Large scale synthesis, characterization and photoluminescence properties of amorphous silica nanowires by thermal evaporation of silicon monoxide

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

Large-scale synthesis, characterization and photoluminescence properties of amorphous silica nanowires by thermal evaporation of silicon monoxide Sanjay K. Srivastava a, Ã , P.K. Singh a , V.N. Singh b , K.N. Sood a , D. Haranath a , Vikram Kumar a a National Physical Laboratory, Dr. K. S. Krishnan Marg, Pusa, New Delhi 110012, India b Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India article info Article history: Received 29 March 2009 Received in revised form 27 April 2009 Accepted 27 April 2009 PACS: 61.46.–w 81.07.–b Keywords: Silicon monoxide Silicon oxide nanowires Thermal evaporation Photoluminescence abstract A single step non-catalytic process based on thermal evaporation of silicon monoxide has been established for large-scale synthesis of silica nanowires. Scanning electron microscopy, high-resolution transmission electron microscopy equipped with energy dispersive X-ray spectrometry (EDAX), X-ray diffractometry were used to characterize the morphology and structure of the material. The as- synthesized nanowires had amorphous structures with diameters in the range 30–100 nm and hundreds of micrometers in length. The EDAX analysis revealed that the nanowires consisted of mainly two elements Si and O in an atomic ratio of approximately 1:2 corresponding to silicon dioxide. Photoluminescence spectra of the silica nanowires showed strong blue emission around 393 nm. Nucleation and growth of silica nanowires has been discussed on the basis of tiny oxide cluster formation that act s as nucleation centers for the nanowires growth. & 2009 Elsevier B.V. All rights reserved. 1. Introduction During past two decades, a lot of attention has been paid to the growth and characterization of one-dimensional (1-D) nanos- tructures such as nanotubes, nanowires, nanobelts because of their distinctive structure, unique properties and applications [1–3]. Silicon-based nanostructures have attracted significant attention due to their potential applications in electronics and opto-electronic devices [4]. For example, silicon oxide (SiO x ) nanowires show intensive blue light emission, which may be a candidate material for high-resolution optical heads of scanning near-field optical microscopes, nanointerconnection integrated optical devices, low-dimensional wave-guides, etc. [5–7]. Several methods such as laser ablation [5,8], thermal evaporation [7,9], carbothermal reduction or carbon-assisted growth [6,10,11], direct thermal oxidation of Si wafers [12–14] have been used to synthesize SiO x nanowires. However, most of these methods employ metal catalysts such as Au [7,8,11,14],Ni[14,15],Fe[5,6], Co [16],Ga[17,18],Cu[19],Sn[20] to assist the synthesis process and consequently, the nanowires have significant presence of embedded residual metallic impurities that may affect their properties. In the recent past, non-catalytic growth of silica nanowires via carbothermal reduction of metal oxides such as MgO, CuO, WO 3 has also been reported. Despite considerable experi- mental efforts, the gr owth m echanism of silica nano wires is not well understood and indeed no consensus about the growt h mechanisms has b een achieved. One school of thoughts b e lieves that vapor– liquid–solid (VLS) [5,6,2 1] or solid–liq uid–solid (S LS) [13,1 5] pro- cesses are the possible mechanisms i n c atal y st-assist ed g r owth o f amorphous SiO x (a-SiO x ) nano w ir es . Other sch ool suggest ed differ- ent chemical reactions and sequences for the a-SiO x nanowires formation [10,11,17 ] to explain their experimental results. Recentl y, Aharonovich and Lifshitz [22] found that metal catalyst is essential for S iO x nanowires growth and proposed an alternative catalyst- assisted mechanism based on preferential adsorption of SiO x on the catalyst droplet without penetration into it. In this paper, we report large-scale synthesis of pure silica (SiO 2 ) nanowires by a non-catalytic approach based on thermal evaporation of SiO under argon ambient with traces of oxygen. The process is simple yet elegant and involves only single process step wherein the SiO vapors are transported from hot zone ($1200 1C) to downstream low-temperature zone where they