- Email: [email protected]
Synthesis and characterization of carbon nanotube grown on flexible and conducting carbon fiber sheet for field emitter
Diamond & Related Materials 18 (2009) 341–344
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Synthesis and characterization of carbon nanotube grown on flexible and conducting carbon fiber sheet for field emitter Yasuhiko Hayashi a,⁎, K. Suzuki b, B. Jang a, T. Tokunaga c, H. Matsumoto b, M. Tanemura a, A. Tanioka b, G.A.J. Amaratunga d a
Department of Frontier Materials, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Okayama, Meguro, Tokyo 152-8552, Japan Department of Quantum Engineering, Nagoya University, Furo, Chikusa, Nagoya 464-8601, Japan d Department of Engineering, University of Cambridge, 9 JJ Thomson Avenue, Cambridge CB3 0DE, United Kingdom b c
a r t i c l e
i n f o
Available online 11 September 2008 Keywords: Multiwall carbon nanotubes (MWCNTs) Conductive flexible carbon fiber (CF) Field emission cathode Fowler–Nordheim (FN) plot
a b s t r a c t We have successfully demonstrated the multiwall carbon nanotube (MWCNT) based field emission cathode fabricated on highly porous, flexible and conductive carbon fiber (CF) sheet without damage of CF sheet by chemical vapor deposition. CF sheet was composed of thin fibers with diameter of about 3 μm prepared by electrospray deposition. Well aligned MWCNTs grown on, or well anchored to, the flexible CF sheet was confirmed by transmission electron microscope. The field enhancement factor for MWCNT emitter fabricated on CF sheet was about 14,000 and 1.6 times higher than that of MWCNT emitter fabricated on Si substrate. Scanning electron microscopy image indicates that the electron emission occurred from the MWCNT field emitter arrays grown on carbon fibers not only at surface but also under layers in CF sheet. Field emission measurements revealed that this flexible MWCNT field emitter array has a great potential for the flexible field emission displays. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The diverse and exemplary properties of carbon nanotubes (CNTs) have inspired a vast range of proposed applications including transistors, interconnects, conductive films, field emission sources, biosensors, scanning probes, nanomechanical devices and hydrogen storage elements [1]. One particularly promising application of CNTs is the electron emitters for field emission displays. Due to its extremely low turn-on fields and high current densities, CNTs have already been proved as one of the most promising electron field emitter [2–4]. One important prerequisite to use CNTs as electron emitters in the flat panel display is to be able to apply them in patterns onto the substrates. This can be realized either by producing the CNTs and subsequent patterning on the substrate or by growing the CNTs directly on a substrate pre-patterned with a catalyst. Jung et al. [5] have reported the assembly of the field emission part of a screen using CNT electrodes embedded in a polymer matrix. Their simple idea could be a big step towards the implementation of large-area displays that are not only flat, but flexible too. Although, lots of researches have been done about the growth of CNTs on patterned substrate, one common feature is that the substrate used for the growth of CNTs is stiff silicon or glass. Flexible electronics such as a flexible display is of great concern as the emerging technique ⁎ Corresponding author. Tel./fax: +81 52 735 5024. E-mail address: [email protected] (Y. Hayashi). 0925-9635/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.09.014
