Temperature Dependence of Morphologies of Aligned Silicon Oxide

composed of a spherical liquid-gallium ball (3 to 5 µm in diameter) and a silicon oxide ... hemisphere surface of the gallium ball. ..... Each badmin...
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NANO LETTERS

Temperature Dependence of Morphologies of Aligned Silicon Oxide Nanowire Assemblies Catalyzed by Molten Gallium

2003 Vol. 3, No. 9 1279-1284

Zhengwei Pan,† Sheng Dai,*,† David B. Beach,† and Douglas H. Lowndes‡ Chemical Sciences and Condensed Matter Sciences DiVisions, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 Received May 16, 2003; Revised Manuscript Received June 12, 2003

ABSTRACT Silicon oxide nanowire assemblies with fishbonelike, gourdlike, spindlelike, badmintonlike, and octopuslike morphologies were synthesized by the chemical vapor deposition of silane at 1150 °C with molten gallium as the catalyst via a vapor−liquid−solid process. The morphologies of the nanowire assemblies were temperature-dependent so that within a specific temperature range nanowire assemblies with a specific morphology were formed. Although the nanowire assemblies formed in different temperature ranges have different morphologies, they all are composed of a spherical liquid-gallium ball (3 to 5 µm in diameter) and a silicon oxide nanowire bunch that grows out from the lowerhemisphere surface of the gallium ball. Branching-growth and batch-growth phenomena were observed in the samples and were believed to be responsible for the formation of the unique morphologies described here. The growth mechanism of the nanowire assemblies is discussed.

One-dimensional nanomaterials in the form of tubes, wires, and belts have attracted much attention in the past decade because of their interesting geometries, novel properties, and potential applications.1-3 Many material systems including carbon,1,4 semiconductors,2,5 oxides,3,6 nitrides,7 and carbides8 have been successfully fabricated by a variety of methods. Among them, silicon oxide nanowires have attracted great attention in recent years because of their intense and stable blue light emission at room temperature and hence their potential applications in future integrated optical devices.9 Various methods have been developed for synthesizing silicon oxide nanowires including simply heating silicon wafers to high temperature10 and the thermal evaporation of silicon-based powders (such as Si, SiO2, SiC, and their mixtures) in the presence of catalysts such as Au and transition metals.6e,9,11 Recently, Pan et al.12 and Zheng et al.13 reported the growth of highly aligned silicon oxide nanowires by using molten Ga as the catalyst and a silicon wafer as the silicon source via a vapor-liquid-solid (VLS) process. Their work demonstrated several novel nanowire growth phenomena and some interesting morphologies of nanowire assemblies that were not observed in the conventional VLS processes.2,14 For example, a micrometer-sized Ga droplet could simultaneously catalyze the growth of hundreds of thousands of aligned silicon oxide nanowires, * Corresponding author. E-mail: [email protected]. † Chemical Sciences Division. ‡ Condensed Matter Sciences Division. 10.1021/nl0343203 CCC: $25.00 Published on Web 07/02/2003

© 2003 American Chemical Society

forming either a carrot-shaped rod that is composed of a large Ga ball and a nanowire bunch12 or a nanowire ball formed around a Ga core.13 One distinctive growth phenomenon that generally occurs in the catalytic growth of silicon oxide nanowires is that the nanowires tend to assemble into various complex morphologies. So far, silicon oxide nanowire assemblies with flowerlike,6e carrotlike,12 cometlike,12 ball-like,13 and brushlike11b morphologies have been successfully fabricated. Herein, we present the temperature-controlled growth of several interesting new silicon oxide nanowire assemblies: fishbonelike, gourdlike, spindlelike, badmintonlike, and octopuslike. These are prepared by the chemical vapor deposition (CVD) of silane (SiH4) with molten Ga as the catalyst. The morphologies of the nanowire assemblies are temperature-dependent, so within a specific temperature range, nanowire assemblies with a specific morphology were formed. This represents an important step toward the design and control of nanostructures. The synthesis was conducted in a tube furnace system similar to that shown in ref 12. In our approach, 1 to 2 g of GaN powder was loaded into an alumina boat and positioned at the center of an alumina tube that was inserted into a horizontal tube furnace. Inside the alumina tube, several alumina plates (60 × 10 mm2) were placed downstream from the GaN powder and acted as substrates for collecting growth products. After evacuating the alumina tube to ∼ 2 × 10-3

Figure 1. Schematic diagram of the position and corresponding temperature range of the five deposition zones inside the reaction chamber. The representative morphologies of the products in these zones are shown.

