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Enhancement of field emission properties of graphite flakes by producing carbon nanotubes on above using thermal chemical vapor deposition
Applied Surface Science 256 (2010) 2409–2413
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Enhancement of field emission properties of graphite flakes by producing carbon nanotubes on above using thermal chemical vapor deposition Wen-Ching Shih a,*, Jian-Min Jeng a, Ming-Hong Tsai a, Jyi-Tsong Lo b a b
Graduate Institute in Electro-Optical Engineering, Tatung University, No. 40, Sec. 3, Chungshan North Road, Taipei 104, Taiwan, ROC FED R&D Division, Tatung Company, No. 22, Sec. 3, Chungshan North Road, Taipei 104, Taiwan, ROC
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 May 2009 Received in revised form 20 October 2009 Accepted 23 October 2009 Available online 31 October 2009
In order to improve the field emission properties of the graphite flakes, the carbon nanotubes (CNTs) are produced on above without the metallic catalyst using mixtures of C2H2 and H2 gases by thermal chemical vapor deposition. We spin the graphite solution on the silicon wafer and dry it, then synthesize the CNTs on the graphite flakes. We change the synthetic time to obtain the optimal conditions for enhancement of field emission properties of graphite flakes. The experimental results show that the density and quality of the CNTs could be controlled significantly by the synthetic time. Besides, the field emission properties of the treated graphite flakes are also affected greatly by it. The emission current density of the treated graphite flakes reaches to 0.5 mA/cm2 at 3 V/mm, and the turn-on field is decreased from 7.7 to 1.9 V/mm after producing the CNTs on above. ß 2009 Elsevier B.V. All rights reserved.
PACS: 79.70.+q 85.45.Db 85.45.Fd Keywords: Graphite flake Carbon nanotube Carbon nanoparticle Thermal chemical vapor deposition Field emission
1. Introduction Recent interest in field emission devices has led to the use of carbon cathodes for applications where large areas of emission are required. Field emission is dependent on the shape and work function of the emitting material or the vacuum environment that affects the effective potential barrier height of the emitting material. Hence, fabrication of the field emission tips becomes a key issue in the vacuum microelectronics. Generally, there are two methods of increasing the current density and decreasing the voltage applied for the field emission. One is to fabricate a sharp edge or tip emission source. Another is to employ a low effective potential barrier height material as the emitter. In addition, stability and uniformity are also important issues for practical applications. Excellent electron field emission characteristics with low turn-on electric field and high-current density have been observed from various materials, for example, carbon nanotube (CNT) [1–3], diamond [4], diamond-like carbon [5,6] and other carbon structures [7,8].
* Corresponding author. Tel.: +886 2 25925252; fax: +886 2 25956393. E-mail address: [email protected] (W.-C. Shih). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.10.076
Since the CNTs were discovered by Iijima [9], many researchers have studied the synthesis of high-quality CNTs using various methods such as arc discharge [9,10], thermal [11,12] or plasmaenhanced chemical vapor deposition (PECVD) [13,14]. The CNTs are well known until now. A variety of synthetic strategies for CNTs have been extensively developed. Especially, CNTs can be facile and commercially produced in large-scale. Also, the properties including field emission of CNTs have also been widely investigated. The growth of CNTs usually requires a catalyst (typically Fe, Co, or Ni). Although these catalysts promote the growth of CNTs, they also require complex post-synthesis purifying treatments (oxidation, acid treatment, filtration, centrifugation, etc.) to remove them, which may damage the final product. Catalyst-free growth of CNTs can be achieved by laser heating in vacuum of amorphous SiC films [15]. A metal-catalyst-free growth method of CNTs has been developed using CVD of CNTs on carbon-implanted SiGe islands on Si substrates [16]. However, high temperatures above 1000 8C or special substrate pretreatment prior to CNT growth were needed. Besides, field emission properties of the prepared catalyst-free CNTs were not reported. In our previous study, the graphite flake was chosen to replace the CNT which cannot abide by the high-electron current pass through itself [17] for its higher electronic and thermal stability