and attracts great research interests. Flexible display is the next generation display, and it requires every component be flexible. Usually, aligned CNTs are grown either by using thin catalyst layers predeposited on substrates or through vapor phase catalyst delivery. However, all of these synthesis methods are based on chemical vapor deposition (CVD), where CNT growth requires temperatures above 600 °C. This high temperature environment places a major restriction on the possible substrates on which CNTs can be grown, thus restricting many interesting applications (i.e. CNTs on flexible plastic substrates). Aligned CNT arrays fabricated on a flexible medium open up new applications in flexible nanoelectronics including switches, displays [5–7] and biocatalytic systems [8]. In this work, we report the fabrication of flexible field emitter arrays of MWCNTs on highly porous and conductive carbon fiber (CF) sheet as a flexible substrate and their field emission properties. It is important to point out that CF sheet has highly conducting nature and this leads to no metallization process for field emission cathode. Moreover, the high temperature synthesis process is applicable for the growth of MWCNTs on CF sheet. Therefore, our approach may offer both substantial cost advantages as well as enable new electronics applications on flexible substrates. 2. Experimental In the present study, CF sheets were prepared by electrospray deposition (EDS), is a straightforward and versatile method for
342
Y. Hayashi et al. / Diamond & Related Materials 18 (2009) 341–344
fabricating thin films by electrostatic force [9–11]. Phenolic resin (novolac type, Mw= 4000–5000) was dissolved in MeOH, and Poly(vinyl butyral) (PVB, Mw= 110,000) was added to the solution [12]. And then it was used for ESD source. The ESD device consists of a syringe-type infusion pump, a high voltage regulated DC power supply and a grounded collector (aluminum sheet, 10 × 10 cm2 area). The as-deposited CF sheets were cured by adding formaldehyde and then carbonized at 900 °C in a ceramic furnace for 2 h under a continuous nitrogen purge to obtain the carbonization yield at around 55%. The MWCNTs were grown by microwave PECVD (MPECVD) using a 2.45-GHz, 1.5-kW microwave power supply, as described elsewhere [13]. The substrate temperature was controlled by a radio-frequency graphite heater. After cleaning the CF sheet, a 1-nm-thick Pd layer was deposited onto the CF sheet (size: 2 × 2 cm2; thickness: 0.12 mm). Although we failed to grow CNTs on CF sheet by using Fe and Co catalyst layers, we have found the growth of CNTs by using Pd layer instead of widely used catalyst layers like Fe, Co or Ni. Therefore, we chose Pd layer in this work. It is seen from our previous work, that the Pd breaks into a small size (about 60–100 nm diameter) uniformly due to surface tension as well as the stress due to mismatch of the thermal expansion coefficients during the initial growth stage [14]. The CF sheet was then transferred into the growth chamber. The chamber was pumped down to a base pressure of 10− 2 Torr. The substrates were heated to 750 °C and held at this temperature for 10 min to sinter the catalyst layers. Hydrogen (H2) gas was then fed into the chamber to maintain a pressure of 20 Torr at the preset temperature and microwave plasma was turned on to 600 W. The feed gas methane (CH4) was introduced, and the H2 gas flow rate was adjusted to achieve a CH4:H2 ratio of 1:1 at a total pressure of 20 Torr. Then, the MWCNT networks were grown for 10 min. The surface morphologies of the CF sheet after carbonization and the MWCNTs grown on CF sheet were observed by scanning electron
Fig. 2. (a) Typical SEM image, (b) cross sectional TEM image of CF sheet and (c) the flexible CF sheet (size: 2 × 2 cm2; thickness: 0.12 mm) mounted onto a cylindrical object (radius = 25 mm) after growth of MWCNTs.
microscopy (SEM) and conventional transmission electron microscopy (TEM). Adsorption isotherms and the Brunauer–Emmet–Teller (BET) specific surface area of the CFs were determined using an adsorption apparatus. The electrical conductivities of the CF sheets were measured by a transmission line model. 3. Results and discussion
Fig. 1. (a) The flexible CF sheet (size: 2 × 2 cm2; thickness: 0.12 mm) mounted onto a cylindrical object (radius = 25 mm) before growth of MWCNTs. The flexible CF sheet was prepared by blend ratio of phenolic resin to PVB (the solute/MeOH (40/60 wt/wt)) at 99/3. (b) The SEM image of carbon fibers.
