Batchwise Growth of Silica Cone Patterns via Self-Assembly of Aligned Nanowires

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Silica nanostructures DOI: 10.1002/smll.200600418

Batchwise Growth of Silica Cone Patterns via SelfAssembly of Aligned Nanowires Shudong Luo, Weiya Zhou,* Weiguo Chu, Jun Shen, Zengxing Zhang, Lifeng Liu, Dongfang Liu, Yanjuan Xiang, Wenjun Ma, and Sishen Xie

Silica-cone patterns self-assembled from well-aligned nanowires are synthesized using gallium droplets as the catalyst and silicon wafers as the silicon source. The cones form a triangular pattern array radially on almost the whole surface of the molten Ga ball. Detailed field-emission scanning electron microscopy (SEM) analysis shows that the cone-pattern pieces frequently slide off and are detached from the molten Ga ball surface, which leads to the exposure of the catalyst surface and the growth of a new batch of silicon oxide nanowires as well as the cone patterns. The processes of growth and detachment alternate, giving rise to the formation of a volcano-like or a flower-like structure with bulkquantity pieces of cone patterns piled up around the Ga ball. Consequently, the cone-patterned layer grows batch by batch until the reaction is terminated. Different to the conventional metal-catalyzed growth model, the batch-by-batch growth of the triangular cone patterns proceeds on the molten Ga balls via alternate growth on and detachment from the catalyst surface of the patterns; the Ga droplet can be used continuously and circularly as an effective catalyst for the growth of amorphous SiOx nanowires during the whole growth period. The intriguing batchwise growth phenomena may enrich our understanding of the vapour–liquid– solid (VLS) growth mechanism for the catalyst growth of nanowires or other nanostructures and may offer a different way of self-assembling novel silica nanostructures.

Keywords: · nanostructures · pattern formation · photoluminescence · self-assembly · silica

1. Introduction [*] S. Luo, Prof. W. Zhou, J. Shen, Z. Zhang, L. Liu, D. Liu, Y. Xiang, W. Ma, Prof. S. Xie Beijing National Laboratory for Condensed Matter Physics Institute of Physics, Chinese Academy of Sciences Beijing 100080 (P. R. China) Fax: (+ 86) 108-264-0215 E-mail: [email protected] Prof. W. Chu The National Center for Nanoscience and Technology of China Beijing 100080 (P. R. China) S. Luo, J. Shen, Z. Zhang, L. Liu, D. Liu, Y. Xiang, W. Ma Graduate School of the Chinese Academy of Sciences Beijing 100039 (P. R. China)

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The vapour–liquid–solid (VLS) growth mechanism has been widely adopted to understand the growth of various kinds of one-dimensional (1D) nanowires and nanotubes of inorganic materials, including elements, oxides, nitrides, carbides, and chalcogenides.[1–3] In the conventional VLS mechanism originally proposed by Wagner and Ellis, a nanoscale alloy droplet acting as a catalytic site absorbs a gas-phase reactant, directs the nucleation and the growth of a nanostructured crystal, and confines the diameter of the crystalline nanowire/nanotube.[4] A common morphological feature of the VLS grown nanowires/nanotubes is that one catalytic

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Self-Assembly of Silica Nanopatterns

