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Fe- or Fe oxide-embedded anodic alumina membrane for nanocarbon growth―fabrication of membrane and observation of initial nanocarbon growth
Microelectronic Engineering 176 (2017) 58–61
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Fe- or Fe oxide-embedded anodic alumina membrane for nanocarbon growth―fabrication of membrane and observation of initial nanocarbon growth Eiichi Kondoh a,⁎, Shigeaki Tamekuni a, Mitsuhiro Watanabe a, Soichiro Okubo b, Takeshi Hikata b, Akira Nakayama b a b
Interdisciplinary Graduate School, University of Yamanashi, Kofu 400-8511, Japan Sumitomo Electric Industries, Ltd., Osaka 554-0024, Japan
a r t i c l e
i n f o
Article history: Received 14 October 2016 Received in revised form 24 January 2017 Accepted 31 January 2017 Available online 01 February 2017 Keywords: Nanocarbon Chemical vapor deposition Fe Fe oxide Catalyst Anodic alumina
a b s t r a c t The present paper investigates the development of a novel process for fabricating catalyst membranes for a carbon transmittance method (CTM). The CTM is a continuous growth method for carbon nanotubes (CNTs) in which one chamber containing carbon source gas and another chamber filled with an inert gas for CNT growth are separated by a catalyst-embedded membrane. In the present study, the membranes were fabricated by filling the nanopores of a self-supporting anodic alumina membrane (AAM) with chemical-vapor-deposited Fe using Fe(CO)5 as a precursor. Nanocarbon (NC) was grown by a (non-CTM) catalytic chemical vapor deposition (CCVD) method, which is a standard technique for CNT growth, at 700 °C using C2H2 as a carbon source. The embedded Fe was oxidized during the temperature ramp prior to C2H2 addition, whereas the oxidized Fe functioned as a catalyst for NC growth. When the pores of the AAM were filled with Fe oxide, the fill Fe oxide functioned as a good catalyst. © 2017 Published by Elsevier B.V.
1. Introduction Continuous growth of high-quality nanocarbon (NC) structures, such as carbon nanotubes (CNTs) and carbon nanofilaments, is gaining interest for its potential application in manufacturing electrical wire. Several carbon growth methods, such as laser vaporization [1,2], arc discharge [3,4], and catalytic chemical vapor deposition (CCVD) [5,6], have been reported. In a previous study, a number of the present authors (Hikata et al.) proposed a carbon transmission method (CTM) [7,8]. In the CTM, a reactor cell is separated into a carbon source gas environment and an inert gas environment by a membrane into which catalyst fibers are embedded. This separation allows the continuous supply of carbon source gas to one end of the fibers so that nanocarbon fibers grow at the other end, which is exposed to the inert gas environment. In the CTM, the diameter of the growing nanocarbon fibers is determined by the diameter of the catalyst fibers. The catalyst fibers were previously fabricated using a wire drawing technique, where the lower limit of the catalyst fiber diameter (approximately 1 μm) [7] was too large to grow single nanocarbon fibers. In a previous study [9], we proposed a novel catalyst structure (Fig. 1) composed of a selfsupporting anodized alumina membrane (AAM) having alumina ⁎ Corresponding author. E-mail address: [email protected] (E. Kondoh).
http://dx.doi.org/10.1016/j.mee.2017.01.039 0167-9317/© 2017 Published by Elsevier B.V.
nano-through-holes filled with a catalyst metal (Ni) using supercritical fluid chemical deposition [9,10]. Originally, Fe was used as a catalyst in developing the CTM [7,8], because Fe is a preferential metal for growing vertical CNTs [11,12]. In addition, in a previous study carried out using Ni, we observed the encroachment or effluence of Ni during NC growth, which is indicative of the chemical instability of Ni. In the present study, Fe or an Fe oxide was embedded into a self-supporting AAM by CVD using Fe(CO)5 as a precursor. The growth of NC was investigated using CCVD.
