Controllable Growth, Structure, and Low Field Emission of Well

Nov 28, 2001 - measurements suggest that the CNx nanotubes began to emit ... The controllable synthesis of well-aligned CNx nanotubes with high N rati...
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J. Phys. Chem. B 2002, 106, 2186-2190

Controllable Growth, Structure, and Low Field Emission of Well-Aligned CNx Nanotubes Xianbao Wang,† Yunqi Liu,*,† Daoben Zhu,*,† Lan Zhang,‡,§ Huizhong Ma,‡ Ning Yao,‡ and Binglin Zhang*,‡ Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. China, Department of Physics, Zhengzhou UniVersity, Zhengzhou 450052, P. R. China, and Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, P. R. China ReceiVed: August 3, 2001; In Final Form: NoVember 28, 2001

A large area and controllable synthesis of well-aligned CNx nanotubes with a high content of nitrogen (xE9%) was carried out by pyrolyzing metal phthalocyanine on an n-type Si(100) substrate. The diameters of the CNx nanotubes range widely from 20 to 200 nm, and the lengths range from 1 to 100 µm. The impressive bamboolike CNx nanotubes consist of a few uniform, small, and well-ordered compartments. Investigation on morphology and elemental composition of the CNx nanotubes suggests that the overall tube morphology depends strongly on the nitrogen concentration. The higher the N content, the more compartmentalized of nanotubes become, which results in the formation of more curved CNx nanotubes. Our studies show that three different types of N atoms can be present in these materials. These are “pyridinic”, “pyrrolic”, and “graphitic” nitrogen with binding energies of 398.1, 401.0, and 405.1 eV, respectively. Field emission measurements suggest that the CNx nanotubes began to emit electrons at an electric field of 1.5 V/µm, and current densities of 80 µA/cm2 have been realized at an applied field as low as 2.6 V/µm. Doping carbon nanotubes with N enhances their electron-conducting properties because of the presence of additional lone pairs of electrons that act as donors with respect to the delocalized π system of the hexagonal framework. The controllable synthesis of well-aligned CNx nanotubes with high N ratio may open a route to improve the field emission properties of nanotubes.

1. Introduction Carbon nanotubes (CNTs) show a variety of electronic behaviors from metallic to semiconducting, depending on composition, chirality, and diameter according to both theoretical1,2 and experimental3 studies. This leads to a diverse spectrum of properties, but it is also highly complex from an application point of view, as the tube chirality and diameter are impossible to control, regardless of some meaningful works,4-6 with the use of current synthesis methods. Synthesis of B- or N-doped CNTs is possibly a method to control the electronic properties of nanotubes in a well-defined way. BC nanotubes offer the possibility of greater electrical conductivity relative to CNTs because the electron deficiency of each boron atom creates a hole-carrier in the valence band. A similar enhancement of the conductivity is expected for CNx nanotubes, because the additional electrons contributed by the nitrogen atoms provide electron carriers for the conduction band. The advantage of such nanotubes is that their electronic properties are primarily determined by composition and are thus relatively easy to control. The synthesis and electronic properties of such heteroatomic nanotubes are topics of current research.7-11 More interestingly, crystalline CNx nanotubes proposed as superhard materials may have a bulk modulus somewhat lower than that of diamond.12,13 CNx nanotubes have resulted in extensive investigations, because these materials are predicted * To whom correspondence should be addressed. E-mail: liuyq@ infoc3.icas.ac.cn. Tel: +86-10-62613253. Fax: +86-10-62559373. † Chinese Academy of Sciences, Beijing. ‡ Zhengzhou University. § Chinese Academy of Sciences, Hefei.

as possible candidates for nanosized electronic and photonic devices and mechanical materials in the future applications. However, to date, the synthesis of crystalline CNx nanotubes remains a challenge, because only the low content of nitrogen has been incorporated into CNTs.8,11,14 Alignment of nanotubes15 is important to enable both fundamental studies and applications, such as scanning probe sensors and nanoelectronics, especially cold cathode flat panel displays. Terrones et al.11 reported the generation of aligned C13Nx (x E 1) nanofibers by pyrolyzing melamine over laserpatterned Fe and Ni substrates. Nath et al.9 reported also that aligned CNx nanotubes can be obtained by carrying out the pyrolysis of pyridine on silica-supported Fe and Co catalysts. However, the alignments are inhomogeneous with some curved nanotubes loosely stacking, and the production of CNx nanotubes with low percentages of nitrogen leads to a complex catalyst deposition process. Recently, well-aligned CNx nanotube films16 have been synthesized by microwave plasma enhanced chemical vapor deposition. Although the electron emission property of these nanotubes was reported with a turn-on field of 0.8 V/µm, the field emission data were not quite consistent with the Fowler-Nordheim theory.17 In the present study, we report a large area and controllable synthesis of well-aligned CNx nanotubes with a high content of nitrogen (x E 9%) by pyrolyzing metal phthalocyanine on an n-type Si(100) substrate. The diameters of the CNx tubes range widely from 20 to 200 nm, and the lengths range from 1 to 100 µm. Investigation on morphology and elemental composition of the CNx nanotubes suggests that the overall tube morphology depends strongly on the nitrogen concentration. The

