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Monolayer graphene on nanostructured Ag for enhancement of surface-enhanced Raman scattering stable platform
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 125603 (http://iopscience.iop.org/0957-4484/26/12/125603) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 132.239.1.231 This content was downloaded on 13/06/2017 at 11:47 Please note that terms and conditions apply.
You may also be interested in: Improved surface-enhanced Raman scattering on arrays of gold quasi-3D nanoholes Weisheng Yue, Yang Yang, Zhihong Wang et al. Enhanced Raman scattering of graphene by silver nanoparticles with different densities and locations Hai-Bin Sun, Can Fu, Yan-Jie Xia et al. Large-area, reproducible and sensitive plasmonic MIM substrates for surface-enhanced Raman scattering Kuanguo Li, Yong Wang, Kang Jiang et al. Surface enhanced Raman scattering of gold nanoparticles supported on copper foil with graphene as a nanometer gap Quan Xiang, Xupeng Zhu, Yiqin Chen et al. A wafer-scale backplane-assisted resonating nanoantenna array SERS device created by tunable thermal dewetting nanofabrication Te-Wei Chang, Manas Ranjan Gartia, Sujin Seo et al. Synthesis of wheatear-like ZnO nanoarrays decorated with Ag nanoparticles and its improved SERS performance through hydrogenation Yufeng Shan, Yong Yang, Yanqin Cao et al. Large-scale uniform Au nanodisk arrays fabricated via x-ray interference lithography for reproducible and sensitive SERS substrate Pingping Zhang, Shumin Yang, Liansheng Wang et al.
Nanotechnology Nanotechnology 26 (2015) 125603 (9pp)
doi:10.1088/0957-4484/26/12/125603
Monolayer graphene on nanostructured Ag for enhancement of surface-enhanced Raman scattering stable platform Zhigao Dai1,6, Fei Mei2,6, Xiangheng Xiao1,5, Lei Liao1, Wei Wu3, Yupeng Zhang1, Jianjian Ying1, Lingbo Wang4, Feng Ren1 and Changzhong Jiang1 1
Department of Physics, Hubei Nuclear Solid Physics Key Laboratory and Center for Ion Beam Application, Wuhan University, Wuhan 430072, People’s Republic of China 2 School of Electrical and Electronic Engineering, Hubei University of Technology, Wuhan 430068, People’s Republic of China 3 School of Printing and Packaging, Wuhan University, Wuhan 430072, People’s Republic of China 4 College of Electronic Information Engineering, Wuhan Polytechnic, Wuhan 430074, People’s Republic of China 5 Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, USA E-mail: [email protected] Received 6 January 2015 Accepted for publication 9 February 2015 Published 6 March 2015 Abstract
We have reported that the Ag nanostructure-based substrate is particularly suitable for surfaceenhanced Raman scattering when it is coated with monolayer graphene, an optically transparent and chemistry-inertness material in the visible range. Ag bowtie nanoantenna arrays and Ag nanogrids were fabricated using plasma-assisted nanosphere lithography. Our measurements show that atmospheric sulfur containing compounds are powerless to break in the monolayer graphene to vulcanize the surfaces of the Ag bowtie nanoantenna arrays and Ag nanogrids by various means, including scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS). Furthermore, the Ag nanostructure substrate coated with the monolayer graphene film shows a larger enhancement of Raman activity and the electromagnetic field than the uncoated substrate. Compared with those of bare Ag nanostructures, the averaged EFs of graphene-film-coated Ag nanostructures were estimated to be about 21 and 5 for Ag bowtie nanoantenna arrays and nanogrids after one month later in air, respectively. These observations are further supported by theoretical calculations. Keywords: Ag nanostructure-based substrate, sulfidation, surface-enhanced Raman scattering (Some figures may appear in colour only in the online journal) 1. Introduction
most influential analytical techniques for label-free single molecule detection and high-resolution vibrational information [7–9]. An ideal platform for a SERS substrate is identified as one with high durability, high repeatability and low cost that is also easy to manufacture and so on [10–14]. The SERS-active substrates are typically built using Ag or gold nanostructures. In the visible regime, Ag nanostructures are normally favored over gold as an energetic SERS element due to their enhancement factor, which is up to two orders of
Incident light can be converted to the nanoscale, restricted by noble metal nanostructures that powerfully improved the optical fields [1–3]. When molecules are located in the ‘hot spot,’ the Raman signals can be dramatically enhanced [4–6]. Surface-enhanced Raman scattering (SERS) is one of the 6
These authors contributed equally to this work
0957-4484/15/125603+09$33.00
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© 2015 IOP Publishing Ltd Printed in the UK
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magnitude larger [15–18]. However, using Ag nanostructures as substrates is always associated with severe drawback in terms of chemical durability. To avoid oxidation or sulfuration, Ag nanoparticles with a thin and inert SiO2 shell were employed [19], yet the thicker SiO2 layer (>2 nm) greatly compromises the SERS activity [20]. Graphene, a single atomic layer of graphite, is a promising candidate for applications in photonics and optoelectronics by virtue of its unique electronic, optical and mechanical properties [21, 22]. Graphene provides the ideal prototype test material for the investigation of SERS [23]. In a graphene-mediated SERS (G-SERS) substrate [24–28], the monolayer graphene provides an atomically flat surface for Raman enhancement. Graphene was revealed to be superior to metallic surfaces in terms of biocompatibility and chemical durability [29–31]. However, the physical mechanism underlying the enhancement and stability of a SERS platform with graphene has not been systematically explored. In our previous articles, extensive-area, uniform Ag bowtie nanoantenna arrays were employed as a highly active SERS platform fabricated by the integration of nanosphere lithography (NSL) and succedent oxygen plasma etching [32, 33]. Here, we introduce an enhancement and a stable SERS-active platform composed of two-dimensional atomic crystal graphene and large-area Ag nanostructures. The thickness of graphene is 0.355 nm [34], which should not impede SERS activity of the Ag nanostructures that are hypersensitized and very localized. With the surface passivated by graphene, the Ag nanostructure can be protected by effectively blocking the diffusion of gas molecules through transferred monolayer graphene. The SERS properties and electromagnetic field enhancement of graphene-coated Ag nanostructures were still excellent. Furthermore, these observations are further supported by our theoretical calculations.
