- Email: [email protected]
Self-catalytic branch growth of SnO2 nanowire junctions
ARTICLE IN PRESS
Journal of Crystal Growth 270 (2004) 505–510 www.elsevier.com/locate/jcrysgro
Self-catalytic branch growth of SnO2 nanowire junctions Y.X. Chen, L.J. Campbell, W.L. Zhou Advanced Materials Research Institute, University of New Orleans, 2000 Lakeshore Drive New Orleans, LA, 70148, USA Received 9 March 2004; accepted 5 July 2004 Communicated by J.M. Redwing Available online 25 August 2004
Abstract Multiple branched SnO2 nanowire junctions have been synthesized by thermal evaporation of SnO powder. Their nanostructures were studied by transmission electron microscopy and field emission scanning electron microcopy. It was observed that Sn nanoparticles generated from decomposition of the SnO powder acted as self-catalysts to control the SnO2 nanojunction growth. Orthorhombic SnO2 was found as a dominate phase in nanojunction growth instead of rutile structure. The branches and stems of nanojunctions were found to be an epitaxial growth by electron diffraction analysis and high-resolution electron microscopy observation. The growth directions of the branched SnO2 nanojunctions were along the orthorhombic [1 1 0] and ½1 1 0. A self-catalytic vapor–liquid–solid growth mechanism is proposed to describe the growth process of the branched SnO2 nanowire junctions. r 2004 Elsevier B.V. All rights reserved. Keywords: A1. Characterization; A1. Crystal structure; A2. Branch growth; B1. Nanomaterials; B1. Oxides; B2. Semiconducting materials
1. Introduction Recently, one-dimensional (1D) nanowire junctions have attracted much attention because of their potential applications in nanoelectronics [1,2]. Until now, 1D nanojunctions have been successfully synthesized by using various methods including laser ablation [1], template-based method [2], thermal chemical vapor deposition [3], and Corresponding author. Tel.:+1-504-280-1068; fax: +1-504-
280-3185. E-mail address: [email protected] (W.L. Zhou).
thermal evaporation [4–6], etc. Thermal evaporation has been demonstrated to be an effective method to synthesize 1D metal oxide semiconductor branched nanojunctions, such as ZnO and In2O3ZnO heterojunctions [4–6]. Among metal oxide semiconductor materials, SnO2 has been actively studied because of its potential applications in optoelectronic devices and chemical sensors [7–9]. 1D SnO2 and In-doped SnO2 with morphologies of nanowire, nanobelt, nanoribbon, and nanotube have been fabricated by thermal evaporation approach [10–13]. 1D SnO nanojunction growth has been reported from direct thermal
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.07.011
ARTICLE IN PRESS 506
Y.X. Chen et al. / Journal of Crystal Growth 270 (2004) 505–510
evaporation of SnO powder [14]. The growth of SnO2 and its doped nanowires is either a direct growth controlled by vapor–solid (VS) mechanism or a catalyst assisted growth controlled by vapor–liquid–solid (VLS) model. Catalysts has been exploited to control 1D nanowire growth, where the diameter and length of 1D nanowires are supposed to be controlled by the catalyst size and growth time. Metallic Sn has been reported to be an effective catalyst to control the growth of ZnO nanowire–nanoribbon junction arrays using thermal evaporation method [4], probably because of its low melting point, i.e. 231.89 1C [15]. Self-catalytic growth of SnO2 nanowires catalyzed by Sn nanoparticles has also been reported [12]. To date, however, the behavior of the Sn nanoparticle catalysts in the thermal evaporation process is not fully understood yet. To explore branched nanojunction growth and get insight into catalytic nature of Sn nanoparticles, we synthesized SnO2 nanostructures by direct thermal evaporation of SnO powder at elevated temperature. Besides the expected SnO2 nanowires and nanobelts, a large quantity of SnO2 nanojunctions with the first and the second branch growth were observed in the products. Sn nanoparticles were found at the tips of branches and stems of the SnO2 nanojunction growth. In this paper, the growth of multiple branch SnO2 nanojunctions are studied by transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). The Sn nanoparticles as self-catalysts in nanojunction growth will also be discussed.