Fig. 1(a) and (b) shows the photograph of free-standing CF sheet (size: 8 × 8 cm2; thickness: 0.12 mm) prepared by blend ratio of phenolic resin to PVB (the solute/MeOH (40/60 wt/wt)) at 99/3 and the SEM image of carbon fibers, respectively. The flexible CF sheet was composed of carbon fibers with a diameter of about 3 μm. The density and the BET specific surface area of the CF sheet were estimated to be around 0.15 g/cm3 and 495 m2/g, respectively. The surface resistivity was varied between 10− 1 and 10− 2 Ω cm, depending on graphitization (carbonization) condition or BET specific surface area. The CF sheet also possesses the high thermal conductivity. We have successfully prepared the flexible and highly conductive CF sheet by ESD method. Fig. 2(a) and (b) shows the typical SEM and TEM images of the CF sheet, respectively. A large number of vertically aligned CNTs were grown on one side of the CF sheet. We estimated the diameters of the CNTs ranging between 50 nm and 100 nm, and that most of the CNTs
Y. Hayashi et al. / Diamond & Related Materials 18 (2009) 341–344
show the tube diameter around 80 nm. All CNT tips were capped with a Pd nanoparticle, where the catalyst particles promote tip-growth mechanism of aligned MWCNTs on CF sheet, as shown in Fig. 2(b). It should be noted that CNTs were well anchored to carbon fibers in CF sheet. And no significant damage in CF sheet and the no notable reduction in the mechanical flexibility of CF sheet after MPECVD process were observed by SEM and bending test, as shown in Fig. 2(a) and (c). Conventional CNT field emitter arrays have only surface layer (i.e. CNTs on Si substrate), but CF sheet allows CNTs to grow on several layers in the CF sheet, leading to an increase in the number of emission sites. The field emission characteristics of MWCNTs on a CF sheet are affected by the catalyst particle at the tip, the conducting CF sheet, and density of MWCNT and MWCNTs grown on under layers (2nd, 3rd layers) in the CF sheet. The field emission measurements were done in a vacuum chamber. The distance between the anode (ITO glass) and the substrate (1 × 1 cm2) were kept at 1.0 mm. Fig. 3(a) shows the typical current density versus the electric field properties for MWCNT field emitter arrays fabricated on CF sheet (1) and Si substrate (2). We also performed the field emission test for only CF sheet to check the effect of CF sheet on field emission property. Field emission did not occur well from CF sheet, which is the control sample. It means that the emission current from field emitter arrays is primarily due to the
343
MWCNTs. The turn-on electric fields of field emitter arrays on CF sheet and Si substrate at the emission current density of 10 μA/cm2 were 500 V/mm and 800 V/mm, respectively. It is clearly shown that MWCNT field emitter arrays on CF sheet shows better field emission property in terms of the turn-on electric field, which is defined as the field required to generate an emission current of 10 μA/cm2 [15]. The low turn-on electric field and the high current density capability for MWCNT field emitter arrays on CF sheet may come from high specific surface area of the CF sheet. Based on Fig. 2(a) the electrons are possibly able to emit from the MWCNT field emitter arrays grown on carbon fibers not only at surface but also under layers in the CF sheet. A CNT-based field emitter should reach 80 μA/cm2 to be used as a cold emission source [16]. The present sample generated this current density at 600 V/mm. Fig. 3(b) shows a Fowler–Nordheim (FN) plot of the FN line to analyze the field emission current [17]. The I–V data of field emitter arrays on CF sheet can be fitted well with the FN model, and it reveals that the emission current of Fig. 2(a) is a typical cold electron emission. The slope of the linear regression is given by Bφ2/3d/β, where B = 6.87 × 109 (V eV− 3/2 m− 1), φ is the work function, and d is the distance between the anode and the cathode [15]. The work function of the MWCNTs in our sample was taken to be that of graphite, 5 eV [15]. The field enhancement factors, β, for field emitter arrays grown on the CF sheet and the Si substrate calculated from the
Fig. 3. (a) Field emission current density as a function of the applied field. (b) The Fowler–Nordheim plot for the field emitter arrays grown on the CF sheet and the Si substrate.
344
Y. Hayashi et al. / Diamond & Related Materials 18 (2009) 341–344
The field emission pattern (1 × 1 cm2) of samples was obtained at the applied electric field of 2000 V/mm between the anode ITO glass with fluorescent material and the substrate. In comparison with the MWCNT field emitter arrays on Si substrate, we found that the uniform emission with high brightness was obtained for MWCNT field emitter arrays on CF sheet, as shown in Fig. 4. This might be also due to the field emission from under layers of CF sheet. 4. Summary In summary, we have successfully fabricated a novel MWCNT field emitter arrays directly on highly porous, flexible and conductive CF sheet by both using ESD and MPCVD techniques. Well aligned MWCNTs well anchored to the flexible CF sheet was confirmed by TEM. The MWCNT based field emitter arrays fabricated on CF sheet showed good field emission properties compared to those of conventional field emitter arrays on Si substrate. This may be due to the electron emission occurred from the MWCNT field emitter arrays grown on carbon fibers not only at surface but also under layers of CF sheet. Our results show promising potential as flexible field emission devices. Advantages of this technique include oblivious of CNT growth temperature, fast fabrication of field emitter arrays, and most notably, the ability to synthesize CNTs on flexible and highly conductive substrates. It is an important advance toward flexible field emitters. Acknowledgments YH would like to thank Prof. T. Soga, T. Jimbo and Dr. Rakesh A. Afre at NIT for useful discussion. References Fig. 4. The emission patterns of field emitter arrays grown on the CF sheet (a) and Si substrate (b). Area of sample area was 1 × 1 cm2; the gap between the anode ITO glass with fluorescent material and the substrate was 1 mm. The applied electric filed was 2000 V/mm.
