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The nature of an amorphous-like species formed on the surface of copper
Scripta Metallurgicae t Materialia.Vol.30, No. 7. pp. 889-894, 1994 Copyright© 1994ElsevierScienceLtd Printedin the USA.All rightsreserved 0956-716X/94 $6.00+ .00
Pergamon
THE NATURE OF AN AMORPHOUS-LIKE SPECIES FORMED ON THE SURFACE OF COPPER Ruo-Nan Guan a, Ri-Sheng Lia'b, Shu-Hua Xu a and Ying-Da Yu a a. Laboratory of Atomic Imaging of Solids, Institute of Metal Research, Academia Sinica, Shenyang 110015, China b. Corrosion Science Laboratory, Institute of Corrosion and Protection of Metals, Chinese Academy of Sciences, Shenyang 110015, China
(Received August 16, 1993) (Revised December 13, 1993) 1. Introduction During the study of the initial stage of copper oxidation by using high resolution electron microscopy(HREM)[1-5], we have observed, in addition to a number of metastable copper oxides, some structureless species. Such species formed at the periphery of a copper sample when the fresh surface of the sample exposed to air, despite the fact that the sample was an electrolytically-thinned or ion-thinned copper film, or a copper particle prepared by the gas evaporation method[6] or by vacuum evaporation[7]. A typical HREM image of such a species is shown in fig.la. We now tentatively call such a species amorphous-like because, from its HREM image, while it seems not completely disordered, it has no usual crystalline structure. More interestingly, such a species can transform into crystalline form during EM observation. To our knowledge, the nature of such a species and its behavior during an EM observation has not been investigated yet. As discussed in this paper, this phenomenon may correlate with the oxidation of copper. Hence, recently we carried out a systematic study and here report our preliminary results. 2. Exoeriments The HREM observations were performed in a JEM 2000EX microscope. The base pressure of the chamber is 104 Pa, and the incident electron beam was operated at 200 kV. The electro-optical magnification was 4-6x10 s and the point-to-point resolution was 0.21 nm. In order to estimate the composition of the amorphous-like species, an Auger electron spectroscopy (ALES) analysis of a mirror surface of copper prepared in air was carried out using a LAS-300 spectrometer made by Riber. The primary electron beam, with a diameter of about 0.2 mm for an AES analysis, was operated at 3 kV and 101aA. The base pressure of the chamber was 6.7x10 ~ Pa. The copper film sample was prepared by Ar-ion-thinning a pure copper sheet with a purity of 99.99 wt.% under a 1.33x10 2 Pa Ar atmosphere. The sample was cleaned with ethanol before placing it on a bare copper sample cup without a carbon microgrid. In order to follow the formation of the amorphous-like species in-situ, some ultrafine copper particles were used as the samples. These particles were obtained during observation of a copper powder agglomerate. The details of the formation of these secondary copper particles have been published in a previous paper[8].
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3. Results and Discussion Figure la shows an H R E M profile image of the so-called amorphous-like species formed on the copper surface at room temperature in air. Since similar amorphous-like species have been observed, not only on the ion-thinned copper film, but also on an electrolytically thinned one, we tentatively assume that such amorphous-like species should be the product of the reaction of the clean copper surface with the ambient atmosphere. This point has been confirmed by the AES analysis of various copper mirror surfaces, which were prepared by mechanically or electrolytic,ally polishing. One such ~ spectra is shown in fig.ld. From fig.ld, we can see that both copper and oxygen are the main components of the surface species, although a small amount S, CI, C and N are also present at the surface. Since the observed amorphous-like species has a thickness of about 10 nm, as shown in fig.la, and the probing depth of the AES has the same order of magnitude, we suggest that the AES analytical result shown in fig.ld should represent, at least qualitatively, the composition of such amorphous-like species. We can see from fig.la that the profile image of the species has a Moire or fingerprint feature, which can more clearly be seen from the front image of a growing amorphous-like species, as shown in fig.lc. Figure lb shows the EDP of the image shown in fig.la, in which the faint diffraction circle near the spot (000) stems from the amorphous-like species, while the sharp diffraction spots obviously stem from the substrate Cu(110). The fringe spacing, as indicated by two black arrows in the figure, can be estimated from the image shown in fig.la, as well as the EDP shown in fig.lb to be 0.35-0.40 nm. It is interesting to note that there is no presence of a crystalline copper oxide, except that the copper-oxygen-containing amorphous-like species could be observed from both HREM image shown in fig.la and the EDP shown in fig.lb. This fact implies that the reaction of Cu with O in air at room temperature may tend to form a copper-oxygen-containing amorphouslike species. We assume that the main components of the amorphous-like species are copper and oxygen. This point has been confirmed not only by AES analysis shown in fig.ld, but also by direct HREM observations of the appearance and disappearance of such species. Figure 2a shows three copper particles