Figure 1. Schematic illustrations of the graphene transfer process
(the size of the atoms is only for visual simplicity and not drawn to scale). 2.2. Transfer of graphene films onto the Ag nanostructures
The monolayer graphene films were transferred onto the Ag nanostructures, as shown in figure 1. High-Quality, monolayer graphene was grown by chemical vapor deposition (CVD) on 25 μm thick polycrystalline Cu foils with CH4 as the carbon source gas [35]. To begin with, the graphenecovered copper foil was spin-coated with a poly(methyl methacrylate) (PMMA) (MicroChem, 950 000 MW, 9–6 wt. % in anisole) film, which was then cured at 120 °C for 30 min. Graphene grew on both sides of the Cu foil. After one side of the Cu/graphene was coated with PMMA, the opposite side of the graphene was etched away by plasma cleaning. Then, a PMMA/graphene membrane was obtained by etching away the copper foil in an aqueous solution of iron chloride (0.4 g mL−1) with a bit of HCl in less than 30 min. The PMMA/graphene stack was washed with an aqueous solution of HCl and was transferred to deionized water to wash away all kinds of residual ions. The aqueous solution of HCl can prevent the hydrolysis of ferric ions. Furthermore, the PMMA/graphene stack was placed on the target substrate and dried at 50 °C for 30 min. Finally, the PMMA layer was dissolved by acetone.
2. Experimental section 2.1. Preparation of Ag bowtie nanoantenna arrays and Ag nanogrids
The Ag nanostructures were fabricated using plasma-assisted NSL. For a more in-depth study of graphene-covered Ag nanostructures used for the SERS platform, the Ag bowtie nanoantenna arrays and Ag nanogrids were fabricated by the integrate of NSL and succedent oxygen plasma etching, respectively. The details of the colloid lithography and subsequent oxygen plasma processing were shown in our previous articles [32, 33]. For the Ag bowtie nanoantenna arrays (Ag nanogrids), the oxygen plasma etching time was 5 min (9 min) on polystyrene (PS) particle masks. Subsequently, a 10 nm thick chromium film was thermally evaporated onto the as-prepared PS particle masks, followed by evaporation of a 60 nm thick Ag film. The PS particles were removed by sonication in chloroform for a few seconds, leaving only the Ag nanostructures on the substrate’s surface.
2.3. Characterization
SEM (FEI Sirion FEG) was used to characterize the surface morphology. The x-ray photoelectron spectroscopy (XPS) analysis was done on a Thermo Scientific ESCALAB 250Xi system with Al kα (1486.6 eV) as the radiation source under a vacuum of 5 × 10−7 Pa. The Raman measurements were performed with a commercial Raman microscope (HR800, Horiba). A laser emitting at 488 nm served as the excitation 2
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Exposed To Atmosphere One Month Later (a1) (a0) 1
(a2) 2 500 nm 1:bare Ag nanostructures 2:covered with the graphene (b1) (b0) 1
(b2) 2 500 nm
Figure 2. SEM images of initial Ag bowtie nanoantenna arrays (a0) and nanogrids (b0); SEM images of bare Ag bowtie nanoantenna arrays (a1) and nanogrids (b1) exposed to the atmosphere a month later; SEM images of Ag bowtie nanoantenna arrays (a2) and nanogrids (b2) covered with graphene exposed to the atmosphere a month later.
Figure 3. (a) AES depth profile of Ag, the S element and the (b) TEM image of Ag bowtie nanoantenna arrays exposed to the atmosphere a
month later.
3
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Figure 4. I2D/IG Raman mapping of graphene covered on the randomly selected SiO2/Si (a), Ag bowtie nanoantenna arrays (b) and nanogrids (c); their corresponding Raman spectra are on the bottom.
3. Results and discussion
source. The laser beam was focused onto the sample through an X100 objective lens. The Raman spectra were collected in the frequency range of 1000 to 2800 cm−1 with an acquisition time of 10 s.
The Ag bowtie nanoantenna arrays and Ag nanogrids were fabricated by the integration of NSL and succedent oxygen plasma etching, as shown in figures 2(a0) and (b0), respectively. A part of the Ag bowtie nanoantenna arrays and nanogrids were left as bare Ag (see figures 2(a0) and (b0)), while the other part was covered with a monolayer graphene. The as-prepared Ag nanostructures were placed in a surrounding laboratory environment for one month. Bare Ag bowtie nanoantenna arrays and nanogrids show large morphological changes in figures 2(a1) and (b1). However, as seen in figures 2(a2) and (b2), the Ag nanostructures coated by graphene show signs of invariabilities; the surfaces of the nanostructures were smooth and retained their original shape. We carried out the thickness of Ag2S by Auger electron spectroscopy (AES). The depth profile monitoring of the elements of Ag and S are shown in figure 3(a). The results indicate that Ag2S almost disappears after the sputtering time of 200 s. The thickness of 60 nm ABNA almost disappeared after the sputtering time of 1800 s on nanostructure Ag after it was exposed to the atmosphere a year later. So, the thickness of Ag2S was estimated to be about ∼200/(1800/60) ∼ 6.5 nm, which is larger than that of monolayer graphene (0.35 nm). Furthermore, the TEM measurement of nanostructured Ag after exposure to the atmosphere a month later is shown in figure 3(b). The thickness of Ag2S is about 5 nm in single ABNA, which well matches with the result of the AES measurement (about 6.5 nm).
