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Nanocrystalline diamond directly transformed from carbon nanotubes under high pressure
Diamond and Related Materials 11 (2002) 87–91
Nanocrystalline diamond directly transformed from carbon nanotubes under high pressure H. Yusa* Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Received 20 June 2000; accepted 2 August 2001
Abstract Multiwalled carbon nanotubes are heated in a diamond anvil cell by a CO2 laser above 17 GPa and 2500 K. The recovered product consists of nano-sized octahedral crystals of less than 50 nm. The tubular structure completely changed to granular and grain sizes corresponded to the diameter of nanotubes. Electron energy loss spectra from the products coincide with that of diamond. No metastable phase or amorphous carbon was detected in the products. The grain size of the diamond suggests that the transformation took place by direct conversion of nanotubes. This result suggests that diamond grain size may be controlled. The products are thought to be sintered compacts of nanocrystalline diamond. If larger sintered compacts are synthesized from nanotubes it may have an advantage in mechanical strength similar to natural polycrystalline diamond such as carbonado. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Diamond; High pressure; High temperature; Nanotubes
1. Introduction Multiwalled carbon nanotubes discovered in carbon soot w1x have been expected to lead to many nano-scale device applications. Carbon nanotubes are thought to be tubular graphite. Multiwalled carbon nanotubes consist of many rolled graphite sheets at regular intervals similar to the (002) graphite plane. The thickness of a wall is limited to approximately 50 sheets, and thicker tubes tend to be polygonized. Graphite has been transformed directly into cubic diamond in a laser heated diamond anvil cell w2x. We reported that direct transformation to diamond without dissolved state preserves the shape of a starting graphite sample. The mechanism for forming cubic diamond via hexagonal diamond w3,4x was also discussed in conjunction with stacking faults along the cubic N111M direction. Considering the structural similarity of nanotubes and graphite, it is thought that direct transformation from nanotubes to diamond may be possible. Nanotube features are expected to influence diamond shape. Diamond formation by laser irradiation to carbon nanotubes on an iron substrate was demonstrated by Wei et al. w5,6x at atmospheric pressure. They * Corresponding author. E-mail address: [email protected] (H. Yusa).
suggest that diamond was formed through an intermediate phase in the Fe-C system. They also reported highpressure and temperature experiments up to 5 GPa and 1500 8C w6x. Resulting products, however, contained a large amount of unreacted material as shown in Raman spectra. Pressure and temperature were too low for conversion to diamond compared to the direct conversion of graphite to diamond w7x. As shown in a previous study, w2x laser-heated DAC can generate sufficient pressure and temperature to enable direct transformation of nanotubes into diamond. We conducted high-pressure and temperature experiments to demonstrate this transformation. 2. Experimental methods Starting samples were multiwalled carbon nanotubes obtained from the Aldrich Chemical Company, Inc., Wisconsin. Samples were checked by transmission electron microscopy (TEM) and high resolution scanning electron microscopy with a field emission gun (FESEM). Nanotubes with closed ends have 10–30 shells and an outer diameter of 5–50 nm, ranging in length from 20 nm to 5 mm. No fullerite was detected in Xray diffraction patterns of the bulk samples. Chemical
0925-9635/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 5 3 2 - 5
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H. Yusa / Diamond and Related Materials 11 (2002) 87–91
Fig. 1. (a) Nanotubes sample at 17.5 GPa in a diamond anvil cell before heating. (b) Sample after heating with a CO2 laser. Nanotubes in the heated area change to a transparent phase. The sample is surrounded by a KBr pressure medium in the hole of a gasket.
