SiO2/Si core-shell nanowires array coated with gap-rich silver nanoparticles were demonstrated as a highly reproducible surface-enhanced Raman scattering (SERS) substrate. SERS detection of a relative standard deviation of 8% for 10−4 M R6G with a spot size of ∼2 μm and 900 spots over an area of 150 × 150 μm2 was reported. The high reproducibility is ascribed to the polarization-independent electrical field distribution among three-dimensional nanowire structure with an optimized thickness of SiO2 shell layer.published_or_final_versio

Huang, J

Liu, XY

Yang, B

Zhang, XJ

Zhu, YY

AIP Advances

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Title Highly reproducible SERS substrate based on polarization-freeAg nanoparticles decorated SiO2/Si core-shell nanowires arrayAuthor(s) Liu, XY; Huang, J; Yang, B; Zhang, XJ; Zhu, YYCitation AIP Advances, 2015, v. 5 n. 5, article no. 057159, p. 057159:1-5Issued Date 2015URL http://hdl.handle.net/10722/210726Rights Creative Commons: Attribution 3.0 Hong Kong LicenseHighly reproducible SERS substrate based on polarization-free Ag nanoparticlesdecorated SiO2/Si core-shell nanowires arrayXiao-Ying Liu, Jian-An Huang, Bo Yang, Xue-Jin Zhang, and Yong-Yuan Zhu  Citation: AIP Advances 5, 057159 (2015); doi: 10.1063/1.4921943 View online: http://dx.doi.org/10.1063/1.4921943 View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/5/5?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in An ultrasensitive, uniform and large-area surface-enhanced Raman scattering substrate based on Ag orAg/Au nanoparticles decorated Si nanocone arrays Appl. Phys. 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See:http://creativecommons.org/licenses/by/3.0/ Downloaded to IP:  147.8.230.137 On: Wed, 24 Jun 2015 03:56:21AIP ADVANCES 5, 057159 (2015)Highly reproducible SERS substrate basedon polarization-free Ag nanoparticles decorated SiO2/ Sicore-shell nanowires arrayXiao-Ying Liu,1,a Jian-An Huang,1,2,a Bo Yang,1 Xue-Jin Zhang,1,band Yong-Yuan Zhu1,c1Key Laboratory of Modern Acoustics and National Laboratory of Solid StateMicrostructures, School of Physics, Nanjing University, Nanjing 210093, China2Department of Electrical and Electronic Engineering, University of Hong Kong,Hong Kong SAR, China(Received 20 March 2015; accepted 12 May 2015; published online 28 May 2015)SiO2/Si core-shell nanowires array coated with gap-rich silver nanoparticles weredemonstrated as a highly reproducible surface-enhanced Raman scattering (SERS)substrate. SERS detection of a relative standard deviation of 8% for 10−4 M R6Gwith a spot size of ∼2 µm and 900 spots over an area of 150 × 150 µm2 was reported.The high reproducibility is ascribed to the polarization-independent electrical fielddistribution among three-dimensional nanowire structure with an optimized thicknessof SiO2 shell layer. C 2015 Author(s). All article content, except where otherwisenoted, is licensed under a Creative Commons Attribution 3.0 Unported License.[http://dx.doi.org/10.1063/1.4921943]Surface enhanced Raman scattering (SERS) relies on hot spots to identify the chemical finger-prints.1 The hot spots have extremely strong electric field and are usually generated by localizedsurface plasmon resonance (LSPR). The electrical field coupling of LSPR in the gap of metalnanoparticle dimmer can result in a field enhancement up to 108.1,2 Although single-molecule SERSdetection could be realized with strong coupled LSPR field, the signals were often accompaniedwith low reproducibility and poor stability.3 The commonly used solid-state SERS substrates offerhighly ordered metal nanostructures for the generation of relatively uniform hot spots, such assilver nanoparticles (AgNPs) array,4–7 silver nanotriangles array,8 and silver film-over-nanosphere(AgFON).1 However, the SERS performance of the above gap-based structures is closely related tothe polarization of incident laser. The highest electric field enhancement appears if the excitationpolarization is parallel to the metal nanoparticle dimmer axis.9 This leads to polarization-dependentSERS signals in spot-to-spot Raman detection, which