (relative to cathode) proportional to the grid spacing between the cathode and anode, which is less than the present grid voltage of 1.38 kV. While experimenting with the single-gridded focusing gun simulation under a high grid voltage, we have found that the accelerated beams between the cathode surface and grids are significantly deflected. Therefore, the focus performance of the electron beams was not as good as the case with the low grid voltage. Simulation predicts that the current transmission ratio defined as (Icathode − Igrid )/ Icathode is 64.2%, which is slightly less than the optical transparency of 72%. We designed a double-gridded focusing gun to enhance the beam focusing at the high grid voltage. Fig. 3 shows a simulation model of the double-gridded focusing gun, with equipotential lines, and electron trajectories obtained from MAGIC2D simulations. Double-grid electrodes are modeled using 20 total

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Fig. 4. PPM field profile for focusing electron beam in TWT and electron trajectories of the double-gridded focusing gun with an applied PPM field.

Fig. 3. (a) Equipotential lines. (b), (c), and (d) electron trajectories of a double-gridded focusing gun for three cases (anode voltage: 4, 8, and 10 kV).

metal rings. The first and second grids are each modeled using ten metal rings. The spacing between the cathode and the first grid electrode is 200 µm, while the second grid is positioned 400-µm away from the first grid. To achieve an electric field of 5 V/µm on the cathode surface, the voltage difference between the cathode and first grid electrode is fixed at 1 kV. The first grid is set to the same potential as the first focus electrode. Likewise, the second grid is set to the same potential as the second focus electrode. The cathode, first grid, second grid, and anode volt-

ages are 0, 1.0, 1.2, and 4 kV, respectively. For the doublegridded cathode, equipotential lines between the two grids are much flat than those of the single-gridded cathode. Simulations predict that the focused beam radius of the double-gridded cathode are reduced by 30% compared to that of the single-gridded cathode, as shown in Fig. 3(b). The focused beam trajectories of the double-gridded gun are shown in Fig. 3(b)–(d) for all three cases. Among the three cases, the cathode, first grid, and second grid voltages are equivalent to those of Fig. 3(a), except for the anode voltage, which is 4, 8, and 10 kV for Fig. 3(b)–(d), respectively. As shown in Fig. 3(b)–(d), we expected the beam focusing to improve under the high anode voltage. In the double-gridded gun, simulation predicts the current transmission to be 63% when the anode voltage is 4 kV. As the anode voltage increased from 4 to 8 kV without the periodic-permanent-magnet (PPM) focusing fields, the focused beam radius reduces from 0.93 to 0.68 mm. However, there is no change in the current transmission of 63%. We also simulated the double-gridded focusing gun with the PPM fields for a focusing electron beam in a TWT. Fig. 4 illustrates the PPM field profile for a focusing electron beam in a TWT and a plot of a focused beam obtained from a double-gridded gun applying a PPM field. The cathode, first grid, second grid, and anode voltages are equivalent to those of Fig. 3(c). A PPM field with a peak field of 2.18 kG was used to focus and confine the beam. The double-gridded focusing gun produces a 5.73-mA electron beam with a current density of 36 mA/cm2 . The grid currents collected from the first grid and second grid electrodes are 1.83 and 0.94 mA, respectively. The transmitted current within the TWT circuit is 2.96 mA when the anode radius is 1.37 mm. In the double-gridded gun simulation with a PPM field, the current transmission reduces from 63% to 51.6%. The double-gridded focusing gun has a nonlaminar beam with a maximum beam radius of 0.92 mm. This radius is larger than the thermionic laminar beam radius of 0.39 mm [10].


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Fig. 5. (a) Experimental setup. (b) Field emission results from CNT paste on a curved cathode.

