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A facile fabrication of Ag-Au-Ag nanostructures with nanogaps for intensified surface-enhanced Raman scattering
Applied Surface Science 389 (2016) 67–72
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
A facile fabrication of Ag-Au-Ag nanostructures with nanogaps for intensified surface-enhanced Raman scattering Liangliang Jin a,b , Guangwei She a,∗ , Jing Li a , Jing Xia a,b , Xiaotian Wang c , Lixuan Mu a , Wensheng Shi a,∗ a Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China b University of Chinese Academy of Sciences, Beijing 100049, China c School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore
a r t i c l e
i n f o
Article history: Received 8 June 2016 Received in revised form 7 July 2016 Accepted 8 July 2016 Available online 14 July 2016 Keywords: Nanogap Surface-enhanced Raman scattering (SERS) Electrodynamics simulation
a b s t r a c t Nanogap between two metallic nanostructures has been demonstrated to be able to efficiently concentrate an incident electromagnetic field into a small space. As a result, the formed strong field localization could extraordinarily enhance the surface-enhanced Raman scattering (SERS). In this study, controllable plasmonic nanogaps are formed by separating two layers of plasmonic Ag nanoparticles (50–100 nm) with small Au nanoparticles (2.5–6 nm). The size of the nanogaps can be readily tuned by altering the size of the Au nanoparticles. Utilizing an SERS substrate with such nanogaps, the SERS performance can be significantly improved. Such improvement could be attributed to the strongly enhanced electric field within the nanogaps, which is demonstrated by the Finite-difference time-domain simulations. The present work provides a facile strategy to rationally fabricate SERS substrates with controllable nanogaps and intensified SERS signals. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Surface-enhanced Raman scattering (SERS) is an optical spectroscopic technique which amplifies the sensitivity of classical Raman spectroscopy. Owing to its high identification and sensitivity, SERS has attracted increasing attention in diverse areas, including chemistry, biology, food safety and biomedical diagnostics [1,2]. The dramatic variations in the SERS intensification have been attributed to the formation of localized plasmons or the so-called “hot spots” [3]. The electromagnetic “hot spot” is a location with strongly enhanced local field, which occurs near sharp asperities or forms at the nanoscale gaps between the metal nanostructures [4]. Shortrange electromagnetic interactions at “hot spots” could lead to enormous Raman enhancement factors, even realize the singlemolecule detection [5,6]. It is well known that the “hot spots” strongly depend on the shape of metal nanoparticles (NPs) as well as the interparticle distance [2,7]. Metallic nanostructures with various morphologies and interparticle spaces, such as nanocubes, nanospheres, nano-octahedrons, nanostars, nanowires and nanopyramids, have been explored to produce the “hot spots” and enhance the Raman signals [6,8–12].
∗ Corresponding authors. E-mail addresses: [email protected] (G. She), [email protected] (W. Shi). http://dx.doi.org/10.1016/j.apsusc.2016.07.066 0169-4332/© 2016 Elsevier B.V. All rights reserved.
Since the nanogaps in SERS substrates can provide abundant “hot spots” and improve the SERS performance, the structures with controllable nanogaps have been given much of interest [13,14]. Various methods have focused on the fabrication of SERS substrates with nanogaps. Several sophisticated lithographic methods, such as electron-beam lithography, nanosphere lithography, focused ion-beam patterning and soft lithography are commonly used to produce tightly spaced plasmonic geometries [15]. Other methods, such as physical/chemical vapor deposition, atomic layer deposition and template technique, are also developed to fabricate nanogap structures [16]. However, although lithographic methods can accurately control the inter-particle distance from tens of nanometers to ∼1 nm, highly specialized lithographic facilities are complex and expensive [17,18]. The other methods are relatively inexpensive, but the tedious preparation and implementation steps require accuracy and time consumption [16,18,19]. It is still a challenge to establish a rational and facile method for the fabrication of high-quality SERS substrates with reliable nanogaps. In this study, we develop a promising strategy to fabricate AgAu-Ag SERS substrates with deliberate nanogaps. In the present strategy, the reductive Si-H bonds on the surface of Si were utilized to reduce Ag+ ions into Ag nanoparticles (Ag NPs) on the surface of Si wafer. Then the uniform Au nanoparticles (Au NPs) with tunable sizes were sputtered onto the Ag NPs-coated Si wafer. Finally, a second layer of Ag NPs prepared by reducing
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AgNO3 with ethylene glycol (EG) was spin-coated onto the Si wafer with Ag and Au nanoparticles. As a result, the nanogaps were deliberately formed between the two layers of Ag NPs by the inset of Au NPs. The nanogaps could be reliably tuned by altering the size and distribution of the Au NPs, which were controlled by the sputtering process. The as-fabricated Ag-Au-Ag SERS substrate with deliberate nanogaps exhibits an improved performance in the detection of 4-Mercaptopyridine (4-MPY) molecules. Finite-difference time-domain (FDTD) simulations indicate that the improved SERS performance could be attributed to the strongly enhanced electric field within the nanogaps. 2. Experimental details 2.1. Preparation of the Ag-Au-Ag SERS substrate The Ag-Au-Ag SERS substrate consists of the first layer of Ag NPs, the interlayer of Au NPs and the second layer of Ag NPs. The fabrication processes are schematically illustrated in Fig. 1a (the
3-dimensional schematic) and Fig. 1b (the cross-sectional schematic). The first layer of Ag NPs was prepared by immersing the cleaned Si wafer into an aqueous solution consisting of 5 mM AgNO3 and 4.8 M HF for 10 s. Si reacted with HF to form the reductive Si-H bonds on the surface of Si wafer [20]. Then, the Ag+ ions were reduced by the Si-H bonds and Ag NPs were formed on the surface of the Si wafer. After rinsing thoroughly with deionized water, the obtained Ag NPs on Si wafer were annealed at 180 ◦ C for 20 min in the Ar atmosphere and the first layer of Ag NPs was obtained. Then the Au NPs were deposited onto the Si wafer with Ag NPs for a determinate time by ion sputter equipment (Hitichi E-1010) with the vacuum at 10 Pa and discharge current at 10 mA. Finally, the second layer of Ag NPs was spin-coated onto the Si wafer with Ag and Au nanoparticles. The Ag NPs used for spincoating were synthesized by reducing AgNO3 with ethylene glycol (EG) in the presence of Poly (N-vinylpyrrolidone) (PVP-K30, Mw & 40,000, the concentration was calculated in terms of the repeating unit). Briefly, a 10 mL EG solution of 0.1 M PVP was injected into 10 mL of a magnetically stirred EG solution of AgNO3 (0.1 M).
