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One-dimensional growth induced by thermal stress
Materials Letters 58 (2004) 1917 – 1919 www.elsevier.com/locate/matlet
One-dimensional growth induced by thermal stress Xiaona Zhang, Chaorong Li *, Ze Zhang Beijing Laboratory of Electron Microscopy, Institute of Physics, Chinese Academy of Sciences, PO Box 603, Beijing 100080, China Received 29 September 2003; accepted 27 November 2003
Abstract Amorphous SiOx nanowires grown on the surface of big Ag particles were synthesized by evaporation of a mixture of SiO and Ag2O. The morphologies at different growth stages hint at a special kind of growth mechanism different from the classical vapor – liquid – solid (V – L – S) mechanism and oxygen assistant mechanism. A layer of SiOx wraps the Ag particles in the cooling process. The SiOx nanowire growth is induced by the thermal stress accumulated in the interface of the outer SiOx layer and Ag particles for different shrinking coefficients. When the thermal stress accumulates to a certain degree, the SiOx layer chaps and peels off from the Ag particles to form SiOx nanowires. D 2004 Elsevier B.V. All rights reserved. Keywords: Deposition; Nanomaterials; Surface
The synthesis of one-dimensional nanostructures has recently reached a new horizon. Up to now, it is not a great challenge in materials science. Many kinds of one-dimensional nanostructures including nanowires, nanotubes, nanobelts, and nanorods have been obtained by different methods [1 –8]. In addition to the synthesis techniques, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray, etc., much attention has been put on the physics and chemical problems in the growth process of one-dimensional nanostructures. Scientists want to understand what induced the one-dimensional growth. Thus the research work on growth mechanism of nanowires is very important. Many authors have reported on the synthesis and growth mechanism of Si and SiOx nanowires [9– 12]. The synthesis methods mainly include chemical vapor deposition, laser ablation, and thermal evaporation. Two typical mechanisms have been put forward to explain the one-dimensional growth process: one is vapor – liquid –solid (V – L –S) mechanism [13] and the other is oxygen assistant mechanism [14]. The V – L – S mechanism emphasizes the role of the metal catalyst, which can form a metal –Si eutectic alloy with a much lower freezing temperature. This eutectic alloy is often liquid in the Si nanowire growth process. The liquid
* Corresponding author. E-mail address: [email protected] (C. Li). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.11.027
droplet is a preferred site for the deposition of Si atoms from the vapor, which causes the liquid to become supersaturated. So the nanowire begins to grow and the liquid alloy droplet remains on the head of the nanowire. The nanowire growth includes two main transport processes: from the vapor to the liquid alloy droplet, and from the liquid alloy droplet to the nanowires (solid phase). The oxygen assistant mechanism points out that the synthesis process does not need any metal catalyst. It emphasizes the role of SiO vapor, which deposits firstly and forms the matrix, and then Si nanoparticles can precipitate from the decomposition of SiO. Preferred oriented nanoparticles can grow fast to form nanowires. We believe that the same nanowires may grow through different mechanisms under different experiment conditions. Here we would report a new one-dimensional growth style of SiOx occurring on the surface of the Ag particles, which is different from the V –L – S mechanism and oxygen assistant mechanism. The cause of nanowire growth is the stress at the interface of two substances, for they have different shrinking coefficients in the cooling process. The one-dimensional Ag/Si composite nanostructures have been synthesized in our laboratory by thermal evaporation of a mixture of SiO and Ag2O [15]. In addition to the one-dimensional nanostructures, we also obtain a byproduct of SiOx nanowires growing from the Ag particles surface. Although these SiOx nanowires are rather thick, their SEM morphology hints at a new growth mechanism.