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[18]. The field emission properties of the graphite flakes have glaring improvement after hydrogen thermal processing owing to the increase of the defect density and the carbon nanoparticles (CNPs) on above. The turn-on field was decreased from 7.7 of the untreated sample to 4.3 V/mm of the treated sample at the optimal processing conditions. However, the turn-on field of the treated sample is still too higher than that of the CNTs. In this paper, in order to further solve the drawback of high turn-on field of the graphite flakes, we attempt to improve the field emission properties of the graphite flakes by producing the CNTs on above without the metallic catalyst using mixtures of C2H2 and H2 gases by thermal CVD. As compared to the reported catalyst-free growth of CNTs [15,16], the synthesis temperature of the proposed method is much lower. Besides, the synthesis process is quite simpler without special substrate pretreatment prior to CNT growth. 2. Experimental procedures After cleaning the n-type silicon (100) substrate, the graphite paste of mixed graphite powders with the binder BP55 and DT-710 is coated on the silicon substrate. The size of graphite is microscale flakes [19]. The binder is removed from the sample by heating in the air at 360 8C for 30 min. Then the sample is put in the thermal chemical vapor deposition (thermal CVD). After the graphite flakes are pretreated by annealing in hydrogen atmosphere at 700 8C for 10 min, the CNTs started to synthesize on the graphite flakes by introducing the reaction gases consisting of acetylene and hydrogen at synthetic temperature of 700 8C for different synthetic time, 1–30 min. The reactor pressure was fixed at 4 mbar throughout the experiments. The Raman spectroscopy (RENISHAW in Via, 514 nm Ar laser excitation) was used to evaluate the variations of structure of the graphite flakes [20,21]. The field emission scanning electron microscope (FE-SEM) (JEOL JSM-6500F), energy-dispersive X-ray spectroscopy (EDS), and high-resolution transmission electron microscope (HR-TEM) (JEOL JEM-1200 EX III) were used to characterize the structures and morphologies of the CNTs grown on the graphite flakes. The field emission properties of the untreated and treated graphite flakes were measured under a pressure of 4 10 6 Torr. The emission current versus the applied voltage was measured using a diode-type structure, while the field emission current was measured using a Keithley 2410 source measurement unit. The ITO glass was used as an anode for an applied voltage and the area of cathode was 1 cm 1 cm. The distance between the anode and the cathode was 150 mm. The turn-on field obtained from the I–V curve is defined as the applied field at which the current density of 0.1 mA/cm2.
Fig. 1. SEM images of carbon nanoparticles on the graphite flakes prepared by hydrogen thermal treatment.
After treating the graphite flakes, the variations of the Raman spectroscopy are shown in Fig. 2. The G-band around 1580 cm 1 indicates the presence of crystalline graphitic carbon with a sp2 bonding structure in the graphite flakes. The D-band around 1355 cm 1 is caused by defects, which is considered as a disordered structure relative to G-band in the graphite crystals [20]. The full width at half maximum intensity (FWHM) and the intensity of the D-band peak of the treated graphite flakes are larger than those of the original graphite flakes. The intensity peak ratio (ID/IG) is increased from 0.23 to 0.27 after treating the graphite flakes. Besides producing the CNPs, it is quite obvious that the heat-treatment in hydrogen atmosphere could increase the defects and minimize the crystal size caused by producing the CNPs on the graphite flakes. The FWHM and the intensity of the Dband peak on the treated graphite flakes possessing more CNPs might be larger than those of the samples possessing less CNPs.
3. Results and discussion The very small CNPs prepared by heat-treatment on the graphite flakes were obtained as shown in Fig. 1. The diameter of the CNP is about 10 nm. We suppose that there are two possible mechanisms. First, the CNPs are produced through hydrogen etching process on the graphite flakes. This is a well-known mechanism of effective carbon etching by hydrogen. The second possible mechanism is the additional carbon source. Hydrogen damaged the graphite flake and took the carbon atoms away during the thermal treatment. The separated carbon atoms changed into the forms of –C and –CHx. Due to the lack of carbon in the ambience, a large fraction of –C and –CHx were taken out of the chamber with the hydrogen. Meanwhile, a small fraction of –C and –CHx were removed on the survived tube surface, which consequently caused many nanoscale particles deposited on the remained graphite flakes [22]. Then the additional carbon was deposited to be the CNPs on the graphite flakes.
