Facile Synthesis and Electrical Properties of Silver Wires through

Dec 1, 2010 - Facile Synthesis and Electrical Properties of Silver Wires through Chemical Reduction by. Polyaniline. Ping Xu,*,†,‡ Sea-Ho Jeon,†...
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J. Phys. Chem. C 2010, 114, 22147–22154

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Facile Synthesis and Electrical Properties of Silver Wires through Chemical Reduction by Polyaniline Ping Xu,*,†,‡ Sea-Ho Jeon,† Hou-Tong Chen,† Hongmei Luo,§ Guifu Zou,† Quanxi Jia,† Marian Anghel,† Christof Teuscher,| Darrick J. Williams,† Bin Zhang,‡ Xijiang Han,‡ and Hsing-Lin Wang*,† Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States, Department of Chemistry, Harbin Institute of Technology, Harbin 150001, China, Department of Chemical Engineering, New Mexico State UniVersity, Las Cruces, New Mexico 88003, United States, and Department of Electrical and Computer Engineering, Portland State UniVersity, Portland, Oregon 97201, United States ReceiVed: September 26, 2010; ReVised Manuscript ReceiVed: NoVember 7, 2010

We demonstrate here for the first time a facile fabrication of silver wire (SW) structures with a wide range of sizes and morphologies through direct chemical reduction by polyaniline (PANI). The synthesis of SW is mostly determined by the nature of the PANI dopant and silver nitrate concentration. Time-resolved optical microscopy allows monitoring the growth of SWs in real time and reveals the possible growth mechanism. Temperature-dependent resistance of a SW with 150 nm diameter by a four-probe method shows typical resistance behavior of silver metal, and the electrical conductivity is 2.1 × 105 S/cm at room temperature. The morphology-dependent electrical properties of these SWs are measured using a two-probe method. The wires comprised of self-assembled silver nanoparticles usually have lower electrical conductivities than those with smooth surfaces, due to the presence of growth defects and enhanced surface scattering. Current-voltage (I-V) curve measurements in a wide potential range either break down or cause surface transformation of the SWs by a synergism of electromigration and surface diffusion. A SW network that shows surface transformation after I-V curve measurement displays a higher resistance. The study of the electrical stability of the SWs opens up a new view of the applicable feasibility of metal nanowires in nanoelectronic devices. Introduction One-dimensional (1D) silver nanostructures such as nanowires, nanotubes, and nanorods have received considerable attention because of their size- and/or shape-dependent mechanical, plasmonic, optical, electrical, and chemical properties.1-5 Because bulk silver exhibits the highest electrical and thermal conductivity among all metals, silver nanowires (AgNWs) are of great importance for applications in shape memory materials,6 plasmonic fibers,7,8 photonic crystals,9 scanning probes,10 electrical interconnectors,5,11 and also molecule detection using surfaceenhanced Raman spectroscopy (SERS).12-19 Although a variety of different approaches have been developed for the preparation of AgNWs via top-down or bottom-up strategies,20 a solutionbased synthesis is considered as one of the most promising methods for the large-scale production of AgNWs.21-24 Most solution processing methods offer AgNWs with limited aspect ratio with the exception of polyol synthesis,25,26 which has been extensively studied to prepare AgNWs with high aspect ratios.1,21,27,28 Despite the success of growing AgNWs using a solution processing route, a need remains to show some level of control over the size and morphology; furthermore, the growth of AgNWs on top of a substrate will have significant implications toward electronic and sensory devices. To these aims, we develop a novel method to fabricate SWs via an electroless * To whom correspondence should be addressed. E-mail: [email protected] (P.X.); [email protected] (H.-L.W.). † Los Alamos National Laboratory. ‡ Harbin Institute of Technology. § New Mexico State University. | Portland State University.

