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Formation of carbon nanofibers from decacyclene by ion beam irradiation
Nuclear Instruments and Methods in Physics Research B 236 (2005) 474–481 www.elsevier.com/locate/nimb
Formation of carbon nanofibers from decacyclene by ion beam irradiation Takahide Kimura a, Hitoshi Koizumi a, Hiroshi Kinoshita b, Tsuneki Ichikawa a
a,*
Division of Materials Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan Center for Advanced Research of Energy Conversion Materials, Hokkaido University, Sapporo 060-8628, Japan
b
Available online 19 May 2005
Abstract Amorphous carbon nanofibers with lengths of 1–20 lm, uniform diameters of 20–100 nm were generated by ion beam-irradiation of decacyclene whiskers that were prepared on a flat plate of any material during vapor-deposition of decacyclene, a polycyclic aromatic compound, in vacuum for only a few minutes. The length of the whiskers did not depend on the duration of the vapor-deposition, whereas the number density of the whiskers increased with increasing the duration and reached 6.9 · 107 cm 2 at 4.5 min after starting the vapor-deposition. Irradiation of more than 1016 ions/cm2 with a 200 keV N+ ion beam converted the whiskers to carbon nanofibers with the electrical conductivity comparable to that of graphite. This novel method of preparation is advantageous for aligning the fibers on a desired location, since the fibers are generated only on the irradiated part at ambient temperature without catalyst. 2005 Elsevier B.V. All rights reserved. PACS: 41.75.A; 61.82.P; 68.70; 79.20.R; 81.05.L; 81.40.R Keywords: Carbon nanofiber; Ion beam; Radiation effect; Decacyclene; Whisker; Electrical conductivity; VLS growth mechanism; Polycyclic aromatic compound
1. Introduction Carbon nanofibers (CNFs) and nanotubes have attracted considerable attention in recent years due to their potential utilities as a hydrogen stor*
Corresponding author. Tel.: +81 11 706 6747; fax: +81 11 706 7897. E-mail address: [email protected] (T. Ichikawa).
age material for fuel cells [1], electron field emitter for optical display [2–6], support for catalysts [7], electrode for lithium ion battery [8], and so on. The mass production of CNFs has usually been carried out by using chemical vapor deposition (CVD) in which CNFs with uniform diameters ranging from 0.01–200 lm are grown from catalyst particles by thermal decomposition of hydrocarbon gas under high temperature [9,10]. Several
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.04.022
T. Kimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 474–481
methods have recently been proposed for increasing the yield of CNFs [11–13], though they are essentially the modifications of the CVD method. Although CVD methods are suitable for the mass production of CNFs, the high temperature treatment and the removal of remaining catalyst particles after the preparation make it difficult to apply CNFs as a micro- or nano-scale component of an integrated electronic or a chemical device. A new method of preparing CNFs at ambient temperature without catalyst has therefore been expected. Carbon nanofibers are possible to be produced by ion beam-irradiation of carbon soot powders or graphite at ambient temperature [14,15]. However, this method of preparation is not necessarily suitable for micro-patterning of carbon nanotubes, because the nanotubes are formed only on the carbonaceous compounds. Highly conducting amorphous carbon can be prepared on any substrate by ion-beam irradiation of organic compounds without catalyst and high temperature. Kaplan et al. found that the irradiation of the thin films of polycyclic aromatic compounds with ion beams gave amorphous carbon with the electrical conductivity of higher than 105 S m, which was comparable to that of graphite and was much higher than that of amorphous carbon prepared by the pyrolysis of polymers [16,17]. However, no study on the formation of CNFs from organic compounds by using ion beam has been reported except for our recent study [18]. During the course of preparing conducting carbon from organic compounds by ion beam-irradiation, we found that the irradiation of vapor-deposited decacyclene, a polycyclic aromatic compound known as a precursor of fullerene, at ambient temperature without catalyst gave amorphous CNFs. The CNFs were generated only in the field of exposure on a flat plate of any materials without careful control of the evaporation and the irradiation conditions, which suggests that our method of preparation makes it possible to set CNFs on a desired location with micro-scale accuracy. Based on the observation that the density of CNFs initially increased with increasing the fluence of ion beams, we suspected that the CNFs were grown from the thin film of decacyclene during the irradiation. However, the mechanism of CNFs formation was
475
not clarified. The present study is aimed at clarifying the mechanism of CNFs formation by detailed observation of pre- and post-irradiated decacyclene films with scanning electron and transmission electron microscopes. It will be shown that the CNFs were not grown from the film but were generated by the conversion of pre-existing whiskers of decacyclene on the film.