purity was confirmed by analysis using energy dispersive X-ray (EDX) analysis. The experimental procedure is similar to that described elsewhere using graphite as a starting material w2x. A stainless steel gasket (200 mm in diameter hole and 110 mm in thickness) was prepared for a sample chamber. The nanotube sample was formed into a platelike shape (5 mm thick and 150 mm edge length) and placed in a gasket with dried KBr as a pressure medium. The sample was enclosed with KBr to prevent thermal contact with anvils. A few grains of small ruby chips were used as a marker to measure pressure before and after heating. The sample was compressed to 16.0– 17.5 GPa at room temperature. The sample was observed to be broken into a few parts during compression in the solid pressure medium. A CO2 laser beam (240 W maximum) with a continuous wave was focused onto the sample at a diameter of 100 mm. The heated sample emitted intense radiation, which was monitored by a spectrometer, indicating temperature of 2500–3000 K. A transparent phase appeared in an area irradiated for a few tens of seconds of heating (Fig. 1b). The laser beam focus was carefully controlled on the center of the sample to avoid heating gasket materials. A rapid phase change was observed by a CCD camera. The sample was quenched by shutting off the laser beam. A 10% pressure drop was measured after heating due to annealing of the solid pressure medium. The sample was recovered at ambient pressure after releasing pressure. To check reproducibility, we repeated experiments under similar P and T conditions four times. A pressurized sample without heating was also recovered for comparison.
3. Results and discussion The sample surrounded by KBr pressure media was removed from the gasket and placed on a grid for TEM. Pressure media was dissolved in water and the sample was held on the grid. The sample remained transparent under an optical microscope after drying at 120 8C for 1 h. The recovered bulk sample was plate-like, almost the same as the starting sample. Raman microprobe spectroscopy identified the phase of the transparent part of the recovered sample as having a broad peak at a wavenumber of 1322–1324 cmy1 which is consistent with the mode of bulk diamond crystal with peak shifted to a slightly lower wavenumber (Fig. 2). These features are explained by a phonon confinement effect due to small grain size w8x. An amorphous phase and graphite with typical broad peaks w9x at 1580 cmy1 and 1350 cmy1 were not found in the transparent region. To examine changes from starting to recovered samples in detail, we took FE-SEM images (Fig. 3). Comparison of the images at the same magnification enabled us to better understand the mechanism behind the phase transformation. Interestingly, the tubular structure completely changed to granular and grain sizes corresponded to the diameter of the nanotubes. Grains were octahedral, common to diamond. No crystals were tubular. We assume that the tubular structure collapsed into nano-sized particles. If carbon atoms crystallize from solution or melt, we would expect crystals larger than the diameter of the starting materials. Fine crystals less than 50 nm in size suggest that these nano-sized diamonds were formed by direct transformation from nanotubes, without going through dissolved or molten
H. Yusa / Diamond and Related Materials 11 (2002) 87–91
Fig. 2. (a) The Raman spectrum of the recovered sample at 1 atm. (b) Raman spectrum of a synthetic large crystalline diamond (GE, 2 mm in size).
state. Diamond aggregates with nano-sized particles have been recovered by shock compression of fullurite w10–13x. Although particle showed sp3 bonding, the crystal structure was not clear due to the large strain on shock compression. They called the poorly defined aggregates amorphous diamond w10,11x. Crystal sizes are controlled by nanotubes used as a starting material in our study where static compression was employed.
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As far as the author knows, the present synthesis is unique for obtaining nanocrystalline diamond having octahedral morphology. For further analysis, we conducted electron energy loss spectroscopy (EELS) using TEM (Hitachi HF2000) with a cold-field emission source and Gatan 666 PEELS system w14,15x. The EELS technique makes it possible to examine hybridization (sp2 or sp3) and the presence of p or s electrons with a resolution of 1 nm. As described above, it was difficult to obtain narrow and unshifted Raman signals due to the grain size effect, EELS analysis was conducted for further confirmation of crystal structure. EELS Plasmon loss and core loss spectra (K-absorption edge) were measured at several points on the samples, both heated and unheated, to clarify the transition mechanism under high pressure. Despite non-hydrostatic compression up to 17 GPa in a solid pressure medium, nanotubes remained tubular (Fig. 4a). The tubular structure thus did not collapse during compression due to the high yield strength of the curled graphitic planes. The fine structure of the carbon Kabsorption edge (Fig. 5a) showed the same features as reported for multiwalled nanotubes w16,17x. No trace of sp3 hybridization was observed with the unheated nanotubes. Based on electron diffraction patterns (EDP) (Fig. 4) no innermost diffraction ring corresponding to the index of the (002) graphite plane was detected in the heated sample, and no diffraction ring other than
Fig. 3. (a), (b) HRSEM images of multiwalled nanotubes as starting materials. (c), (d) HRSEM images of the recovered sample under high P and T conditions. Tubular samples completely converted to nanosize grains.