deposition approach using conducting polymers. It is known that metal ions having a reduction potential higher than that of a conducting polymer can be reduced to form zero-valent metals.29,30 This chemical reduction of metal ions has been applied to obtain composites of conducting polymers and metal nanoparticles, which can be used as effective catalysts31-34 and made into high-performance nonvolatile memory devices.35,36 Our recent works have demonstrated the fabrication and control of silver, gold, palladium, and platinum nanostructures on conducting polymer powders, films, and membranes by modulating the composition, morphology, and chemical nature (dopant) of the conducting polymers.37-43 Herein, we report for the first time the facile fabrication of bulk quantities of silver wires (SWs) through a direct chemical reduction of silver ions by polyaniline (PANI), where no organic solvent and extra reducing agent are required. In this method, both the chemical nature of the PANI film and concentration of silver nitrate solution are crucial in producing SWs as the only product. The size and morphology of the SWs can also be tuned by varying the experimental parameters such as reaction time. Moreover, the mechanism of the nanowire growth has been studied using the time-resolved optical microscopy. The size-, morphology-, and temperature-dependent electrical properties of the SWs are also studied. Experimental Section Chemicals and Materials. N-methyl-2-pyrrolidinone (NMP, 99% Aldrich), heptamethyleneimine (HPMI, 98% Acros), polyaniline (emeraldine base, MW ca. 65 000, Aldrich), AgNO3 (99.9999% Aldrich), citric acid (99.9% Fisher), p-toluene-

10.1021/jp109207d  2010 American Chemical Society Published on Web 12/01/2010

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Figure 1. SEM images of the Ag structures grown on PANI films by immersing the PANI film doped by p-toluenesulfonic acid (a), fluoboric acid (b), R-(-)-mandelic acid (c), and R-(-)-camphorsulfonic acid (d) in 50 mM AgNO3 aqueous solution for 1 min. Scale bar: 5 µm.

sulfonic acid (98.5% Aldrich), R-(-)-camphorsulfonic acid (98% Aldrich), and R-(-)-mandelic acid (99% Aldrich) were used as received. Preparation of Polyaniline (PANI) Films. A typical procedure of fabricating the PANI film is described as follow: 4.14 g of NMP, 0.747 g of HPMI, and 1.15 g of PANI (EB) powder 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 put in an oven at 50 °C for 12 h to evaporate the solvent and form a dense film. The dried film was kept in a water bath to let it peel off from the glass substrate. The resulting film was dried at room temperature and then doped in 0.25 M citric acid, 0.25 M fluoboric acid, 0.25 M p-toluenesulfonic acid, 0.25 M R-(-)-camphorsulfonic acid, or 0.25 M R-(-)-mandelic acid aqueous solution for 2 days. The as-prepared films are as thick as 30 µm, as seen from an SEM iamge of the cross section of a typical PANI film (Supporting Information, Figure S1). Growth of Ag Nanostructures. The doped PANI film was rinsed with distilled water thoroughly to remove the residual dopant acid on its surface. Then it was immersed in AgNO3 aqueous solution to grow Ag nanostructures. Here the effects of AgNO3 solution concentration and growth time on the morphology and size of the produced Ag nanostructures were investigated. Characterization. X-ray diffraction (XRD) measurements were carried out on a Rigka Ultima III diffractometer that uses fine line sealed Cu KR tube (λ ) 1.5406 Å) X-rays. The timeresolved growth of AgNWs on PANI films were recorded on an Olympus BX51 M optical microscope. Scanning electron microscopy (SEM) was carried out on a FEI Inspect F SEM to study the morphology and size of the Ag nanostructures. Bright-

field transmission electron microscopic (TEM) and highresolution TEM (HR-TEM) images were measured on a JEOL 3000F TEM. TEM samples were prepared by scratching the Ag structures off of the PANI film onto a carbon-coated copper grid. The AgNWs were transferred from PANI films onto interdigitated gold electrodes prepatterned on high-resistivity silicon substrate for measuring the electrical properties. The temperature-dependent resistance was measured from 5 to 300 K using a standard four-probe technique on a Quantum Design Physical Property Measurement System (PPMS). Results and Discussion We found in previous works that by altering the surface chemistry of PANI membranes with a skin surface and a porous substructure prepared via a phase inversion method,34 various Ag nanostructures, including micrometer sized leaves, nanocubes, and hemispheric yarn balls comprised of nanosheets, could be achieved on PANI membrane surfaces. However, tailored synthesis of nanostructured metals with various morphologies and structures, especially SWs, on thermally cured PANI dense films remains desirable.34,39 As shown in Figure 1, scanning electron microscopic (SEM) images reveal that only Ag particles with random morphologies are obtained on PANI films doped by p-toluenesulfonic acid, fluoboric acid, R-(-)-mandelic acid, and R-(-)-camphorsulfonic acid. In the case of citric acid doped PANI films, we find that the Ag structures are dominated by the AgNO3 solution concentration (Figure 2). Only AgNPs were generated when the AgNO3 solution concentration was lower than 10 mM (Figures 2a and b). The SWs started to form with the AgNO3 solution concentration exceeding 15 mM. However, a mixture of SWs and AgNPs was obtained with the AgNO3 solution concentration of 15 and 25 mM (Figures 2c and d). Figures 2e and f show

