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Field emission property of N-doped aligned carbon nanotubes grown by pyrolysis of monoethanolamine
Solid State Communications 147 (2008) 15–19
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Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Field emission property of N-doped aligned carbon nanotubes grown by pyrolysis of monoethanolamine Pradip Ghosh ∗ , M. Tanemura, T. Soga, M. Zamri, T. Jimbo Department of Environmental Technology and Urban Planning, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, 466-8555, Japan
article
info
Article history: Received 2 February 2008 Received in revised form 23 April 2008 Accepted 25 April 2008 by S. Miyashita Available online 30 April 2008 PACS: 61.48.De 81.15.Gh 81.07.De 79.70.+q
a b s t r a c t Densely distributed bamboo-shaped nitrogen-doped aligned carbon nanotubes, grown on silicon substrate by thermal decomposition of monoethanolamine/ferrocene mixtures at 900 ◦ C, were investigated for field electron emission. The morphology and crystallinity of the as-grown carbon nanotubes were characterized by SEM, TEM and Raman spectroscopy. X-ray photoelectron spectroscopy was used to analyze the nitrogen concentration on carbon nanotubes and it was observed that nitrogen concentration on nanotubes was 6.6 at.%. Field emission study of as-grown nitrogen-doped carbon nanotubes suggests that they are good emitters with a turn-on and threshold field of 1.8 V/µm and 2.53 V/µm, respectively. The maximum current density was observed to be 6 mA/cm2 at 3 V/µm. It is considered that the nice field emission performance of CNx nanotube is due to the presence of lone pairs of electrons on nitrogen atom that supplies more electrons to the conduction band. © 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Carbon nanotubes B. Chemical vapor deposition D. Field electron emission
1. Introduction N-doped carbon nanotubes are considered as one of the promising materials for potential use in the upcoming field of nanoelectronics. Recently synthesis of N-doped carbon nanotubes have received a growing interest because incorporation of nitrogen atom into carbon nanotubes leads to unique bulk and surface properties on carbon nanotubes and a feasible strategy to tune the electronic property in a well-defined way [1]. On the basis of theoretical [2] and experimental studies [3], it has been predicted that CNx nanotubes have n-type conductivity, low resistance and good field electron emission property. The additional lone pairs of electrons on nitrogen atom with respect to the delocalized π-system of a graphite-like hexagonal network can enhance the conducting properties on N-doped carbon nanotubes [4]. One of the important advantages of this material is that the electronic property of the doped nanotubes is dependant on the dopant concentration and thus it is relatively easy to control this property by changing the dopant concentration into the bulk material. Therefore, considerable attempts have been made to investigate the preparation of nitrogen-doped carbon nanotubes.
∗ Corresponding author. Tel.: +81 52 735 7149; fax: +81 52 735 7120. E-mail address: [email protected] (P. Ghosh). 0038-1098/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2008.04.030
Alignment of carbon nanotube is very important for fundamental study and application in sensors and nanodevices, especially in scanning electron probes and flat panel displays [5]. There are few reports of synthesis of well-aligned nitrogen-doped carbon nanotubes and their application in field electron emission. Recently Srivastava et al. synthesized aligned N-doped carbon nanotubes and showed their field emission property with threshold field in the range of 2.65–3.55V/µm [6]. Zhang and his coworkers prepared aligned CNx nanotubes with a high content of nitrogen (x ≤ 9%) by pyrolyzing metal phthalocyanine on an n-type Si(100) substrate and studied their field emission characteristics [7]. Sharma et al. reported the synthesis of boron and nitrogen-doped carbon nanotubes on tungsten tips and silicon substrates and measured their field emission characteristics [8]. Similarly Che et al. proposed field emission properties of both horizontally aligned and vertically aligned N-doped carbon nanotubes synthesized by C/N feedstock using iron as a catalyst [9]. In this paper we reported the synthesis of well-aligned nitrogen-doped bamboo-shaped carbon nanotubes by pyrolysis of monoethanolamine/ferrocene mixture by chemical vapor deposition method at 900 ◦ C. The Structural characteristics of CNTs have been investigated using electron microscopy and Raman spectroscopy technique. The composition of nanotubes was determined using X-ray photoelectron spectroscopy (XPS). Systematic study of field emission measurements of N-doped carbon nanotubes was carried out.
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P. Ghosh et al. / Solid State Communications 147 (2008) 15–19
Fig. 1. SEM images of as-prepared N-doped aligned carbon nanotubes prepared by thermal decomposition of monoethanolamine/ferrocene mixture at 900 ◦ C (a) low magnification image of aligned N-doped nanotubes (b) high magnification image of N-doped CNTs.