Fig. 1. (a) 3-dimensional and (b) cross-section schematic illustration of the sample preparation process for the Ag-Au-Ag substrate.
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Fig. 2. SEM image (a) and 3D AFM image (b) of Ag1 NP, insert image in (a) shows the amplified SEM image of Ag1 NP. (c) TEM image of Au NPs with the sputtering time of 5s, insert images show the HRTEM image and the SAED pattern of Au NPs. (d) TEM image of Ag1 NP-Au NP, insert image shows the SAED pattern of the sample. SEM image (e) and 3D AFM image (f) of the fabricated Ag1 NP-Au NP-Ag2 NP structure, insert image in (e) shows the amplified SEM image of Ag1 NP-Au NP-Ag2 NP structure.
Afterwards, the solution was put into a 25 mL Teflon-lined autoclave tube. This tube was sealed and maintained at 160 ◦ C for 20 min, followed by natural cooling to room temperature (25 ◦ C). Ag NPs were separated from EG by addition of a large amount of ethanol, followed by sonication and centrifugation. Then the Ag NPs were dispersed in 10 mL ethanol solution for spin-coating. 2.2. Characterization of the nanoparticles and the substrates The morphologies of the samples were characterized by a field emission scanning electron microscopy (FESEM, Hitachi S4800) and atomic force microscopy (Bruker Multimode 8) under tapping-mode. The kind of tip for AFM images was Bruker RTESPA. The structures of the samples were investigated using high
resolution transmission electron microscopy (HRTEM, JEM-2100F, JEOL). Optical absorption spectra were measured using a UV–visNIR Spectrophotometer (Varian, UV–vis-NIR Cary 5000). Raman and SERS spectral measurements were carried out by using a confocal Raman spectrometer (Renishaw inVia Raman Microscope) with 532 nm excitations. The excitation intensity is about 0.5 mW and data acquisition time is 10 s throughout these experiments. The laser beam was focused on a spot about 3 m in diameter by a 50 × microscope objective. The Raman band of a silicon wafer at 520 cm−1 was employed to calibrate the spectrometer. 4-Mercaptopyridine (4-MPY) was adopted as the analyte. The substrates were immersed in 1 mM 4-MPY ethanol solution for 2 h and then thoroughly rinsed for SERS signal measurements.
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Fig. 3. SERS spectra of 1 mM 4-MPY on different substrates, Ag1 NP (curve a), Ag1 NP-Au NP(5s) (curve b), Ag1 NP-Ag2 NP (curve c) and Ag1 NP-Au NP(5s)-Ag2 NP (curve d).
3. Results and discussion For convenience, in the following discussion we shall name the first layer of Ag NPs as ‘Ag1 NP’, Ag1 NP with the interlayer of Au NPs as ‘Ag1 NP-Au NP’ and the Ag-Au-Ag substrate as ‘Ag1 NP-Au NPAg2 NP’. As illustrated by the schematic, the distance between Ag1 NP and Ag2 NP can be tuned by the size and distribution of Au NPs. The distance could be nanoscale when the size of Au NPs is about several nanometers. Fig. 2a–f show the SEM, AFM and TEM images of the samples sequentially obtained. Ag1 NP exhibit sphere-like shape and have an average diameter of about 50 nm, as shown in the SEM image of Fig. 2a. The 3D AFM topographical image of Ag1 NP is identical with the SEM image, as shown in Fig. 2b. The TEM and HRTEM images of the Au NPs which were ion sputtered are shown in Fig. 2c, exhibiting an average size about 2.5 nm. The corresponding SAED pattern of the Au NP is also shown in the insert image in Fig. 2c. The obvious diffraction rings marked as (111), (022) can be indexed to the polycrystalline Au with a cubic structure. The morphologies of Ag1 NP-Au NP can be illustrated in Fig. 2d. The corresponding SAED pattern of Ag1 NP-Au NP is shown in the insert image in Fig. 2d. The diffraction spots marked as (220), (3–11) can be indexed to the single crystalline Ag with a cubic structure. The diffraction rings marked as (111) in the insert image in Fig. 2d can be indexed to the polycrystalline Au. It can be confirmed from Fig. 2d that the Au NPs have been successfully sputtered onto the Ag1 NP. The Au NPs are separately distributed and the distance between each other of sputtered Au NPs is about 5 nm. When the colloid Ag NPs were spin-coated onto the Ag1 NP-Au NP to act as the Ag2 NP, the nanogaps would occur between Ag1 NP and Ag2 NP as a consequence of a spacing function of the sputtered Au NPs, as illustrated in Fig. 1. Fig. 2e and f show the SEM and 3D AFM images of the fabricated Ag1 NP-Au NP(5s)-Ag2 NP hybrid nanoparticles substrate. Both the SEM and AFM topographical images demonstrate that the Ag-Au-Ag SERS substrate has been successfully fabricated. With the second layer Ag NPs and the deliberate nanogaps between the two layers of Ag NPs, it is expected that the substrate has the potential to show enhanced SERS signals. The SERS performance of the as-prepared Ag1 NP-Au NP(5s)Ag2 NP substrate was investigated by using 4-MPY as the analyte, as shown in Fig. 3. For comparison, the Raman spectra of 4-MPY with the same concentration on Ag1 NP, Ag1 NP-Au NP and the substrate consisting of Ag1 NPs and Ag2 NPs but without Au NPs to produce the nanogaps (Ag1 NP-Ag2 NP) were also measured, respectively. The strongest SERS signal was observed from the Ag1 NP-Au NP(5s)-Ag2 NP substrate, which is ten times that from Ag1 NP substrate and twice that from Ag1 NP-Ag2 NP substrate. As
Fig. 4. Absorption spectra of Ag1-Ag2 NP (curve a), Ag1-Au NP(5s)-Ag2 NP (curve b), Ag1-Au NP(10s)-Ag2 NP (curve c) and Ag1-Au NP(15s)-Ag2 NP (curve d).