2. Experimental Procedure Thermal evaporation approach was used to synthesize SnO2 branch growth nanojunctions. The experimental setup is composed of a horizontal high-temperature tube furnace with length of heated zone of about 30 cm, an alumina tube with length of about 110 cm, a rotary pumping system, as well as a gas controlling system. Commercial (Alfa Aesar) SnO powders were used as source materials for synthesis of the SnO2 nanojunctions. The SnO powders were loaded onto a small
alumina boat and then positioned at the center of the alumina tube. After the alumina tube was evacuated to 2 102 Torr, the tube furnace was heated to 1100 1C. Meanwhile, vacuum pressure was increased to 300 Torr, and Ar carrier gas flow rate was kept at 100 sccm (standard cubic centimeters per minute). After the evaporation process started at 1100 1C, the tube furnace temperature was decreased to 1050 1C. Totally, the synthesis time was 20 min. A large yield of gray products were piled onto the alumina collectors located 12 cm downstream from the source materials. The local growth temperature was estimated to be about 700–800 1C. As-synthesized products were characterized by a LEO 1530 VP FESEM equipped with Oxford energy dispersive X-ray spectroscopy (EDS). TEM samples were prepared by dissolving as-synthesized products into ethyl alcohol using ultrasonic cleaner and dispersing one or two droplets of the solution onto TEM carbon grid. TEM observation was performed by a JEOL 2010 transmission electron microscope equipped with EDAX EDS microanalysis.
3. Results and discussion As expected, nanowires and nanobelts are the main components of the products, which will be reported elsewhere [16]. Interestingly, besides nanowires and nanobelts, a large quantity of nanowire junctions with the first and the second branch growth were observed by FESEM, as shown in Fig. 1. Nanoparticles are found located at the tips of the branches and stems. FESEM observations indicate that the diameter of the stems of the nanojunctions ranges from 60 to100 nm, while the length can be as long as several tens of micrometers. The diameter of the branch nanowires is quite uniform, which is about 30–40 nm. However, their length varies from 200 nm to about 1 mm. EDS analysis indicates that the stem and branches of the nanojunction contain Sn and O with a Sn:O ratio close to 1:2 and the nanoparticles at the tips of branches and stem are Sn. It can be deduced from the above observation that Sn nanoparticles may act as catalysts
ARTICLE IN PRESS Y.X. Chen et al. / Journal of Crystal Growth 270 (2004) 505–510
507
second branches is geometrically parallel to that of the stem. The SnOx nanojunctions were further investigated by TEM to obtain their crystallography information. Fig. 3(a) is a TEM image showing SnOx nanobranches grown on a SnOx stem nanowire. A nanoparticle identified as Sn by
Fig. 1. FESEM image showing the first branch growth of SnOx nanojunction. Some second branch nanowires are visible, as denoted by white arrows.
Fig. 2. FESEM image showing large quantity of clear second branch growth of SnOx nanojunction. The growth direction of the second branches is parallel to that of the stem.
controlling SnOx nanojunction growth. In addition to the first branch growth in the SnOx nanojunctions, some second branch growth nanowires are also visible, as denoted by white arrows. A large quantity of clear second branch growth is shown in Fig. 2. The growth direction of the
Fig. 3. (a) TEM image exhibiting a first branch growth nanojunction. (b) SADP taken from the stem and branch of the SnO2 nanojunction. The subscripts of ‘‘o’’ and ‘‘t’’ represent orthorhombic and rutile SnO2, respectively.
ARTICLE IN PRESS 508
Y.X. Chen et al. / Journal of Crystal Growth 270 (2004) 505–510
EDS is clearly visible at the tip of the stem. Most of the branched nanojunctions also have Sn nanoparticles located at their tips except three branches, which we believe that the Sn nanoparticles are dropped during TEM sample preparation. A selected-area diffraction pattern (SADP) taken from the stem and branch of the SnOx nanojunction is shown in Fig. 3(b). Two sets of diffraction pattern are observed in the figure. Compared to the simulated diffraction patterns of SnO2 and SnO, the SADP was indexed as a superimposed diffraction pattern from normal rutile structure and high-temperature orthorhombic structure of SnO2. Therefore, two phases coexist in the nanojunction growth. Normally, SnO2 has a rutile crystal structure with space group of P42/mnm and lattice parameters of a=0.473 nm and c=0.318 nm [17]. However, orthorhombic structured SnO2, with lattice parameters of a=0.471 nm, b=0.572 nm and c=0.521 nm, could be formed at 800oC under high-pressures of 158 kbar [18]. It is not fully understood why the high-temperature and highpressure phase was found at our growth. We think that the fast cooling rate might result in the hightemperature and high-pressure phase remaining to ambient temperature. Orthorhombic SnO2 nanostructure and coexistence with rutile has also been reported in SnO2 nanowire synthesis [10]. In our observation, the orthorhombic SnO2 phase was dominate in the nanojunction growth. An orientation relationship between the orthorhombic and rutile SnO2, i.e., [0 0 1]o // [1 0 2]t and (1 0 0)o // (0 1 0)t, is thus established according to Fig. 3(b), where the subscripts ‘‘o’’ and ‘‘t’’ represent orthorhombic and rutile SnO2, respectively. TEM observation also indicated that no growth orientation relationship between SnO2 and Sn catalyst nanoparticle was formed. From the above analysis, it is concluded that the growth directions of the stem and branches in the SnO2 nanojunction are the orthorhombic ½1 1 0 and [1 1 0] directions, respectively, as denoted in Fig. 3(a). The measured angle between the stem and a branch is about 1011, which is in a good agreement with calculated angle between the orthorhombic [1 1 0] and ½1 1 0 directions of SnO2, i.e., 101.061.
