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Sn-filled Si nanotubes fabricated by the facile DC arc discharge method and their photoluminescence property
ARTICLE IN PRESS Journal of Crystal Growth 310 (2008) 4412–4416
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Sn-filled Si nanotubes fabricated by the facile DC arc discharge method and their photoluminescence property J.J. Feng a, P.X. Yan a,b,, Q. Yang a, J.T. Chen a, D. Yan a a b
School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, People’s Republic of China State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China
a r t i c l e in f o
a b s t r a c t
Article history: Received 28 March 2008 Received in revised form 5 June 2008 Accepted 28 June 2008 Communicated by J.M. Redwing Available online 2 July 2008
High-yield preparation of polycrystalline Si nanotubes (SiNTs) filled with single-crystal Sn was achieved by the DC arc discharge method. The Sn/Si nanocables were identified by X-ray diffraction (XRD), fieldemission scanning electron microscope (FE-SEM), transmission electron microscope (TEM) and photoluminescence (PL). The results show that the Sn/Si coaxial nanocables have homogeneous diameters of about 20–30 nm and lengths ranging from several ten to several hundred nanometers. Most of them are composed of an oval-shaped tip and a tapered hollow body. The possible growth mechanism is vapor–liquid–solid (VLS) model. The PL spectrum shows two characteristic emissions at 491 nm (blue emission) and 572 nm (yellow emission). The origin of luminescence was also discussed. & 2008 Elsevier B.V. All rights reserved.
PACS: 61.05.C 61.82.Fk 78.67.n 78.67.Ch Keywords: A1. Nanostructures A1. Nucleation A2. DC arc discharge method B2. Semiconducting silicon
1. Introduction In recent years, one-dimensional (1D) nanostructure materials have stimulated great interest, because of their unique electronic, optical, and mechanical properties and their potential applications in microscopic physics and the fabrication of nanoscale devices [1,2]. If a multilayer structure comprised of different components in the radial direction can be achieved in the 1D nanostructure, the utility of those materials may be further enhanced by combining uniform electronic properties in the axial direction [3]. After the discovery of carbon nanotubes as one of the products of the arc discharge method [4], even more interest has appeared in the field of nanotechnology [5]. When it was shown to be possible to insert materials into the core volume of carbon nanotubes, in order to produce nanocomposite structure and successfully insert many metals or nonmetals into carbon nanotubes, different methods of synthesis were
Corresponding author at: School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, People’s Republic of China. Tel.: +86 9318912719; fax: +86 9318913554. E-mail address: [email protected] (P.X. Yan).
0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.06.074
attempted and different kinds of elements were tried [6–9]. Various filling and coating techniques, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), pulsed laser deposition (PLD), arc encapsulation and encapsulation via catalytic growth from the solid phase, have been employed to synthesize many kinds of nanocomposite structures [10–14]. Silicon nanotubes would have better compatibility with the present established Si technology and may open up new and exciting possibilities for making different kinds of nanosized heterostructures, by filling the inside (hollow) space with one type of nanomaterial or by decorating the outside surfaces of the nanotubes with another type of nanomaterial. Hu et al. [15] reported that single-crystalline nanotubes of IIB–VI semiconductors were synthesized using Sn nanowires as template under thermal annealing. However, there is no report about SiNTs with this method, and there is no report on the fabrication of coaxial Si nanomaterials by the DC arc discharge method. In this communication, we have prepared Sn-filled SiNTs by the DC arc discharge method with the low melting point metal Sn. The Sn/Si coaxial nanocables were quickly (10 min) synthesized in gram quantities by the feasible and simple DC arc discharge method.