1918
X. Zhang et al. / Materials Letters 58 (2004) 1917–1919
The evaporation experiments were carried out in a closed reaction chamber under the ambient gas of 90% Ar + 10% H2 with a pressure of 0.53 105 Pa. Mixture powder of SiO and Ag2O was used as the evaporation source material. This mixture was put in a closed reaction chamber, in which the Si substrate was located above the evaporation source. Then the chamber was pumped to lower than 10 Pa. After this, 0.53 105 Pa mixed gas of Ar (90%) and H2 (10%) filled the reaction room as ambient gas. Selecting graphite as heating material, the mixture of Ag2O and SiO was heated to about 1473 K with a heating rate of about 50 K/min. Si substrate was used. The distance between the mixture and the substrate was rather close (no more than 1 cm). There was a temperature gradient on the Si substrate surface. The highest temperature of Si substrate was about 1273 K. More detailed experimental steps can be found in Ref. [15]. Sediments obtained on the Si substrate were investigated by SEM XL30. Ag nanowires were formed on the high-temperature part of the substrate. On the low-temperature part of the substrate, Ag particles and SiOx nanowires can be found. It is easily understood that some Ag particles were formed for metals easily condensed as spheres. For using SiO, Ag particles can often be wrapped by SiOx, whose thickness may be up to several tens of nanometers. The ordinary morphology of the Ag particles is shown in Fig. 1. In fact, there is a size difference in the Ag particles. The diameter of small ones is between 100 and 200 nm, which is considered to have a relation with the growth of the Ag nanowires, as discussed in Ref. [15]. The big ones are thought to be related to the growth of the SiOx nanowires, which will be clarified below. A special morphology of Ag particles is shown in Fig. 2, in which the diameter of the Ag particles is about 500 nm. The layer of amorphous SiOx is not continuous and does not have the same thickness as the Ag particles. It seems that some wires twist the particles. From Fig. 3, the wires that twist the particles have extended to form SiOx nanowires.
Fig. 1. The ordinary morphology of the Ag particles wrapped by a layer of SiOx.
Fig. 2. Ag particles wrapped by discontinuous chapped SiOx.
The diameter of these SiOx nanowires is between 100 and 200 nm, and their length is not very long—the longest being no longer than 10 Am. Morphology similar to Fig. 3 has been reported in Ref. [16]. The V – L – S mechanism was used to explain the nanowires’ growth in the Ga –SiOx system. But here the SiOx nanowires forming process is different from the V –L – S mechanism or the oxygen assistant mechanism, and hence a new growth mechanism has to be suggested. Figs. 2 and 3 can be considered as two different periods of SiOx nanowires growth process. From these two pictures, we can confer the whole growth process. The forming of SiOx nanowires is induced by the chap of SiOx on the Ag particles surface. Then SiOx would peel off from the particles to form the SiOx nanowires. The process can be divided into four steps in detail. The sketch map is given in Fig. 4. The first step is the formation of Ag particles, which would act as preferential sites for the deposition of SiOx. Then the SiOx vapor from the decomposition of SiO would condense on the surface of these particles. Therefore, Ag particles would be wrapped by a layer of amorphous SiOx. The thickness of the SiOx layer could be more than 10 nm.
Fig. 3. SiOx nanowires that have been peeled off from the Ag particles.
X. Zhang et al. / Materials Letters 58 (2004) 1917–1919
1919
nanowires would continue to grow and become longer by the V –S mechanism. This is the reason why we could observe some long SiOx nanowires. The details of the growth process, such as where the growth site is located, are not clear yet and need further research. However, not all SiOx nanowires obtained in our experiment are grown by the mechanism discussed above. Some very thin nanowires can also be observed. They are Si nanowires with SiOx sheath. These nanowires are very long—similar to the Si nanowires obtained from the thermal evaporation. The formation of these nanowires has been explained by the oxygen assistant mechanism, as has been discussed in other papers [9]. In conclusion, we found a new growth mechanism of SiOx nanowires. This mechanism is different from the V – L – S and oxygen assistant mechanisms. In this mechanism, SiOx nanowires are induced by the chapping and peeling off of the SiOx layer, which wraps the Ag particles. The essential reasons are the different coefficients of Ag and SiOx in the cooling process. Fig. 4. Sketch map of the SiOx nanowires growth process by chapping and peeling from Ag particles.