results in low signal reproducibility.In this letter, we developed hexagonal-packed SiO2/Si core-shell nanowires array with AgNPsdecoration (AgNPs@SiO2/SiNWs array) as a high reproducible gap-based SERS substrate. Herethe optical mode guided along the direction of nanowire axis participates in the enhancement pro-cess. As a result, strong coupled electrical field can be generated in the gaps between neighboringAgNPs aligned along the direction of nanowire axis. The enhanced field can also be induced inthe gaps of adjacent AgNPs within the cross section of the nanowire which originates from thedirect excitation of incident laser. These two factors will contribute to the SERS. By adjusting theSiO2 thickness, a polarization insensitive gap-based SERS substrate with high reproducibility canbe realized.The silicon nanowires array was first fabricated by nanosphere lithography and metal-assistedchemical etching,10 and then the substrate was annealed at 900˚C and 1 Bar in air for 2 hours toform an oxide layer on silicon surface. After that, the SiO2/SiNWs array was covered with silveraX. Y. Liu and J. A. Huang contributed equally to this work.bEmail: xuejinzh@nju.edu.cncEmail: yyzhu@nju.edu.cn2158-3226/2015/5(5)/057159/5 5, 057159-1 ©Author(s) 2015 All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license. See:http://creativecommons.org/licenses/by/3.0/ Downloaded to IP:  147.8.230.137 On: Wed, 24 Jun 2015 03:56:21057159-2 Liu et al. AIP Advances 5, 057159 (2015)FIG. 1. (a) Cross-sectional SEM image of Ag nanopar ticle decorated SiO2/SiNWs of 150 nm in diameter and ∼1 µm2 inlength and (b) hexagonally packed AgNPs@SiO2/SiNWs at 52˚ visual angle and (c) TEM image of SiO2/Si nanowire after 2hours annealing, the thickness of SiO2 layer is ∼40 nm. Scale bars are 200 nm.by ion beam sputtering at an optimized duration of 4 min under 7 kev and 300 µA. As shown inFig. 1(a), the as-fabricated AgNPs@SiO2/SiNWs was hexagonally packed with an areal density of12.8 wires/ µm2. The wire diameter and length are respectively ∼150 nm and ∼1 µm. The scanningelectron microscope (SEM) image (Fig. 1(b)) indicates that high density AgNPs were uniformlydistributed on the surface of the hexagonal-packed SiO2/SiNWs. The thickness of SiO2 layer is∼40 nm after 2 hours’ annealing, which is confirmed by transmission electron microscope (TEM)measurements as shown in Fig. 1(c).The SERS measurements were performed with a confocal Raman system (Horiba Scien-tific, LabRAM HR Raman spectrometer). A 2 µL aqueous solution of Rhodamine 6G (R6G,Sigma-Aldrich) was dropped onto the substrate, spreading out as a circle of ∼7 mm in diameter.After evaporation, the sample was excited by a He-Ne laser at wavelength of 633 nm via 50× and10× objectives for Raman signal detection.To evaluate the spot-to-spot SERS reproducibility of AgNPs@SiO2/SiNWs array, we carriedout mapping measurements of Raman signals with a spot size of ∼2 µm over 150 × 150 µm2 under a50× objective and ∼4 µm over 450 × 450 µm2 under a 10× objective, respectively. Another orderedgap-based SERS substrate, AgFON, which was made by coating ∼120-nm-thick silver layer onhexagonal-packed polystyrene nanospheres of 300 nm in diameter, was prepared for the controlexperiments. A comparison between them with regard to 1362 cm−1 peak of R6G (10−4 M) isshown in Fig. 2. Figure 2(a) exhibits high uniform R6G spectra from AgNPs@SiO2/SiNWs arraywith RSD of 8.3% under a 50× objective. The RSD value changes a little with the laser spot. Forexample, the RSD was measured to be 8.7% when a 10× objective was used, as shown in Fig. 2(b).In contrast, the RSDs of R6G spectra from the AgFON (Fig. 2(c)) was measured as 32.1% undera 50× objective, whereas the RSD decreased to be 13.9% (Fig. 2(d)) under a 10× objective. Thedifference between Figs. 2(c) and 2(d) can be explained by the averaging effect.11 The above resultsshow that the AgNPs@SiO2/SiNWs array structure is superior to the AgFON structure as for theperformance of reproducibility, which can be maintained at a spot size as small as ∼2 µm. To calcu-late the enhancement factor (EF), 4-Aminothiophonol (4-ABT) was