ARTICLE IN PRESS J.J. Feng et al. / Journal of Crystal Growth 310 (2008) 4412–4416
2. Experimental details The Sn/Si coaxial nanocables were synthesized in a DC arc discharge chamber fabricated by our laboratory, which has been used to synthesize Si nanowires [16]. The structure of it looks like a chamber, which is similar to the design employed by Balasubramanian et al. [17] in their work on the synthesis of the AlN nanowires. A graphite crucible containing a mixture of Si (4 g) and Sn (1 g) was used as the anode and a tungsten filament as the cathode. The chamber was evacuated by a mechanical pump. When the pressure reached 4–6 Pa, pure argon (99.999%) was introduced into the chamber and evacuated to 4–6 Pa over again. This process was used to extrude the residual atmosphere. Finally, the pure argon was introduced to the chamber to achieve an operating pressure of 5 104 Pa. The arc was generated between tungsten cathode and water-cooled graphite anode by applying a voltage of 20–30 V. The arc current varied within the range of 60–70 A during the arc discharge process. Due to the high current density and plasma temperatures involved, the target materials melted immediately. The atom flux was ejected from the molten material surface through the plasma vapor process and subsequently condensed on the wall of a cylindraceous water-cooler around the graphite crucible and the tungsten. After the chamber cooled to room temperature, gray-colored products were collected
Fig. 1. XRD patterns of the sample.
4413
from the inner wall of the water-cooler cylinder. Then, in order to study the photoluminescence (PL) properties of products, the samples were annealed at different temperatures, 400 and 600 1C, respectively, for 10 min in air. The products were characterized by X-ray diffractometer (D/Max-2400X, Rigaku Co, Japan) using Cu Ka radiation (l ¼ 1.54056 nm), field-emission scanning electron microscope (FE-SEM, Hitachi S-4800), and transmission electron microscope (TEM, Hitachi H-600). The room-temperature photoluminescence (PL) property of samples was carried on a fluorescence spectrophotometer (FLS920 T), and was excited by 330 nm.
3. Results and discussion X-ray diffraction (XRD) was employed to study the crystal structures of the as-synthesized products. Fig. 1 shows the XRD pattern of the products. All the reflection peaks can be readily indexed to pure face-centered Si (Joint Committee on Powder Diffraction Standard (JCPDS) Card No. 77-2108: a ¼ 5.419 A˚) and body-centered Sn (JCPDS Card No. 04-0673: a ¼ 5.831 A˚ and c ¼ 3.182 A˚). No other phase was detected in the final products, such as SiO2, SnO, Si3N4, and so on. The FE-SEM images of the products are displayed in Fig. 2. The image shown in Fig. 2(a) indicates large quantities of uniform 1D nanocables, which reveals the general morphology of the assynthesized products. Most of them have homogeneous diameters of about 20–30 nm and lengths ranging from several ten to several hundred nanometers. It can be seen from the high-magnification images in Fig. 2(b) that each nanotube has a particle on the tip. Further insight into the morphology and microstructure of Snfilled Si nanotubes was gained using TEM. Fig. 3(a) shows an overview of the SiNTs, plenty of uniform nanotubes are shown in it. The inset of Fig. 3(a) displays a single SiNT. The hollow structure of the tubes, the tube walls, and the Sn levels inside are clearly shown. The thickness of the wall is about several nanometers. The different contrasts suggest that the Sn-fillings divided the nanotube into several independent and sealed segments, and the inner diameter of the nanotube is consistent with the Sn nanorod. The nanotubes are straight and have relatively uniform dimensions along their entire length. The morphology shown in Fig. 3(a) is dominative in the products, the ratio of which is over 90%. The selected-area electron diffraction (SAED) pattern in Fig. 3(b) was taken from the segments encapsulated Sn nanorod of the nanotube. The sharp, clear diffraction spots indicate that the nanorod is single-crystal Sn. Those spots can be indexed to the
Fig. 2. FE-SEM images of the as-synthesized products. (a) Low-magnification images; (b) high-magnification images.
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Fig. 3. (a) TEM image of Sn/Si nanocables; (b) the selected-area electron diffraction pattern (SAED) taken from segments encapsulated Sn nanorods of SiNTs; (c)–(f) show the TEM images of the different morphologies of SiNTs filled with Sn.