the consistency of the first and second layer Ag NPs for Ag1 NPAu NP(5s)-Ag2 NP and Ag1 NP-Ag2 NP substrates, the significantly intensified SERS could be attributed to the Au NPs interlayer. The introduction of the Au NPs interlayer would produce nanogaps between the two layers of Ag NPs and it may also influence the optical absorption of the substrate. The produced nanogaps and the possible changes of optical absorption are prospective to affect the SERS performance of the substrate. The optical properties of Ag1 NP-Au NP-Ag2 NP substrates have been investigated as a function of a series of sputtering times. Fig. 4a shows the curves of optical absorption versus wavelength for Ag1 NP-Ag2 NP substrate and Ag1 NP-Au NP-Ag2 NP substrates with the sputtering time of 5 s, 10 s, and 15 s, respectively. As we can see, the resonant wavelength of optical absorption is located at 400 nm for Ag1 NP-Ag2 NP substrate, which represents the intrinsic surface plasmon resonance (SPR) of Ag NPs [21]. The resonant wavelength belonging to the intrinsic SPR of Au NPs is generally located at over 500 nm, slightly varying with the nanoparticle sizes [22]. With increasing the sputtering time, the resonant wavelength of optical absorption for the samples moves to longer wavelength. The observed red-shift of the resonant wavelength represents the optical coupling in the hybrid systems [23,24]. It should be mentioned that the absorption of Ag1-Au NP-Ag2 NP substrates at longer wavelength in Fig. 4 became reduced with the increasing sputtering time of the Au NPs. The absorption at longer wavelength corresponds to the absorption of Si wafer, whose absorption spectrum is shown in Fig. S1. The declined absorption of the Au NP samples implies that the thicker film of Au blocked the absorption of Si wafer which was used as the basement of the substrate. The coupling effects in Ag1 NP-Au NP-Ag2 NP substrates are investigated by the electrodynamics simulations. The electrodynamics simulations were performed by using commercial FDTD software (Lumerical Solution) to investigate the coupling effects in Ag1 NP-Ag2 NP and Ag1 NP-Au NP-Ag2 NP hybrids. The simulation configurations are depicted in Fig. 5a and c. Here, the diameter of Ag1 NP is set as 50 nm and Ag2 NPs is set as 100 nm for both Ag1 NP-Ag2 NP and Ag1 NP-Au NP-Ag2 NP, which are in accordance with the experimental diameters of Ag1 NP and Ag2 NP. The diameter of Au NP is set as 2.5 nm and the distance between each other of Au NP is set as 5 nm for Ag1 NP-Au NP(5s)-Ag2 NP hybrid, which are in accordance with the experimental diameter and distribution of sputtered Au NP onto Ag1 NP. As the simulation configuration shown in Fig. 5c, the nanogap between the two layers of Ag NPs can be calculated and the result is 2.26 nm. For the simulation, the refractive indexes of Au and Ag are 0.5328 + 2.194i and 0.0542+ 3.434i at a wavelength of 532 nm, respectively, according to the experimentally measured data from Johnson and Christy
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Fig. 5. (a) Simulation configuration of Ag1 NP-Ag2 NP. Comparing with the actual substrate, the configuration is rotated to be vertical for convenient. And the angle of the incident plane wave is 30◦ accordingly. (b) Simulated electrical field intensity distributions of Ag1 NP-Ag2 NP in the yz plane. The color bar demonstrates the electric field value of ln(|E|)2 which is normalized by the incident field. (c) Simulation configuration of Ag1 NP-Au NP(5)-Ag2 NP. Comparing with the actual substrate, the configuration is rotated to be vertical for convenient. And the angle of the incident plane wave is 30◦ accordingly. The diameter of Au NP is 2.5 nm and the distance between each Au NP is 5 nm. The calculated gap is 2.26 nm. (d) Simulated electrical field intensity distributions of Ag1 NP-Au NP(5s)-Ag2 NP in the yz plane. The color bar demonstrates the electric field value of ln(|E|)2 which is normalized by the incident fiel.
[25]. Fig. 5b and d show the simulated electrical field intensity distributions in the yz plane. It can be clearly seen that the localized electric field is mainly distributed at the nanogaps formed between the Ag NPs for both simulations. However, the electric field has been strongly enhanced for Ag1 NP-Au NP(5s)-Ag2 NP hybrid, leading to a maximal enhancement of 191, while the maximal enhancement of electric field is 85 for Ag1 NP-Ag2 NP without Au NP to tune the nanogaps. The strongly enhancement of electric field represents the enhanced coupling effect in the substrate and could contribute to the SERS enhancement for the Ag1 NP-Au NP-Ag2 NP substrate. Moreover, the nanogap region consisting of enhanced electric field has been expanded for Ag1 NP-Au NP(5s)-Ag2 NP hybrid, as shown in Fig. 5d. More analyte molecules could be within the expanded nanogap region and contribute to the SERS enhancement. In the experiments, the nanogaps would be tuned by the size and distribution of the Au NPs, which were controlled by the sputtering process. The TEM image of the Au NPs with ion sputtering time 15 s is shown in Fig. S2. The distance and size of the Au NPs for Au NP (5 s and 15 s) are different. With increasing the sputtering time, the diameter of the Au NPs was getting bigger and the distance between each of the Au NPs was getting larger. The Au NPs with long sputtering time would induce enlarged nanogap for Ag1 NP-Au NP-Ag2 NP substrate and the plasmonic coupling in the nanogap region would change. A similar electrodynamics simulation was performed for Ag1 NP-Au NP(15s)-Ag2 NP hybrid as shown in Fig. S3. The maximal enhancement of electric field is 94 which is inferior compared with Ag1 NP-Au NP(5s)-Ag2 NP. The intensity of the SERS signal of the target analyte from Ag1 NP-Au NP(15s)-Ag2 NP is also inferior compared with that of Ag1 NP-Au NP(5s)-Ag2 NP substrate, as shown in Fig.S4. The decreased electric field of the enlarged nanogap is responsible for the inferior SERS signal.
4. Conclusion Ag-Au-Ag nanostructures with nanogaps were fabricated by a facile method for intensified SERS signal. The size of nanogaps can be effectively tuned by the size and distribution of the ionsputtered Au NPs. An appropriate size of Au NPs is obtained by controlling the sputtering duration as 5 s and enhanced SERS intensity is achieved. The electrodynamics calculation reveals that the electrical field intensity between two layers of Ag NPs could be significantly enhanced by introducing proper nanogaps. The improved SERS performance is attributed to the strongly enhanced electric field in the nanogaps and the expanded nanogap region which could allow more analyte molecules to be within in the gap region. The proposed method by using small nanoparticles to tune the nanogaps is expected to be a promising strategy for reliable control of nanogaps and intensified SERS signal.
Acknowledgements This work was supported by Chinese Academy of Sciences (Grant KGZD-EW-T02), Most of China (Grant 2016YFA0200800), NSFC (Grants 51272258, 51272302, 91333119 and 61307065) and Youth Innovation Promotion Association CAS.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2016.07. 066.