Acknowledgements The second step is the chapping of SiOx on the Ag particle surface. In the cooling process, the Ag particles and the outer layer of SiOx would not agree very well for they have different shrinking coefficients. Thermal stress would be accumulated at the interface of the Ag particle and the SiOx layer. When the thermal stress accumulates to a certain degree, it needs to be released. The thermal stress may be released by different approaches according to the practical condition. Or the stress released may cause different results. If the thermal stress is not very big and homogenously distributed, it may cause the distortion of the SiOx to form some new nucleation sites similar to the S – K mode in the semiconductor quantum dots growth [17]. If the thermal stress is very big and exceeds the elastic deformation ability of SiOx, the SiOx layer would chap on the Ag particles surface. How the SiOx layer chaps is related to the thermal stress distribution, the radius of the Ag particle, and the thickness of the SiOx layer. When the SiOx layer only chaps along the longitude direction of the particle as shown in Fig. 4, the SiOx can peel off from Ag particles and form nanowires afterwards. The third step is the peeling off of the SiOx from the Ag particles. With the temperature decreasing, the degree of SiOx chapping would become more intense until the SiOx and Ag particles separate thoroughly. The separate SiOx parts have a tendency to deform to a round shape under the function of surface energy. The fourth step is the elongation process of the SiOx nanowires. In fact, the one-dimensional morphology has been formed after the SiOx peeled off from the Ag particles. If there is enough SiO source in the vapor phase, these
This work was financially supported by the National Science Foundation of China under grant no. 50272081 and the Chinese Ministry of Science and Technology under grant no. 2002CD613500.
References [1] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [2] M. Barbic, J.J. Mock, D.R. Smith, S. Schultz, J. Appl. Phys. 91 (2002) 9341. [3] W.C. Hu, D.W. Gong, Z. Chen, Appl. Phys. Lett. 79 (2001) 3083. [4] B.Q. Wei, R. Vajtai, Y. Jung, J. Ward, R. Zhang, G. Ramanath, P.M. Ajayan, Nature 416 (2002) 495. [5] E. Comini, G. Faglia, G. Sberveglieri, Appl. Phys. Lett. 81 (2002) 1869. [6] L. Dai, X.L. Chen, X.N. Zhang, A.Z. Jin, T. Zou, B.Q. Hu, Z. Zhang, J. Appl. Phys. 92 (2002) 1062. [7] Y.K. Liu, C.L. Zheng, W.Z. Wang, C.R. Yin, G.H. Wang, Adv. Mater. 13 (2001) 1883. [8] Y.W. Wang, G.W. Meng, C.H. Liang, G.Z. Wang, L.D. Zhang, Chem. Phys. Lett. 339 (2002) 174. [9] Y.Y. Ying, P.D. Yang, J. Am. Chem. Soc. 123 (2001) 3165. [10] M.K. Sunkara, S. Sharma, R. Miranada, G. Lian, E.C. Dickey, Appl. Phys. Lett. 79 (2001) 1546. [11] Y.J. Chen, J.B. Li, J.H. Dai, Chem. Phys. Lett. 344 (2001) 450. [12] J. Westwater, D.P. Gosain, S. Tomiya, S. Usui, H. Ryda, J. Vac. Sci. Technol., B 15 (1997) 554. [13] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89. [14] N. Wang, Y.H. Tang, Y.F. Zhang, C.S. Lee, S.T. Lee, Phys. Rev., B 58 (1998) R16024. [15] X.N. Zhang, Z. Zhang, C.R. Li (submitted for publication). [16] Z. Pan, S. Dai, D.B. Beach, D.H. Lowndes, Nano Lett. 3 (2003) 1279. [17] F. Heinrichsdorff, A. Krost, M. Grundmann, D. Bimberg, F. Bertram, J. Christen, A. Kosogov, P. Werncr, J. Cryst. Growth 170 (1997) 568.
