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Composite silicon nanostructure arrays fabricated on optical fibre by chemical etching of multicrystal silicon film
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 155601 (http://iopscience.iop.org/0957-4484/26/15/155601) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 149.150.51.237 This content was downloaded on 02/04/2015 at 11:14
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2015) 155601 (7pp)
doi:10.1088/0957-4484/26/15/155601
Composite silicon nanostructure arrays fabricated on optical fibre by chemical etching of multicrystal silicon film Zewen Zuo1,2, Kai Zhu1, Lixin Ning1, Guanglei Cui1, Jun Qu1, Wanxia Huang1, Yi Shi3 and Hong Liu4 1
College of Physics and Electronics Information, and Center for Nanoscience and Technology, Anhui Normal University, Wuhu, 241000, People’s Republic of China 2 National Laboratory of Solid State Microstructures, Nanjing University, Nanjing, 210093, People’s Republic of China 3 School of Electronic Science and Engineering, and Key Laboratory of Photonic and Electronic Materials, Nanjing University, Nanjing, 210093, People’s Republic of China 4 School of Mathematics and Physics, Suzhou University of Science and Technology, Suzhou, 215009, People’s Republic of China E-mail: [email protected] Received 12 November 2014, revised 23 January 2015 Accepted for publication 4 February 2015 Published 24 March 2015 Abstract
Integrating nanostructures onto optical fibers presents a promising strategy for developing newfashioned devices and extending the scope of nanodevices’ applications. Here we report the first fabrication of a composite silicon nanostructure on an optical fiber. Through direct chemical etching using an H2O2/HF solution, multicrystal silicon films with columnar microstructures are etched into a vertically aligned, inverted-cone-like nanorod array embedded in a nanocone array. A faster dissolution rate of the silicon at the void-rich boundary regions between the columns is found to be responsible for the separation of the columns, and thus the formation of the nanostructure array. The morphology of the nanorods primarily depends on the microstructure of the columns in the film. Through controlling the microstructure of the as-grown film and the etching parameters, the structural control of the nanostructure is promising. This fabrication method can be extended to a larger length scale, and it even allows roll-to-roll processing. S Online supplementary data available from stacks.iop.org/NANO/26/155601/mmedia Keywords: silicon nanostructure array, chemical etching, optical fibre 1. Introduction
cladding with a nanostructure array directly fabricated on the surface of the fiber core, the evanescent field will interact many times with the nanostructures [4] and will excite multiple effects, such as photovoltaic [4, 5] and localized surface plasmon resonance [6, 7], etc. Based on this principle, dyesensitized solar cells have been configured with semiconductor nanowire arrays grown on the fiber’s surface, and high energy conversion efficiency has been realized [4, 5]. In addition, cladding-coupled optical fiber sensors with metal nanoparticle or nanowire arrays located on the fiber surface are configurable, which enables an expansion of their applications due to the multiplexing, distributed, real-time, remote, and in vivo sensing capabilities [6, 7]. Therefore, integrating
Nanostructure arrays have been extensively studied for their potential applications in next-generation nanoelectronic, photonic, optoelectronic, and photovoltaic devices, which are generally fabricated on flat, rigid substrates with dimensions that are no more than wafer scale. As a new flexible substrate, optical fibers allow electronic functions to be exploited over a larger length scale and to be woven into large two-dimensional or three-dimensional structures [1–3], which greatly extends the scope of their applications. Moreover, an optical fiber transmits light via total internal reflection at the core/ cladding interface. Therefore, if we substitute the traditional 0957-4484/15/155601+07$33.00
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© 2015 IOP Publishing Ltd Printed in the UK
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Nanotechnology 26 (2015) 155601
nanostructures with optical fibers has great potential to improve performance with a wide variety of applications for the nanodevices. Many techniques have been developed to prepare onedimensional (1D) nanostructures (nanowires, nanorods, nanocones, etc.), such as chemical vapor deposition [8, 9], laser ablation [10], reactive ion etching [11, 12], metalassisted chemical etching [13–16], and others. However, these techniques cannot be easily transferred to optical fibers due to the fibers’ geometries. So far, only limited reports exist on the fabrication of 1D nanostructure arrays on the surface of optical fibers. ZnO nanowire arrays have been prepared via hydrothermal growth using zinc chloride and hexamine as sources [4, 5]. Using so-called oblique angle deposition (OAD), Ag, Al, Ni, Si, and TiO2 nanorod arrays have been successfully fabricated on the fiber surface [17, 18]. OAD is a versatile technique that can be used to fabricate nanorods from various materials. Nevertheless, it is difficult to fabricate nanostructures on a large length scale with this method. 1D silicon nanostructures have been widely investigated for their potential as fundamental building blocks for nanodevices. Although considerable efforts have been devoted to the fabrication [8–16], structural control [8, 9, 14, 19–22], and applications of 1D Si nanostructures [23–26], few results on integrating 1D Si nanostructure arrays on optical fibers have been reported. In view of its great importance to both fundamental research and practical applications, we report herein a new approach for the fabrication of Si nanorod and nanocone arrays on the surface of an optical fiber. The details of the fabrication process, structural properties, and formation mechanism are also provided.
Figure 1. Raman spectra of the as-deposited and annealed Si films
deposited on an optical fiber and on quartz glass, respectively.
