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Temperature dependence of morphology and diameter of silicon nanowires synthesized by laser ablation Y.Q. Chen, K. Zhang, B. Miao, B. Wang, J.G. Hou * Structure Research Laboratory, Center for Physical Sciences, University of Science and Technology of China, Hefei 230026, China Received 15 March 2002 Abstract The silicon nanowires (SiNWs) with different diameters and morphologies were synthesized by laser ablation of a target containing metals over a temperature range 910–1120 °C. The octopus-shaped wires of larger diameters were formed in lower temperature zone (910–960 °C), while SiNWs and silicon nanoparticle chains of smaller diameters in higher temperature zone (960–1120 °C). The distribution of the morphology and diameter of SiNWs as a function of growth temperature differs from that reported by thermal evaporation of SiO powders. The study shows that the morphology and diameter of SiNWs synthesized by laser ablation not only correlate closely with the growth temperature of SiNWs, but also with the nature of a catalyst. By change of nucleation temperature and critical nucleus size of nucleus droplets in vapor–liquid–solid (VLS) growth process, a catalyst can change relationships between the morphology, diameter, and growth temperature of SiNWs. Ó 2002 Elsevier Science B.V. All rights reserved. 1. Introduction One-dimensional nanostructured materials, such as nanotubes [1,2], nanowires [3–8], and nanobelts [9], are a burgeoning and intriguing re- search area both for their fundamental scientific issues in meso-physics phenomena and for poten- tial nano-device application. Silicon is one of the most important electronic materials and holds considerable technological promise for device ap- plications. Therefore much attention has been paid recently to the investigation of SiNWs. Until now, a variety of techniques on synthesizing SiNWs have been developed, including lithography and etching techniques [10,11], scanning tunneling mi- croscopy [12], thermal evaporization [13], and laser ablation [3]. However, controlling the morphology and size of as-grown SiNWs is still a challenging issue. The recent studies [14,15] on SiNWs syn- thesized by thermal evaporation of SiO powders showed that the temperature of substrate for col- lecting SiNWs played a dominant role in control- ling the diameter of SiNWs and the formation of various kinds of silicon nanowire-related mor- phologies. It was demonstrated that the diameters of SiNWs decreased with the descending growth temperature and the morphology of SiNWs was different in different deposition temperature zone. 7 June 2002 Chemical Physics Letters 358 (2002) 396–400 www.elsevier.com/locate/cplett * Corresponding author. Fax: +86-551-360-2803. E-mail address: jghou@ustc.edu.cn (J.G. Hou). 0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 14 (02 )00671-1 Laser ablation synthesis of nanowires is quite popular, by which long, uniform-sized, and single- crystal SiNWs can be readily fabricated in bulk quantities [16]. To our knowledge, there is no re- port to date on the temperature dependence of the morphology of SiNWs synthesized by laser abla- tion. In this Letter, we present the results on this project. Our results show that the morphology and diameter of SiNWs synthesized by laser ablation not only correlate closely with the growth tem- perature of SiNWs, but also with the nature of a catalyst. 2. Experimental The experimental apparatus used for the pre- sent work is similar to the one described previously [3,17]. An alumina tube was mounted inside a horizontal tube furnace. A target was made by compressing Si powders (purity 99.99%) with 5 mol% Zr (purity P 92%; impurities, Mg, Fe, Ge, Ca, Cl). The target was placed at the center inside the furnace. A strip-like Si substrate (68 mm in length and 20 mm in width) was placed at the outlet end, near a cooling copper finger for col- lecting the deposited products. There existed a temperature gradient from center to the gas outlet end of the furnace. A PtRh–Pt thermocouple was used to measure the temperature distribution in the alumina tube. After the tube had been evacu- ated to 0.01 Torr by a mechanical vacuum pump, 5% H 2 –Ar gas mixture, as a carrier gas, was in- troduced and kept flowing at a flow rate of 50 sccm. The pressure in the tube was controlled at 300 Torr. Then the furnace was heated to 1200 °C at the central region. After the temperature and pressure in the tube had been stabilized, pulsed laser beam from an Nd:YAG laser (wavelength 532 nm, pulse width 7–8 nm, frequency 10 Hz, average power 1.7 W) ablated the target for 1 h. When