slope of the FN plots, were 13,859 and 8686, respectively. The field enhancement factor for field emitter arrays on CF sheet was 1.6 times higher than that of field emitter arrays on Si substrate. The typical field enhancement factor obtained for the flat emitter of CF sheet was around 2500. Although the field emission occurred from the CF sheet substrate, the field emission properties were improved in terms of turn-on electric field and current density after growth of MWCNTs on CF sheet, as shown in Fig. 3(a). This result clearly indicates that the efficient field emission of MWCNTs grown on CF sheet is observed. The field enhancement factor is sensitive to radius and height of CNT, inter-tube distance (density of CNT), and tip metal. Therefore, further improvement of the field enhancement factor can be achieved by means of controlling the morphology such as diameter, density, and alignment. Further research is needed to carry out to see the effect of MWCNTs grown on under layers in CF sheet on screening of the electric field, which affects the field enhancement factor, and dipole moment in the particle in the tip top.
[1] A.A. Green, M.C. Hersam, Nano Lett. ASAP Article. [2] W.A. de Heer, A. Châtelain, D. Ugarte, Science 270 (1995) 1179. [3] J.M. Bonard, J.P. Salvetat, T. Stöckli, W.A. de Heer, L. Forró, A. Châtelain, Appl. Phys. Lett. 73 (1998) 918. [4] O.J. Lee, K.H. Lee, Appl. Phys. Lett 82 (2003) 3770. [5] Y.J. Jung, S. Kar, S. Talapatra, C. Soldano, G. Viswanathan, X.L.Z. Yao, F.S. Ou, A. Avadhanula, R. Vajtai, S. Curran, O. Nalamasu, P.M. Ajayan, Nano Lett. 6 (2006) 413. [6] S.H. Hur, O.O. Park, J.A. Rogers, Appl. Phys. Lett 86 (2005) 243502. [7] P.R. Bandaru, C. Daraio, S. Jin, A.M. Rao, Nat. Mater 4 (2005) 663. [8] K. Rege, N.R. Raravikar, D.Y. Kim, L.S. Schadler, P.M. Ajayan, J.S. Dordick, Nano Lett 3 (2003) 829. [9] G.M.H. Meesters, P.H.W. Vercoulen, J.C.M. Marijnissen, B. Scarlett, J. Aerosol Sci 23 (1992) 37. [10] J. Doshi, D.H. Reneker, J. Electrost 35 (1995) 151. [11] A.L. Yarin, S. Koombhongse, D.H. Reneker, J. Appl. Phys 90 (2001) 4836. [12] K. Suzuki, H. Matsumoto, M. Minagawa, M. Kimura, A. Tanioka, Polym. J 39 (2007) 1128. [13] Y. Hayashi, T. Tokunaga, T. Jimbo, Y. Yogata, S. Toh, K. Kaneko, Appl. Phys. Lett 84 (2004) 2886. [14] Y. Hayashi, T. Tokunaga, K. Kaneko, S.J. Henley, V. Stolojan, J.D. Carey, S.R.P. Silva, IEEE Transactions on Nanotechnology 5 (2006) 485–490. [15] J.–M. Bonard, J.P. Salvetat, T. Stockli, L. Forro, A. Chatelain, Appl. Phys. A 69 (1999) 245. [16] L. Nilsson, O. Groening, C. Emmenegger, O. Kuettel, E. Schaller, L. Schlapbach, H. Kind, J.M. Bonard, K. Kern, Appl. Phys. Lett 76 (2000) 2071. [17] J.B. Cui, K. Teo, W.I. Milne, J. Robertson, Appl. Phys. Letts. 77 (2000) 1831.