The surface and the cross-sectional morphology of the samples were characterized using scanning electron microscopy (SEM, Hitachi S-4800). Raman spectroscopy measurements were performed at room temperature on a Raman system (HORIBA Jobin Yvon, LabRAM HR) using a 514 nm Ar+ laser line for excitation. Before the Raman measurement, the spectrum was calibrated with crystalline Si.
3. Results and discussion The broad Raman scattering peak of the as-deposited film shown in figure 1 is around 480 cm−1, indicating an amorphous structure. The annealing process crystallizes the a-Si film, resulting in the formation of a mc-Si film on the fiber. The intensive scattering peak of the annealed film at ∼514 cm−1 evidently deviates from that of bulk Si (∼520 cm−1), while the Raman spectrum from the sameannealed Si film on flat quartz glass shows almost the same peak position as the bulk Si. This result indicates the different microstructure of these two Si films; the nanostructure or high stress may be present within the film on the fiber [27, 28]. Figure 2(a) presents the SEM image of a cleaved cross section of the annealed Si film grown on the surface of the optical fiber, showing that the thickness of the film around the fiber is uniform in spite of the round geometry of the fiber, which is stationary during deposition. Large numbers of gaps and cracks are observed on the film surface (figures 2(b) and (c)), dividing the film into a lot of plots comprising many domains at a size of several hundred nanometers. Through the gaps, it can be identified that the film is made up of closely arranged columns, and the observed domains on the surface are just the top end of these columns. The magnified crosssectional image shown in figure 2(d) further reveals that the columns grow along the radial direction of the fiber and are perpendicular to the fiber surface. The estimated average diameter near the half height of the columns is about 270 nm, which increases to approximately 400 nm at the top surface of the 5.2 μm-thick film. This increase in the diameter along the
2. Experimental details Amorphous silicon (a-Si) film was deposited on the surface of a naked quartz optical fiber by plasma enhanced chemical vapor deposition, using silane (30 sccm, 5% diluted in H2) as the source gas. Prior to the deposition, an optical fiber with a length of 20 cm was washed using acetone to remove the polymer cladding, and then was hung stationary, parallel to the substrate plate. The substrate was not intentionally heated during the deposition. Plasma was produced using an excitation frequency of 13.56 MHz and a power density of 55 mW cm−2. The chamber pressure during the deposition was set constant at 750 mTorr. To obtain a multicrystal silicon (mc-Si) layer, high-temperature annealing was performed to crystallize the as-deposited a-Si in a tube furnace at 900 °C, with a ramping rate of 20 K min−1 and a dwelling of 60 min. The annealing was performed under vacuum, which was obtained by means of a rotary mechanical pump. Then the annealed sample was subjected to chemical etching in a solution containing 4.4 M hydrofluoric acid (HF) and 0.4 M hydrogen peroxide (H2O2) at room temperature [15, 16]. The etching process was also performed in a solution of 0.45 M HF and 0.1 M nitric acid (HNO3). After the etching, the sample was rinsed with deionized water and dried in air. 2
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Nanotechnology 26 (2015) 155601
Figure 2. SEM images of the annealed Si film deposited on the surface of the optical fiber. (a) Cross-sectional image, (b) plan-view image, (c)
magnified plan-view image, and (d) magnified cross-sectional image of the sample.
Further etching induces partial nanorods to peel off from the fiber surface, as shown in figure 3(e). The overall morphology of the nanorods can be clearly observed from the peeled nanorods. The diameter along the nanorod is not uniform, but rather gradually tapers from top to bottom, with a sharp root. Some twinned nanorods with their top portions linked together are also observed, as indicated by the arrow in figure 3(e). The peeling-off of the nanorods leaves the conelike nanostructure on the fiber surface. The magnified image of the nanocone array is shown in figure 3(f), which shows that the nanocones are separated from each other, and their tips are very sharp. We have carried out the same treatment of annealing and etching on the a-Si film deposited on a flat quartz substrate, but without observation of the nanorods or nanocones. SEM investigation indicates that the annealed Si film on the flat substrate is highly compact. No columnar microstructure is observed (see Supplementary Information, figure S1), which is entirely different from the film on the fiber (figure 2(d)). After etching, the morphology of the film has no evident change. Therefore, this composite nanostructure must relate to the particular microstructure of the a-Si film deposited on an optical fiber with a curved surface. Figure 4(b) presents the cross-sectional SEM image of the as-deposited a-Si film on the fiber. We can see an inverted-cone-like columnar structure similar to the annealed film, but with a more compact arrangement of these columns, with void-rich boundary regions between them [29]. Combining the observations shown in figure 3, we can conclude that the silicon film on the fiber exhibits a composite microstructure with
growth direction results in an inverted-cone-shaped geometry. These columns are close to each other, with small interstitials between them. Upon close examination, large numbers of nanoscaled granules bestrew the sidewall of the columns; this may be partially responsible for the deviation of the Raman peak. The mc-Si-covered fiber was immersed into HF/H2O2 solution to perform chemical etching. Figure 3(a) shows the morphology of the sample after etching for 10 min; an array with Si nanorods was formed all over the fiber surface. Compared with the as-annealed sample (figure 2(c)), it can be undoubtedly concluded that the nanorods form as a result of the columns separating from each other in the mc-Si film. However, numerous columns do not effectively separate under these etching conditions, instead forming sheaf-like structures on the micrometer scale. The magnified image shown in figure 3(b) reveals the uniform height of the nanorods in the array. Though some isolated nanorods are observable, most columns are undivided, with a cauliflowerlike morphology at the top, as usually observed on the surface of the mc-Si film. The columns evidently separate from each other after etching the sample for 30 min. Numerous nanorods with round tops and thin bottoms are clearly observed, as seen in figure 3(c). The average diameters of the nanorods are slightly smaller than those of the columns in the mc-Si film, while the sidewalls are still rough. From the cross-sectional image shown in figure 3(d), we can see that a cone-like nanostructure array is presented underneath the nanorod array, and the nanorods are in fact embedded in the nanocone array. 3
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Figure 3. (a) Plan-view image and (b) magnified plan-view image of the sample after etching for 10 min. (c) Plan-view image and (d) cross-
sectional image of the sample after etching for 30 min. (e) Plan-view image and (f) magnified local region of the sample after etching for 60 min.
