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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 of high density SiOx nanowires with Fe 3 O 4 nanoparticles catalyst X.J. Wang a, ⁎ , B. Dong b , Z. Zhou a a School of Electronics Engineering and Computer Science, Peking University, Beijing 100871, China b School of Science, Dalian Nationalities University, Dalian 116600, China abstractarticle info Article history: Received 6 January 2009 Accepted 29 January 2009 Available online 12 February 2009 PACS: 81.05Y 78.55 Keywords: SiOx nanowires Photoluminescence Large scale, high density SiOx nanowires have been synthesized using a novel Fe 3 O 4 nanoparticles catalyst. The lengths of SiOx nanowires are in the range of several tens to hundreds of micrometers, and the diameters of nanowires are 20–80 nm. Transmission electron microscopy and high-resolution transmission electron microscopy show that the SiOx nanowires are amorphous, and energy dispersive X-ray spectrometry analysis reveals that SiOx nanowires consist of Si and O elements in an atomic ratio of approximately x=1.4–1.7. The vapor–liquid–solid (VLS) mechanism is the main formation mechanism of SiOx nanowires. The SiOx nanowires have two broad photoluminescence peaks at about 405 nm and 465 nm when the 250 nm ultraviolet fluorescent light excitation is applied at room temperature. The SiOx nanowires with good photoluminescence properties are promising candidates for ultraviolet–blue optical emitting devices. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Since the discovery of carbon nanotubes by Iijima [1], one- dimensional nanomaterials have stimulated great interest because of their potential fundamental and practical applications in many areas such as material research, chemistry, physics and engineering, etc. Among the one-dimensional nanomaterials, silicon oxide nano- wires are of great significance in the fields of photoluminescence (PL), localization of light, near-field optical microscopy, low-dimensional waveguide, and nano-inter-connection integrated optical devices [2,3]. Several methods, including chemical vapor deposition [4], carbon-assisted methods [5], laser ablation [2], and thermal evapora- tion [6] have been developed to prepare silicon oxide nanowires. Since the majority of silicon oxide nanowires fabrication methods are catalyst-based methods, different kinds of catalysts have been used, such as Au, Pd–Au, Fe, Ga, Ga–In, Ni, In–Ni, Sn, and Co [2,3,7]. Magnetite (Fe 3 O 4 ) is a common magnetic iron oxide that has a cubic inverse spinel structure with oxygen forming a fcc closed packing and iron cations occupying interstitial tetrahedral sites and octahedral sites. In the past decades, Fe 3 O 4 is becoming more and more attractive because of its intrinsic half-metallic ferromagnetic nature, which can be widely used in catalysts, biological assays, chemical sensors, and superparamagnets [8,9]. Recently, efforts have been devoted to preparation of Fe 3 O 4 nanoparticles, nanowires and nanotubes. In 2002, Sun and Zeng [10] prepared the size-controlled monodisperse Fe 3 O 4 nanoparticles with 4–16 nm by high-temperature solution phase reaction of iron (III) acetylacetonate (Fe(acac) 3 ) in the presence of alcohol, oleic acid, and oleylamine. The size-controlled mono- disperse Fe 3 O 4 nanoparticles can be used for a wide range of disciplines, including magnetic fluids, catalysis, biotechnology/bio- medicine, magnetic resonance imaging, data storage, and environ- mental remediation. In the paper, we for the first time prepared the large scale, high density SiOx nanowires using the novel Fe 3 O 4 nanoparticles as ca talyst. Ph otolumines cence of SiOx nanowires fabricated by the method was measured, in order to explore the possibility of application in light emitting devices. 2. Experimental Fe 3 O 4 nanoparticles with diameter less than 10 nm were synthesized by high temperature solution phase reaction of iron (III) acetylacetonate with 1,2-dodecandiol in the presence of oleic acid and oleylamine. The growth of SiOx nanowires was conducted in a horizontal tube furnace with a quartz tube. The mixture of Fe 3 O 4 liquid-drops and silicon powders was deposited on Si (111) wafer. The SiOx nanowires grew by a two-step raising temperature method. In the first procedure, when the temperature of reaction region reached 400 °C, the sample was transferred to reaction region rapidly and kept there for 30 min to eliminate the remained oleic acid and oleylamine. Then, the sample was returned to out of reaction region. Ar gas flow rate kept up 300 sccm through this p rocedure. In the second procedure, when the temperature of reaction region was increased to 1000–1200 °C, the sample was transferred to reaction region Materials Letters 63 (2009) 1149–115 2 ⁎ Corresponding author. Tel.: +86 1062882397. E-mail address: xjwangdlut@hotmail.com (X.J. Wang). