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Characterization and field emission properties of multi-walled carbon nanotubes with fine crystallinity prepared by CO2 laser ablation
Applied Surface Science 258 (2012) 6958–6962
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Characterization and field emission properties of multi-walled carbon nanotubes with fine crystallinity prepared by CO2 laser ablation Ryota Yuge a,∗ , Kiyohiko Toyama a , Toshinari Ichihashi a , Tetsuya Ohkawa b , Yasushi Aoki b , Takashi Manako a a b
Green Innovation Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba 305-8501, Japan Research and Development Department, NEC Lighting, Ltd., 3-1 Nichiden, Minakuchi, Koga, 528-8501, Japan
a r t i c l e
i n f o
Article history: Received 18 December 2011 Received in revised form 21 February 2012 Accepted 25 March 2012 Available online 1 April 2012 Keywords: Carbon nanotube CO2 laser ablation Field emission
a b s t r a c t Multi-walled carbon nanotubes (MWNTs) were synthesized by irradiating of a CO2 laser in continuous wave mode onto a boron-containing graphite target at room temperature. The pressure of Ar atmosphere was controlled in 50, 150, 400, or 760 Torr. The diameter of obtained MWNTs was in the range of 5–40 nm. The quantity and degree of graphitization of synthesized MWNTs increased with the Ar gas pressure. A large quantity of MWNTs with fine crystalline structure has been synthesized preferentially at the condition of 760 Torr. The MWNTs with the fine crystalinity indicated highly oxidative stability in O2 . We also found that a large area field emission device with MWNT cathodes indicated good ˇ value of 3.6 × 104 cm−1 , and sufficient reliability for long term operations over 150 h, suggesting promising application to field emission devices. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of carbon nanotubes (CNTs) by Iijima in 1991 [1,2], there has been much interest in their potential application to electronic devices [3,4], sensing devices [5,6], super strong engineering fibers [7,8], and catalyst supports [9,10]. The CNTs are particularly promising as a cold cathode emitter for field emission displays (FED) [11,12] and field emission lamps (FELs) [13–16] due to their large aspect ratio, high mechanical strength, good electrical conductivity, and possible large-area application by thick film processing. Their practical development should bring about new solutions to energy and environmental problem. For FED and FEL application, the improvement of the lifetime is one of most important subjects, which need the progress of crystallinity of individual CNT [17]. Although the CNTs used as emitters have been prepared by chemical vapor deposition [18–20], arc discharge [1,2,21], and laser ablation method [22,23], the laser ablation method is the most effective for synthesizing CNTs with a high degree of graphitization. Ablation at room temperature using a CO2 laser with high power would enable large scale production of single-walled carbon nanotubes (SWNTs) and single-walled carbon nanohorns (SWNHs) [24–26]. This means that CO2 laser ablation is attractive for not only achieving of CNTs with a fine crystalline structure but also reducing costs.
∗ Corresponding author. Tel.: +81 29 850 1146; fax: +81 29 856 6137. E-mail address: [email protected] (R. Yuge). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.03.143
Iron, cobalt, and nickel are commonly used as catalysts for CNT growth, and boron is used for CNT and graphite growth [27–29]. Recently, multi-walled carbon nanotubes (MWNTs) with several layers have been synthesized using a pulsed or continuous wave (CW) Nd:YAG laser at room temperature [28,29]. However, the obtained MWNTs were small production quantities for low-power laser and their physical properties have not clarified yet. Here we considered that the MWNT growth using the CO2 laser with high power and boron-containing graphite target is a hopeful strategy for accomplishing of large-scale production. Furthermore, it should be noted that a study of field emission (FE) using MWNTs with a fine crystalline structure and non-metal impurity are promising for development of CNT-FE technology. In this study, we succeeded in synthesizing MWNTs in atmospheric pressures at room temperature by CO2 laser ablation on a boron-containing graphite target. This growth method (room temperature, atmospheric pressure, and CO2 laser ablation) is well suited for low cost production. Due to the high crystallinity of the MWNTs, the MWNTs demonstrated stability in O2 at high temperature. In addition, FE measurements of the obtained MWNT cathodes showed that they had a lower turn-on field for FE properties and sufficient stability for long term operations. 2. Experimental Carbonaceous deposits were produced by CO2 laser ablation with CW mode [26]. The target in this study was the composite made of carbon and boron (10 at.% for boron). The CO2 laser
R. Yuge et al. / Applied Surface Science 258 (2012) 6958–6962
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Fig. 1. SEM images of carbonaceous deposits obtained at Ar gas pressures of 50 (a), 150 (b), 400 (c), and 760 Torr (d).