Fig. 4. HREM image viewed along the orthorhombic [0 0 1] zone axis of SnO2. The HREM image was taken from outlined area shown in TEM image inset in top left corner. An enlarged HREM image from outlined area is inset in top right corner.
Fig. 4 is a HREM image of the outlined junction region in the inset TEM image located at top left corner. The HREM image was viewed along the [0 0 1] direction of the orthorhombic SnO2. The HREM image indicates that the lattice of the SnO2 nanojunction is perfect. No defect is observed. The growth directions of the stem and the branch are further confirmed from the HREM image. The inset at top right corner is an enlarged HREM image from outlined area located at the junction region between the stem and the branch in the HREM image. It can be seen that the stem and the branch are single crystal and the SnO2 branch is grown directly from the orthorhombic stem with the same crystallography orientation. The multiple branch growth follows the same growth orientation. Previous study [19] on thermal decomposition of SnO powder at an elevated temperature indicated that the following reactions occurred when the temperature was over 300 1C, 3SnO ðsÞ ! Sn2 O3 ðsÞ þ Sn ðlÞ;
ð1Þ
2Sn2 O3 ðsÞ ! 3SnO2 ðsÞ þ Sn ðlÞ;
ð2Þ
where Sn2O3 was a intermediate phase. In our study, the microstructural observations revealed that the SnO2 nanojunction and Sn catalyst nanoparticles were formed by decomposition of
ARTICLE IN PRESS Y.X. Chen et al. / Journal of Crystal Growth 270 (2004) 505–510
SnO source powders, where Sn nanoparticles acted as catalysts controlling the growth of such multiple branched SnO2 nanojunctions. Based on the above analysis, a self-catalytic VLS growth concerning formation of such branch growth of SnO2 nanojunctions is proposed, as shown in the schematics of Fig. 5. At above 1050 1C, the source SnO powder eventually decomposes into SnO2 and Sn species [19]. Then, liquid Sn droplets are formed because of low melting point of Sn, i.e., 231.89 1C. These nanoscale Sn droplets provide energetically favored sites for absorption of the SnO2 and Sn species, as shown in Fig. 5(a). When the Sn droplets are over saturated, SnO2 nanowires precipitate out and grow along one of the orthorhombic [1 1 0] directions, assumed to be ½1 1 0 as shown in Fig. 5(b). The nanoscale Sn droplets keep melting evaporated SnO2 and Sn species, and continuously control the growth of SnO2 nanowire with longer size and bigger diameter. Meanwhile, some Sn species are carried downstream by Ar gas flow and collided and adhered to newly formed SnO2 nanowire surface, forming Sn nanodroplets. These Sn nanodroplets also absorb SnO2 species and catalyze the growth of the first branch nanowires along the other orthorhombic /1 1 0S directions, assumed to be [1 1 0] as shown in Fig. 5(c). Furthermore, as the Sn nanodroplets adhere to the surface of the first branch nanowires as catalysts, the second branch nanowire growth occurs along the ½1 1 0 direction, which is the same as the stem direction. The growth of multiple branched SnO2 nanowire junctions terminates upon cooling, as shown in Fig. 5(d). Since the self-catalytic VLS growth avoids undesired external transition metal catalyst, it is believed that the self-catalytic VLS growth might open up a possibility for a clear and controlled semiconductor nanojunction synthesis. Further works about controlled nanojunction growth for nanoelectronic building blocks using Sn nanoparticles as catalysts are underway.