are allowed to condense on a substrate. The structure and photo- luminescence (PL) property of the as-deposited material has been investigated and the growth of SiO 2 nanowires is discussed on the basis of tiny SiO 2 cluster formation via direct reaction of SiO vapors with O 2 that subsequently acts as nucleation center for SiO 2 nanowire growth. ARTICLE IN PRESS Contents lists available at ScienceDirect journal hom epage: www.elsevier.com/locate/physe Physica E 1386-9477/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2009.04.032 Ã Corresponding author. Tel.: +9111 4560 8617; fax: +9111 2572 6938. E-mail address: srivassk@mail.nplindia.ernet.in (S.K. Srivastava). Physica E ] (]]]]) ]]]–]]] Please cite this article as: S.K. Srivastava, et al., Physica E (2009), doi:10.1016/j.physe.2009.04.032 2. Experimental The growth process was carried out in a conventional three- zone horizontal quartz tube furnace, the schematic of which is shown in Fig. 1. The requisite amount of source material, i.e., silicon monoxide (SiO) granules (purity $99.9%; Pure Tech Inc., New York, USA) was kept in an alumina boat that was placed in the center of the quartz tube. Ultrasonically cleaned silicon strips of 2 Â 2cm 2 with and without Ni film were placed downstream in the lower temperature zone of the furnace on an alumina sample holder. Thin Ni film (Ni powder, purity 99.99%; CERAC Inc., USA) was deposited on cleaned silicon wafers by thermal evaporation technique at a base pressure of 3.0 Â 10 À6 Torr. The source SiO and the substrates were inserted in the tube and kept at locations at $1200 1C (center zone) and $10 00 1C (in the direction of carrier gas flow), respectively, identified earlier by temperature profiling. The quartz tube system was purged with a carrier gas (argon) flow for three hours before heating up to 1200 1C under a constant argon flow at the rate of 40 l/h. The source SiO was then heated at 1200 1C for 1 h in argon ambient (99.95%) with traces of oxygen and moisture. Therefore, the growth time was 1 h. After completion of the growth process the system was cooled down to room temperature under Ar flow. A thick wool-like spongy, white-colored material was deposited on the silicon substrates (both on Ni coated and without Ni film) as well as on the alumina sample holder. A thick white-colored material was also deposited on the walls of the quartz tube in low-temperature zone. To investigate the role of SiO as source, only silicon substrates were heated at 1000 1C (since the silicon substrates were placed at 1000 1C with source SiO at $1200 1C) for 1 h without SiO granules at the center zone. No deposition was found on the silicon substrates in this case. The as-deposited film on silicon substrates and the material collected from the quartz tube as well as from alumina sample holder were examined by scanning electron microscopy (SEM; LEO 440 VP) operating at 15 kV. Structural analysis of the material was carried out by X-ray diffractometery (XRD) (Bruker, D-8 ADVANCE diffractometer) with CuK a radiation, high-resolution transmission electron microscopy (HRTEM) (FEI, Technai G20- stwin; 200 kV) equipped with energy dispersive X-ray spectro- scopy (EDAX) (EDAX company, USA). A part of the white film was delaminated from the Si wafer and was ultrasonically dispersed in ethanol for 10 min. A few drops of the suspension were placed on carbon-coated copper (Cu) microgrid for TEM and HRTEM investigations. PL measurement of the material was carried out at room temperature using double monochromator-based spec- trometer (PerkinElmer LS55) with xenon flash lamp as excitation source. 3. Results and discussion 3.1. Microstructural analysis A low-magnification SEM micrograph of the as-deposited wool-like thick white film on Ni-coated silicon wafer and its magnified view is shown in Fig. 2(a) and (b), respectively, where high density of 1-D nanostructures in the form of wires having hundreds of micrometers length is clearly seen. Fig. 2(a) also reveals that several layers of nanowires were deposited one over another and total thickness of the film is estimated to be more ARTICLE IN PRESS Ar Outlet 1 23 3- Zone Tube Furnace Quartz Tube SiO Alumina boats Si wafers Fig. 1. Schematic diagram of horizontal furnace set up for the synthesis of silica nanowires. 