10.1021/jp013007r CCC: $22.00 © 2002 American Chemical Society Published on Web 02/09/2002

Well-Aligned CNx Nanotubes

Figure 1. Schematic diagram of the experimental setup used for the measurement of field emission.

impressive bamboo-like CNx nanotubes consist of a few uniform, small, and well-ordered compartments. Field emission measurements suggest that the CNx nanotubes began to emit electrons at an electric field of 1.5 V/µm, and current densities of 80 µA/cm2 have been realized at an applied field as low as 2.6 V/µm. Doping CNTs with N enhances their electronconducting properties because of the presence of additional lone pairs of electrons that act as donors with respect to the delocalized π system of the hexagonal framework. 2. Experimental Section The well-aligned CNx nanotubes were synthesized by pyrolysis of iron (II) phthalocyanine (FePc) in an NH3 flow at 850 °C on an n-type Si (100) substrate. The preparation of the CNx nanotubes is the same method as that of aligned CNTs,4 except where Ar/H2 gases and a quartz glass substrate were replaced by the NH3 atmosphere and Si (100) plate, respectively. An n-type Si (100) plate was placed in the center of a flow reacter consisting of a quartz glass tube and a furnace fitted with an independent temperature controller. A flow of NH3 (40 cm3 min-1) was then introduced into the quartz tube during heating. After the central region of the furnace reached 850 °C, a quartz boat with 0.1 g of FePc was placed in the region where the temperature was about 550 °C. After heating a certain time, CNx nanotubes grew in a direction normal to the substrate

J. Phys. Chem. B, Vol. 106, No. 9, 2002 2187 surface. A H2 flow allows the furnace to cool to room temperature. The weight of FePc and the heating time are crucial in controlling the diameters and lengths of aligned CNx nanotubes. The CNx nanotubes were examined by a scanning electron microscopy (SEM; JSM-6301F) to measure the alignment, configuration, and length of nanotubes. Highly resolution transmission microscopy (HRTEM; JEOL 2010F) equipped with a parallel electron energy loss spectroscopy detector (EELS, Gatan PEELS 666) and X-ray photoelectron spectroscopy (XPS; VG ESCALAB 220-IXL; Al KR) were used to determine the diameter, microstructure, and elemental composition of CNx nanotubes. The field emission measurement was carried out in a vacuum chamber with a 3.75 × 10-7 Torr base pressure by applying voltages up to 1000 V. Figure 1 shows the schematic diagram of the experimental setup. The glass coated with transparent indium-tin-oxide (ITO) was used as an anode to collect electrons from CNx nanotubes. The aligned CNx nanotubes, which were attached to an n-type Si (100) substrate, were used as the cathode. The spacer between the anode and cathode was a 267 µm insulating sheet of polyimide film. The macroscopic electric field was estimated by dividing the applied voltage by the sample-anode separation. The emission current density was calculated from the obtained emission current and the surface area of the CNx nanotube film emitting electron. 3. Results and Discussion 3.1. Morphologies and Structures of the Well-Aligned CNx Nanotubes. Figure 2 is an SEM micrography showing a large area (up to 40 × 60 mm) of well-aligned CNTs perpendicular to the surface of the Si (100) substrate. The dense order pack of the nanotubes, coming from the effect of van der Waals force, results in the formation of aligned CNx nanotubes with uniform diameters and lengths. The observation by SEM indicates that the length of well-aligned CNx nanotubes (namely, the thickness of the alignment film) increases with the increase of the growth

Figure 2. SEM image of well-aligned CNx nanotube films showing uniform diameters and lengths.

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Wang et al.

Figure 3. TEM images of (a) the curved CNx nanotubes consisting of impressive, uniform, and well-ordered bamboo-like compartments and (b) the straight CNTs.