Fig. 2. Raman spectroscopy of the original and pretreated graphite flakes.
W.-C. Shih et al. / Applied Surface Science 256 (2010) 2409–2413
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Fig. 3. SEM photographs of the CNTs synthesized on the graphite flakes for (a) 1 min, (b) 2 min, (c) 5 min, (d) 10 min, (e) 20 min and (f) 30 min.
These high-defect CNPs and defects would be the seed in the synthetic process of the CNTs. After producing the CNPs on the graphite flakes, the acetylene is fed into the chamber to behalf as the carbon source in synthesizing the CNTs. Fig. 3 shows the SEM photographs of the CNTs synthesized on the graphite flakes for different synthetic time. The other synthetic conditions of the CNTs are fixed, such as deposition temperature of 7008C, reactive gas flow ratio (C2H2:H2) of 1:2, and working pressure of 4 mbar. The length of CNTs synthesized for 1 min is short and sparse. When starting the synthesis of the CNTs, they are formed from only a minority of synthetic sites. The density and length of CNTs could be increased when we increase the synthetic time. The composition of the samples is identified by EDS analysis as shown in Fig. 4. Apart from the carbon and silicon peaks, no
metal peak is detected. It means that there is no metallic catalyst partaking during the synthetic process of the CNTs. The I–V curves are shown in Fig. 5. The field emission current is applied from the emission sites of the CNTs. The results show that the field emission properties of the CNTs synthesized for 1 and 2 min are ropy because these two samples have less density of the CNTs as shown in Fig. 3. With increasing the synthetic time, it is obvious that the field emission properties of the treated graphite flakes are improved because of increasing the emission sites of the CNTs. The optimal synthetic time is 10 min. The turn-on field could be optimized by controlling the synthetic time, and reduced from 7.7 for untreated specimen to 1.9 V/mm for the optimal case. The emission current density of the treated graphite flakes reaches to 0.5 mA/cm2 at 3 V/mm. However, when we increase the synthetic
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W.-C. Shih et al. / Applied Surface Science 256 (2010) 2409–2413
Fig. 4. Composition of the CNTs synthesized on the graphite flakes identified by EDS analysis.
Fig. 6. TEM image of the two CNTs with different diameters and lengths.
Fig. 5. I–V curves of the CNTs synthesized on the graphite flakes for different synthetic time. Fig. 7. Schematic growth mechanism of the CNTs on the graphite flakes.
time from 10 to 30 min, the emission properties are deteriorated due to the screen effect. Gohel et al. have demonstrated the enhanced field emission characteristics of CNTs grown on Fe coated Si substrates by hot filament PECVD using C2H2 and H2 as
inlet gases by nitrogen plasma treatment of 20 min [23]. The lowest turn-on voltage (1.73 V/mm) and highest emission current density (0.45 mA/cm2) at 3 V/mm were obtained for the optimal case. Yu et al. presented change of surface morphology and field
Fig. 8. HRTEM images of (a) CNTs on the graphite flakes and (b) the cross section of the CNTs.
