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Field emission from multi-walled carbon nanotubes with various fillers
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Materials Letters 62 (2008) 2795 – 2798 www.elsevier.com/locate/matlet
Field emission from multi-walled carbon nanotubes with various fillers Parlindungan Yonathan, Hyun-Tae Kim, Dang-Hyok Yoon ⁎ School of Materials Science and Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea Received 29 September 2007; accepted 17 January 2008 Available online 26 January 2008
Abstract Pastes for field emission test were prepared by 3-roll milling of multi-walled carbon nanotubes (MWNTs) and UV-sensitive binder solution. The effects of four filler additives, namely two types of indium tin oxide (ITO) powder, glass frit and Ag on the field emission properties of the screen-printed paste were investigated and compared to those without filler using a diode-type configuration. The paste formulation was shown to be adequate for fine patterning using a UV-lithography technique. MWNT pastes containing any type of filler showed better emission properties than the paste without filler, thereby confirming the importance of the filler. The MWNT paste with 1 wt.% glass frit showed the best results with the lowest turn-on field of 1.75 V/μm at 1 μA/cm2, highest emission current density of 78 μA/cm2 at 5 V/μm, and β-factor of 17,000 approximately, which are satisfactory for practical application. © 2008 Elsevier B.V. All rights reserved. Keywords: Electrical properties; Carbon nanotubes; Thick films; Field emission
1. Introduction Recently, carbon nanotubes have been extensively studied as a field emission source for the application of field emission display (FED) [1], backlight unit for liquid crystal display [2] and various lamps [3]. The requirements for these applications include a high emission current density, low turn-on voltage and long-term stability. Since multi-walled carbon nanotubes (MWNTs) generally show better lifetime than single-walled carbon nanotubes, in spite of their relatively lower emission current density, current research efforts are mainly being focused on MWNTs [4]. MWNTs as electron emitters have been fabricated by the direct growth [5], electrophoretic deposition [6], and screen printing methods [1], of which the latter has been successfully adopted to produce a largescale flat panel in a relatively simple and inexpensive process. The paste for screen printing requires various ingredients such as MWNTs, binder resin, solvent, dispersant, photo initiator, and ceramic or metal filler, while the choice for polymeric species is quite restricted due to the use of UV-lithography for micropatterning of the device. In spite of the numerous types of available
⁎ Corresponding author. Tel.: +82 53 810 2561; fax: +82 53 810 4628. E-mail address: [email protected] (D.-H. Yoon). 0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.01.047
fillers, only few materials such as Ag and Al2O3 have been used as a filler for MWNT paste without any systematic approach yet [7]. The filler is one of the important ingredients, as it confers the adhesion strength of the paste and may also change the emission properties at the same time. Based on this background, this work examines the effects of four kinds of fillers on the electron emission properties of MWNT paste, and the results are compared with those of the paste without filler for FED application. 2. Experimental MWNTs grown by catalytic chemical vapor deposition (CVD) method (CMP-310F, Iljin Nanotech, Korea) with a mean diameter of 3–5 nm, length of 10–20 μm, and specific surface area of 797 m2/g were used. In order to synthesize a UVsensitive organic vehicle, acrylic binder resin (Elvacite 2669, Lucite International) was dissolved in texanol (C12H24O3: 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate) and mixed with UV-sensitive monomers, initiators and a filler. Coarse (average particle size of 705 nm) and fine indium tin oxide (ITO) powder (67 nm), glass powder of composition 72 SnO–23 P2O5–5 SiO2/ Al2O3 in wt.% (78 nm), and Ag powder (400 nm) were used as fillers, with 1 wt.% of each filler being added into the paste, except for Ag which was added at 5 wt.% based on the screen-printability