Figure 4. EELS spectrum of the CNx nanotubes which revealed the presence of ionization edges at ca. 284.6 and 400 eV corresponding to the C and N K shell, respectively.

time, which is in agreement with that of the CNTs.4 The length of well-aligned CNx nanotubes can be controlled in a range of 1-100 µm by adjusting the growth time in our sample. A TEM image of the CNx nanotubes shown in Figure 3a suggests that the diameter slightly ranges from 70 to 80 nm in the current sample. The open ends (Figure 3a, arrow) without metal particles indicate that the CNx nanotubes grow in “basegrowth” mechanism.18 The diameters of aligned CNx nanotubes increase with increasing growth time. The CNx nanotube diameters are strongly correlated to the catalyst grain size because catalyst grains act as nucleation seeds in the tube growth. The longer the growth time, the larger the iron catalyst particles by aggregation of iron atom (produced from pyrolysis of FePc) preferentially left on the substrate,18b which accounts for the larger diameters. Therefore, the grain sizes of the iron catalysts play an important role in the CNx diameters. A typical

synthesis experiment of aligned CNx nanotubes with diameters of 70-80 nm shown in Figure 2 and Figure 3a is that pyrolysis of 0.5 g FePc was carried out under a flow of 40 mL/min NH3 at 850 °C on an n-type Si (100) substrate for five minutes. The diameters can be controlled in a range of 20-200 nm by adjusting the growth condition. The remarkable characteristic of the resulting CNx nanotubes shown in Figure 3a is the impressive bamboo-like tube, which consists of uniform, clear, and well-ordered compartments. In contrast to the straight CNTs (Figure 3b) produced by the same method,4 the CNx tubes are curved because of imperfect interlink of the bamboo-like compartments. Clearly, doping with nitrogen modifies the morphology of the nanotubes drastically. We note that the overall nanotube morphology depends strongly on the N concentration, which is consistent with the earlier report.11 The higher the N content, the more compartmentalized of

Well-Aligned CNx Nanotubes

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Figure 5. Wide survey XPS spectrum of the CNx nanotubes. Inset, a Gaussian fit of the π*-type peak of the N 1s spectrum reveals the presence of three peaks at 398.1, 401.0, and 405.1 eV.

Figure 6. Emission current vs field characteristics of the well-aligned CNx nanotubes on an n-type Si(100) substrate (emitting surface area ) 0.5 cm2). The inset shows the corresponding Fowler-Nordheim plot of emission currents.

nanotubes become, which results in the formation of more curved CNx nanotubes. A part of the nitrogen atoms in the CNx nanotubes that belongs to pentagons within the graphites layers as suggested by XPS spectrum (see section 3.2) may provide an explanation as to why nitrogen-doped CNTs do not grow straight along to the tube axis. The incorporation of pentagons or buckling layers induced a curvature of the base planes that prevents growth of straight CNx nanotubes. 3.2. Elemental Composition and the Bonding Character of the Well-Aligned CNx Nanotubes. An EELS spectrum of the CNx nanotubes revealed the presence of ionization edges at ca. 284.6 and 400 eV corresponding to the C and N K shell, respectively (Figure 4). The small peak at 284.6 eV is due to

transitions from the carbon 1s core level to the π* band, and the band starting at 291.5 eV corresponds to transitions to the p orbital projected part of the broad σ* band,19 which come from the sp2 hybridization in the case of carbon. The narrow π* peak and the considerable intensity of the σ* peak compared with the π* peak result from the large diameter and small curvature of the tube.20 The weaker peak of the N K-edge, in contrast to the stronger peak of the C K-edge, suggests that the N content within the CNx nanotubes was ca. E9%, commensurate with C11Nx (x E 1) stoichiometries. To obtain additional information about the elemental composition and bonding character of CNx nanotubes, XPS analysis was carried out. Figure 5 shows the C 1s, N 1s, and O 1s signals,