marked by A, B and C which have been identified to be copper by both their ED pattern and their HREM image, as shown in fig.2b. The details of such identification have been published in the previous paper[8]. We note that particle C in fig.2a was already surrounded by the preformed amorphous-like species, but not yet particles A and B. After observation for 7 minutes, however, particles A and C were nearly incorporated into the amorphous-like species, leaving particle B smaller. After two more minutes, particle B was also nearly incorporated into the amorphous-like species. These results clearly indicate that the amorphous-like species can grow up by incorporating copper material and, accordingly, copper should be the main component. It is also interesting to note that such amorphous-like species can transform into a crystalline copper oxide under electron beam irradiation. A typical example is shown in fig.3, which is the amorphous-like species displayed in fig.la, but photographed 3 hours after electron beam irradiation. We can see from this figure that this amorphous-like species seems to become more disordered, probably due to it being transformed, while the lattice image of substrate Cu(ll0) shown in the bottom is very clear. After 3 hours of observation, we saw that such a transformation became more and more drastic and that a Cu20 crystallite grew up from the amorphous-like species, as shown in fig.3b. Furthermore, we observed that part of this amorphous-like layer collapsed leaving a cavity in the top region and several Cu20 crystailites appeared near the cavity, as shown in fig.3c, which was taken 30 minutes after observation. The fringe spacing marked by two black arrows in the figure is 0.24 nm, which corresponds to the spacing of Cu20{111}. The EDP of fig.3c is shown in fig.3d.
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Comparing fig.3d with fig.lb we can see that the faint circle near the center, which stems from the amorphouslike species, became very weak; in the meantime, some dotted diffraction circles, which are identified as the diffraction from Cu20{lll} and {002}, appeared. Therefore, the EDPs also confirmed that a transformation took place from an amorphous-like species into the crystalline copper oxides. Combining these results with the results shown in fig.l, when a fresh copper surface is in contact with air, a copper-oxygen-containing amorphous-like species, rather than a crystalline copper oxides forms, as demonstrated by fig.la and lb; we therefore suggest that the oxidation of copper at room temperature may proceed by two steps, namely, the interaction of copper with oxygen may tend to form an oxygen-copper-containing amorphous-like species, and then this amorphous-like species transforms into a crystalline copper oxide under appropriate conditions. 4. Conclusion We have investigated the nature of an amorphous-like species formed on the surface of copper by HREM observation and AES analysis. The AES analysis of the copper mirror surface prepared in air indicates that both copper and oxygen may be the main components of such a species. The conclusion of the composition of such a species has been confirmed by H R E M observations, both that an amorphous-like species grew up by incorporating three copper particles and such a species formed in air transformed into crystalline copper oxide under electron beam irradiation. The results observed suggest that such an amorphous-like ~pecies may play a precursor role during copper oxidation, which deserves further investigation. 5. Acknowledgement v
This work is supported by the National Natural Science Foundation of China and the National Material Committee of China. References [1] R. Guan, [2] R. Guan, [3] R. Guan, [4] R. Guan, [5] R. Guan, [6] J. Xu, X. [7] Y.D. Yu, [8] Y.D. Yu,
H. Hashimoto and T. Yoshida, Acta Cryst. B40,109(1984) H. Hashimoto and K.H. Kuo, Acta Cryst. B40,560(1984) H. Hashimoto and K.H. Kuo, Acta Cryst. B41,219(1985) H. Hashimoto, K.H. Kuo and T. Yoshida, Acta Cryst. B43,343(1987) H. Hashimoto, and K.H. Kuo, Acta Cryst. B46,103(1990) Sun, W. Chen, X. Fan and W. Wei, Mater. Sci. Prog. 6,209(1992) S.H. Xu and R. Guan, unpublished R. Guan, X. Sun, J. Xu, W. Chen and W. Wei, Script. Metallurgica 26,25(1992)
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c~ ~
~
c
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c
a
Fig.1 (a) The profile image of an amorphous-like species formed on the surface of an ion-thinned copper film. (b) The electron diffraction pattern of the sample shown in (a). (c) The front image of an amorphouslike species formed on Cu (100). (d) The Auger spectra of a copper mirror surface prepared in air.
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!i!i¸~ii!! •
Fig.2
An amorphous-like species grew up by incorporating three ultrafine copper particles. (a) The initial situation of three particles is marked respectively by A, B and C in the figure. (b) The magnification of the image of particle A shown in fig.2(a). The spacings marked by two black and two white bars are 0.18 and 0.21 nm, which correspond to the spacings of Cu{002} and Cu{lll}, respectively. (c) Three copper particles were incorporated into the amorphous-like species. (d) Accompanying the growth of the amorphous-like species, the images of the copper particles disappeared.
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Fig.3 The transformation process from an amorphous-like species shown in fig.la into several Cu20 crystallites shown in fig.3c.
