J. Phys. Chem. C 2008, 112, 18361–18367
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Silver Nanodisks with Tunable Size by Heat Aging Bin Tang,† Jing An,† Xianliang Zheng,† Shuping Xu,† Dongmei Li,‡ Ji Zhou,† Bing Zhao,† and Weiqing Xu*,† State Key Laboratory of Supramolecular Structure and Materials, Jilin UniVersity, Changchun 130012, P. R. China, and National Laboratory of Superhard Materials, Jilin UniVersity, Changchun 130012, P. R. China ReceiVed: July 23, 2008; ReVised Manuscript ReceiVed: September 11, 2008
A large quantity of well-defined silver nanodisks was prepared by heating silver nanoprisms in aqueous solution. By adjusting only the heating time, the conversion of the morphology and the size of the nanoplates could be easily and effectively controlled. The local surface plasmon resonance (LSPR) of the silver nanoplates could be tuned in a certain range in the heat-aging process. A gradual blue shift in their LSPR bands with the aging time was observed from 655 to 472 nm. The transformation of the morphology during heat aging was captured by the transmission electron microscopy (TEM). The TEM images indicate that the silver atoms on the corners and sides of the nanoprisms dissociate and rearrange during the period of shape transformation. The dissociation of the silver atoms at the sides of the nanoplates is overwhelming when the size shrinks, which is due to the difference in the surface energy and the limiting of citrate stabilization. In addition to the heating effect, we found that the concentration of citrate is another crucial factor for rebuilding the silver nanoplates. 1. Introduction Noble-metal nanoparticles have attracted extensive interest in modern materials chemistry for their unique properties such as optical,1 electronic,2 magnetic,3 and catalytic functions.4 Their local surface plasmon resonance (LSPR) properties are of tremendous importance to applications in biolabeling,5 surfaceenhanced Raman scattering (SERS),6 sensing,7 and the fabrication of nanophotonic devices and circuits.8 LSPR is dependent on several features of the nanoparticles, such as size, shape, composition, and crystallinity, of which size and shape are more significant. Over the past two decades, many attempts have been made to obtain metal nanoparticles of well-controlled size and morphology. For example, silver nanoparticles in the shapes of cubes,9 prisms,10 hexagons,11 disks,12 and wires13 have been synthesized using various chemical approaches including photochemical,10 thermochemical,14 and biochemical techniques.15 So far, the development of new strategies to systematically manipulate the size and shape of nanoparticles is still a great challenge that is critical for optimizing the LSPR properties of nanoparticles for certain applications. In recent years, several reports have focused on the synthesis of silver nanoparticles in disk-like shapes, for instance, by the physical template method,12a the soft templates (organic polymers and/or surfactants) method,12b the structure-controlled solventless method, 12c and the self-seeding co-reduction hydrochemical method.12d Hao et al.12a prepared silver nanodisks by using polystyrene mesospheres as templates in N,N-dimethylformamide (DMF) solution. In that work, polystyrene mesospheres with carboxyl groups first absorbed silver ions to nucleate. Then, the polystyrene mesospheres played the function of the template, as they limited the metal particle growth in one direction and led to anisotropic growth. Wu et al.12c synthesized silver nanodisks by heating silver thiolate precursors * To whom correspondence should be addressed. Tel.: +86-43185159383. Fax: +86-431-85193421. E-mail:
[email protected]. † State Key Laboratory of Supramolecular Structure and Materials. ‡ National Laboratory of Superhard Materials.