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Synthesis and characterization of carbon nanotube grown on flexible and conducting carbon fiber sheet for field emitter Yasuhiko Hayashi a,⁎, K. Suzuki b, B. Jang a, T. Tokunaga c, H. Matsumoto b, M. Tanemura a, A. Tanioka b, G.A.J. Amaratunga d a
Department of Frontier Materials, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Okayama, Meguro, Tokyo 152-8552, Japan Department of Quantum Engineering, Nagoya University, Furo, Chikusa, Nagoya 464-8601, Japan d Department of Engineering, University of Cambridge, 9 JJ Thomson Avenue, Cambridge CB3 0DE, United Kingdom b c
a r t i c l e
i n f o
Available online 11 September 2008 Keywords: Multiwall carbon nanotubes (MWCNTs) Conductive flexible carbon fiber (CF) Field emission cathode Fowler–Nordheim (FN) plot
a b s t r a c t We have successfully demonstrated the multiwall carbon nanotube (MWCNT) based field emission cathode fabricated on highly porous, flexible and conductive carbon fiber (CF) sheet without damage of CF sheet by chemical vapor deposition. CF sheet was composed of thin fibers with diameter of about 3 μm prepared by electrospray deposition. Well aligned MWCNTs grown on, or well anchored to, the flexible CF sheet was confirmed by transmission electron microscope. The field enhancement factor for MWCNT emitter fabricated on CF sheet was about 14,000 and 1.6 times higher than that of MWCNT emitter fabricated on Si substrate. Scanning electron microscopy image indicates that the electron emission occurred from the MWCNT field emitter arrays grown on carbon fibers not only at surface but also under layers in CF sheet. Field emission measurements revealed that this flexible MWCNT field emitter array has a great potential for the flexible field emission displays. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The diverse and exemplary properties of carbon nanotubes (CNTs) have inspired a vast range of proposed applications including transistors, interconnects, conductive films, field emission sources, biosensors, scanning probes, nanomechanical devices and hydrogen storage elements [1]. One particularly promising application of CNTs is the electron emitters for field emission displays. Due to its extremely low turn-on fields and high current densities, CNTs have already been proved as one of the most promising electron field emitter [2–4]. One important prerequisite to use CNTs as electron emitters in the flat panel display is to be able to apply them in patterns onto the substrates. This can be realized either by producing the CNTs and subsequent patterning on the substrate or by growing the CNTs directly on a substrate pre-patterned with a catalyst. Jung et al. [5] have reported the assembly of the field emission part of a screen using CNT electrodes embedded in a polymer matrix. Their simple idea could be a big step towards the implementation of large-area displays that are not only flat, but flexible too. Although, lots of researches have been done about the growth of CNTs on patterned substrate, one common feature is that the substrate used for the growth of CNTs is stiff silicon or glass. Flexible electronics such as a flexible display is of great concern as the emerging technique ⁎ Corresponding author. Tel./fax: +81 52 735 5024. E-mail address: [email protected] (Y. Hayashi). 0925-9635/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.09.014
