Evolution of Catalyst Droplets during VLS Growth and Cooling Process

Dec 3, 2009 - The present studies largely extend the understanding of the VLS mechanism. 1. Introduction. Since it was brought forward by Wagner in 19...
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DOI: 10.1021/cg900587x

Evolution of Catalyst Droplets during VLS Growth and Cooling Process: A Case of Ge/ZnO Nanomatchsticks

2010, Vol. 10 122–127

Dapeng Wei and Qing Chen* Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871, P. R. China Received May 31, 2009; Revised Manuscript Received October 23, 2009

ABSTRACT: Using Ge sphere-capped ZnO nanowires as a model system, we investigate the evolution of catalyst droplets and their influence on nanowire growth in the processes including vapor-liquid-solid (VLS) growth and cooling processes. The diameter ratios between Ge particle and ZnO nanowire have approximately a normal distribution. The atomic ratio of Ge/Zn in the particle during the growing process is found to deviate enormously from the eutectic point in the Ge-Zn binary phase diagram, indicating ternary phase diagram has to be used. Slow cooling allows the elements separation and crystallization of Ge-Zn-O droplets and continuous growth of the nanowire at the nanowire/catalyst sphere interface. The present studies largely extend the understanding of the VLS mechanism.

1. Introduction 1

Since it was brought forward by Wagner in 1960s, the vapor-liquid-solid (VLS) process has been proposed to be the most successful approach for producing single-crystalline semiconductor nanowires.2-5 In the VLS process, an alloy droplet, which acts as a catalytic site for preferential absorption and crystallization of vapor phase reactants, is a vital key to understand the VLS growth process. Hence, much attention has been focused on studying the behaviors of catalyst droplets and their influence on nanowire growth.6-11 Until now, catalyst droplets have been able to directly control the composition,6 size,7 length,8 morphology,12 and growth direction13 of one-dimensional (1D) nanostructures. The catalystinducing growth processes have been in situ observed and studied in detail.7,14,15 Although in most cases the diameter of the 1D nanostructures have roughly the same size as the catalysts, there are several cases where thin nanowires grow from large catalyst particles forming matchstick-like structures.16,17 The ratios between the diameters of the Sn particles and ZnO 1D nanostructures have been demonstrated to change with the orientation of the growth direction of ZnO,18 while the Ge-catalyzed ZnO nanowires seem being less influenced by the Ge particle size.16 On the other hand, Ge is a successful example that semiconductor can act as an effective catalyst for producing semiconductor nanowires through a VLS process. The semiconductor-catalyzed VLS process is promising to realize the one-step synthesis of semiconductor heterostructures. However, until now, the mechanism of semiconductor-catalyzed VLS growth has not been investigated in detail. Furthermore, after the VLS growth, there is a cooling process during which the catalyst droplets are solidified and the structure of the 1D nanostructures near the catalyst may change. Despite over 40 years of study of VLS growth, the cooling process has often been ignored. Here, using Ge sphere-capped ZnO nanowires as a model system, we investigate the evolution of catalyst droplets and *To whom correspondence should be addressed. E-mail: qingchen@pku. edu.cn. pubs.acs.org/crystal