4. Conclusions SnO2 nanojunctions with the first branch growth and the second branch growth were
Sn
SnO2
SnO2 Sn(l)
Sn
509
Sn SnO2
Sn O2
Sn
(a) Sn SnO2
SnO2 Sn
Sn(l)
Sn
SnO2 (b)
SnO2
[110]
Sn Sn SnO2
[110]
Sn(l)
SnO2
[110]
(c)
Sn(s)
SnO2
(d) Fig. 5. Schematic drawing of a self-catalytic VLS growth describing growth process of the multiple branched SnO2 nanojunction. (a) After SnO is decomposed into SnO2 and Sn species, liquid Sn droplets are formed and absorbs SnO2 and Sn species. (b) SnO2 nanowires precipitates out and grow along the orthorhombic ½1 1 0 direction when the droplet is oversaturated. Some Sn species are carried downstream by Ar gas flow and collide and adhere to newly formed SnO2 nanowire surface, forming Sn nanodroplets. (c) These Sn nanodroplets also absorb SnO2 and Sn species and catalyze the growth of the first branch nanowires along the other orthorhombic [1 1 0] direction. (d) For the Sn nanodroplets adhere to the surface of the first branch nanowires, they can catalyze the second branch nanowire growth along the ½1 1 0 direction. The growth of multiple branched SnO2 nanowire junctions terminates upon cooling.
ARTICLE IN PRESS 510
Y.X. Chen et al. / Journal of Crystal Growth 270 (2004) 505–510
synthesized by a direct thermal evaporation of SnO powder. Orthorhombic SnO2 was found as a dominate phase in the nanojunction growth instead of rutile structure. Sn nanoparticles formed by decomposition of SnO powder were located at the tips of the stems and branches of the SnO2 nanojunctions, and catalyzed the growth of the multiple branched SnO2 nanojunctions. The orientation relationship between orthorhombic and rutile SnO2 is [0 0 1]o // [1 0 2]t and (1 0 0)o // (0 1 0)t. The growth directions of the branches and stems in the SnO2 nanojunctions are the orthorhombic / 1 1 0S. No orientation relationship between SnO2 and Sn catalyst nanoparticle was observed. A selfcatalytic VLS growth mechanism was discussed to describe the growth process of the multiple branched SnO2 nanojunction. The observation of such a self-catalytic multiple branch growth nanojunction opens up a possibility for the controlled growth of nanojunction building blocks.
Acknowledgements This work was supported by the AMRI through DARPA Grant No. MDA972-04-1-0029. Part of the work was supported by the Louisiana Board of Regents contract no. LEQSF (2003-06)-RD-B-13. The authors gratefully thank Prof. Z. L. Wang at Georgia Institute of Technology and Dr. Z. W.
Pan at Oak Ridge National Laboratory for the fruitful discussion.
Reference [1] M.S. Gudlksen, L.J. Lauhon, J.F. Wang, D.V. Smith, C.M. Lieber, Nature 415 (2002) 617. [2] C.H. Liu, W.C. Yin, F.C.K. Au, J.X. Ding, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 83 (2003) 3168. [3] J.M. Ting, C.C. Chang, Appl. Phys. Lett. 80 (2002) 324. [4] P.X. Gao, Z.L. Wang, J. Phys. Chem. B 106 (2002) 12653. [5] J.Y. Lao, J.Y. Huang, D.Z. Wang, Z.F. Ren, Nano Lett. 3 (2003) 235. [6] J.Y. Lao, J.G. Wen, Z.F. Ren, Nano Lett. 2 (2002) 1287. [7] Y. Shimizu, M. Egashira, MRS Bull. 24 (1999) 18. [8] G. Williams, G.S.V. Coles, MRS Bull. 24 (1999) 25. [9] C. Tatsuyama, S. Ichimura, Jpn. J. Appl. Phys. 15 (1976) 843. [10] Z.R. Dai, J.L. Gole, J.D. Stout, Z.L. Wang, J. Phys. Chem. B 106 (2002) 1274. [11] P. Nguyen, H.T. Ng, J. Kong, A.M. Cassell, R. Quinn, J. Li, J. Han, M. McNeil, M. Meyyappan, Nano Lett. 3 (2003) 925. [12] Y.Q. Chen, X.F. Cui, K. Zhang, D.Y. Pan, S.Y. Zhang, B. Wang, J.G. Hou, Chem. Phys. Lett. 369 (2003) 16. [13] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [14] Z.L. Wang, Z.W. Pan, Adv. Mater. 14 (2002) 1029. [15] J.A. Dean, Lange’s Handbook of Chemistry, 11th ed, McGraw-Hill, New York, 1973. [16] In preparation. [17] W.H. Baou, Acta Crystallogr. 9 (1956) 515. [18] K. Suito, N. Kawai, Y. Masuda, Mater. Res. Bull. 10 (1975) 677. [19] M.S. Moreno, R.C. Mercader, A.G. Bibiloni, J. Phys.: Condens. Matter 4 (1992) 351.