2.4. Simulation
The electromagnetic field distributions of Ag nanostructures were simulated with the finite difference time domain (FDTD) method. The Ag nanostructures arrays obtained in our experiment could be considered as a periodical structure. We are able to computationally model the rectangular unit cells approximately 1.68 μm2 in size. To model the Ag nanostructures, we use the FDTD method with periodic boundary conditions (PBC). The implementation of PBC is straightforward for normal incidence illumination. The mesh size was 1 nm, and the simulated area was 1.2 μm × 1.4 μm. The electromagnetic field distribution of the initial Ag nanostructures (the thickness of Ag is 60 nm), bare nanostructures (the thicknesses of Ag and Ag2S are 55 nm and 5 nm, respectively) and Ag nanostructures covered with graphene (the thickness of Ag is 60 nm, and the thickness of graphene is 0.35 nm) exposed to the atmosphere a month later were calculated at incident light wavelengths of 488 nm. The dielectric constant of the Ag, Ag2S and graphene were taken from the source program. The propagation direction of the plane waves was along the z-axis. The electric field was assumed to be polarized along the x-axis. 4
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Figure 5. (a) XPS Ag 2p5/2 and Ag 2p3/2 spectra, (b) O 1s spectra, (c) C 1s spectra and (d) S 2p3/2 and S 2p1/2 spectra of initial Ag bowtie nanoantenna arrays ((a0), (b0), (c0) and (d0)); bare Ag bowtie nanoantenna arrays exposed to the atmosphere a month later ((a1), (b1), (c1) and (d1)) and Ag bowtie nanoantenna arrays covered with graphene exposed to the atmosphere a month later ((a2), (b2), (c2) and (d2)), respectively.
To confirm the Ag nanostructures covered with a large monolayer graphene, point-by-point I2D/IG Raman mapping of graphene was collected on SiO2/Si, Ag bowtie nanoantenna arrays and nanogrids with a randomly selected 10 × 10 μm2 area 2 μm in step length, as illustrated in figure 4. The intensity ratio of 2D to G bands was about 1–5 on SiO2/ Si, as shown in figure 4(a), which is characteristic for a monolayer graphene sheet with a relatively low defect density in our CVD-synthesized graphene. After the graphene was transferred onto the Ag nanostructures, most of the intensity ratios of the 2D to G bands were about 1–5 on the Ag bowtie nanoantenna arrays and nanogrids, as illustrated in figures 4(b) and (c). Their corresponding Raman spectra are shown in figure 4 below the I2D/IG Raman mapping. Also, only the G and 2D bands were observed, indicating that the Ag nanostructures did not obviously affect the quality of our synthesized graphene.
Using XPS analysis, the evolution for bare Ag bowtie nanoantenna arrays and the graphene-covered Ag bowtie nanoantenna arrays after one month is given in figure 5. The existence of Ag, O, C and S on the surface of the serial Ag bowtie nanoantenna arrays was investigated. The binding energy values of three samples of the Ag and O elements were similar. The binding energy of the O 1s peak proves that no Ag (I) oxide was formed because the binding energy of the O 1s peak in Ag (I) oxide is 528.6 eV [36], which is much lower than the value shown in the three samples. The XPS for C 1s spectra of initial Ag bowtie nanoantenna arrays (spectrum a0), the C 1s spectra of bare Ag bowtie nanoantenna arrays (spectrum a1) and the graphene-covered Ag bowtie nanoantenna arrays (spectrum a2) after one month are all shown in figure 5(c), respectively. Monolayer graphene is a single atomic layer of carbon, and the increase of peak strength of C 1s spectra proves the existence of graphene in 5
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Figures 6. (a) and (b) The baseline deducted SERS spectra of R6G molecules adsorbed on the initial Ag bowtie nanoantenna arrays (a0) and nanogrids (b0); bare Ag bowtie nanoantenna arrays (a1) and nanogrids (b1) exposed to the atmosphere a month later; Ag bowtie nanoantenna arrays (a2) and nanogrids (b2) covered with graphene exposed to the atmosphere a month later. Comparison of intensities of the bands at 1363, 1508, 1576 and 1650 cm−1 in Ag bowtie nanoantenna arrays (c) and nanogrids (d).
spectrum c2, as shown in figure 5(c). Similar XPS peaks have also been reported for graphene films transferred on SiO2/Si substrates [37]. For the S 2p spectra, the peaks corresponding to S 2p3/2 and S 2p1/2 have binding energies of 161 and 162 eV in the spectrum d1 collected from the bare Ag bowtie nanoantenna arrays after one month, as shown in figure 5(d). These values are reported data for Ag2S [36]. As shown in the previous paper, the surfaces of the Ag nanostructure can be vulcanized by atmospheric sulfur-containing compounds, according to reaction 1. 2Ag + H 2 S → Ag2 S + H 2 .
Also, the O2 and NO2 can enhance the sulfidation process, according to the following reactions: [38] (3)
2Ag + H 2 S + 2NO2 → Ag2 S + 2HNO2 .
(4)
By covering the surface of Ag nanoantennas with graphene, the peaks of S 2p3/2 and S 2p1/2 are extremely weak in the spectrum d2 collected from the graphene-covered Ag bowtie nanoantenna arrays after one month, as shown in figure 5(d). The sulfidation of the Ag surface can be avoided by successfully blocking the diffusion of gas molecules through monolayer graphene. The SERS spectra of Rhodamine 6G (R6G) adsorbed on Ag nanostructures were measured to investigate how successful graphene passivation retained the SERS activities. For the SERS characterization, a concentration of 10−6 M R6G aqueous solution was used. In order to optimize the molecular adsorption, as-prepared substrates were immersed in the solution for 10 h, taken out, rinsed thoroughly with ethanol and finally dried with nitrogen gas. The R6G molecules’ SERS spectra on the series of Ag bowtie nanoantenna arrays
(1)
When no sources of H2S are available, OCS is the principal corrodent of silver in atmospheric conditions. As shown in reaction 2, in the presence of water, OCS rapidly decomposes to form H2S. The corrosion produced by OCS is important since it is the most abundant sulfur species in the atmosphere. OCS + H 2 O → H 2 S + CO2 .
2Ag + H 2 S + 1/2O2 → Ag2 S + H 2 O
(2)
6
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Figure 7. The FDTD calculated local electric field enhancement (log |E|2) of initial Ag bowtie nanoantenna arrays (a0) and nanogrids (b0);
bare Ag bowtie nanoantenna arrays (a1) and nanogrids (b1) exposed to the atmosphere a month later; Ag bowtie nanoantenna arrays (a2) and nanogrids (b2) covered with graphene exposed to the atmosphere a month later.