ablation was over, the fluffy as-deposited products with different colors and appearances were found on the surface of the Si substrate. The morphologies and electron diffraction patterns of the as-deposited products were investigated by transmission electron microscopy (TEM). The chemical composition was analyzed by an energy dispersive X-ray spectrometry (EDS) attached to JEOL-2010 high-resolution TEM. 3. Results and discussion Fig. 1 shows the typical bright-field TEM im- ages of the products, which are, respectively, cor- responding to the different growth temperature zones, A, B1, B2, and C, on the Si substrate (shown in Fig. 2). The appearances and colors of as-deposited products on the Si substrate are dis- tinctly different, just by observing with the naked eyes. A thin dark yellow gauze-like product was formed in zone A, a thick dark yellow fluffy product in Zone B, and a thin light yellow powder- like product in zone C. As can be seen in Fig. 1, both morphology and diameter of as-grown SiNWs are different in different growth tempera- ture zones. In zone A (1080–1120 °C), the nano- wires had a uniform diameter of about 35 nm. The presence of nanoparticles at the tips of the nano- wires implies that growth mechanism of the nanowires in this zone is vapor–liquid–solid (VLS) growth [18]. Energy dispersive spectroscopy (EDS) analysis indicated that the nanoparticles at the tips of the nanowires only contained silicon and oxy- gen. Within the detection limit of EDS measure- ments (% 0:5%), no evidence of existence of Zr or any other elements was detectable on the tip. Even though the metal silicide was not found, it is be- lieved that the function of these Si tips of the nanowires is analogous to that of the metal silicide catalyst in VLS growth process [15,16]. Namely, melt Si nanoparticles may act as a nucleus for the nanowires. In zone B (960–1080 °C), a large quality of what is called silicon nanoparticle chains were formed (Figs. 1b,c). Clearly, the diameter of the silicon nanoparticle chain decreases with re- ducing the growth temperature. Lee et al. [16] have intensively studied the growth mechanism of the silicon nanoparticle chains. They proposed that nucleation and growth occurring alternatively re- sulted in the formation of chains of silicon nano- particles. The formation of the kinks of silicon nanoparticle chains resulted from a change of growth direction of the SiNWs. We examined the kinks by using energy dispersive spectroscopy and Y.Q. Chen et al. / Chemical Physics Letters 358 (2002) 396–400 397 confirmed that the kinks only contained silicon and oxygen. It should be noted that, from zone A to zone B, the temperature dependence of the morphol- ogy and diameter of the SiNWs is similar to that by thermal evaporation, reported recently by Peng et al. [14]. It can be seen that the diameter of SiNWs decreases remarkably with a decrease in growth temperature. They proposed that the variation of the diameter resulted from the Fig. 2. Schematic diagram of the different growth temperature zones on the silicon substrate. Fig. 1. TEM images of the typical morphologies of SiNWs grown in: (a) zone A (1120–1080 °C); (b) zone B 1 (1080–1020 °C); (c) zone B 2 (1020–960 °C); and (d) zone C (960–910 °C). Inset is electron diffraction patterns of the SiNWs. 398 Y.Q. Chen et al. / Chemical Physics Letters 358 (2002) 396–400 variation of the diameter of the droplet nucleated at different temperature. It is recognized that the melting point of nanoparticles decreases with the reduction of their size. When metal silicide nanoparticles of different sizes in the carrier gas were present above the Si substrate, the larger nanoparticles with higher melting points con- densed on the higher temperature position of the substrate, and the smaller nanoparticles with lower melting points on the lower temperature position of the substrate. According to the above discussion, nanostruc- tured morphology with smaller size should have been anticipated in zone C (910–960 °C). However, in addition to some aggregative fine particles, we observed that a large quantity of silicon wires of rather larger diameters (100–150 nm), called oc- topus-shaped silicon nanowires, were formed in zone C, a relatively lower temperature zone (Fig. 1d). It is interesting that the octopus-shaped sili- con nanowires synthesized by thermal evaporation of SiO powders were found at higher growth temperature of more than 1230 °C [15]. Focusing on the octopus-shaped structure, it can be seen that two or more branches share the same tip, which suggests that the tip might act as the nu- cleation site for two or more branches when the