Journal of Crystal Growth 270 (2004) 505–510 www.elsevier.com/locate/jcrysgro
Self-catalytic branch growth of SnO2 nanowire junctions Y.X. Chen, L.J. Campbell, W.L. Zhou Advanced Materials Research Institute, University of New Orleans, 2000 Lakeshore Drive New Orleans, LA, 70148, USA Received 9 March 2004; accepted 5 July 2004 Communicated by J.M. Redwing Available online 25 August 2004
Abstract Multiple branched SnO2 nanowire junctions have been synthesized by thermal evaporation of SnO powder. Their nanostructures were studied by transmission electron microscopy and field emission scanning electron microcopy. It was observed that Sn nanoparticles generated from decomposition of the SnO powder acted as self-catalysts to control the SnO2 nanojunction growth. Orthorhombic SnO2 was found as a dominate phase in nanojunction growth instead of rutile structure. The branches and stems of nanojunctions were found to be an epitaxial growth by electron diffraction analysis and high-resolution electron microscopy observation. The growth directions of the branched SnO2 nanojunctions were along the orthorhombic [1 1 0] and ½1 1 0. A self-catalytic vapor–liquid–solid growth mechanism is proposed to describe the growth process of the branched SnO2 nanowire junctions. r 2004 Elsevier B.V. All rights reserved. Keywords: A1. Characterization; A1. Crystal structure; A2. Branch growth; B1. Nanomaterials; B1. Oxides; B2. Semiconducting materials
1. Introduction Recently, one-dimensional (1D) nanowire junctions have attracted much attention because of their potential applications in nanoelectronics [1,2]. Until now, 1D nanojunctions have been successfully synthesized by using various methods including laser ablation [1], template-based method [2], thermal chemical vapor deposition [3], and Corresponding author. Tel.:+1-504-280-1068; fax: +1-504-
280-3185. E-mail address: [email protected] (W.L. Zhou).
thermal evaporation [4–6], etc. Thermal evaporation has been demonstrated to be an effective method to synthesize 1D metal oxide semiconductor branched nanojunctions, such as ZnO and In2O3ZnO heterojunctions [4–6]. Among metal oxide semiconductor materials, SnO2 has been actively studied because of its potential applications in optoelectronic devices and chemical sensors [7–9]. 1D SnO2 and In-doped SnO2 with morphologies of nanowire, nanobelt, nanoribbon, and nanotube have been fabricated by thermal evaporation approach [10–13]. 1D SnO nanojunction growth has been reported from direct thermal
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.07.011
ARTICLE IN PRESS 506
Y.X. Chen et al. / Journal of Crystal Growth 270 (2004) 505–510
evaporation of SnO powder [14]. The growth of SnO2 and its doped nanowires is either a direct growth controlled by vapor–solid (VS) mechanism or a catalyst assisted growth controlled by vapor–liquid–solid (VLS) model. Catalysts has been exploited to control 1D nanowire growth, where the diameter and length of 1D nanowires are supposed to be controlled by the catalyst size and growth time. Metallic Sn has been reported to be an effective catalyst to control the growth of ZnO nanowire–nanoribbon junction arrays using thermal evaporation method [4], probably because of its low melting point, i.e. 231.89 1C [15]. Self-catalytic growth of SnO2 nanowires catalyzed by Sn nanoparticles has also been reported [12]. To date, however, the behavior of the Sn nanoparticle catalysts in the thermal evaporation process is not fully understood yet. To explore branched nanojunction growth and get insight into catalytic nature of Sn nanoparticles, we synthesized SnO2 nanostructures by direct thermal evaporation of SnO powder at elevated temperature. Besides the expected SnO2 nanowires and nanobelts, a large quantity of SnO2 nanojunctions with the first and the second branch growth were observed in the products. Sn nanoparticles were found at the tips of branches and stems of the SnO2 nanojunction growth. In this paper, the growth of multiple branch SnO2 nanojunctions are studied by transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). The Sn nanoparticles as self-catalysts in nanojunction growth will also be discussed.