Figure 4. (a) Plan-view image and (b) cross-sectional image of the as-deposited a-Si film on the surface of an optical fiber. The inset in (a) is a
low-resolution plan-view image of the sample.
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Figure 5. Schematic of the fabrication process of the composite nanostructure. (a) Annealed Si film is etched in solution. (b) Nanorod array forms as a result of the shrinkage and separation of the columns. (c) Nanocone array unveils as the nanorods peel off.
figure 3(c) and schematically in figure 5(b). During etching, the tensile stress within the film may also play an important role in the separation of the columns. Because the nanorods are embedded in a nanocone array, with a void-rich boundary region between the nanorods and nanocones, further chemical etching causes the boundary regions to be dissolved. Consequently, the nanorods peel off from the valleys of the nanocones, leaving the nanocone array on the fiber surface, as shown in figure 3(e) and schematically in figure 5(c). In summary, there are three steps during the fabrication of the nanostructure: (1) the formation of the columns—the nanorods are born out of the columns in the film, so columnar growth of the film during deposition is a requisite precondition; (2) preliminary detachment of the columns—annealing shrinks the dimension of the columns, resulting in them separating at the top and thus generating the cracks and the void-rich boundary regions between the columns; and (3) further detachment of the columns—a faster dissolution of the Si at the boundary region during etching further detaches the columns, forming a separated nanorod array. According to the previously mentioned formation mechanism, some other materials with curved surfaces maybe also act as substrates to fabricate similar nanostructures. However, the microstructure of the deposited film, which is dependent on substrate surface topography and surface chemistry, should be discrepant [32, 33], thus leading to different structures after etching. For example, we observed some rod-like structures on the NiCr filament surface (see Supplementary Information, figure S3), while we failed to obtain similar structures on Mo filaments (see Supplementary Information, figure S4). Furthermore, other mixed solutions can also work as the etching solution for the fabrication of the previously detailed composite nanostructure, as long as the reduction of the oxidant can supply electric holes for the oxidization of Si, and then the oxidized Si can be dissolved by HF. In fact, similar results have been obtained by using an HNO3/HF solution. We suppose that the nucleation density of the a-Si, the curvature of the fiber, and the annealing process could be the important factors in deciding the microstructure of the film and the separation of the columns. The structural control of the nanostructure could be realized; the height of the nanorods is determined by the thickness of the as-grown a-Si film, while the morphology is principally dependent on the morphology of the columns, which can be controlled by the initial
inverted-cone-shaped columns embedding in a cone-shaped array, as schematically shown in figure 5(a). In fact, fine cone-shaped structures are observable at the film/fiber interface in figures 2(d) and figure 4(b). Moreover, this microstructure comes from growth, rather than annealing. The definite growth mechanism behind this microstructure remains unclear at present, but it could be related to the low diffuse ability of the precursors on the growth surface at low deposition temperatures [30] and the gradually increasing deposition-zone area due to the growth along the radial direction of the fiber. Also, the as-deposited a-Si film on the fiber was etched in the HF/H2O2 solution, but failed to separate the columns to form a similar nanostructure to that of the annealed film (see Supplementary Information, figure S2). By comparing the films with and without annealing, one can see that the surface of the nonannealed film is more compact, with hardly any gaps or cracks (figure 4(a)). Annealing at high temperature leads to the formation of mc-Si, as demonstrated by the Raman spectrum shown in figure 1. Because crystal Si is more compact than a-Si, the reorganization of the Si atoms during annealing shrinks the dimension of the columns and leads to separation at the top, which might be responsible for the formation of the cracks on the surface. Meanwhile, hightemperature annealing relaxes the stress within the film, resulting in the generation of the gaps penetrating the film, as shown in figures 2(b) and (c). During chemical etching, the solution can easily penetrate into the film via the cracks and the void-rich boundary regions between the columns, and then oxidation-reduction reactions occur therein. The reduction of H2O2 generates electronic holes, h+, according to reaction 1 [31]: 2H + + H 2 O2 → 2H 2 O + 2h+
(1)
The generated electronic holes are injected into the valence band of the adjacent Si, leading to the oxidation and then to the dissolution of the Si, according to reaction 2 [31]: Si + 4h+ + 4HF → SiF4 + 4H + and SiF4 + 2HF → H 2 SiF6.
(2)
These reactions dissolve the Si more rapidly at the voidrich boundary regions due to the very loose structure found there, leading the columns to become detached. Meanwhile, the isotropic etching shrinks the size of the column, resulting in a slightly reduced height and diameter. Then, the separated nanorod array forms on the fiber surface, as shown in 5
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Acknowledgments This study was partly supported by the National Natural Science Foundation of the People’s Republic of China under Grant Nos. 61106011 and 11374015, and the Anhui Province Natural Science Foundation under Grant No. 1308085QF109.