nanoparticle usually catalyzes the growth of only one nanowire/nanotube and is attached to one tip of the nanowire/ nanotube. Recently, a feature quite different from those in the conventional VLS process that has been demonstrated in several studies is that one micrometer-sized Ga droplet can simultaneously catalyze the growth of many SiOx nanowires with different nanostructures, such as fishbone-like, gourd-like, spindle-like, badminton-like, octopus-like, and so on.[5–11] However, a catalytic droplet usually promotes the growth of the nanostructure only once. Therefore, the yield of the nanowires is essentially controlled by the number of catalytic droplets involved in the activation reaction. An amazing growth phenomenon has been observed by Pan et al. in the synthesis of silica nanowires, in which the highly aligned nanowires tend to grow batch by batch from one catalytic droplet.[7] It is manifest by one catalytic site directing the batchwise growth of many nanowires repeatedly until the reaction terminates. In Pan6s case, for each batch growth, numerous silica nanowires nucleate simultaneously and grow outwards from the lower-hemisphere surface of a Ga ball, gradually lifting the liquid Ga ball upwards, and finally forming carrot-shaped rods in groups on the silicon wafer. Such a batchwise growth process is apparently different from the conventional VLS mechanism and may extend the scope of the VLS mechanism. However, the amazing growth phenomenon has not been reported by others thereafter and limited knowledge is available from such a single case of the synthetic investigation. In this paper, we present another example of the batchwise growth process of silica nanowires and show a novel pattern of silica cones assembled from the aligned nanowires. In an initial attempt to fabricate GaN nanowires using Ga as a source placed on a Si wafer and NH3 as both the carrying gas and the reactant, a high yield of SiOx nanowires was achieved instead. A similar result occurs frequently in the synthetic investigations of many other researchers under similar experimental conditions.[6–8] However, some novel growth phenomena were observed in our experiments that are different from any previous reports on the VLS growth process. Well-aligned silica nanowires are formed radially on almost the whole surface of the molten Ga balls. The well-aligned silica nanowires cluster and their free-standing tips congregate into conelike structures. The cones are densely populated on the spherical surface of a catalyst ball and assembled into triangular patterns. After the silica-cone layer peels off, another batch of nanowires nucleates and emerges from the exposed catalytic balls. The growth–peeling process takes place alternately and consequently produces many layers stacked around the Ga balls. The intriguing batchwise growth mode of silica cone patterns may enrich our understanding of the VLS growth mechanism and offers a different way of self-assembling silica nanostructures.

2. Results and Discussion After the growth, products that were white in color were found to be distributed on the Si wafer. The representative morphologies of the products were examined by scanning small 2007, 3, No. 3, 444 – 450

electron microscopy (SEM), as shown in Figure 1. Scattered balls were frequently observed on the Si substrate and their diameter varies from several tens to hundreds of micrometers. As demonstrated in Figure 1, a conelike structure is arranged in a triangular pattern on the spherical surface of a ball and the cones extend radially from the ball. A typical image is shown in Figure 1 a. Higher-magnification imaging shown in Figure 1 b indicates that the aligned nanowires with diameters of 10–50 nm cluster into a conelike structure. Figure 1 c exhibits a different morphology of the formed product, which is a volcano-like structure with a small ball at the crater. Detailed SEM observations reveal that a number of cracked pattern pieces (with typical thicknesses ranging from 5 to 15 mm) are piled up around the volcanolike structure. Cone apexes in the pieces spread radially from the center of the volcano. A high-magnification image of several pieces of cone patterns that are stacked radially layer by layer around the volcano is shown in Figure 1 d. From numerous SEM images, it was observed that the cones are regularly arranged in a triangular pattern not only on the surface of a catalyst ball but also in the pieces piled up around the ball. These cones are a few micrometers in size and the apex angle ranges from 30 to 50 degrees. Figure 2 a and b shows the pattern pieces on sloping and planar surfaces, respectively. The majority of cones are closely packed in a triangular pattern, also referred to as a hexagonal pattern, as illustrated in Figure 2 b. Occasional dislocations of cones were found, which is necessary for covering a spherical surface. From the pieces of cone pattern downwards, we judge that the cylindrical segments beneath the cones are composed of well-aligned nanowires, as evidenced in Figure 2 c. Also, this judgment is supported by the side view of the pattern pieces shown in Figure 2 d. Transmission electron microscopy (TEM) images (Figure 3 a) show that the diameters of the nanowires are distributed in the range of 10–50 nm. A selected-area electron diffraction (SAED) pattern reveals that the nanowires are amorphous (inset of Figure 3 a). Energy dispersive X-ray (EDX) analyses (Figure 3 b) indicate that the nanowires are pure amorphous SiOx and no Ga element particles attach to the amorphous nanowires. To understand the growth mechanism of the SiOx cone patterns, a series of experiments with different reaction periods and at different temperatures was performed. The products were analyzed in detail by SEM and EDX. Typical morphologies frequently observed, as shown in Figure 4, informatively suggest the formation sequence responsible for the final structures. Figure 4 a shows the image of a single Ga ball covered with a thin layer of SiOx nanowires. The nanowires on the top surface cluster into a triangular pattern structure, while the nanowires in the crack area are disorderly distributed (high-magnification view of Figure 4 a). Figure 4 b shows that conelike structures form in a triangular pattern and distribute separately on the surface of the Ga ball. In contrast to the full covering of cone patterns shown in Figure 1 a, a partially covered ball is shown in Figure 4 c, which seems to be “rugged”. Cone pattern pieces (indicated by straight arrows) stacked around the ball are arranged in the radial direction of the balls and can reasona-