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References [1] C. Farcau, S. Astilean, Mapping the SERS efficiency and hot-Spots localization on gold film over nanospheres substrates, J. Phys. Chem. C 114 (2010) 11717–11722. [2] W. Y. Zhao, Z. Zeng, P. Tao, Y. Xiong, Y. Zhu Qu, Highly sensitive surface-enhanced Raman scattering based on multi-dimensional plasmonic coupling in Au–graphene–Ag hybrids, Chem. Commun. 51 (2015) 866–869. [3] C.E. Talley, J.B. Jackson, C. Oubre, N.K. Grady, C.W. Hollars, S.M. Lane, T.R. Huser, P. Nordlander, N.J. Halas, Surface-enhanced Raman scattering from individual au nanoparticles and nanoparticle dimer substrates, Nano Lett. 5 (2005) 1569–1574. [4] X. Li, W.C.H. Choy, X. Ren, D. Zhang, H. Lu, Highly intensified surface enhanced raman scattering by using monolayer graphene as the nanospacer of metal film-metal nanoparticle coupling system, Adv. Funct. Mater. 24 (2014) 3114–3122. [5] W. Li, P.H. Camargo, X. Lu, Y. Xia, Dimers of silver nanospheres: facile synthesis and their use as hot spots for surface-enhanced Raman scattering, Nano Lett. 9 (2009) 485–490. [6] M. Rycenga, X. Xia, C.H. Moran, F. Zhou, D. Qin, Z.Y. Li, Y. Xia, Generation of hot spots with silver nanocubes for single-molecule detection by surface-enhanced Raman scattering, Angew. Chem. 50 (2011) 5473–5477. [7] Z. Rong, R. Xiao, C. Wang, D. Wang, S. Wang, Plasmonic Ag core-satellite nanostructures with a tunable silica-spaced nanogap for surface-enhanced raman scattering, Langmuir 31 (2015) 8129–8137. [8] L. Li, T. Hutter, U. Steiner, S. Mahajan, Single molecule SERS and detection of biomolecules with a single gold nanoparticle on a mirror junction, Analyst 138 (2013) 4574–4578. [9] X. Xia, J. Zeng, B. McDearmon, Y. Zheng, Q. Li, Y. Xia, Silver nanocrystals with concave surfaces and their optical and surface-enhanced Raman scattering properties, Angew. Chem. 50 (2011) 12542–12546. [10] J. Lee, B. Hua, S. Park, M. Ha, Y. Lee, Z. Fan, H. Ko, Tailoring surface plasmons of high-density gold nanostar assemblies on metal films for surface-enhanced Raman spectroscopy, Nanoscale 6 (2014) 616–623. [11] A. Tao, F. Kim, C. Hess, J. Goldberger, R. He, Y. Sun, Y. Xia, P. Yang, Langmuir-Blodgett silver nanowire monolayers for molecular sensing using surface-enhanced raman spectroscopy, Nano Lett. 3 (2003) 1229–1233. [12] P. Wang, O. Liang, W. Zhang, T. Schroeder, Y.H. Xie, Ultra-sensitive graphene-plasmonic hybrid platform for label-free detection, Adv. Mater. 25 (2013) 4918–4924. [13] J. Wang, L. Huang, L. Yuan, L. Zhao, X. Feng, W. Zhang, L. Zhai, J. Zhu, Silver nanostructure arrays abundant in sub-5 nm gaps as highly Raman-enhancing substrates, Appl. Surf. Sci. 258 (2012) 3519–3523.
[14] J. Wang, L. Huang, L. Zhai, L. Yuan, L. Zhao, W. Zhang, D. Shan, A. Hao, X. Feng, J. Zhu, Hot spots engineering in hierarchical silver nanocap array for surface-enhanced Raman scattering, Appl. Surf. Sci. 261 (2012) 605–609. [15] Y. Shin, J. Song, D. Kim, T. Kang, Facile preparation of ultrasmall void metallic nanogap from self-assembled gold-Silica core-Shell nanoparticles monolayer via kinetic control, Adv. Mater. 27 (2015) 4344–4350. [16] Q. Fu, Z. Zhan, J. Dou, X. Zheng, R. Xu, M. Wu, Y. Lei, Highly reproducible and sensitive SERS substrates with Ag inter-nanoparticle gaps of 5 nm fabricated by ultrathin aluminum mask technique, ACS Appl. Mater. Interfaces 7 (2015) 13322–13328. [17] Z.Q. Cheng, F. Nan, D.J. Yang, Y.T. Zhong, L. Ma, Z.H. Hao, L. Zhou, Q.Q. Wang, Plasmonic nanorod arrays of a two-segment dimer and a coaxial cable with 1 nm gap for large field confinement and enhancement, Nanoscale 7 (2015) 1463–1470. [18] H. Liu, X. Zhang, T. Zhai, T. Sander, L. Chen, P.J. Klar, Centimeter-scale-homogeneous SERS substrates with seven-order global enhancement through thermally controlled plasmonic nanostructures, Nanoscale 6 (2014) 5099–5105. [19] H. Im, K.C. Bantz, N.C. Lindquist, C.L. Haynes, S.H. Oh, Vertically oriented sub-10-nm plasmonic nanogap arrays, Nano Lett. 10 (2010) 2231–2236. [20] X.T. Wang, W.S. Shi, G.W. She, L.X. Mu, S.T. Lee, High-performance surface-enhanced Raman scattering sensors based on Ag nanoparticles-coated Si nanowire arrays for quantitative detection of pesticides, Appl. Phys. Lett. 96 (2010) 053104. [21] N.F. Adegboyega, V.K. Sharma, K. Siskova, R. Zboril, M. Sohn, B.J. Schultz, S. Banerjee, Interactions of aqueous Ag+ with fulvic acids: mechanisms of silver nanoparticle formation and investigation of stability, Environ. Sci. Technol. 47 (2013) 757–764. [22] P.K. Jain, X. Huang, I.H. El-Sayed, M.A. El-Sayed, Review of some interesting surface plasmon resonance-enhanced properties of noble metal nanoparticles and their applications to biosystems, Plasmonics 2 (2007) 107–118. [23] J. Wang, D. Song, L. Wang, H. Zhang, H. Zhang, Y. Sun, Design and performances of immunoassay based on SPR biosensor with Au/Ag alloy nanocomposites, Sens. Actuators B 157 (2011) 547–553. [24] Y.-W. Ma, L.-H. Zhang, Z.-W. Wu, M.-F. Yi, J. Zhang, G.-S. Jian, The study of tunable local surface plasmon resonances on Au-Ag and Ag-Au core-shell alloy nanostructure particles with DDA method, Plasmonics 10 (2015) 1791–1800. [25] P.B. Johnson, R.W. Christy, Optical constants of noble metals, Phys. Rev. B 6 (1972) 4370–4379.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
A facile fabrication of Ag-Au-Ag nanostructures with nanogaps for intensified surface-enhanced Raman scattering Liangliang Jin a,b , Guangwei She a,∗ , Jing Li a , Jing Xia a,b , Xiaotian Wang c , Lixuan Mu a , Wensheng Shi a,∗ a Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China b University of Chinese Academy of Sciences, Beijing 100049, China c School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore
a r t i c l e
i n f o
Article history: Received 8 June 2016 Received in revised form 7 July 2016 Accepted 8 July 2016 Available online 14 July 2016 Keywords: Nanogap Surface-enhanced Raman scattering (SERS) Electrodynamics simulation