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of the resultant paste. The MWNT amount was fixed at 1.6 wt.% in all pastes for the comparison. The five paste compositions, including that without filler, were pre-mixed using a stirrer and subjected to 3-roll milling. 3-roll mill uses the shear force created by three horizontally positioned rolls rotating at opposite directions with different speeds relative to each other to homogenize viscous materials by decreasing the gaps between rolls. The MWNT pastes were screen-printed once or twice onto an ITO-coated soda lime glass to a thickness of approximately 2–3 μm and dried at 90 °C for 10 min in air. Screen printing in twice was achieved by reprinting the paste on the dried preprinted area in order to increase the surface-exposed MWNT density. Since the residue of organic materials in the pastes causes problems such as out-gassing and arching during the field emission measurement, binder burnout
was performed at 450 °C in a nitrogen atmosphere, followed by an activation process comprising the vertical alignment of MWNTs using a sticky tape. The field emission characteristics of the pastes were measured in a vacuum chamber with a parallel diode-type configuration at a pressure of 10− 6 Torr using a pulse generator with 1/500 duty. The gap between the anode and cathode was 200 μm, and each sample had an area of 1.1× 1.1 cm2. In order to check the UV-development properties of these pastes for practical application, 25 μm-diameter dots at a spacing of 400 μm were formed simultaneously using UV-lithography technique. The MWNTs and printed morphologies were investigated using a field emission scanning electron microscope (FESEM: S-4100, Hitachi) and high resolution transmission electron microscope (HRTEM: H-7600, Hitachi).
Fig. 1. (a) SEM and TEM (inset) images of MWNTs and (b) photograph of MWNT cathode dot patterns formed by UV-lithography.
P. Yonathan et al. / Materials Letters 62 (2008) 2795–2798
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3. Results and discussion The filler in the paste plays two roles: enhancing the adhesion strength of MWNTs to the substrate and offering an electronconducting path from the cathode to MWNTs. In this respect, the Ag filler and glass frit are closely related with the latter and former, respectively, while ITO may act for both purposes. Since single crystalline ITO itself was reported for the field emitter due to its metallic conductivity [8], two types of ITO powder with different particle size were utilized to check ITO's size effect in this study. The as-received MWNTs had a highly entangled morphology, as shown in Fig. 1(a), which needed to be dispersed to facilitate the electron emission from their tips. The HRTEM image of the MWNT in the inset of the same figure presents four graphene layers with a diameter of 4.8 nm. The MWNT dot patterns formed by the UV-lithography technique present a clear boundary, as shown in Fig. 1(b), indicating the adequate paste formulation for fine patterning using this technique. Fig. 2 shows the side-view of the paste with 1 wt.% glass frit, which was screen-printed twice on ITO glass, representing the high, surfaceexposed, MWNT density after the activation process. Since electrons are emitted from the MWNT tips, a higher surface-exposed density is desirable to achieve a high emission current density. Fig. 3(a) and (b) shows the current–voltage (I–V) characteristics and the corresponding Fowler–Nordheim (F–N) plots for the pastes containing 1 wt.% fine ITO and the same amount of glass frit, respectively. In both cases, the pastes screen-printed twice showed a higher emission current density than those printed once. The F–N plots [insets in Fig. 3(a) and (b)] show almost linear slopes, implying the field emission of electrons from MWNTs. The F–N model expresses the relationships among the emission current density, local electrical field, and work function (Φ) of the emitter in the field emission, where ln(J/E2) vs. 1/E shows a linear relationship based on the following equation [9]: ln
J E2
¼ ln
ab2 U
3
0:95bU2 bE
ð1Þ
where J is the emission current density (A/m2), E the applied electric field (V/m), β the field enhancement factor, Φ the work function of MWNT which is assumed to be 5 eV, and a and b are constants
Fig. 3. Current vs. voltage characteristics and corresponding Fowler–Nordheim (F–N) plots (inset) for the pastes screen-printed once and twice with (a) 1 wt.% fine ITO, and (b) 1 wt.% glass frit.