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the ball (indicated by the curved arrow). Figure 4 d shows a more complete, flowerlike growth feature. The flower “bud” is the catalyst ball with the initially grown cone pattern on the surface and the “petals” consist of those detached pattern pieces piled up around the ball layer by layer. This image suggests that the growth process mainly involves two alternate stages: the catalytic growth of the cone pattern on the spherical ball surface and the detachment of the pattern piece from the ball. A high-magnification side view of a cone, shown in the inset of Figure 1. Scanning electron microscopy (SEM) images of the representative morphologies of the syntheFigure 4 d, demonstrates that sized SiOx cone patterns. a) Low-magnification image of a ball covered by a layer of regular cones in a the nanowires on the wall of triangular pattern; b) high-magnification images revealing cone architecture composed of clustered the cone are well-aligned but nanowires; c) volcano-like structure with bulk-quantity pieces of cone patterns piled up around a Ga not uniform in length, that is, ball; d) high-magnification image of several pieces of cone pattern radially stacked layer by layer. the longest part is in the centre along the axis and the shortest part is on the outside around the cone. These phenomena imply that the nanowires comprising a cone are not grown simultaneously. As an almost-complete cone pattern layer drops off the Ga ball, the catalyst surface is exposed to the atmosphere again (as shown in Figure 4 e), and new nanowires immediately grow on the fresh ball surface (inset A of Figure 4 e), which suggests the beginning of the growth of a new batch of the cone patterns. The results of EDX analyses of regions A and B, shown in the insets of Figure 4 e, demonstrate that the ball is Ga and the nanowires Figure 2. SEM images of the representative morphologies of a piece of cone-pattern layer piled up are SiOx with an O:Si atomic around a catalyst ball. a) Top view of a sloping cone-pattern piece showing the regular arrangement of ratio of 1.5–2.2:1. the cones. b) Top view of a piece showing well-assembled cones arranged uniformly in a triangular patThe low-melting-point tern. c) Bottom view of a pattern piece with the cone downwards showing well-aligned nanowires. metal Ga is known as an efd) Side view of a pattern piece showing the cylindrical segment composed of clustered well-aligned fective catalyst for the nanowires beneath each cone and the nanowire cylinders crowded against one another but arranged growth of silicon oxide nanoregularly. wires.[6, 7, 12, 13] It is generally accepted that silicon oxide nanowires are formed at active sites on the surface of the bly be assigned as the pieces peeled off from the molten Ga ball via multiple nucleation when Si atoms precipitate ball. The assumption is confirmed by the observations that out of the supersaturated Ga–Si melt and react with oxygen. some pieces on the ball have fractured but still adhere to

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Figure 3. a) TEM image of SiOx nanowires. Inset: SAED pattern showing the amorphous nature of the nanowires. b) Typical EDX spectrum of the SiOx nanowires.

As argued in References [7, 8, 12], the authors of which obtained the silicon oxide nanostructures in an experimental setup and under synthesis conditions similar to ours, the tiny amount of oxygen required for the formation of SiOx may originate from air leakage into the reaction system during growth. This argument seems applicable to our experiments since the pressure of the chamber was sustained at 500 Torr during growth. With respect to the Si source, besides that due to the etching behavior of Ga droplets, the possibility that Si gas evaporated from the substrate could be involved in the formation of SiOx nanowires can not be ruled out, as argued in Reference [7]. Therefore, a large number of silicon oxide nanowires grow outwards from the Ga ball and even cover the whole surface. According to the analyses of the observed results typically shown in Figures 1, 2, 3, and 4, a growth model is proposed for the formation of small 2007, 3, No. 3, 444 – 450