2. Experimental Decacyclene (Fig. 1), supplied by Aldrich, was used as received. Decacyclene purified by re-crystallization from hexane solution was occasionally used for examining the effect of impurity. About 300 mg of decacyclene was put on a tungsten heater and vapor-deposited on a flat plate composed of brass, copper, amorphous carbon, glass or stainless steel by using a compact vacuum coater (ULVAC, VPC-260). The distance between the heater and the plates located above it was 6 cm. The weights of the decacyclene films were monitored by using a quartz crystal oscillator-type deposition controller (ULVAC, CRTM-6000). The vapor-deposited plates were observed by using a scanning electron microscope (SEM: JEOL, JSM-6500F, operated at 5 keV) as a function of the heating time. The vapor-deposited decacyclene was also observed by using a high-resolution transmission electron microscope (TEM: JEOL,
Fig. 1. Molecular structure of decacyclene.
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ARM1300, operated at 1250 keV) after detaching it from a plate by scratching. The electronic absorption spectra of decacyclene before and after the deposition were measured with a UV–Visible spectrophotometer (SHIMADZU, UV-3101PC). All the irradiations were carried out at ambient temperature with 200 keV N+ ion beam at a flux of 9.6 · 1012 ions/cm2 in vacuum. The temperature of samples during the irradiation was estimated to be lower than 450 K. In order to examine the degree of carbonization of decacyclene films by ion beam-irradiation, the resistivity of the films on glass plates were measured with a LCZ meter (NF ELECTRONIC INSTRUMENTS, LCZ2321) using a silver paste as two electrodes. The resistances of the samples were more than two orders of magnitude higher than the contact resistance of the electrodes, so that the contact resistance did not affect the measurement. The resistivity of decacyclene films was more than 1010 X cm before the irradiation and was too high for the correct measurement. The film irradiated to a fluence of 2 · 1016 ions/cm2 was observed with the scanning electron microscope. The CNFs generated after the irradiation was detached from the plate and was observed with the transmission electron microscope to obtain information on the inner structure of the CNFs. In order to examine the possibility of micro-patterning of CNFs, a decacyclene film was irradiated with the ion beam through a slit on a copper foil to a fluence of 2 · 1016 ions/cm2. The irradiated sample was soaked in aniline at 333 K for 30 min and was then observed with the scanning electron microscope.
3. Results and discussion 3.1. Formation of whisker Fig. 2 shows the SEM images of decacyclene films that were generated on flat copper plates after turning-on the heater for the vapor-deposition. The vapor-deposition was started around 1.5 min after turning-on the heater. The plate was covered with hemispheres of decacyclene with diameters of less than 500 nm and cylindrical decacyclene whiskers with uniform diameters of 20–
100 nm and lengths of 1–20 m. Whiskers generated from purified decacyclene were straighter, which indicates that impurities in the whiskers caused the change of the direction of crystal growth. As shown in Fig. 3, the number density of the whiskers increased with increasing the duration of vapor-deposition. However, the length of the whiskers did not depend on the duration. The growth of the whiskers was therefore very fast and was completed probably within a few second. The evaporation was completed at 4.5 min after turning-on the heater. The number density of the whiskers at 4.5 min was 7 · 107 fibers/cm2. The total weight of the decacyclene whisker estimated from the SEM image was more than 1.5 · 10 5 g/cm2. The total weight of the vapor-deposited decacyclene was roughly 1 · 10 4 g/cm2 at 4.5 min, so that the ratio of the weight of the decacyclene whiskers to the total weight of the vapor-deposited decacyclene was more than 15%. The decacyclene whiskers were also formed on a plate composed of brass, amorphous carbon, glass or stainless steel. Fig. 4 shows the TEM image of decacyclene whiskers detached from the flat copper plate. A layered structure was observed inside of the whiskers. The thickness of the layers was 0.6–0.7 nm. Fig. 5 shows the electronic absorption spectra of decacyclene before and after the vapor-deposition. Although the spectrum of decacyclene after the vapor-deposition was not similar to that before the vapor-deposition, it was similar to the spectrum of recrystalized decacyclene. This result indicates that decacyclene was purified during the vapordeposition. The shape of the whiskers we observed was much different from that reported by Ho and Pascal [19]. They obtained ribbon-like helical crystals by sublimation of decacyclene in vacuum for 12 h. Taking the co-existence of the spheres of decacyclene and the cylindrical structure of whiskers into account, the whiskers are considered to be formed by so-called vapor–liquid–solid (VLS) growth mechanism [20,21]. According to this mechanism, a whisker is grown by the solidification of a liquid hemisphere that is attached on the top of the whisker. The liquid must contain some pre-existing impurity for decreasing the melting point of host molecules. The condensation of
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Fig. 2. SEM images of vapor-deposited decacyclene on flat cupper plates at: (a) 0 min, (b) 1.5 min and (c) 4.5 min after turning-on the heater for the deposition and (d) of vapor-deposited purified decacyclene.
