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Physica E 23 (2004) 1 – 4 www.elsevier.com/locate/physe Tiny SiO 2 nano-wires synthesized on Si (1 1 1) wafer Junjie Niu a , Jian Sha a; b , Niansheng Zhang b , Yujie Ji a , Xiangyang Ma a , Deren Yang a;∗ a State Key Lab of Silicon Materials, Department of Material Science and Engineering, Zhejiang University, Zheda Lu 38, Hangzhou 310027, People’s Republic of China b Department of Physics, Zhejiang University, Hangzhou 310027, People’s Republic of China Received 8 November 2003; accepted 27 November 2003 Abstract Tiny SiO 2 nano-wires (SiO 2 -NWs) were synthesized on a p-Si (1 1 1) wafer by the chemical-vapor-deposition method. The minimum diameter of the nano-wires was around 9 nm, and the length was longer than 10 m. The results of transmission electron microscopy shows that the amorphous nano-wires were composed of Si and O with an approximate atomic ratio of 1:2. Furthermore, the photoluminescence behavior of the SiO 2 -NWs has been also checked. ? 2004 Elsevier B.V. All rights reserved. PACS: 71.55.Cn; 81.05.Ys; 81.15.Gh Keywords: Tiny SiO 2 nano-wires; Synthesis; PL spectrum 1. Introduction Quasi-one dimensional nano-materials have stim- ulated much interest for their potential applications in nano-electronics, optics, at face display, etc. [1–9]. Due to the requirement of the quantum con- ÿnement eect, the diameter of the one-dimensional nano-materials should be very small (e.g. 1 nm). It has been reported that the minimum diameter of car- bon tubes could be 0:4nm [10], and that of silicon nano-wires (SiNWs) was 1 nm [11]. As an important potential photoluminescence and wave-guide mate- rial, silica nano-wires (SiO 2 -NWs) have attracted attention [12]. Several techniques have been used to fabricate SiO 2 -NWs, such as sol–gel, laser ablation, ∗ Corresponding author. Tel.: +86-571-8795-1667; fax: +86- 571-8795-2322. E-mail address: mseyang@dial.zju.edu.cn (D. Yang). catalyzed thermal decomposition, carbothermal re- duction, chemical-vapor-deposition (CVD) and so on [13–17]. By means of CVD method, it was reported that the diameter of the SiO 2 -NWs has reached to 50 nm [18]. In this paper, we report the synthesis of the large scale tiny (about 9 nm in diameter) and long (∼ m) SiO 2 -NWs by a CVD process. For the consideration of compatibility with integrated circuits, silicon sub- strates were used in our experiments. The nano-wires were checked by means of a scanning electron mi- croscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), and photoluminescence (PL) spectroscopy. 2. Experiment The heavily boron-doped p-type Si (1 1 1) wafers as substrates was ÿrst cleaned for 30 min in the acetone 1386-9477/$ - see front matter ? 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2003.11.274 2 J. Niu et al. / Physica E 23 (2004) 1 – 4 by ultrasound. The substrates were about 20 mm in width and 40 mm in length. Next, a magnetic sputter- ing method was used to deposit Ni as a catalyst on the substrates. Then the substrates were placed in a quartz tube furnace. The furnace chamber was pumped down to 10 Pa and heated. When the temperature reached 1000 ◦ C, a mixture gas of argon, hydrogen, and silane (ow ratio 100:20:15) was allowed into the chamber. The pressure and temperature in the chamber were kept at 2000 Pa and 1000 ◦ C during the deposition. After that, the substrates were removed from the fur- nace for the SEM (JSM-T20, JEOL) and PL (F-4500, Hitachi) measurement, respectively. The PL spectra of the deposited matters on the substrates were mea- sured at room temperature in the spectral range of 200–900 nm using a general Xe-light source with a wavelength 206 nm as the excitation source. Further- more, the deposited matters on the substrates were dis- solved in an ethanol solution, and then the dropwise was placed on a copper grid covered with a very thin carbon ÿlm, so that the deposited materials could be analyzed with a TEM (Phillip CM200) equipped with an EDX. 3. Results and discussion Top view of the large-scale entangled tiny nano-wires synthesized on the silicon substrates is shown in Fig. 1. The as-grown nano-wires have length Fig. 1. Top view SEM image of the densely tiny nano-wires that grew on a silicon substrate. Fig. 2. TEM image of the as-grown SiO 2 -NWs. Most of the smooth SiO 2 -NWs have uniform diameter of about 9 nm, while the others have a diameter of 20 nm. The corresponding EDX data (upper right inset) of the SiO 2 -NWs shows that they are composed of Si and O with an approximate atomic ratio of 1:2. C and Cu peaks originated from Cu grid for TEM analysis. up to tens of micrometers, and most of them have di- ameter around 9 nm and a few of them have diam- eter around 20 nm, as shown in Fig. 2. The smooth and homogeneity structure of the SiO 2 -NWs was ob- served. The EDX spectrum (Fig. 2, upper right) shows that the nano-wires were composed of Si and O with an approximate atomic ratio of 1:2. C and Cu peaks in the spectrum originated from the Cu grid used for TEM analysis. The