2. Experimental The thickness of the AAM used in the preset study was 0.3 mm. The nanoholes formed in the alumina membrane have a diameter of 10–20 nm and pass through the membrane. Fig. 2 shows the experimental setup of the CVD reactor used for Fe growth. Fe(CO)5, a yellow liquid compound purchased from Sigma-Aldrich, was further purified by Gas-Phased Growth Ltd. (Japan) and was used as an Fe precursor. Fe(CO)5 was charged in a stainless-steel reactor at a volume of 9 μL/cm3 together with an AAM specimen, and gaseous CO2 was then introduced at atmospheric pressure. The reactor was then heated to 300 °C and the temperature and pressure were maintained for 15 min. O2 was used as a deposition gas when Fe oxide was grown. After Fe or
E. Kondoh et al. / Microelectronic Engineering 176 (2017) 58–61
59
Fig. 1. Schematic diagram of the membrane fabrication process.
Fe oxide deposition, both sides of the AAM were polished using diamond slurry. Finally, the specimen was cleaned with acetone. Nanocarbon was grown by atmospheric thermal CCVD using a 5%C2H2–95%Ar gas mixture. The specimen was placed in a fused-silica tube having an O-ring flange at both ends. The gas was admitted at one end and was exhausted at the other end through long piping terminated by a water seal check valve. The deposition temperature was 700 °C. In our preliminary experiments, the growth of NCs (carbon nanotubes, filaments, and rods) on an Fe film catalyst on a SiO2 substrate was confirmed under these conditions. 3. Results and discussion 3.1. Fe filling Fig. 3 shows surface and longitudinal secondary electron microcopy (SEM) images of an AAM before and after Fe filling. Fig. 3(a) and (b) show a pristine (as-received) AAM, where pores are indicated in black. As shown in Fig. 3(b) pores are longitudinally discontinuous, because the cut plane is not perfectly parallel to the longitudinal direction of the pores and presumably because the pores have a slight curvature. Fig. 3(c) and (d) are AAM images captured after Fe filling and surface polishing. The polished Fe exhibits a closely packed geometry with tracing the pore structure. The diameter of the top of the filled Fe, however, seems larger than the original pore diameter (Fig. 3(c)), indicating that
the surface Fe was laterally expanded during polishing. Indeed, in the longitudinal section (Fig. 3(d)), the diameter of the Fe (white) agrees with the original pore diameter. The growth of NC on the Fe surface is described later herein. 3.2. Mutual coupling effects between Fe and AAM In our NC CCVD experiments on the Fe-embedded AAM, the CCVD reactor was heated to the target temperature by a pure Ar gas flow, and C2H2 was then admitted. The Fe was heated in an ambient Ar environment, and the stability of the Fe at elevated temperatures was investigated. Continuous Fe films were used as specimens in order to investigate the effects of heating. Fe was deposited by an electron beam evaporator on SiO2 glass substrates. The film thickness was approximately 15 nm. The films were annealed in an ambient Ar environment at different temperatures and were examined by X-ray diffraction analysis. Fe2O3 and/or Fe3O4 peaks were detected when the film was annealed at more than 400 °C (data not shown). This means that the Fe is at least partly oxidized before NC growth. The origin of the oxygen is not clear and is a serious concern. Residual oxygen in the reactor system is most likely source. Water trapped in the AAM during anodization and adsorbed water are also possible oxygen sources. Fig. 4 shows XRD patterns of Fe-embedded AAM after polishing. The embedded Fe exhibits a broad peak, suggesting poor crystallinity. After annealing at 700 °C, the Fe was oxidized and new Fe3O4 peaks appeared. The crystallization of amorphous alumina was also confirmed. At an elevated temperature of 850 °C, the Fe peaks disappeared, and peaks of alumina and Fe3O4 developed. The embedded Fe was found to be mostly converted to Fe oxide at 700 °C, accompanying alumina crystallization. When Fe oxide was deposited from an Fe(CO)5-oxygen mixture, we confirmed the deposition of Fe3O4, as indicated by the XRD pattern (Fig. 5). The effect of Fe oxidation on NC growth is discussed in the following section. 3.3. Fe versus Fe oxide for NC growth
Fig. 2. Schematic diagram of the Fe CVD reactor used in the present study.