References [1] He R, Day T D, Krishnamurthi M, Sparks J R, Sazio P J A, Gopalan V and Badding J V 2013 Silicon p-i-n junction fibers Adv. Mater. 25 1461–7 [2] Sugawara Y, Uraoka Y, Yano H, Hatayama T, Fuyuki T, Nakamura T, Toda S, Koaizawa H, Mimura A and Suzuki K 2007 Polycrystalline silicon thin-film transistors on quartz fiber Appl. Phys. Lett. 91 203518 [3] Zou D, Wang D, Chu Z, Lv Z and Fan X 2010 Fiber-shaped flexible solar cells Coordin. Chem. Rev. 254 1169–78 [4] Weintraub B, Wei Y and Wang Z L 2009 Optical fiber/ nanowire hybrid structures for efficient three-dimensional dye-sensitized solar cells Angew. Chem. Int. Ed. 48 1–6 [5] Wei Y, Xu C, Xu S, Li C, Wu W and Wang Z L 2010 Planar waveguide-nanowire integrated three-dimensional dyesensitized solar cells Nano Lett. 10 2092–6 [6] Stoddart P R and White D J 2009 Optical fibre SERS sensors Anal. Bioanal. Chem. 394 1761–74 [7] Zhang Y, Gu C, Schwartzberg A M and Zhang J Z 2005 Surface-enhanced Raman scattering based on D-shaped fiber Appl. Phys. Lett. 87 123105 [8] Cui Y, Lauhon L J, Gudiksen M S, Wang J and Lieber C M 2001 Diameter-controlled synthesis of single-crystal silicon nanowires Appl. Phys. Lett. 78 2214–6 [9] Kayes B M, Filler M A, Putnam M C, Kelzenberg M D, Lewis N S and Atwater H A 2007 Growth of vertically aligned Si wire arrays over large areas (>1 cm2) with Au and Cu catalysts Appl. Phys. Lett. 91 103110 [10] Zhang Y F, Tang Y H, Wang N, Yu D P, Lee C S, Bello I and Lee S T 1998 Silicon nanowires prepared by laser ablation at high temperature App. Phys. Lett. 72 1835–7 [11] Tang J, Ou F S, Kuo H P, Hu M, Stickle W F, Li Z and Williams R S 2009 Silver-coated Si nanograss as highly sensitive surface-enhanced Raman spectroscopy substrates Appl. Phys. A 96 793–7 [12] Zhu J, Yu Z, Burkhard G F, Hsu C M, Connor S T, Xu Y, Wang Q, McGehee M, Fan S and Cui Y 2009 Optical absorption enhancement in amorphous silicon nanowire and nanocone arrays Nano Lett. 9 279–82 [13] Chartier C, Bastide S and Lévy-Clément C 2008 Metal-assisted chemical etching of silicon in HF-H2O2 Electrochim. Acta 53 5509–16 [14] Sun L, Fan Y, Wang X, Susantyoko R A and Zhang Q 2014 Large scale low cost fabrication of diameter controllable silicon nanowire arrays Nanotechnology 25 255302 [15] Zuo Z W, Cui G L, Shi Y, Liu Y S and Ji G B 2013 Goldthickness-dependent Schottky barrier height for charge transfer in metal-assisted chemical etching of silicon Nanoscal. Res. Lett. 8 193 [16] Ho J W, Wee Q, Dumond J, Tay A and Cha S J 2013 Versatile pattern generation of periodic, high aspect ratio Si nanostructure arrays with sub-50 nm resolution on a wafer scale Nanoscal. Res. Lett. 8 506 [17] Fan J G and Zhao Y P 2005 Direct deposition of aligned nanorod array onto cylindrical objects J. Vac. Sci. Technol. B 23 947–53
Figure 6. Plan-view SEM image of an array of nanorods with a
sunflower-seed appearance, which was transferred onto a conductive tape by directly rolling the fiber on the tape.
nucleation and the growth process. For example, using a power density of 75 mW cm−2 during the a-Si deposition, followed by the same annealing and chemical etching, an array of nanorods with a sunflower-seed appearance was obtained. Figure 6 shows the transferred array on a conductive tape, attained by directly rolling the fiber on the tape. Moreover, the dimensions can be further regulated by controlling the duration and the rate of the isotropic etching, which is related to the mixture ratio of the solution and the temperature. At present, due to the limitation of the chamber area, the length of the optical fiber that was covered with Si film is about 20 cm, but the scale could be significantly elongated by coiling the fiber. By using proper equipment like a pipe furnace, the fabrication could be extended to a larger length scale, and even roll-to-roll processing is possible. Further studies will be devoted to elucidating the growth mechanism of the a-Si film deposited on the optical fiber surface, which is critical for structural control. Also, the etching conditions—including the temperature, mixture ratio of the solution, duration, and so on—will be further optimized.
4. Conclusions In summary, we have prepared a composite Si nanostructure with a vertically aligned, inverted-cone-like nanorod array embedded in a nanocone array, through direct chemical etching of the mc-Si film deposited on optical fiber surface. A reasonable mechanism was proposed to elucidate the formation process of this composite structure. The morphology (and the height and diameter) of the nanorods was found to be primarily dependent on the microstructure of the columns in the mc-Si film, and it can be regulated by isotropic etching. This study presents a new method of fabricating nanostructures on optical fibers, and it might open a new pathway for integrating nanostructures with fibers in order to extend the application scope of nanodevices.
