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Facile Fabrication of Homogeneous 3D Silver Nanostructures on Gold-Supported Polyaniline Membranes as Promising SERS Substrates Ping Xu,†,‡ Nathan H. Mack,† Sea-Ho Jeon,† Stephen K. Doorn,† Xijiang Han,*,‡ and Hsing-Lin Wang*,† † Physical Chemistry and Spectroscopy, Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, and ‡Department of Chemistry, Harbin Institute of Technology, Harbin 150001, China
Received December 9, 2009. Revised Manuscript Received January 27, 2010 We report a facile synthesis of large-area homogeneous three-dimensional (3D) Ag nanostructures on Au-supported polyaniline (PANI) membranes through a direct chemical reduction of metal ions by PANI. The citric acid absorbed on the Au nuclei that are prefabricated on PANI membranes directs Ag nanoaprticles (AgNPs) to self-assemble into 3D Ag nanosheet structures. The fabricated hybrid metal nanostructures display uniform surface-enhanced Raman scattering (SERS) responses throughout the whole surface area, with an average enhancement factor of 106-107. The nanocavities formed by the stereotypical stacking of these Ag nanosheets and the junctions and gaps between two neighboring AgNPs are believed to be responsible for the strong SERS response upon plasmon absorption. These homogeneous metal nanostructure decorated PANI membranes can be used as highly efficient SERS substrates for sensitive detection of chemical and biological analytes.
1. Introduction Surface-enhanced Raman scattering (SERS) is currently recognized as one of the most promising spectroscopic probes for ultrasensitive detection of chemical and biological analytes.1-5 Electromagnetic hot spots, which typically reside in interstitial voids of metal nanoparticles and metal structures with intersections, bifurcations, and high radius of curvatures, are believed to be primarily responsible for the huge amplifications seen in single molecule SERS. However, controlled synthesis of these “hot spots” is difficult in most SERS active materials as it requires exquisite preparation of metal nanoparticles with certain sizes and morphology and delicate manipulation of the nanoparticle assemblies. To this aim, developing synthetic approaches to making metal nanoparticles with well-defined structures is greatly desired for realizing highly sensitive SERS devices.6-8 Though a number
of promising silver nanostructures have been found to be efficient SERS-active platforms,9-17 fabrication of highly regular and reproducible SERS substrates remains a great challenge. Recent studies have shown an extremely strong local field enhancement in the gap between two closely spaced silver nanoparticles.18-22 Formation of such dimeric structures usually requires the use of organic and biological molecules to bridge two nanoparticles; however, the bridging molecules also prevent the target analyte from further being absorbed onto the metal surface.23-29 Furthermore, preparation of uniform and large-area SERS active substrates consisting of metal nanoparticle dimers for commercial applications is simply not practical. Recently, we have shown that through a direct chemical reduction of metal ions by polyaniline (PANI), metal nanostructures with well-defined morphology and size can be deposited on PANI membranes through controlled modulation of the chemical nature (dopant) of the PANI substrate.30 Platinum and palladium
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(1) Nie, S.; Emory, S. R. Science 1997, 275, 1102. (2) Zhang, J.; Gao, Y.; Alvarez-Puebla, R. A.; Buriak, J. M.; Fenniri, H. Adv. Mater. 2006, 18, 3233. (3) Schmuck, C.; Wich, P.; K€ustner, B.; Kiefer, W.; Schl€ucker, S. Angew. Chem., Int. Ed. 2007, 46, 4786. (4) Doering, W. E.; Piotti, M. E.; Natan, M. J.; Freeman, R. G. Adv. Mater. 2007, 19, 3100. (5) Stiles, P. L.; Dieronger, J. A.; Shah, N. C.; Van Duyne, R. P. Annu. Rev. Anal. Chem. 2008, 1, 601. (6) Katz, E.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 6042–6108. (7) Courty, A.; Henry, A.-I.; Goubet, N.; Pileni, M.-P. Nat. Mater. 2007, 6, 900– 907. (8) Linh Tran, M.; Centeno, S. P.; Hutchison, J. A.; Engelkamp, H.; Liang, D.; Van Tendeloo, G.; Sels, B. F.; Hofkens, J.; Uji-i, H. J. Am. Chem. Soc. 2008, 130, 17240. (9) Tao, A. R.; Kim, F.; Hess, C.; Goldberger, J.; He, R.; Sun, Y.; Xia, Y.; Yang, P. Nano Lett. 2003, 3, 1229. (10) Svedberg, F.; Li, Z.; Xu, H.; K€all, M. Nano Lett. 2006, 6, 2639. (11) McLellan, J. M.; Li, Z.-Y.; Siekkinen, A. R.; Xia, Y. Nano Lett. 2007, 7, 1013. (12) Wiley, B. J.; Chen, Y.; McLellan, J. M.; Xiong, Y.; Li, Z.-Y.; Ginger, D.; Xia, Y. Nano Lett. 2007, 7, 1032. (13) Braun, G.; Lee, S. J.; Dante, M.; Nguyen, T.-Q.; Moskovits, M.; Reich, N. J. Am. Chem. Soc. 2007, 129, 6378. (14) Fang, Y.; Seong, N.-H.; Dlott, D. D. Science 2008, 321, 388. (15) Yoon, I.; Kang, T.; Choi, W.; Kim, J.; Yoo, Y.; Joo, S.-W.; Park, Q.-H.; Ihee, H.; Kim, B. J. Am. Chem. Soc. 2009, 131, 758.