2190 J. Phys. Chem. B, Vol. 106, No. 9, 2002 corresponding to the main peaks centered at 284.5, 400.7, and 532.9 eV, respectively. A sharp peak at 284.5 eV, corresponding to a π* feature associated with sp2 hybridized carbon, confirms that the nanotubes mainly contain carbon. The peak at 532.9 eV arises from oxygen absorbed on the surface of the CNx nanotubes. A Gaussian fit of the π*-type peak of the N 1s spectrum (Figure 5. inset) reveals the presence of three peaks at 398.1, 401.0, and 405.1 eV. This suggests the N atom in the three different bonding characters inserted into the graphite network. These are “pyridinic”, “pyrrolic”, and “graphitic” nitrogen with binding energies of 398.1, 401.0, and 405.1 eV, respectively, according to theoretical calculation and experimental report.21 Here, we must point out that the terms pyridinic and pyrrolic are used in a rather broad sense; the first one is used to refer to N atoms which contribute to the π system with one p electron, whereas the second refers to N atoms with two p electrons on the π system although not necessarily in a fivemembered ring as in pyrrole. The “graphitic” nitrogen corresponds to highly coordinated N atoms substituting inner C atoms on the graphite layers. The pyrrolic or the substitutional N atoms in a graphite sheet strongly favor the formation of pentagons and curvatures, which is responsible for the impressive bamboolike morphologies and the bending CNx nanotubes. The carbonto-nitrogen ratio, estimated by taking the ratio of the integrated peak areas under the C 1s and N 1s signals and dividing them by the respective photoionization cross-sections, can be turned out to be up to 11:1, in agreement with the above results by EELS. 3.3. Field Emission Properties of the Well-Aligned CNx Nanotubes. The field emission current was measured as a function of applied voltage at a pressure of 3.75 × 10-7 Torr for a spacer of 267 µm between a cathode of the CNx nanotube devices and an anode of indium-tin-oxide (ITO) coated glass plate shown in Figure 6. The Fowler-Nordheim model17 was used to analyze our field emission data. A linear relation was observed in the ln(I/V2) versus 1/V characteristics (Figure 6 inset), suggesting that the current indeed results from the field emission of the CNx nanotubes. The turn-on field for wellaligned CNx nanotube emitters was as low as 1.5 V/µm, and current densities of 80 µA/cm2 were observed at an applied field of 2.6 V/µm. It is well-known that the field emission performances of the nanotubes are strongly correlated to the morphologies of the tube tips, such as a radius of curvature and open/closed ends, etc. Recently, Yuan et al.22 and Saito et al.23 showed that the open tubes began to emit electrons at a lower applied voltage than the closed ones. Their reasoning is that the open-end nanotubes have circular sharp edges, which may have a smaller radius of curvature than the closed-end nanotubes. However, our experimental results indicated that the closedend CNx nanotubes with large diameters of 70-80 nm have a lower turn-on field and a higher emission current density at the same electric field than the open-end nanotubes with small diameters of 55 nm.22 The most reasonable explanation is that good field emission performances of the CNx nanotubes are derived from doping with N atoms. To confirm this argument, we compare the emission properties of the CNx tubes with those of the CNTs produced by the same synthesis method. It is found that the CNx nanotubes have lower turn-on field and higher current density than the CNTs (CNTs: turn-on field of 2.6 V/µm, 14 µA/cm2 at 4.8 V/µm). Doping graphite-like C structures with N may enhance their electron-conducting properties because of the presence of additional lone pairs of electrons that act as donors with respect to the delocalized π system of the hexagonal framework.11