vaporized molecules on the liquid causes super saturation of the solution, so that the excess molecules are solidified at the boundary between the solid and the liquid. The condensation or the solidification of the vapor on the solid is prohibited since the affinity of the vapor is much lower on the solid than on the liquid. The whisker is cylindrical with a uniform diameter, since the whisker is grown from the droplet with a constant diameter that is determined by the amount of the impurity and the contact angle of the liquid with respect to the solid. The VLS growth mechanism is, however, not directly applicable to decacyclene. Formation of
the whiskers on any plate indicates that no preexisting impurity is necessary for the formation. Even an impurity in decacyclene is not necessary, since the evaporation of purified decacyclene gave similar whiskers. Another discrepancy with the regular VLS growth mechanism is its abnormally fast rate of whisker formation. Formation of whiskers of 100 lm generally needs a few hours or even a few days [22]. The formation of decacyclene whiskers of the same length takes less than a few minutes. We propose a new VLS growth mechanism that the cylindrical whisker of decacyclene with a uniform diameter is generated from a super-cooled
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Fig. 5. Electronic absorption spectra of decacyclene dissolved in tetrahydrofuran.
Fig. 3. Effect of vapor-deposition time on the number density of decacyclene whiskers. The heater for the vapor-deposition is turned-on at t = 0.
Fig. 6. Schematic representation of the formation of decacyclene whisker by a proposed VLS growth mechanism.
Fig. 4. TEM image of decacyclene whiskers detached from a flat cupper plate. Solid lines and allows are added to the picture for emphasizing the width of the whiskers.
liquid droplet of decacyclene. Fig. 6 depicts the formation mechanism of the whiskers. The first step is the formation of a super-cooled liquid droplet on a flat plate by the condensation of decacyclene vapor. The droplet either solidifies as a whole to make a sphere of decacyclene or changes to a decacyclene whisker. The second step is the formation of a cylindrical seed whisker at the boundary between the droplet and the plate. Assuming that a
whisker is grown layer-by-layer and the wettability of the surface of the layer to the liquid is quite high, with the aid of gravity, the super-cooled liquid droplet is hung on the seed crystal. The diameter of the seed whisker may be determined by the wettability and the size and the surface tension of the liquid droplet. The third step is the growth of the seed whisker. The growing plane of the whisker is completely covered with super-cooled liquid, so that the diameter of the whisker is kept constant during the growth. A liquid hemisphere with the diameter of, for example, 400 nm is possible to give a cylindrical whisker with the diameter of 50 nm and the length of 8.5 lm. A liquid droplet gives a solid hemisphere unless it converts to a whisker. Solid hemispheres with the diameters of less than 500 nm are therefore observed with the whiskers. In regular VLS mechanisms, the rate of vapor condensation on a liquid phase must be the same
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as the rate of solidification on the liquid–solid boundary. Moreover, the rate of the removal of the latent heat of crystallization is also the same as the rate of solidification. Careful controlling of the environmental temperature and the rate of vaporization is therefore necessary for keeping whiskers growing. Growing of a decacyclene whisker from super-cooled liquid simply needs the presence of liquid that covers the entire surface of the growing crystal plane, so that the careful controlling is not necessary for the growing. The whiskers are therefore possible to grow within a few seconds. 3.2. Formation of carbon nanofibers Prolonged irradiation of decacyclene films with 200 keV N+ ion beams changed the color of the film from orange to dark brown and then black at a fluence of more than 1016 ions/cm2. Fig. 7 compares the SEM images of the whiskers before and after the irradiation of 2 · 1016 ions/cm2. Although the diameters and the number density of the whiskers slightly decreased after the irradiation, the overall shapes of the plates were quite similar with each other. These results indicate that the irradiation of the whiskers up to 1016 ions/cm2 caused the selective ejection of hydrogen atoms from the whiskers to generate CNFs. As shown in Fig. 8, the electrical resistance of a decacyclene film decreased with increasing fluence, and reached
Fig. 8. Effect of fluence on the electrical resistance of decacyclene film with the area of 0.5 cm · 1.0 cm and the weight of 4.3 · 10 5 g/cm2.
