Stable and high emission current from carbon nanotube paste with ...

Stable and high emission current from carbon nanotube paste with spin on glass Jae-Hong Park, Jin-San Moon, Jae-Hee Han, Alexander S. Berdinsky, Ji-Beom Yoo,a兲 and Chong-Yun Park Center for Nanotubes and Nanostructured Composites, Sungkyunkwan University, 300 Chunchun-Dong, Jangan-Gu, Suwon, 440-746, Korea

Joong-Woo Nam, Jonghwan Park, Chun Gyoo Lee, and Deok Hyeon Choe Technology Development 3, Corporate R&D Center, Samsung SDI 428-5, Gongse-Ri, Kiheung-Eup, Yongin, 449-902, Korea

共Received 1 October 2004; accepted 31 January 2005; published 6 April 2005兲 A carbon nanotube 共CNT兲 paste was synthesized by mixing multiwalled CNTs 共MWNTs兲, organic vehicles, and inorganic binder. The paste with spin on glass 共SOG兲 showed improved uniformity, dispersion, and adhesion characteristics of the CNT paste layer. The emission characteristics of CNT paste with SOG were improved, and compared to those of a CNT paste with a glass frit. When the organic vehicle was changed from ethyl cellulose to acryl solution, current density of CNT paste increased. The firing condition for CNT pastes was investigated and optimized. It was found that firing at 450 °C under N2 was the most suitable condition for pastes with MWNTs. We obtained stable and high emission current of 100 mA at an electric field of 8.35 V / ␮m from CNT paste with SOG printed on a Ni plate. © 2005 American Vacuum Society. 关DOI: 10.1116/1.1880192兴

I. INTRODUCTION Carbon nanotubes 共CNT兲 exhibit excellent field emission characteristics and are one of the most actively studied materials for cold cathode field emitters due to CNTs’ high aspect ratios, small tip radii of curvature, mechanical strength, and good chemical and thermal stability.1,2 Field emitter arrays have been regarded as attractive electron sources in various applications such as field emission displays,3–5 x-ray tubes,6 lighting devices,7 microwave power amplifiers, and electron microscopy. Recently, both the direct growth method, such as chemical vapor deposition 共CVD兲, and the screen-printing method using CNT paste have been recognized as two promising techniques for the fabrication of the field emission array. However, the CVD method has several problems such as low throughput, high cost, and high growth temperature in large area cold cathode fabrication. However, the screen-printing process using CNT paste has been adopted as an effective method for fabricating the large area cold cathode because of low cost, simple process, uniform emission site, and mass production advantages.3,5 CNT paste was synthesized by mixing CNT powders, organic vehicles, and inorganic binder. For the stable, uniform, and high emission current of the screen-printed cold cathode field emitter, uniformity and adhesion of CNT films to the substrate is necessary. In order to improve the adhesion between CNT paste film and the substrate after the firing process, a glass frit was conventionally used as an inorganic binder. However, CNT paste with a glass frit has shown poor uniformity in cathode film formation and emission characteristics. In this work, through the optimization of paste composition and a兲

Corresponding author; electronic mail: [email protected]

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firing condition, we studied the formation of the CNT paste layer for high field emission current. CNT pastes with different inorganic binders and organic vehicles were prepared and tested. The field emission characteristics were investigated for CNT pastes with different compositions after firing under various conditions. II. EXPERIMENT CNT paste was synthesized by mixing CNT powders, organic vehicles, and inorganic binder. Multiwalled carbon nanotubes 共MWNTs兲 powders grown by CVD and ethyl cellulose 共EC兲 solution 共or acryl solution兲 were used as an electron emission source and organic vehicle, respectively. We prepared CNT pastes with different inorganic binders such as glass frit and spin on glass 共SOG兲. Figure 1 shows the schematic diagram of the CNT paste formation process. The mixture of CNT powders, organic vehicles, and inorganic binder was premixed through the solder paste softener for 15 min. Then the three-roll mill process was carried out for mixing and dispersion of CNT powders in the organic vehicle used as a polymer matrix. Mechanically well-dispersed CNT paste was printed onto 2 ⫻ 2 cm2 of the indium tin oxide 共ITO兲 coated soda lime glass. In order to remove the solvents in the CNT paste, heat treatment was performed in a forced convection oven for 15 min at 90 °C in an air atmosphere. An organic vehicle plays an important role for the rheological properties of CNT paste such as viscosity, thixotropy, and printability. However, residue of the organic vehicle causes problems such as outgassing and arcing during the field emission. Therefore, the organic vehicles in CNT paste must be removed in order to obtain stable emission current. The CNT paste film was fired in air or nitrogen 共N2兲 at temperatures ranging from 400 to 450 °C.