ablation was operated at 3.5 kW. During 10 s laser ablation, the target was rotated at rotation speed of 6 rpm, which made the continuous production possible. The laser-power-density of 50 kW/cm2 was estimated from the size of the laser spot (about 2.6 mm) on the target surface. The gas pressure in the growth chamber was controlled by changing the evacuating power while holding the flow rate of the Ar buffer gas constant. The pressure was about 50, 150, 400, or 760 Torr, and the gas flow rate was 10 L/min. All the laser vaporization experiments were performed at room temperature. The structures of the specimens were observed with a scanning electron microscope (SEM) (Hitachi S-4800), and transmission electron microscope (TEM) (Topcon EM-002B). The SEM was operated at 1 kV, and the TEM at 120 kV. The Raman spectra were measured with a laser Raman spectrophotometer (JASCO NRS-2000). The excitation wavelength of the Ar ion laser was 488 nm. Thermogravimetric analysis (TGA) (BRUKER AXS TG-DTA2000SA) was performed to characterize the oxidative stability of the obtained materials. The TGA conditions were in an oxygen atmosphere at temperatures ranging from room temperature to 1000 ◦ C at a ramp rate of 10 ◦ C/min. A constant amount (ca. 1.5 mg) of sample was used for the TGA. The carbonaceous deposits (50 mg) were mixed with ethylcellulose (100 mg), glass frit (400 mg), and ␣-terpineol (10 ml), and sufficiently stirred with roll-milling for about 6 h. The resulting pastes were screen-printed on an indium tin oxide (ITO) glass substrate (2 cm × 2 cm) through a 400 mesh net. The paste/ITO was first dried at 80 ◦ C in air and then sintered at 500 ◦ C in N2 , mainly to remove organic materials in the paste and to fix the granular materials. A peeling process with tape was used to activate the cathode [17]. The resulting cathode was placed under an ITO glass plate (anode) at a distance of 3 mm. Field emission measurements were carried out in a vacuum chamber (5 × 10−7 Pa, room temperature). To investigate the durability of MWNT emitters, we measured the time dependence of the field emission current density. The initial current density was set to 20 A/cm2 .
3. Results and discussion 3.1. Characterization of carbonaceous deposits Fig. 1 shows typical SEM images of carbonaceous deposits obtained at Ar pressures of 50, 150, 400, and 760 Torr. The each SEM image indicated the presence of CNTs, together with large amount of non-CNTs materials. The quantities of CNTs increased with the Ar gas pressure. Comparison with the SWNTs and/or MWNTs formed using other methods suggests that CNT formation readily occurred at room temperature and atmospheric pressure. As shown in Fig. 1d, CNTs with a needle-shaped structure were observed mainly for 760 Torr. This structure results in CNTs with the fine crystallinity. The structure of the non-CNT materials strongly depended on the Ar pressure. The uncertain structure such as amorphous carbon was formed abundantly for Ar pressure of 50 Torr (Fig. 1a). On the other hands, mixtures of spherical and/or plate-like structures were formed for Ar gas pressure of 150, 400, and 760 Torr. It was clearly seen that adsorbents on CNT surface for 760 Torr become small quantity than those for 50, 150, and 400 Torr. Fig. 2 shows typical TEM images of the carbonaceous deposits obtained at Ar gas pressures of 50 and 760 Torr. For 760 Torr, we found that carbonaceous deposits involved in MWNTs (blue arrow), nanocapsules (sky blue arrow), spherical polyhedral graphite (green arrow), and graphite sheets (red arrow) (Fig. 2a–d). The MWNTs were 5–40 nm in diameter. The absence of defects on the MWNT walls indicate that they would have highly oxidative stability in air and have sufficient mechanical strength for a high degree of graphitization. Large particles of boron carbide [28,29], i.e. ∼20 nm, deposited inside of the tip site of the MWNTs were also observed (Fig. 2b). The nanocapsule as shown in Fig. 2d was composed of 2–8-nm graphite layers and encapsulated a crystalline particles of boron carbide, which had a diameter of about 10–40 nm. We did not find any SWNTs in the carbonaceous deposits. Although MWNTs were formed for 50 Torr, the quantity was much less than that for 760 Torr, which agreed with the SEM results. The graphene sheet layers of MWNTs for 50 Torr were less than
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R. Yuge et al. / Applied Surface Science 258 (2012) 6958–6962 Table 1 G/D intensity ratio and FWHM of D and G band in Raman spectra of carbonaceous deposits obtained at Ar pressures of 50, 150, 400, and 760 Torr. Values were estimated by peak fitting assuming a Lorentzian shape.