2.7 · 103 X at a fluence of 4 · 1016 ions/cm2. Although this measurement did not give the electrical conductivity of the CNFs, assuming that the film was flat and the conductivity of the CNFs was the same as that of the film, the conductivity of the CNFs were estimated to be 2.5 · 103 S m 1. The actual conductivity of the CNFs
Fig. 7. Comparison of the SEM images of decacyclene whiskers: (a) before and (b) after the irradiation to the fluences of 2.0 · 1016 ions/cm2.
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must be much higher than 2.5 · 103 S m 1, since the film had an island structure with many whiskers. The conductivity of the CNFs might be similar to that of carbon films generated from polycyclic aromatic compounds by ion beam-irradiation (105S m 1) [16]. The electrical conductivity of carbon films generated at 1173 K by the CVD of decacyclene are 4–8 · 104 S m 1 [23], which compares well with that of the carbon films generated by the ion beam-irradiation. The carbon films prepared by the CVD of aromatic compounds generally show high electrical conductivity because of the easy development of graphitic structures [24–26]. Fig. 9 shows the TEM image and the electron diffraction pattern of CNFs formed by the irradiation to a fluence of 2 · 1016 ions/cm2. Although the CNFs were amorphous as evidenced by the absence of a diffraction pattern, distorted layered structures were remained in the CNFs. These results indicate that graphitic structures were also developed by the irradiation at ambient temperature. Cyclodehydrogenation is known to take place by flash vacuum pyrolysis of decacyclene [27,28]. The cyclodehydrogenation may also be responsible for the efficient carbonization and graphitization by ion beam-irradiation. Fig. 10 shows the SEM image of a vapor-deposited cupper plate that was partially irradiated through a slit of a cupper plate and then soaked in aniline. The CNFs was formed only within the field of irradiation, which certifies that the present
Fig. 10. SEM image of partially-irradiated decacyclene film after soaking in aniline at 333 K for 30 min.
method of CNFs formation is suitable for the micro-patterning of CNFs. In conclusion, CNFs with high electrical conductivity were prepared at ambient temperature by ion beam-irradiation of decacyclene whiskers that were generated during vapor-deposition of decacyclene in vacuum for only a few minutes. This method of preparation is advantageous for aligning the CNFs on a desired location within lm-scale accuracy.
Fig. 9. (a) TEM image and (b) electron diffraction pattern of CNFs formed by ion beam-irradiation to a fluence of 2 · 1016 ions/cm2.
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Acknowledgement The present work is supported by the Grantin-Aid for Scientific Research (KAKENHI) in Priority Area ‘‘Molecular Nano Dynamics’’ from Ministry of Education, Culture, Sports, Science and Technology.
References [1] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kinang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377. [2] Q.H. Wang, T.D. Corrigan, J.Y. Dai, R.P.H. Chang, Appl. Phys. Lett. 70 (1997) 3308. [3] P.G. Collins, A. Zettl, Appl. Phys. Lett. 69 (1996) 1969. [4] W.A. de Heer, A. Chtelain, D. Ugarte, Science 270 (1999) 1179. [5] Y. Saito, S. Uemura, K. Hamaguchi, Jpn. J. Appl. Phys. 37 (1998) L346. [6] Y. Nakayama, S. Akita, Synth. Met. 117 (2001) 207. [7] P. Serp, M. Corriaas, P. Kalck, Appl. Catal. 337 (2003) A253. [8] M. Endo, Science 265 (1994) 22. [9] T. Koyama, M. Endo, Y. Onuma, J. Appl. Phys. 11 (1972) 445. [10] A. Oberlin, M. Endo, T. Koyama, J. Cryst. Growth 32 (1976) 335. [11] M. Mbangiseni, B. Richard, N. Christakis, L.X. Ying, W. Michael, C. Neil, Carbon 40 (2002) 2737.