W.-C. Shih et al. / Applied Surface Science 256 (2010) 2409–2413
emission property of CNTs by hydrogen plasma post-treatment. The turn-on field (at 0.1 mA/cm2) decreases from 3.9 to 1.5 V/mm after the sample is treated with hydrogen plasma for 30 min. For comparison, the turn-on voltage and emission current density at 3 V/mm of the proposed method without post-treatment is close to those of the reported catalytic growth of CNTs with post-treatment of nitrogen or hydrogen plasma treatment [23,24]. Fig. 6 shows the TEM images of the two CNTs which have different diameters and lengths. The tip of the shorter CNT is not an irregular half sphericity and has no CNPs inside, so we argued that the growth mechanism is the base growth model which was different from the conventional tip growth model in thermal CVD. Fig. 7 shows the proposed schematic growth mechanism of the CNTs on the graphite flake. After thermal treating the graphite flake in the hydrogen atmosphere, the more defect groups and CNPs are appeared on the surface of the graphite flake as explained in Fig. 7(b). Afterward acetylene acted as the atomic carbon source was fed into the furnace. The gas acetylene is pyrolyzed into hydrocarbon or carbon molecules and then approached to the surface. The hydrocarbon or carbon molecules source near the defects and CNPs would be the effective carbon source for synthesis of the CNTs as explained in Fig. 7(c). During synthesizing the CNTs, the distance between two defects in defect group plays a critical role. The distance between two defects must be near 0.34 nm in defect group for arrangement of multi-graphene wall [9]. If the difference between two defects is very large, the probability of synthesis and the inner diameters of the CNTs are very small. On the other hand, the length of CNTs would be longer if the distance between two defects is near 0.34 nm in defect group. The formation mechanism of CNTs on graphite flake could be explained that why these CNTs have different diameters and lengths. HRTEM is used to observe the interior and the wall structures of the CNTs. Fig. 8 shows a typical HRTEM image of the CNTs on the graphite flakes. The outside diameter of the CNT is about 30 nm. It clearly shows that the CNT is a multi-walled tube with individual cylindrical layers, so it is not a solid fiber. The arrangement of graphene layers is very well and amorphous carbon exists outside the tube. 4. Conclusion The experimental results show that the field emission properties of the graphite flakes have significant improvement after producing the CNTs on above. The turn-on field is decreased from 7.7 to 1.9 V/mm. The CNPs play an important role during the
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growth of the CNTs. We change the synthetic time to obtain the optimal conditions for enhancement of field emission property of graphite flakes. The optimal synthetic time and turn on field are 10 min and 1.9 V/mm, respectively. The outside diameter of the CNT observed by HRTEM is about 30 nm.
Acknowledgments This work was supported by the FED R&D Division at Tatung Company, Taiwan, ROC and Chunghwa Picture Tubes Company, Taiwan, ROC. The authors deeply appreciated the financial and technical support. References [1] K.B.K. Teo, S.B. Lee, M. Chhowalla1, V. Semet, V. Thien Binh, O. Groening, M. Castignolles, A. Loiseau, G. Pirio, P. Legagneux, D. Pribat, D.G. Hasko, H. Ahmed, G.A.J. Amaratunga, W.I. Milne, Nanotechnology 14 (2003) 204. [2] M. Hirakawa, S. Sonoda, C. Tanaka, H. Murakami, H. Yamakawa, Appl. Surf. Sci. 169 (2001) 662. [3] L. Xinghui, Z. Changchun, L. Weihua, Z. Fanguang, T. Changhui, Mater. Chem. Phys. 93 (2005) 473. [4] R.S. Takalkar, J.L. Davidson, W.P. Kang, A. Wisitsora-At, Diamond Relat. Mater. 