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(16) Baik, J. M.; Lee, S. J.; Moskovits, M. Nano Lett. 2009, 9, 672. (17) Pietrobon, B.; McEachran, M.; Kitaev, V. ACS Nano 2009, 3, 21. (18) Sawai, Y.; Takimoto, B.; Nabika, H.; Ajito, K.; Murakoshi, K. J. Am. Chem. Soc. 2007, 129, 1658. (19) Ringler, M.; Klar, T. A.; Schwemer, A.; Susha, A. S.; Stehr, J.; Raschke, G.; Funk, S.; Borowski, M.; Nichtl, A.; K€urzinger, K.; Phillips, R. T.; Feldmann, J. Nano Lett. 2007, 7, 2753. (20) Camden, J. P.; Dieringer, J. A.; Wang, Y.; Masiello, D. J.; Marks, L. D.; Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc. 2008, 130, 12616. (21) Camargo, P. H. C.; Rycenga, M.; Au, L.; Xia, Y. Angew. Chem., Int. Ed. 2009, 48, 2180. (22) Li, W.; Camargo, P. H. C.; Lu, X.; Xia, Y. Nano Lett. 2009, 9, 485. (23) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (24) Alivisatos, A. P.; Johnsson, K. P.; Peng, X. G.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P.; Schultz, P. G. Nature 1996, 382, 609. (25) Loweth, C. J.; Caldwell, W. B.; Peng, X. G.; Alivisatos, A. P.; Schultz, P. G. Angew. Chem., Int. Ed. 1999, 38, 1808. (26) Novak, J. P.; Feldheim, D. L. J. Am. Chem. Soc. 2000, 122, 3979. (27) Sardar, R.; Heap, T. B.; Shumaker-Parry, J. S. J. Am. Chem. Soc. 2007, 129, 5356. (28) Park, S. Y.; Lytton-Jean, A. K. R.; Lee, B.; Weigand, S.; Schatz, G. C.; Mirkin, C. A. Nature 2008, 451, 553. (29) Wang, X.; Li, G.; Chen, T.; Yang, M.; Zhang, Z.; Wu, T.; Chen, H. Nano Lett. 2008, 8, 2643. (30) Wang, H.-L.; Li, W.; Jia, Q. X.; Akhadov, E. Chem. Mater. 2007, 19, 520.
Published on Web 02/16/2010
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Figure 1. SEM images of (a) Au nanoparticles grown on a PANI membrane (doped by citric acid) by immersing the PANI membrane in 10 mM AuCl3 aqueous solution for 10 s, and the Ag nanostructures produced by immersing the Au nanolayer-supported PANI membrane in 50 mM AgNO3 aqueous solution for (b) 10 s, (c) 30 s, (d) 1 min, (e) 10 min, and (f) 60 min. The scale bar is 500 nm.