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[26] Kempa T J, Tian B, Kim D R, Hu J S, Zheng X and Lieber C M 2008 Single and tandem axial p-i-n nanowire photovoltaic devices Nano Lett. 8 3456–60 [27] Smit C, Swaaij R, Donker H, Petit A, Kessels W and Sanden M 2003 Determining the material structure of microcrystalline silicon from Raman spectra J. Appl. Phys. 94 3582–8 [28] Islam M N, Pradhan A and Kumar S 2005 Effects of crystallite size distribution on the Raman-scattering profiles of silicon nanostructures J. Appl. Phys. 98 024309 [29] Müllerová J, Šutta P, Elzakker G, Zeman M and Mikula M 2008 Microstructure of hydrogenated silicon thin films prepared from silane diluted with hydrogen Appl. Surf. Sci. 254 3690–5 [30] Matsuda A, Takai M, Nishimoto T and Kondo M 2003 Control of plasma chemistry for preparing highly stabilized amorphous silicon at high growth rate Sol. Energy Mater. Sol. Cells 78 3–26 [31] Huang Z P, Geyer N, Werner P, Boor J and Gösele U 2011 Metal-assisted chemical etching of silicon: a review Adv. Mater. 23 285–308 [32] Roca I, Cabarrocas P, Layadi N, Heitz T, Drévillon B and Solomon I 1998 Substrate selectivity in the formation of microcrystalline silicon: mechanisms and technological consequences Appl. Phys. Lett. 66 3609–11 [33] Vallat-Sauvain E, Bailat J, Meier J, Niquille X, Kroll U and Shah A 2005 Influence of the substrate’s surface morphology and chemical nature on the nucleation and growth of microcrystalline silicon Thin Solid Films 485 77–81
[18] Chu H Y, Liu Y, Huang Y and Zhao Y 2007 A high sensitive fiber SERS probe based on silver nanorod arrays Opt. Express 15 12230–9 [19] Zhong X, Qu Y, Lin Y C, Liao L and Duan X F 2011 Unveiling the formation pathway of single crystalline porous silicon nanowires ACS Appl. Mater. Interfaces 3 261–70 [20] Kim J, Han H, Kim Y H, Choi S H, Kim J C and Lee W 2011 Au/Ag bilayered metal mesh as a Si etching catalyst for controlled fabrication of Si nanowires ACS Nano 5 3222–9 [21] Azeredo B P et al 2013 Silicon nanowires with controlled sidewall profile and roughness fabricated by thin-film dewetting and metal-assisted chemical etching Nanotechnology 24 225305 [22] Lombardi I, Hochbaum A I, Yang P, Carraro C and Maboudian R 2006 Synthesis of high density, sizecontrolled Si nanowire arrays via porous anodic alumina mask Chem. Mater. 18 988–91 [23] Shao M W, Ma D and Lee S T 2010 Silicon nanowires— synthesis, properties, and applications Eur. J. Inorg. Chem. 2010 4264–78 [24] Zhang B H, Wang H S, Lu L H, Ai K L, Zhang G and Cheng X L 2008 Large-area silver-coated silicon nanowire arrays for molecular sensing using surface-enhanced Raman spectroscopy Adv. Funct. Mater. 18 2348–55 [25] Tian B, Zheng X, Kempa T J, Fang Y, Yu N, Yu G, Huang J and Lieber C M 2007 Coaxial silicon nanowires as solar cells and nanoelectronic sources Nature 449 885–90
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Composite silicon nanostructure arrays fabricated on optical fibre by chemical etching of multicrystal silicon film
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 155601 (http://iopscience.iop.org/0957-4484/26/15/155601) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 149.150.51.237 This content was downloaded on 02/04/2015 at 11:14
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2015) 155601 (7pp)
doi:10.1088/0957-4484/26/15/155601
Composite silicon nanostructure arrays fabricated on optical fibre by chemical etching of multicrystal silicon film Zewen Zuo1,2, Kai Zhu1, Lixin Ning1, Guanglei Cui1, Jun Qu1, Wanxia Huang1, Yi Shi3 and Hong Liu4 1
College of Physics and Electronics Information, and Center for Nanoscience and Technology, Anhui Normal University, Wuhu, 241000, People’s Republic of China 2 National Laboratory of Solid State Microstructures, Nanjing University, Nanjing, 210093, People’s Republic of China 3 School of Electronic Science and Engineering, and Key Laboratory of Photonic and Electronic Materials, Nanjing University, Nanjing, 210093, People’s Republic of China 4 School of Mathematics and Physics, Suzhou University of Science and Technology, Suzhou, 215009, People’s Republic of China E-mail: [email protected] Received 12 November 2014, revised 23 January 2015 Accepted for publication 4 February 2015 Published 24 March 2015 Abstract
Integrating nanostructures onto optical fibers presents a promising strategy for developing newfashioned devices and extending the scope of nanodevices’ applications. Here we report the first fabrication of a composite silicon nanostructure on an optical fiber. Through direct chemical etching using an H2O2/HF solution, multicrystal silicon films with columnar microstructures are etched into a vertically aligned, inverted-cone-like nanorod array embedded in a nanocone array. A faster dissolution rate of the silicon at the void-rich boundary regions between the columns is found to be responsible for the separation of the columns, and thus the formation of the nanostructure array. The morphology of the nanorods primarily depends on the microstructure of the columns in the film. Through controlling the microstructure of the as-grown film and the etching parameters, the structural control of the nanostructure is promising. This fabrication method can be extended to a larger length scale, and it even allows roll-to-roll processing. S Online supplementary data available from stacks.iop.org/NANO/26/155601/mmedia Keywords: silicon nanostructure array, chemical etching, optical fibre 1. Introduction
cladding with a nanostructure array directly fabricated on the surface of the fiber core, the evanescent field will interact many times with the nanostructures [4] and will excite multiple effects, such as photovoltaic [4, 5] and localized surface plasmon resonance [6, 7], etc. Based on this principle, dyesensitized solar cells have been configured with semiconductor nanowire arrays grown on the fiber’s surface, and high energy conversion efficiency has been realized [4, 5]. In addition, cladding-coupled optical fiber sensors with metal nanoparticle or nanowire arrays located on the fiber surface are configurable, which enables an expansion of their applications due to the multiplexing, distributed, real-time, remote, and in vivo sensing capabilities [6, 7]. Therefore, integrating