NC CCVD was carried out using Fe- and Fe oxide-embedded AAMs. When our standard deposition time was used (typically 15–60 min) no significant difference was observed by either visual inspection or SEM. Therefore, the initial carbon growth was investigated by admitting the C2H2-Ar gas mixture for a short time (1–10 s) after the temperature ramp with flowing Ar. Immediately after the impulse addition of C2H2, the furnace heating was terminated, and the reactor was naturally cooled. Fig. 6 compares NC grown on an Fe-embedded AAM (Fig. 6(a)) and on an Fe oxide-embedded AAM (Fig. 6(b)). The amount of NC grown on the Fe oxide-embedded AAM was greater than, or at least equal to that, grown on the Fe-embedded AAM. Recent studies have demonstrated
60
E. Kondoh et al. / Microelectronic Engineering 176 (2017) 58–61
Fig. 3. SEM images of the anodic alumina membrane (AAM). (a) Top view and (b) longitudinal-section of pristine AAM. (c) Top view of a transverse-section after Fe filling.
the superior performance of Fe oxides as a catalyst for CNT growth, as compared to conventional Fe [13,14,15]. This superior performance occurs because Fe oxides are more chemically stable than Fe and are thus easy to handle. Moreover, the reduction of Fe oxide by hydrocarbons
eases the incorporation of C into Fe [13,15]. Our observations agree with this background knowledge. In our case, the Fe was oxidized during the temperature ramp. However, the Fe may not be completely oxidized, and controlling the degree of oxidation is not straightforward from a process point of view. Therefore, the use of oxidized Fe is favorable in view of process controllability and experimental reproducibility. Note that NC did not grow from all of the AAM nanoholes, although Fe or Fe oxide remained in the nanoholes. Presumably, the surfaces of
Fig. 4. X-ray diffraction patterns of Fe-embedded AAM (after polishing).
Fig. 5. X-ray diffraction patterns of Fe oxide-embedded AAM (after polishing).
E. Kondoh et al. / Microelectronic Engineering 176 (2017) 58–61
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Fig. 6. NC grown on (a) Fe-embedded AAM and (b) Fe oxide-embedded AAM.
the catalysts were covered with residue from polishing slurry, as the AAMs were only cleaned with acetone. (However, we did not observe any residual diamond particles.) The surfaces were inactivated by the residue, and NC growth began on “cleaner” catalyst. We expect more uniform and denser NC growth by applying, for instance, plasma cleaning to the AAMs.
4. Conclusions The nanopores of AAM were filled with Fe by atmospheric pressure chemical vapor deposition (CVD). The AAM surface was polished in order to remove the excess blanket film. Chemical vapor deposition Fe exhibited good filling capability. The embedded Fe was partly oxidized during temperature ramp in Ar before adding the carbon source gas for NC CCVD, and the oxidation did not deteriorate the catalytic ability of Fe. Indeed, NC exhibited similar growth on Fe oxide. In view of the difficulty of the controllability of Fe oxidation before NC deposition, the use of CVD Fe oxide is preferable to fabricating catalyst-embedded AAMs for NC growth.