One-dimensional growth induced by thermal stress Xiaona Zhang, Chaorong Li *, Ze Zhang Beijing Laboratory of Electron Microscopy, Institute of Physics, Chinese Academy of Sciences, PO Box 603, Beijing 100080, China Received 29 September 2003; accepted 27 November 2003
Abstract Amorphous SiOx nanowires grown on the surface of big Ag particles were synthesized by evaporation of a mixture of SiO and Ag2O. The morphologies at different growth stages hint at a special kind of growth mechanism different from the classical vapor – liquid – solid (V – L – S) mechanism and oxygen assistant mechanism. A layer of SiOx wraps the Ag particles in the cooling process. The SiOx nanowire growth is induced by the thermal stress accumulated in the interface of the outer SiOx layer and Ag particles for different shrinking coefficients. When the thermal stress accumulates to a certain degree, the SiOx layer chaps and peels off from the Ag particles to form SiOx nanowires. D 2004 Elsevier B.V. All rights reserved. Keywords: Deposition; Nanomaterials; Surface
The synthesis of one-dimensional nanostructures has recently reached a new horizon. Up to now, it is not a great challenge in materials science. Many kinds of one-dimensional nanostructures including nanowires, nanotubes, nanobelts, and nanorods have been obtained by different methods [1 –8]. In addition to the synthesis techniques, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray, etc., much attention has been put on the physics and chemical problems in the growth process of one-dimensional nanostructures. Scientists want to understand what induced the one-dimensional growth. Thus the research work on growth mechanism of nanowires is very important. Many authors have reported on the synthesis and growth mechanism of Si and SiOx nanowires [9– 12]. The synthesis methods mainly include chemical vapor deposition, laser ablation, and thermal evaporation. Two typical mechanisms have been put forward to explain the one-dimensional growth process: one is vapor – liquid –solid (V – L –S) mechanism [13] and the other is oxygen assistant mechanism [14]. The V – L – S mechanism emphasizes the role of the metal catalyst, which can form a metal –Si eutectic alloy with a much lower freezing temperature. This eutectic alloy is often liquid in the Si nanowire growth process. The liquid
* Corresponding author. E-mail address: [email protected] (C. Li). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.11.027
droplet is a preferred site for the deposition of Si atoms from the vapor, which causes the liquid to become supersaturated. So the nanowire begins to grow and the liquid alloy droplet remains on the head of the nanowire. The nanowire growth includes two main transport processes: from the vapor to the liquid alloy droplet, and from the liquid alloy droplet to the nanowires (solid phase). The oxygen assistant mechanism points out that the synthesis process does not need any metal catalyst. It emphasizes the role of SiO vapor, which deposits firstly and forms the matrix, and then Si nanoparticles can precipitate from the decomposition of SiO. Preferred oriented nanoparticles can grow fast to form nanowires. We believe that the same nanowires may grow through different mechanisms under different experiment conditions. Here we would report a new one-dimensional growth style of SiOx occurring on the surface of the Ag particles, which is different from the V –L – S mechanism and oxygen assistant mechanism. The cause of nanowire growth is the stress at the interface of two substances, for they have different shrinking coefficients in the cooling process. The one-dimensional Ag/Si composite nanostructures have been synthesized in our laboratory by thermal evaporation of a mixture of SiO and Ag2O [15]. In addition to the one-dimensional nanostructures, we also obtain a byproduct of SiOx nanowires growing from the Ag particles surface. Although these SiOx nanowires are rather thick, their SEM morphology hints at a new growth mechanism.
