NANO LETTERS
In-situ Formation of Ultrathin Ge Nanobelts Bonded with Nanotubes
2005 Vol. 5, No. 7 1419-1422
Wei-Qiang Han,*,† Lijun Wu,† Yimei Zhu,† and Myron Strongin‡ Center for Functional Nanomaterials, and Department of Physics, BrookhaVen National Laboratory, Upton, New York 11973-5000 Received April 25, 2005; Revised Manuscript Received May 26, 2005
ABSTRACT A novel nanostructure of ultrathin Ge nanobelts bonded with nanotubes has been fabricated and characterized. Nanotubes (either carbon or BN) are first coated with amorphous germanium and then heated and observed by an in-situ TEM. The thickness, down to 2 nm, and the width of the Ge nanobelts are determined by the thickness of this amorphous Ge coating and the diameter of nanotubes, respectively.
One-dimensional nanomaterials, such as nanotubes, nanowires, and nanobelts, have been investigated for potential applications because of their unique electrical, optical, and mechanical properties.1-5 Besides one-dimensional nanostructures of a single-component, a variety of nanostructures with two- or multicomponent phases have also been synthesized, including nanocables6-11 and nanoheterojunctions.12-14 These novel structures are of great importance to nanotechnology. Germanium (Ge) nanowires have been synthesized via several synthesis routes,15-18 and carbon nanotubes filled with Ge nanowires or nanoparticles have also been reported.19,20 Carbon nanotubes (CNTs) have also been used as templates to synthesize filled and coated nanotubes, as well as nanowires and BN nanotubes.21-23 During the formation of nanowires and BN nanotubes through the CNT-confined reaction, CNTs were used as templates and were fully or partially consumed as carbon oxides. In contrast to the above work, here we report a thickness controllable method, in which nanotubes (either carbon or boron nitride nanotubes) are used as templates, without using a catalyst and without consumption of these nanotubes, to grow Ge nanobelts with thicknesses of only about 2-5 nm. To our knowledge, there are no previous reports about Ge nanobelts and there are also no controlled methods to make other nanobelts, and in particular, those that are ultrathin. More interestingly, these crystalline Ge nanobelts are parallel and are bonded to the nanotubes in a new and unusual nanostructure. Our approach to the growth of a Ge nanobelt bonded with a nanotube includes several procedures and is schematically shown in Figure 1. In the present work, multiwalled carbon nanotubes (made by arc-discharge) or BN nanotubes (made * Corresponding author. E-mail
[email protected]. † Center for Functional Nanomaterials. ‡ Department of Physics. 10.1021/nl050770e CCC: $30.25 Published on Web 06/04/2005
© 2005 American Chemical Society
Figure 1. Schematics of (a) nanaotube; (b) amorphous coated nanotube; (c) nanobelt bonded with a nanotube. Note, only the topside of the nanotube is coated by the amorphous Ge.
by a carbon nanotube-substitution reaction23) are first dispersed on copper TEM grids covered with lacy carbon (Figure 1a). The copper grids were then glued to the substrate in a high vacuum chamber (pressure ∼ 2 × 10-8 Torr) where they were coated with amorphous Ge (a-Ge) at 78 K by a standard thermal vapor deposition technique, where the Ge was evaporated from a tungsten filament. During this heating, Ge is evaporated onto the substrate. The thickness is monitored by a quartz crystal oscillator. Only the up-side of the nanotube is coated with a-Ge (Figure 1b) because only this side is exposed to the Ge evaporation direction. Hence, the down-side of the nanotube faces the substrate and is not exposed to the evaporated Ge. The copper grids with a-Ge coated nanotubes were taken off the substrate and studied in a JEOL-3000F TEM equipped with a Gatan heating stage. After heat-treatment, the a-Ge coating becomes a flat crystalline nanobelt (Figure 1c). The thickness of the a-Ge coating used in this study is typically 4 nm for the carbon nanotubes. We have checked many coated NTs and no a-Ge coating thickness is over 4.5 nm. Figure 2 shows a low-magnification TEM image of a a-Ge coated carbon nanotube. The inset shows the highresolution image (HREM) of a sector of this a-Ge coated carbon nanotube. It is seen that the coating is quite uniform
Figure 2. Morphology of an amorphous Ge coated carbon nanotube. The boundaries between the nanotube and amorphous Ge are marked by lines B and C. Lines A and D indicate the surface of the amorphous Ge. The thickness of the amorphous layer is measured by AB or DC, which is ∼4 nm. The inset at the topright is the high-resolution image of the nanotube, where the feature of the multiwall carbon nanotube is visible.
