- Email: [email protected]
Surface defects observed with a scanning tunneling microscope on microcrystalline graphite coated by silicone grease
Journal of Nuclear North-Holland
Materials
189 (1992) 152-154
Letter to the Editors
Surface defects observed with a scanning tunneling microscope on microcrystalline graphite coated by silicone grease T. Sumomogi, T. Endo, H. Yamada and K. Kuwahara Hiroshima-Denki Institute of Technology, Nakano 6-20-1, Aki-ku. Hiroshima 739-03, Japan
S. Morita and T. Kino Department of Physics, Hiroshima Unirtersity, Hiroshima 730, Japan Received
8 August
1991; accepted
15 February
1992
In the field of nuclear fusion technology, numerous graphite materials with different structures have been used because of their low-Z characteristics [ll. Their radiation-induced damage structures have been investigated mainly by using a transmission electron microscope 121. For microcrystalline graphite materials consisting of grains with diameter below several nanometers, however, there have been no definite ways to characterize small defects of such grains. Since the electron configuration in graphite structures is extremely localized, the characterization has to be performed not only with the direct microscopic observation but also with some means of utilizing electronic information near the defects.
A scanning tunneling microscope (STM) is a very powerful tool for the study on the atomic structure of the surface, with atomic resolution. On the other hand, a scanning tunneling spectroscope (STS) has other several important merits. Electronic behavior with atomic resolution is detectable by the spectroscopic measurement (tunneling current versus tunneling bias characteristics, and its first and second differential characteristics). Although material species to be observable using STM/STS has been limited so far, its wide applications for the material science and technology have been recently developed, as seen for an in the 5th International Conference on example, STM/STS (STM ‘90, Baltimore) [3]. For the observation of the atomic configuration on material surfaces using STM/STS, the fresh surface and the surface flatness have an important role. In the previous papers [4,5], we showed that the fresh and partially flat surface could be obtained for bulk metals, when the sample was polished in oil such as silicone oil used in a diffusion pump, and was kept in the oil without exposure in atmosphere, for the direct STM/STS observation. Recently, we have found that the surface coated by silicone grease maintained the freshness for a longer period, compared with a case 0022-3115/92/$05.00
0 1992 - Elsevier
Science
Publishers
keeping the surface in the oil. In this case, the surface was covered by silicone grease in the oil bath just after the polishing and cleaning. For the satisfactory observation of bulk surfaces with atomic resolution using STM, the flatness of surface is most important. According to our experiments. the flatness required is below several ten nanometers. A surface of bulk microcrystalline graphite (commercial name of “glassy carbon” obtained from Showa Denko Co., Ltd.) has a flatness within about 50 nm after polishing (maybe, by cleaving) with emery papers. The sample is composed of microcrystals with several nanometers grains in diameter having graphite structures. The c-axis is highly oriented according to X-ray analysis. The grain contains a high defect density. So, it is presumed that the glassy carbon is a typical material for the investigation of defects in graphite structures using STM. In this paper, we show that the structural small defects of glassy carbon coated by silicone grease arc visible using STM. The sample was prepared as follows, we exposed the fresh surface of the bulk glassy carbon by cleaving with emergy papers in a diffusion pump oil bath, and washed it ultrasonically in the same new oil. After coating the surface by silicone grease to
B.V. All rights
reserved
T. Sumomogi et al. / Surface defects on microcrystalline graphite
Fig. 1. STM image (3.0 nmX3.0 nm) of HOPG graphite coated by silicone grease. For all STM images in this paper, the gray tone spans approximately 0.7 nm from the base of the
153
Fig. 2. An example of STM image (6.0 nm X 6.0 nm) on glassy carbon coated by silicone grease. The flat surface over a large area is seen.
figure.