those of MWNTs for 760 Torr. The nanocapsules or spherical polyhedral graphites were not observed as shown in Fig. 2f. Fig. 2e shows the high resolution TEM image of an individual MWNT for 760 Torr and the selected area electron diffraction (SAED) pattern. The walls are composed of graphite sheets aligned parallel to the tube axis. The spots of SAED indexed as (0 0 2) and (0 0 4) diffractions from graphite sheets were seen clearly, which indicated that the nanotube was well-crystallized. The SAED of MWNTs for 50 Torr was also observed, but clear diffraction spots indexed as (0 0 2) were not found. Fig. 3 shows the Raman spectra of the carbonaceous deposits obtained at Ar pressures of 50, 150, 400, and 760 Torr, which can analyze the electronic and vibration properties. Two main Raman modes were observed for all samples, exhibiting a broad G band (∼1590 cm−1 ) and D band (∼1350 cm−1 ). The G band is originated from the E2g symmetry in-plane stretching mode, which is observed in an ordered graphite structure. The D band is associated with inplane vibration resulting from structure imperfection. It has been attributed to the Raman inactive A1g mode by a finite crystallite size effect and, more generally, to the breakdown of selection rules due to the loss of symmetry in the disordered structure. The full-width
50
150
400
760
G/D ratio (arb. units) FWHM of D band (cm−1 ) FWHM of G band (cm−1 )
1.16 115.7 67.1
1.20 79.2 50.2
1.19 75.2 49.8
1.20 63.7 45.2
at half maximum (FWHM) of the D and G bands appears to be narrow with an increase in the Ar pressure (Table 1), meaning that the crystalline structure of the carbonaceous deposits was improved by increasing the Ar pressure. The peak intensity ratio between the G and D bands (G/D ratio) did not depend on the gas pressure (Table 1). The meaning of this is unclear. However, we thought that the quantity of disorder in the hexagonal lattice is probably due to the same. Here, it should be noted that the intensity of each D band was too large in comparison with those of conventional MWNTs and polyhedral graphite [30,31] although the MWNTs had a fine crystalline structure, as shown by the SEM and TEM observations. Considering the previous reports [29,32], we concluded finally that a small quantity of the boron was present in the MWNT lattice at a concentration below the detection limits of EELS and that it degraded the overall regularity of the hexagonal network in MWNTs, resulting in increased D band intensity. The results of the TGA are shown in Fig. 4a. From the peaktemperatures of the combustion for derivative TG curves, we understood that the oxidative stability in O2 increased with the Ar pressure. The weight of the carbonaceous deposits formed at Ar pressures of 150, 400, and 760 Torr did not changed till about 500 ◦ C. This shows the excellent oxidative stability in O2 in comparison with the combustion temperature of commercial MWNTs [33]. We also carried out the SEM observation of the residues after the carbonaceous deposits prepared at 700 Torr were burned in O2 up to 500 ◦ C (Fig. 4b). The result showed that MWNTs were not combusted and deformed structurally. The weight increased temporarily at 500 ◦ C and then decreased. This means that the particles of boron carbide existed in MWNTs and nanocapsules were oxidized to form boron oxides, and thereafter carbon components combusted for temperature of above 600 ◦ C. The weight of the deposit formed at an Ar pressure of 50 Torr decreased gradually with the temperature. This may be because the uncertain structure such as amorphous carbons other than the graphitic components, as shown in Fig. 2f, started to combust before oxidation of boron carbide. We found finally from SEM and TEM observation or TGA that a large quantity of MWNTs have been synthesized with fine 600 500
Intensity (arb. units)
Fig. 2. TEM images of carbonaceous deposits obtained at Ar gas pressures of 760 Torr (a–e) and 50 Torr (f).
Gas pressure (Torr)
50 Torr 150 Torr 400 Torr 760 Torr
400 300 200 100 0 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 -1
Raman shift (cm ) Fig. 3. Raman spectra of carbonaceous deposits obtained at Ar pressures of 50, 150, 400, and 760 Torr. Each spectrum was normalized to the G band intensity.
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Fig. 6. J–T curve for emitters of carbonaceous deposits obtained at Ar pressure of 760 Torr. SEM images of cathode electrode before (a) and after (b) J–T measurement are shown in insets. The large spherical and flat lumps observed in SEM image show glass frit in the paste.
Fig. 4. (a) TG and derivative TG profiles of carbonaceous deposits obtained at Ar pressures of 50, 150, 400, and 760 Torr. (b) SEM image of the residue after combustion in O2 up to 500 ◦ C by TGA.
crystalline structure at the condition of 760 Torr. The fine crystalline structure was probably formed due to high-temperature growth of MWNTs by CO2 laser ablation method even if the boron atom has been involved in graphene lattice of MWNTs. The growth conditions we used (room temperature, atmospheric pressure, and CO2 laser ablation) is well suited for commercial manufacture of MWNTs. 3.2. Field emission properties of carbonaceous deposits The FE properties are shown in Fig. 5 for the carbonaceous deposits formed at Ar pressures of 50 and 760 Torr, which differed considerably in terms of the quantity of MWNTs. The emission
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2
10 10 10 10
-2
-3
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50 Torr 760 Torr
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-4 2
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50 Torr 760 Torr
ln(I/V )
Emission current (mA/cm )
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-28 -32 -36 0.0002
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1/V
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Electric field (kV/mm) Fig. 5. J–E curves and F–N plots (inset) for emitters of carbonaceous deposits obtained at Ar pressure of 50 and 760 Torr. F–N plots are composed of linear relationships with the slopes of each sample.