a b s t r a c t Nanogap between two metallic nanostructures has been demonstrated to be able to efficiently concentrate an incident electromagnetic field into a small space. As a result, the formed strong field localization could extraordinarily enhance the surface-enhanced Raman scattering (SERS). In this study, controllable plasmonic nanogaps are formed by separating two layers of plasmonic Ag nanoparticles (50–100 nm) with small Au nanoparticles (2.5–6 nm). The size of the nanogaps can be readily tuned by altering the size of the Au nanoparticles. Utilizing an SERS substrate with such nanogaps, the SERS performance can be significantly improved. Such improvement could be attributed to the strongly enhanced electric field within the nanogaps, which is demonstrated by the Finite-difference time-domain simulations. The present work provides a facile strategy to rationally fabricate SERS substrates with controllable nanogaps and intensified SERS signals. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Surface-enhanced Raman scattering (SERS) is an optical spectroscopic technique which amplifies the sensitivity of classical Raman spectroscopy. Owing to its high identification and sensitivity, SERS has attracted increasing attention in diverse areas, including chemistry, biology, food safety and biomedical diagnostics [1,2]. The dramatic variations in the SERS intensification have been attributed to the formation of localized plasmons or the so-called “hot spots” [3]. The electromagnetic “hot spot” is a location with strongly enhanced local field, which occurs near sharp asperities or forms at the nanoscale gaps between the metal nanostructures [4]. Shortrange electromagnetic interactions at “hot spots” could lead to enormous Raman enhancement factors, even realize the singlemolecule detection [5,6]. It is well known that the “hot spots” strongly depend on the shape of metal nanoparticles (NPs) as well as the interparticle distance [2,7]. Metallic nanostructures with various morphologies and interparticle spaces, such as nanocubes, nanospheres, nano-octahedrons, nanostars, nanowires and nanopyramids, have been explored to produce the “hot spots” and enhance the Raman signals [6,8–12].
∗ Corresponding authors. E-mail addresses: [email protected] (G. She), [email protected] (W. Shi). http://dx.doi.org/10.1016/j.apsusc.2016.07.066 0169-4332/© 2016 Elsevier B.V. All rights reserved.
Since the nanogaps in SERS substrates can provide abundant “hot spots” and improve the SERS performance, the structures with controllable nanogaps have been given much of interest [13,14]. Various methods have focused on the fabrication of SERS substrates with nanogaps. Several sophisticated lithographic methods, such as electron-beam lithography, nanosphere lithography, focused ion-beam patterning and soft lithography are commonly used to produce tightly spaced plasmonic geometries [15]. Other methods, such as physical/chemical vapor deposition, atomic layer deposition and template technique, are also developed to fabricate nanogap structures [16]. However, although lithographic methods can accurately control the inter-particle distance from tens of nanometers to ∼1 nm, highly specialized lithographic facilities are complex and expensive [17,18]. The other methods are relatively inexpensive, but the tedious preparation and implementation steps require accuracy and time consumption [16,18,19]. It is still a challenge to establish a rational and facile method for the fabrication of high-quality SERS substrates with reliable nanogaps. In this study, we develop a promising strategy to fabricate AgAu-Ag SERS substrates with deliberate nanogaps. In the present strategy, the reductive Si-H bonds on the surface of Si were utilized to reduce Ag+ ions into Ag nanoparticles (Ag NPs) on the surface of Si wafer. Then the uniform Au nanoparticles (Au NPs) with tunable sizes were sputtered onto the Ag NPs-coated Si wafer. Finally, a second layer of Ag NPs prepared by reducing
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AgNO3 with ethylene glycol (EG) was spin-coated onto the Si wafer with Ag and Au nanoparticles. As a result, the nanogaps were deliberately formed between the two layers of Ag NPs by the inset of Au NPs. The nanogaps could be reliably tuned by altering the size and distribution of the Au NPs, which were controlled by the sputtering process. The as-fabricated Ag-Au-Ag SERS substrate with deliberate nanogaps exhibits an improved performance in the detection of 4-Mercaptopyridine (4-MPY) molecules. Finite-difference time-domain (FDTD) simulations indicate that the improved SERS performance could be attributed to the strongly enhanced electric field within the nanogaps. 2. Experimental details 2.1. Preparation of the Ag-Au-Ag SERS substrate The Ag-Au-Ag SERS substrate consists of the first layer of Ag NPs, the interlayer of Au NPs and the second layer of Ag NPs. The fabrication processes are schematically illustrated in Fig. 1a (the
3-dimensional schematic) and Fig. 1b (the cross-sectional schematic). The first layer of Ag NPs was prepared by immersing the cleaned Si wafer into an aqueous solution consisting of 5 mM AgNO3 and 4.8 M HF for 10 s. Si reacted with HF to form the reductive Si-H bonds on the surface of Si wafer [20]. Then, the Ag+ ions were reduced by the Si-H bonds and Ag NPs were formed on the surface of the Si wafer. After rinsing thoroughly with deionized water, the obtained Ag NPs on Si wafer were annealed at 180 ◦ C for 20 min in the Ar atmosphere and the first layer of Ag NPs was obtained. Then the Au NPs were deposited onto the Si wafer with Ag NPs for a determinate time by ion sputter equipment (Hitichi E-1010) with the vacuum at 10 Pa and discharge current at 10 mA. Finally, the second layer of Ag NPs was spin-coated onto the Si wafer with Ag and Au nanoparticles. The Ag NPs used for spincoating were synthesized by reducing AgNO3 with ethylene glycol (EG) in the presence of Poly (N-vinylpyrrolidone) (PVP-K30, Mw & 40,000, the concentration was calculated in terms of the repeating unit). Briefly, a 10 mL EG solution of 0.1 M PVP was injected into 10 mL of a magnetically stirred EG solution of AgNO3 (0.1 M).