diameter of tip was large enough. Moreover, a bifurcation phenomenon of the silicon nanowires was also observed, which may be attributed to renucleation of the crystal silicon in growth pro- cess. On the other hand, the diameter of each branch of the octopus-shaped nanowires decreases gradually as the distance from the tip increases. TEM investigations revealed that there existed two kinds of structures in the entire length of the branch. One was a crystal silicon core sheathed with a thick amorphous outer layer of silicon ox- ide, which originated from the tip and terminated at reaching a certain length. The other, the rest of the branch, was a complete amorphous silicon oxide. Fig. 3 is the HRTEM image of the interface (arrow in Fig. 1d) of two kinds of structures along the axis of the branch. It can be seen that the growth direction of the crystal silicon was h 111i, which is consistent with the fact that the growth direction of SiNWs synthesized by metal catalyzed VLS growth is predominantly h111i [3,14]. The peculiar feature of the branch suggests that there may exist a competitive growth between crystal silicon core and outer layer of silicon oxide. When the forming rate of outer layer of silicon oxide exceeded the growth rate of crystal silicon, outer layer of silicon oxide will surround the crystal sil- icon. As a result, the growth of crystal silicon ceased and silicon oxide of outer layer coalesced together and extended. In order to investigate the reason why the SiNWs of such larger diameters were deposited in zone C, we analyzed the chemical composition of the tip of the octopus-shaped wires in the zone C by EDS. EDS analysis showed that the tip con- tained Si, Mg, Ge, and O (oxygen came from the outer layer of the tip), as shown in Fig. 4. From Fig. 3. HRTEM image of the interface between the crystal silicon and amorphous silicon oxides,along the axis of the branch of the octopus-shaped wires.The arrow shown in Fig. 1d indicates the interface. Fig. 4. EDS spectrum taken from the tip of the octopus-shaped SiNWs. Y.Q. Chen et al. / Chemical Physics Letters 358 (2002) 396–400 399 Mg–Si, Si–Ge, and Mg–Ge binary phase diagrams [19,20], it is evident that addition of Mg and Ge into Si can reduce the melting point of the silicon solid solution. Moreover, the melting points of nanoparticles are usually lower than the corre- sponding bulk material. All of these factors col- lectively could effectively reduce the melting points of the tips of larger diameter in zone C. This then explains why these octopus-shaped wires could be deposited in zone C, a relatively lower temperature zone. Furthermore, it is known that the diameters of the nanowires formed in VLS process have re- lation to the critical diameter dc of the liquid droplets nucleated in VLS process. Droplets larger than d c will become stable nuclei, whereas droplets smaller than d c will disappear gradually. The crit- ical nucleus size can be expressed as r à ¼ ðÀ2cÞ=DF v ; where c is the specific interfacial free energy of the condensate–vapor interface and DF v is the bulk free energy change per unit volume [21,22]. We assume that the addition of Mg and Ge into Si nanoparticles would increase interfacial free energy c. Therefore r à becomes larger, which means that the droplets with larger diameters (> r à ) can grow to form nanowires. This may be the reason why the diameters of nanowires in zone C are larger than those in zone A or zone B. 4. Conclusions In summary, the diameter and morphology of SiNWs synthesized by laser ablation not only correlate closely with growth temperature, but also with the nature of a catalyst. The nature of a catalyst has a direct influence upon the nucleation temperature and critical nucleus size of the drop- lets in VLS growth process. The addition of Mg and Ge into Si tips gave rise to the deposition of octopus-shaped SiNWs of larger diameters in the lower temperature zone. Furthermore, it is sug- gested in our research that there is a corresponding correlation between morphology and diameter of nanowires. Nanowires of larger diameter (100–150 nm) were inclined to be octopus-shaped, while nanowires of smaller diameter (10–15 nm) were inclined to be nanoparticle-chain-shaped. Acknowledgements This work was supported by the NSF of China (59972036, 10074059, and 19904012) and the ICQS of Chinese Academy of Sciences. References [1] S. Iijima, Nature 354 (1991) 917. [2] N.G. Chopra, R.J. 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Zhang,. between morphology and diameter of nanowires. Nanowires of larger diameter (100–150 nm) were inclined to be octopus-shaped, while nanowires of smaller diameter