nanostructures chemically deposited on our PANI membranes can be used as highly efficient catalysts for regioselective hydrosilylation reaction and selective hydrogenation of alkynes and cinnamaldehyde, respectively.31,32 Similarly, metal nanoparticles embedded onto PANI nanowires exhibit catalytic functions and are understood as pseudo homogeneous catalysts for Suzuki coupling between aryl chlorides and phenylboronic acid, and for phenol formation from aryl halides and potassium hydroxide in water and air.33 This nanowire/metal nanoparticle composite was also incorporated as a component of a nonvolatile memory device.34 Thus, the PANI substrate is emerging as an effective medium for generation of complex, functional metal nanostructures. Herein, we demonstrate the facile synthesis of homogeneous three-dimensional (3D) Ag nanostructures on Au-supported PANI membranes; a means to generate SERS substrates with a dense collection of electromagnetic hot spots and hence strong SERS responses over the whole surface area. The Ag nanosheets that form the 3D structures are actually single thin Ag layers assembled by uniform Ag nanoparticles (AgNPs). The nanocavities formed by the stereotypical stacking of these Ag nanosheets and the junctions and gaps between two neighboring AgNPs are believed to be responsible for strong SERS response upon plasmon absorption.
2. Results and Discussion We have previously shown that microscale Ag yarn-balls grown on citric acid doped PANI membranes are actually comprised of an ensemble of Ag nanosheets.30 However, direct (31) Gao, Y.; Chen, C.-A.; Gau, H.-M.; Bailey, J. A.; Akhadov, E.; Williams, D.; Wang, H.-L. Chem. Mater. 2008, 20, 2839. (32) Shih, H.-H.; Williams, D.; Mack, N. H.; Wang, H.-L. Macromolecules 2009, 42, 14. (33) Gallon, B. J.; Kojima, R. W.; Kaner, R. B.; Diaconescu, P. L. Angew. Chem., Int. Ed. 2007, 46, 7251. (34) Tseng, R. J.; Huang, J.; Ouyang, J.; Kaner, R. B.; Yang, Y. Nano Lett. 2005, 5, 1077.
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immersion of citric acid doped PANI membranes in AgNO3 aqueous solutions only produce scattered Ag microspheres with poor surface coverage on the PANI membrane (Figure S1 (Supporting Information)). A homogeneous SERS substrate should have a complete surface coverage and SERS response that is location independent. To this aim, we strive to prepare Ag structures that meet the above requirements via an electroless deposition method. In a typical experiment, a citric acid doped PANI membrane was first immersed in 10 mM AuCl3 aqueous solution for 10 s, where a shiny gold layer was immediately visualized on the PANI membrane (Au-PANI). Scanning electron microscopy (SEM) image of the gold layer (Figure 1a) shows a wide range of size distribution (10-100 nm) for the Au nanoparticles (AuNPs) and aggregates. Although the PANI membrane surface was primarily covered with metallic AuNPs, which themselves cannot spontaneously reduce silver ions because of the mismatch in their reduction potentials (1.5 V for Au3þ/Au0 versus 0.799 V for Agþ/Ag0), the underlying PANI substrate can readily act as a reducing agent with the AuNPs acting as an intermediate conductor in the redox reaction. When the Au-layer supported PANI membrane was immersed in a 50 mM AgNO3 aqueous solution, the AuNPs were coated with a conformal thin layer of Ag at early reaction times (Figure 1b). This bimetallic layer forms with the Au nanoparticles acting as high surface area nucleation sites for Ag reduction and results in an overall ripening of the deposited Ag particles into a near continuous Ag thin film. The conductive metallic nature of the Au seed particles makes them ideal electrochemical electrodes located away from the PANI surface, inhibiting Ag formation directly on the PANI substrate. Additionally, Au and Ag readily alloy, making Ag reduction at the Au surface a logical starting point for subsequent Ag deposition. Longer reaction times lead to the growth of uniform 20 nm Ag nanospheres on the newly formed dense Ag coated Au nanolayer (Figure 1c). It is interesting to note that sheet-like structures vertical to the PANI substrate start to form by way of self-assembling Ag nanospheres (Figure 1d). These nanosheets DOI: 10.1021/la904617p