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©2005 American Vacuum Society

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FIG. 3. J–E plots of EC-based CNT paste depending on the inorganic binders. Inset is the F-N plot.

FIG. 1. Schematic diagram of CNT paste formation process.

Field emission scanning electron microscopy 共FESEM兲 was employed for the characterization of CNT paste morphology. The emission characteristics of CNT paste were measured in a vacuum chamber with a parallel diode-type configuration at pressure of 5 ⫻ 10−6 Torr after surface activation treatment with adhesive tape. The distance between the cathode and the anode was 200 µm. For the measurement of the emission current, we used both a direct current 共dc兲 power supply and a pulsed voltage power supply with a duty cycle of 1 / 500. III. RESULTS AND DISCUSSION Two types of CNT paste were synthesized. One type was composed of CNT powder, EC solution, and a glass frit. The other type was prepared by using SOG instead of a glass frit. Figure 2 shows FESEM images of CNT pastes with different

FIG. 2. SEM images of EC-based CNT paste with different inorganic binders; 共a兲 cross sectional and 共b兲 top view of CNT paste with glass frit, 共c兲 cross sectional and 共d兲 top view of CNT paste with SOG. JVST B - Microelectronics and Nanometer Structures

inorganic binder. When we changed an inorganic binder from a glass frit to SOG, the morphology of the cathode layer significantly changed after firing at 450 °C under an air atmosphere. Figures 2共a兲 and 2共b兲 are cross sectional and top views of CNT paste with a glass frit, respectively. In the case of CNT paste with a glass frit, we observed a small amount of CNTs and partially melted glass frits. The low density and poor uniformity of CNT emitters were due to the fact that both organic materials and MWNTs were exposed to air during firing at 450 °C. Therefore, in the case of CNT paste with a glass frit, the optimization of firing conditions such as the gas ambient and firing temperature, was required to obtain the uniform cathode layer with a reasonable density of CNT emitters. Figures 2共c兲 and 2共d兲 are cross sectional and top views of CNT paste with SOG, respectively. As shown in Figs. 2共c兲 and 2共d兲, uniform and thick cathode layers with a reasonable CNT emitter were obtained after the same firing process as that used in the CNT paste with a glass frit. The film thickness was about 7.1 µm. The emission characteristics of these CNT pastes were measured in a vacuum chamber with a parallel diode-type configuration at a pressure of 5 ⫻ 10−6 torr. We used a pulsed voltage power supply with a duty cycle of 1 / 500. To obtain high emission current density and uniform emission site from the screen-printed CNT emitters, special activation treatments such as laser irradiation,8 ion irradiation,9 and adhesive tape method10 were reported. Before current-voltage 共I–V兲 measurement, we carried out the activation treatment by using adhesive tape. Figure 3 shows the current densityelectric field 共J–E兲 curve and the Fowler–Nordheim 共F-N兲 plot of the activated CNT paste films with a glass frit and SOG. The F-N plot confirmed the electron emission from the quantum mechanical tunneling. The emission current density at an applied field of 7.95 V / ␮m increased from 133 to 265 ␮A / cm2 when the inorganic binder changed from a glass frit to SOG. The organic vehicle makes a significant effect on the emission characteristics and rheological properties of CNT

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FIG. 4. SEM images of acryl-based CNT paste with 共a兲 glass frit and 共b兲 SOG. The J–E plot of the CNT paste with different inorganic binder is represented in 共c兲. Inset of 共c兲 indicates the F-N plot.