current density (J) is the emission current per unit emitter area (2 cm × 2 cm) and electric field (E) is the applied voltage per unit electrode distance between anode and cathode. Turn-on fields were 1.6 and 0.7 kV/mm, respectively, defined as electric field at a current of 1 A/cm2 . The corresponding Fowler–Nordheim (F–N) plots, in which the I–V curves are plotted as ln(I/V2 ) vs. 1/V, are shown in the inset. Each plot is a straight line, indicating a cathode electrode made of carbonaceous deposits follow F–N theory as behavior [34,35]. The slope of the F–N plots depends on the work function and field enhancement factor ˇ. Here, we estimated ˇ with the hypothesis that the work function is similar to that of graphite (∼5 eV) [36], and found ˇ = 2.1 × 104 and 3.6 × 104 cm−1 for 50 and 760 Torr, respectively. The ˇ value of 3.6 × 104 cm−1 was larger than that of MWNT emitter for other FEL application [37]. There are two possible reasons for the significant enhancement of the ˇ values for the 760-Torr sample. One is the difference in local morphology around a MWNT such as the tip structure, aspect ratio, and stand-up height from the ground plane, which should drastically change the electric field near the MWNT tips and efficiency of field emission [38,39]. The other is the difference in the electrical properties of the MWNT itself. Since the boron doping to MWNTs changes their electron filling, the conductivity of sample could be drastically altered by the doping level. Such a difference in transport property could cause a wide diversity in the efficiency of field emission. More work is needed to identify which of these two possible reasons played a bigger role in our results. This work includes, for example, precise compositional analysis and/or direct observation of the electrode surface structure. To investigate the durability of MWNTs emitters, we measured time dependence of the field emission current density (J–T) as shown in Fig. 6. The initial current density of 20 A/cm2 remained constant over 150 h. Considering previously reported results [17,40], we concluded that MWNTs have sufficient reliability to be used as emitters. The long life time of the cathode was mainly due to the higher degree of graphitization of the MWNTs in the carbonaceous deposits, which is recognized from the SEM images of the cathode (inset of (a and b) in Fig. 6). In fact, the structure of each MWNTs shows no evidence of damage after the FE measurements. 4. Conclusion We synthesized MWNTs by irradiating a CO2 laser onto a boron-containing graphite target in several pressures at room
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temperature. The obtained MWNTs in diameter were 5–40 nm. The large quantity of MWNTs preferentially has been synthesized with fine crystalline structure at the condition of 760 Torr. FE measurements of large-area MWNT cathodes showed the low electron emission threshold of 0.7 kV/mm, the excellent ˇ value of 3.6 × 104 cm−1 and sufficient reliability for long term operations of about 150 h, suggesting promising application to FE devices. We believe that the practical synthesis of MWNTs by CO2 laser ablation method at room temperature and atmospheric pressure will enable large scale production, which will reduce their cost. References [1] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58. [2] S. Iijima, T. Ichihashi, Single-shell carbon nanotubes of 1-nm diameter, Nature 363 (1993) 603–605. [3] S.J. Tans, A.R.M. Verschueren, C. Dekker, Room-temperature transistor based on a single carbon nanotube, Nature 393 (1998) 49–52. [4] S. Frank, P. Poncharal, Z.L. Wang, W.A. Heer, Carbon nanotube quantum resistors, Science 280 (1998) 1744–1746. [5] M.E. Itkis, F. Borondics, A. Yu, R.C. Haddon, Bolometric infrared photoresponse of suspended single-walled carbon nanotube films, Science 312 (2006) 413–416. [6] C. Stampfer, T. Helbling, D. Obergfell, B. Schöberle, M.K. Tripp, A. Jungen, S. Roth, V.M. Bright, C. Hierold, Fabrication of single-walled carbon-nanotube-based pressure sensors, Nano Lett. 6 (2006) 233–237. [7] A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y.H. Lee, S.G. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tománek, J.E. Fischer, R.E. Smalley, Crystalline ropes of metallic carbon nanotubes, Science 273 (1996) 483–487. [8] B. Vigolo, A. Pénicaud, C. Coulon, C. Sauder, R. Pailler, C. Journet, P. Bernier, P. Poulin, Macroscopic fibers and ribbons of oriented carbon nanotubes, Science 290 (2000) 1331–1334. [9] R.V. Hull, L. Li, Y. Xing, C.C. Chusuei, Pt nanoparticle binding on functionalized multiwalled carbon nanotubes, Chem. Mater. 