avoid the exposure of sample surface in air, the sample was set under a tunneling tip of a Pt-Ir wire. At the initial stage of the STM observation, the image was noisy and an image with atomic resolution was not obtained. After many trials of scanning of the tip over the sample surface, however, atomic rows could be observed stably and the state was continuously kept for several weeks. The reason may be due to the removal of some kinds of contaminations or dust loosely combined around the tunneling tip. All STM images shown in this paper are current images with no image processing, under the condition of the tip bias Vr = + 27 mV and the tunneling current I, = 13 nA. One image made of 256 X 12.5 dots is obtained in about 8 s. In fig. 1, an STM image of HOPG graphite coated by silicone grease is shown as a standard case. We can not find any differences between the STM images observed in air and in grease, which indicates that the grease coating has no influence on STM observation with atomic resolution. In fig. 2, an example of STM image on glassy carbon is shown. The surface is satisfactory flat so as to observe atomic configurations using STM, in spite of the polishing with emery paper. We can see atomic configurations in wide surface ranges. However, the atomic configuration at the lower right hand side in the figure is different from those of other sides. The features can be seen more clearly in fig. 3 with
the same scale as in fig. 2. In the figure, we can observe a discrepancy of atomic array directions. Usually, the angles between directions of atomic arrays on graphite should be 60” and/or 120” in STM images. However, we can see the angle of about 135” in the left hand side of the figure. The origin of these discrepancies is unknown at present. In figs. 4 and 5, we show an extra atomic row and a vacancy-like point defect in the STM image. The structures and the characterizations of such disordered arrays are left in further investigations, because of two reasons described below.
Fig. 3. Unusual atomic arrays with the angle of 135” on glassy carbon with the same scale as in fig. 2.
T. Sumomogi et al. / Surface defects on microcrystalline graphite
154
First, on an STM image of graphite, observable atoms on the first hexagonal basal plane are restricted to the atoms that have the second layer atoms just under them. If these second layer atoms were displaced by some reason, disordered atomic configurations like these should be observable in STM images. So, the extra atomic row or the vacancy-like defects in the STM image in figs. 4 and 5 may not mean the dislocation or vacancy. Second, as for the defect structures on graphite, several models have been proposed at present [6]. A usual isotropic model for lattice defects, such as in metals, is not applicable for graphite because of its layered structure. Even the existence of edge or screw dislocations in graphite structures [7,8] has not been confirmed yet. The present achievement indicates the possibilities of the application of STM as a tool for the study of defect structures and their characterizations with atomic resolutions (especially, for small defects not to be observable by transmission electron microscopy). In that case, the investigations on electronic structures near the defects using STS may become powerful. because the STS measurements may offer the information on C-C bonding near the defects. This is our main target at present. In conclusion, we found several kinds of defects in graphite structures with atomic resolution using STM.
Fig. 5. Vacancy-like
point defect in STM image (3.0 nmx3.0 nm) of glassy carbon.
These achievements indicate that STM is applicable for the investigation of defect structures and their characterizations for graphite. Acknowledgement
The support by the Science Research Promotion Fund from Japan Private School Promotion Foundation is gratefully acknowledged.
References
Fig. 4. Extra
atomic
rows in STM image (3.0 nm X 3.0 nm) of glassy carbon
[l] A. Miyahara and T. Tanabe, J. Nucl. Mater. 155-157 (1988) 49. [2] G.R. Millward and D.A. Jefferson, Chem. and Phys. Carbon 14 (1978) 1. [3] Proc. 5th Int. Conf. on Scanning Tunneling Microscope/ Spectroscope & 1st Int. Conf. on NAN0 I, eds. R.J. Colton, R.K. Marrian and J.A. Stroscio (American Vacuum Society, New York, 1990). [4] T. Endo, H. Yamada, T. Sumomogi, K. Kuwahara, T. Fujita and S. Morita, J. Vat. Sci. Technol. A8 (1990) 468. [5] T. Endo, H. Yamada, T. Sumomogi, K. Kohno, K. Kuwahara, T. Fujita and S. Morita, J. Appl. Phys. 68 (1990) 2528. [6] B.T. Kelly, Carbon 20 (1982) 3. [7] K. Sone, et al, J. Nucl. Mater. 71 (1977) 82. [S] M. Saidoh, R. Yamada and K. Nakamura, J. Nucl. Mater. 102 (1981) 97.
