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Physica E 38 (2007) 27–30 Flower-like silicon nanostructures Zhihong Liu a , Jia Sha a,b , Qing Yang a , Zixue Su a,b , Hui Zhang a , Deren Yang a,Ã a State Key Laboratory of Silicon Materials, Zhejiang University, Zheda Road 38, Hangzhou 310027, PR China b Department of Physics, Zhejiang University, Zheda Road 38, Hangzhou 310027, PR China Available online 17 December 2006 Abstract In this paper we present a flower-like silicon nanostructure grown by combining the oxidation-assisted growth (OAG) mechanism and the vapor–liquid–solid (VLS) growth mechanism. It is found that the flower-like silicon nanostructures are nucleated initially via the VLS mechanism and then grown on silicon wafer via the OAG mechanism. Furthermore, light emission was observed, which is considered to be the enhanced photothermal effect. r 2007 Elsevier B.V. All rights reserved. PACS: 68.65.Àk; 78.20.Nv; 78.67.Àn Keywords: Silicon; Flower-like; Photothermal; Nanowires 1. Introduction During recent years, silicon nanostructures have attracted much attention due to their potential applications in interconnection and basic blocks for future nanoscale electronic and optoelectronics devices [1–3],andsensor applications [4–6]. There are two major synthesis methods: vapor–liquid–solid (VLS) and oxide-assisted growth (OAG). The VLS process was originally developed by Wagner [7] and co-workers, recently Lieber, Yang, and many other research groups used it to generate silicon nanowires and other n anowires [8–11]. The VLS process is a well-controlled method, while the OAG method [12–15] can produce large quantity SiNWs by the simple thermal evaporation of silicon monoxide powders. Furthermore, different morphol- ogies such as wires, rods, chains, coaxial cables, and ribbon structures can be produced in the OAG process. In this paper we present a flower-like silicon nanos- tructures grown by combining the thermal evaporation of silicon monoxide (SiO) powders with the VLS growth mechanism. Enhanced photothermal effect is observed. 2. Experiment The flower-like silicon nanostructure was synthesized by thermal evaporation SiO powders at 1100 1Cinan evacuated quartz tube. SiO powders (99.99%, Shanghai Chemical Co.) were put in the center of the furnace, and several n-type (1 1 1) silicon wafers with a resistivity of about 0.01 O cm were placed in the downstream. The silicon wafers were cleaned by the standard RCA process, then covered with a 10 nm thick Au film. The furnace was evacuated to 30 Pa by a mechanical pump, meanwhile the temperature of the furnace was raised up to 1100 1Cata heating rate of 20 1C/s. Then a mixed gas of Ar (80%) and H 2 (20%) at 10 4 Pa was kept flowing at a flow rate of 250 standard cubic centimeters per minute (sccm) through the tube. After 3 h of growth, the flower-like silicon nanos- tructure was formed on the silicon substrates of the downstream at the area of about 700 1C. For comparison, the silicon wafers without Au film deposition were also prepared, and the same experiments were carried out. The as-grown specimens were analyzed by a field emission scanning electronic microscope (FESEM, FEI, Sirion), a transmission electronic microscope (TEM, JEM-2010, JEO L), and a fiber optical spectrometer (Tensail TS100A). ARTICLE IN PRESS www.elsevier.com/locate/physe 1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2006.12.028 Ã Corresponding author. Tel.: +86 571 87951667; fax: +86 571 87952322. E-mail address: mseyang@zju.edu.cn (D. Yang). 3. Result and discussion Fig. 1 is the SEM image of an as-grown specimen. The flower-like nanostructures with the diameter of 500–1000 nm and the length of 3–5 mm on the silicon substrate can be observed. A typical flower is composed of an intertwined stem, a bulbous head consisting of a tight bundle of several nanowires, and a single catalyst particle attached at the top end of the flower-like nanostructures. The TEM image of a flower-like silicon nanostructure on a holey carbon grid is given in Fig. 2. The upper right inset of Fig. 2 is the selected area electron diffraction (SAED) ARTICLE IN PRESS Fig. 2. TEM image of a flower-like nanostructure. The upper right inset is the SAED pattern of the stem, which is marked with an arrow b. Fig. 1. SEM image of an as-grown specimen. The surface of the silicon substrate is covered with flower-like silicon nanostructures. The white arrow reveals a single catalyst particle attached at the top end of a flower-like nanostructure. Z. Liu et al. / Physica E 38 (2007) 27–3028 pattern of the stem, which is marked as b with an arrow. It indicates that the flower-like silicon nanostructures are crystalline in nature. Figs. 3a and b are