Torr, high-purity Ar gas was introduced at a rate of 50 sccm. The GaN powder was heated to 1150 °C in ambient Ar at a pressure of 300 Torr. A flow of 50 sccm of 2% SiH4 in Ar was then introduced into the reaction chamber to start the nanowire growth, and the pressure was maintained at 300 Torr throughout the process. At the reaction temperature of 1150 °C, a dense smokelike vapor due to the decomposition of GaN powder and the pyrolysis of SiH4 gas was observed through a window located at one end of the alumina tube. The vapors mixed and were transferred to the downstream part of the alumina tube by the Ar carrier gas, and silicon oxide nanowires were grown on the alumina substrates. Nanowire growth was terminated by switching off the SiH4 gas when the GaN powder was completely evaporated, which took about 1 h. The sample was then cooled in ambient Ar to room temperature. During growth, the temperature distribution along the deposition region was measured in situ by a sheathed thermocouple so that the relationship between the temperature and morphologies of the products could be readily obtained. The morphology of the as-grown products was examined by a field-emission scanning electron microscope (SEM, Philips XL-30) operated at an accelerating voltage of 10 kV. The composition of the nanowire assemblies was analyzed by an energy-dispersive X-ray spectroscope (EDS) attached to the SEM. Transmission electron microscopy (TEM) investigations were performed on a Philips CM-200 fieldemission electron microscope operated at 200 kV. After the growth, white-colored products were formed on the alumina substrates, covering a substrate length of ∼45 mm that corresponds to the temperature range of 950-1100 °C. The substrate, together with the products on it, was transferred into the SEM for imaging without destroying the location and orientation of the products on the substrate. SEM observations reveal that from the highest deposition temperature of 1100 °C to the lowest deposition temperature of 950 °C, the morphology, distribution, and density of the products vary gradually with their positions on the alumina substrate. Although there is no apparent boundary between adjacent regions, five distinctive deposition zones can be identified on the basis of the morphologies of the products. Figure 1 schematically depicts the position and the corresponding temperature range of the five zones inside the reaction chamber, with the representative morphologies of 1280

the products also shown. From 1100 to 950 °C, the morphologies of the products change from fishbonelike (zone I, 1100-1050 °C) to gourdlike (zone II, 1050-1030 °C), spindlelike (zone III, 1030-1010 °C), badmintonlike (zone IV, 1010-980 °C), and finally octopuslike (zone V, 950980 °C). However, although the products that formed in different temperature zones have different morphologies, they share some similar structural and compositional features. For example, as shown in Figure 1, all of the products are composed of a spherical liquid-Ga ball and a silicon oxide nanowire bunch. Figure 2a shows an SEM image of the products formed in zone I. The length of this zone is about 20 mm, corresponding to a temperature range of ∼1050-1100 °C. The products formed in zone I have a fishbonelike morphology composed of a spherical ball with a diameter of 3-5 µm and an aligned nanowire bunch with a length of up to 50 µm. EDS analyses reveal that the ball is Ga and the wires are SiOx with a Si/O atomic ratio of ∼1.5 (inset of Figure 2a). Figure 2b is a TEM image of the tip of a fishbonelike structure, showing many aligned SiOx nanowires growing out from the lower-hemisphere surface of a Ga ball. An interesting phenomenon observed during TEM observation is that when we suddenly converge the electron beam onto the Ga ball a fraction of the molten Ga will erupt off, because of the thermal and charging effects from electron beam irradiation, to produce a monolayer of Ga nanoparticles that are homogeneously distributed onto the carbon supporting film around the Ga ball (Figure 2c). From Figure 2a, we note that the amount and volume of the nanowires are varied along the axis of the nanowire bunch and that the bunch seems to be composed of many batches of nanowires that are ricked one by one along the bunch’s growth direction. These morphological features can be well explained by two novel nanowire growth phenomena that were mainly observed in silicon oxide nanowire growth. One growth phenomenon is branching growth6e,12 (i.e., one nanowire splits into two branches, and the newly formed branch also splits into two subbranches and so on (Figure 2d)). In some cases, this branching growth proceeds quickly and severely, resulting in the amount and volume of nanowires increasing dramatically within a short distance (inset of Figure 2d). The other growth phenomenon is that the nanowires within the bunch tend to grow batch-byNano Lett., Vol. 3, No. 9, 2003