and attracts great research interests. Flexible display is the next generation display, and it requires every component be flexible. Usually, aligned CNTs are grown either by using thin catalyst layers predeposited on substrates or through vapor phase catalyst delivery. However, all of these synthesis methods are based on chemical vapor deposition (CVD), where CNT growth requires temperatures above 600 °C. This high temperature environment places a major restriction on the possible substrates on which CNTs can be grown, thus restricting many interesting applications (i.e. CNTs on flexible plastic substrates). Aligned CNT arrays fabricated on a flexible medium open up new applications in flexible nanoelectronics including switches, displays [5–7] and biocatalytic systems [8]. In this work, we report the fabrication of flexible field emitter arrays of MWCNTs on highly porous and conductive carbon fiber (CF) sheet as a flexible substrate and their field emission properties. It is important to point out that CF sheet has highly conducting nature and this leads to no metallization process for field emission cathode. Moreover, the high temperature synthesis process is applicable for the growth of MWCNTs on CF sheet. Therefore, our approach may offer both substantial cost advantages as well as enable new electronics applications on flexible substrates. 2. Experimental In the present study, CF sheets were prepared by electrospray deposition (EDS), is a straightforward and versatile method for
342
Y. Hayashi et al. / Diamond & Related Materials 18 (2009) 341–344
fabricating thin films by electrostatic force [9–11]. Phenolic resin (novolac type, Mw= 4000–5000) was dissolved in MeOH, and Poly(vinyl butyral) (PVB, Mw= 110,000) was added to the solution [12]. And then it was used for ESD source. The ESD device consists of a syringe-type infusion pump, a high voltage regulated DC power supply and a grounded collector (aluminum sheet, 10 × 10 cm2 area). The as-deposited CF sheets were cured by adding formaldehyde and then carbonized at 900 °C in a ceramic furnace for 2 h under a continuous nitrogen purge to obtain the carbonization yield at around 55%. The MWCNTs were grown by microwave PECVD (MPECVD) using a 2.45-GHz, 1.5-kW microwave power supply, as described elsewhere [13]. The substrate temperature was controlled by a radio-frequency graphite heater. After cleaning the CF sheet, a 1-nm-thick Pd layer was deposited onto the CF sheet (size: 2 × 2 cm2; thickness: 0.12 mm). Although we failed to grow CNTs on CF sheet by using Fe and Co catalyst layers, we have found the growth of CNTs by using Pd layer instead of widely used catalyst layers like Fe, Co or Ni. Therefore, we chose Pd layer in this work. It is seen from our previous work, that the Pd breaks into a small size (about 60–100 nm diameter) uniformly due to surface tension as well as the stress due to mismatch of the thermal expansion coefficients during the initial growth stage [14]. The CF sheet was then transferred into the growth chamber. The chamber was pumped down to a base pressure of 10− 2 Torr. The substrates were heated to 750 °C and held at this temperature for 10 min to sinter the catalyst layers. Hydrogen (H2) gas was then fed into the chamber to maintain a pressure of 20 Torr at the preset temperature and microwave plasma was turned on to 600 W. The feed gas methane (CH4) was introduced, and the H2 gas flow rate was adjusted to achieve a CH4:H2 ratio of 1:1 at a total pressure of 20 Torr. Then, the MWCNT networks were grown for 10 min. The surface morphologies of the CF sheet after carbonization and the MWCNTs grown on CF sheet were observed by scanning electron
Fig. 2. (a) Typical SEM image, (b) cross sectional TEM image of CF sheet and (c) the flexible CF sheet (size: 2 × 2 cm2; thickness: 0.12 mm) mounted onto a cylindrical object (radius = 25 mm) after growth of MWCNTs.
microscopy (SEM) and conventional transmission electron microscopy (TEM). Adsorption isotherms and the Brunauer–Emmet–Teller (BET) specific surface area of the CFs were determined using an adsorption apparatus. The electrical conductivities of the CF sheets were measured by a transmission line model. 3. Results and discussion
Fig. 1. (a) The flexible CF sheet (size: 2 × 2 cm2; thickness: 0.12 mm) mounted onto a cylindrical object (radius = 25 mm) before growth of MWCNTs. The flexible CF sheet was prepared by blend ratio of phenolic resin to PVB (the solute/MeOH (40/60 wt/wt)) at 99/3. (b) The SEM image of carbon fibers.