References [1] T. Guo, O. Nikolaev, A. Thess, D.T. Colbert, R.E. Smalley, Chem. Phys. Lett. 243 (1995) 49. [2] F. Kokai, I. Nozaki, A. Koshio, Diam. Relat. Mater. 24 (2012) 25. [3] D. He, T. Zhao, Y. Liu, J. Zhu, G. Yu, L. Ge, Diam. Relat. Mater. 16 (2007) 1722. [4] X. Sun, W. Bao, Y. Lv, J. Deng, X. Wang, Mater. Lett. 61 (2007) 3956. [5] H. Liu, D. Takagi, H. Ohno, S. Chiashi, T. Chokan, Y. Homma, Appl. Phys. Express 1 (2008) 014001. [6] A. Takahashi, Y. Izumi, E. Ikenaga, T. Ohkochi, M. Kotsugi, T. Matsuhita, T. Muro, A. Kawabata, T. Murakami, M. Nihei, N. Yokoyama, IUCrJ 1 (2014) 221. [7] T. Hikata, K. Hayashi, T. Mizukoshi, Y. Sakurai, I. Ishigami, T. Aoki, T. Seki, J. Matsuo, Appl. Phys. Express 1 (2008) 034002. [8] T. Hikata, SEI Technical Rev 66 (2008) 81 (English Edition). [9] M. Watanabe, K. Osada, E. Kondoh, S. Okubo, T. Hikata, A. Nakayama, APL Mater. 2 (2014) 100701. [10] J.M. Blackburn, D.P. Long, A. Cabañas, J.J. Watkins, Science 294 (2001) 141. [11] G. Zhong, T. Iwasaki, K. Honda, Y. Furukawa, I. Ohdomari, H. Kawarada, Jpn. J. Appl. Phys. 44 (2005) 1558. [12] N.T. Alvarez, C.E. Hamilton, C.L. Pint, A. Orbaek, J. Yao, A.L. Frosinini, A.R. Barron, J.M. Tour, R.H. Hauge, ACS Appl. Mater. Interfaces 2 (2010) 1851. [13] C.J. Unrau, R.L. Axelbaum, P. Fraundorf, J. Nanopart. Res. 12 (2010) 2125. [14] T. Hikata, S. Okubo, Y. Higashi, T. Matsuba, R. Utsunomiya, S. Tsurekawa, K. Murakami, J. Fujita, AIP Adv. 3 (2013) 042127. [15] D.-M. Tang, C. Liu, W.-J. Yu, L.-L. Zhang, P.-X. Hou, J.-C. Li, F. Li, Y. Bando, D. Golberg, H.-M. Cheng, ACS Nano 8 (2014) 292.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Fe- or Fe oxide-embedded anodic alumina membrane for nanocarbon growth―fabrication of membrane and observation of initial nanocarbon growth Eiichi Kondoh a,⁎, Shigeaki Tamekuni a, Mitsuhiro Watanabe a, Soichiro Okubo b, Takeshi Hikata b, Akira Nakayama b a b
Interdisciplinary Graduate School, University of Yamanashi, Kofu 400-8511, Japan Sumitomo Electric Industries, Ltd., Osaka 554-0024, Japan
a r t i c l e
i n f o
Article history: Received 14 October 2016 Received in revised form 24 January 2017 Accepted 31 January 2017 Available online 01 February 2017 Keywords: Nanocarbon Chemical vapor deposition Fe Fe oxide Catalyst Anodic alumina
a b s t r a c t The present paper investigates the development of a novel process for fabricating catalyst membranes for a carbon transmittance method (CTM). The CTM is a continuous growth method for carbon nanotubes (CNTs) in which one chamber containing carbon source gas and another chamber filled with an inert gas for CNT growth are separated by a catalyst-embedded membrane. In the present study, the membranes were fabricated by filling the nanopores of a self-supporting anodic alumina membrane (AAM) with chemical-vapor-deposited Fe using Fe(CO)5 as a precursor. Nanocarbon (NC) was grown by a (non-CTM) catalytic chemical vapor deposition (CCVD) method, which is a standard technique for CNT growth, at 700 °C using C2H2 as a carbon source. The embedded Fe was oxidized during the temperature ramp prior to C2H2 addition, whereas the oxidized Fe functioned as a catalyst for NC growth. When the pores of the AAM were filled with Fe oxide, the fill Fe oxide functioned as a good catalyst. © 2017 Published by Elsevier B.V.