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H. Yusa / Diamond and Related Materials 11 (2002) 87–91
Fig. 4. TEM images and EDP of recovered samples. (a) Sample without heating at 17.0 GPa (b) the sample heated by a CO2 laser at 17.5 GPa.
those for diamond was found. No additional spots derived from structural imperfection were seen, indicating that the nanocrystalline diamonds contained few extended defects. The TEM image (Fig. 4b) of the heated sample showed no tubular structure or intermediate onion-like structure w18x as suggested by Wei et al. w6x. A p-electron plasmon peak at 285 eV observed in initial nanotubes was lost in the heated sample (Fig. 5b). Plasmon loss spectra (Fig. 6b) gave direct evidence that the phase has a s-electron state of sp3 hybridization equivalent to that of typical diamond w11x. No chemical composition other than carbon was detected on the samples by EELS (K-absorption edge) or EDX. Based on the above analysis, we concluded that the products were sintered compact with nanocrystalline grains of diamond. The grain size was controlled by the initial nanotube diameter. It is expected that sintered
compacts with nanocrystalline diamond may have high mechanical strength similar to natural polycrystalline diamond such as carbonado w19x. Synthesis of large sintered compacts using a large-volume high-pressure apparatus appears to be great interest in this respect. Acknowledgements The author thanks H. Matsumoto for his help with EELS experiments. This work was supported by the Center of Excellence Project at the National Institute for Materials Science. References w1x S. Iijima, Nature 354 (1991) 56. w2x H. Yusa, K. Takemura, Y. Matsui, H. Morishima, K. Watanabe, H. Yamawaki, K. Aoki, Appl. Phys. Lett. 72 (1998) 1843.
Fig. 5. EELS spectra (K-edge absorption) of recovered samples. (a) Laser-heated sample at 17.5 GPa. (b) Sample subjected to 17 GPa at room temperature.
H. Yusa / Diamond and Related Materials 11 (2002) 87–91
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Fig. 6. EELS spectra (plasmon loss) of recovered samples. (a) Laser-heated sample at 17.5 GPa. (b) Sample subjected to 17 GPa at room temperature. w3x W. Utsumi, T. Yagi, Science 252 (1991) 1542. w4x T. Yagi, W. Utsumi, M. Yamakata, T. Kikegawa, O. Shimomura, Phys. Rev. B46 (1992) 6031. w5x B. Wei, J. Zhang, J. Liang, W. Liu, Z. Gao, D. Wu, J. Mat. Sci, Lett. 16 (1997) 402. w6x B. Wei, J. Zhang, J. Liang, D. Wu, Carbon 36 (1998) 997. w7x F.P. Bundy, J. Geophys. Res. 85 (1980) 6930. w8x M. Yoshikawa, Y. Mori, M. Maegawa, G. Katagiri, H. Ishida, A. Ishitani, Appl. Phys. Lett. 62 (1993) 3114. w9x P.V. Huong, Diamond Relat. Mater. 1 (1991) 33. w10x H. Hirai, K. Kondo, T. Ohwada, Carbon 31 (1993) 1095. w11x H. Hirai, K. Kondo, N. Yoshizawa, M. Shiraishi, Appl. Phys. Lett. 64 (1994) 1797.
w12x C.S. Yoo, W.J. Nellis, Science 254 (1991) 1489. w13x T. Sekine, 1992 Proceedings Japanese Academic Series, B68 95. w14x Hitachi Scientific Instrument Technical Data, No. 69, (1990). w15x T. Kogure, K. Saiki, M. Konno, T. Kamino, Mat. Res. Soc. Symp. Proc. 504 (1998) 183. w16x Y. Kiyose, S. Horiuchi, M. Kyotani, M. Yumura, K. Uchida, S. Ohshima, Y. Kuriki, F. Ikazaki, N. Yamahira, Thin Solid Films 273 (1996) 222. w17x R. Kuzuo, M. Terauchi, M. Tanaka, Jpn. J. Appl. Phys. 31 (1992) L1484. w18x F. Banhart, M. Ajayan, Nature 382 (1996) 433. w19x L.F. Trueb, E.C. deWys, Science 165 (1969) 799.