(a = 1.54 × 10− 6 Ad eV/V2, b = 6.83 × 109 eV3/2), The slope of ln(J/E2) vs. 1/E from Eq. (1) is given by: 3
Slope ¼
Fig. 2. Side-view of the screen-printed MWNT paste containing 1 wt.% glass frit and screen-printed twice on the ITO-coated glass substrate.
d ð ln ð J =E2 ÞÞ 0:95bU2 ¼ d ð1=EÞ b
ð2Þ
Therefore, β can be calculated from Eq. (2) since the other parameters are known. Table 1 summarizes the turn-on field for 1 μA/cm2, emission current density and β factor for the samples investigated in this study. In all cases, the samples screen-printed twice showed a higher current density and β factor than those printed once, due to the higher surfaceexposed, MWNT density. It is notable that the turn-on field was not only decided by the material system in the paste, but also by the number of screen-printings. Moreover, samples with any type of filler showed better emission properties than the MWNT paste without filler. The paste containing fine ITO powder showed better emission properties than that with a coarse powder, which was attributed to the enhanced contact between the fine ITO and MWNTs. The overall field emission efficiency of the MWNT pastes with four fillers was demonstrated in the order of glass frit N fine ITO N Ag N no filler, based on the experimental results. Especially, the MWNT paste with 1 wt.% glass
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Table 1 Turn-on field, emission current density and β-factor values for MWNTs pastes containing no filler and four different fillers Filler type and amount
Number of Turn-on field for Current density β-factor printing 1 μA/cm2 (V/μm) at 5 V/μm (μA/cm2)
No filler
1 2 1 wt.% ITO 1 (coarse) 2 1 wt.% ITO 1 (fine) 2 1 wt.% glass 1 frit 2 5 wt.% Ag 1 2
2.60 2.05 2.65 1.95 2.20 1.80 1.75 1.75 2.40 2.05
28.9 51.1 31.4 62.2 53.4 73.6 78.4 87.8 40.0 55.7
6728 13,109 5930 13,654 7224 13,927 17,227 17,448 6815 13,958
frit showed the lowest turn-on field of 1.75 V/μm at 1 μA/cm2, highest emission current density of 78 μA/cm2 at 5 V/μm, and β-factor of 17,000, which are perfectly satisfactory for commercial purpose.
4. Conclusion We investigated the effects of four fillers on the emission properties of MWNT paste screen-printed on ITO substrate with an area of 1.1 × 1.1 cm2 for FED application. The higher surface-exposed, MWNT density that was achieved by printing twice improved the emission properties compared to that printed only once. Filler addition efficiently increased the
current density and decreased the turn-on field compared to that without filler. The best filler results were obtained with 1 wt.% glass frit addition. The results also suggested that the UV-sensitive binder system can be used for fine patterning. Acknowledgements This work was supported by the Korean Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2006-331-D00255). References [1] J.M. Kim, W.B. Choi, N.S. Lee, J.E. Jung, Diam. Relat. Mater. 9 (2000) 1184–1189. [2] Y.C. Kim, E.H. Yoo, Jpn. J. Appl. Phys. 44 (2005) L454–L456. [3] Y. Cho, S. Lee, M. An, D. Kim, Phys. Status Solidi, A Appl. Res. 204 (2007) 1804–1807. [4] J.M. Bonard, J.P. Salvetat, T. Stöckli, L. Forró, A. Châtelain, Appl. Phys. A. 69 (1999) 245–254. [5] C.J. Lee, J. Park, S.Y. Kang, J.H. Lee, Chem. Phys. Lett. 326 (2000) 175–180. [6] K. Yamamoto, S. Akita, Y. Nakayama, J. Phys. D. Appl. Phys. 31 (1998) L34–L36. [7] J.W. Nam, S.H. Cho, Y.C. Choi, J.S. Ha, J.H. Park, D.H. Choe, J.B. Yoo, J.H. Park, Diam. Relat. Mater. 14 (2005) 2089–2093. [8] H.S. Jang, D.H. Kim, H.R. Lee, S.Y. Lee, Mater. Lett. 59 (2005) 1526–1529. [9] J.W. Gadzuk, E.W. Plummer, Rev. Mod. Phys. 45 (1973) 487–551.