(2 0 0) and (3 0 1) reflections of a bcc structure of Sn, which agrees with XRD analysis. The diffraction rings can be indexed to the (2 2 0) and (4 2 2) reflections of polycrystalline silicon, which
means that the silicon nanotubes are polycrystalline. When the SAED pattern (inserted in Fig. 3(b)) was taken from the hollow segments of the tube, there were only diffraction rings but no
ARTICLE IN PRESS J.J. Feng et al. / Journal of Crystal Growth 310 (2008) 4412–4416
diffraction spots, which further confirms that the as-synthesized nanotubes are Si polycrystalline. Considering the XRD pattern which shows that the product is composed of only Sn and Si, all these results indicate that the as-prepared samples are polycrystalline Si nanotubes filled with Sn single crystal. Several typical morphologies of the products were displayed in Fig. 3(c–f). All products in Fig. 3(c and d) have a big tip particle, just like tadpoles. Fig. 3(c) shows the products with a tapered hollow tube body, the tip particle seems to have a trend to expand into the hollow tube body. Fig. 3(d) shows the tadpoles with a longer tail, the tapered hollow tubes are longer, and parts of Sn particles have expanded into the hollow tube. The tapers are smaller than those tubes shown in Fig. 3(c). The tubes shown in Fig. 3(e) have no spherical particles at their tip end, and all the Sn particles have entirely expanded into the tube channel. The diameters of those nanotubes gradually decrease along their axes, but the change is not distinct compared with that of Fig. 3(c and d). In the last picture, Fig. 3(f), the nanotubes are filled with continuous Sn rod, whose tips have no particle and all Sn are expanded into the tubular channel. Both ends of the nanotubes are sealed and have one or several hollow segments.
4. Growth mechanism Considering the fact that most SiNTs are sealed with a Sn particle at one end, the vapor–liquid–solid (VLS) process may be involved in the growth process of Sn single crystal-filled Si nanotubes. Fig. 4 gives the proposed illustration of the growth mechanism. The nucleation of the present tubes is not yet fully understood, but Sn is believed to play an important role in the nucleation and the growth process of SiNTs. Although no Sn–Si phase diagram is available to illustrate the solubility of Si in Sn, the results suggest that Sn may be served as a solvent for absorbing Si under reaction conditions. It was reported that Sn nanowires were believed as a template when preparing single-crystalline nanotubes [15]. In our work, although the function of Sn is different from that of the report, it is similar to that. Fig. 4 shows Sn/Si nanocables growth process. In order to illuminate expressly, we give four TEM images of representative stages chosen from a mass of products. During the initial stage of vaporization, at the spot where the arc focused (the temperature can reach about 3000 1C instantaneously), the mixed Sn and Si powders vaporized at the same time. The material flux ejected from the molten mixture surface and started to grow under highly non-equilibrium conditions, and numerous tiny liquid droplets of Sn/Si condensed from the vapor of Si and Sn precursors (Fig. 4(a)). Then, the Si phase would be precipitated
4415
from the droplets at the liquid–solid interface to form Si nanotubes, due to the drastic change of the temperature when the droplets leave the arc center (Fig. 4(b)). Meanwhile, liquid Sn in the center of the droplets was sucked in a nanometer-sized cavity due to capillary action (Fig. 4(c)) [14,18], and this may be the reason for that part segment of SiNT being hollow. During the entire growth process, the droplets absorb the Si vapors to sustain the growth of the SiNT. Finally, the products condensed on the inner wall of the water cooler. Fig. 4(d) gives the final morphology of a single Sn-filled SiNT. A few nanotubes with different morphologies (Fig. 3(c–f)) have been prepared at one time, the reason for which may be that the growth time of these nanocables are not same. If Sn/Si liquid droplets were scattered by more Ar atoms when they got away from the arc-spot, the growth time would be longer. Therefore, more Sn vapors would be absorbed which resulted in liquid Sn not only filling the cavity but also expanding into the tubular channel. Based on the discussion above, we considered that the nanocables shown in Fig. 4(a–d) indicated the different growth step of the nanocables although the products were obtained simultaneously in one experiment. Therefore, we believe that the growth mechanism of Sn/Si coaxial nanocables would be VLS process.
