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Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học
Preparation and photoluminescence properties of amorphous silica nanowires X.C. Wu * , W.H. Song, K.Y. Wang, T. Hu, B. Zhao, Y.P. Sun, J.J. Du Laboratory of Internal Friction and Defects in Solids, Institute of Solid State Physics, Academia Sinica, P.O. Box 1129, Hefei 230031, People's Republic of China Received 10 October 2000; in ®nal form 4 January 2001 Abstract Bulk-quantity amorphous silica nanowires (SiONWs) have been synthesized by carbothermal reduction reaction between silicon dioxide and active carbons. Transmission electron microscopy (TEM) image shows the formation of the nanowires at a diameter of 60±110 nm and a length up to hundreds micrometers. Besides most smooth-surface polyp- shaped nanowires, two other forms of nanowires, named amoeba-shaped and frog-egg-shaped nanowires, have also been observed. The nanowires can emit stable and high brightness blue light at 435 nm (2.85 eV) under excitation at 260 nm (4.77 eV). The formation of the nanowires into dierent shapes may be explained by the vapor±liquid±solid (VLS) mechanism. Ó 2001 Published by Elsevier Science B.V. 1. Introduction One-dimensional quantum wires are of great scienti®c interest due to their great potential for testing and understanding fundamental concepts about the roles of dimensionality in mesoscopic physics and for applications in nanodevices [1,2]. For instance, nanotweezers made of carbon na- notubes can be used to manipulate submicron clusters and nanowires [3]. GaAs and InAs nano- wires have found applications in developing one- dimensional high-speed ®eld eect transistor, or laser working at low-threshold current density and high gain [4]. GaN nanowires may be fabricated into one-dimensional nanoscale luminescence di- odes [5]. With the development of mesoscopic science and the advances in integrated optical technology, it is important to synthesize nanowires with optical properties that can meet the demands of further applications. Silica as photolumines- cence (PL) materials has long been concerned for, and people have continually reported PL bands with peak energies around 1.9±4.3 eV for bulk SiO 2 or SiO 2 ®lms [6±12]. Recently, Yu et al. [13] have also reported fabrication and intensive blue light emission phenomenon of silica nanowires (SiONWs), and pointed out their potential ap- plications in high-resolution optical heads of scanning near-®eld optical microscope or nanoin- terconnections in future integrated optical devices. Therefore, SiO 2 nanowires are a promising one- dimensional luminescence materials, but they have only been synthesized by using excimer laser ab- lation. In this Letter, we report the carbothermal reduction synthesis and characterization and PL properties of the nanowires. Their growth process has also been discussed. 9 March 2001 Chemical Physics Letters 336 (2001) 53±56 www.elsevier.nl/locate/cplett * Corresponding author. Fax: +86-551-5591434. E-mail address: jjdu@mail.issp.ac.cn (X.C. Wu). 0009-2614/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 063-X 2. Experimental The mixtures of SiO 2 4g,FeNO 3 3 Á 9H 2 O (250 mg) and active carbons (4 g) were ball-milled for 20 h in ethanol media. After desiccated, they were pressed into several circular pellets (U1cm 0:5 cm) under 10 Mpa. The pellets were placed at the center of conventional horizontal furnace with a sintered alumina tube (U2:5cm 100 cm) and calcined at 1350°C for 3 h in ¯owing argon (40 ml/min). A white product was found to deposit on the surface of the pellets and the ther- malcouple. TEM images of the products were taken with a JEM-200CX transmission electron micro- scope. The composition of SiONWs was deter- mined by the X-ray photoemission spectra (XPS), which were recorded on a VGESCALAB MKII X- ray photoelectron spectrometer. XPS data were collected in the constant analyzer energy (CAE) mode at 20 eV. Mg Ka (hm 1253:6 eV) radiation was employed as excitation source with an anode voltage of 12 KV and an emission current of 20 mA. PL spectrum was measured in a Hitachi 850 ¯uorescence spectrophotometer with a Xe lamp at room temperature. The excitation wavelength was 260 nm, and the ®lter wavelength was 310 nm. 3. Results and discussion TEM micrography shows the general morphol- ogy and dimension of SiONWs. The nanowires shown in Fig. 1(a) look like polyp with trunks and branches, but their surface is smooth. The trunk has 110 nm in diameter and hundreds micrometers in length by scanning throughout the