Wang et al. 4. Conclusions In summary, we report a large area and controllable synthesis of well-aligned CNx nanotubes with a high content of nitrogen (x E 9%) by pyrolyzing metal phthalocyanine on an n-type Si(100) substrate. The diameters of the CNx tubes range widely from 20 to 200 nm, and the lengths range from 1 to 100 µm. The impressive bamboo-like CNx nanotubes consist of a few uniform, small, and well-ordered compartments. Our studies show that three different types of N atoms can be present in these materials. These are “pyridinic”, “pyrrolic”, and “graphitic” nitrogen with binding energies of 398.1, 401.0, and 405.1 eV, respectively. Field emission measurements suggest that doping CNTs with N enhances their electron-conducting properties because of the presence of additional lone pairs of electrons that act as donors with respect to the delocalized π system of the hexagonal framework. The controllable synthesis of wellaligned CNx nanotubes with high N ratio may open a route to improve the field emission properties of nanotubes. Acknowledgment. The authors gratefully acknowledge financial supports from the National Natural Science Foundation of China (NNSFC), the Major State Basic Research Development Program, and the Chinese Academy of Sciences. References and Notes (1) Hamada, N.; Sawada, S.; Oshiyama, A. Phys. ReV. Lett. 1992, 68, 1579. (2) Saito, K.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Appl. Phys. Lett. 1992, 60, 2204. (3) Odom, T. W.; Huang, J.-L.; Kim, P.; Lieber, C. M. J. Phys. Chem. B 2000, 104, 2794. (4) Wang, X. B.; Liu, Y. Q.; Zhu, D. B. Chem. Phys. Lett. 2001, 340, 419. (5) Willems, I.; Konya, Z.; Colomer, J.-F.; Tendeloo, G. V.; Nagaraju, N.; Fonseca, A.; Nagy, J. B. Chem Phys. Lett. 2000, 71, 317. (6) Choi, Y. G.; Shin, Y. M.; Lee, Y. H.; Lee, B. S.; Park, G.-S.; Choi, W. B.; Lee, N. S.; Kim, J. M. Appl. Phys. Lett. 2000, 76, 2367. (7) Chopa, N. G.; Luyken, R. J.; Cherrey, K.; Crespi, V. H.; Cohen, M. L.; Louie, S. G.; Zettl, A. Science 1995, 269, 966. (8) (a) Suenaga, K.; Johansson, M. P.; Hellgren, N.; Broitman, E.; Wallenberg, L. R.; Colliex, C.; Sundgren, J.-E.; Hutman, L. Chem. Phys. Lett. 1999, 300, 695. (b) Suenaga, K.; Colliex, C.; Demoncy, N.; Loiseau, A.; Pascard, H.; Willaime, F. Science 1997, 278, 653. (9) Nath, M.; Satishkumar, B. C.; Govindaraj, A.; Vinod, C. P.; Rao, C. N. R. Chem. Phys. Lett. 2000, 322, 333. (10) Han, W. Q.; Cumings, J.; Zettl, A. Appl. Phys. Lett. 2001, 78, 2769. (11) Terrones, M.; Redlich, P.; Grobert, N.; Trasobares, S.; Hsu, W.K.; Terrones, H.; Zhu, Y.-Q.; Hare, J. P.; Reeves, C. L.; Cheetham, A. K.; Ru¨hle, M.; Kroto, H. W.; Walton, D. R. M. AdV. Mater. 1999, 11, 655. (12) Liu, A. Y.; Cohen, M. L. Science 1989, 245, 841. (13) Teter, D. M.; Hemley, R. J. Science 1996, 271, 53 (14) Sen, R.; Satishkumar, B. C.; Govindaraj, A.; Harikumar, K. R.; Raina, G.; Zhang, J.-P.; Cheetham, A. K.; Rao, C. N. R. Chem. Phys. Lett. 1998, 287, 671. (15) (a) Li, W. Z.; Xie, S. S.; Qian, L. X.; Chang, B. H.; Zou, B. S.; Zhou, W. Y.; Zhao, R. A.; Wang, G. Science 1996, 274, 1701. (b) Ren, Z. F.; Huang, Z. P.; Xu, J. W.; Wang, J. H.; Bush, P.; Siegal, M. P.; Provencio, P. N. Science 1998, 282, 1105. (c) Fan, S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassall, A. M.; Dai, H. Science 1999, 283, 512. (16) Zhong, D. Y.; Liu, S.; Zhang, G. Y.; Wang, E. G. J. Appl. Phys. 2001, 89, 5939. (17) Fowler, R. H.; Nordheim, L. W. Proc. R. Soc. London A 1928, 119, 173. (18) (a) Amelinckx, S.; Zhang, X. B.; Bernaerts, D.; Zhang, X. F.; Ivanov, V.; Nagy, J. B. Science 1994, 265, 635. (b) Wang, X. B.; Hu, W. P.; Liu, Y. Q.; Long, C. F.; Xu, Y.; Zhou, S. Q.; Zhu, D. B.; Dai, L. M. Carbon 2001, 39, 1533. (19) Ste´phan, O.; Ajayan, P. M.; Colliex, C.; Cyrot-Lackmann, F.; Sandre´, E¨ . Phys. ReV. B 1996, 53, 13824. (20) Suenaga, K.; Colliex, C.; Iijima, S. Appl. Phys. Lett. 2001, 78, 70. (21) Casanovas, J.; Ricart, J. M.; Rubio, J.; Illas, F.; Jimenez-Mateos, J. M. J. Am. Chem. Soc. 1996, 118, 8071. (22) Yuan, Z. H.; Huang, H.; Dang, H. Y.; Cao, J. E.; Hu, B. H.; Fan, S. S. Appl. Phys. Lett. 2001, 78, 3127. (23) Saito, Y.; Hamaguchi, K.; Hata, K.; Uchida, K.; Tasaka, Y.; Ikazaki, F.; Yumura, M.; Kasuya, A.; Nishina, Y. Nature 1997, 389, 554.