Nanostructure arrays have been extensively studied for their potential applications in next-generation nanoelectronic, photonic, optoelectronic, and photovoltaic devices, which are generally fabricated on flat, rigid substrates with dimensions that are no more than wafer scale. As a new flexible substrate, optical fibers allow electronic functions to be exploited over a larger length scale and to be woven into large two-dimensional or three-dimensional structures [1–3], which greatly extends the scope of their applications. Moreover, an optical fiber transmits light via total internal reflection at the core/ cladding interface. Therefore, if we substitute the traditional 0957-4484/15/155601+07$33.00
1
© 2015 IOP Publishing Ltd Printed in the UK
Z Zuo et al
Nanotechnology 26 (2015) 155601
nanostructures with optical fibers has great potential to improve performance with a wide variety of applications for the nanodevices. Many techniques have been developed to prepare onedimensional (1D) nanostructures (nanowires, nanorods, nanocones, etc.), such as chemical vapor deposition [8, 9], laser ablation [10], reactive ion etching [11, 12], metalassisted chemical etching [13–16], and others. However, these techniques cannot be easily transferred to optical fibers due to the fibers’ geometries. So far, only limited reports exist on the fabrication of 1D nanostructure arrays on the surface of optical fibers. ZnO nanowire arrays have been prepared via hydrothermal growth using zinc chloride and hexamine as sources [4, 5]. Using so-called oblique angle deposition (OAD), Ag, Al, Ni, Si, and TiO2 nanorod arrays have been successfully fabricated on the fiber surface [17, 18]. OAD is a versatile technique that can be used to fabricate nanorods from various materials. Nevertheless, it is difficult to fabricate nanostructures on a large length scale with this method. 1D silicon nanostructures have been widely investigated for their potential as fundamental building blocks for nanodevices. Although considerable efforts have been devoted to the fabrication [8–16], structural control [8, 9, 14, 19–22], and applications of 1D Si nanostructures [23–26], few results on integrating 1D Si nanostructure arrays on optical fibers have been reported. In view of its great importance to both fundamental research and practical applications, we report herein a new approach for the fabrication of Si nanorod and nanocone arrays on the surface of an optical fiber. The details of the fabrication process, structural properties, and formation mechanism are also provided.
Figure 1. Raman spectra of the as-deposited and annealed Si films
deposited on an optical fiber and on quartz glass, respectively.
The surface and the cross-sectional morphology of the samples were characterized using scanning electron microscopy (SEM, Hitachi S-4800). Raman spectroscopy measurements were performed at room temperature on a Raman system (HORIBA Jobin Yvon, LabRAM HR) using a 514 nm Ar+ laser line for excitation. Before the Raman measurement, the spectrum was calibrated with crystalline Si.
3. Results and discussion The broad Raman scattering peak of the as-deposited film shown in figure 1 is around 480 cm−1, indicating an amorphous structure. The annealing process crystallizes the a-Si film, resulting in the formation of a mc-Si film on the fiber. The intensive scattering peak of the annealed film at ∼514 cm−1 evidently deviates from that of bulk Si (∼520 cm−1), while the Raman spectrum from the sameannealed Si film on flat quartz glass shows almost the same peak position as the bulk Si. This result indicates the different microstructure of these two Si films; the nanostructure or high stress may be present within the film on the fiber [27, 28]. Figure 2(a) presents the SEM image of a cleaved cross section of the annealed Si film grown on the surface of the optical fiber, showing that the thickness of the film around the fiber is uniform in spite of the round geometry of the fiber, which is stationary during deposition. Large numbers of gaps and cracks are observed on the film surface (figures 2(b) and (c)), dividing the film into a lot of plots comprising many domains at a size of several hundred nanometers. Through the gaps, it can be identified that the film is made up of closely arranged columns, and the observed domains on the surface are just the top end of these columns. The magnified crosssectional image shown in figure 2(d) further reveals that the columns grow along the radial direction of the fiber and are perpendicular to the fiber surface. The estimated average diameter near the half height of the columns is about 270 nm, which increases to approximately 400 nm at the top surface of the 5.2 μm-thick film. This increase in the diameter along the
2. Experimental details Amorphous silicon (a-Si) film was deposited on the surface of a naked quartz optical fiber by plasma enhanced chemical vapor deposition, using silane (30 sccm, 5% diluted in H2) as the source gas. Prior to the deposition, an optical fiber with a length of 20 cm was washed using acetone to remove the polymer cladding, and then was hung stationary, parallel to the substrate plate. The substrate was not intentionally heated during the deposition. Plasma was produced using an excitation frequency of 13.56 MHz and a power density of 55 mW cm−2. The chamber pressure during the deposition was set constant at 750 mTorr. To obtain a multicrystal silicon (mc-Si) layer, high-temperature annealing was performed to crystallize the as-deposited a-Si in a tube furnace at 900 °C, with a ramping rate of 20 K min−1 and a dwelling of 60 min. The annealing was performed under vacuum, which was obtained by means of a rotary mechanical pump. Then the annealed sample was subjected to chemical etching in a solution containing 4.4 M hydrofluoric acid (HF) and 0.4 M hydrogen peroxide (H2O2) at room temperature [15, 16]. The etching process was also performed in a solution of 0.45 M HF and 0.1 M nitric acid (HNO3). After the etching, the sample was rinsed with deionized water and dried in air. 2
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Figure 2. SEM images of the annealed Si film deposited on the surface of the optical fiber. (a) Cross-sectional image, (b) plan-view image, (c)
magnified plan-view image, and (d) magnified cross-sectional image of the sample.