Materials Letters 62 (2008) 2795 – 2798 www.elsevier.com/locate/matlet
Field emission from multi-walled carbon nanotubes with various fillers Parlindungan Yonathan, Hyun-Tae Kim, Dang-Hyok Yoon ⁎ School of Materials Science and Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea Received 29 September 2007; accepted 17 January 2008 Available online 26 January 2008
Abstract Pastes for field emission test were prepared by 3-roll milling of multi-walled carbon nanotubes (MWNTs) and UV-sensitive binder solution. The effects of four filler additives, namely two types of indium tin oxide (ITO) powder, glass frit and Ag on the field emission properties of the screen-printed paste were investigated and compared to those without filler using a diode-type configuration. The paste formulation was shown to be adequate for fine patterning using a UV-lithography technique. MWNT pastes containing any type of filler showed better emission properties than the paste without filler, thereby confirming the importance of the filler. The MWNT paste with 1 wt.% glass frit showed the best results with the lowest turn-on field of 1.75 V/μm at 1 μA/cm2, highest emission current density of 78 μA/cm2 at 5 V/μm, and β-factor of 17,000 approximately, which are satisfactory for practical application. © 2008 Elsevier B.V. All rights reserved. Keywords: Electrical properties; Carbon nanotubes; Thick films; Field emission
1. Introduction Recently, carbon nanotubes have been extensively studied as a field emission source for the application of field emission display (FED) [1], backlight unit for liquid crystal display [2] and various lamps [3]. The requirements for these applications include a high emission current density, low turn-on voltage and long-term stability. Since multi-walled carbon nanotubes (MWNTs) generally show better lifetime than single-walled carbon nanotubes, in spite of their relatively lower emission current density, current research efforts are mainly being focused on MWNTs [4]. MWNTs as electron emitters have been fabricated by the direct growth [5], electrophoretic deposition [6], and screen printing methods [1], of which the latter has been successfully adopted to produce a largescale flat panel in a relatively simple and inexpensive process. The paste for screen printing requires various ingredients such as MWNTs, binder resin, solvent, dispersant, photo initiator, and ceramic or metal filler, while the choice for polymeric species is quite restricted due to the use of UV-lithography for micropatterning of the device. In spite of the numerous types of available
⁎ Corresponding author. Tel.: +82 53 810 2561; fax: +82 53 810 4628. E-mail address: [email protected] (D.-H. Yoon). 0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.01.047
fillers, only few materials such as Ag and Al2O3 have been used as a filler for MWNT paste without any systematic approach yet [7]. The filler is one of the important ingredients, as it confers the adhesion strength of the paste and may also change the emission properties at the same time. Based on this background, this work examines the effects of four kinds of fillers on the electron emission properties of MWNT paste, and the results are compared with those of the paste without filler for FED application. 2. Experimental MWNTs grown by catalytic chemical vapor deposition (CVD) method (CMP-310F, Iljin Nanotech, Korea) with a mean diameter of 3–5 nm, length of 10–20 μm, and specific surface area of 797 m2/g were used. In order to synthesize a UVsensitive organic vehicle, acrylic binder resin (Elvacite 2669, Lucite International) was dissolved in texanol (C12H24O3: 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate) and mixed with UV-sensitive monomers, initiators and a filler. Coarse (average particle size of 705 nm) and fine indium tin oxide (ITO) powder (67 nm), glass powder of composition 72 SnO–23 P2O5–5 SiO2/ Al2O3 in wt.% (78 nm), and Ag powder (400 nm) were used as fillers, with 1 wt.% of each filler being added into the paste, except for Ag which was added at 5 wt.% based on the screen-printability
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of the resultant paste. The MWNT amount was fixed at 1.6 wt.% in all pastes for the comparison. The five paste compositions, including that without filler, were pre-mixed using a stirrer and subjected to 3-roll milling. 3-roll mill uses the shear force created by three horizontally positioned rolls rotating at opposite directions with different speeds relative to each other to homogenize viscous materials by decreasing the gaps between rolls. The MWNT pastes were screen-printed once or twice onto an ITO-coated soda lime glass to a thickness of approximately 2–3 μm and dried at 90 °C for 10 min in air. Screen printing in twice was achieved by reprinting the paste on the dried preprinted area in order to increase the surface-exposed MWNT density. Since the residue of organic materials in the pastes causes problems such as out-gassing and arching during the field emission measurement, binder burnout
was performed at 450 °C in a nitrogen atmosphere, followed by an activation process comprising the vertical alignment of MWNTs using a sticky tape. The field emission characteristics of the pastes were measured in a vacuum chamber with a parallel diode-type configuration at a pressure of 10− 6 Torr using a pulse generator with 1/500 duty. The gap between the anode and cathode was 200 μm, and each sample had an area of 1.1× 1.1 cm2. In order to check the UV-development properties of these pastes for practical application, 25 μm-diameter dots at a spacing of 400 μm were formed simultaneously using UV-lithography technique. The MWNTs and printed morphologies were investigated using a field emission scanning electron microscope (FESEM: S-4100, Hitachi) and high resolution transmission electron microscope (HRTEM: H-7600, Hitachi).