the corresponding energy dispersive X-ray spectroscopy (EDX) data of the head (marked as a with an arrow in Fig. 2) and the stem (marked as b with an arrow in Fig. 2) of the flower-like silicon nanostructure. The EDX spectra show that the stem only contains Si and O elements, while the head contains 2.8 at% Au besides Si and O. On the basis of the results of SEM and TEM, it is considered that the growth of those flower-like silicon nanostructures is via both of the VLS process and the OAG process. In the initial stage, the Au film is dissolved as liquid drops on silicon substrate, and then those Au drops act as catalysts to enhance the nucleation of silicon nanostructures, which is described as the VLS process [8]. Later, silicon nanostructures are grown up on the nucleation sites via SiO vapor, which is the so-called OAG process [16]. The growth may take place at the periphery of the Au liquid drops. As a result, the Au ball is pushed away from the silicon substrate and lifts upward by the growing nanowires. In the silicon substrate without Au film, no flower-like structures were observed besides individual nanowires. Under the irradiation of a 980 nm laser of 100 mW, the as-grown specimen emits a visible light. Fig. 4 is the optical spectrum. It can be seen that the wavelength of the light is mainly from 520 to 860 nm. It is believed that the emitting of the light is due to the enhanced photothermal effect of silicon nanostructures which was report ed by N. Wang et al. [17]. The special structure of flower-like silicon nanostructures enhances the optical absorption, and raises the temperature of the nanostructures, so that the silicon nanostructures emit visible light. ARTICLE IN PRESS Element Weight% Spectrum 3 Atomic% O K 31.04 52.12 Si K 44.31 42.37 Ca K 4.03 2.70 Au M 20.62 2.81 Element Weight% Atomic% O K 34.31 47.83 Si K 52.17 0 Ca O Au Si a b Au Au Au Au Au Ca Ca Full Scale 368 cts Cursor: -0.102 keV (0 cts) 12345678 910 keV 0 O Si Full Scale 368 cts Cursor: -0.102 keV (0 cts) 12345678910 keV 65.69 Spectrum 2 Fig. 3. (a) is the corresponding EDX data of the head (marked as a with an arrow in Fig. 2) and (b) is the corresponding EDX data of the stem (marked as b with an arrow in Fig. 2). The upper right insets in the spectra are the element rate table, respectively. Z. Liu et al. / Physica E 38 (2007) 27–30 29 4. Conclusions We have demonstrated the synthesis of flower-like silicon nanostructures. The synthesis can be controlled at optimal experimental condition consistent with a VLS growth mechanism. The optical characterization of the as-pr epared flower-like silicon nanostructures has been carried out, and light emission into visible range was observed, which is believed to be due to the enhanced photothermal effect. Acknowledgments The authors would like to thank the Natural Science Foundation of China (Grant No. 60225010) for financial support. References [1] Y. Cui, X.F. Duan, J.T. Hu, C.M. Lieber, J. Phys. Chem. B 104 (2000) 5213. [2] G. Zheng, W. Lu, S. Jin, C.M. Lieber, Adv. Mater. 16 (2004) 1890. [3] Y. Cui, Z. Zhong, D. Wang, W.U. Wang, C.M. Lieber, Nano Lett. 3 (2003) 149. [4] F. Patolsky, C.M. Lieber, Mater. Today 8 (2005) 20. [5] J I. Hahm, C.M. Lieber, Nano Lett. 4 (2004) 51. [6] Y. Cui, Q. Wei, H. Park, C.M. Lieber, Science 293 (2001) 1289. [7] R.S. Wanger, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89. [8] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [9] Y. Wu, Y. Cui, L. Huynh, C.J. Barrelet, D.C. Bell, C.M. Lieber, Nano Lett. 4 (2004) 433. [10] A.I. Hochbaum, R. Fan, R. He, P. Yang, Nano Lett. 5 (2005) 457. [11] J. Niu, J. Sha, X. Ma, J. Xu, D. Yang, Chem. Phys. Lett. 367 (2003) 528. [12] J. Niu, J. Sha, D. Yang, Physica E 23 (2004) 131. [13] H.Y. Peng, Z.W. Pan, L. Xu, X.H. Fan, N. Wang, C.S. Lee, S.T. Lee, Adv. Mater. 13 (2001) 317. [14] Z.W. Pan, Z.R. Dai, L. Xu, S.T. Lee, Z.L. Wang, J. Phys. Chem. B 105 (2001) 2507. [15] W.S. Shi, H.Y. Peng, N. Wang, C.P. Li, L. Xu, C.S. Lee, R. Kalish, S.T. Lee, J. Am. Chem. Soc 123 (2001) 11095. [16] R.Q. Zhang, Y. Lifshitz, S.T. Lee, Adv. Mater. 15 (2003) 635. [17] N. Wang, B.D. Yao, Y.F. Chan, X.Y. Zhang, Nano Lett. 3 (2003) 475. ARTICLE IN PRESS 400 600 800 0 100 200 300 400 intensity (a.u.) λ nm Fig. 4. Optical spectrum of flower-like nanostructure excited by a 980 nm laser under a power of 100 mW. Z. Liu et al. / Physica E 38 (2007) 27–3030 . 27–30 Flower-like silicon nanostructures Zhihong Liu a , Jia Sha a,b , Qing Yang a , Zixue Su a,b , Hui Zhang a , Deren Yang a,Ã a State Key Laboratory of Silicon. 68.65.Àk; 78.20.Nv; 78.67.Àn Keywords: Silicon; Flower-like; Photothermal; Nanowires 1. Introduction During recent years, silicon nanostructures have attracted much