5. Photoluminescence Fig. 5 shows the PL spectra of the as-prepared sample and the samples annealed at 400 and 600 1C, respectively. The pristine sample (Fig. 5) shows two characteristic emissions at 491 and 572 nm. The intensity of the peak increases with the increasing of oxidation temperature, without any change of the peak position, as shown in Fig. 5. The weak peak at 572 nm is attributed to the thin oxide layer covering the SiNTs’ surface [19,20]. Because SiNTs would be oxidized in the air consequentially, the SiNTs would be covered by a thin oxide layer. However, the layer of SiO2 is very thin, so no characteristic peaks of SiO2 appeared in the previous XRD pattern. In addition, the origin of the blue emission centered at 491 nm could be derived from two possible reasons: (1) small diameters of SiNT or (2) neutral oxygen vacancy in silicon oxide or Si–O-related emitting centers [21]. In fact, the morphologies of samples have not changed after annealing, and we did not observe any shift of the emission peak. Hence, we can exclude the theory of quantum confinement effect. We consider the observed blue luminescence results from the existence of defects in silicon oxides. These defects, such as e.g. twofold-coordinated silicon such as –O–Si–O– and neutral oxygen vacancy defects, which formed mainly at the interface between SiNT and SiO2 layer, are
Fig. 4. Illustrations of Sn/Si nanocables growth process.
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hundred nanometers. SiNTs have large inner/outer diameter ratio. Both ends of Sn/Si coaxial nanocables are sealed and nearly all of them are sealed with the Sn sphere particles. The growth process of the product is depicted by the vapor–liquid–solid (VLS) mechanism. Sn is believed to be a viable template for high-yield growth of the Sn/Si coaxial nanocables. The PL spectra show two characteristic peaks. The origin of luminescence is suggested to be due to oxide and defects in silicon oxide at the SiNT/SiO2 interface. The Sn/Si nanocables may have wide applications in optoelectronic devices. The results also suggest that DC arc discharge would be an effective method for synthesis of other semiconductor nanotubes filled with the low-melting-point metal, and our work may open a new area to synthesize nanotubes and promote potential applications of nanotubes.
Acknowledgments
Fig. 5. PL spectra of the products. (a) The pristine sample; (b) the sample oxidized at 400 1C for 10 min; (c) the sample oxidized at 600 1C for 10 min.
believed to be the origin of emission and as the luminescence centers. As the annealing temperature increases, the number of SiO2 nanocrystals on the surface of SiNTs increases. The higher the thickness of SiO2 is, the higher the second correspondent PL peak energy is. The first peak’s intensity is also enhanced by the annealing treatment. A possible reason is that, after hightemperature annealing, the interface would be clearer due to the atomic diffusion, so oxygen vacancy defects would be increased and the number of Si–O-related emission centers increases [20–22]. Therefore, as luminescence centers increases, the intensity of PL increases accordingly. Up to now, the exact nature of PL of the present nanocables remains unclear, and requires systematic investigation.
6. Conclusions In summary, we have synthesized Sn-filled SiNTs using a simple DC arc discharge method. The result shows that the Sn/Si coaxial nanocables have homogeneous diameters of about 20–30 nm and lengths ranging from several tens to several