50 µm 10 µm 10 µm 2 µm 25 µm Fig. 2. SEM micrographs of (a) nanowires deposited on Ni-coated Si wafers (low magnification), (b) magnified view of (a), (c) nanowires deposited on Si wafers without catalyst Ni film, (d) product collected from quartz tube magnified view of which is shown in the inset. S.K. Srivastava et al. / Physica E ] (]]]]) ]]]–]]]2 Please cite this article as: S.K. Srivastava, et al., Physica E (2009), doi:10.1016/j.physe.2009.04.032 than 100 m m. Fig. 2(b) gives an idea of nanowires diameter distribution which, indeed, is quite uniform. The nanowires formation is straight, free from wrinkles and chain-like morphology. It is to be noted that a thick white layer was deposited over the entire area of the Ni-coated Si wafers even on the side edges. To study the role of Ni on the growth of nanowires, we performed identical experiment on silicon wafers without Ni film as well as on alumina substrate holder. A high density of nanowires was also observed on Si wafers without Ni film (see Fig. 2(c)) similar to that observed with Ni film. This clearly indicates the non-catalytic growth of nanowires, which was further confirmed by the investigations of the white spongy material deposited on the inner surface of the quartz tube and also on the alumina sample holder. Fig. 2(d) shows the low-magnification SEM image of the substance collected from the quartz tube, which shows agglomerated clusters consisting of high density of nanowires. The magnified view of a single cluster is depicted in the inset of Fig. 2(d). These observations suggest that growth of nanowires in the present process is probably not governed by the VLS or SLS mechanism, which essentially is a catalytic process. The present results are different from that of Aharonovich et al. [8,22] where metal catalyst (Au or Ni) was found to be essential for the SiO x nanowires growth and also different from bi-cycle chain-like morphology of silica nanowires observed by Kar and Chaudhuri [14] during carbon-assisted non-catalytic growth. It is also to be remarked here that no nanowire growth was observed on silicon substrates heated at 100 0 1C without SiO source at center of tube (1200 1C), which clearly indicates that the nanowires growth in the present process does not take place due to silicon substrates heating in presence of oxygen traces or moisture. The growth of thick nanowire film on alumina substrate holder is also evidence that SiO is the main source for silicon nanowires growth. Fig. 3(a) shows a typical TEM micrograph of nanowires deposited on Ni-coated Si wafers. The nanowires have diameter in the range 30–100 nm with center of the distribution at $50 nm. The diameters remain nearly constant throughout the length of the nanowires. The nanowires have remarkably clean and smooth surface. It is important to note here that the nanowires have circular cross-section revealing the cylindrical nature (shown by circles in Fig. 3(a)) and no metal particles are seen at either end of the wires. The TEM investigations of the material further confirm our view that growth of nanowires is essentially non-catalytic in the present process. 3.2. Structural and compositional analyses The XRD patterns (not shown here) revealed amorphous character of the deposited nanowires film, which was further confirmed by the HRTEM (Fig. 3(b)) study. The selected area electron diffraction (SAED) pattern (shown in the inset of Fig. 3(b)) recorded from a single nanowire where only diffusive rings reveal the amorphous nature of the nanowires. No lattice fringes could be resolved in the HRTEM across the diameter of the nanowires (Fig. 3(b)). Furthermore, no Si-SiO 2 core-shell kind of structure as seen by Park and Yong [13] and Zhang et al. [23] was observed. This reveals that the nanowires have uniform amorphous structure across the length and diameter. EDAX spectra of the nanowires were also recorded during TEM investigation to examine their chemical composition. The EDAX spectrum shown in Fig. 4 for a single nanowire reveals presence of only two elements Si and O with an atomic ratio of approximately 1:2 (strong C and Cu signals are attributed to the carbon-coated Cu microgrid). Based on the above observations, we may conclude that the nanowires are amorphous silicon dioxide (SiO 2 ). 