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[12] T.G. Ros, A.J. van Dillen, J.W. Geus, D.C. Koningsberger, Chem. Phys. Chem. 2002 (2002) 209. [13] D.J. Krijn, G. John, Catal. Rev. Sci. Eng. 42 (2000) 481. [14] K. Yamamoto, Y. Koga, S. Fujiwara, M. Kubota, Appl. Phys. Lett. 69 (1996) 4174. [15] L.P. Biro´, G.I. Ma´rk, J. Gyulai, K. Havancsa´k, S. Lipp, Ch. Lehrer, L. Frey, H. Ryssel, Nucl. Instr. and Meth. B 147 (1999) 142. [16] M.L. Kaplan, S.R. Forrest, P.H. Schmidt, T. Venkatesan, J. Appl. Phys. 55 (1984) 732. [17] A.J. Lovinger, S.R. Forrest, M.L. Kaplan, P.H. Schmidt, T. Venkatesan, J. Appl. Phys. 55 (1984) 476. [18] T. Kimura, H. Koizumi, H. Kinoshita, H. Takahashi, T. Ichikawa, Jpn. J. Appl. Phys. 43 (2004) L863. [19] D.M. Ho, R.A. Pascal Jr., Chem. Mater 5 (1993) 1358. [20] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89. [21] R.S. Wagner, W.C. Ellis, Trans. Mettalurgical Soc., AIME 233 (1965) 1053. [22] F.R.N. Nabarro, P.J. Jackson, Growth of Crystal Whiskers; Growth and Perfection of Crystals, John Wiley, 1958, p. 13. [23] Y. Nakadoi, T. Kaneko, S. Otani, TANSO 171 (1996) 36. [24] H.A. Yu, T. Kaneko, S. Otani, S. Yoshimura, A. Oya, TANSO 178 (1997) 101. [25] K. Kokaji, A. Oya, K. Maruyama, Y. Yamada, M. Shiraishi, Carbon 35 (1997) 253. [26] H.A. Yu, T. Kaneko, S. Otani, Y. Sasaki, S. Yoshimura, Carbon 36 (1998) 137. [27] L.T. Scott, M.S. Bratcher, S. Hagen, J. Am. Chem. Soc. 118 (1996) 8743. [28] R.M.B. Ansems, L.T. Scott, J. Am. Chem. Soc. 122 (2000) 2719.
Formation of carbon nanofibers from decacyclene by ion beam irradiation Takahide Kimura a, Hitoshi Koizumi a, Hiroshi Kinoshita b, Tsuneki Ichikawa a
a,*
Division of Materials Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan Center for Advanced Research of Energy Conversion Materials, Hokkaido University, Sapporo 060-8628, Japan
b
Available online 19 May 2005
Abstract Amorphous carbon nanofibers with lengths of 1–20 lm, uniform diameters of 20–100 nm were generated by ion beam-irradiation of decacyclene whiskers that were prepared on a flat plate of any material during vapor-deposition of decacyclene, a polycyclic aromatic compound, in vacuum for only a few minutes. The length of the whiskers did not depend on the duration of the vapor-deposition, whereas the number density of the whiskers increased with increasing the duration and reached 6.9 · 107 cm 2 at 4.5 min after starting the vapor-deposition. Irradiation of more than 1016 ions/cm2 with a 200 keV N+ ion beam converted the whiskers to carbon nanofibers with the electrical conductivity comparable to that of graphite. This novel method of preparation is advantageous for aligning the fibers on a desired location, since the fibers are generated only on the irradiated part at ambient temperature without catalyst. 2005 Elsevier B.V. All rights reserved. PACS: 41.75.A; 61.82.P; 68.70; 79.20.R; 81.05.L; 81.40.R Keywords: Carbon nanofiber; Ion beam; Radiation effect; Decacyclene; Whisker; Electrical conductivity; VLS growth mechanism; Polycyclic aromatic compound
1. Introduction Carbon nanofibers (CNFs) and nanotubes have attracted considerable attention in recent years due to their potential utilities as a hydrogen stor*
Corresponding author. Tel.: +81 11 706 6747; fax: +81 11 706 7897. E-mail address: [email protected] (T. Ichikawa).