Published on Web 12/03/2009

their influence on nanowire growth in the processes including alloying, nucleating, VLS growth, cooling, and solidification. Our observations show that the diameter ratios between Ge particle and ZnO nanowire have approximately a normal distribution and are different for different cooling processes. We also observed that the atomic ratio of Ge/Zn in the catalyst particle during the growing process deviates enormously from the eutectic point in Ge-Zn binary phase diagram, indicating the Ge-catalyzed ZnO growth cannot be explained by Ge-catalyzed Zn growth followed by Zn oxidation. For the slowing cooling specimen, the composition of the nanowire near the interfaces between the Ge particles and the ZnO nanowires is also observed to be various. 2. Experimental Section The synthesis was conducted in a horizontal tube furnace with a 25 mm inner-diameter quartz tube mounted inside. The high purity Ge (99.999%) and Zn (99.999%) powders were placed in a quartz boat and were introduced into the quartz tube. A clean Si wafer was also introduced downstream in the tube. First, the tube was pumped to a base pressure of 1.5  10-2 Torr, and then a carrier of high-purity Ar was fed at a flowing rate of 100 sccm (standard cubic centimeter per minute). The pressure was maintained at 20 Torr during the whole process. Second, the temperature at the furnace central region was ramped to 1000 °C at a rate of 20 °C min-1 and maintained for 45 min. At the same time, the Zn and Ge powders were placed in the 600-650 °C and central region, respectively, while the Si wafer was in the region of 550-650 °C along the downstream of the gas flow. After the reaction, the specimens were cooled to room temperature in the furnace. We also prepared some quenching specimens by pulling them out of the furnace directly after the reaction using a homemade magnetic tool. The morphologies of the products were first characterized using scanning electron microscopy (SEM, using a FEI XL 30F). Then the products were scraped off the Si substrates and collected; these powders were used to examine the crystal structure of the products by X-ray diffraction (XRD, Rigaku X-ray diffractometer). Detailed structures and elemental distributions were characterized using transmission electron microscopy (TEM, using a Tecnai G20), and energy dispersive X-ray spectrometry (EDS, using an EDAX spectroscope attached to TEM). r 2009 American Chemical Society

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3. Results and Discussion

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Figure 1. (a) A typical X-ray diffraction pattern of the powder products. (b) SEM image of nanomatchstick arrays on the substrate. The inset is a higher magnification image, and the scale bar is 2 μm. (c) Statistics charts of the diameters of the nanowires and the spheres, and the ratios between them in the products experienced slow cooling.

catalyst particle have a fixed orientation, which might be caused by the following reason: during the cooling process, the catalyst particle crystallized with a preferred orientation of (001)ZnO (111)Ge to lower the interface energy. The EDS analyses on the nanomatchsticks confirm that the nanowire is ZnO and the sphere is Ge (Figure 2d). However, quantitative EDS analysis show two different results. In some nanomatchsticks, the Zn/O atom ratios of the nanowires near the Ge spheres remain about 1. But in other nanomatchsticks the end part of the nanowire mainly contains Zn and has little oxygen (Figure 2e). The lengths of the O-lacked end parts range from 0 to 200 nm. Furthermore, SAED patterns recorded from the interface area (Figure S2, Supporting Information) confirm that the O-lacked end part still remains the crystal structure of ZnO. The HRTEM image in Figure 2f shows that the lattice spacing of 0.52 nm corresponds to the (001) plane of ZnO, and the crystal orientation relationship is still (001)ZnO (111)Ge. The reason for this phenomenon will be discussed later. A series of experiments were performed to investigate the VLS growth process of the Ge/ZnO nanomatchsticks. After different time reactions, specimens were immediately pulled out of the furnace to be quenched to room temperature. The quenching process can prevent the diffusion and crystal growth happening in slow cooling process, and the quenched products can provide information during the reaction. SEM images of three products with different reaction times are shown in Figure 3. At the beginning, hemispherical particles formed on the substrate (Figure 3a). After several minutes, nanowires start to nucleate (Figure 3b). The diameter of the nanowire is thin at the beginning and become thicker later, forming a tapered morphology. Except the end part, the nanowires have uniform diameter along their growth direction in the long-time reaction specimens (Figure 3c). The tapered morphology has been observed over the first few micrometers of the nanowires in a previous report,16 probably due to the significant increase of the size of the catalyst particle )