Figure 3. Gourdlike SiOx nanowire assemblies formed in zone II (1030-1050 °C). (a) SEM image of aligned gourdlike nanowire assemblies. (b) Low-magnification TEM image showing four entire gourdlike structures. High-magnification TEM images of the tip (c) and tail (d) of a gourdlike nanowire assembly. The inset in d shows an enlarged image of the tail.

Figure 2. Fishbonelike SiOx nanowire assemblies formed in zone I (1050-1100 °C). (a) Low-magnification SEM image showing many fishbonelike nanowire assemblies. The inset is an EDS spectrum of the nanowires. (b) TEM image of the tip of a fishbonelike structure showing many SiOx nanowires growing out from the lower-hemisphere surface of a Ga ball. (c) Tip in b after strong irradiation by a converging electron beam, producing a layer of Ga nanoparticles around the Ga ball because of the eruption of liquid Ga. (d) TEM image of a nanowire bundle displaying branching-growth phenomena. The inset shows a nanowire bundle in which the amount and volume of the nanowires increase dramatically within a short distance (∼10 µm) through branching growth. The arrows represent the nanowire’s growth direction. (e) High-resolution TEM image of the amorphous SiOx nanowires. The inset is an electron diffraction pattern showing the amorphous nature of the nanowires.

batch;12 that is, for each batch, many nanowires simultaneously nucleate on the Ga ball’s lower hemisphere, grow at nearly the same rate, and simultaneously detach from the Ga ball and halt their growth because of the force exerted by the newly formed batch above (refer to the growth model proposed in ref 12 and the discussion below). As a result of batch growth, the Ga ball is pushed away from the alumina substrate and lifted upward by the growing nanowire batch. From this point of view, the nanowire bunch likes a rick piled up with nanowires. It should be noted that branching growth and batch growth not only account for the formation of the fishbonelike morphology but also are responsible for Nano Lett., Vol. 3, No. 9, 2003

the growth of the gourdlike, spindlelike, and badmintonlike morphologies that will be described below. TEM studies show that the SiOx nanowires have a uniform diameter along their entire length and a narrow diameter distribution (Figure 2d). The diameter and length of the nanowires are in the range of 40-80 nm and 10-20 µm, respectively. High-resolution TEM observations show that the nanowires are amorphous and homogeneous without crystal Si cores (Figure 2e). The amorphous feature of the wires is further confirmed by the highly dispersed electron diffraction pattern inserted into Figure 2e. As the temperature decreases to ∼1030-1050 °C (zone II), gourdlike SiOx nanowire assemblies are formed, as shown in the typical SEM image in Figure 3a, covering a substrate length of about 5 mm. Like the fishbonelike assemblies, the gourdlike assemblies also have a Ga ball above with a diameter of 3-4 µm and a nanowire bunch composed of aligned SiOx nanowires. It is interesting that these gourdlike nanowire assemblies are perpendicular to the substrate surface, forming an aligned gourd array. The morphological features of the gourdlike SiOx nanowire assemblies are clearly shown in Figure 3b, in which four entire gourds are displayed. The length of the gourd is up to 40 µm. Parts c and d of Figure 3 are enlarged TEM images of the tip part and tail part of a gourd, respectively, showing branching-growth and batch-growth features. The SiOx nanowires are of uniform diameter with values of ∼40-80 nm. SiOx nanowire assemblies with spindlelike morphology are grown in very high yield in zone III (Figure 4a), covering a 1281