14 (2005) 692. [5] J.T. Ryu, S.I. Honda, M. Katayama, K. Oura, Curr. Appl. Phys. 5 (2005) 387. [6] S.R.P. Silva, J.D. Carey, X. Guo, W.M. Tsang, C.H.P. Poa., Thin Solid Films 482 (2005) 79. [7] J.J. Wang, M.Y. Zhu, R.A. Outlaw, X. Zhao, D.M. Manos, B.C. Holloway, V.P. Mammana, Appl. Phys. Lett. 85 (2004) 1265. [8] W.Y. Yeh, J. Hwang, A.P. Lee, C.S. Kou, H. Chang, Appl. Phys. Lett. 79 (2001) 3609. [9] S. Iijima, Nature 354 (1991) 56. [10] D.S. Bethune, C.H. Kiang, M.S. De Vries, G. Gorman, R. Savoy, J. Vazquez, R. Beyers, Nature 363 (1993) 605. [11] H. Cui, G. Eres, J.Y. Howe, A. Puretkzy, M. Varela, D.B. Geohegan, D.H. Lowndes, Chem. Phys. Lett. 374 (2003) 222. [12] C.J. Lee, J. Park, Carbon 39 (2001) 1891. [13] Z.F. Ren, Z.P. Huang, J.W. Xu, J.H. Wang, P. Bush, M.P. Siegal, P.N. Provencio, Science 282 (1998) 1105. [14] M. Chhowalla, K.B.K. Teo, C. Ducati, N.L. Rupesinghe, G.A.J. Amaratunga, A.C. Ferrari, D. Roy, J. Robertson, W.I. Milne, J. Appl. Phys. 90 (2001) 5308. [15] S. Botti, L.S. Asilyan, R. Ciardi, F. Fabbri, S. Loreti, A. Santoni, S. Orlanducci, Chem. Phys. Lett. 396 (2004) 1. [16] T. Uchino, K.N. Bourdakos, C.H. de Groot, P. Ashburn, M.E. Kiziroglou, G.D. Dilliway, D.C. Smith, Appl. Phys. Lett. 86 (2005) 233110. [17] Y. Saito, K. Seko, J. Kinoshita, Diamond Relat. Mater. 14 (2005) 1843. [18] W.C. Shih, J.M. Jeng, M.H. Tsai, J.T. Lo, Appl. Surf. Sci. 255 (2009) 7679–7682. [19] C.E. Hunt, O.J. Glembocki, Y. Wang, S.M. Prokes, Appl. Phys. Lett. 86 (2005) 163112. [20] F. Tuinatra, J.L. Koenic, J. Chem. Phys. 53 (1970) 1126. [21] A.C. Ferrari, J. Robertson, Phil. Res. Rep. 362 (2004) 2477. [22] J. Zhang, T. Feng, W. Yu, X. Liu, X. Wang, Q. Li, Diamond Relat. Mater. 13 (2004) 54. [23] A. Gohel, K.C. Chin, Y.W. Zhu, C.H. Sow, A.T.S. Wee, Carbon 43 (2005) 2530. [24] K. Yu, Z. Zhu, Y. Zhang, Q. Li, W. Wang, L. Luo, X. Yu., H. Ma, Z. Li, T. Feng, Appl. Surf. Sci. 225 (2004) 380–388.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Enhancement of field emission properties of graphite flakes by producing carbon nanotubes on above using thermal chemical vapor deposition Wen-Ching Shih a,*, Jian-Min Jeng a, Ming-Hong Tsai a, Jyi-Tsong Lo b a b
Graduate Institute in Electro-Optical Engineering, Tatung University, No. 40, Sec. 3, Chungshan North Road, Taipei 104, Taiwan, ROC FED R&D Division, Tatung Company, No. 22, Sec. 3, Chungshan North Road, Taipei 104, Taiwan, ROC
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 May 2009 Received in revised form 20 October 2009 Accepted 23 October 2009 Available online 31 October 2009
In order to improve the field emission properties of the graphite flakes, the carbon nanotubes (CNTs) are produced on above without the metallic catalyst using mixtures of C2H2 and H2 gases by thermal chemical vapor deposition. We spin the graphite solution on the silicon wafer and dry it, then synthesize the CNTs on the graphite flakes. We change the synthetic time to obtain the optimal conditions for enhancement of field emission properties of graphite flakes. The experimental results show that the density and quality of the CNTs could be controlled significantly by the synthetic time. Besides, the field emission properties of the treated graphite flakes are also affected greatly by it. The emission current density of the treated graphite flakes reaches to 0.5 mA/cm2 at 3 V/mm, and the turn-on field is decreased from 7.7 to 1.9 V/mm after producing the CNTs on above. ß 2009 Elsevier B.V. All rights reserved.