to 180-225 °C in a nitrogen atmosphere. The disk-like shape was considered to be derived from the structures of the precursors. Chen et al.12b obtained silver nanodisks from truncated triangular silver nanoplates at 40 °C in the presence of hexadecyl trimethyl ammonium bromide (CTAB). A monolayer of CTAB on the basal face was suggested to account for the anisotropic growth. However, the products obtained by this method had a wide size distribution. Their LSPR tuning was also limited (to the range of only 420-560 nm). Yu et al.12d obtained silver nanodisks through aging of silver nanoprisms at room temperature, where the nanoprisms were synthesized by a self-seeding co-reduction hydrochemical method. They attributed the shape transformation to the difference in the interaction potential energies between bis(2-ethylhexyl) sulfosuccinate (AOT) and silver facets. In this system, three kinds of reducing agents were required. In our experiments, we chose a simple reaction system that contains only three reagents: AgNO3, trisodium citrate, and NaBH4. First, silver nanoprisms were prepared to be the nanodisk precursors. Then, by a heat-aging treatment, the silver nanoprisms were dissociated and rearranged on their corners and sides under the stabilization of citrate. Numerous welldefined silver nanodisks were obtained as the products. Transmission electron microscopy (TEM) images and ultraviolet-visible (UV-vis) spectra were used to monitor the transformation of the morphology and size of the nanoplates during the heating process. In previous studies, the diameter control of silver nanodisks was limited even though the silver nanodisks were prepared by several methods.12 In addition, obtaining optimal SERS activity of silver nanoplates as substrates requires tuning the LSPR of the silver nanoplates to appropriate wavelengths. Herein, we report a thermal aging method that yields a large quantity of regular silver disks through evolution of the shape from prisms to disks. Importantly, the method allows one to deliberately tailor the diameter and tune the LSPR of the silver nanodisks by controlling the heating time.
10.1021/jp806486f CCC: $40.75 2008 American Chemical Society Published on Web 10/31/2008
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Figure 1. Evolution of the color of the silver-nanoparticle-containing solution. (A-C) Silver seed solution exposed to the light for 0, 4, and 7 h, respectively. (D-E) Silver nanoplates heated at 95 °C for 30, 50, and 70 min, respectively.
2. Experimental Section 2.1. Materials. AgNO3 (99.8%) was obtained from First Shanghai Chemical Reagent Plant. NaBH4 (96.0%) was obtained from Sinopharm Chemical Reagent Co., Ltd. Trisodium citrate (99.0%) was purchased from Beijing Chemical Reagent Plant. All chemicals were of analytic grade and were used without further purification. MilliQ water (18.6 Ω · cm-1) was used to make aqueous solutions. 2.2. Preparation of Triangular Silver Nanoplate Precursor. A typical experiment was carried out by the light-irradiated seed-growth method.11b First, silver seeds were prepared by dropwise addition of NaBH4 solution (8.0 mM, 2.0 mL) to an aqueous solution of AgNO3 (0.2 mM, 100 mL) in the presence of trisodium citrate (0.4 mM) under vigorous stirring. Then, the yellow silver seeds (100 mL in a glass conical flask) were irradiated under a 70-W sodium lamp for 7 h. The color of the silver nanoparticles changed from yellow, to green, to blue during the irradiation process (Figure 1). UV-vis spectroscopy was used to monitor the evolution of the LSPR bands of the silver nanoplates. The final products were triangle-shaped silver nanoplates (or nanoprisms) with a blue color. The yield was over 95%. The average edge length of these nanoplates was 74.5 ((4.6) nm. The silver-nanoplate-containing solution was used as the stock solution in the subsequent heating process without any further treatment. 2.3. Preparation of Silver Nanodisks. The triangular silver nanoplate colloids were kept in a 95 °C water bath for several hours. During the heat-aging treatment, the color of the particle solution changed from blue to pink and finally to bright yellow (Figure 1). The triangular silver nanoplates were converted to nanodisks during this process. The heat-aging time affects the morphology and size of the nanodisks. Many small nanoparticles were generated in the solution. The silver-nanodisk-containing solutions were purified by centrifuge at a speed of 8000 rpm for 3 min before the images were taken. 2.4. Instruments. A 70-W sodium lamp (Shanghai Yaming Co., Ltd.) was used as the light source for the photoinduced process. UV-vis spectra were recorded on a Shimadzu UV3100 spectrophotometer. TEM images were measured with a Hitachi H-8100 IV instrument operating at 200 kV. Samples for TEM analysis were prepared by dripping a drop of silver nanoparticle colloid onto carbon-coated copper grids and drying them in air at room temperature. High-resolution TEM (HRTEM) images were measured with a JEOL 3010 high-resolution