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comprised of assemblies of many smaller AgNPs have become discernible at a reaction time of 10 min (Figure 1e). After immersion for 1 h, homogeneous and well-defined 3D Ag nanosheet assemblies are ultimately obtained on top of the gold layer (Ag-Au-PANI, Figure 1f). Close examination of the SEM images reveals that the nanosheets are ensembles of 40-60 nm AgNPs, and there are ample nanocavities formed by the repetitive stacking of the individual nanosheets. This process results in a hybrid metal layer of 500 nm thickness over the entire PANI surface (several square centimeters), however, the thickness and size of this silver nanosheet layer, to some extent, can be modulated with the reaction time. Inspection of SEM micrographs shows an increase in overall thickness with longer reaction times. Though only elemental Ag was detected from a surface energy-dispersive X-ray analysis (EDAX) of the Ag-Au-PANI, both elemental Au and Ag can be detected from the cross section of the hybrid metal layer (Figure S2 (Supporting Information)). This result suggests a complete surface coverage of AgNPs on top of the original Au layer, consistent with our proposed growth model. It is important to note that the 3D Ag nanostructures formed on the PANI membrane are highly sensitive to the chemical nature of the PANI itself, namely the dopants used. The morphology of the underlying Au particles will to some extent dictate the macroscopic morphology of the overall metallic film, however, the fine sheet-like nanostructures observed in the Ag layers is believed to result from preferential binding of the citric acid dopant to specific Ag crystal faces on the metal surface. Similar random AuNP assemblies were formed on PANI membranes doped by citric acid, R-(-)-camphorsulfonic acid, and R-(-)-mandelic acid, however, the subsequent Ag nanosheet structures were only observed on PANI membranes doped by citric acid. In contrast, featureless Ag blocks were produced on the R-(-)-camphorsulfonic acid and R-(-)-mandelic acid doped PANI membranes (Figures S3 and S4 (Supporting Information)). More importantly, no conformal Ag film was observed on the Au particles with these other dopants, as the gold nanoparticle layer is clearly distinguishable at intermediate growth times (Figure S5 (Supporting Information)). These results were consistent for a wide range of different doping acids (data not shown); only citric acid resulted in the formation of Ag nanosheets. These results suggest a limited morphological tunability of Ag particles simply by varying the PANI dopant; a result consistent with our previous report.30 To elucidate how citric acid impacts the nucleation and growth of Ag structures on PANI membranes, we immersed R-(-)camphorsulfonic acid and R-(-)-mandelic acid doped PANI membranes with Au-supported nanolayers into a solution of AgNO3 and citric acid ([Agþ]:[citric acid] = 20:1). Surprisingly, Ag nanosheet structures are now observed (Figure 2). Nanosheet structures assembled with Ag nanoparticles are produced on both Au layer-supported PANI membranes, regardless of the original doping acid used. These results suggest that citric acid plays a key role in the formation of Ag nanosheets by directing Ag growth through selective binding to Ag crystal faces. Additionally, during these studies, we found no dependence of the Ag morphology on the citric acid concentration. Presumably, only a surface adsorbed monolayer of citric acid needs to be present on the growing Ag surface to dictate Ag growth. All citric acid concentrations used were well above those typically associated with monolayer coverage. To evaluate the SERS activity of the above structures, we immersed the substrates in a 4 mmol/L mercaptobenzoic acid (MBA) ethanol solution and used a 785 nm Raman apparatus in 8884 DOI: 10.1021/la904617p
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Figure 2. SEM images of Ag nanostructures grown on the Au layer-supported PANI membrane doped by (a) R-(-)-camphorsulfonic acid and (b) R-(-)-mandelic acid by immersing the goldsupported PANI membranes in a mixture of AgNO3 aqueous solution and citric acid aqueous solution for 1 h.
backscatter configuration. Figure 3a shows the SERS spectra of MBA absorbed on Au-PANI and Ag-Au-PANI. The SERS spectrum of MBA is dominated by the ν8a (∼1590 cm-1) and ν12 (∼1080 cm-1) aromatic ring vibrations; other weak bands at ∼1150 and ∼1180 cm-1 are attributed to the C-H deformation modes.35-37 MBA, an organic molecule with a thiol group on one end and a carboxylic acid on the other end, has strong chemical interactions with Ag/Au metal surfaces, the strong covalent bond between MBA and Au/Ag however does not lead to strong electronic coupling with the surface plasmon;14 therefore, the SERS response mainly results from an electromagnetic enhancement which is common in metal structures with sharp edges, intersections and bifurcations, where nanoscaled roughness creates high local fields.11,38 Here, poor SERS response was found on the randomly assembled AuNPs, while the SERS signal is dramatically enhanced on the 3D silver nanosheet assemblies (35) Michota, A.; Bukowska, J. J. Raman Spectrosc. 2003, 34, 21. (36) Sun, Z.; Zhao, B.; Lombardi, J. R. Appl. Phys. Lett. 2007, 91, 221106. (37) Yang, L.; Jiang, X.; Ruan, W.; Zhao, B.; Xu, W.; Lombardi, J. R. J Phys. Chem. C 2008, 112, 20095. (38) Kneipp, K.; Kneipp, H.; Kneipp, J. Acc. Chem. Res. 2006, 39, 443.