age material for fuel cells [1], electron field emitter for optical display [2–6], support for catalysts [7], electrode for lithium ion battery [8], and so on. The mass production of CNFs has usually been carried out by using chemical vapor deposition (CVD) in which CNFs with uniform diameters ranging from 0.01–200 lm are grown from catalyst particles by thermal decomposition of hydrocarbon gas under high temperature [9,10]. Several
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.04.022
T. Kimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 474–481
methods have recently been proposed for increasing the yield of CNFs [11–13], though they are essentially the modifications of the CVD method. Although CVD methods are suitable for the mass production of CNFs, the high temperature treatment and the removal of remaining catalyst particles after the preparation make it difficult to apply CNFs as a micro- or nano-scale component of an integrated electronic or a chemical device. A new method of preparing CNFs at ambient temperature without catalyst has therefore been expected. Carbon nanofibers are possible to be produced by ion beam-irradiation of carbon soot powders or graphite at ambient temperature [14,15]. However, this method of preparation is not necessarily suitable for micro-patterning of carbon nanotubes, because the nanotubes are formed only on the carbonaceous compounds. Highly conducting amorphous carbon can be prepared on any substrate by ion-beam irradiation of organic compounds without catalyst and high temperature. Kaplan et al. found that the irradiation of the thin films of polycyclic aromatic compounds with ion beams gave amorphous carbon with the electrical conductivity of higher than 105 S m, which was comparable to that of graphite and was much higher than that of amorphous carbon prepared by the pyrolysis of polymers [16,17]. However, no study on the formation of CNFs from organic compounds by using ion beam has been reported except for our recent study [18]. During the course of preparing conducting carbon from organic compounds by ion beam-irradiation, we found that the irradiation of vapor-deposited decacyclene, a polycyclic aromatic compound known as a precursor of fullerene, at ambient temperature without catalyst gave amorphous CNFs. The CNFs were generated only in the field of exposure on a flat plate of any materials without careful control of the evaporation and the irradiation conditions, which suggests that our method of preparation makes it possible to set CNFs on a desired location with micro-scale accuracy. Based on the observation that the density of CNFs initially increased with increasing the fluence of ion beams, we suspected that the CNFs were grown from the thin film of decacyclene during the irradiation. However, the mechanism of CNFs formation was
475
not clarified. The present study is aimed at clarifying the mechanism of CNFs formation by detailed observation of pre- and post-irradiated decacyclene films with scanning electron and transmission electron microscopes. It will be shown that the CNFs were not grown from the film but were generated by the conversion of pre-existing whiskers of decacyclene on the film.
2. Experimental Decacyclene (Fig. 1), supplied by Aldrich, was used as received. Decacyclene purified by re-crystallization from hexane solution was occasionally used for examining the effect of impurity. About 300 mg of decacyclene was put on a tungsten heater and vapor-deposited on a flat plate composed of brass, copper, amorphous carbon, glass or stainless steel by using a compact vacuum coater (ULVAC, VPC-260). The distance between the heater and the plates located above it was 6 cm. The weights of the decacyclene films were monitored by using a quartz crystal oscillator-type deposition controller (ULVAC, CRTM-6000). The vapor-deposited plates were observed by using a scanning electron microscope (SEM: JEOL, JSM-6500F, operated at 5 keV) as a function of the heating time. The vapor-deposited decacyclene was also observed by using a high-resolution transmission electron microscope (TEM: JEOL,
Fig. 1. Molecular structure of decacyclene.
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ARM1300, operated at 1250 keV) after detaching it from a plate by scratching. The electronic absorption spectra of decacyclene before and after the deposition were measured with a UV–Visible spectrophotometer (SHIMADZU, UV-3101PC). All the irradiations were carried out at ambient temperature with 200 keV N+ ion beam at a flux of 9.6 · 1012 ions/cm2 in vacuum. The temperature of samples during the irradiation was estimated to be lower than 450 K. In order to examine the degree of carbonization of decacyclene films by ion beam-irradiation, the resistivity of the films on glass plates were measured with a LCZ meter (NF ELECTRONIC INSTRUMENTS, LCZ2321) using a silver paste as two electrodes. The resistances of the samples were more than two orders of magnitude higher than the contact resistance of the electrodes, so that the contact resistance did not affect the measurement. The resistivity of decacyclene films was more than 1010 X cm before the irradiation and was too high for the correct measurement. The film irradiated to a fluence of 2 · 1016 ions/cm2 was observed with the scanning electron microscope. The CNFs generated after the irradiation was detached from the plate and was observed with the transmission electron microscope to obtain information on the inner structure of the CNFs. In order to examine the possibility of micro-patterning of CNFs, a decacyclene film was irradiated with the ion beam through a slit on a copper foil to a fluence of 2 · 1016 ions/cm2. The irradiated sample was soaked in aniline at 333 K for 30 min and was then observed with the scanning electron microscope.