III. EXPERIMENTAL RESULTS To fabricate a CNT-based cathode and obtain a high field emission current, we used CNT paste instead of the conventional Spindt-type field emission array [11]. The CNT paste was prepared using a mixture of multiwalled CNT powders synthesized by CVD, organic vehicles, and inorganic binders. A well-dispersed CNT paste was printed onto a specially designed cathode whose diameter and spherical radius were 4.3 and 9.73 mm, respectively. Then, the CNT paste was dried and pressed by a metallic rod with the same spherical radius. Details on the CNT growth and paste are described in [12]. Fig. 5(a) and (b) shows the experimental setup and field emission results of the curved CNT cathode. A stainless-steel anode with the same curvature radius was positioned 480-µm away from the cathode. From the CNT cathode with a coating area of 0.15 cm2 , we repeatedly obtained a cold beam emission of about 11 mA, corresponding to a current density of approximately 73 mA/cm2 at the anode–cathode voltage difference of 2400 V (electric field = 5 V/µm). Data were taken during four separate measurements at a pressure of 3 × 10−6 torr in the dc mode. The field emission current was approximately one quarter of the thermionic cathode current, which we have previously reported in TWT experiments [13]. We also fabricated two gridded focusing guns pasted with CNTs for a TWT application. The scanning electron microscope (SEM) image of the surface morphology of the activated CNT emitter is shown in Fig. 6(a). Numerous CNTs, which act as electron emitters, are seen at the film surface. Fig. 6(b)

Fig. 6. (a) SEM image of the surface morphology of CNT paste after activation treatment. (b) Photograph of gridded CNT focusing gun. (c) Experimental configuration of a diode-type field emission test.

and (c) shows a detailed view of the gridded CNT focusing gun and the experimental configuration of the diode-type field emission test on the gridded CNT focusing gun. The grid is consist of a molybdenum mesh that was positioned 200-µm away from the cathode surface. The grid mesh thickness is 0.038 mm, and the dimensions of the opening hole are 0.216 by 0.216 mm. The transparency of the grid mesh is 72%. In the first experiments,

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of 5 ± 0.3 eV at the tip. This is very similar to the value of graphite. In this paper, we assume the work function of the CNTs to be 5 eV, similar to that of graphite. The dimensionless field enhancement factor γ is defined as F = γV /d, where F is the local field at the emitter surface and d is the interelectrode distance. Eventually, γ and A can be calculated from the slope m of the F–N plot (ln I/V 2 = ln b + m/V , where ln b is the ordinate in the F–N plot), as follows: 6.44 × 109 φ1.5 d m 2 m b 10.4 −14 A = 1.07 × 10 × . × exp − √ φ φ γ=−

Fig. 7. Current–voltage (I–V ) characteristics and F–N fittings for the curved CNT cathode and the gridded CNT focusing gun. Open triangles and squares indicate I–V characteristics of the curved CNT cathode and the gridded CNT focusing gun. Filled triangles and squares show F–N fittings of the curved CNT cathode and the gridded CNT focusing gun.

the diode-type field emission current from the gridded CNT focusing gun was measured under dc operation. The cathode was biased at a negative voltage, while the grid, anode, and collector were grounded. The field emission properties of the gridded CNT focusing gun were measured in a vacuum chamber at approximately 1 × 10−7 –2 × 10−7 torr. Fig. 7 shows the current–voltage (I–V ) characteristics and the F–N fittings for the gridded CNT focusing gun and curved CNT cathode. The field emission current from the gridded CNT focusing gun was measured to be 2 mA when the applied electric field was 4.2 V/µm. We observed three characteristic regions from the experimental I–V curves and their corresponding F–N plots. The I–V curve fits well to the F–N relation at the low-voltage region. The I–V curve deviated from the linear F–N plot due to the current saturation of CNT emitters at the intermediate-voltage region. The emission current increased again after passing the intermediate-voltage region (at the higher voltage region). Several groups have also observed the similar behavior from CNTs and suggested that the change in the F–N slope is caused mainly by the variations in the surface chemical and adsorption states of CNTs during the field emission [14], [15]. Therefore, we conjecture that the reason for the F–N slope deviation from our measured CNT paste emitter curves is attributed to the gas adsorbate effect introduced by the organic residue of CNT paste. We also calculated the field enhancement factor γ and the effective emission area A from the I–V relationship according to the F–N model as follows [16]: 2 10.4 1.5 × 10−6 V 2 √ γ exp I=A φ d φ −6.44 × 109 φ1.5 d exp (1) γV where A is the emitting area. The work function θ is in electronvolt. Generally, multiwalled CNTs have a work function