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Figure 1a shows a typical XRD pattern of the powder products obtained through slow cooling after VLS growth. The main diffraction peaks could be indexed to hexagonal ZnO (JCPDS No. 36-1451) and diamond-cubic Ge (JCPDS No. 04-0545), indicating that the products are composed of ZnO and Ge crystal. Two tiny peaks at about 38° and 65° could be indexed to (300) and (422) reflections of cubic GeO2 (JCPDS No. 23-0999), which might result from the surface oxidation of Ge crystal. The SEM image in Figure 1b shows that the products contain a lot of matchstick-like nanostructures vertically aligned on the Si substrate. The typical diameter of the nanowires is 50-150 nm, and their lengths are 5-10 μm. The microspheres on the tips have diameter distributions in the range of 0.4-1.2 μm. Figure 1c shows that the diameters of the nanowires and the spheres have approximately normal distribution with the peak values of about 90 and 600 nm, respectively. In a previous report, the diameters of the microspheres were reported to be different for the nanowires having the same diameter,16 and we also observed the same phenomena. If the diameter of the microsphere does not affect the diameter of the nanowire, the ratio between the two diameters would have a random distribution. However, we observed that the ratio between the diameter of the microspheres and the diameter of the nanowires has approximately normal distribution with a peak value of about 6.5 (as shown in Figure 1c), indicating the diameter of the nanowires is still affected by the diameter of the microspheres. It is well-known that the diameter of the nanowires can be controlled by the diameter of the catalyst droplets in Au-catalyzed VLS growth. The reason that the diameter of the microspheres affecting the diameter of the nanowires might be similar to but not exactly the same as the Au-catalyzed VLS growth case. Further study is needed. Detailed structural and chemical analyses of the products were carried out using TEM and EDS. Figure 2a shows a TEM image of a matchstick-like nanostructure. Clearly, the surface of the sphere is smooth, and the straight nanowire has a uniform diameter along its axis. Clear interface between sphere and nanowire is easily observed. The insets of Figure 2a show the corresponding selective area electron diffraction (SAED) patterns recorded from the center of the spheres, the nanowires, and the connecting area, respectively. The SAED pattern of the nanowire can be indexed to the [010] zone axis of hexagonal ZnO, and the SAED pattern of the center part of the microsphere belongs to diamond-cubic Ge crystal. The growth direction of ZnO nanowire is [001], and the correspondingly parallel direction of Ge sphere is [111]. Figure 2b is a high-resolution TEM (HRTEM) image showing the interface between the ZnO nanowire and the Ge sphere. The lattice spacing of the planes perpendicular to the growth direction of the nanowire is 0.52 nm, corresponding to the (001) plane of ZnO. On the sphere side, the lattice spacing of 0.33 nm corresponds to the (111) plane of Ge. Hence, the orientation relationship between the ZnO nanowire and the Ge sphere is (001)ZnO (111)Ge. However, careful observation shows that each Ge sphere is not one single crystal, and clear grain boundaries can be observed near the surface of the spheres, as shown in Figure 2c. While the inset of Figure 2a only shows one set of diffraction pattern from the center of individual crystal particle, indicating that the central part of the sphere is a single crystal and occupies large volume of the sphere. The nanowire and the large part of the

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Figure 2. (a) a TEM image of a nanomatchstick experienced a slow cooling process. The insets are corresponding SAED patterns recorded from (1) the microsphere, (2) the nanowire, and (3) the joint of them, respectively. (b) a HRTEM image of the interface. The lattice spacings of 0.33 and 0.52 nm correspond to the (111) plane of Ge and the (001) plane of ZnO, respectively. (c) a HRTEM image of the area outlined in the inset, showing the boundary between two Ge crystal grains near the surface of the Ge sphere. The scale bar in the inset is 200 nm. (d) EDS spectra recorded from (1) the microsphere, (2) the joint area, and (3) the nanowire in (a), respectively. (e) line profiles of different elements along the white dashed line in the inset. (f) An HRTEM image of the interface between a Ge sphere and the O-lacked nanowire end part.

Figure 3. SEM images of the products after growth time of (a) 3, (b) 10, and (c) 45 min. The arrows point to the coalescence of catalyst particles.

at the initial growth stage. Coalescences were observed in Figure 3a-c, indicating that the spheres or hemispheres are in a liquid or semisolid state during the reaction and supporting that the reaction is a VLS process. The surfaces of the spheres or hemispheres in Figure 3 are all rough, which is different from that shown in Figures 1 and 2. A possible reason is that

when quenching from high temperature to room temperature, many crystals nucleate simultaneously from the liquid droplet and there is no time to form a low energy smooth surface; the diffraction pattern recorded from the quenched sphere (inset (1) in Figure 4a, which shows a polycrystal pattern) supports this proposal. While in the slow cooling process, crystals can