18 (2006) 1780–1788. [10] J. Chen, M. Wang, B. Liu, Z. Fan, K. Cui, Y. Kuang, Platium catalysts prepared with functional carbon nanotube defects and its improved catalytic performance for methanol oxidation, J. Phys. Chem. B 110 (2006) 11775–11779. [11] A.G. Rinzler, J.H. Hafner, P. Nikolaev, P. Nordlander, D.T. Colbert, R.E. Smalley, L. Lou, S.G. Kim, D. Tománek, Unraveling nanotubes: field emission from an atomic wire, Science 269 (1995) 1550–1553. [12] W.A. Heer, A. Châtelain, D. Ugarte, A carbon nanotube field-emission electron source, Science 270 (1995) 1179–1180. [13] J. Yu, J. Chen, S.Z. Deng, N.S. Xu, Field emission characteristics of screen-printed carbon nanotube cold cathode by hydrogen plasma treatment, Appl. Surf. Sci. 258 (2011) 738–742. [14] J.M. Bonard, T. Stöckli, O. Noury, A. Châtelain, Field emission from cylindrical carbon nanotube cathodes: possibilities for luminescent tubes, Appl. Phys. Lett. 78 (2001) 2775–2777. [15] J. Chen, X.H. Liang, S.Z. Deng, N.S. Xu, Flat-panel luminescent lamp using carbon nanotube cathodes, J. Vac. Sci. Technol. B 21 (2003) 1727–1729. [16] G. Delepierre, R. Mahfouz, F.J.C.S. Aires, J. Dijon, Green backlighting for TV liquid crystal display using carbon nanotubes, Appl. Surf. Sci. 108 (2010) 0443081–044308-8. [17] S.I. Jung, S.H. Jo, H.S. Moon, J.M. Kim, D.S. Zang, C.J. Lee, Improved crystallinity of double-walled carbon nantubes after a high-temperature thermal annealing and their enhanced field emisson properties, J. Phys. Chem. C 111 (2007) 4175–4179. [18] H. Dai, A.G. Rinzler, P. Nikolaev, A. Thess, D.T. Colbert, R.E. Smalley, Single-wall nanotubes produced by metal-catalyzed disproportionation of carbon monoxide, Chem. Phys. Lett. 260 (1996) 471–475.
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Characterization and field emission properties of multi-walled carbon nanotubes with fine crystallinity prepared by CO2 laser ablation Ryota Yuge a,∗ , Kiyohiko Toyama a , Toshinari Ichihashi a , Tetsuya Ohkawa b , Yasushi Aoki b , Takashi Manako a a b
Green Innovation Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba 305-8501, Japan Research and Development Department, NEC Lighting, Ltd., 3-1 Nichiden, Minakuchi, Koga, 528-8501, Japan
a r t i c l e
i n f o
Article history: Received 18 December 2011 Received in revised form 21 February 2012 Accepted 25 March 2012 Available online 1 April 2012 Keywords: Carbon nanotube CO2 laser ablation Field emission
a b s t r a c t Multi-walled carbon nanotubes (MWNTs) were synthesized by irradiating of a CO2 laser in continuous wave mode onto a boron-containing graphite target at room temperature. The pressure of Ar atmosphere was controlled in 50, 150, 400, or 760 Torr. The diameter of obtained MWNTs was in the range of 5–40 nm. The quantity and degree of graphitization of synthesized MWNTs increased with the Ar gas pressure. A large quantity of MWNTs with fine crystalline structure has been synthesized preferentially at the condition of 760 Torr. The MWNTs with the fine crystalinity indicated highly oxidative stability in O2 . We also found that a large area field emission device with MWNT cathodes indicated good ˇ value of 3.6 × 104 cm−1 , and sufficient reliability for long term operations over 150 h, suggesting promising application to field emission devices. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of carbon nanotubes (CNTs) by Iijima in 1991 [1,2], there has been much interest in their potential application to electronic devices [3,4], sensing devices [5,6], super strong engineering fibers [7,8], and catalyst supports [9,10]. The CNTs are particularly promising as a cold cathode emitter for field emission displays (FED) [11,12] and field emission lamps (FELs) [13–16] due to their large aspect ratio, high mechanical strength, good electrical conductivity, and possible large-area application by thick film processing. Their practical development should bring about new solutions to energy and environmental problem. For FED and FEL application, the improvement of the lifetime is one of most important subjects, which need the progress of crystallinity of individual CNT [17]. Although the CNTs used as emitters have been prepared by chemical vapor deposition [18–20], arc discharge [1,2,21], and laser ablation method [22,23], the laser ablation method is the most effective for synthesizing CNTs with a high degree of graphitization. Ablation at room temperature using a CO2 laser with high power would enable large scale production of single-walled carbon nanotubes (SWNTs) and single-walled carbon nanohorns (SWNHs) [24–26]. This means that CO2 laser ablation is attractive for not only achieving of CNTs with a fine crystalline structure but also reducing costs.