Figure 4. Spindlelike SiOx nanowire assemblies formed in zone III (1010-1030 °C). (a) Low-magnification SEM image showing a large number of spindlelike structures growing in groups. (b) High-magnification SEM image of a group of spindlelike structures. (c) Side-view SEM image showing the shape and features of the spindlelike structure. (d) High-magnification SEM image of the top part of two spindlelike structures.

substrate length of ∼6 mm that corresponds to a temperature range of ∼1010-1030 °C. The spindlelike assemblies are closely packed, highly aligned, and tend to grow in groups; for each group, several tens of spindles grow radially upward from a central point, with a Ga ball (4 to 5 µm in diameter) atop each spindle (Figure 4b). The spindles have lengths of up to 40 µm and maximum diameters in the range of 1520 µm (Figure 4c). Figure 4d is a side-view high-magnification SEM image of the top part of two spindles showing numerous SiOx nanowires growing downward from the Ga ball’s lower-hemisphere surface. The spindlelike SiOx nanowire assemblies are very similar in both growth habits and morphological and structural characteristics to the Gacatalyzed carrotlike SiOx rods prepared by using a Si wafer as the Si source,12 suggesting a similar growth mechanism. In addition, the wire density in the spindlelike structures is much higher than in the fishbonelike (Figure 2) and gourdlike (Figure 3) structures. With further decreases in the deposition temperature, the length and density of the nanowire assemblies decrease gradually, and the morphologies of the products change from spindlelike to badmintonlike and finally to octopuslike. Shown in Figure 5a is a typical low-magnification SEM image of the products formed in zone IV (covering a temperature range of ∼980-1010 °C with a substrate length of ∼7 mm) in which large quantities of badmintonlike SiOx nanowire assemblies are formed. Each badminton (“shuttlecock”) has a perfectly spherical Ga ball with a diameter of 4 to 5 µm and an expanded nanowire bunch with a height of around 10 µm (Figure 5b and c) exhibiting typical branching-growth features. Figure 6 shows two SEM images of the octopuslike products formed in zone V (covering the temperature range of ∼950-980 °C and substrate length of ∼7 mm), with Figure 6a taken in the higher-temperature part and Figure 6b taken in the lower-temperature part. It is apparent that the octopuslike products formed in zone V have a much 1282

Figure 5. Badmintonlike SiOx nanowire assemblies formed in zone IV (980-1010 °C). (a) Low-magnification SEM image showing many badmintonlike structures. (b) High-magnification SEM image of two badmintonlike nanowire assemblies. (c) TEM image of a badmintonlike structure.

lower wire density and a larger wire diameter than the structures formed in zones I-IV. Especially for the octopi shown in Figure 6b, only several tens of SiOx nanowires with diameters of 100-200 nm and lengths of 5-8 µm sprout from the Ga ball’s lower-hemisphere surface. The above results show that by the CVD of SiH4 at 1150 °C with molten Ga as the catalyst SiOx nanowire assemblies with a variety of interesting morphologies were formed over a temperature range of 950-1100 °C and a substrate length of ∼45 mm. Our extensive experiments at varied furnace Nano Lett., Vol. 3, No. 9, 2003

Figure 6. Octopuslike SiOx nanowire assemblies formed in zone V (950-980 °C). SEM images of the octopuslike structures formed (a) at the higher temperature (near 980 °C) and (b) at the lower temperature (near 950 °C). The inset in a shows a TEM image of an octopuslike structure.