Further etching induces partial nanorods to peel off from the fiber surface, as shown in figure 3(e). The overall morphology of the nanorods can be clearly observed from the peeled nanorods. The diameter along the nanorod is not uniform, but rather gradually tapers from top to bottom, with a sharp root. Some twinned nanorods with their top portions linked together are also observed, as indicated by the arrow in figure 3(e). The peeling-off of the nanorods leaves the conelike nanostructure on the fiber surface. The magnified image of the nanocone array is shown in figure 3(f), which shows that the nanocones are separated from each other, and their tips are very sharp. We have carried out the same treatment of annealing and etching on the a-Si film deposited on a flat quartz substrate, but without observation of the nanorods or nanocones. SEM investigation indicates that the annealed Si film on the flat substrate is highly compact. No columnar microstructure is observed (see Supplementary Information, figure S1), which is entirely different from the film on the fiber (figure 2(d)). After etching, the morphology of the film has no evident change. Therefore, this composite nanostructure must relate to the particular microstructure of the a-Si film deposited on an optical fiber with a curved surface. Figure 4(b) presents the cross-sectional SEM image of the as-deposited a-Si film on the fiber. We can see an inverted-cone-like columnar structure similar to the annealed film, but with a more compact arrangement of these columns, with void-rich boundary regions between them [29]. Combining the observations shown in figure 3, we can conclude that the silicon film on the fiber exhibits a composite microstructure with
growth direction results in an inverted-cone-shaped geometry. These columns are close to each other, with small interstitials between them. Upon close examination, large numbers of nanoscaled granules bestrew the sidewall of the columns; this may be partially responsible for the deviation of the Raman peak. The mc-Si-covered fiber was immersed into HF/H2O2 solution to perform chemical etching. Figure 3(a) shows the morphology of the sample after etching for 10 min; an array with Si nanorods was formed all over the fiber surface. Compared with the as-annealed sample (figure 2(c)), it can be undoubtedly concluded that the nanorods form as a result of the columns separating from each other in the mc-Si film. However, numerous columns do not effectively separate under these etching conditions, instead forming sheaf-like structures on the micrometer scale. The magnified image shown in figure 3(b) reveals the uniform height of the nanorods in the array. Though some isolated nanorods are observable, most columns are undivided, with a cauliflowerlike morphology at the top, as usually observed on the surface of the mc-Si film. The columns evidently separate from each other after etching the sample for 30 min. Numerous nanorods with round tops and thin bottoms are clearly observed, as seen in figure 3(c). The average diameters of the nanorods are slightly smaller than those of the columns in the mc-Si film, while the sidewalls are still rough. From the cross-sectional image shown in figure 3(d), we can see that a cone-like nanostructure array is presented underneath the nanorod array, and the nanorods are in fact embedded in the nanocone array. 3
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Figure 3. (a) Plan-view image and (b) magnified plan-view image of the sample after etching for 10 min. (c) Plan-view image and (d) cross-
sectional image of the sample after etching for 30 min. (e) Plan-view image and (f) magnified local region of the sample after etching for 60 min.
Figure 4. (a) Plan-view image and (b) cross-sectional image of the as-deposited a-Si film on the surface of an optical fiber. The inset in (a) is a
low-resolution plan-view image of the sample.
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Figure 5. Schematic of the fabrication process of the composite nanostructure. (a) Annealed Si film is etched in solution. (b) Nanorod array forms as a result of the shrinkage and separation of the columns. (c) Nanocone array unveils as the nanorods peel off.