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Figure 2. SEM images of the Ag nanostructures grown on PANI films by immersing the PANI films doped by citric acid in 5 mM (a), 10 mM (b), 15 mM (c), 25 mM (d), 50 mM (e), and 100 mM (f) AgNO3 aqueous solution for 1 min.

that only SWs are formed on the PANI film using AgNO3 solutions with concentration g50 mM. These as-prepared SWs can be as long as more than 100 µm. With increasing AgNO3 concentration, the diameter of the SWs becomes larger. The growth of SWs on the PANI film surface can be monitored by optical microscopy. A recorded real-time video showing the growth of SWs on PANI films can be found in the Supporting Information. Initially, the silver nucleates on the PANI surface as many small nanoparticles, and these particles elongate with time; meanwhile, the distribution density of silver particles on PANI surface increases. These elongated silver particles grow further into silver nanorods with concomitant increase in the distribution density of silver particles. Continuous propagation of these nanorods results in high aspect ratio SWs. These SWs not only grow on the PANI surface, they also grow perpendicular to the plane of the PANI surface. Magnified SEM and transmission electron microscopic (TEM) images reveal that most of the SWs yielded by immersing the PANI film (doped by citric acid) in 50 mM AgNO3 solution for 1 min are round-shaped, smooth nanowires, with diameters in the range of 100-200 nm (Figures 3a and b). High-resolution TEM (HR-TEM) in Figure 3c indicates that these SWs are single crystalline growing along the [111] direction, consistent with the AgNWs synthesized by other strategies.44,45 X-ray diffraction (XRD) measurement further confirms that [111] crystalline plane is the favorable growth direction of these SWs (Figure 3d), where the other four diffraction peaks corresponding to the (200), (220), (311), and (222) planes of face-centered cubic (fcc) silver are greatly restrained. However, upon close examination of these SWs produced on the PANI films, we find a wide range of interesting structures, as shown in Figure 4. With similar size and relative smooth surface, obvious growth defects can be found on some SWs (designated by the arrows in Figures 4a and b). A SW decorated with tiny AgNPs was also produced (Figure 4c). There are different kinds of SWs that are constructed of self-assembly of

AgNPs into 1D structures, including a SW that consists of Ag nanospheres (Figure 4d), straight SW formed by densely packed even smaller AgNPs (Figures 4e and f), necklace-like SWs formed by assembly of grainlike AgNPs (Figure 4g), and interestingly, a SW assembled by AgNPs with a square cross section (Figure 4h). A HR-TEM study shows that the AgNPs that form these SWs are exclusively single crystalline along the [111] plane (Supporting Information, Figures S2 and S3). This may explain why SWs with various morphologies have only one dominant (111) diffraction peak in the XRD pattern. The exact cause of how these AgNPs self-assemble into specific 1D morphology is currently under study. At the present time, the SW shown in Figure 3a (round-shaped smooth nanowires) appears to be the dominant one among all possible morphologies. When the reaction period was extended to 1 h, as shown in Figure 5a, PANI film would be fully covered with extremely long silver wires (up to ∼1 mm), and it was found that apart from the size growth of the SWs, the morphologies also changed greatly. Ag wires with perfect round cross section were rarely seen among these wires. The SWs obtained were actually dominated by morphologies as shown in Figures 5b and c. SWs with a diameter of about 1 µm were produced, and we can also see larger SWs with polygonal cross sections, and the surface of the wires is decorated with AgNPs. Another interesting structure is comprised of Ag nanosheets, with an average width of 1-2 µm. The backbones of these wires are very thin Ag nanosheet assemblies, and some of the nanosheets can grow along the backbones. We think this morphology can be rationalized by the fact that some citric acid molecules may diffuse into the AgNO3 solution when doped PANI film was immersed in solution for a long time, as our previous work shows that the generation of Ag nanosheet assemblies are the typical product on citric acid doped PANI membrane.38,41 Although there is a structural diversity of the as-prepared SWs by this method, the yield of SWs with different sizes and morphologies renders a great opportunity to study the size- and

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Figure 3. SEM image (a), TEM image (b), HR-TEM image (c), and XRD pattern (d) of the SWs yielded by immersing the PANI film doped by citric acid in 50 mM AgNO3 aqueous solution for 1 min.