1918
X. Zhang et al. / Materials Letters 58 (2004) 1917–1919
The evaporation experiments were carried out in a closed reaction chamber under the ambient gas of 90% Ar + 10% H2 with a pressure of 0.53 105 Pa. Mixture powder of SiO and Ag2O was used as the evaporation source material. This mixture was put in a closed reaction chamber, in which the Si substrate was located above the evaporation source. Then the chamber was pumped to lower than 10 Pa. After this, 0.53 105 Pa mixed gas of Ar (90%) and H2 (10%) filled the reaction room as ambient gas. Selecting graphite as heating material, the mixture of Ag2O and SiO was heated to about 1473 K with a heating rate of about 50 K/min. Si substrate was used. The distance between the mixture and the substrate was rather close (no more than 1 cm). There was a temperature gradient on the Si substrate surface. The highest temperature of Si substrate was about 1273 K. More detailed experimental steps can be found in Ref. [15]. Sediments obtained on the Si substrate were investigated by SEM XL30. Ag nanowires were formed on the high-temperature part of the substrate. On the low-temperature part of the substrate, Ag particles and SiOx nanowires can be found. It is easily understood that some Ag particles were formed for metals easily condensed as spheres. For using SiO, Ag particles can often be wrapped by SiOx, whose thickness may be up to several tens of nanometers. The ordinary morphology of the Ag particles is shown in Fig. 1. In fact, there is a size difference in the Ag particles. The diameter of small ones is between 100 and 200 nm, which is considered to have a relation with the growth of the Ag nanowires, as discussed in Ref. [15]. The big ones are thought to be related to the growth of the SiOx nanowires, which will be clarified below. A special morphology of Ag particles is shown in Fig. 2, in which the diameter of the Ag particles is about 500 nm. The layer of amorphous SiOx is not continuous and does not have the same thickness as the Ag particles. It seems that some wires twist the particles. From Fig. 3, the wires that twist the particles have extended to form SiOx nanowires.
Fig. 1. The ordinary morphology of the Ag particles wrapped by a layer of SiOx.
Fig. 2. Ag particles wrapped by discontinuous chapped SiOx.
The diameter of these SiOx nanowires is between 100 and 200 nm, and their length is not very long—the longest being no longer than 10 Am. Morphology similar to Fig. 3 has been reported in Ref. [16]. The V – L – S mechanism was used to explain the nanowires’ growth in the Ga –SiOx system. But here the SiOx nanowires forming process is different from the V –L – S mechanism or the oxygen assistant mechanism, and hence a new growth mechanism has to be suggested. Figs. 2 and 3 can be considered as two different periods of SiOx nanowires growth process. From these two pictures, we can confer the whole growth process. The forming of SiOx nanowires is induced by the chap of SiOx on the Ag particles surface. Then SiOx would peel off from the particles to form the SiOx nanowires. The process can be divided into four steps in detail. The sketch map is given in Fig. 4. The first step is the formation of Ag particles, which would act as preferential sites for the deposition of SiOx. Then the SiOx vapor from the decomposition of SiO would condense on the surface of these particles. Therefore, Ag particles would be wrapped by a layer of amorphous SiOx. The thickness of the SiOx layer could be more than 10 nm.
Fig. 3. SiOx nanowires that have been peeled off from the Ag particles.
X. Zhang et al. / Materials Letters 58 (2004) 1917–1919
1919
nanowires would continue to grow and become longer by the V –S mechanism. This is the reason why we could observe some long SiOx nanowires. The details of the growth process, such as where the growth site is located, are not clear yet and need further research. However, not all SiOx nanowires obtained in our experiment are grown by the mechanism discussed above. Some very thin nanowires can also be observed. They are Si nanowires with SiOx sheath. These nanowires are very long—similar to the Si nanowires obtained from the thermal evaporation. The formation of these nanowires has been explained by the oxygen assistant mechanism, as has been discussed in other papers [9]. In conclusion, we found a new growth mechanism of SiOx nanowires. This mechanism is different from the V – L – S and oxygen assistant mechanisms. In this mechanism, SiOx nanowires are induced by the chapping and peeling off of the SiOx layer, which wraps the Ag particles. The essential reasons are the different coefficients of Ag and SiOx in the cooling process. Fig. 4. Sketch map of the SiOx nanowires growth process by chapping and peeling from Ag particles.