1. Introduction Continuous growth of high-quality nanocarbon (NC) structures, such as carbon nanotubes (CNTs) and carbon nanofilaments, is gaining interest for its potential application in manufacturing electrical wire. Several carbon growth methods, such as laser vaporization [1,2], arc discharge [3,4], and catalytic chemical vapor deposition (CCVD) [5,6], have been reported. In a previous study, a number of the present authors (Hikata et al.) proposed a carbon transmission method (CTM) [7,8]. In the CTM, a reactor cell is separated into a carbon source gas environment and an inert gas environment by a membrane into which catalyst fibers are embedded. This separation allows the continuous supply of carbon source gas to one end of the fibers so that nanocarbon fibers grow at the other end, which is exposed to the inert gas environment. In the CTM, the diameter of the growing nanocarbon fibers is determined by the diameter of the catalyst fibers. The catalyst fibers were previously fabricated using a wire drawing technique, where the lower limit of the catalyst fiber diameter (approximately 1 μm) [7] was too large to grow single nanocarbon fibers. In a previous study [9], we proposed a novel catalyst structure (Fig. 1) composed of a selfsupporting anodized alumina membrane (AAM) having alumina ⁎ Corresponding author. E-mail address: [email protected] (E. Kondoh).
http://dx.doi.org/10.1016/j.mee.2017.01.039 0167-9317/© 2017 Published by Elsevier B.V.
nano-through-holes filled with a catalyst metal (Ni) using supercritical fluid chemical deposition [9,10]. Originally, Fe was used as a catalyst in developing the CTM [7,8], because Fe is a preferential metal for growing vertical CNTs [11,12]. In addition, in a previous study carried out using Ni, we observed the encroachment or effluence of Ni during NC growth, which is indicative of the chemical instability of Ni. In the present study, Fe or an Fe oxide was embedded into a self-supporting AAM by CVD using Fe(CO)5 as a precursor. The growth of NC was investigated using CCVD.
2. Experimental The thickness of the AAM used in the preset study was 0.3 mm. The nanoholes formed in the alumina membrane have a diameter of 10–20 nm and pass through the membrane. Fig. 2 shows the experimental setup of the CVD reactor used for Fe growth. Fe(CO)5, a yellow liquid compound purchased from Sigma-Aldrich, was further purified by Gas-Phased Growth Ltd. (Japan) and was used as an Fe precursor. Fe(CO)5 was charged in a stainless-steel reactor at a volume of 9 μL/cm3 together with an AAM specimen, and gaseous CO2 was then introduced at atmospheric pressure. The reactor was then heated to 300 °C and the temperature and pressure were maintained for 15 min. O2 was used as a deposition gas when Fe oxide was grown. After Fe or
E. Kondoh et al. / Microelectronic Engineering 176 (2017) 58–61
59
Fig. 1. Schematic diagram of the membrane fabrication process.
Fe oxide deposition, both sides of the AAM were polished using diamond slurry. Finally, the specimen was cleaned with acetone. Nanocarbon was grown by atmospheric thermal CCVD using a 5%C2H2–95%Ar gas mixture. The specimen was placed in a fused-silica tube having an O-ring flange at both ends. The gas was admitted at one end and was exhausted at the other end through long piping terminated by a water seal check valve. The deposition temperature was 700 °C. In our preliminary experiments, the growth of NCs (carbon nanotubes, filaments, and rods) on an Fe film catalyst on a SiO2 substrate was confirmed under these conditions. 3. Results and discussion 3.1. Fe filling Fig. 3 shows surface and longitudinal secondary electron microcopy (SEM) images of an AAM before and after Fe filling. Fig. 3(a) and (b) show a pristine (as-received) AAM, where pores are indicated in black. As shown in Fig. 3(b) pores are longitudinally discontinuous, because the cut plane is not perfectly parallel to the longitudinal direction of the pores and presumably because the pores have a slight curvature. Fig. 3(c) and (d) are AAM images captured after Fe filling and surface polishing. The polished Fe exhibits a closely packed geometry with tracing the pore structure. The diameter of the top of the filled Fe, however, seems larger than the original pore diameter (Fig. 3(c)), indicating that