PACS: 79.70.+q 85.45.Db 85.45.Fd Keywords: Graphite flake Carbon nanotube Carbon nanoparticle Thermal chemical vapor deposition Field emission
1. Introduction Recent interest in field emission devices has led to the use of carbon cathodes for applications where large areas of emission are required. Field emission is dependent on the shape and work function of the emitting material or the vacuum environment that affects the effective potential barrier height of the emitting material. Hence, fabrication of the field emission tips becomes a key issue in the vacuum microelectronics. Generally, there are two methods of increasing the current density and decreasing the voltage applied for the field emission. One is to fabricate a sharp edge or tip emission source. Another is to employ a low effective potential barrier height material as the emitter. In addition, stability and uniformity are also important issues for practical applications. Excellent electron field emission characteristics with low turn-on electric field and high-current density have been observed from various materials, for example, carbon nanotube (CNT) [1–3], diamond [4], diamond-like carbon [5,6] and other carbon structures [7,8].
* Corresponding author. Tel.: +886 2 25925252; fax: +886 2 25956393. E-mail address: [email protected] (W.-C. Shih). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.10.076
Since the CNTs were discovered by Iijima [9], many researchers have studied the synthesis of high-quality CNTs using various methods such as arc discharge [9,10], thermal [11,12] or plasmaenhanced chemical vapor deposition (PECVD) [13,14]. The CNTs are well known until now. A variety of synthetic strategies for CNTs have been extensively developed. Especially, CNTs can be facile and commercially produced in large-scale. Also, the properties including field emission of CNTs have also been widely investigated. The growth of CNTs usually requires a catalyst (typically Fe, Co, or Ni). Although these catalysts promote the growth of CNTs, they also require complex post-synthesis purifying treatments (oxidation, acid treatment, filtration, centrifugation, etc.) to remove them, which may damage the final product. Catalyst-free growth of CNTs can be achieved by laser heating in vacuum of amorphous SiC films [15]. A metal-catalyst-free growth method of CNTs has been developed using CVD of CNTs on carbon-implanted SiGe islands on Si substrates [16]. However, high temperatures above 1000 8C or special substrate pretreatment prior to CNT growth were needed. Besides, field emission properties of the prepared catalyst-free CNTs were not reported. In our previous study, the graphite flake was chosen to replace the CNT which cannot abide by the high-electron current pass through itself [17] for its higher electronic and thermal stability
2410
W.-C. Shih et al. / Applied Surface Science 256 (2010) 2409–2413
[18]. The field emission properties of the graphite flakes have glaring improvement after hydrogen thermal processing owing to the increase of the defect density and the carbon nanoparticles (CNPs) on above. The turn-on field was decreased from 7.7 of the untreated sample to 4.3 V/mm of the treated sample at the optimal processing conditions. However, the turn-on field of the treated sample is still too higher than that of the CNTs. In this paper, in order to further solve the drawback of high turn-on field of the graphite flakes, we attempt to improve the field emission properties of the graphite flakes by producing the CNTs on above without the metallic catalyst using mixtures of C2H2 and H2 gases by thermal CVD. As compared to the reported catalyst-free growth of CNTs [15,16], the synthesis temperature of the proposed method is much lower. Besides, the synthesis process is quite simpler without special substrate pretreatment prior to CNT growth. 2. Experimental procedures After cleaning the n-type silicon (100) substrate, the graphite paste of mixed graphite powders with the binder BP55 and DT-710 is coated on the silicon substrate. The size of graphite is microscale flakes [19]. The binder is removed from the sample by heating in the air at 360 8C for 30 min. Then the sample is put in the thermal chemical vapor deposition (thermal CVD). After the graphite flakes are pretreated by annealing in hydrogen atmosphere at 700 8C for 10 min, the CNTs started to synthesize on the graphite flakes by introducing the reaction gases consisting of acetylene and hydrogen at synthetic temperature of 700 8C for different synthetic time, 1–30 min. The reactor pressure was fixed at 4 mbar throughout the experiments. The Raman spectroscopy (RENISHAW in Via, 514 nm Ar laser excitation) was used to evaluate the variations of structure of the graphite flakes [20,21]. The field emission scanning electron microscope (FE-SEM) (JEOL JSM-6500F), energy-dispersive X-ray spectroscopy (EDS), and high-resolution transmission electron microscope (HR-TEM) (JEOL JEM-1200 EX III) were used to characterize the structures and morphologies of the CNTs grown on the graphite flakes. The field emission properties of the untreated and treated graphite flakes were measured under a pressure of 4 10 6 Torr. The emission current versus the applied voltage was measured using a diode-type structure, while the field emission current was measured using a Keithley 2410 source measurement unit. The ITO glass was used as an anode for an applied voltage and the area of cathode was 1 cm 1 cm. The distance between the anode and the cathode was 150 mm. The turn-on field obtained from the I–V curve is defined as the applied field at which the current density of 0.1 mA/cm2.