Figure 4. SEM images of other SWs with different morphologies obtained by immersing the PANI film doped by citric acid in 50 mM AgNO3 aqueous solution for 1 min. Scale bar: 1 µm.

morphology-dependent electrical properties. The electrical property and environmental stability of a SW are particularly important as they may impact the nanowire device properties should it be used as an interconnect. We study the properties of individual SWs by transferring the SWs on PANI films onto interdigitated gold electrodes prepatterned on high-resistivity silicon substrates. The temperature-dependent resistance is determined by using a typical four-probe method in a bathtype helium cryostat in the temperature range of 5-300 K, and the room-temperature current-voltage (I-V) measurements were also carried out using a two-probe method.1 Figure 6 shows a temperature-dependent resistance curve of a smooth SW with 150 nm diameter, and the inset displays the same resistance plotted with a logarithmic scale for the temperature axis to better reveal the low-temperature behavior

of the resistance. One can see a typical resistance vs temperature behavior of a metal, including a fairly linear temperature dependence of resistance down to about 30 K, and a flat line (which means a residual resistance) below that point. This indicates there is no significant disorder or defect in the SW that can generate localization effects.46 The value of the temperature coefficient of the resistivity β ) 1/R(dR/dT) is calculated to be ∼2.0 × 10-3 K-1 at 300 K, which matches well with the value for high-purity Ag (3.8 × 10-3 K-1).47 Figure 7a shows a smooth SW with a diameter of 120 nm on the gold electrodes. When the current was measured in a dc potential range between -0.05 and +0.05 V at a rate of 0.01 V/s, a linear I-V curve, I (mA) ) 0.005 + 5.526 V (V), was obtained (Figure 7c). Using a two-probe method, an electrical conductivity of 0.356 × 105 S/cm was measured for this SW,

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Figure 5. SEM images of the SWs obtained by immersing the PANI film doped by citric acid in 50 mM AgNO3 aqueous solution for 1 h (a). (b-d) are magnified SEM images of the typical SWs from (a).

Figure 6. Temperature-dependent resistance of a smooth SW with 150 nm diameter.

which represents a reasonable value compared with previous studies.1,48 However, this conductivity measured using a twoprobe method is clearly lower than that measured by using a four-probe method. The conductivity of the smooth SW with a 150 nm diameter (∼2.1 × 105 S/cm) measured at 300 K using the four-probe method agrees well with the conductivity of bulk silver (6.25 × 105 S/cm). The difference in conductivity from two- and four-probe methods is believed to be due to contact resistance between a SW and gold electrode surface. This result suggests the feasibility of using the smooth SWs fabricated on PANI films as interconnects in fabricating nanoelectronic devices. When the I-V curve was measured in a dc potential range of 0-4 V at a rate of 0.2 V/s (Figure 7d), a linear feature was found from 0 to 0.9 V, followed by a nonlinear jump in the range between 0.9 and 1.5 V and a sudden current drop to 0 at about 1.5 V, after which no current was detected. This result indicates that the SW breaks down during measurement in the higher voltage range. SEM image of this SW after I-V curve measurement in 0-4 V reveals that two sections of the SW were melted with micrometer-sized gaps (Figure 7b). As indicated by the arrows, the meltdown of the SW might have