the surface Fe was laterally expanded during polishing. Indeed, in the longitudinal section (Fig. 3(d)), the diameter of the Fe (white) agrees with the original pore diameter. The growth of NC on the Fe surface is described later herein. 3.2. Mutual coupling effects between Fe and AAM In our NC CCVD experiments on the Fe-embedded AAM, the CCVD reactor was heated to the target temperature by a pure Ar gas flow, and C2H2 was then admitted. The Fe was heated in an ambient Ar environment, and the stability of the Fe at elevated temperatures was investigated. Continuous Fe films were used as specimens in order to investigate the effects of heating. Fe was deposited by an electron beam evaporator on SiO2 glass substrates. The film thickness was approximately 15 nm. The films were annealed in an ambient Ar environment at different temperatures and were examined by X-ray diffraction analysis. Fe2O3 and/or Fe3O4 peaks were detected when the film was annealed at more than 400 °C (data not shown). This means that the Fe is at least partly oxidized before NC growth. The origin of the oxygen is not clear and is a serious concern. Residual oxygen in the reactor system is most likely source. Water trapped in the AAM during anodization and adsorbed water are also possible oxygen sources. Fig. 4 shows XRD patterns of Fe-embedded AAM after polishing. The embedded Fe exhibits a broad peak, suggesting poor crystallinity. After annealing at 700 °C, the Fe was oxidized and new Fe3O4 peaks appeared. The crystallization of amorphous alumina was also confirmed. At an elevated temperature of 850 °C, the Fe peaks disappeared, and peaks of alumina and Fe3O4 developed. The embedded Fe was found to be mostly converted to Fe oxide at 700 °C, accompanying alumina crystallization. When Fe oxide was deposited from an Fe(CO)5-oxygen mixture, we confirmed the deposition of Fe3O4, as indicated by the XRD pattern (Fig. 5). The effect of Fe oxidation on NC growth is discussed in the following section. 3.3. Fe versus Fe oxide for NC growth
Fig. 2. Schematic diagram of the Fe CVD reactor used in the present study.
NC CCVD was carried out using Fe- and Fe oxide-embedded AAMs. When our standard deposition time was used (typically 15–60 min) no significant difference was observed by either visual inspection or SEM. Therefore, the initial carbon growth was investigated by admitting the C2H2-Ar gas mixture for a short time (1–10 s) after the temperature ramp with flowing Ar. Immediately after the impulse addition of C2H2, the furnace heating was terminated, and the reactor was naturally cooled. Fig. 6 compares NC grown on an Fe-embedded AAM (Fig. 6(a)) and on an Fe oxide-embedded AAM (Fig. 6(b)). The amount of NC grown on the Fe oxide-embedded AAM was greater than, or at least equal to that, grown on the Fe-embedded AAM. Recent studies have demonstrated
60
E. Kondoh et al. / Microelectronic Engineering 176 (2017) 58–61
Fig. 3. SEM images of the anodic alumina membrane (AAM). (a) Top view and (b) longitudinal-section of pristine AAM. (c) Top view of a transverse-section after Fe filling.
the superior performance of Fe oxides as a catalyst for CNT growth, as compared to conventional Fe [13,14,15]. This superior performance occurs because Fe oxides are more chemically stable than Fe and are thus easy to handle. Moreover, the reduction of Fe oxide by hydrocarbons
eases the incorporation of C into Fe [13,15]. Our observations agree with this background knowledge. In our case, the Fe was oxidized during the temperature ramp. However, the Fe may not be completely oxidized, and controlling the degree of oxidation is not straightforward from a process point of view. Therefore, the use of oxidized Fe is favorable in view of process controllability and experimental reproducibility. Note that NC did not grow from all of the AAM nanoholes, although Fe or Fe oxide remained in the nanoholes. Presumably, the surfaces of
Fig. 4. X-ray diffraction patterns of Fe-embedded AAM (after polishing).