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.01.084 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet rapidly again. The growth of SiOx nanowires started. The reaction lasted for 2 h in this procedure under a constant 50 sccm flow rate of Ar gas. After the furnace was cooled to room temperature, the white product was found on the surface of Si(111) substrate. Field-emission scanning electron microscope (FESEM) (XL-SFEG, FEI Corp.) with energy-dispersive X-ray spectroscopy (EDS) was used for morpholo- gical observation and element composition analysis of SiOx nano- wires. X-ray diffraction (XRD) (D/Max-2400, Rigaku Corp.) with CuKα radiation was assumed for phase structure of Fe 3 O 4 nanopar- ticles. Transmission electron microscopy (TEM) (Tecnai-20, PHILIPS Corp.) and high-resolution transmission electron microscopy (HRTEM) (Tecnai F20, FEI Corp.) with electron energy loss spectro- meter (EELS) were employed to perform the microanalysis of SiOx nanowires. Photoluminescence measurement was performed at a fluorescence spectrometer (F4500, HITACHI corp.) with a resolution of 1.0 nm. 3. Results and discussion Fig. 1(a) shows TEM image of Fe 3 O 4 nanoparticles. It can be seen that the monodisperse Fe 3 O 4 nanoparticles with diameter about 8 nm were observed from the TEM image. X-ray diffraction is used to obtain structure information of Fe 3 O 4 nanoparticles, as shown in Fig. 1(b). It indicated the highly crystalline cubic spinel structure was detected from the XRD pattern. The reflection peak positions and relative Fig. 1. (a) TEM image of Fe 3 O 4 nanoparticles, (b) XRD pattern of Fe 3 O 4 nanoparticles. Fig. 2. SEM images and EDS of SiOx nanowires deposited on the Si(111) substrate at the reaction temperature of 1000 °C for 2 h. (a) and (b), The lower magnification SEM images of the nanowires, (c) The higher magnification SEM image of nanowire tops, (d) EDS of the nanowires. 115 0 X.J. Wang et al. / Materials Letters 63 (2009) 1149–115 2 intensities of Fe 3 O 4 nanoparticles agree well with standard Fe 3 O 4 XRD card (JCPDS No. 85-1436). Fig. 2(a)–(c) shows the SEM images of SiOx nanowires deposited on the Si(111) substrate at the reaction temperature of 1000 °C for 2 h. Large scale, high density nanowires are uniformly covered on the surface of Si(111) substrate, as illustrated in Fig. 2(a) and (b). The lengths of these nanowires are in the range of several tens to hundreds of micrometers. Fig. 2(c) shows the higher magnification SEM image of nanowire tops. It can be clearly seen that, at every one top of these nanowires, there is a droplet whose diameter is much larger than that of related nanowire. Similar products were also observed at reaction temperatures of 1100 and 1200 °C. The composition analysis of the products was achieved using EDS. Fig. 2(d) shows EDS spectrum of the nanowires, indicating that the products only consist of silicon and oxygen elements. Quantitative analysis shows that the atomic ratio of Si:O is 1:(1.4–1.7), suggesting that the SiO 1.4–1.7 nanowires have been synthesized by the method. The EDS measurement achieved on the nanowire top displays the existence of a small amount of elemental iron. Further sample characterization was carried out using transmis- sion electron microscopy and high-resolution transmission electron microscopy. Fig. 3(a) shows the low magnification TEM images of SiOx nanowires. The diameters of SiOx nanowires are in the range of 20– 80 nm. The global droplets on nanowire tops were not observed because the gl obal droplets have no strong connection with nanowires, and separated from related nanowires when these nanowires were ultrasonically dispersed in ethanol. Fig. 3(b) shows the HRTEM images of the nanowire. No crystal structure was observed. Electron diffraction was done to determine its structure, as shown in the inset of Fig. 3(b), suggested that the nanowire is amorphous. Chemical composition of the nanowire was analyzed by electron energy