2. Experimental Nitrogen-doped carbon nanotubes were prepared by chemical vapor deposition of monoethanolamine (NH2 CH2 CH2 OH) and ferrocene mixture on Si(100) substrate at atmospheric pressure. A 1 m long and 25 mm of inner diameter wide quartz tube was used as a reactor. The tube was placed inside the two horizontal electric furnaces. Before carbon nanotubes deposition, n-type Si(100) substrate was ultrasonically cleaned in acetone for 5 min followed by water and placed in a quartz boat and kept in the center of the second furnace. The mixture of ferrocene and monoethanolamine was kept in the first furnace. To expel air from the tube, nitrogen gas was sent through the tube at a flow rate of 50 ml/min for 15 min. The temperature of the second furnace was 900 ◦ C whereas temperature of the first furnace was 100 ◦ C. When both furnaces attained the desire temperature, the flow rate of the nitrogen gas was increased from 50 ml/min to 100 ml/min. The reaction was maintained for 60 min followed by 10 min annealing at the same temperature. The two furnaces were cooled down to room temperature under nitrogen atmosphere and the as-prepared material was collected for characterization. The as-grown materials were characterized by scanning electron microscopy (Hitachi S-3000H, scanning electron microscope), transmission electron microscopy (HITACHI HF 2000 with an acceleration voltage 200 kV) and Raman spectroscopy (JASCO, NRS1500W, green laser with an exciton wavelength of 532 nm), Xray photoelectron spectroscopy (SSX-100 XPS spectrometer using Al Kα X-ray source (1486.6 eV) under high vacuum condition of about 10−10 Torr). Field emission measurements were performed in a vacuum chamber at a pressure less than 3 × 10−9 Torr within UHV-SEM (JAMP-7100). The cathode consisted of as-grown Ndoped CNTs on n-type Si substrate, and anode was a polished stainless steel rod (1 mm in diameter). The distance between cathode and anode was 300 µm and it was carefully monitored in situ with the aid of UHV-SEM images. 3. Results and discussions 3.1. Morphology and structure of the well-aligned N-doped carbon nanotubes Fig. 1a and b shows the SEM images of the nitrogendoped carbon nanotubes synthesized at 900 ◦ C by catalytic decomposition of monoethanolamine at atmospheric pressure. SEM images show dense and well-aligned carbon nanotubes with respect to the Si substrate. The dense order pack of nanotubes comes from the effect of Van der walls force among
the neighboring nanotubes which results the formation of aligned nanotubes with almost uniform length. The length of the asprepared carbon nanotubes was approximately 50 µm. TEM/HRTEM images of N-doped carbon nanotubes are displayed in Figs. 2a–d. The as-prepared carbon nanotubes were dispersed in ethanol and a drop of suspension was put onto the holey carbon grid. TEM images clearly revealed that as-prepared nanotubes mostly exhibited bamboo-like structure with wellseparated transverse bridge compartments (Fig. 2a). The detailed structures of the N-doped carbon nanotubes are given by HRTEM images. HRTEM image clearly indicates the defects and roughness within the wall of carbon nanotubes (Fig. 2b). The defects are attributed to the presence of nitrogen atom by formation of pyridinelike bond with carbon atom [10]. It is observed that incorporation of nitrogen atom into carbon nanotubes network not only promotes bamboo-shaped structure but also introduce defects into the tube. Fig. 2c shows the presence of cone structure in tube that is common for N-doped bamboo-shaped carbon nanotubes. HRTEM image also indicates the presence of amorphous carbon coating on the outer surface of the carbon nanotubes (Fig. 2d). Fig. 2d shows that walls are constructed from ∼10 graphene layers with inner diameter ∼15 nm. 3.2. XPS characterization To obtain the information about the elemental composition and bonding environment of N-doped carbon nanotubes, XPS analysis was carried out. The measured XPS spectra indicate the presence of carbon, nitrogen and oxygen in the as-prepared material. Apart from these elements, peak of iron was also observed in the asgrown N-doped carbon nanotubes. The strong intensity of the C 1s peak indicates the presence of carbon nanotubes in the asprepared material. The oxygen peak might arise from SiO2 barrier film on the Si substrate or air adsorbed at the specimen surface. The amount of oxygen was found to be 1.06 at.%. Fig. 3a shows the wide scan XPS spectra of N-doped carbon nanotubes containing 6.6 at.% of nitrogen, prepared at 900 ◦ C by decomposition of monoethanolamine using ferrocene as a catalyst. Figs. 3b and c shows the C 1s and N 1s signal of N-doped carbon nanotubes respectively. The C 1s spectrum of N-doped carbon nanotubes consist of two components at 284.6 and 285.3 eV which is shown in Fig. 3b. The strongest peak at 284.6 eV indicates that carbon is mostly in the form of graphite and assigned to sp2 aromatic hydrocarbons [11,12]. The peak at 285.3 eV is assigned to C–N bond [13]. Fig. 3c displays the N 1s spectra of as-prepared N-doped carbon nanotubes. From XPS spectra, the nitrogen concentration on asgrown carbon nanotubes was calculated to be 6.6 at.%. N 1s signal
P. Ghosh et al. / Solid State Communications 147 (2008) 15–19
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Fig. 2. TEM/HRETM images of as-grown N-doped nanotubes (a) low magnification image of N-doped carbon nanotubes, showing bamboo-like morphology (b) HRTEM image of carbon nanotubes, showing outer wall of N-doped carbon nanotubes and arrow mark indicates the defects within the wall (c) cone structure of N-doped carbon nanotubes (d) HRTEM image of N-doped carbon nanotubes which shows the amorphous carbon coating on the outer wall of carbon nanotubes.