2. Experimental Procedure Thermal evaporation approach was used to synthesize SnO2 branch growth nanojunctions. The experimental setup is composed of a horizontal high-temperature tube furnace with length of heated zone of about 30 cm, an alumina tube with length of about 110 cm, a rotary pumping system, as well as a gas controlling system. Commercial (Alfa Aesar) SnO powders were used as source materials for synthesis of the SnO2 nanojunctions. The SnO powders were loaded onto a small
alumina boat and then positioned at the center of the alumina tube. After the alumina tube was evacuated to 2 102 Torr, the tube furnace was heated to 1100 1C. Meanwhile, vacuum pressure was increased to 300 Torr, and Ar carrier gas flow rate was kept at 100 sccm (standard cubic centimeters per minute). After the evaporation process started at 1100 1C, the tube furnace temperature was decreased to 1050 1C. Totally, the synthesis time was 20 min. A large yield of gray products were piled onto the alumina collectors located 12 cm downstream from the source materials. The local growth temperature was estimated to be about 700–800 1C. As-synthesized products were characterized by a LEO 1530 VP FESEM equipped with Oxford energy dispersive X-ray spectroscopy (EDS). TEM samples were prepared by dissolving as-synthesized products into ethyl alcohol using ultrasonic cleaner and dispersing one or two droplets of the solution onto TEM carbon grid. TEM observation was performed by a JEOL 2010 transmission electron microscope equipped with EDAX EDS microanalysis.
3. Results and discussion As expected, nanowires and nanobelts are the main components of the products, which will be reported elsewhere [16]. Interestingly, besides nanowires and nanobelts, a large quantity of nanowire junctions with the first and the second branch growth were observed by FESEM, as shown in Fig. 1. Nanoparticles are found located at the tips of the branches and stems. FESEM observations indicate that the diameter of the stems of the nanojunctions ranges from 60 to100 nm, while the length can be as long as several tens of micrometers. The diameter of the branch nanowires is quite uniform, which is about 30–40 nm. However, their length varies from 200 nm to about 1 mm. EDS analysis indicates that the stem and branches of the nanojunction contain Sn and O with a Sn:O ratio close to 1:2 and the nanoparticles at the tips of branches and stem are Sn. It can be deduced from the above observation that Sn nanoparticles may act as catalysts
ARTICLE IN PRESS Y.X. Chen et al. / Journal of Crystal Growth 270 (2004) 505–510
507
second branches is geometrically parallel to that of the stem. The SnOx nanojunctions were further investigated by TEM to obtain their crystallography information. Fig. 3(a) is a TEM image showing SnOx nanobranches grown on a SnOx stem nanowire. A nanoparticle identified as Sn by
Fig. 1. FESEM image showing the first branch growth of SnOx nanojunction. Some second branch nanowires are visible, as denoted by white arrows.
Fig. 2. FESEM image showing large quantity of clear second branch growth of SnOx nanojunction. The growth direction of the second branches is parallel to that of the stem.
controlling SnOx nanojunction growth. In addition to the first branch growth in the SnOx nanojunctions, some second branch growth nanowires are also visible, as denoted by white arrows. A large quantity of clear second branch growth is shown in Fig. 2. The growth direction of the
Fig. 3. (a) TEM image exhibiting a first branch growth nanojunction. (b) SADP taken from the stem and branch of the SnO2 nanojunction. The subscripts of ‘‘o’’ and ‘‘t’’ represent orthorhombic and rutile SnO2, respectively.
ARTICLE IN PRESS 508
Y.X. Chen et al. / Journal of Crystal Growth 270 (2004) 505–510
EDS is clearly visible at the tip of the stem. Most of the branched nanojunctions also have Sn nanoparticles located at their tips except three branches, which we believe that the Sn nanoparticles are dropped during TEM sample preparation. A selected-area diffraction pattern (SADP) taken from the stem and branch of the SnOx nanojunction is shown in Fig. 3(b). Two sets of diffraction pattern are observed in the figure. Compared to the simulated diffraction patterns of SnO2 and SnO, the SADP was indexed as a superimposed diffraction pattern from normal rutile structure and high-temperature orthorhombic structure of SnO2. Therefore, two phases coexist in the nanojunction growth. Normally, SnO2 has a rutile crystal structure with space group of P42/mnm and lattice parameters of a=0.473 nm and c=0.318 nm [17]. However, orthorhombic structured SnO2, with lattice parameters of a=0.471 nm, b=0.572 nm and c=0.521 nm, could be formed at 800oC under high-pressures of 158 kbar [18]. It is not fully understood why the high-temperature and highpressure phase was found at our growth. We think that the fast cooling rate might result in the hightemperature and high-pressure phase remaining to ambient temperature. Orthorhombic SnO2 nanostructure and coexistence with rutile has also been reported in SnO2 nanowire synthesis [10]. In our observation, the orthorhombic SnO2 phase was dominate in the nanojunction growth. An orientation relationship between the orthorhombic and rutile SnO2, i.e., [0 0 1]o // [1 0 2]t and (1 0 0)o // (0 1 0)t, is thus established according to Fig. 3(b), where the subscripts ‘‘o’’ and ‘‘t’’ represent orthorhombic and rutile SnO2, respectively. TEM observation also indicated that no growth orientation relationship between SnO2 and Sn catalyst nanoparticle was formed. From the above analysis, it is concluded that the growth directions of the stem and branches in the SnO2 nanojunction are the orthorhombic ½1 1 0 and [1 1 0] directions, respectively, as denoted in Fig. 3(a). The measured angle between the stem and a branch is about 1011, which is in a good agreement with calculated angle between the orthorhombic [1 1 0] and ½1 1 0 directions of SnO2, i.e., 101.061.