Fig. 5. X-ray diffraction patterns of Fe oxide-embedded AAM (after polishing).
E. Kondoh et al. / Microelectronic Engineering 176 (2017) 58–61
61
Fig. 6. NC grown on (a) Fe-embedded AAM and (b) Fe oxide-embedded AAM.
the catalysts were covered with residue from polishing slurry, as the AAMs were only cleaned with acetone. (However, we did not observe any residual diamond particles.) The surfaces were inactivated by the residue, and NC growth began on “cleaner” catalyst. We expect more uniform and denser NC growth by applying, for instance, plasma cleaning to the AAMs.
4. Conclusions The nanopores of AAM were filled with Fe by atmospheric pressure chemical vapor deposition (CVD). The AAM surface was polished in order to remove the excess blanket film. Chemical vapor deposition Fe exhibited good filling capability. The embedded Fe was partly oxidized during temperature ramp in Ar before adding the carbon source gas for NC CCVD, and the oxidation did not deteriorate the catalytic ability of Fe. Indeed, NC exhibited similar growth on Fe oxide. In view of the difficulty of the controllability of Fe oxidation before NC deposition, the use of CVD Fe oxide is preferable to fabricating catalyst-embedded AAMs for NC growth.
References [1] T. Guo, O. Nikolaev, A. Thess, D.T. Colbert, R.E. Smalley, Chem. Phys. Lett. 243 (1995) 49. [2] F. Kokai, I. Nozaki, A. Koshio, Diam. Relat. Mater. 24 (2012) 25. [3] D. He, T. Zhao, Y. Liu, J. Zhu, G. Yu, L. Ge, Diam. Relat. Mater. 16 (2007) 1722. [4] X. Sun, W. Bao, Y. Lv, J. Deng, X. Wang, Mater. Lett. 61 (2007) 3956. [5] H. Liu, D. Takagi, H. Ohno, S. Chiashi, T. Chokan, Y. Homma, Appl. Phys. Express 1 (2008) 014001. [6] A. Takahashi, Y. Izumi, E. Ikenaga, T. Ohkochi, M. Kotsugi, T. Matsuhita, T. Muro, A. Kawabata, T. Murakami, M. Nihei, N. Yokoyama, IUCrJ 1 (2014) 221. [7] T. Hikata, K. Hayashi, T. Mizukoshi, Y. Sakurai, I. Ishigami, T. Aoki, T. Seki, J. Matsuo, Appl. Phys. Express 1 (2008) 034002. [8] T. Hikata, SEI Technical Rev 66 (2008) 81 (English Edition). [9] M. Watanabe, K. Osada, E. Kondoh, S. Okubo, T. Hikata, A. Nakayama, APL Mater. 2 (2014) 100701. [10] J.M. Blackburn, D.P. Long, A. Cabañas, J.J. Watkins, Science 294 (2001) 141. [11] G. Zhong, T. Iwasaki, K. Honda, Y. Furukawa, I. Ohdomari, H. Kawarada, Jpn. J. Appl. Phys. 44 (2005) 1558. [12] N.T. Alvarez, C.E. Hamilton, C.L. Pint, A. Orbaek, J. Yao, A.L. Frosinini, A.R. Barron, J.M. Tour, R.H. Hauge, ACS Appl. Mater. Interfaces 2 (2010) 1851. [13] C.J. Unrau, R.L. Axelbaum, P. Fraundorf, J. Nanopart. Res. 12 (2010) 2125. [14] T. Hikata, S. Okubo, Y. Higashi, T. Matsuba, R. Utsunomiya, S. Tsurekawa, K. Murakami, J. Fujita, AIP Adv. 3 (2013) 042127. [15] D.-M. Tang, C. Liu, W.-J. Yu, L.-L. Zhang, P.-X. Hou, J.-C. Li, F. Li, Y. Bando, D. Golberg, H.-M. Cheng, ACS Nano 8 (2014) 292.