(2) (3)

When the gridded CNT focusing gun was fabricated, the field enhancement factor of γ at the fourth measurement increased from 956 to 4498. However, the corresponding A decreased significantly from 1.80 × 10−7 to 1.22 × 10−16 m2 . One possible explanation is that there were numerous effective CNT emitters that could participate in the field emission at the fourth measurement before assembly, thus resulting in a high population density and causing a decrease in γ due to the screening effect. However, the emission density decreased significantly after fabrication, resulting in an increment of γ. It is likely that a handling process during fabrication on the gridded CNT focusing gun, for example, laser brazing and exposure to air, deteriorated the CNT film quality on the cathode. In the second experiment, we examined the triode field emission characteristics with two accelerating stages. Fig. 8 illustrates the triode-type field emission setup in a pulsed operation. A negative high voltage was applied to the cathode while the anode was grounded. A resistive divider was used to apply and vary the grid voltage. The pulsewidth was set to 100 µs, and the pulse repetition frequency was set to 10 Hz. The pulsed emission current and the collected current were measured by Pearson current transformers. Fig. 9(a) shows the applied cathode voltage measured by the oscilloscope. Fig. 9(b) and (c) depicts a pulsed emission current from the gridded CNT cathode and a pulsed current collected on the anode block. The pulsed emission and anode currents were also measured by Pearson current transformers. When the cathode voltage was −4 kV and the cathode–grid potential difference was 1.38 kV, we measured a pulsed cathode current Icathode of 2.8 mA. The anode current Ianode , which was collected on the anode and collector, was measured to be 1.8 mA. The current transmission ratio was 64%, which is in positive agreement with 64.2%, which was obtained from the MAGIC2D simulations. The field emission properties of the gridded CNT focusing gun were tested at a low vacuum pressure of 2 × 10−8 torr. Fig. 10 shows the I–V characteristics of the second gridded CNT focusing gun at 100 µs. The cathode and anode currents were measured without breakdown as a function of the voltage difference between the cathode and the grid ranging from 0.98 to 1.38 kV. When the pulsed cathode–grid bias was 1.38 kV, the cathode current at 20 µs was measured to be higher than that at 100 µs by 1.8 mA. This increment is possibly due to the adsorption of the remaining gas molecules to CNTs. When the applied voltage difference between the cathode and the


Fig. 8.

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Triode-type field emission setup of a gridded CNT focusing gun in pulsed operation.

Fig. 10. Pulsed cathode current (filled circle) and anode current (open circle) measured on an oscilloscope as a function of the voltage difference between cathode and grid.

Fig. 9. Three pulsed waveforms measured on an oscilloscope. (a) Applied cathode voltage (5 kV/div) for the gridded CNT gun. (b) Emitted cathode current (2 mA/div) for the gridded CNT gun. (c) Collected anode current (2 mA/div) for the gridded CNT gun.

grid was 1.4 kV, a breakdown occurred. The beam emission characteristic of the CNT pasted cathode was slightly degraded after the cathode breakdown. Furthermore, even after an arc, the gridded CNT cathode still emitted current, which is a good property for a cathode in consideration for a potential use in a vacuum electronic device such as a TWT. Proper conditioning of CNT cathodes is critical to their performance; therefore, the development of effective methods for improved degassing is under constant progress.