The authors thank the Engineers S. Wang and Y.X. Li who came from Testing and Analytic Center, Gansu Academy of Science for TEM measurement. References [1] D. Snoke, Science 273 (1996) 1351. [2] Y.N. Xia, P.D. Yang, G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, Y.Q. Yan, Adv. Mater. 15 (2003) 353. [3] P. Szuromi, Science 281 (1998) 973. [4] S. Iijima, Nature 354 (1991) 56. [5] R.S. Ruoff, Nature 372 (1994) 731. [6] C. Guerret-Plecourt, Y. Le Bouar, A. Loiseau, H. Pascard, Nature 372 (1994) 761. [7] A. loiseau, H. Pascared, Chem. Phy. Lett. 256 (1996) 246. [8] J.Q. Hu, Y. Bando, J. Zhan, Z. Liu, Adv. Mater. 17 (2005) 975. [9] J.Q. Hu, Y. Bando, J. Zhan, D. Golberg, Angew. Chem. Int. Ed. 43 (2004) 4606. [10] J. Jiao, S. Seraphin, J. Appl. Phys. 83 (1998) 2442. [11] T. Lkuno, T. Yasuda, S. Honda, K. Oura, M. Katayama, J. Appl. Phys. 98 (2005) 114305. [12] Y.B. Li, Y. Bando, D. Golberg, Z.W. Liu, Appl. Phys. Lett. 83 (2003) 999. [13] J.Q. Hu, X.M. Meng, Y. Jiang, C.S. Lee, S.T. Lee, Adv. Mater. 15 (2003) 70. [14] Y.B. Li, Y. Bando, D. Golberg, Adv. Mater. 16 (2004) 37. [15] J.Q. Hu, Y. Baodo, J.H. Zhan, M.Y. Liao, Appl. Phys. Lett. 87 (2005) 113107. [16] J.B. Chang, J.Z. Liu, P.X. Yan, Mater. Lett. 60 (2006) 2125. [17] C. Balasubramanian, V.P. Godbole, V.K. Rohatgi, A.K. Das, S.V. Bhoraskar, Nanotechnology 15 (2004) 370. [18] P.M. Ajayan, S. Iijima, Nature 361 (1993) 333. [19] S.Y. Ma, B.R. Zhang, G.G. Qin, Mater. Res. Bull. 32 (1997) 1427. [20] S.Y. Jeong, J.Y. Kim, H.D. Yang, Adv. Mater. 15 (2003) 1172. [21] Y.P. Guo, J.C. Zheng, A.T.S. Wee, Chem. Phys. Lett. 339 (2001) 319. [22] X.M. Liu, K.F. Yao, Nanotechnology 16 (2005) 2932.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Sn-filled Si nanotubes fabricated by the facile DC arc discharge method and their photoluminescence property J.J. Feng a, P.X. Yan a,b,, Q. Yang a, J.T. Chen a, D. Yan a a b
School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, People’s Republic of China State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China
a r t i c l e in f o
a b s t r a c t
Article history: Received 28 March 2008 Received in revised form 5 June 2008 Accepted 28 June 2008 Communicated by J.M. Redwing Available online 2 July 2008
High-yield preparation of polycrystalline Si nanotubes (SiNTs) filled with single-crystal Sn was achieved by the DC arc discharge method. The Sn/Si nanocables were identified by X-ray diffraction (XRD), fieldemission scanning electron microscope (FE-SEM), transmission electron microscope (TEM) and photoluminescence (PL). The results show that the Sn/Si coaxial nanocables have homogeneous diameters of about 20–30 nm and lengths ranging from several ten to several hundred nanometers. Most of them are composed of an oval-shaped tip and a tapered hollow body. The possible growth mechanism is vapor–liquid–solid (VLS) model. The PL spectrum shows two characteristic emissions at 491 nm (blue emission) and 572 nm (yellow emission). The origin of luminescence was also discussed. & 2008 Elsevier B.V. All rights reserved.
PACS: 61.05.C 61.82.Fk 78.67.n 78.67.Ch Keywords: A1. Nanostructures A1. Nucleation A2. DC arc discharge method B2. Semiconducting silicon
1. Introduction In recent years, one-dimensional (1D) nanostructure materials have stimulated great interest, because of their unique electronic, optical, and mechanical properties and their potential applications in microscopic physics and the fabrication of nanoscale devices [1,2]. If a multilayer structure comprised of different components in the radial direction can be achieved in the 1D nanostructure, the utility of those materials may be further enhanced by combining uniform electronic properties in the axial direction [3]. After the discovery of carbon nanotubes as one of the products of the arc discharge method [4], even more interest has appeared in the field of nanotechnology [5]. When it was shown to be possible to insert materials into the core volume of carbon nanotubes, in order to produce nanocomposite structure and successfully insert many metals or nonmetals into carbon nanotubes, different methods of synthesis were
Corresponding author at: School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, People’s Republic of China. Tel.: +86 9318912719; fax: +86 9318913554. E-mail address: [email protected] (P.X. Yan).