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Figure 4. (a) a TEM image of a nanomatchstick produced by quenching. The insets: the corresponding SAED patterns recorded from (1) the sphere and (2) the nanowire. The electron diffraction rings indicated by lines 1-5 correspond to (110)ZnO, (101)ZnO, (100)ZnO, (111)Ge, and (220)Ge, respectively. (b) The corresponding EDS spectra recorded from (1) the sphere and (2) nanowire. (c) Four typical EDS spectra recorded from the spheres in the same specimen showing different contents of O, Zn, and Ge. (d) The statistics charts of the diameters of nanowires, the diameters of spheres, and the ratios between them after quenching, indicating approximately normal distribution with the peak values of about 9 nm, 90 nm, and 0.85 μm, respectively.

grow to large crystals and form a roughly smooth surface to lower the total energy. EDS analysis (shown in Figure 4b) on the sample shown in Figure 3c indicate that the nanowire is ZnO, while the particles contain Ge, Zn, and O. As the amount of the samples shown in Figure 3a,b is too small to be collected for TEM observation, we only examined these samples by SEM and did EDS analysis in SEM. Our results show that the particles shown in Figure 3a,b also contain Ge, Zn, and O. Therefore, during the growth process, alloy droplets not only contain Zn and Ge, but also contain a considerable amount of dissolved oxygen. Furthermore, the amount of Ge, Zn, and O varied from particle to particle, as shown in Figure 4c that obtained from a 45 min reaction specimen. As quenching can prevent element diffusion, the composition of the particles in the quenched specimen is roughly the same as the composition during the reaction. Therefore, the droplets may have different composition during the reaction. Figure 4d indicates that catalyst particles after quenching generally have larger diameters than those after slow cooling, but the diameters of ZnO nanowires are almost the same in both quenched and slow cooling specimens, resulting in a larger diameter ratio in the quenched specimen than in the slow cooling specimen. Therefore, the volumes of the droplets were gradually decreasing during the slow cooling. Ge-Zn binary phase diagram and in situ oxidation of Zn into ZnO during the growth process have been used to explain the Ge-catalyzed ZnO nanowire growth.16 According to the Ge-Zn binary phase diagram, solid Zn would precipitate from the Ge-Zn droplet if the amount of Zn in the liquid is higher than 94.7%at and the temperature is between 394 and 419.58 °C. However, so far, we did not find any particle containing over 90%at Zn. Furthermore, the substrate temperature in our experiments is 550-650 °C. Therefore, Ge-Zn binary phase diagram cannot explain the growth of

the Ge/ZnO nanomatchsticks. We propose oxygen plays an important role in the reaction and ternary phase diagram is needed to explain the ZnO growth mechanism. Unfortunately, the Ge-Zn-O ternary phase diagram is still unavailable. However, the process might be understood to some extent based on the following facts. It is known that Zn has a stronger reducibility than Ge. Hence, it is reasonable that oxygen reacted first with Zn to form ZnO, which has a melting point (MP) around 1975 °C. Being a typical ionic compound, ZnO has a much higher melting point than Zn (MP around 419 °C), Ge (MP around 938 °C), and GeO2 (MP around 1115 °C). So, ZnO is easier to precipitate from the droplet containing Ge, Zn, and O during the VLS growth process than the other three materials. The fact that the diffraction pattern of the particle after quenching contains ZnO and Ge (the inset (1) in Figure 4a) supports our proposal. Another interesting phenomenon is that catalyst particles after quenching generally have larger volumes than that after slow cooling, while the diameter of the nanowires remains almost the same, resulting in a larger sphere to nanowire diameter ratio. The statistic charts are shown in Figure 4d. Figure 5 shows that the evolution of catalyst tips during the growth and cooling processes. After ending the reaction, the catalyst droplets will experience different processes for slow or fast cooling. If the nanomatchsticks were slowly cooled, residual Zn and O species within the droplets would first continuously precipitate at the interfaces to form the end part of the ZnO nanowire, due to its higher melting point than Ge, and then the Ge droplets were solidified into Ge microspheres. Generally, Ge microsphere inclines to crystallize along the direction that brings the least lattice mismatch at interface.14 That small Ge crystals with a different orientation exist on the surface of microspheres might result from the earlier crystallization of the surface layer. In the meantime, crystallization of surface layer also hindered the outside Zn and O from entering

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Figure 5. The scheme exhibiting the evolvement of catalyst droplets during the VLS growth and cooling processes.