occurred at those parts with higher resistancesa smaller local diameter or growth defect. As the temperature increases, the defect sites become thinner and thinner and eventually break down into separate segments. The perturbation of the electrical conductivity at ∼0.9 V is likely due to the premeltdown of SW and contact resistance variation. Electromigration, where the metal ions counterpropagate against the flow of electrons in a metal structure under bias, can be accounted for by the degradation of the SWs because electromigration occurs faster at high-resistance junctions which eventually become the loci of the gaps. Meanwhile, surface diffusion, a process dominated by thermal effect, in which material diffuses out of hightemperature regions and deposits at lower-temperature locations, may also play a significant role in thinning out high-resistance sections in a nanowire.17,47 For a SW assembled by AgNPs with a polygonal cross section and an edge length of 500 nm (Figure 8a), a linear I-V curve, I (mA) ) 0.065 + 26.171 V (V), was also obtained in a dc potential range between -0.05 and +0.05 at 0.01 V/s, from which an electrical conductivity of 0.182 × 105 S/cm was calculated. It is not surprising that a thicker silver wire has a lower conductivity than a thinner one; it may be rationalized by the fact that a thicker wire assembled by AgNPs has a rough surface, which leads to structural defects and enhanced surface scattering, hence making the SW less conductive. This phenomenon can also be found for a straight silver wire assembled by densely packed smaller AgNPs (440 nm in diameter), which has a conductivity of 0.109 × 105 S/cm (Supporting Information, Figure S4). When we measured the I-V curve in a wider dc potential range at 0.2 V/s (Figure 8d), a linear feature was found from 0 to 1.4 V, and a meltdown potential was observed at 1.4 V. As shown in Figure 8b, a melted submicrometer gap can be seen, and it is interesting to find in the magnified SEM image (inset of Figure 8b) that the surface near the melted part becomes very smooth and no AgNPs can be distinguished, and the wire transforms from a square shape to a smooth rounded structure. We believe that due to high temperature, surface diffusion of

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Figure 7. SEM images of a smooth SW (diameter ) 120 nm) on gold electrode before (a) and after (b) I-V curve measurements and I-V curves of the SW in the range of -0.05 to +0.05 V (scanning rate: 0.01 V/s) (c) and 0-4 V (scanning rate: 0.2 V/s) (d).

Figure 8. SEM images of a SW with square cross section (edge length: 500 nm) on gold electrode before (a) and after (b) I-V curve measurements, and I-V curves of the SW in the range of -0.05 and +0.05 V (scanning rate: 0.01 V/s) (c) and 0-4 V (scanning rate: 0.2 V/s) (d).

melted Ag to nearby lower-temperature parts could be the main factor leading to this surface transformation. We also managed to form a simple SW network, where several junction points are in contact with the Au electrode (Figure 9a). It can be seen that the SW in part I has a shape of a triangular prism, and those in part II and III are similar to that in Figure 8a, with a polygonal cross section. A linear I-V curve, I (mA) ) 0.197 + 58.105 V (V), was obtained in a dc potential range between -0.05 and +0.05 at 0.01 V/s (Figure 9c), indicating a lower resistance of the SW network than that of a single SW, presumably due to the current flow through multichannels. Figure 9d shows a linear I-V curve, I (mA) )

0.078 + 63.442 V (V), in the potential range between 0 and 1.5 V, and a sudden current drop from 95 to 30 mA at 1.5 V. However, another linear I-V curve emerges with a smaller slope, I (mA) ) 3.112 + 18.652 V (V), from 1.5 to 5.0 V, indicating that the resistance of the SW network system was increased about 3.4 times. Interestingly, the SEM image after the I-V curve measurements shows that all the SWs remain intact (Figure 9b). A close examination reveals that the morphologies of the SWs in parts II and III were almost the same as those prior to I-V measurement; however, the SW in part I was melted to form a smooth surface by surface diffusion. This may suggest that when the surface temperature of the SW

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Figure 9. SEM images of a simple SW network formed on gold electrode before (a) and after (b) I-V curve measurements, and I-V curves of the SW network in the range of -0.05 to +0.05 V (scanning rate: 0.01 V/s) (c) and 0-5 V (scanning rate: 0.2 V/s) (d).