type experiments, when the cathode-to-grid voltage was set at 1.38 kV, we observed a cathode current of 4.6 mA at 20 µs. It is likely that a handling process, including a laser brazing during the fabrication of the gridded CNT gun, deteriorated the CNT paste quality on the cathode. We also designed and simulated single- and double-gridded focusing guns to predict and enhance the beam focusing at high grid voltage using MAGIC2D. Simulations predict that the current transmission ratio of the single-gridded focusing gun is 64.2% when the cathode voltage is zero and grid voltage is 1.38 kV. This is in positive agreement with the 64% transmission ratio obtained from triode-type field emission measurements of the gridded CNT focusing gun. Focusing performance is expected to be much improved by designing a double-gridded focusing gun with a PPM field. The CNT pasted cathode remains a promising candidate for practical TWT applications.

IV. CONCLUSION We stably obtained a cold beam emission of about 11 mA corresponding to 73 mA/cm2 at 5 V/µm from a curved CNT cathode. We were also able to examine the field emission characteristics on a gridded CNT cathode. In the triode-

ACKNOWLEDGMENT The authors would like to thank Dr. L. Ludeking at ATK Mission Research for the many useful discussions on MAGIC2D simulations.

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R EFERENCES [1] H. Makishimia, H. Imura, M. Takahashi, H. Fukui, and A. Okamoto, “Remarkable improvements of microwave electron tubes through the development of cathode materials,” in Proc. 10th Int. Vac. Microelectron. Conf., Kyongju, Korea, 1997, pp. 194–199. [2] D. R. Whaley, B. M. Gannon, C. R. Smith, C. M. Amstrong, and C. A. Spindt, “Application of field emitter arrays to microwave power amplifiers,” IEEE Trans. Plasma Sci., vol. 28, no. 3, pp. 727–747, Jun. 2000. [3] W. I. Milne, K. B. K. Teo, G. A. J. Amaratunga, P. Legagneux, L. Gangloff, J.-P. Schnell, V. Semet, V. T. Binh, and O. Groening, “Carbon nanotubes as field emission sources,” J. Mater. Chem., vol. 14, no. 6, pp. 933–943, 2004. [4] N. L. Rupesinghe, M. Chhowalla, K. B. K. Teo, and G. A. J. Amaratunga, “Field emission vacuum power switch using vertically aligned carbon nanotubes,” J. Vac. Sci. Technol. B, Microelectron. Process. Phenom., vol. 21, no. 1, pp. 338–343, Jan. 2003. [5] J. H. Han, T. Y. Lee, D. Y. Kim, J. B. Yoo, C. Y. Park, J. J. Choi, T. W. Jung, I. T. Han, and J. M. Kim, “Field emission properties of carbon nanotubes grown on Co/TiN coated Ta substrate for cathode in microwave power amplifier,” Diam. Relat. Mater., vol. 13, no. 4–8, pp. 987–993, Apr.–Aug. 2004. [6] K. B. K. Teo, E. Minoux, L. Hudanski, F. Peauger, J. P. Schnell, L. Gangloff, P. Legagneus, D. Dieumegard, G. A. J. Amaratunga, and W. I. Milne, “Microwave devices; Carbon nanotubes as cold cathodes,” Nature, vol. 437, no. 7061, p. 968, Oct. 2005. [7] B. Goplen, L. Ludeking, D. Smithe, and G. Warren, “User-configurable MAGIC code for electromagnetic PIC simulations,” Comput. Phys. Commun., vol. 87, no. 1/2, pp. 54–86, May 1995. [8] L. Ludeking, private communication, 2005. [9] H. J. Kim, W. B. Seo, J. J. Choi, J. H. Han, and J. B. Yoo, “Beam emission test on carbon nanotube cathode of a gridded Pierce gun,” in Proc. 7th Int. Vac. Electron. and 6th Int. Vac. Electron Sources Conf., Monterey, CA, 2006, pp. 479–480. [10] H. J. Kim, L. B. Jang, W. B. Seo, and J. J. Choi, “Experimental investigation of broadband vaned helix traveling-wave tube,” Jpn. J. Appl. Phys., vol. 45, no. 1A, pp. 292–299, 2006. [11] C. A. Spindt, C. E. Holland, P. R. Schwoebel, and I. Brodie, “Field emitter array development for microwave applications,” J. Vac. Sci. Technol. B, Microelectron. Process. Phenom., vol. 14, no. 3, pp. 1986–1989, May 1996. [12] J. H. Park, J. S. Moon, J. H. Han, A. S. Berdinsky, J. B. Yoo, C. Y. Park, J. W. Nam, J. H. Park, C. G. Lee, and D. H. Choe, “Stable and high emission current from carbon nanotube paste with spin on glass,” J. Vac. Sci. Technol. B, Microelectron. Process. Phenom., vol. 23, no. 2, pp. 702–706, Mar. 2005. [13] Y. H. Na, J. J. Choi, and R. Kim, “Development of X-band and Q-band traveling-wave tube amplifiers,” in Proc. 4th Int. Vac. Electron. Conf., Seoul, Korea, 2003, pp. 88–89. [14] C. Dong and M. C. Gupta, “Influences of the surface reactions on the field emission from multiwall carbon nanotubes,” Appl. Phys. Lett., vol. 83, no. 1, pp. 159–161, Jul. 2003. [15] D. H. Kim, C. D. Kim, and H. R. Lee, “Effects of the ion irradiation of screen-printed carbon nanotubes for use in field emission display applications,” Carbon, vol. 42, no. 8-9, pp. 1807–1812, 2004. [16] J.-M. Bonard, C. Klinke, K. A. Dean, and B. F. Coll, “Degradation and failure of carbon nanotube field emitters,” Phys. Rev. B, Condens. Matter, vol. 67, no. 11, pp. 115406/1–115406/10, Mar. 2003.