temperatures (1100-1200 °C) show that the products are strongly related to the temperature distribution inside the reaction chamber. Within a certain temperature range, products with a specific morphology can be obtained. Because the temperature distribution inside the reaction chamber has a dominant influence on the morphologies of the products, it may be possible to control the growth of SiOx nanowire assemblies with a specific morphology by modifying the temperature distribution. However, it is not clear at present (i) why the temperature has such a great influence that the morphologies of the products change so much within just a 20 °C range or a 5-mm distance and (ii) what is the underlying growth mechanism. The temperature may take effect through influencing the distribution and activity of the reactants over the deposition area, the concentration and solubility of the reactant in the Ga droplets, the sizes and number of SiOx nuclei, and the growth kinetics of the nanowires. Although there exist striking, systematic differences in morphology among the nanowire assemblies formed in different temperature zones, these products also exhibit some common structural and compositional features. First, a spherical liquid-Ga ball 3-5 µm in diameter is consistently present at the top of a nanowire assembly composed of highly aligned SiOx nanowires; that is, a micrometer-scale Ga ball can simultaneously catalyze the growth of many SiOx Nano Lett., Vol. 3, No. 9, 2003

nanowires. Second, branching-growth and batch-growth phenomena are observed in the samples. Third, both highresolution TEM observations and electron diffraction analyses reveal that all nanowires are amorphous and homogeneous without any crystalline domains. Fourth, EDS analyses show that the composition of the wires is SiOx with x in the range of 1.5-2. These features are fully consistent with the results obtained in the Ga-catalyzed synthesis of carrotlike and cometlike SiOx nanowire assemblies, as described in ref 12. This suggests that the growth model proposed in ref 12 can account for the formation of the nanowire assemblies described here. Considering the different Si sources used in the two studies, the growth of the nanowire assemblies by using SiH4 as the Si source can be briefly described as follows. At the reaction temperature of 1150 °C, a dense, hot vapor of Ga and Si species is created through the thermal decomposition of GaN and the pyrolysis of SiH4. The hot vapor condenses into small liquid clusters as the Ga and Si species cool through collision with the buffer gas. At the same time, oxygen, which comes mainly from leakage into the reaction system,15 is also absorbed by the Ga-containing clusters because of the large solubility of O in liquid Ga.16 These Ga clusters are transferred to the downstream part of the reaction chamber by the carrier gas, deposited onto the alumina substrates, and grow into small Ga-containing balls as the oncoming clusters are continuously absorbed from the vapor. When the concentrations of Si and O in the Ga ball are high enough, O will preferentially react with Si rather than Ga to form many SiOx nanoparticles on the lowerhemisphere surface of the Ga ball because the Si-O bond (185 kJ/mol) is stronger than the Ga-O bond (59 kJ/mol).17 These particles act as nucleation sites and initiate the growth of the first batch of SiOx nanowires. The Ga ball is then pushed away from the alumina substrate by the growing nanowires and grows larger by continuously absorbing reactants from the vapor. Branching growth begins as soon as the nanowires begin to grow and continues during the entire growth period. As this first batch of nanowires begins to grow, a second batch of nanowires nucleates and grows above the first, exerting a force on the batch below. When the force is great enough, the second batch of nanowires will lift the Ga ball upward, thereby detaching the first batch of nanowires from the Ga ball and halting their growth. Repetition of the growth and detachment processes allows for the formation of various complex and interesting nanowire assemblies as displayed in Figures 2-5. In summary, we have synthesized a variety of interesting SiOx nanowire assemblies by using molten Ga as the catalyst and SiH4 as the Si source via a VLS process. The morphologies of the products are temperature-dependent, offering a potential way of designing and controlling nanostructures. Acknowledgment. This research was sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory (ORNL), managed by UTBattelle, LLC, for the U.S. Department of Energy under contract no. DE-AC05-00OR22725. Z.P. acknowledges support from the ORNL Research Associates Program, admin1283

istered jointly by ORNL and the Oak Ridge Institute for Science and Education. We also acknowledge the ORNL SHaRE Collaborative Research Center, which provided the SEM and TEM analyses. References (1) (2) (3) (4)

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NL0343203

Nano Lett., Vol. 3, No. 9, 2003