∗ Corresponding author. Tel.: +81 29 850 1146; fax: +81 29 856 6137. E-mail address: [email protected] (R. Yuge). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.03.143
Iron, cobalt, and nickel are commonly used as catalysts for CNT growth, and boron is used for CNT and graphite growth [27–29]. Recently, multi-walled carbon nanotubes (MWNTs) with several layers have been synthesized using a pulsed or continuous wave (CW) Nd:YAG laser at room temperature [28,29]. However, the obtained MWNTs were small production quantities for low-power laser and their physical properties have not clarified yet. Here we considered that the MWNT growth using the CO2 laser with high power and boron-containing graphite target is a hopeful strategy for accomplishing of large-scale production. Furthermore, it should be noted that a study of field emission (FE) using MWNTs with a fine crystalline structure and non-metal impurity are promising for development of CNT-FE technology. In this study, we succeeded in synthesizing MWNTs in atmospheric pressures at room temperature by CO2 laser ablation on a boron-containing graphite target. This growth method (room temperature, atmospheric pressure, and CO2 laser ablation) is well suited for low cost production. Due to the high crystallinity of the MWNTs, the MWNTs demonstrated stability in O2 at high temperature. In addition, FE measurements of the obtained MWNT cathodes showed that they had a lower turn-on field for FE properties and sufficient stability for long term operations. 2. Experimental Carbonaceous deposits were produced by CO2 laser ablation with CW mode [26]. The target in this study was the composite made of carbon and boron (10 at.% for boron). The CO2 laser
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Fig. 1. SEM images of carbonaceous deposits obtained at Ar gas pressures of 50 (a), 150 (b), 400 (c), and 760 Torr (d).
ablation was operated at 3.5 kW. During 10 s laser ablation, the target was rotated at rotation speed of 6 rpm, which made the continuous production possible. The laser-power-density of 50 kW/cm2 was estimated from the size of the laser spot (about 2.6 mm) on the target surface. The gas pressure in the growth chamber was controlled by changing the evacuating power while holding the flow rate of the Ar buffer gas constant. The pressure was about 50, 150, 400, or 760 Torr, and the gas flow rate was 10 L/min. All the laser vaporization experiments were performed at room temperature. The structures of the specimens were observed with a scanning electron microscope (SEM) (Hitachi S-4800), and transmission electron microscope (TEM) (Topcon EM-002B). The SEM was operated at 1 kV, and the TEM at 120 kV. The Raman spectra were measured with a laser Raman spectrophotometer (JASCO NRS-2000). The excitation wavelength of the Ar ion laser was 488 nm. Thermogravimetric analysis (TGA) (BRUKER AXS TG-DTA2000SA) was performed to characterize the oxidative stability of the obtained materials. The TGA conditions were in an oxygen atmosphere at temperatures ranging from room temperature to 1000 ◦ C at a ramp rate of 10 ◦ C/min. A constant amount (ca. 1.5 mg) of sample was used for the TGA. The carbonaceous deposits (50 mg) were mixed with ethylcellulose (100 mg), glass frit (400 mg), and ␣-terpineol (10 ml), and sufficiently stirred with roll-milling for about 6 h. The resulting pastes were screen-printed on an indium tin oxide (ITO) glass substrate (2 cm × 2 cm) through a 400 mesh net. The paste/ITO was first dried at 80 ◦ C in air and then sintered at 500 ◦ C in N2 , mainly to remove organic materials in the paste and to fix the granular materials. A peeling process with tape was used to activate the cathode [17]. The resulting cathode was placed under an ITO glass plate (anode) at a distance of 3 mm. Field emission measurements were carried out in a vacuum chamber (5 × 10−7 Pa, room temperature). To investigate the durability of MWNT emitters, we measured the time dependence of the field emission current density. The initial current density was set to 20 A/cm2 .