Nanocrystalline diamond directly transformed from carbon nanotubes under high pressure H. Yusa* Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Received 20 June 2000; accepted 2 August 2001
Abstract Multiwalled carbon nanotubes are heated in a diamond anvil cell by a CO2 laser above 17 GPa and 2500 K. The recovered product consists of nano-sized octahedral crystals of less than 50 nm. The tubular structure completely changed to granular and grain sizes corresponded to the diameter of nanotubes. Electron energy loss spectra from the products coincide with that of diamond. No metastable phase or amorphous carbon was detected in the products. The grain size of the diamond suggests that the transformation took place by direct conversion of nanotubes. This result suggests that diamond grain size may be controlled. The products are thought to be sintered compacts of nanocrystalline diamond. If larger sintered compacts are synthesized from nanotubes it may have an advantage in mechanical strength similar to natural polycrystalline diamond such as carbonado. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Diamond; High pressure; High temperature; Nanotubes
1. Introduction Multiwalled carbon nanotubes discovered in carbon soot w1x have been expected to lead to many nano-scale device applications. Carbon nanotubes are thought to be tubular graphite. Multiwalled carbon nanotubes consist of many rolled graphite sheets at regular intervals similar to the (002) graphite plane. The thickness of a wall is limited to approximately 50 sheets, and thicker tubes tend to be polygonized. Graphite has been transformed directly into cubic diamond in a laser heated diamond anvil cell w2x. We reported that direct transformation to diamond without dissolved state preserves the shape of a starting graphite sample. The mechanism for forming cubic diamond via hexagonal diamond w3,4x was also discussed in conjunction with stacking faults along the cubic N111M direction. Considering the structural similarity of nanotubes and graphite, it is thought that direct transformation from nanotubes to diamond may be possible. Nanotube features are expected to influence diamond shape. Diamond formation by laser irradiation to carbon nanotubes on an iron substrate was demonstrated by Wei et al. w5,6x at atmospheric pressure. They * Corresponding author. E-mail address: [email protected] (H. Yusa).
suggest that diamond was formed through an intermediate phase in the Fe-C system. They also reported highpressure and temperature experiments up to 5 GPa and 1500 8C w6x. Resulting products, however, contained a large amount of unreacted material as shown in Raman spectra. Pressure and temperature were too low for conversion to diamond compared to the direct conversion of graphite to diamond w7x. As shown in a previous study, w2x laser-heated DAC can generate sufficient pressure and temperature to enable direct transformation of nanotubes into diamond. We conducted high-pressure and temperature experiments to demonstrate this transformation. 2. Experimental methods Starting samples were multiwalled carbon nanotubes obtained from the Aldrich Chemical Company, Inc., Wisconsin. Samples were checked by transmission electron microscopy (TEM) and high resolution scanning electron microscopy with a field emission gun (FESEM). Nanotubes with closed ends have 10–30 shells and an outer diameter of 5–50 nm, ranging in length from 20 nm to 5 mm. No fullerite was detected in Xray diffraction patterns of the bulk samples. Chemical
0925-9635/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 5 3 2 - 5
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H. Yusa / Diamond and Related Materials 11 (2002) 87–91
Fig. 1. (a) Nanotubes sample at 17.5 GPa in a diamond anvil cell before heating. (b) Sample after heating with a CO2 laser. Nanotubes in the heated area change to a transparent phase. The sample is surrounded by a KBr pressure medium in the hole of a gasket.