Materials
189 (1992) 152-154
Letter to the Editors
Surface defects observed with a scanning tunneling microscope on microcrystalline graphite coated by silicone grease T. Sumomogi, T. Endo, H. Yamada and K. Kuwahara Hiroshima-Denki Institute of Technology, Nakano 6-20-1, Aki-ku. Hiroshima 739-03, Japan
S. Morita and T. Kino Department of Physics, Hiroshima Unirtersity, Hiroshima 730, Japan Received
8 August
1991; accepted
15 February
1992
In the field of nuclear fusion technology, numerous graphite materials with different structures have been used because of their low-Z characteristics [ll. Their radiation-induced damage structures have been investigated mainly by using a transmission electron microscope 121. For microcrystalline graphite materials consisting of grains with diameter below several nanometers, however, there have been no definite ways to characterize small defects of such grains. Since the electron configuration in graphite structures is extremely localized, the characterization has to be performed not only with the direct microscopic observation but also with some means of utilizing electronic information near the defects.
A scanning tunneling microscope (STM) is a very powerful tool for the study on the atomic structure of the surface, with atomic resolution. On the other hand, a scanning tunneling spectroscope (STS) has other several important merits. Electronic behavior with atomic resolution is detectable by the spectroscopic measurement (tunneling current versus tunneling bias characteristics, and its first and second differential characteristics). Although material species to be observable using STM/STS has been limited so far, its wide applications for the material science and technology have been recently developed, as seen for an in the 5th International Conference on example, STM/STS (STM ‘90, Baltimore) [3]. For the observation of the atomic configuration on material surfaces using STM/STS, the fresh surface and the surface flatness have an important role. In the previous papers [4,5], we showed that the fresh and partially flat surface could be obtained for bulk metals, when the sample was polished in oil such as silicone oil used in a diffusion pump, and was kept in the oil without exposure in atmosphere, for the direct STM/STS observation. Recently, we have found that the surface coated by silicone grease maintained the freshness for a longer period, compared with a case 0022-3115/92/$05.00
0 1992 - Elsevier
Science
Publishers
keeping the surface in the oil. In this case, the surface was covered by silicone grease in the oil bath just after the polishing and cleaning. For the satisfactory observation of bulk surfaces with atomic resolution using STM, the flatness of surface is most important. According to our experiments. the flatness required is below several ten nanometers. A surface of bulk microcrystalline graphite (commercial name of “glassy carbon” obtained from Showa Denko Co., Ltd.) has a flatness within about 50 nm after polishing (maybe, by cleaving) with emery papers. The sample is composed of microcrystals with several nanometers grains in diameter having graphite structures. The c-axis is highly oriented according to X-ray analysis. The grain contains a high defect density. So, it is presumed that the glassy carbon is a typical material for the investigation of defects in graphite structures using STM. In this paper, we show that the structural small defects of glassy carbon coated by silicone grease arc visible using STM. The sample was prepared as follows, we exposed the fresh surface of the bulk glassy carbon by cleaving with emergy papers in a diffusion pump oil bath, and washed it ultrasonically in the same new oil. After coating the surface by silicone grease to
B.V. All rights
reserved
T. Sumomogi et al. / Surface defects on microcrystalline graphite
Fig. 1. STM image (3.0 nmX3.0 nm) of HOPG graphite coated by silicone grease. For all STM images in this paper, the gray tone spans approximately 0.7 nm from the base of the
153
Fig. 2. An example of STM image (6.0 nm X 6.0 nm) on glassy carbon coated by silicone grease. The flat surface over a large area is seen.
figure.