3. Results and discussion 3.1. Characterization of carbonaceous deposits Fig. 1 shows typical SEM images of carbonaceous deposits obtained at Ar pressures of 50, 150, 400, and 760 Torr. The each SEM image indicated the presence of CNTs, together with large amount of non-CNTs materials. The quantities of CNTs increased with the Ar gas pressure. Comparison with the SWNTs and/or MWNTs formed using other methods suggests that CNT formation readily occurred at room temperature and atmospheric pressure. As shown in Fig. 1d, CNTs with a needle-shaped structure were observed mainly for 760 Torr. This structure results in CNTs with the fine crystallinity. The structure of the non-CNT materials strongly depended on the Ar pressure. The uncertain structure such as amorphous carbon was formed abundantly for Ar pressure of 50 Torr (Fig. 1a). On the other hands, mixtures of spherical and/or plate-like structures were formed for Ar gas pressure of 150, 400, and 760 Torr. It was clearly seen that adsorbents on CNT surface for 760 Torr become small quantity than those for 50, 150, and 400 Torr. Fig. 2 shows typical TEM images of the carbonaceous deposits obtained at Ar gas pressures of 50 and 760 Torr. For 760 Torr, we found that carbonaceous deposits involved in MWNTs (blue arrow), nanocapsules (sky blue arrow), spherical polyhedral graphite (green arrow), and graphite sheets (red arrow) (Fig. 2a–d). The MWNTs were 5–40 nm in diameter. The absence of defects on the MWNT walls indicate that they would have highly oxidative stability in air and have sufficient mechanical strength for a high degree of graphitization. Large particles of boron carbide [28,29], i.e. ∼20 nm, deposited inside of the tip site of the MWNTs were also observed (Fig. 2b). The nanocapsule as shown in Fig. 2d was composed of 2–8-nm graphite layers and encapsulated a crystalline particles of boron carbide, which had a diameter of about 10–40 nm. We did not find any SWNTs in the carbonaceous deposits. Although MWNTs were formed for 50 Torr, the quantity was much less than that for 760 Torr, which agreed with the SEM results. The graphene sheet layers of MWNTs for 50 Torr were less than
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R. Yuge et al. / Applied Surface Science 258 (2012) 6958–6962 Table 1 G/D intensity ratio and FWHM of D and G band in Raman spectra of carbonaceous deposits obtained at Ar pressures of 50, 150, 400, and 760 Torr. Values were estimated by peak fitting assuming a Lorentzian shape.
those of MWNTs for 760 Torr. The nanocapsules or spherical polyhedral graphites were not observed as shown in Fig. 2f. Fig. 2e shows the high resolution TEM image of an individual MWNT for 760 Torr and the selected area electron diffraction (SAED) pattern. The walls are composed of graphite sheets aligned parallel to the tube axis. The spots of SAED indexed as (0 0 2) and (0 0 4) diffractions from graphite sheets were seen clearly, which indicated that the nanotube was well-crystallized. The SAED of MWNTs for 50 Torr was also observed, but clear diffraction spots indexed as (0 0 2) were not found. Fig. 3 shows the Raman spectra of the carbonaceous deposits obtained at Ar pressures of 50, 150, 400, and 760 Torr, which can analyze the electronic and vibration properties. Two main Raman modes were observed for all samples, exhibiting a broad G band (∼1590 cm−1 ) and D band (∼1350 cm−1 ). The G band is originated from the E2g symmetry in-plane stretching mode, which is observed in an ordered graphite structure. The D band is associated with inplane vibration resulting from structure imperfection. It has been attributed to the Raman inactive A1g mode by a finite crystallite size effect and, more generally, to the breakdown of selection rules due to the loss of symmetry in the disordered structure. The full-width
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400
760
G/D ratio (arb. units) FWHM of D band (cm−1 ) FWHM of G band (cm−1 )
1.16 115.7 67.1
1.20 79.2 50.2
1.19 75.2 49.8
1.20 63.7 45.2
at half maximum (FWHM) of the D and G bands appears to be narrow with an increase in the Ar pressure (Table 1), meaning that the crystalline structure of the carbonaceous deposits was improved by increasing the Ar pressure. The peak intensity ratio between the G and D bands (G/D ratio) did not depend on the gas pressure (Table 1). The meaning of this is unclear. However, we thought that the quantity of disorder in the hexagonal lattice is probably due to the same. Here, it should be noted that the intensity of each D band was too large in comparison with those of conventional MWNTs and polyhedral graphite [30,31] although the MWNTs had a fine crystalline structure, as shown by the SEM and TEM observations. Considering the previous reports [29,32], we concluded finally that a small quantity of the boron was present in the MWNT lattice at a concentration below the detection limits of EELS and that it degraded the overall regularity of the hexagonal network in MWNTs, resulting in increased D band intensity. The results of the TGA are shown in Fig. 4a. From the peaktemperatures of the combustion for derivative TG curves, we understood that the oxidative stability in O2 increased with the Ar pressure. The weight of the carbonaceous deposits formed at Ar pressures of 150, 400, and 760 Torr did not changed till about 500 ◦ C. This shows the excellent oxidative stability in O2 in comparison with the combustion temperature of commercial MWNTs [33]. We also carried out the SEM observation of the residues after the carbonaceous deposits prepared at 700 Torr were burned in O2 up to 500 ◦ C (Fig. 4b). The result showed that MWNTs were not combusted and deformed structurally. The weight increased temporarily at 500 ◦ C and then decreased. This means that the particles of boron carbide existed in MWNTs and nanocapsules were oxidized to form boron oxides, and thereafter carbon components combusted for temperature of above 600 ◦ C. The weight of the deposit formed at an Ar pressure of 50 Torr decreased gradually with the temperature. This may be because the uncertain structure such as amorphous carbons other than the graphitic components, as shown in Fig. 2f, started to combust before oxidation of boron carbide. We found finally from SEM and TEM observation or TGA that a large quantity of MWNTs have been synthesized with fine 600 500
Intensity (arb. units)
Fig. 2. TEM images of carbonaceous deposits obtained at Ar gas pressures of 760 Torr (a–e) and 50 Torr (f).