Acknowledgements The second step is the chapping of SiOx on the Ag particle surface. In the cooling process, the Ag particles and the outer layer of SiOx would not agree very well for they have different shrinking coefficients. Thermal stress would be accumulated at the interface of the Ag particle and the SiOx layer. When the thermal stress accumulates to a certain degree, it needs to be released. The thermal stress may be released by different approaches according to the practical condition. Or the stress released may cause different results. If the thermal stress is not very big and homogenously distributed, it may cause the distortion of the SiOx to form some new nucleation sites similar to the S – K mode in the semiconductor quantum dots growth [17]. If the thermal stress is very big and exceeds the elastic deformation ability of SiOx, the SiOx layer would chap on the Ag particles surface. How the SiOx layer chaps is related to the thermal stress distribution, the radius of the Ag particle, and the thickness of the SiOx layer. When the SiOx layer only chaps along the longitude direction of the particle as shown in Fig. 4, the SiOx can peel off from Ag particles and form nanowires afterwards. The third step is the peeling off of the SiOx from the Ag particles. With the temperature decreasing, the degree of SiOx chapping would become more intense until the SiOx and Ag particles separate thoroughly. The separate SiOx parts have a tendency to deform to a round shape under the function of surface energy. The fourth step is the elongation process of the SiOx nanowires. In fact, the one-dimensional morphology has been formed after the SiOx peeled off from the Ag particles. If there is enough SiO source in the vapor phase, these
This work was financially supported by the National Science Foundation of China under grant no. 50272081 and the Chinese Ministry of Science and Technology under grant no. 2002CD613500.
References [1] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [2] M. Barbic, J.J. Mock, D.R. Smith, S. Schultz, J. Appl. Phys. 91 (2002) 9341. [3] W.C. Hu, D.W. Gong, Z. Chen, Appl. Phys. Lett. 79 (2001) 3083. [4] B.Q. Wei, R. Vajtai, Y. Jung, J. Ward, R. Zhang, G. Ramanath, P.M. Ajayan, Nature 416 (2002) 495. [5] E. Comini, G. Faglia, G. Sberveglieri, Appl. Phys. Lett. 81 (2002) 1869. [6] L. Dai, X.L. Chen, X.N. Zhang, A.Z. Jin, T. Zou, B.Q. Hu, Z. Zhang, J. Appl. Phys. 92 (2002) 1062. [7] Y.K. Liu, C.L. Zheng, W.Z. Wang, C.R. Yin, G.H. Wang, Adv. Mater. 13 (2001) 1883. [8] Y.W. Wang, G.W. Meng, C.H. Liang, G.Z. Wang, L.D. Zhang, Chem. Phys. Lett. 339 (2002) 174. [9] Y.Y. Ying, P.D. Yang, J. Am. Chem. Soc. 123 (2001) 3165. [10] M.K. Sunkara, S. Sharma, R. Miranada, G. Lian, E.C. Dickey, Appl. Phys. Lett. 79 (2001) 1546. [11] Y.J. Chen, J.B. Li, J.H. Dai, Chem. Phys. Lett. 344 (2001) 450. [12] J. Westwater, D.P. Gosain, S. Tomiya, S. Usui, H. Ryda, J. Vac. Sci. Technol., B 15 (1997) 554. [13] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89. [14] N. Wang, Y.H. Tang, Y.F. Zhang, C.S. Lee, S.T. Lee, Phys. Rev., B 58 (1998) R16024. [15] X.N. Zhang, Z. Zhang, C.R. Li (submitted for publication). [16] Z. Pan, S. Dai, D.B. Beach, D.H. Lowndes, Nano Lett. 3 (2003) 1279. [17] F. Heinrichsdorff, A. Krost, M. Grundmann, D. Bimberg, F. Bertram, J. Christen, A. Kosogov, P. Werncr, J. Cryst. Growth 170 (1997) 568.