the special structures in our experiment, as depicted schematically in Figure 5. The new batchwise growth model also obeys the VLS growth mechanism, although it is apparently different from the conventional VLS processes described in the Introduction. In detail, a Ga droplet catalyzes the multiple nucleations of thousands of SiOx nanowires randomly distributing on the surface of the molten Ga ball (strictly speaking, the Ga–Si–O alloy spherical layer), as shown in inset A of Figure 4 e. The complete absence of a gallium droplet at the nanowire tip indicates the growth of these nuclei into 1D nanowires via basal attachment.[14, 15] Those proximate nanowires gradually aggregate or bundle into a separate cluster (shown in inset A of Figure 4 e); a lot of clusters thus form on the spherical surface of the molten ball. The driving force for the aggregation of nanowires might be the van der Waals force between the thin SiOx nanowires[12] similar to bundled carbon nanotubes or might be due to the strain developed on the spherical surface.[16] During growth, numerous clusters are prone to self-assemble into a triangular pattern on the spherical surface of a droplet (shown in region A of Figure 4 a and region B of Figure 4 d). The triangular patterning, also referred to as hexagonal close-packing, probably corresponds to the configuration with the lowest surface energy, as reported for the formation of SiOx patterns on a Ag droplet.[16] Each cluster forms the central part of a cone. At this stage, there exist some spare regions between the small clusters. Meanwhile, the nucleation and/ or growth of numerous nanowires continues over the whole molten Ga surface. The newly grown nanowires around the cluster are prone to attaching to the cluster (see region A of Figure 4 d) and therefore the central clusters grow continuously both in diameter and in length and form separated cones (see Figure 4 b). When the cones, maintaining the triangular pattern, become large enough and fill the exposed Ga surface as illustrated in Figures 1 a and 2 a and b, the growth of the diameter of the cluster or cone halts. However, the thickness of the triangularly patterned cone layer would be increased by the continual growth of the wellaligned nanowires comprising the cones (Figure 2 d), that is, the cone-structure layer would be pushed up by the radial array of nanowires (Figure 2 d) that build up the part beneath the cone and grow continuously. Therefore, this layer consists of cone-shaped structures on the top surface shown in (Figure 2 a, b, and d) and the densely aligned nanowires cylinder beneath the cone (Figure 2 c and d). Afterwards, the cone-patterned layer with a certain thickness is detached from the Ga droplet (see Figure 4 c) possibly because of the differences in thermal expansion or strain between the SiOx nanowires and the melted Ga droplet. After detachment, the fresh surface of the Ga droplet is exposed to continuously catalyze further nucleation. Consequently, the conepatterned layer grows batch by batch until the reaction is terminated. The processes of growth and detachment alternate and result in the formation of a volcano-like (Figure 1 c) or flower-like (Figure 4 d) structure with bulk-quantity pieces of cone patterns piled up around the Ga ball. However, although the formation process discussed above is consistent with the numerous observations of dif-

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Figure 4. SEM and EDX measurements on the products at different growth stages. a) Image of a ball on which SiOx nanowires are formed in a triangular pattern and some clustered closely; the corresponding high-magnification of region A is shown below. b) Image of a ball on which many cones are grown in sunder and form a triangular pattern. c) Image of a ball on which many cones have grown and some pattern pieces are dropped nearby (indicated by straight arrows). d) Image showing that a Ga ball is visibly shelled and surrounded by a few cone-pattern pieces. A high-magnification side view of a cone (A) and top view of the triangular pattern on the surface of the ball (B) are shown below. e) Image of two Ga balls that are nearly separated from the cone-pattern pieces; shown below are a high-magnification SEM image of region A and EDX spectra of regions A and B, respectively.

ferent morphologies of cone-shaped structures, several questions remain: what is the driving force for the nanowires to cluster into the cone-shaped structure? Why does the conepattern layer not extend from the Ga ball throughout the whole reaction period but peel off the ball? What is the driving force for the detachment? These are still challenging issues and open to further study.

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Light-emitting property was measured at room temperature on a Raman system using a HeCd laser as the excitation source; the photoluminescence (PL) spectrum is shown in Figure 6. An intense green-light-emission peak was observed centered at around 555 nm (ca. 2.2 eV). This green-light emission is generally observed in amorphous bulk silica,[17–19] nanowires,[20–22] and Si/SiOx composite nanowires.[23] Measurements on different pieces of the cone patterns transferred to a Ge or Cu supporter confirm the constancy of the PL peak position. As a comparison, silica-nanowire arrays synthesized in N2 also show an emission peak at 530 nm. Itoh et al. ascribed this 2.2 eV emission to the radiative decay of self-trapped excitons, which is governed by the structure of tetrahedron SiO4.[18] Further, Nishikawa et al. suggested that this band is sample dependent and surplus oxygen plays a role in its appearance.[19] Thus, in our case we can reason that the origin of the 2.2 eV band is associated with the nonstoichiometry in the as-prepared silica product. In addition, we note that the silica nanowires, mostly grown and post-treated in a reduction atmosphere, show a green-light emission as previously reported.[20, 21, 23] Therefore, the involvement of hydrogen in the form of either SiOH or SiH cannot be ruled out as the factor determining the emission band.[23–25]