Fig. 1. (a) 3-dimensional and (b) cross-section schematic illustration of the sample preparation process for the Ag-Au-Ag substrate.
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Fig. 2. SEM image (a) and 3D AFM image (b) of Ag1 NP, insert image in (a) shows the amplified SEM image of Ag1 NP. (c) TEM image of Au NPs with the sputtering time of 5s, insert images show the HRTEM image and the SAED pattern of Au NPs. (d) TEM image of Ag1 NP-Au NP, insert image shows the SAED pattern of the sample. SEM image (e) and 3D AFM image (f) of the fabricated Ag1 NP-Au NP-Ag2 NP structure, insert image in (e) shows the amplified SEM image of Ag1 NP-Au NP-Ag2 NP structure.
Afterwards, the solution was put into a 25 mL Teflon-lined autoclave tube. This tube was sealed and maintained at 160 ◦ C for 20 min, followed by natural cooling to room temperature (25 ◦ C). Ag NPs were separated from EG by addition of a large amount of ethanol, followed by sonication and centrifugation. Then the Ag NPs were dispersed in 10 mL ethanol solution for spin-coating. 2.2. Characterization of the nanoparticles and the substrates The morphologies of the samples were characterized by a field emission scanning electron microscopy (FESEM, Hitachi S4800) and atomic force microscopy (Bruker Multimode 8) under tapping-mode. The kind of tip for AFM images was Bruker RTESPA. The structures of the samples were investigated using high
resolution transmission electron microscopy (HRTEM, JEM-2100F, JEOL). Optical absorption spectra were measured using a UV–visNIR Spectrophotometer (Varian, UV–vis-NIR Cary 5000). Raman and SERS spectral measurements were carried out by using a confocal Raman spectrometer (Renishaw inVia Raman Microscope) with 532 nm excitations. The excitation intensity is about 0.5 mW and data acquisition time is 10 s throughout these experiments. The laser beam was focused on a spot about 3 m in diameter by a 50 × microscope objective. The Raman band of a silicon wafer at 520 cm−1 was employed to calibrate the spectrometer. 4-Mercaptopyridine (4-MPY) was adopted as the analyte. The substrates were immersed in 1 mM 4-MPY ethanol solution for 2 h and then thoroughly rinsed for SERS signal measurements.
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Fig. 3. SERS spectra of 1 mM 4-MPY on different substrates, Ag1 NP (curve a), Ag1 NP-Au NP(5s) (curve b), Ag1 NP-Ag2 NP (curve c) and Ag1 NP-Au NP(5s)-Ag2 NP (curve d).
3. Results and discussion For convenience, in the following discussion we shall name the first layer of Ag NPs as ‘Ag1 NP’, Ag1 NP with the interlayer of Au NPs as ‘Ag1 NP-Au NP’ and the Ag-Au-Ag substrate as ‘Ag1 NP-Au NPAg2 NP’. As illustrated by the schematic, the distance between Ag1 NP and Ag2 NP can be tuned by the size and distribution of Au NPs. The distance could be nanoscale when the size of Au NPs is about several nanometers. Fig. 2a–f show the SEM, AFM and TEM images of the samples sequentially obtained. Ag1 NP exhibit sphere-like shape and have an average diameter of about 50 nm, as shown in the SEM image of Fig. 2a. The 3D AFM topographical image of Ag1 NP is identical with the SEM image, as shown in Fig. 2b. The TEM and HRTEM images of the Au NPs which were ion sputtered are shown in Fig. 2c, exhibiting an average size about 2.5 nm. The corresponding SAED pattern of the Au NP is also shown in the insert image in Fig. 2c. The obvious diffraction rings marked as (111), (022) can be indexed to the polycrystalline Au with a cubic structure. The morphologies of Ag1 NP-Au NP can be illustrated in Fig. 2d. The corresponding SAED pattern of Ag1 NP-Au NP is shown in the insert image in Fig. 2d. The diffraction spots marked as (220), (3–11) can be indexed to the single crystalline Ag with a cubic structure. The diffraction rings marked as (111) in the insert image in Fig. 2d can be indexed to the polycrystalline Au. It can be confirmed from Fig. 2d that the Au NPs have been successfully sputtered onto the Ag1 NP. The Au NPs are separately distributed and the distance between each other of sputtered Au NPs is about 5 nm. When the colloid Ag NPs were spin-coated onto the Ag1 NP-Au NP to act as the Ag2 NP, the nanogaps would occur between Ag1 NP and Ag2 NP as a consequence of a spacing function of the sputtered Au NPs, as illustrated in Fig. 1. Fig. 2e and f show the SEM and 3D AFM images of the fabricated Ag1 NP-Au NP(5s)-Ag2 NP hybrid nanoparticles substrate. Both the SEM and AFM topographical images demonstrate that the Ag-Au-Ag SERS substrate has been successfully fabricated. With the second layer Ag NPs and the deliberate nanogaps between the two layers of Ag NPs, it is expected that the substrate has the potential to show enhanced SERS signals. The SERS performance of the as-prepared Ag1 NP-Au NP(5s)Ag2 NP substrate was investigated by using 4-MPY as the analyte, as shown in Fig. 3. For comparison, the Raman spectra of 4-MPY with the same concentration on Ag1 NP, Ag1 NP-Au NP and the substrate consisting of Ag1 NPs and Ag2 NPs but without Au NPs to produce the nanogaps (Ag1 NP-Ag2 NP) were also measured, respectively. The strongest SERS signal was observed from the Ag1 NP-Au NP(5s)-Ag2 NP substrate, which is ten times that from Ag1 NP substrate and twice that from Ag1 NP-Ag2 NP substrate. As
Fig. 4. Absorption spectra of Ag1-Ag2 NP (curve a), Ag1-Au NP(5s)-Ag2 NP (curve b), Ag1-Au NP(10s)-Ag2 NP (curve c) and Ag1-Au NP(15s)-Ag2 NP (curve d).