R6G). As shown in figures 6(c) and (d), using I1 and I2 of the peaks at 1363, 1508, 1576 and 1650 cm−1 of R6G, the averaged EF of graphene-film-coated Ag nanostructures is estimated to be about 21 and 5 for bare Ag bowtie nanoantenna arrays (figure 6(c)) and nanogrids (figure 6(d)) exposed to the atmosphere a month later, respectively. The AgS2 layer can greatly degrade the local electromagnetic field, resulting in weak SERS activity on the nanostructure’s surface. In order to further explore the physical mechanism of SERS enhancements of graphene-film-coated Ag nanostructures, the FDTD method, based on PBC, was applied to compute the spatial distributions of the electromagnetic field intensity for Ag bowtie nanoantenna arrays and nanogrids (theoretical calculation details in Experimental section 2.4). Figure 7(a0) shows the local electric field enhancement log E 2 distribution of the initial Ag bowtie nanoantenna arrays, where, |E| = |Elocal/Ein| and Elocal and Ein are the local and incident electric fields, respectively. The local electric field enhancement of bare Ag bowtie nanoantenna arrays and Ag bowtie nanoantenna arrays covered with graphene exposed to the atmosphere a month later are shown in figures 7(a1) and (a2), respectively. The input light is polarized along the x-axis. When exposed to the atmosphere a month later, the electromagnetic enhancement |E|2 of the graphene-film-coated Ag nanostructures is stronger than that of the bare Ag nanostructures (1.5 × 103 vs 3.8 × 101) at
(spectrum a0 to a2) and nanogrids (spectrum b0 to b2) are shown in figures 6(a) and (b), respectively; the baselines of the spectra are all deducted. The detection of G (∼1590 cm−1, superposed with the vcc of R6G) and G’ (∼2690 cm−1 ) bands testifies the presence of graphene (see Raman spectra a2 and b2). Over a period of one month, the SERS signals decreased dramatically for unpassivated Ag bowtie nanoantenna arrays (spectrum a1) and nanogrids (spectrum b1), as shown in figures 6(a) and (b). In contrast, the graphene-covered Ag bowtie nanoantenna arrays (spectrum a2) and nanogrids (spectrum b2) showed a stronger protection of initial SERS properties (spectra a0 and b0). The SERS enhancement factors (EF) of graphene-filmcoated Ag nanostructures (bowtie nanoantenna arrays and nanogrids) were estimated using the following equation [39, 40]:
(
EF = ( I2 / C2 ) ( I1/ C1), where I1 is the Raman intensity with a concentration C1 on unpassivated Ag nanostructures exposed to the atmosphere a month later. I2 can be obtained from Ag nanostructures covered with graphene exposed to the atmosphere a month later with an analyte concentration of C2. In the studies, the laser wavelength and power, microscopic magnification and spectrometer were identical. Also, C1 is equal to C2 (approximate and same monolayer R6G molecules, 10−6 M 7
)
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incident light wavelengths of 488 nm. As for SERS, |E|4 is the Raman enhancement factor of the electromagnetic effect [13]. The maximum SERS enhancements on the graphene-filmcoated Ag nanostructures and bare Ag nanostructures correspond to the theoretical values of 2.3 × 106 and 1.4 × 103, respectively. Furthermore, the local electric field enhancement (log |E|2) distribution of the initial Ag nanogrids is shown in figure 7(b0). Similar to the result of Ag bowtie nanoantenna arrays, the electromagnetic enhancement |E|2 of the graphene-film-coated Ag nanogrids is stronger than that of the bare Ag nanogrids (1.0 × 102 vs 4 × 100) at incident light wavelengths of 488 nm after exposure to the atmosphere a month later. The thicker Ag2S layer greatly compromises the local electromagnetic field and SERS activity.
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4. Conclusion In summary, we have reported that after being coated with a high-quality, CVD-synthesized graphene, which is an optically transparent and chemistry-inertness material in the visible region, a Ag nanostructure-based substrate showed excellent SERS properties. Ag bowtie nanoantenna arrays and Ag nanogrids were fabricated using plasma-assisted NSL. Serving as the protective layer, graphene efficiently protected the sulfidation of the Ag bowtie nanoantenna arrays and Ag nanogrids surfaces in air. The surfaces of the Ag nanostructures covered by graphene were smooth and retained their original shape after one month later in air. In addition, the Ag nanostructure substrate coated with a monolayer graphene film showed a higher enhancement of Raman signals and the electromagnetic field than the uncoated substrate. The averaged EFs of the graphene-film-coated Ag nanostructures were estimated to be about 21 and 5 for Ag bowtie nanoantenna arrays and nanogrids exposed to the atmosphere a month later, respectively. The experimental results were confirmed by theoretical calculations. The approach of graphene-coated metal nanostructures has the prospect to be commonly applied to other plasmonic devices such as Cu or Al nanostructures. The wavelength ranges of Cu are in the near-IR spectrum, while Al is in the deep-UV spectrum. Thus, graphene-coated metal nanostructures have the potential to develop a wide range of plasmonic applications.
Acknowledgments This work was partially supported by the NSFC (51171132, U1260102, 51201115, 51371079, 51371131 and 11375134), the NCET (12-0418), the China Postdoctoral Science Foundation (2014M550406), the Hubei Provincial Natural Science Foundation (2011CDB270, 2012FFA042), the Jiangsu Provincial Natural Science Foundation (BK20141217), the Wuhan Planning Project of Science and Technology (2014010101010019) and the experimental technology project of Wuhan University. 8
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[36] Moulder J F, Chastain J and King R C 1995 Handbook of XRay Photoelectron Spectroscopy: a Reference Book of Standard Spectra for Identification and Interpretation of XPS Data (Eden Prairie, MN: Physical Electronics) [37] Bae S et al 2010 Nat Nano 5 574–8 [38] Elechiguerra J L, Larios-Lopez L, Liu C, Garcia-Gutierrez D, Camacho-Bragado A and Yacaman M J 2005 Chem. Mater. 17 6042–52 [39] Liu R, Liu J, Zhou X, Sun M T and Jiang G 2011 Anal. Chem. 83 9131–5 [40] Le Ru E, Blackie E, Meyer M and Etchegoin P 2007 J. Phys. Chem. C 111 13794–803
[31] Li X, Li J, Zhou X, Ma Y, Zheng Z, Duan X and Qu Y 2014 Carbon 66 713–9 [32] Dai Z, Xiao X, Liao L, Zheng J, Mei F, Wu W, Ying J, Ren F and Jiang C 2013 Appl. Phys. Lett. 103 041903–7 [33] Dai Z, Xiao X, Zhang Y, Ren F, Wu W, Zhang S, Zhou J, Mei F and Jiang C 2012 Nanotechnology 23 335701–6 [34] Xu Y, Bai H, Lu G, Li C and Shi G 2008 J. Am. Chem. Soc. 130 5856–7 [35] Li X, Cai W, An J, Kim S, Nah J, Yang D, Piner R, Velamakanni A, Jung I and Tutuc E 2009 Science 324 1312–4
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Monolayer graphene on nanostructured Ag for enhancement of surface-enhanced Raman scattering stable platform
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 125603 (http://iopscience.iop.org/0957-4484/26/12/125603) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 132.239.1.231 This content was downloaded on 13/06/2017 at 11:47 Please note that terms and conditions apply.