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Pergamon
THE NATURE OF AN AMORPHOUS-LIKE SPECIES FORMED ON THE SURFACE OF COPPER Ruo-Nan Guan a, Ri-Sheng Lia'b, Shu-Hua Xu a and Ying-Da Yu a a. Laboratory of Atomic Imaging of Solids, Institute of Metal Research, Academia Sinica, Shenyang 110015, China b. Corrosion Science Laboratory, Institute of Corrosion and Protection of Metals, Chinese Academy of Sciences, Shenyang 110015, China
(Received August 16, 1993) (Revised December 13, 1993) 1. Introduction During the study of the initial stage of copper oxidation by using high resolution electron microscopy(HREM)[1-5], we have observed, in addition to a number of metastable copper oxides, some structureless species. Such species formed at the periphery of a copper sample when the fresh surface of the sample exposed to air, despite the fact that the sample was an electrolytically-thinned or ion-thinned copper film, or a copper particle prepared by the gas evaporation method[6] or by vacuum evaporation[7]. A typical HREM image of such a species is shown in fig.la. We now tentatively call such a species amorphous-like because, from its HREM image, while it seems not completely disordered, it has no usual crystalline structure. More interestingly, such a species can transform into crystalline form during EM observation. To our knowledge, the nature of such a species and its behavior during an EM observation has not been investigated yet. As discussed in this paper, this phenomenon may correlate with the oxidation of copper. Hence, recently we carried out a systematic study and here report our preliminary results. 2. Exoeriments The HREM observations were performed in a JEM 2000EX microscope. The base pressure of the chamber is 104 Pa, and the incident electron beam was operated at 200 kV. The electro-optical magnification was 4-6x10 s and the point-to-point resolution was 0.21 nm. In order to estimate the composition of the amorphous-like species, an Auger electron spectroscopy (ALES) analysis of a mirror surface of copper prepared in air was carried out using a LAS-300 spectrometer made by Riber. The primary electron beam, with a diameter of about 0.2 mm for an AES analysis, was operated at 3 kV and 101aA. The base pressure of the chamber was 6.7x10 ~ Pa. The copper film sample was prepared by Ar-ion-thinning a pure copper sheet with a purity of 99.99 wt.% under a 1.33x10 2 Pa Ar atmosphere. The sample was cleaned with ethanol before placing it on a bare copper sample cup without a carbon microgrid. In order to follow the formation of the amorphous-like species in-situ, some ultrafine copper particles were used as the samples. These particles were obtained during observation of a copper powder agglomerate. The details of the formation of these secondary copper particles have been published in a previous paper[8].
889
890
SURFACE OF COPPER
Vol.
30, No.
3. Results and Discussion Figure la shows an H R E M profile image of the so-called amorphous-like species formed on the copper surface at room temperature in air. Since similar amorphous-like species have been observed, not only on the ion-thinned copper film, but also on an electrolytically thinned one, we tentatively assume that such amorphous-like species should be the product of the reaction of the clean copper surface with the ambient atmosphere. This point has been confirmed by the AES analysis of various copper mirror surfaces, which were prepared by mechanically or electrolytic,ally polishing. One such ~ spectra is shown in fig.ld. From fig.ld, we can see that both copper and oxygen are the main components of the surface species, although a small amount S, CI, C and N are also present at the surface. Since the observed amorphous-like species has a thickness of about 10 nm, as shown in fig.la, and the probing depth of the AES has the same order of magnitude, we suggest that the AES analytical result shown in fig.ld should represent, at least qualitatively, the composition of such amorphous-like species. We can see from fig.la that the profile image of the species has a Moire or fingerprint feature, which can more clearly be seen from the front image of a growing amorphous-like species, as shown in fig.lc. Figure lb shows the EDP of the image shown in fig.la, in which the faint diffraction circle near the spot (000) stems from the amorphous-like species, while the sharp diffraction spots obviously stem from the substrate Cu(110). The fringe spacing, as indicated by two black arrows in the figure, can be estimated from the image shown in fig.la, as well as the EDP shown in fig.lb to be 0.35-0.40 nm. It is interesting to note that there is no presence of a crystalline copper oxide, except that the copper-oxygen-containing amorphous-like species could be observed from both HREM image shown in fig.la and the EDP shown in fig.lb. This fact implies that the reaction of Cu with O in air at room temperature may tend to form a copper-oxygen-containing amorphouslike species. We assume that the main components of the amorphous-like species are copper and oxygen. This point has been confirmed not only by AES analysis shown in fig.ld, but also by direct HREM observations of the appearance and disappearance of such species. Figure 2a shows three copper particles marked by A, B and C which have been identified to be copper by both their ED pattern and their HREM image, as shown in fig.2b. The details of such identification have been published in the previous paper[8]. We note that particle C in fig.2a was already surrounded by the preformed amorphous-like species, but not yet particles A and B. After observation for 7 minutes, however, particles A and C were nearly incorporated into the amorphous-like species, leaving particle B smaller. After two more minutes, particle B was also nearly incorporated into the amorphous-like species. These results clearly indicate that the amorphous-like species can grow up by incorporating copper material and, accordingly, copper should be the main component. It is also interesting to note that such amorphous-like species can transform into a crystalline copper oxide under electron beam irradiation. A typical example is shown in fig.3, which is the amorphous-like species displayed in fig.la, but photographed 3 hours after electron beam irradiation. We can see from this figure that this amorphous-like species seems to become more disordered, probably due to it being transformed, while the lattice image of substrate Cu(ll0) shown in the bottom is very clear. After 3 hours of observation, we saw that such a transformation became more and more drastic and that a Cu20 crystallite grew up from the amorphous-like species, as shown in fig.3b. Furthermore, we observed that part of this amorphous-like layer collapsed leaving a cavity in the top region and several Cu20 crystailites appeared near the cavity, as shown in fig.3c, which was taken 30 minutes after observation. The fringe spacing marked by two black arrows in the figure is 0.24 nm, which corresponds to the spacing of Cu20{111}. The EDP of fig.3c is shown in fig.3d.