3.3. Photoluminescence Fig. 5 shows the PL spectrum of SiO 2 nanowires recorded at room temperature with 241 nm excitation. A strong blue luminescence is observed with peak position at 393 nm ($3.15 eV). The PL properties of silica nanowires have been investigated before showing single or two PL bands depending on their structural properties [5,7,24]. For example, Yu et al. [5] observed two broad PL peaks of SiO 2 nanowires at 470 nm ($2.65 eV) and 420 nm ($3.0 eV), whereas Zhu et al. [24] ARTICLE IN PRESS 100 nm 5 nm Fig. 3. (a) TEM micrographs of the nanowires showing metal-free ends with circular cross-section (indicated by circles), (b) HRTEM micrograph of a nanowire showing amorphous structure. The SAED pattern of the nanowire is shown in the inset of (b). S.K. Srivastava et al. / Physica E ] (]]]]) ]]]–]]] 3 Please cite this article as: S.K. Srivastava, et al., Physica E (2009), doi:10.1016/j.physe.2009.04.032 reported that two broad PL peaks of SiO x were at around 570 nm ($2.2 eV) and 430 nm ($2.88 eV). On the other hand, Wang et al. [7] observed single broad PL peak at $446 nm ($2.78 eV) from SiO x nanowires. These emissions have been attributed to the structural defects related to oxygen deficiency in the silica nanowires that act as radiative recombination centers. Nishikawa et al. [25] have observed several luminescence bands in the range 1.9–4.3 eV in various types of high-purity silica glasses where the band at 3.1 eV was attributed to some intrinsic diamagnetic defect centers, such as twofold coordinated silicon lone pair centers (O–Si–O) caused by high oxygen deficiency in the samples. Therefore, the observed blue light emission from the silica nanowires in the present study could have its origin to the structural defects such as oxygen deficiency, which might have been generated during the nanowires growth. 3.4. Growth mechanism The growth mechanism in 1-D nanostructures has been explained by the screw dislocation model and the VLS model in the past. The former is not appropriate in case of amorphous nanowires whereas the latter is based on the three-step mechan- ism involving (i) diffusion of Si/SiO x vapors into the metal particles, (ii) formation of liquid droplet of metal and Si/SiO x and (iii) the precipitation in the form of solid nanowires after super-saturation of liquid metal–Si/SiO x droplet at the liquid– solid interface. Thus a metal nanoparticle at one end of the nanowires is usually considered as the evidence for the operation of the VLS model [26]. But no metal particles were found at either end of the nanowires in the present study as confirmed by SEM and TEM results. Therefore, VLS or SLS mechanism is not pertinent to explain the present experimental observations. Further, growth of SiO 2 nanowires on silicon wafers without Ni film, alumina holder and quartz tube rules out the metal catalyst-assisted growth mechanism by Aharonovich and Lifshitz [22]. In the present case, the growth of the SiO 2 nanowires cannot be explained using the oxide-assisted growth (OAG) process pro- posed by Lee et al. [26–28] for the crystalline Si nanowires. The OAG process presumes an oxide cluster rather than a metal particle after the VLS mechanism to assist the formation of the nanowires wherein an outer oxide shell formation is essential to prevent the growth along the lateral direction that results in 1-D Si-SiO x core-shell nanowire [26] formation. However, the nano- wires grown in the present study are amorphous across the diameter instead of crystalline Si core and amorphous SiO 2 outer shell structure as confirmed by HRTEM image and SAED pattern. On the other hand, the formation of crystalline Si–SiO 2 core-shell nanowires first and then complete oxidation of the structure to result SiO 2 nanowires is also not practically possible as discussed by Buttner and Zacharias [29]. Therefore, what could be the driving force for the formation of 1-D amorphous SiO 2 nanowires? Since no crystalline Si embedded in SiO 2 outer shell was observed unlike some earlier reports on growth of Si nanowires by SiO evaporation [23], the concept that SiO first disproportionate into Si and SiO 2 to form Si nanowires with SiO 2 outer layer may not be applied. Therefore, the following reaction mechanism may be proposed to explain our observations. The SiO vapors, generated at temperature $1200 1C, are transported downstream towards the substrates by the carrier gas and during traversal they get converted into SiO 2 molecules directly by reacting with O 2 according to the