Fig. 4. HREM image viewed along the orthorhombic [0 0 1] zone axis of SnO2. The HREM image was taken from outlined area shown in TEM image inset in top left corner. An enlarged HREM image from outlined area is inset in top right corner.
Fig. 4 is a HREM image of the outlined junction region in the inset TEM image located at top left corner. The HREM image was viewed along the [0 0 1] direction of the orthorhombic SnO2. The HREM image indicates that the lattice of the SnO2 nanojunction is perfect. No defect is observed. The growth directions of the stem and the branch are further confirmed from the HREM image. The inset at top right corner is an enlarged HREM image from outlined area located at the junction region between the stem and the branch in the HREM image. It can be seen that the stem and the branch are single crystal and the SnO2 branch is grown directly from the orthorhombic stem with the same crystallography orientation. The multiple branch growth follows the same growth orientation. Previous study [19] on thermal decomposition of SnO powder at an elevated temperature indicated that the following reactions occurred when the temperature was over 300 1C, 3SnO ðsÞ ! Sn2 O3 ðsÞ þ Sn ðlÞ;
ð1Þ
2Sn2 O3 ðsÞ ! 3SnO2 ðsÞ þ Sn ðlÞ;
ð2Þ
where Sn2O3 was a intermediate phase. In our study, the microstructural observations revealed that the SnO2 nanojunction and Sn catalyst nanoparticles were formed by decomposition of
ARTICLE IN PRESS Y.X. Chen et al. / Journal of Crystal Growth 270 (2004) 505–510
SnO source powders, where Sn nanoparticles acted as catalysts controlling the growth of such multiple branched SnO2 nanojunctions. Based on the above analysis, a self-catalytic VLS growth concerning formation of such branch growth of SnO2 nanojunctions is proposed, as shown in the schematics of Fig. 5. At above 1050 1C, the source SnO powder eventually decomposes into SnO2 and Sn species [19]. Then, liquid Sn droplets are formed because of low melting point of Sn, i.e., 231.89 1C. These nanoscale Sn droplets provide energetically favored sites for absorption of the SnO2 and Sn species, as shown in Fig. 5(a). When the Sn droplets are over saturated, SnO2 nanowires precipitate out and grow along one of the orthorhombic [1 1 0] directions, assumed to be ½1 1 0 as shown in Fig. 5(b). The nanoscale Sn droplets keep melting evaporated SnO2 and Sn species, and continuously control the growth of SnO2 nanowire with longer size and bigger diameter. Meanwhile, some Sn species are carried downstream by Ar gas flow and collided and adhered to newly formed SnO2 nanowire surface, forming Sn nanodroplets. These Sn nanodroplets also absorb SnO2 species and catalyze the growth of the first branch nanowires along the other orthorhombic /1 1 0S directions, assumed to be [1 1 0] as shown in Fig. 5(c). Furthermore, as the Sn nanodroplets adhere to the surface of the first branch nanowires as catalysts, the second branch nanowire growth occurs along the ½1 1 0 direction, which is the same as the stem direction. The growth of multiple branched SnO2 nanowire junctions terminates upon cooling, as shown in Fig. 5(d). Since the self-catalytic VLS growth avoids undesired external transition metal catalyst, it is believed that the self-catalytic VLS growth might open up a possibility for a clear and controlled semiconductor nanojunction synthesis. Further works about controlled nanojunction growth for nanoelectronic building blocks using Sn nanoparticles as catalysts are underway.