Fig. 1. (a) SEM and TEM (inset) images of MWNTs and (b) photograph of MWNT cathode dot patterns formed by UV-lithography.
P. Yonathan et al. / Materials Letters 62 (2008) 2795–2798
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3. Results and discussion The filler in the paste plays two roles: enhancing the adhesion strength of MWNTs to the substrate and offering an electronconducting path from the cathode to MWNTs. In this respect, the Ag filler and glass frit are closely related with the latter and former, respectively, while ITO may act for both purposes. Since single crystalline ITO itself was reported for the field emitter due to its metallic conductivity [8], two types of ITO powder with different particle size were utilized to check ITO's size effect in this study. The as-received MWNTs had a highly entangled morphology, as shown in Fig. 1(a), which needed to be dispersed to facilitate the electron emission from their tips. The HRTEM image of the MWNT in the inset of the same figure presents four graphene layers with a diameter of 4.8 nm. The MWNT dot patterns formed by the UV-lithography technique present a clear boundary, as shown in Fig. 1(b), indicating the adequate paste formulation for fine patterning using this technique. Fig. 2 shows the side-view of the paste with 1 wt.% glass frit, which was screen-printed twice on ITO glass, representing the high, surfaceexposed, MWNT density after the activation process. Since electrons are emitted from the MWNT tips, a higher surface-exposed density is desirable to achieve a high emission current density. Fig. 3(a) and (b) shows the current–voltage (I–V) characteristics and the corresponding Fowler–Nordheim (F–N) plots for the pastes containing 1 wt.% fine ITO and the same amount of glass frit, respectively. In both cases, the pastes screen-printed twice showed a higher emission current density than those printed once. The F–N plots [insets in Fig. 3(a) and (b)] show almost linear slopes, implying the field emission of electrons from MWNTs. The F–N model expresses the relationships among the emission current density, local electrical field, and work function (Φ) of the emitter in the field emission, where ln(J/E2) vs. 1/E shows a linear relationship based on the following equation [9]: ln
J E2
¼ ln
ab2 U
3
0:95bU2 bE
ð1Þ
where J is the emission current density (A/m2), E the applied electric field (V/m), β the field enhancement factor, Φ the work function of MWNT which is assumed to be 5 eV, and a and b are constants
Fig. 3. Current vs. voltage characteristics and corresponding Fowler–Nordheim (F–N) plots (inset) for the pastes screen-printed once and twice with (a) 1 wt.% fine ITO, and (b) 1 wt.% glass frit.