You may also be interested in: Improved surface-enhanced Raman scattering on arrays of gold quasi-3D nanoholes Weisheng Yue, Yang Yang, Zhihong Wang et al. Enhanced Raman scattering of graphene by silver nanoparticles with different densities and locations Hai-Bin Sun, Can Fu, Yan-Jie Xia et al. Large-area, reproducible and sensitive plasmonic MIM substrates for surface-enhanced Raman scattering Kuanguo Li, Yong Wang, Kang Jiang et al. Surface enhanced Raman scattering of gold nanoparticles supported on copper foil with graphene as a nanometer gap Quan Xiang, Xupeng Zhu, Yiqin Chen et al. A wafer-scale backplane-assisted resonating nanoantenna array SERS device created by tunable thermal dewetting nanofabrication Te-Wei Chang, Manas Ranjan Gartia, Sujin Seo et al. Synthesis of wheatear-like ZnO nanoarrays decorated with Ag nanoparticles and its improved SERS performance through hydrogenation Yufeng Shan, Yong Yang, Yanqin Cao et al. Large-scale uniform Au nanodisk arrays fabricated via x-ray interference lithography for reproducible and sensitive SERS substrate Pingping Zhang, Shumin Yang, Liansheng Wang et al.
Nanotechnology Nanotechnology 26 (2015) 125603 (9pp)
doi:10.1088/0957-4484/26/12/125603
Monolayer graphene on nanostructured Ag for enhancement of surface-enhanced Raman scattering stable platform Zhigao Dai1,6, Fei Mei2,6, Xiangheng Xiao1,5, Lei Liao1, Wei Wu3, Yupeng Zhang1, Jianjian Ying1, Lingbo Wang4, Feng Ren1 and Changzhong Jiang1 1
Department of Physics, Hubei Nuclear Solid Physics Key Laboratory and Center for Ion Beam Application, Wuhan University, Wuhan 430072, People’s Republic of China 2 School of Electrical and Electronic Engineering, Hubei University of Technology, Wuhan 430068, People’s Republic of China 3 School of Printing and Packaging, Wuhan University, Wuhan 430072, People’s Republic of China 4 College of Electronic Information Engineering, Wuhan Polytechnic, Wuhan 430074, People’s Republic of China 5 Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, USA E-mail: [email protected] Received 6 January 2015 Accepted for publication 9 February 2015 Published 6 March 2015 Abstract
We have reported that the Ag nanostructure-based substrate is particularly suitable for surfaceenhanced Raman scattering when it is coated with monolayer graphene, an optically transparent and chemistry-inertness material in the visible range. Ag bowtie nanoantenna arrays and Ag nanogrids were fabricated using plasma-assisted nanosphere lithography. Our measurements show that atmospheric sulfur containing compounds are powerless to break in the monolayer graphene to vulcanize the surfaces of the Ag bowtie nanoantenna arrays and Ag nanogrids by various means, including scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS). Furthermore, the Ag nanostructure substrate coated with the monolayer graphene film shows a larger enhancement of Raman activity and the electromagnetic field than the uncoated substrate. Compared with those of bare Ag nanostructures, the averaged EFs of graphene-film-coated Ag nanostructures were estimated to be about 21 and 5 for Ag bowtie nanoantenna arrays and nanogrids after one month later in air, respectively. These observations are further supported by theoretical calculations. Keywords: Ag nanostructure-based substrate, sulfidation, surface-enhanced Raman scattering (Some figures may appear in colour only in the online journal) 1. Introduction
most influential analytical techniques for label-free single molecule detection and high-resolution vibrational information [7–9]. An ideal platform for a SERS substrate is identified as one with high durability, high repeatability and low cost that is also easy to manufacture and so on [10–14]. The SERS-active substrates are typically built using Ag or gold nanostructures. In the visible regime, Ag nanostructures are normally favored over gold as an energetic SERS element due to their enhancement factor, which is up to two orders of
Incident light can be converted to the nanoscale, restricted by noble metal nanostructures that powerfully improved the optical fields [1–3]. When molecules are located in the ‘hot spot,’ the Raman signals can be dramatically enhanced [4–6]. Surface-enhanced Raman scattering (SERS) is one of the 6
These authors contributed equally to this work
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magnitude larger [15–18]. However, using Ag nanostructures as substrates is always associated with severe drawback in terms of chemical durability. To avoid oxidation or sulfuration, Ag nanoparticles with a thin and inert SiO2 shell were employed [19], yet the thicker SiO2 layer (>2 nm) greatly compromises the SERS activity [20]. Graphene, a single atomic layer of graphite, is a promising candidate for applications in photonics and optoelectronics by virtue of its unique electronic, optical and mechanical properties [21, 22]. Graphene provides the ideal prototype test material for the investigation of SERS [23]. In a graphene-mediated SERS (G-SERS) substrate [24–28], the monolayer graphene provides an atomically flat surface for Raman enhancement. Graphene was revealed to be superior to metallic surfaces in terms of biocompatibility and chemical durability [29–31]. However, the physical mechanism underlying the enhancement and stability of a SERS platform with graphene has not been systematically explored. In our previous articles, extensive-area, uniform Ag bowtie nanoantenna arrays were employed as a highly active SERS platform fabricated by the integration of nanosphere lithography (NSL) and succedent oxygen plasma etching [32, 33]. Here, we introduce an enhancement and a stable SERS-active platform composed of two-dimensional atomic crystal graphene and large-area Ag nanostructures. The thickness of graphene is 0.355 nm [34], which should not impede SERS activity of the Ag nanostructures that are hypersensitized and very localized. With the surface passivated by graphene, the Ag nanostructure can be protected by effectively blocking the diffusion of gas molecules through transferred monolayer graphene. The SERS properties and electromagnetic field enhancement of graphene-coated Ag nanostructures were still excellent. Furthermore, these observations are further supported by our theoretical calculations.