4. Conclusions SnO2 nanojunctions with the first branch growth and the second branch growth were
Sn
SnO2
SnO2 Sn(l)
Sn
509
Sn SnO2
Sn O2
Sn
(a) Sn SnO2
SnO2 Sn
Sn(l)
Sn
SnO2 (b)
SnO2
[110]
Sn Sn SnO2
[110]
Sn(l)
SnO2
[110]
(c)
Sn(s)
SnO2
(d) Fig. 5. Schematic drawing of a self-catalytic VLS growth describing growth process of the multiple branched SnO2 nanojunction. (a) After SnO is decomposed into SnO2 and Sn species, liquid Sn droplets are formed and absorbs SnO2 and Sn species. (b) SnO2 nanowires precipitates out and grow along the orthorhombic ½1 1 0 direction when the droplet is oversaturated. Some Sn species are carried downstream by Ar gas flow and collide and adhere to newly formed SnO2 nanowire surface, forming Sn nanodroplets. (c) These Sn nanodroplets also absorb SnO2 and Sn species and catalyze the growth of the first branch nanowires along the other orthorhombic [1 1 0] direction. (d) For the Sn nanodroplets adhere to the surface of the first branch nanowires, they can catalyze the second branch nanowire growth along the ½1 1 0 direction. The growth of multiple branched SnO2 nanowire junctions terminates upon cooling.
ARTICLE IN PRESS 510
Y.X. Chen et al. / Journal of Crystal Growth 270 (2004) 505–510
synthesized by a direct thermal evaporation of SnO powder. Orthorhombic SnO2 was found as a dominate phase in the nanojunction growth instead of rutile structure. Sn nanoparticles formed by decomposition of SnO powder were located at the tips of the stems and branches of the SnO2 nanojunctions, and catalyzed the growth of the multiple branched SnO2 nanojunctions. The orientation relationship between orthorhombic and rutile SnO2 is [0 0 1]o // [1 0 2]t and (1 0 0)o // (0 1 0)t. The growth directions of the branches and stems in the SnO2 nanojunctions are the orthorhombic / 1 1 0S. No orientation relationship between SnO2 and Sn catalyst nanoparticle was observed. A selfcatalytic VLS growth mechanism was discussed to describe the growth process of the multiple branched SnO2 nanojunction. The observation of such a self-catalytic multiple branch growth nanojunction opens up a possibility for the controlled growth of nanojunction building blocks.
Acknowledgements This work was supported by the AMRI through DARPA Grant No. MDA972-04-1-0029. Part of the work was supported by the Louisiana Board of Regents contract no. LEQSF (2003-06)-RD-B-13. The authors gratefully thank Prof. Z. L. Wang at Georgia Institute of Technology and Dr. Z. W.
Pan at Oak Ridge National Laboratory for the fruitful discussion.
Reference [1] M.S. Gudlksen, L.J. Lauhon, J.F. Wang, D.V. Smith, C.M. Lieber, Nature 415 (2002) 617. [2] C.H. Liu, W.C. Yin, F.C.K. Au, J.X. Ding, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 83 (2003) 3168. [3] J.M. Ting, C.C. Chang, Appl. Phys. Lett. 80 (2002) 324. [4] P.X. Gao, Z.L. Wang, J. Phys. Chem. B 106 (2002) 12653. [5] J.Y. Lao, J.Y. Huang, D.Z. Wang, Z.F. Ren, Nano Lett. 3 (2003) 235. [6] J.Y. Lao, J.G. Wen, Z.F. Ren, Nano Lett. 2 (2002) 1287. [7] Y. Shimizu, M. Egashira, MRS Bull. 24 (1999) 18. [8] G. Williams, G.S.V. Coles, MRS Bull. 24 (1999) 25. [9] C. Tatsuyama, S. Ichimura, Jpn. J. Appl. Phys. 15 (1976) 843. [10] Z.R. Dai, J.L. Gole, J.D. Stout, Z.L. Wang, J. Phys. Chem. B 106 (2002) 1274. [11] P. Nguyen, H.T. Ng, J. Kong, A.M. Cassell, R. Quinn, J. Li, J. Han, M. McNeil, M. Meyyappan, Nano Lett. 3 (2003) 925. [12] Y.Q. Chen, X.F. Cui, K. Zhang, D.Y. Pan, S.Y. Zhang, B. Wang, J.G. Hou, Chem. Phys. Lett. 369 (2003) 16. [13] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [14] Z.L. Wang, Z.W. Pan, Adv. Mater. 14 (2002) 1029. [15] J.A. Dean, Lange’s Handbook of Chemistry, 11th ed, McGraw-Hill, New York, 1973. [16] In preparation. [17] W.H. Baou, Acta Crystallogr. 9 (1956) 515. [18] K. Suito, N. Kawai, Y. Masuda, Mater. Res. Bull. 10 (1975) 677. [19] M.S. Moreno, R.C. Mercader, A.G. Bibiloni, J. Phys.: Condens. Matter 4 (1992) 351.