avoid the exposure of sample surface in air, the sample was set under a tunneling tip of a Pt-Ir wire. At the initial stage of the STM observation, the image was noisy and an image with atomic resolution was not obtained. After many trials of scanning of the tip over the sample surface, however, atomic rows could be observed stably and the state was continuously kept for several weeks. The reason may be due to the removal of some kinds of contaminations or dust loosely combined around the tunneling tip. All STM images shown in this paper are current images with no image processing, under the condition of the tip bias Vr = + 27 mV and the tunneling current I, = 13 nA. One image made of 256 X 12.5 dots is obtained in about 8 s. In fig. 1, an STM image of HOPG graphite coated by silicone grease is shown as a standard case. We can not find any differences between the STM images observed in air and in grease, which indicates that the grease coating has no influence on STM observation with atomic resolution. In fig. 2, an example of STM image on glassy carbon is shown. The surface is satisfactory flat so as to observe atomic configurations using STM, in spite of the polishing with emery paper. We can see atomic configurations in wide surface ranges. However, the atomic configuration at the lower right hand side in the figure is different from those of other sides. The features can be seen more clearly in fig. 3 with
the same scale as in fig. 2. In the figure, we can observe a discrepancy of atomic array directions. Usually, the angles between directions of atomic arrays on graphite should be 60” and/or 120” in STM images. However, we can see the angle of about 135” in the left hand side of the figure. The origin of these discrepancies is unknown at present. In figs. 4 and 5, we show an extra atomic row and a vacancy-like point defect in the STM image. The structures and the characterizations of such disordered arrays are left in further investigations, because of two reasons described below.
Fig. 3. Unusual atomic arrays with the angle of 135” on glassy carbon with the same scale as in fig. 2.
T. Sumomogi et al. / Surface defects on microcrystalline graphite
154
First, on an STM image of graphite, observable atoms on the first hexagonal basal plane are restricted to the atoms that have the second layer atoms just under them. If these second layer atoms were displaced by some reason, disordered atomic configurations like these should be observable in STM images. So, the extra atomic row or the vacancy-like defects in the STM image in figs. 4 and 5 may not mean the dislocation or vacancy. Second, as for the defect structures on graphite, several models have been proposed at present [6]. A usual isotropic model for lattice defects, such as in metals, is not applicable for graphite because of its layered structure. Even the existence of edge or screw dislocations in graphite structures [7,8] has not been confirmed yet. The present achievement indicates the possibilities of the application of STM as a tool for the study of defect structures and their characterizations with atomic resolutions (especially, for small defects not to be observable by transmission electron microscopy). In that case, the investigations on electronic structures near the defects using STS may become powerful. because the STS measurements may offer the information on C-C bonding near the defects. This is our main target at present. In conclusion, we found several kinds of defects in graphite structures with atomic resolution using STM.
Fig. 5. Vacancy-like
point defect in STM image (3.0 nmx3.0 nm) of glassy carbon.
These achievements indicate that STM is applicable for the investigation of defect structures and their characterizations for graphite. Acknowledgement
The support by the Science Research Promotion Fund from Japan Private School Promotion Foundation is gratefully acknowledged.
References
Fig. 4. Extra
atomic
rows in STM image (3.0 nm X 3.0 nm) of glassy carbon
[l] A. Miyahara and T. Tanabe, J. Nucl. Mater. 155-157 (1988) 49. [2] G.R. Millward and D.A. Jefferson, Chem. and Phys. Carbon 14 (1978) 1. [3] Proc. 5th Int. Conf. on Scanning Tunneling Microscope/ Spectroscope & 1st Int. Conf. on NAN0 I, eds. R.J. Colton, R.K. Marrian and J.A. Stroscio (American Vacuum Society, New York, 1990). [4] T. Endo, H. Yamada, T. Sumomogi, K. Kuwahara, T. Fujita and S. Morita, J. Vat. Sci. Technol. A8 (1990) 468. [5] T. Endo, H. Yamada, T. Sumomogi, K. Kohno, K. Kuwahara, T. Fujita and S. Morita, J. Appl. Phys. 68 (1990) 2528. [6] B.T. Kelly, Carbon 20 (1982) 3. [7] K. Sone, et al, J. Nucl. Mater. 71 (1977) 82. [S] M. Saidoh, R. Yamada and K. Nakamura, J. Nucl. Mater. 102 (1981) 97.