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Comparing fig.3d with fig.lb we can see that the faint circle near the center, which stems from the amorphouslike species, became very weak; in the meantime, some dotted diffraction circles, which are identified as the diffraction from Cu20{lll} and {002}, appeared. Therefore, the EDPs also confirmed that a transformation took place from an amorphous-like species into the crystalline copper oxides. Combining these results with the results shown in fig.l, when a fresh copper surface is in contact with air, a copper-oxygen-containing amorphous-like species, rather than a crystalline copper oxides forms, as demonstrated by fig.la and lb; we therefore suggest that the oxidation of copper at room temperature may proceed by two steps, namely, the interaction of copper with oxygen may tend to form an oxygen-copper-containing amorphous-like species, and then this amorphous-like species transforms into a crystalline copper oxide under appropriate conditions. 4. Conclusion We have investigated the nature of an amorphous-like species formed on the surface of copper by HREM observation and AES analysis. The AES analysis of the copper mirror surface prepared in air indicates that both copper and oxygen may be the main components of such a species. The conclusion of the composition of such a species has been confirmed by H R E M observations, both that an amorphous-like species grew up by incorporating three copper particles and such a species formed in air transformed into crystalline copper oxide under electron beam irradiation. The results observed suggest that such an amorphous-like ~pecies may play a precursor role during copper oxidation, which deserves further investigation. 5. Acknowledgement v
This work is supported by the National Natural Science Foundation of China and the National Material Committee of China. References [1] R. Guan, [2] R. Guan, [3] R. Guan, [4] R. Guan, [5] R. Guan, [6] J. Xu, X. [7] Y.D. Yu, [8] Y.D. Yu,
H. Hashimoto and T. Yoshida, Acta Cryst. B40,109(1984) H. Hashimoto and K.H. Kuo, Acta Cryst. B40,560(1984) H. Hashimoto and K.H. Kuo, Acta Cryst. B41,219(1985) H. Hashimoto, K.H. Kuo and T. Yoshida, Acta Cryst. B43,343(1987) H. Hashimoto, and K.H. Kuo, Acta Cryst. B46,103(1990) Sun, W. Chen, X. Fan and W. Wei, Mater. Sci. Prog. 6,209(1992) S.H. Xu and R. Guan, unpublished R. Guan, X. Sun, J. Xu, W. Chen and W. Wei, Script. Metallurgica 26,25(1992)
892
SURFACE OF COPPER
Vol.
30, No.
d
c~ ~
~
c
Aq
g
c
a
Fig.1 (a) The profile image of an amorphous-like species formed on the surface of an ion-thinned copper film. (b) The electron diffraction pattern of the sample shown in (a). (c) The front image of an amorphouslike species formed on Cu (100). (d) The Auger spectra of a copper mirror surface prepared in air.
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30, No.
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SURFACE
OF COPPER
893
!i!i¸~ii!! •
Fig.2
An amorphous-like species grew up by incorporating three ultrafine copper particles. (a) The initial situation of three particles is marked respectively by A, B and C in the figure. (b) The magnification of the image of particle A shown in fig.2(a). The spacings marked by two black and two white bars are 0.18 and 0.21 nm, which correspond to the spacings of Cu{002} and Cu{lll}, respectively. (c) Three copper particles were incorporated into the amorphous-like species. (d) Accompanying the growth of the amorphous-like species, the images of the copper particles disappeared.
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OF COPPER
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Fig.3 The transformation process from an amorphous-like species shown in fig.la into several Cu20 crystallites shown in fig.3c.
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