Figure 4. (a) High-resolution images taken from the a Ge-coated carbon nanotube heated at (a) 450 °C and (b) 650 °C, respectively. The straight (111) fringes of the crystalline Ge region are present in the amorphous matrix.
Figure 3. Morphology of an amorphous Ge coated BN nanotube with (a) low and (b) high magnification. The lines indicate the boundaries between nanotube and amorphous Ge, and surface of the amorphous Ge, respectively. The thickness of the amorphous coating is about 2 nm.
and well attached to the nanotubes. Although the carbon nanotube is covered by a-Ge, it is still visible as shown in the inset. The boundary between the Ge and carbon nanotube is very sharp as marked by lines B and C. Notice, there are no vacancies in the boundary. The thickness of the Ge coating is measured to be about 4 nm by measuring the distance between the surface (line A) and the boundary (line B). For the coated BN nanotubes used in this study, the amorphous Ge coating is also uniform but the thickness is 2 nm as shown in Figure 3a and b. We varied the temperature to monitor the crystallization of the a-Ge coating. When the heating temperature was raised to 350 C°, crystalline Ge nanoparticles start to form within the a-Ge. The size and amount of the crystalline Ge nanoparticles increased with increasing heating temperature. Figures 4a and b are the high-resolution images taken of the carbon nanotubes heated at 450 °C and 650 °C, respectively. The fringes corresponding to the (111) plane of a crystalline Ge nanoparticle are shown in the amorphous matrix. At the end, the sample was kept at 750 °C for 30 min for complete crystallization of a-Ge, and then cooled naturally. After the heat treatment, all the a-Ge is crystallized and some nanobelts are single crystals. Figures 5a and b are the images taken from a coated carbon nanotube after the heat treatment with 1420
Figure 5. (a) Low- and (b) high-magnification images taken from the Ge-coated carbon nanotube after heat treatment (750 °C for 30 min). The Ge is completely crystallized. The multiwalled carbon nanotube indicated by AC has the same features as those before heating. The projection of the crystalline Ge nanobelt is marked by lines B and D. (c) Schematic drawing of the geometry of the nanobelt bonded with the nanotube.
low- and high- magnification, respectively. The straight lines represent the (111) fringes of a Ge single crystal. We performed the same heating process for a-Ge coated BN nanotubes and similar crystallization of the a-Ge was observed. Figures 6a and b are high-resolution images taken from two BN nanotubes after the heat treatment. From the high resolution images shown in Figures 5 and 6, the features in both carbon and BN nanotubes are clearly seen, and they are not changed with the heat treatment. For the Ge coating, however, in addition to the crystallization of a-Ge, the geometry of the Ge coating was changed as well and is indicative of the crystalline Ge lifting off the nanotube. For example, in the right side in Figure 5b, the distance between the boundary of the nanotube (line C) and surface (lines D) Nano Lett., Vol. 5, No. 7, 2005
Figure 6. High-resolution images taken from the Ge-coated BN nanotubes after heat treatment (750 °C for 30 min). The BN nanotubes are marked by lines A and C.