Wei and Chen

these polyhedral shaped particles are crystalline Zn2GeO4 and the nanowires are crystalline ZnO with the growth direction in [001]. In general, the faceted surfaces of the catalyst particles and lower growth temperature than eutectic point are direct evidence for the vapor-solid-solid (VSS) process.19 The VSS growth, as another important process for the liquid or solid catalyst-induced vapor growth, has been observed in TiSi2-catalyzed Si nanowires,20 Au or Cu-catalyzed Ge nanowires,21,22 and AuZn-catalyzed ZnO nanowires.23 It is known that Zn2GeO4 remains in the solid state at 400-500 °C.24 Thus, in the present case, Zn and O vapor atoms were introduced into the Zn2GeO4 crystal and diffused slowly, resulting in slow growth velocity of ZnO nanowires. Hence, the nanowires were generally shorter than the ZnO nanowires grown via the VLS process. These facts suggest that the growth of Zn2GeO4-capped nanowires is likely to be governed by the VSS process. 4. Conclusion

Figure 6. (a) An SEM image of the products obtained in the temperature zone of 400-500 °C. The inset: a higher magnification SEM image exhibiting nanowires capped by polyhedral shaped particles. The scale bar is 2 μm. (b) A TEM image of a nanomatchstick, and corresponding SAED patterns recorded from the polyhedral shaped particles (inset 1) and the nanowire (inset 2). (c) The corresponding EDS spectra from the particle (line 1) and the nanowire (line 2), revealing that the particle contains Zn, Ge, and O, while the nanowire contains Zn and O.

into these spheres. Therefore, the precipitation of Zn and O results in a decrease of the volumes of the droplets. When Zn and O were exhausted in the droplet, Ge was separated from ZnO species. If the catalyst droplets contain less oxygen than Zn, O-lacked ZnO part near the interface between the nanowires and the sphere would form after exhausting the residual oxygen in the catalyst droplet. Hence, the content of oxygen in the droplet determines the composition of the end part and the length of the O-lacked part in the nanowire. On the contrary, quenching could keep the Zn and O species within the catalyst particle, because the species do not have enough time to precipitate at the droplet-nanowire interface. Ge and residual Zn and O species in the droplet would solidify fast to form tiny Ge and ZnO crystals. Hence, the volumes of Ge-Zn-O spheres after quenching are generally larger than that of Ge sphere experienced slow cooling. In the lower temperature zone (about 400-500 °C), some nanowires capped by polyhedral shaped particles were observed in the quenched products, as shown in Figure 6a. These nanowires were generally shorter than the ZnO nanowires capped by Ge spheres. Furthermore, the corresponding SAED patterns and EDS spectra in Figure 6b,c indicate that

In summary, using Ge sphere-capped ZnO nanowires as a model system, we studied the VLS growth process. We found that in this semiconductor catalyzed oxide nanowire growth process, the diameter of the ZnO nanowires is also affected by the diameter of the catalyst particles. There are Ge, Zn, and O in the catalyst particle during the VLS growth and the Ge/Zn atomic ratio deviates enormously from that given by the Ge-Zn binary phase diagram, indicating that the ternary phase diagram is needed to explain the nanowire growth. In the slow cooling process, which can effectively drive the elements separation and crystallization of Ge-Zn-O contained droplets, the ZnO nanowires grow continuously, and Ge spheres formed with most of their volume being a single crystal epitaxially connected with the ZnO nanowire. Larger polycrystal particles containing Ge, Zn, and O and having a rough surface were observed to connect with the ZnO nanowires in the quenched specimens. ZnO nanowires capped by Zn2GeO4 polyhedral shaped particles were observed in the specimens prepared at a lower temperature, and the growth process may be governed by the VSS mechanism. Supporting Information Available: Figure S1: The experimental setup for synthesizing Ge/ZnO nanomatchsticks. Figure S2: (a) a TEM image of a Ge/ZnO nanomatchstick with the O-lacked ZnO end part. (b-d) selective area electron diffraction (SAED) patterns recorded from the sphere, nanowire, and interface. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgment. This work was supported by NSF of China (60771005, 60728102) and Beijing NSF (4092023).

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