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Figure 3. (a) SERS spectra of mercaptobenzoic acid (MBA) excited with the 785 nm laser line on Au-PANI and Ag-Au-PANI substrates, corresponding to Figures 1, parts a and f, respectively (accumulation time = 10 s, laser intensity = 1 mW). (b) A SERS map (40 μm by 40 μm) of the 1080 cm-1 band of MBA observed from Ag-Au-PANI substrate (step size = 1 μm, accumulation time = 1 s, laser intensity = 0.1 mW). Inset shows the histogram of the Raman signal from the SERS map.
Figure 4. (a) SEM image and (b) corresponding EDAX spectrum of the Ag-Au-PANI substrate after immersion in 4 mmol/L MBA ethanol solution for 30 min prior to SERS measurement.
on top of the Au nanolayer (Figure 3a). SERS spectra were recorded for substrates with long reaction times to ensure formation of equilibrium Ag nanostructures. All shorter reaction times resulted in decreased SERS intensity (data not shown). A strong band at ∼1400 cm-1 found on Au-PANI substrate could be ascribed to COO- groups resulting from deprotonation of MBA molecules by regions of bare PANI.35 A SERS spatial map composed of the intensity of the 1080 cm-1 band of MBA absorbed on the Ag-Au-PANI substrate is shown in Figure 3b. Here, one can find a homogeneous SERS response throughout the whole surface, with a scale of several tens of micrometers. The histogram of these SERS intensities indicates a relatively narrow distribution with only a ∼10% deviation from the mean. An average enhancement factor of 106-107 is calculated for this Ag-Au-PANI substrate, which is comparable to previously reported Ag substrates.9,39 The substrates shown in Figure 2 also exhibit strong SERS responses, with almost identical en(39) Luo, W.; van der Veer, W.; Chu, P.; Mills, D. L.; Penner, R. M.; Hemminger, J. C. J. Phys. Chem. C 2008, 112, 11609.
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hancement factors to that of the above Ag-Au-PANI substrate (Figure S6 (Supporting Information)). It is interesting to find that an SEM image of the Ag-Au-PANI substrate after immersion in the MBA ethanol solution prior to SERS measurement (Figure 4a) shows a very different and modified morphology as compared to that in Figure 1f. The surface was fully and uniformly covered by a layer of MBA molecules, as confirmed by the detection of C, O, and S elements in the EDAX spectrum (Figure 4b). This complete coverage of the Ag structures by MBA explains the homogeneous SERS response over the whole surface. Such uniform substrates are ideally suited for use as chemical and biological sensors. We think the strong SERS response from our substrates can be rationalized by the following: (i) the gaps and junctions between neighboring AgNPs in the nanosheets may give a similar electromagnetic enhancement as found in metal dimers;18-22 (ii) the nanocavities formed between silver nanosheets presents a uniform nanoscale roughness, and the electromagnetic wave excites localized surface plasmons on the surface, resulting in amplification of the electromagnetic fields near the metal surface.5 DOI: 10.1021/la904617p
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3. Conclusions In summary, homogeneous silver nanosheet assemblies with well-defined 3D structures can be fabricated on PANI membranes through an extremely simple methodology (direct chemical reduction by PANI). Both the prefabricated gold layer and surface chemistry of the PANI membrane play pivotal roles in the formation of Ag nanosheet structures. Citric acid absorbed onto the prefabricated gold nanoparticles can direct the growth of silver nanosheet assemblies with complete surface coverage over the PANI membrane. The fabricated hybrid structures show strong and uniform SERS response with an average enhancement factor of 106-107 and the histogram has a narrow distribution with only a 10.6% deviation from the mean. The nanocavities formed by the silver nanosheet stacks and the junctions and gaps between two silver nanoparticles may serve as effective SERS hot spots. We believe these highly regular and reproducible structures can be used as highly efficient SERS substrates for sensitive detection of chemical and biological analytes.