Fig. 3. XPS spectra of CNx nanotubes (a) wide scan XPS spectra of as-prepared sample (b) C 1s and (c) N 1s core level spectrum.
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Fig. 4. Raman spectra of as-prepared N-doped carbon nanotubes.
was deconvoluted into three bands at 398.4, 401 and 404.8 eV. The diverse electronic structures of N atoms including the “graphitic-” and “pyridine-” like structures and N–O bonds were identified by XPS [14–16]. According to the theoretical calculation and experimental results, the band at 398.4 and 401 eV corresponds to the “pyridinic” and “graphitic” nitrogen respectively [17,18] and the band at 404.8 eV corresponds to some oxidized N-species i.e. N–O bonds [19]. 3.3. Raman spectroscopy analysis To obtain the information regarding the degree of crystallinity of carbon nanotubes, Raman spectroscopy was employed. The Raman spectrum of as-prepared N-doped carbon nanotube is shown in Fig. 4. The spectrum shows two bands at 1357 cm−1 (Dband) and 1591 cm−1 (G-band). The G-band corresponds to the C–C stretching (E2g ) mode for typical graphite and D-band is ascribed to the defects and disorder in graphene sheets. The defects and disorders are related to the presence of nitrogen atom in the asprepared carbon nanotubes [20]. Due to presence of nitrogen atom defects and disorders are introduced into the carbon nanotubes. The ratio of ID /IG is directly related to the extent of disorders in the as-prepared nanotubes. From Raman spectrum ID /IG was found to be 0.97. The high ID /IG value indicates that the crystalline perfection of the as-prepared nanotubes was less. The presence of nitrogen atom tends to introduce disorders in the graphene planes.
3.4. Field emission property of N-doped CNTs The field emission current density vs. electric field of N-doped carbon nanotube is shown in Fig. 5. For the reliability of the data, field emission measurements were performed for two times: while increasing the voltage from zero to maximum (1st cycle up) and while decreasing the voltage from maximum to zero (1st cycle down). It is clear from Fig. 5a that N-doped carbon nanotubes showed good field emission characteristics with turn-on field, Eto (corresponding to the current density of 10 µA/cm2 ) and threshold field, Ethr (corresponding to the current density of 1 mA/cm2 ) is 1.8 V/µm and 2.53 V/µm respectively. The good field emission performances of the CNx nanotubes are attributed to the presence of nitrogen atoms in the framework of CNx nanotubes. In nitrogendoped carbon nanotubes, nitrogen atoms substitute the carbon atoms on the graphite layer and modify the conductance property of N-doped nanotubes by donating electrons [16]. The as-prepared N-doped nanotubes show high current density at low electric field,
Fig. 5. (a) Current density versus macroscopic field plot of N-doped carbon nanotubes (b) the corresponding Fowler–Nordheim plot.
which is suitable for various field emission devices. The maximum current density was found to be 6 mA/cm2 at 3 V/µm. Again for undoped carbon nanotubes it has been observed that the tiny metal particles encapsulated into the cavities of carbon nanotubes play an important role to improve the field electron emission [21,22]. Apart from the contribution of nitrogen atom, iron particle on the surface and cavities of nanotubes may also contribute additional electron density that leads to good field emission current. The corresponding Fowler–Nordheim plot is shown in Fig. 5b. It can be found from the F–N plot that ln(J/E2 ) versus 1/E yields straight line, which indicates that emission behavior follows the Fowler–Nordheim model. The field enhancement factor was calculated according to the Fowler–Nordheim equation: ! β2 E 2 J=A exp −Bφ3/2 /βE
φ
where J is emission current density, β is the field enhancement factor, φ is the work function and its value is assumed to be 5.0 eV, E is the electric field, A and B are constant (A = 1.54 × 10−6 AV−2 eV, B = 6.83 × 109 eV−3/2 V m−1 ). The field enhancement factor of the N-doped carbon nanotubes with nitrogen concentration of 6.6 at.% was approximately 2000 and this value is sufficient for applications of field emission devices. 4. Conclusion In summary, by simple chemical vapor deposition method at 900 ◦ C at atmospheric pressure we have prepared nitrogendoped bamboo-shaped aligned carbon nanotubes by pyrolyzing
P. Ghosh et al. / Solid State Communications 147 (2008) 15–19
monoethanolamine/ferrocene mixture. SEM and TEM/HRTEM indicated that as-prepared nanotubes are dense, well-aligned and have bamboo-like morphology. XPS analysis suggests that asprepared nitrogen-doped nanotubes contain 6.6 at.% of nitrogen content with three different types of nitrogen environment. Field emission measurements suggest that N-doped carbon nanotubes show nice field emission properties with a turn-on field and threshold field is 1.8 V/µm and 2.53 V/µm respectively and current density of 6 mA/cm2 was achieved at an applied field as low as 3 V/µm. The nitrogen-doped carbon nanotube with 6.6 at.% of nitrogen helps to enhance the conducting property due to the presence of lone pairs of electrons on nitrogen atom. The nitrogen-doped carbon nanotubes with high nitrogen content may be suitable to fabricate field emitter for various field emission devices. Using this inexpensive and effective method, it may be possible to control the dopant concentration on nanotubes by varying experimental parameters (temperature, flow rate of gas etc.) and hence the field emission performance. Acknowledgements One of the authors Pradip Ghosh is grateful to JASSO scholarship and 21st century COE program for financial support to carry out this work. The authors also wish to thank Dr. R. Katoh for his help in HRTEM measurements. References [1] O. Stephan, P.M. Ajayan, C. Colliex, P. Redlich, J.M. Lambert, P. Bernier, P. Lefin, Science 266 (1994) 1683. [2] Y. Miyamoto, M.L. Cohen, S.G. Louie, Solid State Commun. 102 (1997) 605.