is 6 nm, which is larger than the maximum thickness of the a-Ge coating (4.5 nm) for the sample used in this study. If the Ge was still on the nanotube, we would only see a Ge region that was the layer thickness. Another example is Figure 6b, where the distance between the boundary (line C) and surface (lines D) is 5 nm, which is much larger than the thickness of the original a-Ge coating (2 nm). Hence, these examples imply that the Ge coating is no longer completely attached to the nanotubes after its crystallization. Moreover, the straight (111) fringes of crystalline Ge indicate that the (111) plane of the crystalline Ge is flat. This indicates that after crystallization the Ge coating is no longer a curved plane that is attached to the nanotube; instead, it becomes a flat plane (so-called nanobelt) with a line interface where the Ge is bonded to the nanotube, as shown in Figure 1c. In Figure 5c we show a schematic drawing of the geometric configuration of the Ge-coated nanotubes. The length of nanobelts is typically several hundred nanometers and can be up to a couple of micrometers. About 20% of the nanotubes form such a structure. To further understand the microstructure of the nanobelt, we performed tilting experiments. Figure 7a is the HREM of a Ge coated carbon nanotube after heat treatment. Figures 7b and 7c are the HREMs of the nanobelt after tilting 20° clockwise and 15° counterclockwise along the axis of the nanotube from Figure 7a, respectively. The projections (AB) of the Ge coating are 13.1 nm in Figure 7a, 14.8 nm in Figure Nano Lett., Vol. 5, No. 7, 2005
Figure 7. (a) High-resolution image of a Ge coated carbon nanotube after heat treatment. (b,c) High-resolution images after tilting (b) 20° clockwise and (c) 15° counterclockwise along the axis of the nanotube from (a), respectively. The projections of the nanobelt marked by AB are 13.1 nm in (a), 14.8 nm in (b), and 10.8 nm in (c), respectively. The real width of the nanobelt is therefore calculated to be 14.9 nm, and the angles between the nanobelt and beam are 118.7°, 98.7°, and 133.7°, respectively. The insets in the figures show the geometric configurations.
7b, and 10.8 nm in Figure 7c, respectively. Based on our calculations, only a flat plane can yield such projections. The width of the Ge coating is 14.9 nm, and the angles between 1421
the nanobelt and beam are 118.7°, 98.7°, and 133.7°, respectively. The insets in the figures are the schematics of the corresponding geometric configurations. During the Ge deposition, the substrate was held at liquid nitrogen temperature, which makes the coating both amorphous and uniform. The amorphous coating, which is completely attached to the surface of the carbon nanotubes, is stable up to room temperature. We did not observe a significant volume change of the Ge coating during heattreatment, which is consistent with the X-ray-diffraction absorption measurements by T. B. Light.24 The melting temperature of the Ge is 938.3 °C, and Ge does not form an alloy with C and BN during our reaction temperature range. Ge atoms can only move within a very local area. Our HREMs show that the fringes which mostly correspond to the (111) plane of crystalline Ge are always straight, even at the earliest stages of the crystallization of a-Ge (Figure 4). This indicates that the flat Ge layer starts with the initial crystallization. With the growth of crystalline Ge, more Ge comes off the nanotube. There may be two factors that make the Ge coating flat: (i) the different coeffectients of expansion between Ge and the nanotube, and (ii) the surface energy of crystalline Ge on the nanotubes may be higher than that of amorphous Ge due to the bonding constraints in crystalline Ge and the difficulty in conforming to the curvature of the nanotube with linear bonds. Another related way of saying this is that to keep crystalline Ge on the curved nanotube, the crystal planes, e.g. (111), must be enormously distorted. This will yield a high strain field and apparently this is not stable. Although