(a = 1.54 × 10− 6 Ad eV/V2, b = 6.83 × 109 eV3/2), The slope of ln(J/E2) vs. 1/E from Eq. (1) is given by: 3
Slope ¼
Fig. 2. Side-view of the screen-printed MWNT paste containing 1 wt.% glass frit and screen-printed twice on the ITO-coated glass substrate.
d ð ln ð J =E2 ÞÞ 0:95bU2 ¼ d ð1=EÞ b
ð2Þ
Therefore, β can be calculated from Eq. (2) since the other parameters are known. Table 1 summarizes the turn-on field for 1 μA/cm2, emission current density and β factor for the samples investigated in this study. In all cases, the samples screen-printed twice showed a higher current density and β factor than those printed once, due to the higher surfaceexposed, MWNT density. It is notable that the turn-on field was not only decided by the material system in the paste, but also by the number of screen-printings. Moreover, samples with any type of filler showed better emission properties than the MWNT paste without filler. The paste containing fine ITO powder showed better emission properties than that with a coarse powder, which was attributed to the enhanced contact between the fine ITO and MWNTs. The overall field emission efficiency of the MWNT pastes with four fillers was demonstrated in the order of glass frit N fine ITO N Ag N no filler, based on the experimental results. Especially, the MWNT paste with 1 wt.% glass
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P. Yonathan et al. / Materials Letters 62 (2008) 2795–2798
Table 1 Turn-on field, emission current density and β-factor values for MWNTs pastes containing no filler and four different fillers Filler type and amount
Number of Turn-on field for Current density β-factor printing 1 μA/cm2 (V/μm) at 5 V/μm (μA/cm2)
No filler
1 2 1 wt.% ITO 1 (coarse) 2 1 wt.% ITO 1 (fine) 2 1 wt.% glass 1 frit 2 5 wt.% Ag 1 2
2.60 2.05 2.65 1.95 2.20 1.80 1.75 1.75 2.40 2.05
28.9 51.1 31.4 62.2 53.4 73.6 78.4 87.8 40.0 55.7
6728 13,109 5930 13,654 7224 13,927 17,227 17,448 6815 13,958
frit showed the lowest turn-on field of 1.75 V/μm at 1 μA/cm2, highest emission current density of 78 μA/cm2 at 5 V/μm, and β-factor of 17,000, which are perfectly satisfactory for commercial purpose.
4. Conclusion We investigated the effects of four fillers on the emission properties of MWNT paste screen-printed on ITO substrate with an area of 1.1 × 1.1 cm2 for FED application. The higher surface-exposed, MWNT density that was achieved by printing twice improved the emission properties compared to that printed only once. Filler addition efficiently increased the
current density and decreased the turn-on field compared to that without filler. The best filler results were obtained with 1 wt.% glass frit addition. The results also suggested that the UV-sensitive binder system can be used for fine patterning. Acknowledgements This work was supported by the Korean Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2006-331-D00255). References [1] J.M. Kim, W.B. Choi, N.S. Lee, J.E. Jung, Diam. Relat. Mater. 9 (2000) 1184–1189. [2] Y.C. Kim, E.H. Yoo, Jpn. J. Appl. Phys. 44 (2005) L454–L456. [3] Y. Cho, S. Lee, M. An, D. Kim, Phys. Status Solidi, A Appl. Res. 204 (2007) 1804–1807. [4] J.M. Bonard, J.P. Salvetat, T. Stöckli, L. Forró, A. Châtelain, Appl. Phys. A. 69 (1999) 245–254. [5] C.J. Lee, J. Park, S.Y. Kang, J.H. Lee, Chem. Phys. Lett. 326 (2000) 175–180. [6] K. Yamamoto, S. Akita, Y. Nakayama, J. Phys. D. Appl. Phys. 31 (1998) L34–L36. [7] J.W. Nam, S.H. Cho, Y.C. Choi, J.S. Ha, J.H. Park, D.H. Choe, J.B. Yoo, J.H. Park, Diam. Relat. Mater. 14 (2005) 2089–2093. [8] H.S. Jang, D.H. Kim, H.R. Lee, S.Y. Lee, Mater. Lett. 59 (2005) 1526–1529. [9] J.W. Gadzuk, E.W. Plummer, Rev. Mod. Phys. 45 (1973) 487–551.