was high enough under ambient conditions, a thin layer of AgO could be formed, hence reducing the SW conductivity. Melting of the nanowire could also lead to change in crystallinity. Nevertheless, we believe this decrease in conductance observed after the surface transformation can also be due to a small degree of resistive heating of the nanowire system, because it is known that metals present a higher resistance at a higher temperature.49 Conclusions In summary, silver wires (SWs) were successfully fabricated on polyaniline (PANI) films through an extremely facile methodology (direct chemical reduction by PANI). It was found out that SWs could be produced on citric acid doped PANI films; In contrast, silver particles were yielded using other dopants (acids). Meanwhile, the silver ion concentration was found to be critical in preparing SWs. Time-resolved optical micrographs reveal that growth of SWs actually starts with silver nanoparticle (AgNP) nuclei, which then elongate to form nanorods and further propagate into high aspect ratio SWs. Yield of SWs with a wide range of structures renders an opportunity to study the size- and morphology-dependent electrical properties. It was found that the electrical conductivity of a SW formed by assembled AgNPs commonly is lower than that of a smooth SW. All SWs displayed linear I-V characteristics in a potential range between -0.05 and +0.05 V; however, performing the I-V measurement in a wider potential range (to several volts) would either break down or transform the morphology of the SWs, caused by a synergetic effect of electromigration and surface diffusion. Our results suggest the feasibility and stability of using SWs as interconnects in electronic devices. The fabrication of SWs by simple chemical reduction of PANI via self-assembly method presents an alternative approach to fabricating nanowires with a wide range of unique structures and morphologies that were previously inaccessible. Acknowledgment. H.-L.W. acknowledges the financial support from Laboratory Directed Research and Development (LDRD) fund under the auspices of DOE, BES Office of Science, and 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). P.X. is thankful for the support from the Joint Educational Ph.D. Program of Chinese Scholarship Council (CSC), NSF of China (No. 20776032, 21071037), and Special Fund of Harbin Technological Innovation (2010RFXXG012). Supporting Information Available: Additional SEM images and one video showing the real-time growth of silver wires on PANI film. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Sun, Y.; Yin, Y.; Mayers, B. T.; Herricks, T.; Xia, Y. Chem. Mater. 2002, 14, 4736–4745. (2) Schider, G.; Krenn, J. R.; Hohenau, A.; Diltbacher, H.; Leitner, A.; Aussenegg, F. R.; Schaich, W. L.; Puscasu, I.; Monacelli, B.; Boreman, G. Phys. ReV. B 2003, 68, 155427. (3) Jeong, D. H.; Zhang, Y. X.; Moskovits, M. J. Phys. Chem. B 2004, 108, 12724–12728. (4) Gunawidjaja, R.; Jiang, C.; Peleshanko, S.; Ornatska, M.; Singamaneni, S.; Tsukruk, V. V. AdV. Funct. Mater. 2006, 16, 2024–2034. (5) Zhang, W.; Chen, P.; Gao, Q.; Zhang, Y.; Tang, Y. Chem. Mater. 2008, 20, 1699–1703. (6) Park, H. S.; Ji, C. J. Acta Mater. 2006, 54, 2645–2654. (7) Sanders, A. W.; Routenberg, D. A.; Wiley, B. J.; Xia, Y.; Dufresne, E. R.; Reed, M. A. Nano Lett. 2006, 6, 1822–1826. (8) Ditlbacher, H.; Hohenau, A.; Wagner, D.; Kreibig, U.; Rogers, M.; Hofer, F.; Aussenegg, F. R.; Krenn, J. R. Phys. ReV. Lett. 2005, 95, 257403. (9) Hu, X.; Chan, C. T. Appl. Phys. Lett. 2004, 85, 1520–1522. (10) Jing, G. Y.; Duan, H. L.; Sun, X. M.; Zhang, Z. S.; Xu, J.; Li, Y. D.; Wang, J. X.; Yu, D. P. Phys. ReV. B 2006, 73, 235409. (11) Graff, A.; Wagner, D.; Ditlbacher, H.; Kreibig, U. Eur. Phys. J. D 2005, 34, 263–269. (12) Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R.; Sun, Y.; Xia, Y.; Yang, P. Nano Lett. 2003, 3, 1229–1233. (13) Pazos-Pe´rez, N.; Barbosa, S.; Rodrı´guez-Lorenzo, L.; AldeanuevaPotel, P.; Pe´rez-Juste, J.; Pastoriza-Santos, I.; Alvarez-Puebla, R. A.; LizMarza´n, L. M. J. Phys. Chem. Lett. 2010, 1, 24–27. (14) Lee, S. J.; Morrill, A. R.; Moskovits, M. J. Am. Chem. Soc. 2006, 128, 2200–2201.

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