most areas of the crystalline Ge came off the nanotube, there appears to be a linear interface where the Ge is bonded to the nanotube. The nanostructure that is made here is effectively a nanobelt which is ultrathin and has a region that is bonded to the nanotube. The thickness of nanobelts is determined by the thickness of the amorphous Ge coating, which is very easily controlled from 2 nm to several nanometers by the ultrahigh vacuum thermal deposition system. The width of the nanobelts is determined by the diameter of nanotubes so we can control the width of the nanobelts by selecting different diameter nanotubes. This is an easy, efficient and controllable way to grow ultrathin nanobelts. The thickness of the nanobelts made by previous methods usually was over 10 nm. The ultrathin nanobelts made by the present method are expected to have physical properties that are different from the thick nanobelts. One possible application is a semiconducting Ge nanobelt and a metallic or semiconducting carbon nanotube parallel and bonded together to form a new type of heterojunction. This novel route can be adapted so it can be used for
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nanotubes and nanobelts of other materials (especially other amorphous materials such as a-Si). Nanotubes can be semiconducting, metallic, or insulating, while these nanobelts can be semiconducting, metallic, insulating, magnetic, or ferroelectric. This novel nanostructure combining nanotubes and nanobelts with different properties is expected to have great nanoscientific opportunities and nanotechnology applications. In summary, we report a simple and efficient route for the formation of novel nanostructures composed of nanobelts bonded to nanotubes (either carbon or BN) and which are expected to have novel physical properties and important applications. This route is not only good for the formation of Ge-nanobelt/nanotube parallel nanostructures, but is probably good for the formation of other nanobelt/nanotube parallel nanostructures. Acknowledgment. This work is support by the U. S. DOE under contract DE-AC02-98CH10886. References (1) Iijima, S. Nature 1991, 354, 56-58. (2) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435445. (3) Han, W. Q.; Fan, S.; Li, Q.; Hu, Y. Science 1997, 277, 1287-1289. (4) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947-1949. (5) Wang, Z. L. Annu. ReV. Phys. Chem. 2004, 55, 159-196. (6) Lauhon, L. J.; Gudiksen, M. S.; Wang, C. L.; Lieber, C. M. Nature 2002, 420, 57-61. (7) Wang, Z. L.; Dai, Z. R.; Gao, R. P.; Bai, Z. G.; Gole, J. L. Appl. Phys. Lett. 2000, 77, 3349-3351. (8) Manna, L.; Scher, E. C.; Li, L.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 7136-7145. (9) Goldberger, J.; He, R.; Lee, S.; Zhang, Y.; Yan, H.; Choi, H.; Yang, P. Nature 2003, 422, 599-602. (10) Han, W. Q.; Zettl, A. Appl. Phys. Lett. 2002, 81, 5051-5053. (11) Ajayan, P. M.; Iijima, S. Nature 1993, 361, 333-334. (12) Zhang, Y.; Ichihashi, T.; Landree, E.; Nihey, F.; Iijima, S. Science 1999, 285, 1719-1922. (13) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617-620. (14) Wu, Y. Y.; Fan, R.; Yang, P. D. Nano Lett. 2002, 2, 83-86. (15) Hanrath, T.; Korgel, B. A. J. Am. Chem. Soc. 2002, 124, 14241429. (16) Kamins, T. I.; Li, X.; Williams, R. S. Nano Lett. 2004, 4, 503-506. (17) Gu, G.; Burghard, M.; Kim, G. T.; Dusberg, G. S.; Chiu, P. W.; Krstic, V.; Roth, S.; Han, W. Q. J. Appl. Phys. 2001, 90, 57455751. (18) Wu, Y.; Yang, P. J. Am. Chem. Soc. 2001, 123, 3165-3166. (19) Loiseau, A.; Pascard, H. Chem. Phys. Lett. 1996, 256, 246-252. (20) Wu, Y. Y.; Yang, P. D. AdV. Mater. 2001, 13, 520-522. (21) Dai, H. J.; Wong, E. W.; Lu, Y. Z.; Fan, S. S.; Lieber, C. M. Nature 1995, 375, 769-772. (22) Han, W. Q.; Fan, S. S.; Li, Q. Q.; Gu, B. L.; Zhang, X. B.; Yu, D. P. Appl. Phys. Lett. 1997, 71, 2271-2273. (23) Han, W. Q.; Bando, Y.; Kurashima, K.; Sato, S. Appl. Phys. Lett. 1998, 73, 3085-3087. (24) Light, T. B. Phy. RVe. Lett. 1969, 22, 999-1001.
NL050770E
Nano Lett., Vol. 5, No. 7, 2005