high-magniÿcation TEM image of a SiO 2 -NW is given in Fig. 3. A clear Ni particle can be seen as the catalyst attached to the tip of the SiO 2 -NW. The SAED pattern (inset of Fig. 3) shows no diraction spots, indicating the amorphous nature of the SiO 2 -NW, which was of the same nature as that in the previous work [18]. The PL measurements at an excitation wave- length of 206 nm were carried out with a general Xe-light source. Fig. 4 shows the PL spectrum of the SiO 2 -NWs measured at room temperature. It reveals a normal emission band at around 544 nm, which is believed to be due to neutral oxygen vacancies of SiO 2 -NWs [18,19]. Furthermore, a new emission band at 595 nm with very weak density could be also observed. At present, the exact mechanism of the PL of the SiO 2 -NWs is not clear. The systemic experiments will be needed. According to the vapor–liquid–solid (VLS) mecha- nism, the catalyst as islands can induce the deposition J. Niu et al. / Physica E 23 (2004) 1 – 4 3 Fig. 3. TEM image of a single SiO 2 -NWs. A Ni–Si droplet attached to the tip of the SiO 2 -NW (the white arrow, lower inset). The SAED pattern in the upper right indicates the amorphous nature of the SiO 2 -NW. Fig. 4. PL spectrum of SiO 2 -NWs measured at room temperature. atoms to form droplets so that nano-wires can grow [20]. The diameter of the nanowires is dependent on the size of the catalyst. In most of the cases these two sizes are very close. In our experiments, the size of the catalyst was uniform and very small (¡ 10 nm in di- Fig. 5. The sketch graphs of the SiO 2 -NWs growth. ameter as shown in Fig. 3). Those small catalyst par- ticles could easily form nuclei so that the nano-wires with smaller diameter could be grown on them. In the beginning of the nano-wire growth, the round Ni particle and the Si atoms deposited from silane form a mixed Si–Ni eutectic droplet (Fig. 5a); and then with more and more Si atoms melting, the droplets gradually reach supersaturation. The Si atoms in the droplet will segregate when more Si atoms joined. The segregation has an equal probability in the 360 ◦ area around the droplets that place on the smooth silicon substrates. Therefore, the segregated Si atoms would grow around the droplet with the crystal directions Si (1 1 1), Si (2 2 0), Si (3 1 1), etc. (Fig. 5b). According to the lowest energy theory, the Si (1 1 1) direction will dominate the ÿnal growth, for it is the lowest en- ergy. Because the consumed Si atoms, due to the earli- est growth of Si (1 1 1) direction, gradually to reach a homeostasis with the deposited Si atoms, the growth of other new Si (1 1 1) directions will not appear around the droplet (Fig. 5c). Thus with the prolonged grow- ing time, the Si atoms along with (1 1 1) direction con- tinually grow to SiNWs (Fig. 5d). Here the droplet moved forward slowly accompanying the growth of the SiNWs. One thing must be mentioned: the newly formed SiNWs will be oxidized rapidly to amorphous SiO 2 by the remaining oxygen due to the low vacuum degree and the impure reaction gases in the quartz tube. Since the temperature (∼ 1000 ◦ C) is much lower than the crystalline temperature of the SiO 2 -NWs, this 4 J. Niu et al. / Physica E 23 (2004) 1 – 4 induces the original nano-silicon structure to an amor- phous SiO 2 -NWs and round-belt SiO 2 structure (see the TEM image in Fig. 3). In the primary phase of the SiNWs, the droplets are pushed forward slowly with the continued growth. If only one droplet exists on the silicon wafer, the droplet will be pushed to one direction straightly. Therefore, the SiNWs will be uniform and straight growing under this condition (Fig. 5d). In fact, hundreds and thou- sands of droplets on the substrates collide unavoidably during the co-instantaneous growth. When a moving droplet encounters another moving droplet, the two droplets will commix to a bigger droplet (Fig. 5e, be- cause the outside silicon ring is very thin, we ignore its very weak eect). When Si atoms drop in, the new droplet will reach supersaturation again to segregate and will keep the former SiNWs to grow continually. But the growth velocity (Fig. 5f) has been slower in comparison with the original SiNWs (Fig. 5c). A cer- tain angle (in Fig. 5f) between the two dierent di- rectional SiNWs induces the SiNWs to grow curly (see Fig. 5g). The SiNWs formed under this condition are commonly curving (see the circular regions in Fig. 2). Obviously, the number of commixed SiNWs droplets is very small. Even the encountering of three or more than three droplets is much less (Fig. 5h). The major- ity is that one droplet forms a SiNW and its diameter is relatively straight, as shown Fig. 2. A deeper under- standing of the growth mechanism of the SiO 2 -NWs might contribute to the successful synthesis and de- vice application of one-dimensional quantum wires. 4. Conclusions Tiny SiO 2 nano-wires with a minimum diameter of about 9 nm and a length of more than 10 m were synthesized on a p-Si (1 1 1) wafer. 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