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[3] D. Golberg, P.S. Dorozhkin, Y. Bando, Z.C. Dong, C.C. Tang, Y. Uemura, N. Grobert, M. Reyes-Reyes, H. Terrones, M. Terrones, Appl. Phys. A 76 (2003) 499. [4] M. Terrones, P.M. Ajayan, F. Banhart, X. Blase, D.L. Carroll, J.C. Charlier, R. Czerw, B. Foley, N. Grobert, R. Kamalakaran, P. Kohler-Redlich, M. Ruhle, T. Seeger, H. Terrones, Appl. Phys. A Mater. 74 (2002) 355. [5] S. Fan, M.G. Chapline, N.R. Franklin, T.W. Tombler, A.M. Cassell, H. Dai, Science 283 (1999) 512. [6] S.K. Srivastava, V.D. Vankar, D.V. Sridhar Rao, V. Kumar, Thin Solid Films 515 (2006) 1851. [7] X. Wang, Y. Liu, D. Zhu, L. Zhang, H. Ma, N. Yao, B. Zhang, J. Phys. Chem. B 106 (2002) 2186. [8] R.B. Sharma, D.J. Late, D.S. Joag, A. Govindaraj, C.N.R. Rao, Chem. Phys. Lett. 428 (2006) 102. [9] R.C. Che, L.M. Peng, M.S. Wang, Appl. Phys. Lett. 85 (2004) 4753. [10] M. Terrones, H. Terrones, N. Grobert, W.K. Hsu, Y.Q. Zhu, J.P. Hare, H.W. Kroto, D.R.M. Walton, Ph. Kohler-Redlich, M. Rühle, J.P. Zhang, A.K. Cheetham, Appl. Phys. Lett. 75 (1999) 3932. [11] R. Droppa Jr, C.T.M. Ribeiro, A.R. Zanatta, M.C. dos Santos, F. Alvarez, Phys. Rev. B 69 (2004) 045405. [12] T.-Y. Kim, K.-R. Lee, K.Y. Eun, K.-H. Oh, Chem. Phys. Lett. 372 (2003) 603. [13] S.Y. Kim, J. Lee, C.W. Na, J. Park, K. Seo, B. Kim, Chem. Phys. Lett. 413 (2005) 300. [14] H.C. Choi, J. Park, B. Kim, J. Phys. Chem. B 109 (2005) 4333. [15] M. Nath, B.C. Satishkumar, A. Govindaraj, C.P. Vinod, C.N.R. Rao, Chem. Phys. Lett. 322 (2000) 333. [16] M. Terrones, P. Redlich, N. Grobert, S. Trasobares, W.K. Hsu, H. Terrones, Y.Q. Zhu, J.P. Hare, C.L. Reeves, A.K. Cheetham, M. Rühle, H.W. Kroto, D.R.M. Walton, Adv. Mater. 11 (1999) 655. [17] Y. Xia, R. Mokaya, Adv. Mater. 16 (2004) 1553. [18] Y. Xia, R. Mokaya, Chem. Mater. 17 (2005) 1553. [19] J.R. Pels, F. Kapteijn, J.A. Moulijn, Q. Zhu, K.M. Thomas, Carbon 33 (1995) 1641. [20] M.J. Pelletier, Analytical Applications of Raman Spectroscopy, Blackwell Science, Malden, MA, 1999. [21] M. Kumar, T. Okazaki, M. Hiramatsu, Y. Ando, Carbon 45 (2007) 1899. [22] K.A. Dean, B.R. Chalamala, Appl. Phys. Lett. 76 (2000) 375.