Gas pressure (Torr)
50 Torr 150 Torr 400 Torr 760 Torr
400 300 200 100 0 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 -1
Raman shift (cm ) Fig. 3. Raman spectra of carbonaceous deposits obtained at Ar pressures of 50, 150, 400, and 760 Torr. Each spectrum was normalized to the G band intensity.
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Fig. 6. J–T curve for emitters of carbonaceous deposits obtained at Ar pressure of 760 Torr. SEM images of cathode electrode before (a) and after (b) J–T measurement are shown in insets. The large spherical and flat lumps observed in SEM image show glass frit in the paste.
Fig. 4. (a) TG and derivative TG profiles of carbonaceous deposits obtained at Ar pressures of 50, 150, 400, and 760 Torr. (b) SEM image of the residue after combustion in O2 up to 500 ◦ C by TGA.
crystalline structure at the condition of 760 Torr. The fine crystalline structure was probably formed due to high-temperature growth of MWNTs by CO2 laser ablation method even if the boron atom has been involved in graphene lattice of MWNTs. The growth conditions we used (room temperature, atmospheric pressure, and CO2 laser ablation) is well suited for commercial manufacture of MWNTs. 3.2. Field emission properties of carbonaceous deposits The FE properties are shown in Fig. 5 for the carbonaceous deposits formed at Ar pressures of 50 and 760 Torr, which differed considerably in terms of the quantity of MWNTs. The emission
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2
10 10 10 10
-2
-3
-20
50 Torr 760 Torr
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-4 2
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ln(I/V )
Emission current (mA/cm )
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-28 -32 -36 0.0002
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Electric field (kV/mm) Fig. 5. J–E curves and F–N plots (inset) for emitters of carbonaceous deposits obtained at Ar pressure of 50 and 760 Torr. F–N plots are composed of linear relationships with the slopes of each sample.
current density (J) is the emission current per unit emitter area (2 cm × 2 cm) and electric field (E) is the applied voltage per unit electrode distance between anode and cathode. Turn-on fields were 1.6 and 0.7 kV/mm, respectively, defined as electric field at a current of 1 A/cm2 . The corresponding Fowler–Nordheim (F–N) plots, in which the I–V curves are plotted as ln(I/V2 ) vs. 1/V, are shown in the inset. Each plot is a straight line, indicating a cathode electrode made of carbonaceous deposits follow F–N theory as behavior [34,35]. The slope of the F–N plots depends on the work function and field enhancement factor ˇ. Here, we estimated ˇ with the hypothesis that the work function is similar to that of graphite (∼5 eV) [36], and found ˇ = 2.1 × 104 and 3.6 × 104 cm−1 for 50 and 760 Torr, respectively. The ˇ value of 3.6 × 104 cm−1 was larger than that of MWNT emitter for other FEL application [37]. There are two possible reasons for the significant enhancement of the ˇ values for the 760-Torr sample. One is the difference in local morphology around a MWNT such as the tip structure, aspect ratio, and stand-up height from the ground plane, which should drastically change the electric field near the MWNT tips and efficiency of field emission [38,39]. The other is the difference in the electrical properties of the MWNT itself. Since the boron doping to MWNTs changes their electron filling, the conductivity of sample could be drastically altered by the doping level. Such a difference in transport property could cause a wide diversity in the efficiency of field emission. More work is needed to identify which of these two possible reasons played a bigger role in our results. This work includes, for example, precise compositional analysis and/or direct observation of the electrode surface structure. To investigate the durability of MWNTs emitters, we measured time dependence of the field emission current density (J–T) as shown in Fig. 6. The initial current density of 20 A/cm2 remained constant over 150 h. Considering previously reported results [17,40], we concluded that MWNTs have sufficient reliability to be used as emitters. The long life time of the cathode was mainly due to the higher degree of graphitization of the MWNTs in the carbonaceous deposits, which is recognized from the SEM images of the cathode (inset of (a and b) in Fig. 6). In fact, the structure of each MWNTs shows no evidence of damage after the FE measurements. 4. Conclusion We synthesized MWNTs by irradiating a CO2 laser onto a boron-containing graphite target in several pressures at room
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