Figure 1. Schematic illustrations of the graphene transfer process
(the size of the atoms is only for visual simplicity and not drawn to scale). 2.2. Transfer of graphene films onto the Ag nanostructures
The monolayer graphene films were transferred onto the Ag nanostructures, as shown in figure 1. High-Quality, monolayer graphene was grown by chemical vapor deposition (CVD) on 25 μm thick polycrystalline Cu foils with CH4 as the carbon source gas [35]. To begin with, the graphenecovered copper foil was spin-coated with a poly(methyl methacrylate) (PMMA) (MicroChem, 950 000 MW, 9–6 wt. % in anisole) film, which was then cured at 120 °C for 30 min. Graphene grew on both sides of the Cu foil. After one side of the Cu/graphene was coated with PMMA, the opposite side of the graphene was etched away by plasma cleaning. Then, a PMMA/graphene membrane was obtained by etching away the copper foil in an aqueous solution of iron chloride (0.4 g mL−1) with a bit of HCl in less than 30 min. The PMMA/graphene stack was washed with an aqueous solution of HCl and was transferred to deionized water to wash away all kinds of residual ions. The aqueous solution of HCl can prevent the hydrolysis of ferric ions. Furthermore, the PMMA/graphene stack was placed on the target substrate and dried at 50 °C for 30 min. Finally, the PMMA layer was dissolved by acetone.
2. Experimental section 2.1. Preparation of Ag bowtie nanoantenna arrays and Ag nanogrids
The Ag nanostructures were fabricated using plasma-assisted NSL. For a more in-depth study of graphene-covered Ag nanostructures used for the SERS platform, the Ag bowtie nanoantenna arrays and Ag nanogrids were fabricated by the integrate of NSL and succedent oxygen plasma etching, respectively. The details of the colloid lithography and subsequent oxygen plasma processing were shown in our previous articles [32, 33]. For the Ag bowtie nanoantenna arrays (Ag nanogrids), the oxygen plasma etching time was 5 min (9 min) on polystyrene (PS) particle masks. Subsequently, a 10 nm thick chromium film was thermally evaporated onto the as-prepared PS particle masks, followed by evaporation of a 60 nm thick Ag film. The PS particles were removed by sonication in chloroform for a few seconds, leaving only the Ag nanostructures on the substrate’s surface.
2.3. Characterization
SEM (FEI Sirion FEG) was used to characterize the surface morphology. The x-ray photoelectron spectroscopy (XPS) analysis was done on a Thermo Scientific ESCALAB 250Xi system with Al kα (1486.6 eV) as the radiation source under a vacuum of 5 × 10−7 Pa. The Raman measurements were performed with a commercial Raman microscope (HR800, Horiba). A laser emitting at 488 nm served as the excitation 2
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Exposed To Atmosphere One Month Later (a1) (a0) 1
(a2) 2 500 nm 1:bare Ag nanostructures 2:covered with the graphene (b1) (b0) 1
(b2) 2 500 nm
Figure 2. SEM images of initial Ag bowtie nanoantenna arrays (a0) and nanogrids (b0); SEM images of bare Ag bowtie nanoantenna arrays (a1) and nanogrids (b1) exposed to the atmosphere a month later; SEM images of Ag bowtie nanoantenna arrays (a2) and nanogrids (b2) covered with graphene exposed to the atmosphere a month later.
Figure 3. (a) AES depth profile of Ag, the S element and the (b) TEM image of Ag bowtie nanoantenna arrays exposed to the atmosphere a
month later.
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Figure 4. I2D/IG Raman mapping of graphene covered on the randomly selected SiO2/Si (a), Ag bowtie nanoantenna arrays (b) and nanogrids (c); their corresponding Raman spectra are on the bottom.
3. Results and discussion
source. The laser beam was focused onto the sample through an X100 objective lens. The Raman spectra were collected in the frequency range of 1000 to 2800 cm−1 with an acquisition time of 10 s.
The Ag bowtie nanoantenna arrays and Ag nanogrids were fabricated by the integration of NSL and succedent oxygen plasma etching, as shown in figures 2(a0) and (b0), respectively. A part of the Ag bowtie nanoantenna arrays and nanogrids were left as bare Ag (see figures 2(a0) and (b0)), while the other part was covered with a monolayer graphene. The as-prepared Ag nanostructures were placed in a surrounding laboratory environment for one month. Bare Ag bowtie nanoantenna arrays and nanogrids show large morphological changes in figures 2(a1) and (b1). However, as seen in figures 2(a2) and (b2), the Ag nanostructures coated by graphene show signs of invariabilities; the surfaces of the nanostructures were smooth and retained their original shape. We carried out the thickness of Ag2S by Auger electron spectroscopy (AES). The depth profile monitoring of the elements of Ag and S are shown in figure 3(a). The results indicate that Ag2S almost disappears after the sputtering time of 200 s. The thickness of 60 nm ABNA almost disappeared after the sputtering time of 1800 s on nanostructure Ag after it was exposed to the atmosphere a year later. So, the thickness of Ag2S was estimated to be about ∼200/(1800/60) ∼ 6.5 nm, which is larger than that of monolayer graphene (0.35 nm). Furthermore, the TEM measurement of nanostructured Ag after exposure to the atmosphere a month later is shown in figure 3(b). The thickness of Ag2S is about 5 nm in single ABNA, which well matches with the result of the AES measurement (about 6.5 nm).