Fig. 1(a) and (b) shows the photograph of free-standing CF sheet (size: 8 × 8 cm2; thickness: 0.12 mm) prepared by blend ratio of phenolic resin to PVB (the solute/MeOH (40/60 wt/wt)) at 99/3 and the SEM image of carbon fibers, respectively. The flexible CF sheet was composed of carbon fibers with a diameter of about 3 μm. The density and the BET specific surface area of the CF sheet were estimated to be around 0.15 g/cm3 and 495 m2/g, respectively. The surface resistivity was varied between 10− 1 and 10− 2 Ω cm, depending on graphitization (carbonization) condition or BET specific surface area. The CF sheet also possesses the high thermal conductivity. We have successfully prepared the flexible and highly conductive CF sheet by ESD method. Fig. 2(a) and (b) shows the typical SEM and TEM images of the CF sheet, respectively. A large number of vertically aligned CNTs were grown on one side of the CF sheet. We estimated the diameters of the CNTs ranging between 50 nm and 100 nm, and that most of the CNTs
Y. Hayashi et al. / Diamond & Related Materials 18 (2009) 341–344
show the tube diameter around 80 nm. All CNT tips were capped with a Pd nanoparticle, where the catalyst particles promote tip-growth mechanism of aligned MWCNTs on CF sheet, as shown in Fig. 2(b). It should be noted that CNTs were well anchored to carbon fibers in CF sheet. And no significant damage in CF sheet and the no notable reduction in the mechanical flexibility of CF sheet after MPECVD process were observed by SEM and bending test, as shown in Fig. 2(a) and (c). Conventional CNT field emitter arrays have only surface layer (i.e. CNTs on Si substrate), but CF sheet allows CNTs to grow on several layers in the CF sheet, leading to an increase in the number of emission sites. The field emission characteristics of MWCNTs on a CF sheet are affected by the catalyst particle at the tip, the conducting CF sheet, and density of MWCNT and MWCNTs grown on under layers (2nd, 3rd layers) in the CF sheet. The field emission measurements were done in a vacuum chamber. The distance between the anode (ITO glass) and the substrate (1 × 1 cm2) were kept at 1.0 mm. Fig. 3(a) shows the typical current density versus the electric field properties for MWCNT field emitter arrays fabricated on CF sheet (1) and Si substrate (2). We also performed the field emission test for only CF sheet to check the effect of CF sheet on field emission property. Field emission did not occur well from CF sheet, which is the control sample. It means that the emission current from field emitter arrays is primarily due to the
343
MWCNTs. The turn-on electric fields of field emitter arrays on CF sheet and Si substrate at the emission current density of 10 μA/cm2 were 500 V/mm and 800 V/mm, respectively. It is clearly shown that MWCNT field emitter arrays on CF sheet shows better field emission property in terms of the turn-on electric field, which is defined as the field required to generate an emission current of 10 μA/cm2 [15]. The low turn-on electric field and the high current density capability for MWCNT field emitter arrays on CF sheet may come from high specific surface area of the CF sheet. Based on Fig. 2(a) the electrons are possibly able to emit from the MWCNT field emitter arrays grown on carbon fibers not only at surface but also under layers in the CF sheet. A CNT-based field emitter should reach 80 μA/cm2 to be used as a cold emission source [16]. The present sample generated this current density at 600 V/mm. Fig. 3(b) shows a Fowler–Nordheim (FN) plot of the FN line to analyze the field emission current [17]. The I–V data of field emitter arrays on CF sheet can be fitted well with the FN model, and it reveals that the emission current of Fig. 2(a) is a typical cold electron emission. The slope of the linear regression is given by Bφ2/3d/β, where B = 6.87 × 109 (V eV− 3/2 m− 1), φ is the work function, and d is the distance between the anode and the cathode [15]. The work function of the MWCNTs in our sample was taken to be that of graphite, 5 eV [15]. The field enhancement factors, β, for field emitter arrays grown on the CF sheet and the Si substrate calculated from the
Fig. 3. (a) Field emission current density as a function of the applied field. (b) The Fowler–Nordheim plot for the field emitter arrays grown on the CF sheet and the Si substrate.
344
Y. Hayashi et al. / Diamond & Related Materials 18 (2009) 341–344
The field emission pattern (1 × 1 cm2) of samples was obtained at the applied electric field of 2000 V/mm between the anode ITO glass with fluorescent material and the substrate. In comparison with the MWCNT field emitter arrays on Si substrate, we found that the uniform emission with high brightness was obtained for MWCNT field emitter arrays on CF sheet, as shown in Fig. 4. This might be also due to the field emission from under layers of CF sheet. 4. Summary In summary, we have successfully fabricated a novel MWCNT field emitter arrays directly on highly porous, flexible and conductive CF sheet by both using ESD and MPCVD techniques. Well aligned MWCNTs well anchored to the flexible CF sheet was confirmed by TEM. The MWCNT based field emitter arrays fabricated on CF sheet showed good field emission properties compared to those of conventional field emitter arrays on Si substrate. This may be due to the electron emission occurred from the MWCNT field emitter arrays grown on carbon fibers not only at surface but also under layers of CF sheet. Our results show promising potential as flexible field emission devices. Advantages of this technique include oblivious of CNT growth temperature, fast fabrication of field emitter arrays, and most notably, the ability to synthesize CNTs on flexible and highly conductive substrates. It is an important advance toward flexible field emitters. Acknowledgments YH would like to thank Prof. T. Soga, T. Jimbo and Dr. Rakesh A. Afre at NIT for useful discussion. References Fig. 4. The emission patterns of field emitter arrays grown on the CF sheet (a) and Si substrate (b). Area of sample area was 1 × 1 cm2; the gap between the anode ITO glass with fluorescent material and the substrate was 1 mm. The applied electric filed was 2000 V/mm.