sample. The brunches have the diameter about 60 nm and a length up to 5 lm. Electron diraction micro- graphs (inset) show that the nanowires are amor- phous. Fig. 1(b) indicates the amoeba-shape of the nanowires, with the diameter of about 70 nm and a length up to tens of micrometers. Fig. 1(c) shows frog-egg morphology of the nanowires, which is similar to wire-like silica nanosphere agglomerates [14]. Circular nanoparticles are connected via necks to form pearl-like chains. The largest parti- cles have the diameter about 100 nm, while the smallest particles have the diameter about 60 nm. Due to low reaction temperature, no silicon car- bide nanowires were observed. The reactants were suciently ball-milled to accelerate chemical reaction. If the time grinding the reactants were reduced, we could still obtain the SiONWs. Further evidence for the formation of SiO 2 nanowires can obtained through XPS. The two strong peaks at 103.35 and 532.65 eV as shown in Fig. 1. TEM morphologies of SiONWs. (a) polyp-shaped SiONWs; (b) amoeba-shaped SiONWs; (c) frog-egg-shaped SiONWs. 54 X.C. Wu et al. / Chemical Physics Letters 336 (2001) 53±56 Fig. 2(b) and (c) correspond to the binding ener- gies of Si(2p) and O(1s) for SiO 2 , respectively. No obvious Si peaks (Si2p 98.7 eV in Si) are observed. The quanti®cation of the peaks reveals that atomic ratio of Si to O is 1:2.41. Obviously, the observa- tion of oxygen must be due to adsorption and surface contamination of the sample. The survey spectrum in Fig. 2(a) also displays C(1s) (at 284.65 eV) peak, which can be attributed to a small amount of the residual graphite (284.3 eV for C1s in graphite). As is shown in Fig. 3, a stable and strong blue light emission is revealed at 435 nm (2.85 eV) at room temperature under excitation at 260 nm while ultraviolet and blue light emission at 350 nm (3.54 eV), 420 nm (3.0 eV), and 465 nm (2.7 eV) can also be observed. Compared with [13], intensive peak at 420 nm in [13] changes into shoulder in Fig. 3 while shoulder at 435 nm into intensive peak. The shoulder at 465 nm in Fig. 3 approaches the peak at 470 nm in [13]. Ultraviolet light emission at 350 nm was observed in oxidized porous silicon and annealed SiO 2 [15]. The PL spectrum is also dierent from that of oxidized Si nanowires [16]. The growth process of the nanowires can be explained by the vapor±liquid±solid (VLS) mech- anism, since little droplets can be seen at the tops of the nanowires [17]. The growth of the nanowires Fig. 2. XPS of the sample. (a) survey spectrum of the sample; (b) Si2p binding energy spectrum; (c) O1s binding energy spectrum. Fig. 3. PL spectrum of the SiONWs at room temperature under excitation at 260 nm. X.C. Wu et al. / Chemical Physics Letters 336 (2001) 53±56 55 could be divided into three steps. The ®rst step is that silica is reduced by active carbon to silicon and silicon monoxide, and then silicon reacts with iron to form FeSi 2 [18]. The second step is that FeSi 2 is evaporated on the surface of the pellets and the thermalcouple to become liquid-phase catalytic growth center. The third step is that the vapor of silicon and silicon monoxide is trans- ported to the catalytic center to form SiO and Si nanowires by VLS mechanism while both silicon and silicon monoxide are all oxidized to amor- phous SiO 2 during cooling. In the above-growth model, the nucleation step can be further divided into monocentric and polycentric nucleation, and growth step into periodic stable growth and peri- odic unstable growth [19]. Thus the combination of dierent nucleation and growth processes can give rise to dierent forms of SiONWs, which is similar to the growth model of Si nanowires [20]. The formation process of polyp-shaped SiONWs is considered to be due to the coexistence of monocentric and polycentric nucleation and of the periodic stable growth on the basis of the trunks and the branches with even diameters. When trunks stably grow in monocentric nucleus, some FeSi 2 nanoparticles deposit on the surface of the trunks to become many new growth centers, namely, polycentre, resulting in the formation of branch-shaped nanowires. The branch-shaped nanowires can grow stably, but the stability is only relative. Amoeba-shaped nanowires are attributed to monocentric nucleation and periodic unstable growth to exhibit a typical periodic instability of diameter. The block dots of the polyp-shaped and amoeba-shaped SiONWs are the sites of nuclei. However, the periodicity is not strict and diers for various nanowires. 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