paste. CNT paste was synthesized by using the acryl solution as an organic vehicle instead of the EC solution to study the effects of the organic vehicle on the properties of CNT paste. A glass frit and SOG were used as inorganic binders. Figures 4共a兲 and 4共b兲 show SEM images of acryl based CNT paste with different inorganic binders. When acryl solution was used as organic vehicle, the thickness of CNT pastes was increased. Figure 4共c兲 shows the J–E curve of the paste with different inorganic binders. The inset of Fig. 4共c兲 shows the F-N plot. When the organic vehicle changed from the EC solution to the acryl solution, the emission current density at an applied electric field of 5 V / ␮m increased from 54 to 228.5 ␮A / cm2 in the paste with a glass frit, and from 61 to 302.5 ␮A / cm2 in the paste with SOG. The current density increased by five times in magnitude after the organic vehicle changed from the EC solution to the acryl solution irrespective of the type of an inorganic binder. However, the film uniformity and emission current density of the CNT paste with SOG was better than that of the CNT paste with a glass frit when the acryl solution was used as the organic J. Vac. Sci. Technol. B, Vol. 23, No. 2, Mar/Apr 2005

vehicle. These results suggest that the acryl solution is more suitable and is a better organic vehicle than the EC solution for field emission applications. To obtain high emission current from printed CNT emitters, the effect of firing condition was investigated. CNT paste films using the acryl solution were prepared and fired at temperature ranging from 400 to 450 °C in air or N2. Figure 5 shows the effects of the firing condition on the emission properties of acryl based CNT paste with SOG. The best emission properties of CNT paste were obtained when the CNT paste was fired at 450 °C in an N2 ambient. The current density at an applied electric field of 5 V / ␮m was 302 ␮A / cm2. The electric field at the current density of 300 ␮A / cm2 was 4.4 V / ␮m. For high electron emission applications ITO-coated soda lime glass as a cathode electrode is not suitable because of low electrical conductivity. Therefore, CNT paste was printed onto a Ni plate. Figure 6 shows scanning electron microscopy 共SEM兲 images of the acryl based CNT paste with SOG printed onto a Ni plate. As shown in the inset of

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FIG. 5. The emission characteristics of acryl-based CNT paste with SOG depending on different firing conditions.

Fig. 6共a兲, uniform CNT paste film was formed with a thickness of about 20 µm. Figure 6共b兲 shows CNTs protruded from the CNT paste surface by surface treatment using adhesive tape. J–E measurement was carried out using dc voltage in a high vacuum chamber. For the emission current measurement, a stainless steel plate was used, as an anode instead of ITO coated glass, because ITO coated glass is not strong enough to endure a high density of electron bombardment at a high electric field. The emission area was 1 ⫻ 1 cm2. The distance between anode and cathode was 170 µm. Figure 7

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FIG. 7. Cycled J–E plot from acryl-based CNT paste with SOG printed onto a Ni plate fired at 450 °C in N2. Inset indicates the F-N plot.

shows the cycled J–E plot from acryl-based CNT paste with SOG printed onto a Ni plate. The CNT paste shows stable emission current after the third measurement. The F-N plot is shown in the inset of Fig. 7, indicating that the electron emission comes from the quantum mechanical tunneling. We obtained a very high emission current of about 100 mA at the electric field of 8.35 V / ␮m from CNT paste film printed on the Ni plate.

IV. CONCLUSION CNT paste was synthesized by using SOG as an inorganic binder instead of a glass frit. We improved the emission properties of CNT paste through optimization of the paste composition and the firing condition. CNT paste with SOG showed enhanced emission characteristics compared to CNT paste with a glass frit. When the organic vehicle changed from the EC solution to the acryl solution, the current density of CNT paste increases by about five times in magnitude. Firing at 450 °C under N2 is the most suitable condition for CNT paste. From CNT paste with SOG, we obtained stable and high emission current of 100 mA at an electric field of 8.35 V / ␮m. Our experiments have shown that CNT paste with SOG can be used as an efficient electron emitter in vacuum nanoelectronics applications.

ACKNOWLEDGMENTS

FIG. 6. SEM images of acryl-based CNT paste with SOG printed on a Ni plate: 共a兲 cross sectional and 共b兲 tilt view of CNT paste after firing at 450 °C in N2. Inset of 共a兲 shows the thickness of the CNT paste. JVST B - Microelectronics and Nanometer Structures

This research was supported by Grant No. M1-02-KR-010001-02-K18-01-016-1-2 from the Information Display Research and Development Center, one of the 21st Century Frontier Research and Development programs funded by the Ministry of Science and Technology of Korea, and Korea Science and Engineering Foundation 共KOSEF兲 through the Center for Nanotubes and Nanostructured Composites 共CNNC兲 at Sungkyunkwan University.

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