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Field emission property of N-doped aligned carbon nanotubes grown by pyrolysis of monoethanolamine Pradip Ghosh ∗ , M. Tanemura, T. Soga, M. Zamri, T. Jimbo Department of Environmental Technology and Urban Planning, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, 466-8555, Japan
article
info
Article history: Received 2 February 2008 Received in revised form 23 April 2008 Accepted 25 April 2008 by S. Miyashita Available online 30 April 2008 PACS: 61.48.De 81.15.Gh 81.07.De 79.70.+q
a b s t r a c t Densely distributed bamboo-shaped nitrogen-doped aligned carbon nanotubes, grown on silicon substrate by thermal decomposition of monoethanolamine/ferrocene mixtures at 900 ◦ C, were investigated for field electron emission. The morphology and crystallinity of the as-grown carbon nanotubes were characterized by SEM, TEM and Raman spectroscopy. X-ray photoelectron spectroscopy was used to analyze the nitrogen concentration on carbon nanotubes and it was observed that nitrogen concentration on nanotubes was 6.6 at.%. Field emission study of as-grown nitrogen-doped carbon nanotubes suggests that they are good emitters with a turn-on and threshold field of 1.8 V/µm and 2.53 V/µm, respectively. The maximum current density was observed to be 6 mA/cm2 at 3 V/µm. It is considered that the nice field emission performance of CNx nanotube is due to the presence of lone pairs of electrons on nitrogen atom that supplies more electrons to the conduction band. © 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Carbon nanotubes B. Chemical vapor deposition D. Field electron emission
1. Introduction N-doped carbon nanotubes are considered as one of the promising materials for potential use in the upcoming field of nanoelectronics. Recently synthesis of N-doped carbon nanotubes have received a growing interest because incorporation of nitrogen atom into carbon nanotubes leads to unique bulk and surface properties on carbon nanotubes and a feasible strategy to tune the electronic property in a well-defined way [1]. On the basis of theoretical [2] and experimental studies [3], it has been predicted that CNx nanotubes have n-type conductivity, low resistance and good field electron emission property. The additional lone pairs of electrons on nitrogen atom with respect to the delocalized π-system of a graphite-like hexagonal network can enhance the conducting properties on N-doped carbon nanotubes [4]. One of the important advantages of this material is that the electronic property of the doped nanotubes is dependant on the dopant concentration and thus it is relatively easy to control this property by changing the dopant concentration into the bulk material. Therefore, considerable attempts have been made to investigate the preparation of nitrogen-doped carbon nanotubes.
∗ Corresponding author. Tel.: +81 52 735 7149; fax: +81 52 735 7120. E-mail address: [email protected] (P. Ghosh). 0038-1098/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2008.04.030
Alignment of carbon nanotube is very important for fundamental study and application in sensors and nanodevices, especially in scanning electron probes and flat panel displays [5]. There are few reports of synthesis of well-aligned nitrogen-doped carbon nanotubes and their application in field electron emission. Recently Srivastava et al. synthesized aligned N-doped carbon nanotubes and showed their field emission property with threshold field in the range of 2.65–3.55V/µm [6]. Zhang and his coworkers prepared aligned CNx nanotubes with a high content of nitrogen (x ≤ 9%) by pyrolyzing metal phthalocyanine on an n-type Si(100) substrate and studied their field emission characteristics [7]. Sharma et al. reported the synthesis of boron and nitrogen-doped carbon nanotubes on tungsten tips and silicon substrates and measured their field emission characteristics [8]. Similarly Che et al. proposed field emission properties of both horizontally aligned and vertically aligned N-doped carbon nanotubes synthesized by C/N feedstock using iron as a catalyst [9]. In this paper we reported the synthesis of well-aligned nitrogen-doped bamboo-shaped carbon nanotubes by pyrolysis of monoethanolamine/ferrocene mixture by chemical vapor deposition method at 900 ◦ C. The Structural characteristics of CNTs have been investigated using electron microscopy and Raman spectroscopy technique. The composition of nanotubes was determined using X-ray photoelectron spectroscopy (XPS). Systematic study of field emission measurements of N-doped carbon nanotubes was carried out.
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Fig. 1. SEM images of as-prepared N-doped aligned carbon nanotubes prepared by thermal decomposition of monoethanolamine/ferrocene mixture at 900 ◦ C (a) low magnification image of aligned N-doped nanotubes (b) high magnification image of N-doped CNTs.