following equation: 2SiOðgÞþ2O 2 ðgÞ!2SiO 2 ðsÞ It may be remarked here that in the present experiment no special arrangement was used to remove residual O 2 from the carrier gas or from the process chamber (either by vacuum or hydrogen gas or O 2 traps) consequently, the traces of residual O 2 in the chamber could be present. The SiO 2 molecules so formed condense on the substrate or quartz tube wall in the low- temperature zone ($1000 1 C) to form SiO 2 nanoclusters that then act as nucleation center for the growth of SiO 2 nanowires. Subsequently, SiO 2 nanoclusters may aggregate to induce 1-D SiO 2 nanostructures to minimize its systemic energy [30]. The proposed mechanism may find theoretical support by Zhang and Zhang [31] who found that growth of energetically favored anisotropic 1-D silica nanowires may occur without metal catalyst template by short-range ordering of building blocks such as (SiO 2 ) 8 clusters. They showed that silica cluster (SiO 2 ) 8 is geometrically highly symmetric structure, energetically highly stable with high chemically reactive ends of SiQO groups, and thus make it easy to be assembled into larger linearly extended ARTICLE IN PRESS Fig. 4. EDAX spectrum of a nanowire (shown in the inset) showing Si and O as main detected elements. The quantitative data is also shown in the inset. 300 393 nm PL Intensity (a.u.) Wavelength (nm) 350 400 450 500 550 600 650 700 Fig. 5. Room temperature PL spectrum of the SiO 2 nanowires recorded with 241 nm excitation. S.K. Srivastava et al. / Physica E ] (]]]]) ]]]–]]]4 Please cite this article as: S.K. Srivastava, et al., Physica E (2009), doi:10.1016/j.physe.2009.04.032 clusters and hence into 1-D s ilica nanowires. The pr esent study shows that no p r eferential sit e or m orphology o f the su bstrat e is required for the formation of the SiO 2 nanowires. Though ex act formation mechanism of s ilica nanowires from SiO vapors, factors controlling the diameters of the nanowires in the present approach, remains still unclear and needs f u rther detail investigations, the present work is useful due to its simplicity and the low cost. 4. Conclusions Amorphous silicon dioxide nanowires of several hundred microns in length and tens of nanometers in diameter have been synthesized in bulk by a non-catalytic single step process using thermal evaporation of silicon monoxide under argon atmosphere with traces of oxygen. The nanowires were free from metal contaminations and showed blue photoluminescence at room temperature. It is proposed that in-situ formation of SiO 2 vapors via reaction of SiO vapors with O 2 leads to the formation of SiO 2 nanoclusters, which consequently results in the formation of large nanowires. The present simple and low-cost process of producing pure silica nanowires (free from metallic contaminations) in bulk may lead to potential applications in nanoelectronics and optical devices. Acknowledgements The authors wish to thank Ms. Manisha and Dr. S.K. Halder for XRD measurements of the samples and the Director, NPL for his permission to publish this work. References [1] C.N.R. Rao, F.L. Deepak, G. Gundiah, A. Govindaraj, Prog. Solid State Chem. 31 (2003) 5. [2] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Adv. Mater. 15 (2003) 353. [3] C.N.R. Rao, A. Govindaraj, S.R.C. Vivekchand, Annu. Rep. Prog. Chem. Sect. A 102 (2006) 20. [4] L.J. Chen, J. Mater. Chem. 17 (2007) 4639. [5] D.P. Yu, L. Hang, Y. ding, H.Z. Zhang, Z.G. Bai, J.J. Wang, Y.H. Zou, W. 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Lee, Y.F. Zhang, N. Wang, Y.H. Tang, I. Bello, C.S. Lee, Y.W. Chung, J. Mater Res. 14 (1999) 4503. [29] C.C. Buttner, M. Zacharias, Appl. Phys. Lett. 89 (2006) 263106. [30] Y. Zhang, N. Wang, R. He, J. Liu, X. Zhang, J. Zhu, J. Cryst. Growth 233 (2001) 803. [31] D. Zhang, R.Q. Zhang, J. Phys. Chem. B 110 (2006) 1338. ARTICLE IN PRESS S.K. Srivastava et al. / Physica E ] (]]]]) ]]]–]]] 5 Please cite this article as: S.K. Srivastava, et al., Physica E (2009), doi:10.1016/j.physe.2009.04.032 . Large-scale synthesis, characterization and photoluminescence properties of amorphous silica nanowires by thermal evaporation of silicon monoxide Sanjay. 2009 PACS: 61.46.–w 81.07.–b Keywords: Silicon monoxide Silicon oxide nanowires Thermal evaporation Photoluminescence abstract A single step non-catalytic process based on thermal evaporation