0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.06.074
attempted and different kinds of elements were tried [6–9]. Various filling and coating techniques, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), pulsed laser deposition (PLD), arc encapsulation and encapsulation via catalytic growth from the solid phase, have been employed to synthesize many kinds of nanocomposite structures [10–14]. Silicon nanotubes would have better compatibility with the present established Si technology and may open up new and exciting possibilities for making different kinds of nanosized heterostructures, by filling the inside (hollow) space with one type of nanomaterial or by decorating the outside surfaces of the nanotubes with another type of nanomaterial. Hu et al. [15] reported that single-crystalline nanotubes of IIB–VI semiconductors were synthesized using Sn nanowires as template under thermal annealing. However, there is no report about SiNTs with this method, and there is no report on the fabrication of coaxial Si nanomaterials by the DC arc discharge method. In this communication, we have prepared Sn-filled SiNTs by the DC arc discharge method with the low melting point metal Sn. The Sn/Si coaxial nanocables were quickly (10 min) synthesized in gram quantities by the feasible and simple DC arc discharge method.
ARTICLE IN PRESS J.J. Feng et al. / Journal of Crystal Growth 310 (2008) 4412–4416
2. Experimental details The Sn/Si coaxial nanocables were synthesized in a DC arc discharge chamber fabricated by our laboratory, which has been used to synthesize Si nanowires [16]. The structure of it looks like a chamber, which is similar to the design employed by Balasubramanian et al. [17] in their work on the synthesis of the AlN nanowires. A graphite crucible containing a mixture of Si (4 g) and Sn (1 g) was used as the anode and a tungsten filament as the cathode. The chamber was evacuated by a mechanical pump. When the pressure reached 4–6 Pa, pure argon (99.999%) was introduced into the chamber and evacuated to 4–6 Pa over again. This process was used to extrude the residual atmosphere. Finally, the pure argon was introduced to the chamber to achieve an operating pressure of 5 104 Pa. The arc was generated between tungsten cathode and water-cooled graphite anode by applying a voltage of 20–30 V. The arc current varied within the range of 60–70 A during the arc discharge process. Due to the high current density and plasma temperatures involved, the target materials melted immediately. The atom flux was ejected from the molten material surface through the plasma vapor process and subsequently condensed on the wall of a cylindraceous water-cooler around the graphite crucible and the tungsten. After the chamber cooled to room temperature, gray-colored products were collected
Fig. 1. XRD patterns of the sample.
4413
from the inner wall of the water-cooler cylinder. Then, in order to study the photoluminescence (PL) properties of products, the samples were annealed at different temperatures, 400 and 600 1C, respectively, for 10 min in air. The products were characterized by X-ray diffractometer (D/Max-2400X, Rigaku Co, Japan) using Cu Ka radiation (l ¼ 1.54056 nm), field-emission scanning electron microscope (FE-SEM, Hitachi S-4800), and transmission electron microscope (TEM, Hitachi H-600). The room-temperature photoluminescence (PL) property of samples was carried on a fluorescence spectrophotometer (FLS920 T), and was excited by 330 nm.
3. Results and discussion X-ray diffraction (XRD) was employed to study the crystal structures of the as-synthesized products. Fig. 1 shows the XRD pattern of the products. All the reflection peaks can be readily indexed to pure face-centered Si (Joint Committee on Powder Diffraction Standard (JCPDS) Card No. 77-2108: a ¼ 5.419 A˚) and body-centered Sn (JCPDS Card No. 04-0673: a ¼ 5.831 A˚ and c ¼ 3.182 A˚). No other phase was detected in the final products, such as SiO2, SnO, Si3N4, and so on. The FE-SEM images of the products are displayed in Fig. 2. The image shown in Fig. 2(a) indicates large quantities of uniform 1D nanocables, which reveals the general morphology of the assynthesized products. Most of them have homogeneous diameters of about 20–30 nm and lengths ranging from several ten to several hundred nanometers. It can be seen from the high-magnification images in Fig. 2(b) that each nanotube has a particle on the tip. Further insight into the morphology and microstructure of Snfilled Si nanotubes was gained using TEM. Fig. 3(a) shows an overview of the SiNTs, plenty of uniform nanotubes are shown in it. The inset of Fig. 3(a) displays a single SiNT. The hollow structure of the tubes, the tube walls, and the Sn levels inside are clearly shown. The thickness of the wall is about several nanometers. The different contrasts suggest that the Sn-fillings divided the nanotube into several independent and sealed segments, and the inner diameter of the nanotube is consistent with the Sn nanorod. The nanotubes are straight and have relatively uniform dimensions along their entire length. The morphology shown in Fig. 3(a) is dominative in the products, the ratio of which is over 90%. The selected-area electron diffraction (SAED) pattern in Fig. 3(b) was taken from the segments encapsulated Sn nanorod of the nanotube. The sharp, clear diffraction spots indicate that the nanorod is single-crystal Sn. Those spots can be indexed to the
Fig. 2. FE-SEM images of the as-synthesized products. (a) Low-magnification images; (b) high-magnification images.