2. Experimental Nitrogen-doped carbon nanotubes were prepared by chemical vapor deposition of monoethanolamine (NH2 CH2 CH2 OH) and ferrocene mixture on Si(100) substrate at atmospheric pressure. A 1 m long and 25 mm of inner diameter wide quartz tube was used as a reactor. The tube was placed inside the two horizontal electric furnaces. Before carbon nanotubes deposition, n-type Si(100) substrate was ultrasonically cleaned in acetone for 5 min followed by water and placed in a quartz boat and kept in the center of the second furnace. The mixture of ferrocene and monoethanolamine was kept in the first furnace. To expel air from the tube, nitrogen gas was sent through the tube at a flow rate of 50 ml/min for 15 min. The temperature of the second furnace was 900 ◦ C whereas temperature of the first furnace was 100 ◦ C. When both furnaces attained the desire temperature, the flow rate of the nitrogen gas was increased from 50 ml/min to 100 ml/min. The reaction was maintained for 60 min followed by 10 min annealing at the same temperature. The two furnaces were cooled down to room temperature under nitrogen atmosphere and the as-prepared material was collected for characterization. The as-grown materials were characterized by scanning electron microscopy (Hitachi S-3000H, scanning electron microscope), transmission electron microscopy (HITACHI HF 2000 with an acceleration voltage 200 kV) and Raman spectroscopy (JASCO, NRS1500W, green laser with an exciton wavelength of 532 nm), Xray photoelectron spectroscopy (SSX-100 XPS spectrometer using Al Kα X-ray source (1486.6 eV) under high vacuum condition of about 10−10 Torr). Field emission measurements were performed in a vacuum chamber at a pressure less than 3 × 10−9 Torr within UHV-SEM (JAMP-7100). The cathode consisted of as-grown Ndoped CNTs on n-type Si substrate, and anode was a polished stainless steel rod (1 mm in diameter). The distance between cathode and anode was 300 µm and it was carefully monitored in situ with the aid of UHV-SEM images. 3. Results and discussions 3.1. Morphology and structure of the well-aligned N-doped carbon nanotubes Fig. 1a and b shows the SEM images of the nitrogendoped carbon nanotubes synthesized at 900 ◦ C by catalytic decomposition of monoethanolamine at atmospheric pressure. SEM images show dense and well-aligned carbon nanotubes with respect to the Si substrate. The dense order pack of nanotubes comes from the effect of Van der walls force among
the neighboring nanotubes which results the formation of aligned nanotubes with almost uniform length. The length of the asprepared carbon nanotubes was approximately 50 µm. TEM/HRTEM images of N-doped carbon nanotubes are displayed in Figs. 2a–d. The as-prepared carbon nanotubes were dispersed in ethanol and a drop of suspension was put onto the holey carbon grid. TEM images clearly revealed that as-prepared nanotubes mostly exhibited bamboo-like structure with wellseparated transverse bridge compartments (Fig. 2a). The detailed structures of the N-doped carbon nanotubes are given by HRTEM images. HRTEM image clearly indicates the defects and roughness within the wall of carbon nanotubes (Fig. 2b). The defects are attributed to the presence of nitrogen atom by formation of pyridinelike bond with carbon atom [10]. It is observed that incorporation of nitrogen atom into carbon nanotubes network not only promotes bamboo-shaped structure but also introduce defects into the tube. Fig. 2c shows the presence of cone structure in tube that is common for N-doped bamboo-shaped carbon nanotubes. HRTEM image also indicates the presence of amorphous carbon coating on the outer surface of the carbon nanotubes (Fig. 2d). Fig. 2d shows that walls are constructed from ∼10 graphene layers with inner diameter ∼15 nm. 3.2. XPS characterization To obtain the information about the elemental composition and bonding environment of N-doped carbon nanotubes, XPS analysis was carried out. The measured XPS spectra indicate the presence of carbon, nitrogen and oxygen in the as-prepared material. Apart from these elements, peak of iron was also observed in the asgrown N-doped carbon nanotubes. The strong intensity of the C 1s peak indicates the presence of carbon nanotubes in the asprepared material. The oxygen peak might arise from SiO2 barrier film on the Si substrate or air adsorbed at the specimen surface. The amount of oxygen was found to be 1.06 at.%. Fig. 3a shows the wide scan XPS spectra of N-doped carbon nanotubes containing 6.6 at.% of nitrogen, prepared at 900 ◦ C by decomposition of monoethanolamine using ferrocene as a catalyst. Figs. 3b and c shows the C 1s and N 1s signal of N-doped carbon nanotubes respectively. The C 1s spectrum of N-doped carbon nanotubes consist of two components at 284.6 and 285.3 eV which is shown in Fig. 3b. The strongest peak at 284.6 eV indicates that carbon is mostly in the form of graphite and assigned to sp2 aromatic hydrocarbons [11,12]. The peak at 285.3 eV is assigned to C–N bond [13]. Fig. 3c displays the N 1s spectra of as-prepared N-doped carbon nanotubes. From XPS spectra, the nitrogen concentration on asgrown carbon nanotubes was calculated to be 6.6 at.%. N 1s signal
P. Ghosh et al. / Solid State Communications 147 (2008) 15–19
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Fig. 2. TEM/HRETM images of as-grown N-doped nanotubes (a) low magnification image of N-doped carbon nanotubes, showing bamboo-like morphology (b) HRTEM image of carbon nanotubes, showing outer wall of N-doped carbon nanotubes and arrow mark indicates the defects within the wall (c) cone structure of N-doped carbon nanotubes (d) HRTEM image of N-doped carbon nanotubes which shows the amorphous carbon coating on the outer wall of carbon nanotubes.