used as the probe moleculefor it can form a monolayer on the Ag surface.4 The experimental Raman EF was calculated to be6 × 105.12We further studied the influence of the concentrations of R6G aqueous solution on the SERSspectra of on AgNPs@SiO2/SiNW array. Figure 3(a) shows the SERS spectra for four differentconcentrations of R6G aqueous solution. The integration time is 5s. Figure 3(b) illustrates a mapp-ing measurement with 10−6 M R6G aqueous solution, and the RSD is calculated as ∼7%. Aboveresults shows that the reproducibility can keep very high for the concentration of R6G aqueoussolution as low as 10−6 M.The core-shell structure of AgNPs@SiO2/SiNWs SERS array was investigated by the simula-tion based on three-dimensional finite-difference time-domain (FDTD) software (Lumerical FDTDSolutions). The structures and geometrical parameters correspond to those in experiments, i.e.,SiO2/Si core-shell nanowires of 150 nm in outside diameter and 1 µm in length with different thick-nesses of SiO2 layer are close packed with AgNPs. The diameter and interval distance of AgNPs are All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license. See:http://creativecommons.org/licenses/by/3.0/ Downloaded to IP:  147.8.230.137 On: Wed, 24 Jun 2015 03:56:21057159-3 Liu et al. AIP Advances 5, 057159 (2015)FIG. 2. Raman mapping images of 1362 cm−1 band of R6G from substrates of AgNPs@SiO2/SiNWs array under (a) a 50×objective and (b) a 10× objective, and AgFON under (c) a 50× objective and (d) a 10× objective.set as 20 nm and 13 nm respectively. The light source is an x-polarized plane wave at wavelength of633 nm. Figure 4 shows the electrical field intensity distribution for three representative nanowires.For silicon nanowire, the coupled LSPR electrical field in the gap of AgNPs is degenerated inthe cross section of an AgNPs@SiO2/SiNW, as shown in Fig. 4(a). At wavelength of 633 nm, therefractive index of Si is about 3.88, while that of SiO2 is about 1.46. Thus the AgNPs@SiO2/SiNWswith thin SiO2 shell layer, i.e. thick Si core, is likely to support guided modes.13 The degenerationof electrical field in the cross section (Fig. 4(a)) comes from the generation of guided mode alongthe nanowire, which results in the redistribution of the electrical field. Being associated with theguided mode, strong coupled electrical field can distribute in the gap of AgNPs dimmers alignedalong the nanowire axis direction, which is confirmed by Fig. 4(b). By comparison, for a SiO2nanowire, the coupled LSPR electrical field occurs in the gap of AgNPs, consistent with the incidentpolarization, as shown in Fig. 4(c). Figure 4(d) presents weak electrical field distributed along thenanowire axis direction in the absence of guided modes. It is expected that there is an intermediatethickness of SiO2 shell layer that the above two factors work together to generate a uniform fieldenhancement around the nanowire surface. Figures 4(e) and 4(f) refer to an intermediate case ofcore-shell SiO2/SiNWs with 45 nm SiO2 shell layer. It can be seen that both the electrical fieldFIG. 3. (a) Spectra of evaporated R6G aqueous solution with different concentrations on AgNPs@SiO2/SiNWs array. (b)Raman mapping image of 10−6M R6G 1362 cm−1 band of R6G from AgNPs@SiO2/SiNWs array under 50× objective. All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license. See:http://creativecommons.org/licenses/by/3.0/ Downloaded to IP:  147.8.230.137 On: Wed, 24 Jun 2015 03:56:21057159-4 Liu et al. AIP Advances 5, 057159 (2015)FIG. 4. Electrical field intensity distribution in the cross-sectional and vertical directions of (a, b) Si nanowire and (c, d) SiO2nanowire and (e, f) SiO2/Si core-shell nanowire. The excitation light is x-polarized.intensities in the cross-sectional and vertical directions are rather strong. The strong field enhancesthe Raman scattering. The Raman photons emitted from R6G inside the gap of AgNPs wouldbe either absorbed or experience multiple scattering. Those escaped from the hexagonal-packednanowire array would thus contribute to the detected Raman signal. Figures 4(e) and 4(f) also showthat the