3. Results and discussion 3.1. Formation of whisker Fig. 2 shows the SEM images of decacyclene films that were generated on flat copper plates after turning-on the heater for the vapor-deposition. The vapor-deposition was started around 1.5 min after turning-on the heater. The plate was covered with hemispheres of decacyclene with diameters of less than 500 nm and cylindrical decacyclene whiskers with uniform diameters of 20–
100 nm and lengths of 1–20 m. Whiskers generated from purified decacyclene were straighter, which indicates that impurities in the whiskers caused the change of the direction of crystal growth. As shown in Fig. 3, the number density of the whiskers increased with increasing the duration of vapor-deposition. However, the length of the whiskers did not depend on the duration. The growth of the whiskers was therefore very fast and was completed probably within a few second. The evaporation was completed at 4.5 min after turning-on the heater. The number density of the whiskers at 4.5 min was 7 · 107 fibers/cm2. The total weight of the decacyclene whisker estimated from the SEM image was more than 1.5 · 10 5 g/cm2. The total weight of the vapor-deposited decacyclene was roughly 1 · 10 4 g/cm2 at 4.5 min, so that the ratio of the weight of the decacyclene whiskers to the total weight of the vapor-deposited decacyclene was more than 15%. The decacyclene whiskers were also formed on a plate composed of brass, amorphous carbon, glass or stainless steel. Fig. 4 shows the TEM image of decacyclene whiskers detached from the flat copper plate. A layered structure was observed inside of the whiskers. The thickness of the layers was 0.6–0.7 nm. Fig. 5 shows the electronic absorption spectra of decacyclene before and after the vapor-deposition. Although the spectrum of decacyclene after the vapor-deposition was not similar to that before the vapor-deposition, it was similar to the spectrum of recrystalized decacyclene. This result indicates that decacyclene was purified during the vapordeposition. The shape of the whiskers we observed was much different from that reported by Ho and Pascal [19]. They obtained ribbon-like helical crystals by sublimation of decacyclene in vacuum for 12 h. Taking the co-existence of the spheres of decacyclene and the cylindrical structure of whiskers into account, the whiskers are considered to be formed by so-called vapor–liquid–solid (VLS) growth mechanism [20,21]. According to this mechanism, a whisker is grown by the solidification of a liquid hemisphere that is attached on the top of the whisker. The liquid must contain some pre-existing impurity for decreasing the melting point of host molecules. The condensation of
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Fig. 2. SEM images of vapor-deposited decacyclene on flat cupper plates at: (a) 0 min, (b) 1.5 min and (c) 4.5 min after turning-on the heater for the deposition and (d) of vapor-deposited purified decacyclene.
vaporized molecules on the liquid causes super saturation of the solution, so that the excess molecules are solidified at the boundary between the solid and the liquid. The condensation or the solidification of the vapor on the solid is prohibited since the affinity of the vapor is much lower on the solid than on the liquid. The whisker is cylindrical with a uniform diameter, since the whisker is grown from the droplet with a constant diameter that is determined by the amount of the impurity and the contact angle of the liquid with respect to the solid. The VLS growth mechanism is, however, not directly applicable to decacyclene. Formation of
the whiskers on any plate indicates that no preexisting impurity is necessary for the formation. Even an impurity in decacyclene is not necessary, since the evaporation of purified decacyclene gave similar whiskers. Another discrepancy with the regular VLS growth mechanism is its abnormally fast rate of whisker formation. Formation of whiskers of 100 lm generally needs a few hours or even a few days [22]. The formation of decacyclene whiskers of the same length takes less than a few minutes. We propose a new VLS growth mechanism that the cylindrical whisker of decacyclene with a uniform diameter is generated from a super-cooled
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Fig. 5. Electronic absorption spectra of decacyclene dissolved in tetrahydrofuran.
Fig. 3. Effect of vapor-deposition time on the number density of decacyclene whiskers. The heater for the vapor-deposition is turned-on at t = 0.
Fig. 6. Schematic representation of the formation of decacyclene whisker by a proposed VLS growth mechanism.
Fig. 4. TEM image of decacyclene whiskers detached from a flat cupper plate. Solid lines and allows are added to the picture for emphasizing the width of the whiskers.