Fig. 1. SEM images of carbon nanoparticles on the graphite flakes prepared by hydrogen thermal treatment.
After treating the graphite flakes, the variations of the Raman spectroscopy are shown in Fig. 2. The G-band around 1580 cm 1 indicates the presence of crystalline graphitic carbon with a sp2 bonding structure in the graphite flakes. The D-band around 1355 cm 1 is caused by defects, which is considered as a disordered structure relative to G-band in the graphite crystals [20]. The full width at half maximum intensity (FWHM) and the intensity of the D-band peak of the treated graphite flakes are larger than those of the original graphite flakes. The intensity peak ratio (ID/IG) is increased from 0.23 to 0.27 after treating the graphite flakes. Besides producing the CNPs, it is quite obvious that the heat-treatment in hydrogen atmosphere could increase the defects and minimize the crystal size caused by producing the CNPs on the graphite flakes. The FWHM and the intensity of the Dband peak on the treated graphite flakes possessing more CNPs might be larger than those of the samples possessing less CNPs.
3. Results and discussion The very small CNPs prepared by heat-treatment on the graphite flakes were obtained as shown in Fig. 1. The diameter of the CNP is about 10 nm. We suppose that there are two possible mechanisms. First, the CNPs are produced through hydrogen etching process on the graphite flakes. This is a well-known mechanism of effective carbon etching by hydrogen. The second possible mechanism is the additional carbon source. Hydrogen damaged the graphite flake and took the carbon atoms away during the thermal treatment. The separated carbon atoms changed into the forms of –C and –CHx. Due to the lack of carbon in the ambience, a large fraction of –C and –CHx were taken out of the chamber with the hydrogen. Meanwhile, a small fraction of –C and –CHx were removed on the survived tube surface, which consequently caused many nanoscale particles deposited on the remained graphite flakes [22]. Then the additional carbon was deposited to be the CNPs on the graphite flakes.
Fig. 2. Raman spectroscopy of the original and pretreated graphite flakes.
W.-C. Shih et al. / Applied Surface Science 256 (2010) 2409–2413
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Fig. 3. SEM photographs of the CNTs synthesized on the graphite flakes for (a) 1 min, (b) 2 min, (c) 5 min, (d) 10 min, (e) 20 min and (f) 30 min.
These high-defect CNPs and defects would be the seed in the synthetic process of the CNTs. After producing the CNPs on the graphite flakes, the acetylene is fed into the chamber to behalf as the carbon source in synthesizing the CNTs. Fig. 3 shows the SEM photographs of the CNTs synthesized on the graphite flakes for different synthetic time. The other synthetic conditions of the CNTs are fixed, such as deposition temperature of 7008C, reactive gas flow ratio (C2H2:H2) of 1:2, and working pressure of 4 mbar. The length of CNTs synthesized for 1 min is short and sparse. When starting the synthesis of the CNTs, they are formed from only a minority of synthetic sites. The density and length of CNTs could be increased when we increase the synthetic time. The composition of the samples is identified by EDS analysis as shown in Fig. 4. Apart from the carbon and silicon peaks, no
metal peak is detected. It means that there is no metallic catalyst partaking during the synthetic process of the CNTs. The I–V curves are shown in Fig. 5. The field emission current is applied from the emission sites of the CNTs. The results show that the field emission properties of the CNTs synthesized for 1 and 2 min are ropy because these two samples have less density of the CNTs as shown in Fig. 3. With increasing the synthetic time, it is obvious that the field emission properties of the treated graphite flakes are improved because of increasing the emission sites of the CNTs. The optimal synthetic time is 10 min. The turn-on field could be optimized by controlling the synthetic time, and reduced from 7.7 for untreated specimen to 1.9 V/mm for the optimal case. The emission current density of the treated graphite flakes reaches to 0.5 mA/cm2 at 3 V/mm. However, when we increase the synthetic
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Fig. 4. Composition of the CNTs synthesized on the graphite flakes identified by EDS analysis.