purity was confirmed by analysis using energy dispersive X-ray (EDX) analysis. The experimental procedure is similar to that described elsewhere using graphite as a starting material w2x. A stainless steel gasket (200 mm in diameter hole and 110 mm in thickness) was prepared for a sample chamber. The nanotube sample was formed into a platelike shape (5 mm thick and 150 mm edge length) and placed in a gasket with dried KBr as a pressure medium. The sample was enclosed with KBr to prevent thermal contact with anvils. A few grains of small ruby chips were used as a marker to measure pressure before and after heating. The sample was compressed to 16.0– 17.5 GPa at room temperature. The sample was observed to be broken into a few parts during compression in the solid pressure medium. A CO2 laser beam (240 W maximum) with a continuous wave was focused onto the sample at a diameter of 100 mm. The heated sample emitted intense radiation, which was monitored by a spectrometer, indicating temperature of 2500–3000 K. A transparent phase appeared in an area irradiated for a few tens of seconds of heating (Fig. 1b). The laser beam focus was carefully controlled on the center of the sample to avoid heating gasket materials. A rapid phase change was observed by a CCD camera. The sample was quenched by shutting off the laser beam. A 10% pressure drop was measured after heating due to annealing of the solid pressure medium. The sample was recovered at ambient pressure after releasing pressure. To check reproducibility, we repeated experiments under similar P and T conditions four times. A pressurized sample without heating was also recovered for comparison.
3. Results and discussion The sample surrounded by KBr pressure media was removed from the gasket and placed on a grid for TEM. Pressure media was dissolved in water and the sample was held on the grid. The sample remained transparent under an optical microscope after drying at 120 8C for 1 h. The recovered bulk sample was plate-like, almost the same as the starting sample. Raman microprobe spectroscopy identified the phase of the transparent part of the recovered sample as having a broad peak at a wavenumber of 1322–1324 cmy1 which is consistent with the mode of bulk diamond crystal with peak shifted to a slightly lower wavenumber (Fig. 2). These features are explained by a phonon confinement effect due to small grain size w8x. An amorphous phase and graphite with typical broad peaks w9x at 1580 cmy1 and 1350 cmy1 were not found in the transparent region. To examine changes from starting to recovered samples in detail, we took FE-SEM images (Fig. 3). Comparison of the images at the same magnification enabled us to better understand the mechanism behind the phase transformation. Interestingly, the tubular structure completely changed to granular and grain sizes corresponded to the diameter of the nanotubes. Grains were octahedral, common to diamond. No crystals were tubular. We assume that the tubular structure collapsed into nano-sized particles. If carbon atoms crystallize from solution or melt, we would expect crystals larger than the diameter of the starting materials. Fine crystals less than 50 nm in size suggest that these nano-sized diamonds were formed by direct transformation from nanotubes, without going through dissolved or molten
H. Yusa / Diamond and Related Materials 11 (2002) 87–91
Fig. 2. (a) The Raman spectrum of the recovered sample at 1 atm. (b) Raman spectrum of a synthetic large crystalline diamond (GE, 2 mm in size).
state. Diamond aggregates with nano-sized particles have been recovered by shock compression of fullurite w10–13x. Although particle showed sp3 bonding, the crystal structure was not clear due to the large strain on shock compression. They called the poorly defined aggregates amorphous diamond w10,11x. Crystal sizes are controlled by nanotubes used as a starting material in our study where static compression was employed.
89
As far as the author knows, the present synthesis is unique for obtaining nanocrystalline diamond having octahedral morphology. For further analysis, we conducted electron energy loss spectroscopy (EELS) using TEM (Hitachi HF2000) with a cold-field emission source and Gatan 666 PEELS system w14,15x. The EELS technique makes it possible to examine hybridization (sp2 or sp3) and the presence of p or s electrons with a resolution of 1 nm. As described above, it was difficult to obtain narrow and unshifted Raman signals due to the grain size effect, EELS analysis was conducted for further confirmation of crystal structure. EELS Plasmon loss and core loss spectra (K-absorption edge) were measured at several points on the samples, both heated and unheated, to clarify the transition mechanism under high pressure. Despite non-hydrostatic compression up to 17 GPa in a solid pressure medium, nanotubes remained tubular (Fig. 4a). The tubular structure thus did not collapse during compression due to the high yield strength of the curled graphitic planes. The fine structure of the carbon Kabsorption edge (Fig. 5a) showed the same features as reported for multiwalled nanotubes w16,17x. No trace of sp3 hybridization was observed with the unheated nanotubes. Based on electron diffraction patterns (EDP) (Fig. 4) no innermost diffraction ring corresponding to the index of the (002) graphite plane was detected in the heated sample, and no diffraction ring other than
Fig. 3. (a), (b) HRSEM images of multiwalled nanotubes as starting materials. (c), (d) HRSEM images of the recovered sample under high P and T conditions. Tubular samples completely converted to nanosize grains.