liquid droplet of decacyclene. Fig. 6 depicts the formation mechanism of the whiskers. The first step is the formation of a super-cooled liquid droplet on a flat plate by the condensation of decacyclene vapor. The droplet either solidifies as a whole to make a sphere of decacyclene or changes to a decacyclene whisker. The second step is the formation of a cylindrical seed whisker at the boundary between the droplet and the plate. Assuming that a
whisker is grown layer-by-layer and the wettability of the surface of the layer to the liquid is quite high, with the aid of gravity, the super-cooled liquid droplet is hung on the seed crystal. The diameter of the seed whisker may be determined by the wettability and the size and the surface tension of the liquid droplet. The third step is the growth of the seed whisker. The growing plane of the whisker is completely covered with super-cooled liquid, so that the diameter of the whisker is kept constant during the growth. A liquid hemisphere with the diameter of, for example, 400 nm is possible to give a cylindrical whisker with the diameter of 50 nm and the length of 8.5 lm. A liquid droplet gives a solid hemisphere unless it converts to a whisker. Solid hemispheres with the diameters of less than 500 nm are therefore observed with the whiskers. In regular VLS mechanisms, the rate of vapor condensation on a liquid phase must be the same
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as the rate of solidification on the liquid–solid boundary. Moreover, the rate of the removal of the latent heat of crystallization is also the same as the rate of solidification. Careful controlling of the environmental temperature and the rate of vaporization is therefore necessary for keeping whiskers growing. Growing of a decacyclene whisker from super-cooled liquid simply needs the presence of liquid that covers the entire surface of the growing crystal plane, so that the careful controlling is not necessary for the growing. The whiskers are therefore possible to grow within a few seconds. 3.2. Formation of carbon nanofibers Prolonged irradiation of decacyclene films with 200 keV N+ ion beams changed the color of the film from orange to dark brown and then black at a fluence of more than 1016 ions/cm2. Fig. 7 compares the SEM images of the whiskers before and after the irradiation of 2 · 1016 ions/cm2. Although the diameters and the number density of the whiskers slightly decreased after the irradiation, the overall shapes of the plates were quite similar with each other. These results indicate that the irradiation of the whiskers up to 1016 ions/cm2 caused the selective ejection of hydrogen atoms from the whiskers to generate CNFs. As shown in Fig. 8, the electrical resistance of a decacyclene film decreased with increasing fluence, and reached
Fig. 8. Effect of fluence on the electrical resistance of decacyclene film with the area of 0.5 cm · 1.0 cm and the weight of 4.3 · 10 5 g/cm2.
2.7 · 103 X at a fluence of 4 · 1016 ions/cm2. Although this measurement did not give the electrical conductivity of the CNFs, assuming that the film was flat and the conductivity of the CNFs was the same as that of the film, the conductivity of the CNFs were estimated to be 2.5 · 103 S m 1. The actual conductivity of the CNFs
Fig. 7. Comparison of the SEM images of decacyclene whiskers: (a) before and (b) after the irradiation to the fluences of 2.0 · 1016 ions/cm2.
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must be much higher than 2.5 · 103 S m 1, since the film had an island structure with many whiskers. The conductivity of the CNFs might be similar to that of carbon films generated from polycyclic aromatic compounds by ion beam-irradiation (105S m 1) [16]. The electrical conductivity of carbon films generated at 1173 K by the CVD of decacyclene are 4–8 · 104 S m 1 [23], which compares well with that of the carbon films generated by the ion beam-irradiation. The carbon films prepared by the CVD of aromatic compounds generally show high electrical conductivity because of the easy development of graphitic structures [24–26]. Fig. 9 shows the TEM image and the electron diffraction pattern of CNFs formed by the irradiation to a fluence of 2 · 1016 ions/cm2. Although the CNFs were amorphous as evidenced by the absence of a diffraction pattern, distorted layered structures were remained in the CNFs. These results indicate that graphitic structures were also developed by the irradiation at ambient temperature. Cyclodehydrogenation is known to take place by flash vacuum pyrolysis of decacyclene [27,28]. The cyclodehydrogenation may also be responsible for the efficient carbonization and graphitization by ion beam-irradiation. Fig. 10 shows the SEM image of a vapor-deposited cupper plate that was partially irradiated through a slit of a cupper plate and then soaked in aniline. The CNFs was formed only within the field of irradiation, which certifies that the present
Fig. 10. SEM image of partially-irradiated decacyclene film after soaking in aniline at 333 K for 30 min.
method of CNFs formation is suitable for the micro-patterning of CNFs. In conclusion, CNFs with high electrical conductivity were prepared at ambient temperature by ion beam-irradiation of decacyclene whiskers that were generated during vapor-deposition of decacyclene in vacuum for only a few minutes. This method of preparation is advantageous for aligning the CNFs on a desired location within lm-scale accuracy.
Fig. 9. (a) TEM image and (b) electron diffraction pattern of CNFs formed by ion beam-irradiation to a fluence of 2 · 1016 ions/cm2.
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Acknowledgement The present work is supported by the Grantin-Aid for Scientific Research (KAKENHI) in Priority Area ‘‘Molecular Nano Dynamics’’ from Ministry of Education, Culture, Sports, Science and Technology.
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