Fig. 6. TEM image of the two CNTs with different diameters and lengths.
Fig. 5. I–V curves of the CNTs synthesized on the graphite flakes for different synthetic time. Fig. 7. Schematic growth mechanism of the CNTs on the graphite flakes.
time from 10 to 30 min, the emission properties are deteriorated due to the screen effect. Gohel et al. have demonstrated the enhanced field emission characteristics of CNTs grown on Fe coated Si substrates by hot filament PECVD using C2H2 and H2 as
inlet gases by nitrogen plasma treatment of 20 min [23]. The lowest turn-on voltage (1.73 V/mm) and highest emission current density (0.45 mA/cm2) at 3 V/mm were obtained for the optimal case. Yu et al. presented change of surface morphology and field
Fig. 8. HRTEM images of (a) CNTs on the graphite flakes and (b) the cross section of the CNTs.
W.-C. Shih et al. / Applied Surface Science 256 (2010) 2409–2413
emission property of CNTs by hydrogen plasma post-treatment. The turn-on field (at 0.1 mA/cm2) decreases from 3.9 to 1.5 V/mm after the sample is treated with hydrogen plasma for 30 min. For comparison, the turn-on voltage and emission current density at 3 V/mm of the proposed method without post-treatment is close to those of the reported catalytic growth of CNTs with post-treatment of nitrogen or hydrogen plasma treatment [23,24]. Fig. 6 shows the TEM images of the two CNTs which have different diameters and lengths. The tip of the shorter CNT is not an irregular half sphericity and has no CNPs inside, so we argued that the growth mechanism is the base growth model which was different from the conventional tip growth model in thermal CVD. Fig. 7 shows the proposed schematic growth mechanism of the CNTs on the graphite flake. After thermal treating the graphite flake in the hydrogen atmosphere, the more defect groups and CNPs are appeared on the surface of the graphite flake as explained in Fig. 7(b). Afterward acetylene acted as the atomic carbon source was fed into the furnace. The gas acetylene is pyrolyzed into hydrocarbon or carbon molecules and then approached to the surface. The hydrocarbon or carbon molecules source near the defects and CNPs would be the effective carbon source for synthesis of the CNTs as explained in Fig. 7(c). During synthesizing the CNTs, the distance between two defects in defect group plays a critical role. The distance between two defects must be near 0.34 nm in defect group for arrangement of multi-graphene wall [9]. If the difference between two defects is very large, the probability of synthesis and the inner diameters of the CNTs are very small. On the other hand, the length of CNTs would be longer if the distance between two defects is near 0.34 nm in defect group. The formation mechanism of CNTs on graphite flake could be explained that why these CNTs have different diameters and lengths. HRTEM is used to observe the interior and the wall structures of the CNTs. Fig. 8 shows a typical HRTEM image of the CNTs on the graphite flakes. The outside diameter of the CNT is about 30 nm. It clearly shows that the CNT is a multi-walled tube with individual cylindrical layers, so it is not a solid fiber. The arrangement of graphene layers is very well and amorphous carbon exists outside the tube. 4. Conclusion The experimental results show that the field emission properties of the graphite flakes have significant improvement after producing the CNTs on above. The turn-on field is decreased from 7.7 to 1.9 V/mm. The CNPs play an important role during the
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growth of the CNTs. We change the synthetic time to obtain the optimal conditions for enhancement of field emission property of graphite flakes. The optimal synthetic time and turn on field are 10 min and 1.9 V/mm, respectively. The outside diameter of the CNT observed by HRTEM is about 30 nm.
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