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H. Yusa / Diamond and Related Materials 11 (2002) 87–91
Fig. 4. TEM images and EDP of recovered samples. (a) Sample without heating at 17.0 GPa (b) the sample heated by a CO2 laser at 17.5 GPa.
those for diamond was found. No additional spots derived from structural imperfection were seen, indicating that the nanocrystalline diamonds contained few extended defects. The TEM image (Fig. 4b) of the heated sample showed no tubular structure or intermediate onion-like structure w18x as suggested by Wei et al. w6x. A p-electron plasmon peak at 285 eV observed in initial nanotubes was lost in the heated sample (Fig. 5b). Plasmon loss spectra (Fig. 6b) gave direct evidence that the phase has a s-electron state of sp3 hybridization equivalent to that of typical diamond w11x. No chemical composition other than carbon was detected on the samples by EELS (K-absorption edge) or EDX. Based on the above analysis, we concluded that the products were sintered compact with nanocrystalline grains of diamond. The grain size was controlled by the initial nanotube diameter. It is expected that sintered
compacts with nanocrystalline diamond may have high mechanical strength similar to natural polycrystalline diamond such as carbonado w19x. Synthesis of large sintered compacts using a large-volume high-pressure apparatus appears to be great interest in this respect. Acknowledgements The author thanks H. Matsumoto for his help with EELS experiments. This work was supported by the Center of Excellence Project at the National Institute for Materials Science. References w1x S. Iijima, Nature 354 (1991) 56. w2x H. Yusa, K. Takemura, Y. Matsui, H. Morishima, K. Watanabe, H. Yamawaki, K. Aoki, Appl. Phys. Lett. 72 (1998) 1843.
Fig. 5. EELS spectra (K-edge absorption) of recovered samples. (a) Laser-heated sample at 17.5 GPa. (b) Sample subjected to 17 GPa at room temperature.
H. Yusa / Diamond and Related Materials 11 (2002) 87–91
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Fig. 6. EELS spectra (plasmon loss) of recovered samples. (a) Laser-heated sample at 17.5 GPa. (b) Sample subjected to 17 GPa at room temperature. w3x W. Utsumi, T. Yagi, Science 252 (1991) 1542. w4x T. Yagi, W. Utsumi, M. Yamakata, T. Kikegawa, O. Shimomura, Phys. Rev. B46 (1992) 6031. w5x B. Wei, J. Zhang, J. Liang, W. Liu, Z. Gao, D. Wu, J. Mat. Sci, Lett. 16 (1997) 402. w6x B. Wei, J. Zhang, J. Liang, D. Wu, Carbon 36 (1998) 997. w7x F.P. Bundy, J. Geophys. Res. 85 (1980) 6930. w8x M. Yoshikawa, Y. Mori, M. Maegawa, G. Katagiri, H. Ishida, A. Ishitani, Appl. Phys. Lett. 62 (1993) 3114. w9x P.V. Huong, Diamond Relat. Mater. 1 (1991) 33. w10x H. Hirai, K. Kondo, T. Ohwada, Carbon 31 (1993) 1095. w11x H. Hirai, K. Kondo, N. Yoshizawa, M. Shiraishi, Appl. Phys. Lett. 64 (1994) 1797.
w12x C.S. Yoo, W.J. Nellis, Science 254 (1991) 1489. w13x T. Sekine, 1992 Proceedings Japanese Academic Series, B68 95. w14x Hitachi Scientific Instrument Technical Data, No. 69, (1990). w15x T. Kogure, K. Saiki, M. Konno, T. Kamino, Mat. Res. Soc. Symp. Proc. 504 (1998) 183. w16x Y. Kiyose, S. Horiuchi, M. Kyotani, M. Yumura, K. Uchida, S. Ohshima, Y. Kuriki, F. Ikazaki, N. Yamahira, Thin Solid Films 273 (1996) 222. w17x R. Kuzuo, M. Terauchi, M. Tanaka, Jpn. J. Appl. Phys. 31 (1992) L1484. w18x F. Banhart, M. Ajayan, Nature 382 (1996) 433. w19x L.F. Trueb, E.C. deWys, Science 165 (1969) 799.