figure 3(c) and schematically in figure 5(b). During etching, the tensile stress within the film may also play an important role in the separation of the columns. Because the nanorods are embedded in a nanocone array, with a void-rich boundary region between the nanorods and nanocones, further chemical etching causes the boundary regions to be dissolved. Consequently, the nanorods peel off from the valleys of the nanocones, leaving the nanocone array on the fiber surface, as shown in figure 3(e) and schematically in figure 5(c). In summary, there are three steps during the fabrication of the nanostructure: (1) the formation of the columns—the nanorods are born out of the columns in the film, so columnar growth of the film during deposition is a requisite precondition; (2) preliminary detachment of the columns—annealing shrinks the dimension of the columns, resulting in them separating at the top and thus generating the cracks and the void-rich boundary regions between the columns; and (3) further detachment of the columns—a faster dissolution of the Si at the boundary region during etching further detaches the columns, forming a separated nanorod array. According to the previously mentioned formation mechanism, some other materials with curved surfaces maybe also act as substrates to fabricate similar nanostructures. However, the microstructure of the deposited film, which is dependent on substrate surface topography and surface chemistry, should be discrepant [32, 33], thus leading to different structures after etching. For example, we observed some rod-like structures on the NiCr filament surface (see Supplementary Information, figure S3), while we failed to obtain similar structures on Mo filaments (see Supplementary Information, figure S4). Furthermore, other mixed solutions can also work as the etching solution for the fabrication of the previously detailed composite nanostructure, as long as the reduction of the oxidant can supply electric holes for the oxidization of Si, and then the oxidized Si can be dissolved by HF. In fact, similar results have been obtained by using an HNO3/HF solution. We suppose that the nucleation density of the a-Si, the curvature of the fiber, and the annealing process could be the important factors in deciding the microstructure of the film and the separation of the columns. The structural control of the nanostructure could be realized; the height of the nanorods is determined by the thickness of the as-grown a-Si film, while the morphology is principally dependent on the morphology of the columns, which can be controlled by the initial
inverted-cone-shaped columns embedding in a cone-shaped array, as schematically shown in figure 5(a). In fact, fine cone-shaped structures are observable at the film/fiber interface in figures 2(d) and figure 4(b). Moreover, this microstructure comes from growth, rather than annealing. The definite growth mechanism behind this microstructure remains unclear at present, but it could be related to the low diffuse ability of the precursors on the growth surface at low deposition temperatures [30] and the gradually increasing deposition-zone area due to the growth along the radial direction of the fiber. Also, the as-deposited a-Si film on the fiber was etched in the HF/H2O2 solution, but failed to separate the columns to form a similar nanostructure to that of the annealed film (see Supplementary Information, figure S2). By comparing the films with and without annealing, one can see that the surface of the nonannealed film is more compact, with hardly any gaps or cracks (figure 4(a)). Annealing at high temperature leads to the formation of mc-Si, as demonstrated by the Raman spectrum shown in figure 1. Because crystal Si is more compact than a-Si, the reorganization of the Si atoms during annealing shrinks the dimension of the columns and leads to separation at the top, which might be responsible for the formation of the cracks on the surface. Meanwhile, hightemperature annealing relaxes the stress within the film, resulting in the generation of the gaps penetrating the film, as shown in figures 2(b) and (c). During chemical etching, the solution can easily penetrate into the film via the cracks and the void-rich boundary regions between the columns, and then oxidation-reduction reactions occur therein. The reduction of H2O2 generates electronic holes, h+, according to reaction 1 [31]: 2H + + H 2 O2 → 2H 2 O + 2h+
(1)
The generated electronic holes are injected into the valence band of the adjacent Si, leading to the oxidation and then to the dissolution of the Si, according to reaction 2 [31]: Si + 4h+ + 4HF → SiF4 + 4H + and SiF4 + 2HF → H 2 SiF6.
(2)
These reactions dissolve the Si more rapidly at the voidrich boundary regions due to the very loose structure found there, leading the columns to become detached. Meanwhile, the isotropic etching shrinks the size of the column, resulting in a slightly reduced height and diameter. Then, the separated nanorod array forms on the fiber surface, as shown in 5
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Acknowledgments This study was partly supported by the National Natural Science Foundation of the People’s Republic of China under Grant Nos. 61106011 and 11374015, and the Anhui Province Natural Science Foundation under Grant No. 1308085QF109.
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Figure 6. Plan-view SEM image of an array of nanorods with a
sunflower-seed appearance, which was transferred onto a conductive tape by directly rolling the fiber on the tape.
nucleation and the growth process. For example, using a power density of 75 mW cm−2 during the a-Si deposition, followed by the same annealing and chemical etching, an array of nanorods with a sunflower-seed appearance was obtained. Figure 6 shows the transferred array on a conductive tape, attained by directly rolling the fiber on the tape. Moreover, the dimensions can be further regulated by controlling the duration and the rate of the isotropic etching, which is related to the mixture ratio of the solution and the temperature. At present, due to the limitation of the chamber area, the length of the optical fiber that was covered with Si film is about 20 cm, but the scale could be significantly elongated by coiling the fiber. By using proper equipment like a pipe furnace, the fabrication could be extended to a larger length scale, and even roll-to-roll processing is possible. Further studies will be devoted to elucidating the growth mechanism of the a-Si film deposited on the optical fiber surface, which is critical for structural control. Also, the etching conditions—including the temperature, mixture ratio of the solution, duration, and so on—will be further optimized.
4. Conclusions In summary, we have prepared a composite Si nanostructure with a vertically aligned, inverted-cone-like nanorod array embedded in a nanocone array, through direct chemical etching of the mc-Si film deposited on optical fiber surface. A reasonable mechanism was proposed to elucidate the formation process of this composite structure. The morphology (and the height and diameter) of the nanorods was found to be primarily dependent on the microstructure of the columns in the mc-Si film, and it can be regulated by isotropic etching. This study presents a new method of fabricating nanostructures on optical fibers, and it might open a new pathway for integrating nanostructures with fibers in order to extend the application scope of nanodevices.
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