the consistency of the first and second layer Ag NPs for Ag1 NPAu NP(5s)-Ag2 NP and Ag1 NP-Ag2 NP substrates, the significantly intensified SERS could be attributed to the Au NPs interlayer. The introduction of the Au NPs interlayer would produce nanogaps between the two layers of Ag NPs and it may also influence the optical absorption of the substrate. The produced nanogaps and the possible changes of optical absorption are prospective to affect the SERS performance of the substrate. The optical properties of Ag1 NP-Au NP-Ag2 NP substrates have been investigated as a function of a series of sputtering times. Fig. 4a shows the curves of optical absorption versus wavelength for Ag1 NP-Ag2 NP substrate and Ag1 NP-Au NP-Ag2 NP substrates with the sputtering time of 5 s, 10 s, and 15 s, respectively. As we can see, the resonant wavelength of optical absorption is located at 400 nm for Ag1 NP-Ag2 NP substrate, which represents the intrinsic surface plasmon resonance (SPR) of Ag NPs [21]. The resonant wavelength belonging to the intrinsic SPR of Au NPs is generally located at over 500 nm, slightly varying with the nanoparticle sizes [22]. With increasing the sputtering time, the resonant wavelength of optical absorption for the samples moves to longer wavelength. The observed red-shift of the resonant wavelength represents the optical coupling in the hybrid systems [23,24]. It should be mentioned that the absorption of Ag1-Au NP-Ag2 NP substrates at longer wavelength in Fig. 4 became reduced with the increasing sputtering time of the Au NPs. The absorption at longer wavelength corresponds to the absorption of Si wafer, whose absorption spectrum is shown in Fig. S1. The declined absorption of the Au NP samples implies that the thicker film of Au blocked the absorption of Si wafer which was used as the basement of the substrate. The coupling effects in Ag1 NP-Au NP-Ag2 NP substrates are investigated by the electrodynamics simulations. The electrodynamics simulations were performed by using commercial FDTD software (Lumerical Solution) to investigate the coupling effects in Ag1 NP-Ag2 NP and Ag1 NP-Au NP-Ag2 NP hybrids. The simulation configurations are depicted in Fig. 5a and c. Here, the diameter of Ag1 NP is set as 50 nm and Ag2 NPs is set as 100 nm for both Ag1 NP-Ag2 NP and Ag1 NP-Au NP-Ag2 NP, which are in accordance with the experimental diameters of Ag1 NP and Ag2 NP. The diameter of Au NP is set as 2.5 nm and the distance between each other of Au NP is set as 5 nm for Ag1 NP-Au NP(5s)-Ag2 NP hybrid, which are in accordance with the experimental diameter and distribution of sputtered Au NP onto Ag1 NP. As the simulation configuration shown in Fig. 5c, the nanogap between the two layers of Ag NPs can be calculated and the result is 2.26 nm. For the simulation, the refractive indexes of Au and Ag are 0.5328 + 2.194i and 0.0542+ 3.434i at a wavelength of 532 nm, respectively, according to the experimentally measured data from Johnson and Christy
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Fig. 5. (a) Simulation configuration of Ag1 NP-Ag2 NP. Comparing with the actual substrate, the configuration is rotated to be vertical for convenient. And the angle of the incident plane wave is 30◦ accordingly. (b) Simulated electrical field intensity distributions of Ag1 NP-Ag2 NP in the yz plane. The color bar demonstrates the electric field value of ln(|E|)2 which is normalized by the incident field. (c) Simulation configuration of Ag1 NP-Au NP(5)-Ag2 NP. Comparing with the actual substrate, the configuration is rotated to be vertical for convenient. And the angle of the incident plane wave is 30◦ accordingly. The diameter of Au NP is 2.5 nm and the distance between each Au NP is 5 nm. The calculated gap is 2.26 nm. (d) Simulated electrical field intensity distributions of Ag1 NP-Au NP(5s)-Ag2 NP in the yz plane. The color bar demonstrates the electric field value of ln(|E|)2 which is normalized by the incident fiel.
[25]. Fig. 5b and d show the simulated electrical field intensity distributions in the yz plane. It can be clearly seen that the localized electric field is mainly distributed at the nanogaps formed between the Ag NPs for both simulations. However, the electric field has been strongly enhanced for Ag1 NP-Au NP(5s)-Ag2 NP hybrid, leading to a maximal enhancement of 191, while the maximal enhancement of electric field is 85 for Ag1 NP-Ag2 NP without Au NP to tune the nanogaps. The strongly enhancement of electric field represents the enhanced coupling effect in the substrate and could contribute to the SERS enhancement for the Ag1 NP-Au NP-Ag2 NP substrate. Moreover, the nanogap region consisting of enhanced electric field has been expanded for Ag1 NP-Au NP(5s)-Ag2 NP hybrid, as shown in Fig. 5d. More analyte molecules could be within the expanded nanogap region and contribute to the SERS enhancement. In the experiments, the nanogaps would be tuned by the size and distribution of the Au NPs, which were controlled by the sputtering process. The TEM image of the Au NPs with ion sputtering time 15 s is shown in Fig. S2. The distance and size of the Au NPs for Au NP (5 s and 15 s) are different. With increasing the sputtering time, the diameter of the Au NPs was getting bigger and the distance between each of the Au NPs was getting larger. The Au NPs with long sputtering time would induce enlarged nanogap for Ag1 NP-Au NP-Ag2 NP substrate and the plasmonic coupling in the nanogap region would change. A similar electrodynamics simulation was performed for Ag1 NP-Au NP(15s)-Ag2 NP hybrid as shown in Fig. S3. The maximal enhancement of electric field is 94 which is inferior compared with Ag1 NP-Au NP(5s)-Ag2 NP. The intensity of the SERS signal of the target analyte from Ag1 NP-Au NP(15s)-Ag2 NP is also inferior compared with that of Ag1 NP-Au NP(5s)-Ag2 NP substrate, as shown in Fig.S4. The decreased electric field of the enlarged nanogap is responsible for the inferior SERS signal.
4. Conclusion Ag-Au-Ag nanostructures with nanogaps were fabricated by a facile method for intensified SERS signal. The size of nanogaps can be effectively tuned by the size and distribution of the ionsputtered Au NPs. An appropriate size of Au NPs is obtained by controlling the sputtering duration as 5 s and enhanced SERS intensity is achieved. The electrodynamics calculation reveals that the electrical field intensity between two layers of Ag NPs could be significantly enhanced by introducing proper nanogaps. The improved SERS performance is attributed to the strongly enhanced electric field in the nanogaps and the expanded nanogap region which could allow more analyte molecules to be within in the gap region. The proposed method by using small nanoparticles to tune the nanogaps is expected to be a promising strategy for reliable control of nanogaps and intensified SERS signal.
Acknowledgements This work was supported by Chinese Academy of Sciences (Grant KGZD-EW-T02), Most of China (Grant 2016YFA0200800), NSFC (Grants 51272258, 51272302, 91333119 and 61307065) and Youth Innovation Promotion Association CAS.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2016.07. 066.