4. Experimental Section Materials. Polyaniline (PANI) emeraldine base (EB) powder was obtained from Aldrich. N-Methyl-2-pyrrolidone (NMP, 99% Aldrich), heptamethylenimine (HPMI, 98% Acros), AuCl3 (99% Aldrich), AgNO3 (99.9999% Aldrich), citric acid (99.9% Fisher), R-(-)-camphorsulfonic acid (98% Aldrich), R-(-)-mandelic acid (99% Aldrich), and mercaptobenzoic acid (MBA, Aldrich 90%) were used as received. Fabrication of PANI Membranes. PANI membranes are achieved by employing a phase inversion method using water as the coagulation bath.[6,7] In a typical experiment, 1.15 g of PANI (EB) powder, 4.14 g of NMP, and 0.747 g of HPMI were mixed in a 12 mL Teflon vial. The mixture was stirred for 0.5-1 h to form a homogeneous solution, followed by being poured onto a glass substrate and spread into a wet film using a gardener’s blade (Pompano Beach, FL) with a controlled thickness. The wet film was then immersed into a water bath and kept in the water bath for at least 24 h. The resulting membrane was then dried at room temperature for 12 h before doping in 0.25 M citric acid, 0.25 M R-(-)-camphorsulfonic acid, and 0.25 M R-(-)mandelic acid by immersion. The PANI membranes are doped in organic acid for 3 days before being used for preparation of metal nanoparticles. Preparation of Metal Nanostructures on PANI Membrane. (1) Preparation of silver nanostructures on gold layersupported PANI membrane: a piece of doped PANI membrane was first immersed in a 10 mM AuCl3 aqueous solution for 10 s until a shiny gold layer is visualized. The rinsed gold layersupported PANI membrane was then immersed in 50 mM AgNO3 aqueous solution for different time periods in order to
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study the structural evolution of silver nanostructures as a function of time. For comparison, pure PANI membranes (with no AuNPs) were also immersed in 50 mM AgNO3 aqueous solution for identical time periods. (2) To study the effect of citric acid on the growth of silver nanoparticles on PANI membrane, a solution mixture with 50 mM AgNO3 and 0.25 M citric acid (volume ratio: 100:1) was used. Au layer-supported PANI membranes doped by R-(-)-camphorsulfonic acid and R-(-)-mandelic acid were immersed in the above solution for 1 h. The PANI membranes after growing metal nanostructures were rinsed with water and dried in air before any characterization. Characterization. Scanning electron microscopic (SEM) images were taken on a FEI Inspect SEM, and the chemical composition was analyzed by energy dispersive X-ray analysis (EDAX Inc.). The metal-supported PANI membrane was immersed in an MBA ethanol solution (4 mmol/L) for 30 min before the surface-enhanced Raman scattering (SERS) response was determined. The SERS spectra were recorded on a Kaiser Raman spectrometer through a 20 (0.50 NA) microscope objective, coupled with a liquid-nitrogen-cooled charge-coupled device (CCD) detector (wavelength: 785 nm). The incident laser power was kept at 1 mW and total accumulation times of 10 s were employed. SERS intensity maps were collected on a micro Raman apparatus using a Ti:sapphire excitation laser tuned to 785 nm. The Raman spectra were recorded through a 100 microscope objective, filtered through a triple grating monochromator and dispersed onto a liquid nitrogen cooled CCD detector. The intensity map was generated from the integrated SERS intensity of the 1080 cm-1 band of MBA as the sample was rastered parallel to the microscope imaging plane. The incident laser power was kept constant at 0.1 mW with total accumulation times of 1 s and a step size of 1 μm was used.
Acknowledgment. P.X. thanks the support from the Joint Educational Ph.D. Program of Chinese Scholarship Council (CSC) and NSF of China (No. 20776032) and helpful discussions with Prof. Younan Xia, Prof. George C. Schatz, and Prof. Vladimir Kitaev about the growth of metal structures. H.L.W. acknowledges the financial support from Laboratory Directed Research and Development (LDRD) fund under the auspices of DOE. Partial financial support by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering, and by the National Nanotechnology Enterprise Development Center (NNEDC). This work was performed in part at the U.S. Department of Energy, Center for Integrated Nanotechnologies, at Los Alamos National Laboratory (Contract DE-AC52-06NA25396) and Sandia National Laboratories (Contract DE-AC04-94AL85000). Supporting Information Available: Figures showing additional SEM images and SERS spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
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