electrical field is distributed with weakened anisotropy three-dimensionally. It will lead tothe polarization independence and consequently the high SERS reproducibility.The above analyses suggest an optimum thickness of SiO2 shell layer that gives rise to uni-formly distributed hot spots around the SiO2/Si core-shell nanowire surface, hence the most repro-ducible SERS detection. This was experimentally verified by several SiO2/SiNWs array substrates.The samples were treated by thermal annealing with annealing periods varied from 0 to 4 hrsat 900˚C and 1 Bar in air, resulting in different thickness of SiO2 shell layer. Other experimentalprocesses are the same as the above-mentioned. SERS measurements were taken with a spot size of∼2 µm over 150 × 150 µm2 under a 50× objective. Figure 5 shows that the highest reproducibilitywith the RSD value as low as 8% takes place for the annealing duration of 2-3 hrs. It also showsthat the SERS-active capability of AgNPs@SiO2/SiNWs array (Fig. 5(a)) is better than that ofAgNPs/SiNWs array (Fig. 5(b)). The polarization-independent “hot spots” generation may accountfor the above behavior. All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license. See:http://creativecommons.org/licenses/by/3.0/ Downloaded to IP:  147.8.230.137 On: Wed, 24 Jun 2015 03:56:21057159-5 Liu et al. AIP Advances 5, 057159 (2015)FIG. 5. Raman mapping images of 1362 cm−1 band of R6G from substrates of (a) AgNPs@SiO2/SiNWs array (2 hrsannealing), and (b) AgNPs/SiNWs array (0 hrs annealing). (c) Relationship between the annealing period and the measuredRSDs.In conclusion, we characterized the RSD of SERS signals from AgNPs@SiO2/SiNWs arraysubstrates. With the optimization of SiO2 shell layer, the RSD decreases to 8% from original 13% ofAgNPs@SiNWs array substrates. This stems from the tunability of three-dimensional distributionof hot spots around the nanowire surface. The developed AgNPs@SiO2/SiNWs array substratesmay serve as practical SERS detection with high reproducibility.ACKNOWLEDGEMENTThis work was supported by the State Key Program for Basic Research of China (GrantNo.2012CB921502), by the National Natural Science Foundation of China (Grant Nos. 11274159,11174128 and 11374150) and by PAPD.1 S. L. Kleinman, R. R. Frontiera, A. I. Henry, J. A. Dieringer, and R. P. Van Duyne, Phys. Chem. Chem. Phys. 15, 21 (2013).2 E. Petryayeva and U. J. Krull, Anal. Chim. Acta 706, 8 (2011).3 S. Lal, N. K. Grady, J. Kundu, C. S. Levin, J. B. Lassiter, and N. J. Halas, Chem. Soc. Rev. 37, 898 (2008).4 H. Wang, C. S. Levin, and N. J. Halas, J. Am. Chem. Soc 123, 14992 (2005).5 J. P. Camden, J. A. Dieringer, J. Zhao, and R. P. Van Duyne, Acc. Chem. Res. 41, 1653 (2008).6 L. B. Luo, L. M. Chen, M. L. Zhang, Z. B. He, W. F. Zhang, G. D. Yuan, W. J. Zhang, and S. T. Lee, J. Phys. Chem. C 113,9191 (2009).7 L. F. He, J. A. Huang, T. T. Xu, L. M. Chen, K. Zhang, S. T. Han, Y. He, and S. T. Lee, J. Mater. Chem 22, 1370 (2012).8 S. Fayyaz, M. Tabatabaei, R. Hou, and F. Lagugne-Labarthet, J. Phys. Chem. C 116, 11665 (2012).9 M. Ringler, T. A. Klar, A. Schwemer, A. S. Susha, J. Stehr, G. Raschke, S. Funk, M. Borowski, A. Nichtl, K. Kurzinger, R.T. Phillips, and J. Feldmann, Nano Lett. 7, 2753 (2007).10 J. A. Huang, Y. Q. Zhao, X. J. Zhang, L. B. Luo, Y. K. Liu, J. A. Zapien, C. Surya, and S. T. Lee, Appl. Phys. Lett. 98,183108 (2011).11 Y. J. Oh and K. H. Jeong, Adv. Mater. 24, 2234 (2012).12 E. C. Le Ru, E. Blackie, M. Meyer, and P. G. Etchegoin, J. Phys. Chem. C 111, 13794-13803 (2007).13 K. Seo, M. Wober, P. Steinvurzel, E. Schonbrun, Y. Dan, T. Ellenbogen, and K. B. Crozier, Nano Lett. 11, 1851 (2011). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license. 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Highly reproducible SERS substrate based on polarization-free Ag nanoparticles decorated SiO2/Si core-shell nanowires array

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Highly reproducible SERS substrate based on polarization-free Ag nanoparticles decorated SiO2/Si core-shell nanowires array

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