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References [1] C. Farcau, S. Astilean, Mapping the SERS efficiency and hot-Spots localization on gold film over nanospheres substrates, J. Phys. Chem. C 114 (2010) 11717–11722. [2] W. Y. Zhao, Z. Zeng, P. Tao, Y. Xiong, Y. Zhu Qu, Highly sensitive surface-enhanced Raman scattering based on multi-dimensional plasmonic coupling in Au–graphene–Ag hybrids, Chem. Commun. 51 (2015) 866–869. [3] C.E. Talley, J.B. Jackson, C. Oubre, N.K. Grady, C.W. Hollars, S.M. Lane, T.R. Huser, P. Nordlander, N.J. Halas, Surface-enhanced Raman scattering from individual au nanoparticles and nanoparticle dimer substrates, Nano Lett. 5 (2005) 1569–1574. [4] X. Li, W.C.H. Choy, X. Ren, D. Zhang, H. Lu, Highly intensified surface enhanced raman scattering by using monolayer graphene as the nanospacer of metal film-metal nanoparticle coupling system, Adv. Funct. Mater. 24 (2014) 3114–3122. [5] W. Li, P.H. Camargo, X. Lu, Y. Xia, Dimers of silver nanospheres: facile synthesis and their use as hot spots for surface-enhanced Raman scattering, Nano Lett. 9 (2009) 485–490. [6] M. Rycenga, X. Xia, C.H. Moran, F. Zhou, D. Qin, Z.Y. Li, Y. Xia, Generation of hot spots with silver nanocubes for single-molecule detection by surface-enhanced Raman scattering, Angew. Chem. 50 (2011) 5473–5477. [7] Z. Rong, R. Xiao, C. Wang, D. Wang, S. Wang, Plasmonic Ag core-satellite nanostructures with a tunable silica-spaced nanogap for surface-enhanced raman scattering, Langmuir 31 (2015) 8129–8137. [8] L. Li, T. Hutter, U. Steiner, S. Mahajan, Single molecule SERS and detection of biomolecules with a single gold nanoparticle on a mirror junction, Analyst 138 (2013) 4574–4578. [9] X. Xia, J. Zeng, B. McDearmon, Y. Zheng, Q. Li, Y. Xia, Silver nanocrystals with concave surfaces and their optical and surface-enhanced Raman scattering properties, Angew. Chem. 50 (2011) 12542–12546. [10] J. Lee, B. Hua, S. Park, M. Ha, Y. Lee, Z. Fan, H. Ko, Tailoring surface plasmons of high-density gold nanostar assemblies on metal films for surface-enhanced Raman spectroscopy, Nanoscale 6 (2014) 616–623. [11] A. Tao, F. Kim, C. Hess, J. Goldberger, R. He, Y. Sun, Y. Xia, P. Yang, Langmuir-Blodgett silver nanowire monolayers for molecular sensing using surface-enhanced raman spectroscopy, Nano Lett. 3 (2003) 1229–1233. [12] P. Wang, O. Liang, W. Zhang, T. Schroeder, Y.H. Xie, Ultra-sensitive graphene-plasmonic hybrid platform for label-free detection, Adv. Mater. 25 (2013) 4918–4924. [13] J. Wang, L. Huang, L. Yuan, L. Zhao, X. Feng, W. Zhang, L. Zhai, J. Zhu, Silver nanostructure arrays abundant in sub-5 nm gaps as highly Raman-enhancing substrates, Appl. Surf. Sci. 258 (2012) 3519–3523.
[14] J. Wang, L. Huang, L. Zhai, L. Yuan, L. Zhao, W. Zhang, D. Shan, A. Hao, X. Feng, J. Zhu, Hot spots engineering in hierarchical silver nanocap array for surface-enhanced Raman scattering, Appl. Surf. Sci. 261 (2012) 605–609. [15] Y. Shin, J. Song, D. Kim, T. Kang, Facile preparation of ultrasmall void metallic nanogap from self-assembled gold-Silica core-Shell nanoparticles monolayer via kinetic control, Adv. Mater. 27 (2015) 4344–4350. [16] Q. Fu, Z. Zhan, J. Dou, X. Zheng, R. Xu, M. Wu, Y. Lei, Highly reproducible and sensitive SERS substrates with Ag inter-nanoparticle gaps of 5 nm fabricated by ultrathin aluminum mask technique, ACS Appl. Mater. Interfaces 7 (2015) 13322–13328. [17] Z.Q. Cheng, F. Nan, D.J. Yang, Y.T. Zhong, L. Ma, Z.H. Hao, L. Zhou, Q.Q. Wang, Plasmonic nanorod arrays of a two-segment dimer and a coaxial cable with 1 nm gap for large field confinement and enhancement, Nanoscale 7 (2015) 1463–1470. [18] H. Liu, X. Zhang, T. Zhai, T. Sander, L. Chen, P.J. Klar, Centimeter-scale-homogeneous SERS substrates with seven-order global enhancement through thermally controlled plasmonic nanostructures, Nanoscale 6 (2014) 5099–5105. [19] H. Im, K.C. Bantz, N.C. Lindquist, C.L. Haynes, S.H. Oh, Vertically oriented sub-10-nm plasmonic nanogap arrays, Nano Lett. 10 (2010) 2231–2236. [20] X.T. Wang, W.S. Shi, G.W. She, L.X. Mu, S.T. Lee, High-performance surface-enhanced Raman scattering sensors based on Ag nanoparticles-coated Si nanowire arrays for quantitative detection of pesticides, Appl. Phys. Lett. 96 (2010) 053104. [21] N.F. Adegboyega, V.K. Sharma, K. Siskova, R. Zboril, M. Sohn, B.J. Schultz, S. Banerjee, Interactions of aqueous Ag+ with fulvic acids: mechanisms of silver nanoparticle formation and investigation of stability, Environ. Sci. Technol. 47 (2013) 757–764. [22] P.K. Jain, X. Huang, I.H. El-Sayed, M.A. El-Sayed, Review of some interesting surface plasmon resonance-enhanced properties of noble metal nanoparticles and their applications to biosystems, Plasmonics 2 (2007) 107–118. [23] J. Wang, D. Song, L. Wang, H. Zhang, H. Zhang, Y. Sun, Design and performances of immunoassay based on SPR biosensor with Au/Ag alloy nanocomposites, Sens. Actuators B 157 (2011) 547–553. [24] Y.-W. Ma, L.-H. Zhang, Z.-W. Wu, M.-F. Yi, J. Zhang, G.-S. Jian, The study of tunable local surface plasmon resonances on Au-Ag and Ag-Au core-shell alloy nanostructure particles with DDA method, Plasmonics 10 (2015) 1791–1800. [25] P.B. Johnson, R.W. Christy, Optical constants of noble metals, Phys. Rev. B 6 (1972) 4370–4379.