slope of the FN plots, were 13,859 and 8686, respectively. The field enhancement factor for field emitter arrays on CF sheet was 1.6 times higher than that of field emitter arrays on Si substrate. The typical field enhancement factor obtained for the flat emitter of CF sheet was around 2500. Although the field emission occurred from the CF sheet substrate, the field emission properties were improved in terms of turn-on electric field and current density after growth of MWCNTs on CF sheet, as shown in Fig. 3(a). This result clearly indicates that the efficient field emission of MWCNTs grown on CF sheet is observed. The field enhancement factor is sensitive to radius and height of CNT, inter-tube distance (density of CNT), and tip metal. Therefore, further improvement of the field enhancement factor can be achieved by means of controlling the morphology such as diameter, density, and alignment. Further research is needed to carry out to see the effect of MWCNTs grown on under layers in CF sheet on screening of the electric field, which affects the field enhancement factor, and dipole moment in the particle in the tip top.
[1] A.A. Green, M.C. Hersam, Nano Lett. ASAP Article. [2] W.A. de Heer, A. Châtelain, D. Ugarte, Science 270 (1995) 1179. [3] J.M. Bonard, J.P. Salvetat, T. Stöckli, W.A. de Heer, L. Forró, A. Châtelain, Appl. Phys. Lett. 73 (1998) 918. [4] O.J. Lee, K.H. Lee, Appl. Phys. Lett 82 (2003) 3770. [5] Y.J. Jung, S. Kar, S. Talapatra, C. Soldano, G. Viswanathan, X.L.Z. Yao, F.S. Ou, A. Avadhanula, R. Vajtai, S. Curran, O. Nalamasu, P.M. Ajayan, Nano Lett. 6 (2006) 413. [6] S.H. Hur, O.O. Park, J.A. Rogers, Appl. Phys. Lett 86 (2005) 243502. [7] P.R. Bandaru, C. Daraio, S. Jin, A.M. Rao, Nat. Mater 4 (2005) 663. [8] K. Rege, N.R. Raravikar, D.Y. Kim, L.S. Schadler, P.M. Ajayan, J.S. Dordick, Nano Lett 3 (2003) 829. [9] G.M.H. Meesters, P.H.W. Vercoulen, J.C.M. Marijnissen, B. Scarlett, J. Aerosol Sci 23 (1992) 37. [10] J. Doshi, D.H. Reneker, J. Electrost 35 (1995) 151. [11] A.L. Yarin, S. Koombhongse, D.H. Reneker, J. Appl. Phys 90 (2001) 4836. [12] K. Suzuki, H. Matsumoto, M. Minagawa, M. Kimura, A. Tanioka, Polym. J 39 (2007) 1128. [13] Y. Hayashi, T. Tokunaga, T. Jimbo, Y. Yogata, S. Toh, K. Kaneko, Appl. Phys. Lett 84 (2004) 2886. [14] Y. Hayashi, T. Tokunaga, K. Kaneko, S.J. Henley, V. Stolojan, J.D. Carey, S.R.P. Silva, IEEE Transactions on Nanotechnology 5 (2006) 485–490. [15] J.–M. Bonard, J.P. Salvetat, T. Stockli, L. Forro, A. Chatelain, Appl. Phys. A 69 (1999) 245. [16] L. Nilsson, O. Groening, C. Emmenegger, O. Kuettel, E. Schaller, L. Schlapbach, H. Kind, J.M. Bonard, K. Kern, Appl. Phys. Lett 76 (2000) 2071. [17] J.B. Cui, K. Teo, W.I. Milne, J. Robertson, Appl. Phys. Letts. 77 (2000) 1831.