Fig. 3. XPS spectra of CNx nanotubes (a) wide scan XPS spectra of as-prepared sample (b) C 1s and (c) N 1s core level spectrum.
P. Ghosh et al. / Solid State Communications 147 (2008) 15–19
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Fig. 4. Raman spectra of as-prepared N-doped carbon nanotubes.
was deconvoluted into three bands at 398.4, 401 and 404.8 eV. The diverse electronic structures of N atoms including the “graphitic-” and “pyridine-” like structures and N–O bonds were identified by XPS [14–16]. According to the theoretical calculation and experimental results, the band at 398.4 and 401 eV corresponds to the “pyridinic” and “graphitic” nitrogen respectively [17,18] and the band at 404.8 eV corresponds to some oxidized N-species i.e. N–O bonds [19]. 3.3. Raman spectroscopy analysis To obtain the information regarding the degree of crystallinity of carbon nanotubes, Raman spectroscopy was employed. The Raman spectrum of as-prepared N-doped carbon nanotube is shown in Fig. 4. The spectrum shows two bands at 1357 cm−1 (Dband) and 1591 cm−1 (G-band). The G-band corresponds to the C–C stretching (E2g ) mode for typical graphite and D-band is ascribed to the defects and disorder in graphene sheets. The defects and disorders are related to the presence of nitrogen atom in the asprepared carbon nanotubes [20]. Due to presence of nitrogen atom defects and disorders are introduced into the carbon nanotubes. The ratio of ID /IG is directly related to the extent of disorders in the as-prepared nanotubes. From Raman spectrum ID /IG was found to be 0.97. The high ID /IG value indicates that the crystalline perfection of the as-prepared nanotubes was less. The presence of nitrogen atom tends to introduce disorders in the graphene planes.
3.4. Field emission property of N-doped CNTs The field emission current density vs. electric field of N-doped carbon nanotube is shown in Fig. 5. For the reliability of the data, field emission measurements were performed for two times: while increasing the voltage from zero to maximum (1st cycle up) and while decreasing the voltage from maximum to zero (1st cycle down). It is clear from Fig. 5a that N-doped carbon nanotubes showed good field emission characteristics with turn-on field, Eto (corresponding to the current density of 10 µA/cm2 ) and threshold field, Ethr (corresponding to the current density of 1 mA/cm2 ) is 1.8 V/µm and 2.53 V/µm respectively. The good field emission performances of the CNx nanotubes are attributed to the presence of nitrogen atoms in the framework of CNx nanotubes. In nitrogendoped carbon nanotubes, nitrogen atoms substitute the carbon atoms on the graphite layer and modify the conductance property of N-doped nanotubes by donating electrons [16]. The as-prepared N-doped nanotubes show high current density at low electric field,
Fig. 5. (a) Current density versus macroscopic field plot of N-doped carbon nanotubes (b) the corresponding Fowler–Nordheim plot.
which is suitable for various field emission devices. The maximum current density was found to be 6 mA/cm2 at 3 V/µm. Again for undoped carbon nanotubes it has been observed that the tiny metal particles encapsulated into the cavities of carbon nanotubes play an important role to improve the field electron emission [21,22]. Apart from the contribution of nitrogen atom, iron particle on the surface and cavities of nanotubes may also contribute additional electron density that leads to good field emission current. The corresponding Fowler–Nordheim plot is shown in Fig. 5b. It can be found from the F–N plot that ln(J/E2 ) versus 1/E yields straight line, which indicates that emission behavior follows the Fowler–Nordheim model. The field enhancement factor was calculated according to the Fowler–Nordheim equation: ! β2 E 2 J=A exp −Bφ3/2 /βE
φ
where J is emission current density, β is the field enhancement factor, φ is the work function and its value is assumed to be 5.0 eV, E is the electric field, A and B are constant (A = 1.54 × 10−6 AV−2 eV, B = 6.83 × 109 eV−3/2 V m−1 ). The field enhancement factor of the N-doped carbon nanotubes with nitrogen concentration of 6.6 at.% was approximately 2000 and this value is sufficient for applications of field emission devices. 4. Conclusion In summary, by simple chemical vapor deposition method at 900 ◦ C at atmospheric pressure we have prepared nitrogendoped bamboo-shaped aligned carbon nanotubes by pyrolyzing
P. Ghosh et al. / Solid State Communications 147 (2008) 15–19
monoethanolamine/ferrocene mixture. SEM and TEM/HRTEM indicated that as-prepared nanotubes are dense, well-aligned and have bamboo-like morphology. XPS analysis suggests that asprepared nitrogen-doped nanotubes contain 6.6 at.% of nitrogen content with three different types of nitrogen environment. Field emission measurements suggest that N-doped carbon nanotubes show nice field emission properties with a turn-on field and threshold field is 1.8 V/µm and 2.53 V/µm respectively and current density of 6 mA/cm2 was achieved at an applied field as low as 3 V/µm. The nitrogen-doped carbon nanotube with 6.6 at.% of nitrogen helps to enhance the conducting property due to the presence of lone pairs of electrons on nitrogen atom. The nitrogen-doped carbon nanotubes with high nitrogen content may be suitable to fabricate field emitter for various field emission devices. Using this inexpensive and effective method, it may be possible to control the dopant concentration on nanotubes by varying experimental parameters (temperature, flow rate of gas etc.) and hence the field emission performance. Acknowledgements One of the authors Pradip Ghosh is grateful to JASSO scholarship and 21st century COE program for financial support to carry out this work. The authors also wish to thank Dr. R. Katoh for his help in HRTEM measurements. References [1] O. Stephan, P.M. Ajayan, C. Colliex, P. Redlich, J.M. Lambert, P. Bernier, P. Lefin, Science 266 (1994) 1683. [2] Y. Miyamoto, M.L. Cohen, S.G. Louie, Solid State Commun. 102 (1997) 605.