2.4. Simulation
The electromagnetic field distributions of Ag nanostructures were simulated with the finite difference time domain (FDTD) method. The Ag nanostructures arrays obtained in our experiment could be considered as a periodical structure. We are able to computationally model the rectangular unit cells approximately 1.68 μm2 in size. To model the Ag nanostructures, we use the FDTD method with periodic boundary conditions (PBC). The implementation of PBC is straightforward for normal incidence illumination. The mesh size was 1 nm, and the simulated area was 1.2 μm × 1.4 μm. The electromagnetic field distribution of the initial Ag nanostructures (the thickness of Ag is 60 nm), bare nanostructures (the thicknesses of Ag and Ag2S are 55 nm and 5 nm, respectively) and Ag nanostructures covered with graphene (the thickness of Ag is 60 nm, and the thickness of graphene is 0.35 nm) exposed to the atmosphere a month later were calculated at incident light wavelengths of 488 nm. The dielectric constant of the Ag, Ag2S and graphene were taken from the source program. The propagation direction of the plane waves was along the z-axis. The electric field was assumed to be polarized along the x-axis. 4
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Figure 5. (a) XPS Ag 2p5/2 and Ag 2p3/2 spectra, (b) O 1s spectra, (c) C 1s spectra and (d) S 2p3/2 and S 2p1/2 spectra of initial Ag bowtie nanoantenna arrays ((a0), (b0), (c0) and (d0)); bare Ag bowtie nanoantenna arrays exposed to the atmosphere a month later ((a1), (b1), (c1) and (d1)) and Ag bowtie nanoantenna arrays covered with graphene exposed to the atmosphere a month later ((a2), (b2), (c2) and (d2)), respectively.
To confirm the Ag nanostructures covered with a large monolayer graphene, point-by-point I2D/IG Raman mapping of graphene was collected on SiO2/Si, Ag bowtie nanoantenna arrays and nanogrids with a randomly selected 10 × 10 μm2 area 2 μm in step length, as illustrated in figure 4. The intensity ratio of 2D to G bands was about 1–5 on SiO2/ Si, as shown in figure 4(a), which is characteristic for a monolayer graphene sheet with a relatively low defect density in our CVD-synthesized graphene. After the graphene was transferred onto the Ag nanostructures, most of the intensity ratios of the 2D to G bands were about 1–5 on the Ag bowtie nanoantenna arrays and nanogrids, as illustrated in figures 4(b) and (c). Their corresponding Raman spectra are shown in figure 4 below the I2D/IG Raman mapping. Also, only the G and 2D bands were observed, indicating that the Ag nanostructures did not obviously affect the quality of our synthesized graphene.
Using XPS analysis, the evolution for bare Ag bowtie nanoantenna arrays and the graphene-covered Ag bowtie nanoantenna arrays after one month is given in figure 5. The existence of Ag, O, C and S on the surface of the serial Ag bowtie nanoantenna arrays was investigated. The binding energy values of three samples of the Ag and O elements were similar. The binding energy of the O 1s peak proves that no Ag (I) oxide was formed because the binding energy of the O 1s peak in Ag (I) oxide is 528.6 eV [36], which is much lower than the value shown in the three samples. The XPS for C 1s spectra of initial Ag bowtie nanoantenna arrays (spectrum a0), the C 1s spectra of bare Ag bowtie nanoantenna arrays (spectrum a1) and the graphene-covered Ag bowtie nanoantenna arrays (spectrum a2) after one month are all shown in figure 5(c), respectively. Monolayer graphene is a single atomic layer of carbon, and the increase of peak strength of C 1s spectra proves the existence of graphene in 5
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Figures 6. (a) and (b) The baseline deducted SERS spectra of R6G molecules adsorbed on the initial Ag bowtie nanoantenna arrays (a0) and nanogrids (b0); bare Ag bowtie nanoantenna arrays (a1) and nanogrids (b1) exposed to the atmosphere a month later; Ag bowtie nanoantenna arrays (a2) and nanogrids (b2) covered with graphene exposed to the atmosphere a month later. Comparison of intensities of the bands at 1363, 1508, 1576 and 1650 cm−1 in Ag bowtie nanoantenna arrays (c) and nanogrids (d).
spectrum c2, as shown in figure 5(c). Similar XPS peaks have also been reported for graphene films transferred on SiO2/Si substrates [37]. For the S 2p spectra, the peaks corresponding to S 2p3/2 and S 2p1/2 have binding energies of 161 and 162 eV in the spectrum d1 collected from the bare Ag bowtie nanoantenna arrays after one month, as shown in figure 5(d). These values are reported data for Ag2S [36]. As shown in the previous paper, the surfaces of the Ag nanostructure can be vulcanized by atmospheric sulfur-containing compounds, according to reaction 1. 2Ag + H 2 S → Ag2 S + H 2 .
Also, the O2 and NO2 can enhance the sulfidation process, according to the following reactions: [38] (3)
2Ag + H 2 S + 2NO2 → Ag2 S + 2HNO2 .
(4)
By covering the surface of Ag nanoantennas with graphene, the peaks of S 2p3/2 and S 2p1/2 are extremely weak in the spectrum d2 collected from the graphene-covered Ag bowtie nanoantenna arrays after one month, as shown in figure 5(d). The sulfidation of the Ag surface can be avoided by successfully blocking the diffusion of gas molecules through monolayer graphene. The SERS spectra of Rhodamine 6G (R6G) adsorbed on Ag nanostructures were measured to investigate how successful graphene passivation retained the SERS activities. For the SERS characterization, a concentration of 10−6 M R6G aqueous solution was used. In order to optimize the molecular adsorption, as-prepared substrates were immersed in the solution for 10 h, taken out, rinsed thoroughly with ethanol and finally dried with nitrogen gas. The R6G molecules’ SERS spectra on the series of Ag bowtie nanoantenna arrays
(1)
When no sources of H2S are available, OCS is the principal corrodent of silver in atmospheric conditions. As shown in reaction 2, in the presence of water, OCS rapidly decomposes to form H2S. The corrosion produced by OCS is important since it is the most abundant sulfur species in the atmosphere. OCS + H 2 O → H 2 S + CO2 .