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Fig. 3. (a) TEM image of Sn/Si nanocables; (b) the selected-area electron diffraction pattern (SAED) taken from segments encapsulated Sn nanorods of SiNTs; (c)–(f) show the TEM images of the different morphologies of SiNTs filled with Sn.
(2 0 0) and (3 0 1) reflections of a bcc structure of Sn, which agrees with XRD analysis. The diffraction rings can be indexed to the (2 2 0) and (4 2 2) reflections of polycrystalline silicon, which
means that the silicon nanotubes are polycrystalline. When the SAED pattern (inserted in Fig. 3(b)) was taken from the hollow segments of the tube, there were only diffraction rings but no
ARTICLE IN PRESS J.J. Feng et al. / Journal of Crystal Growth 310 (2008) 4412–4416
diffraction spots, which further confirms that the as-synthesized nanotubes are Si polycrystalline. Considering the XRD pattern which shows that the product is composed of only Sn and Si, all these results indicate that the as-prepared samples are polycrystalline Si nanotubes filled with Sn single crystal. Several typical morphologies of the products were displayed in Fig. 3(c–f). All products in Fig. 3(c and d) have a big tip particle, just like tadpoles. Fig. 3(c) shows the products with a tapered hollow tube body, the tip particle seems to have a trend to expand into the hollow tube body. Fig. 3(d) shows the tadpoles with a longer tail, the tapered hollow tubes are longer, and parts of Sn particles have expanded into the hollow tube. The tapers are smaller than those tubes shown in Fig. 3(c). The tubes shown in Fig. 3(e) have no spherical particles at their tip end, and all the Sn particles have entirely expanded into the tube channel. The diameters of those nanotubes gradually decrease along their axes, but the change is not distinct compared with that of Fig. 3(c and d). In the last picture, Fig. 3(f), the nanotubes are filled with continuous Sn rod, whose tips have no particle and all Sn are expanded into the tubular channel. Both ends of the nanotubes are sealed and have one or several hollow segments.
4. Growth mechanism Considering the fact that most SiNTs are sealed with a Sn particle at one end, the vapor–liquid–solid (VLS) process may be involved in the growth process of Sn single crystal-filled Si nanotubes. Fig. 4 gives the proposed illustration of the growth mechanism. The nucleation of the present tubes is not yet fully understood, but Sn is believed to play an important role in the nucleation and the growth process of SiNTs. Although no Sn–Si phase diagram is available to illustrate the solubility of Si in Sn, the results suggest that Sn may be served as a solvent for absorbing Si under reaction conditions. It was reported that Sn nanowires were believed as a template when preparing single-crystalline nanotubes [15]. In our work, although the function of Sn is different from that of the report, it is similar to that. Fig. 4 shows Sn/Si nanocables growth process. In order to illuminate expressly, we give four TEM images of representative stages chosen from a mass of products. During the initial stage of vaporization, at the spot where the arc focused (the temperature can reach about 3000 1C instantaneously), the mixed Sn and Si powders vaporized at the same time. The material flux ejected from the molten mixture surface and started to grow under highly non-equilibrium conditions, and numerous tiny liquid droplets of Sn/Si condensed from the vapor of Si and Sn precursors (Fig. 4(a)). Then, the Si phase would be precipitated
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from the droplets at the liquid–solid interface to form Si nanotubes, due to the drastic change of the temperature when the droplets leave the arc center (Fig. 4(b)). Meanwhile, liquid Sn in the center of the droplets was sucked in a nanometer-sized cavity due to capillary action (Fig. 4(c)) [14,18], and this may be the reason for that part segment of SiNT being hollow. During the entire growth process, the droplets absorb the Si vapors to sustain the growth of the SiNT. Finally, the products condensed on the inner wall of the water cooler. Fig. 4(d) gives the final morphology of a single Sn-filled SiNT. A few nanotubes with different morphologies (Fig. 3(c–f)) have been prepared at one time, the reason for which may be that the growth time of these nanocables are not same. If Sn/Si liquid droplets were scattered by more Ar atoms when they got away from the arc-spot, the growth time would be longer. Therefore, more Sn vapors would be absorbed which resulted in liquid Sn not only filling the cavity but also expanding into the tubular channel. Based on the discussion above, we considered that the nanocables shown in Fig. 4(a–d) indicated the different growth step of the nanocables although the products were obtained simultaneously in one experiment. Therefore, we believe that the growth mechanism of Sn/Si coaxial nanocables would be VLS process.