loss spectroscopy. Si and O peaks were observed, and no iron, carbon and other impurities could be detected, revealing that the nanowire is pure SiOx nanowire, which is consistent with result of EDS spectrum. The vapor–liquid–solid (VLS), solid–liquid–solid (SLS), and oxide- assisted (OA) growth mechanisms have been used to explain the growth of silicon nanowires and silicon oxide nanowires. In our experiments, Fe 3 O 4 nan oparticles are employed as catalysts to fabricate SiOx nanowires. We have also tried to synthesize SiOx nanowires without Fe 3 O 4 catalysts, but no one-dimensional SiOx nanomaterials were found on t he surface of Si(111) substrate. Moreover, EDS also detected the existence of iron only at the top of SiOx nanowires. The fact suggested that the Fe 3 O 4 catalysts play a key role in the formation of SiOx nanowires, and a VLS mechanism is the most probable growth mechanism. Here, we give a description for the possible formation of SiOx nanowires. Firstly, with the temperature increased from 400 °C to 1000 °C, Si vapor is generated at high temperature by the vaporization of silicon powder. Meanwhile, Fe–Si– O nanoclusters form on the surface of Si(111) substrate, which act as nuclei for the formation of SiOx nanowires. As the droplets become supersaturated, amorphous silicon nanomires are formed by the reaction between Si and O. Oxygen source may come from two factors, one is the oxygen in Fe 3 O 4 nanoparticles, the other is the remainder oxygen in the reaction chamber. The presence of a small amount of O is not expected to change the Fe–Si phase diagram significantly, but in the meanwhile it acts as the oxygen source during the silicon oxide growth. Fig. 4 shows the room-temperature PL spectrum of SiOx nanowires under the 250 nm ultraviolet fluorescent light excitation. Two broad PL emission peaks are clearly observed at the center wavelengths of about 405 nm (3.06 eV) and 465 nm (2.67 eV). The PL properties of various silica nanowires have been studied extensively [2]. It has been suggested that the 2.7 eV band is attributed to the neutral oxygen vacancy (≡Si–Si≡), while the 3.0 eV band corresponds to some intrinsic diamagnetic defect center, such as twofold coordinated silicon lone-pair centers (O–Si–O). Therefore, we believe that the PL emissions from the SiOx nanowires should be attributed to the above- mentioned defect centers, which arise from the oxygen deficiency. The ultraviolet–blue emission properties of SiOx nanowires are of significant interest for their potential ultraviolet–blue emitting device applications. 4. Conclusions Large scale, high density SiOx nanowires have been synthesized by using a novel 8 nm Fe 3 O 4 nanoparticles catalyst. The SiOx nanowires with leng ths of several tens to hundreds of micrometers and diameters of 20–80 nm grew by a two-step raising temperature process based on the VLS mechanism. TEM and HRTEM show that the Fig. 3. TEM and HRTEM images of SiOx nanowires. Fig. 4. The room-temperature photoluminescence spectrum of SiOx nanowires under the 250 nm ultraviolet fluorescent light excitation. 1151X.J. Wang et al. / Materials Letters 63 (2009) 1149–115 2 SiOx nanowires are amorphous, and EDS analysis reveals that SiOx nanowires consist of Si and O elements in an atomic ratio of approximately x=1.4–1.7. Two broad ultraviolet–blue PL peaks at 405 nm and 465 nm were observed at room temperature. References [1] Iijima S. Nature 1991;354:56. [2] Yu DP, Hang QL, Ding Y, Zhang HZ, Bai ZG, Wang JJ, et al. Appl Phys Lett 1998;73:3076. [3] Wang YW, Liang CH, Meng GW, Peng XS, Zhang LD. J Mater Chem 2002;12:651. [4] Pan ZW, Dai ZR, Ma C, Wang ZL. J Am Chem Soc 2002;124:1817. [5] Wenger KS, Cornu D, Chassagneux F, Epicier T, Miele PJ. Mater Chem 2003;13:3058. [6] Chen YJ, Li JB, Han YS, Wei QM, Dai JH. Appl Phys A 2002;74:433. [7] Li SH, Zhu XF, Zhao YP. J Phys Chem B 200 4;108:17032. [8] Keane MA. J Catal 1997;166:347. [9] Decuyper M, Joniau M. Prog Colloid Polym Sci 1990;82:353. [10] Sun SH, Zeng H. J Am Chem Soc 2002;124:8204. 115 2 X.J. Wang et al. / Materials Letters 63 (2009) 1149–115 2 . Preparation and photoluminescence of high density SiOx nanowires with Fe 3 O 4 nanoparticles catalyst X.J. Wang a, ⁎ , B. Dong b , Z. Zhou a a School of. 2009 PACS: 81.05Y 78.55 Keywords: SiOx nanowires Photoluminescence Large scale, high density SiOx nanowires have been synthesized using a novel Fe 3 O 4 nanoparticles catalyst. The