3. Conclusions In summary, the main findings of this study are as follows: (i) Each Ga droplet can catalyze multiple nucleation and growth of hundreds of thousands of amorphous SiOx nanowires with diameters of 10–50 nm; (ii) the nanowires

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(v) the Ga droplet can be used as an effective catalyst for the growth of amorphous SiOx nanowires continuously and circularly during the whole growth period. These interesting findings are apparently different from the conventional VLS processes but follow a new batchwise growth model, which may enrich our understanding of the VLS growth mechanism for the catalytic growth of nanowires and other nanostructures.

Figure 5. Schematic representation of the batchwise growth model proposed for the growth of SiOx cone patterns. a) Ga droplet presented as a ball on the Si substrate. b) The Ga ball etches the Si substrate to form a GaSi alloy at the elevated temperature. The formed alloy covers the surface of the molten Ga ball. c) Multiple nucleation occurs when the solubility of Si in the GaSi alloy solution is supersatACHTUNGREurated (the solubility of Si in Ga is 13 at. % at 950 8C). Thereafter SiOx nanowires emerge on the nucleation sites (cf. region A of Figure 4 e). d) SiOx nanowires are so populated that they cluster into a triangularly patterned structure (cf. region A of Figure 4 a and region B of Figure 4 d); the thin conelike-pattern layer forms initially on the surface of the catalyst ball. e) SiOx nanowires are well aligned and remain as the self-assembled triangular cone patterns during growth (see Figures 1 a and 4 c). f) The cracked layer of cone patterns on the surface of the molten ball slides and peels off the ball, leading to the exposure of the molten GaSi alloy sheathing the Ga ball (see Figure 4 e). After that, the exposed ball makes it possible to grow SiOx nanowires and a new batch of cone patterns. The experimental observations clearly indicate that more than one batch of SiOx cone patterns are usually formed under these synthesis conditions (see Figures 1 c and 4 d).

4. Experimental Section N-type siliconACHTUNGRE(100) wafers were ultrasonically cleaned in acetone for 30 min and rinsed in deionized water before use. Molten gallium was plastered onto a clean wafer. The wafer-containing gallium droplet was placed in an alumina boat, which was transferred to the center of a long quartz tube mounted horizontally in a tubular furnace. The system was purged with nitrogen (99.999 %) for a few minutes and evacuated by a mechanical pump (limit pressure 2 2 10 3 Torr) alternately three times. The system was then switched to an ammonia (99.999 %) flow at 60 sccm (sccm = standard cubic centimeters per minute) and the chamber pressure was maintained at 500 Torr. The furnace was heated at a rate of 10 8C per minute. Different growth temperatures (from 700 to 950 8C) and growth durations (from 10 min to 8 h) were tested. Finally, the samples were slowly cooled to room temperature in the furnace under the ammonia flux. The as-grown product was characterized by a field-emission SEM (Hitachi S-5200) and TEM (Tecnai F20 and JOEL 2010) operating at 200 kV. The chemical compositions were determined by the EDX spectrometer attached to the SEM or TEM instruments. The PL spectrum was recorded at room temperature on a Renishaw R-1000 Raman system using a 325.0 nm HeCd laser as the excitation source.

Acknowledgements

Figure 6. Room-temperature PL spectrum of a piece of the SiOx cone pattern.

This work is supported by the National Natural Science Foundation of China (Grant No. 10334060) and the “973” National Key Basic Research Program (Grant No. 2005CB623602). We thank Prof. G. Wang, Ms. C.Y. Wang, and Mr. X.A. Yang of the Institue of Physics, CAS, for their help with the SEM and TEM measurements, and Dr. H.T. Yang of Tsinghua University for technical assistance with PL measurements.

aggregate into triangularly patterned clusters and grow into cone-shaped structure layers via self-assembly on a Ga droplet. Each cone-shaped cluster consists of thousands of aligned SiOx nanowires; (iii) the cone-pattern layer slides off and is detached from the Ga droplet. The fresh surface of the Ga droplet is exposed and catalyzes the nucleation of new SiOx nanowires repeatedly; (iv) the layer of SiOx nanowires containing the cone patterns is grown batch by batch;

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