2Ag + H 2 S + 1/2O2 → Ag2 S + H 2 O
(2)
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Figure 7. The FDTD calculated local electric field enhancement (log |E|2) of initial Ag bowtie nanoantenna arrays (a0) and nanogrids (b0);
bare Ag bowtie nanoantenna arrays (a1) and nanogrids (b1) exposed to the atmosphere a month later; Ag bowtie nanoantenna arrays (a2) and nanogrids (b2) covered with graphene exposed to the atmosphere a month later.
R6G). As shown in figures 6(c) and (d), using I1 and I2 of the peaks at 1363, 1508, 1576 and 1650 cm−1 of R6G, the averaged EF of graphene-film-coated Ag nanostructures is estimated to be about 21 and 5 for bare Ag bowtie nanoantenna arrays (figure 6(c)) and nanogrids (figure 6(d)) exposed to the atmosphere a month later, respectively. The AgS2 layer can greatly degrade the local electromagnetic field, resulting in weak SERS activity on the nanostructure’s surface. In order to further explore the physical mechanism of SERS enhancements of graphene-film-coated Ag nanostructures, the FDTD method, based on PBC, was applied to compute the spatial distributions of the electromagnetic field intensity for Ag bowtie nanoantenna arrays and nanogrids (theoretical calculation details in Experimental section 2.4). Figure 7(a0) shows the local electric field enhancement log E 2 distribution of the initial Ag bowtie nanoantenna arrays, where, |E| = |Elocal/Ein| and Elocal and Ein are the local and incident electric fields, respectively. The local electric field enhancement of bare Ag bowtie nanoantenna arrays and Ag bowtie nanoantenna arrays covered with graphene exposed to the atmosphere a month later are shown in figures 7(a1) and (a2), respectively. The input light is polarized along the x-axis. When exposed to the atmosphere a month later, the electromagnetic enhancement |E|2 of the graphene-film-coated Ag nanostructures is stronger than that of the bare Ag nanostructures (1.5 × 103 vs 3.8 × 101) at
(spectrum a0 to a2) and nanogrids (spectrum b0 to b2) are shown in figures 6(a) and (b), respectively; the baselines of the spectra are all deducted. The detection of G (∼1590 cm−1, superposed with the vcc of R6G) and G’ (∼2690 cm−1 ) bands testifies the presence of graphene (see Raman spectra a2 and b2). Over a period of one month, the SERS signals decreased dramatically for unpassivated Ag bowtie nanoantenna arrays (spectrum a1) and nanogrids (spectrum b1), as shown in figures 6(a) and (b). In contrast, the graphene-covered Ag bowtie nanoantenna arrays (spectrum a2) and nanogrids (spectrum b2) showed a stronger protection of initial SERS properties (spectra a0 and b0). The SERS enhancement factors (EF) of graphene-filmcoated Ag nanostructures (bowtie nanoantenna arrays and nanogrids) were estimated using the following equation [39, 40]:
(
EF = ( I2 / C2 ) ( I1/ C1), where I1 is the Raman intensity with a concentration C1 on unpassivated Ag nanostructures exposed to the atmosphere a month later. I2 can be obtained from Ag nanostructures covered with graphene exposed to the atmosphere a month later with an analyte concentration of C2. In the studies, the laser wavelength and power, microscopic magnification and spectrometer were identical. Also, C1 is equal to C2 (approximate and same monolayer R6G molecules, 10−6 M 7
)
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incident light wavelengths of 488 nm. As for SERS, |E|4 is the Raman enhancement factor of the electromagnetic effect [13]. The maximum SERS enhancements on the graphene-filmcoated Ag nanostructures and bare Ag nanostructures correspond to the theoretical values of 2.3 × 106 and 1.4 × 103, respectively. Furthermore, the local electric field enhancement (log |E|2) distribution of the initial Ag nanogrids is shown in figure 7(b0). Similar to the result of Ag bowtie nanoantenna arrays, the electromagnetic enhancement |E|2 of the graphene-film-coated Ag nanogrids is stronger than that of the bare Ag nanogrids (1.0 × 102 vs 4 × 100) at incident light wavelengths of 488 nm after exposure to the atmosphere a month later. The thicker Ag2S layer greatly compromises the local electromagnetic field and SERS activity.
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4. Conclusion In summary, we have reported that after being coated with a high-quality, CVD-synthesized graphene, which is an optically transparent and chemistry-inertness material in the visible region, a Ag nanostructure-based substrate showed excellent SERS properties. Ag bowtie nanoantenna arrays and Ag nanogrids were fabricated using plasma-assisted NSL. Serving as the protective layer, graphene efficiently protected the sulfidation of the Ag bowtie nanoantenna arrays and Ag nanogrids surfaces in air. The surfaces of the Ag nanostructures covered by graphene were smooth and retained their original shape after one month later in air. In addition, the Ag nanostructure substrate coated with a monolayer graphene film showed a higher enhancement of Raman signals and the electromagnetic field than the uncoated substrate. The averaged EFs of the graphene-film-coated Ag nanostructures were estimated to be about 21 and 5 for Ag bowtie nanoantenna arrays and nanogrids exposed to the atmosphere a month later, respectively. The experimental results were confirmed by theoretical calculations. The approach of graphene-coated metal nanostructures has the prospect to be commonly applied to other plasmonic devices such as Cu or Al nanostructures. The wavelength ranges of Cu are in the near-IR spectrum, while Al is in the deep-UV spectrum. Thus, graphene-coated metal nanostructures have the potential to develop a wide range of plasmonic applications.
Acknowledgments This work was partially supported by the NSFC (51171132, U1260102, 51201115, 51371079, 51371131 and 11375134), the NCET (12-0418), the China Postdoctoral Science Foundation (2014M550406), the Hubei Provincial Natural Science Foundation (2011CDB270, 2012FFA042), the Jiangsu Provincial Natural Science Foundation (BK20141217), the Wuhan Planning Project of Science and Technology (2014010101010019) and the experimental technology project of Wuhan University. 8
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