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[3] D. Golberg, P.S. Dorozhkin, Y. Bando, Z.C. Dong, C.C. Tang, Y. Uemura, N. Grobert, M. Reyes-Reyes, H. Terrones, M. Terrones, Appl. Phys. A 76 (2003) 499. [4] M. Terrones, P.M. Ajayan, F. Banhart, X. Blase, D.L. Carroll, J.C. Charlier, R. Czerw, B. Foley, N. Grobert, R. Kamalakaran, P. Kohler-Redlich, M. Ruhle, T. Seeger, H. Terrones, Appl. Phys. A Mater. 74 (2002) 355. [5] S. Fan, M.G. Chapline, N.R. Franklin, T.W. Tombler, A.M. Cassell, H. Dai, Science 283 (1999) 512. [6] S.K. Srivastava, V.D. Vankar, D.V. Sridhar Rao, V. Kumar, Thin Solid Films 515 (2006) 1851. [7] X. Wang, Y. Liu, D. Zhu, L. Zhang, H. Ma, N. Yao, B. Zhang, J. Phys. Chem. B 106 (2002) 2186. [8] R.B. Sharma, D.J. Late, D.S. Joag, A. Govindaraj, C.N.R. Rao, Chem. Phys. Lett. 428 (2006) 102. [9] R.C. Che, L.M. Peng, M.S. Wang, Appl. Phys. Lett. 85 (2004) 4753. [10] M. Terrones, H. Terrones, N. Grobert, W.K. Hsu, Y.Q. Zhu, J.P. Hare, H.W. Kroto, D.R.M. Walton, Ph. Kohler-Redlich, M. Rühle, J.P. Zhang, A.K. Cheetham, Appl. Phys. Lett. 75 (1999) 3932. [11] R. Droppa Jr, C.T.M. Ribeiro, A.R. Zanatta, M.C. dos Santos, F. Alvarez, Phys. Rev. B 69 (2004) 045405. [12] T.-Y. Kim, K.-R. Lee, K.Y. Eun, K.-H. Oh, Chem. Phys. Lett. 372 (2003) 603. [13] S.Y. Kim, J. Lee, C.W. Na, J. Park, K. Seo, B. Kim, Chem. Phys. Lett. 413 (2005) 300. [14] H.C. Choi, J. Park, B. Kim, J. Phys. Chem. B 109 (2005) 4333. [15] M. Nath, B.C. Satishkumar, A. Govindaraj, C.P. Vinod, C.N.R. Rao, Chem. Phys. Lett. 322 (2000) 333. [16] M. Terrones, P. Redlich, N. Grobert, S. Trasobares, W.K. Hsu, H. Terrones, Y.Q. Zhu, J.P. Hare, C.L. Reeves, A.K. Cheetham, M. Rühle, H.W. Kroto, D.R.M. Walton, Adv. Mater. 11 (1999) 655. [17] Y. Xia, R. Mokaya, Adv. Mater. 16 (2004) 1553. [18] Y. Xia, R. Mokaya, Chem. Mater. 17 (2005) 1553. [19] J.R. Pels, F. Kapteijn, J.A. Moulijn, Q. Zhu, K.M. Thomas, Carbon 33 (1995) 1641. [20] M.J. Pelletier, Analytical Applications of Raman Spectroscopy, Blackwell Science, Malden, MA, 1999. [21] M. Kumar, T. Okazaki, M. Hiramatsu, Y. Ando, Carbon 45 (2007) 1899. [22] K.A. Dean, B.R. Chalamala, Appl. Phys. Lett. 76 (2000) 375.