Hae Jin Kim received the B.S. degree in electronic engineering and the M.S. degree in radio science and engineering from Kwangwoon University, Seoul, Korea, in 2000 and 2002, respectively, where he is currently working toward the Ph.D. degree in the same field of study. His research interests include the simulation and design of high-power vacuum devices, including TWT, magnetron, and klystron.

Jin Joo Choi received the B.S. degree in physics from Seoul National University, Seoul, Korea, in 1983, the M.S. degree in physics from Georgia State University, Atlanta, in 1985, and the Ph.D. degree in nuclear engineering from University of Michigan, Ann Arbor, in 1991. In 1991, he joined the Vacuum Electronics Branch, Electronic Science and Technology Division, U.S. Naval Research Laboratory (NRL), Washington, DC. His work at NRL was on the development of highpower millimeterwave gyro-amplifiers for electronic warfare and radar applications. In 1997, he moved to Kwangwoon University, Seoul, where he is currently an Associate Professor with the School of Electronics Engineering. His research interests include high-power vacuum electronics and passive and active solid state devices.

Jae-Hee Han received the M.S. and Ph.D. degrees in materials science and engineering from Sungkyunkwan University, Suwon, Korea, in 2001 and 2005, respectively. Currently, he is working as a Postdoctoral Fellow with the Department of Chemical Engineering, University of Illinois, Urbana–Champaign. He is focused on biomedical applications of CNTs.

Jae Hong Park received the B.S. degree from Keimyung University, Dae-gu, Korea, in 2003 and the M.S. degree in materials science and engineering from Sungkyunkwan University, Suwon, Korea, where he is currently working toward the Ph.D. degree in advanced materials science and engineering. His research is focused on device-compatible applications of CNT paste.

Ji-Beom Yoo received the Ph.D. degree in electronic materials from Stanford University, Stanford, CA, in 1989. In 1989, he was a Senior Researcher with the Electronic and Telecommunication Research Institute (ETRI), Korea. He was invited as a Professor with the Sungkyunkwan University, Suwon, Korea, in 1994. Currently, he is working as a Vice-Dean of the School of Advanced Materials Science and Engineering. He had published over 100 articles in reputed international journals.