5. Photoluminescence Fig. 5 shows the PL spectra of the as-prepared sample and the samples annealed at 400 and 600 1C, respectively. The pristine sample (Fig. 5) shows two characteristic emissions at 491 and 572 nm. The intensity of the peak increases with the increasing of oxidation temperature, without any change of the peak position, as shown in Fig. 5. The weak peak at 572 nm is attributed to the thin oxide layer covering the SiNTs’ surface [19,20]. Because SiNTs would be oxidized in the air consequentially, the SiNTs would be covered by a thin oxide layer. However, the layer of SiO2 is very thin, so no characteristic peaks of SiO2 appeared in the previous XRD pattern. In addition, the origin of the blue emission centered at 491 nm could be derived from two possible reasons: (1) small diameters of SiNT or (2) neutral oxygen vacancy in silicon oxide or Si–O-related emitting centers [21]. In fact, the morphologies of samples have not changed after annealing, and we did not observe any shift of the emission peak. Hence, we can exclude the theory of quantum confinement effect. We consider the observed blue luminescence results from the existence of defects in silicon oxides. These defects, such as e.g. twofold-coordinated silicon such as –O–Si–O– and neutral oxygen vacancy defects, which formed mainly at the interface between SiNT and SiO2 layer, are
Fig. 4. Illustrations of Sn/Si nanocables growth process.
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hundred nanometers. SiNTs have large inner/outer diameter ratio. Both ends of Sn/Si coaxial nanocables are sealed and nearly all of them are sealed with the Sn sphere particles. The growth process of the product is depicted by the vapor–liquid–solid (VLS) mechanism. Sn is believed to be a viable template for high-yield growth of the Sn/Si coaxial nanocables. The PL spectra show two characteristic peaks. The origin of luminescence is suggested to be due to oxide and defects in silicon oxide at the SiNT/SiO2 interface. The Sn/Si nanocables may have wide applications in optoelectronic devices. The results also suggest that DC arc discharge would be an effective method for synthesis of other semiconductor nanotubes filled with the low-melting-point metal, and our work may open a new area to synthesize nanotubes and promote potential applications of nanotubes.
Acknowledgments
Fig. 5. PL spectra of the products. (a) The pristine sample; (b) the sample oxidized at 400 1C for 10 min; (c) the sample oxidized at 600 1C for 10 min.
believed to be the origin of emission and as the luminescence centers. As the annealing temperature increases, the number of SiO2 nanocrystals on the surface of SiNTs increases. The higher the thickness of SiO2 is, the higher the second correspondent PL peak energy is. The first peak’s intensity is also enhanced by the annealing treatment. A possible reason is that, after hightemperature annealing, the interface would be clearer due to the atomic diffusion, so oxygen vacancy defects would be increased and the number of Si–O-related emission centers increases [20–22]. Therefore, as luminescence centers increases, the intensity of PL increases accordingly. Up to now, the exact nature of PL of the present nanocables remains unclear, and requires systematic investigation.
6. Conclusions In summary, we have synthesized Sn-filled SiNTs using a simple DC arc discharge method. The result shows that the Sn/Si coaxial nanocables have homogeneous diameters of about 20–30 nm and lengths ranging from several tens to several
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