transmission electron microscope operating at 300 kV. Specimens for HRTEM were prepared by depositing a drop of colloidal solution onto holey-carbon-coated Cu grids. A water bath pot (HH-2 model, Shanghai Yifei Technology Co., Ltd.) was employed to heat the silver colloids in the aging process. A TGL-16A model centrifuge (Jintan Hongkai Instrument Factory) was used for the sample purification. 3. Results and Discussion 3.1. Nanodisk Formation. Blue silver nanoprisms were fabricated through a photoinduced growth in the presence of citrate in aqueous solutions, as described in a previous work.11b These silver nanoprisms were then heated at 95 °C in a water bath. The color of the colloid turned sequentially from blue to purple, pink, and yellow in the heat-aging process (Figure 1). The colors reflect changes in the LSPR bands. The morphological changes of the silver nanoprisms were characterized by TEM. Figure 2 shows the morphologies of the nanoplates (A) before and (B,C) after the heat treatment. After the 30-min heating, the nanoprisms were slightly truncated (Figure 2B). The sharp corners of the nanoprisms nearly disappeared. The truncation process finished after the 60-min heating. The shape transformation from triangular to disk-like occurred through rebuilding of the silver atoms on the vertexes and sides of the prisms. It should be mentioned that many small silver nanoparticles were generated during the shape conversion. We successfully purified the large nanoplates from the small silver nanoparticles by using centrifuge method (A-D in Figure 3). 3.2. Size Tuning of Nanodisks. To clarify the extent of the size of the nanodisks, TEM images after different heating periods (40, 50, 60, and 70 min) were taken and compared (A-D in Figure 3). The diameter and thickness of these disklike nanoparticles were measured, as shown in the middle and right panels of Figure 3. After 40-70 min of heat aging, the diameter was 57.2 ((5.3), 48.1 ((4.4), 42.3 ((4.0), and 39.3 ((4.5) nm, and the thickness was 13.6 ((1.0), 13.4 ((0.9), 13.9 ((1.2), and 13.6 ((0.7) nm, respectively. The size and the aspect ratio (i.e., the ratio of the longer diameter to the shorter thickness) of the disk-like silver nanoparticles decreased gradually with the heating time. It is obvious that the thickness of the silver nanoplates is unchanged as the diameter is decreased. Because of the shrinking size and unchanged thickness, we can conclude that the size reduction of the nanodisks in the heat-aging process occurs through the dis-
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Figure 2. TEM images of nanoplates in the heat-aging process: (A) initial nanoprisms before heating and nanoparticles after (B) 30 and (C) 60 min of heating. The insets are TEM images of individual nanoprisms and nanodisks and their corresponding electron diffraction patterns.
Figure 3. TEM images of nanoparticles heat-aged for (A) 40, (B) 50, (C) 60, and (D) 70 min, along with corresponding statistical schematics of the diameter (A1-D1) and thickness (A2-D2) of the nanodisks.
sociation of the silver atoms on the side faces of the nanoplates. In other words, the heating time can be controlled to easily adjust the nanodisk size between 57 to 39 nm.
3.3. LSPR Spectra of Nanodisks. UV-vis spectroscopy was used to monitor the evolution of the nanodisks during heating. Figure 4 shows UV-vis spectra of silver nanoplates at heating
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Figure 4. (A) Normalized UV-vis spectra of silver nanoplates colloid after heat aging for 0-70 min. Curves 1-6 correspond to 0, 30, 40, 50, 60, and 70 min, respectively. (B) Enlarged UV-vis spectra from A in the range of 250-500 nm. (C) Relationship between the in-plane dipole plasmon resonance peak of nanoplates and the heating time.
SCHEME 1: Illustration of (I) the Shape Transformation of Nanoplates and (II) the Reduction of Nanodisks in the Process of Heat Aging
times of 0, 30, 40, 50, 60, and 70 min. Before heating, four LSPR bands were observed from this spectrum (see the enlarged view, Figure 4B). Three of them centered at 331, 478, and 655 nm are assigned to the out-of-plane quadrupole, in-plane quadrupole, and in-plane dipole plasmon resonance modes, respectively, of silver nanoprisms.10,16 The peak at 392 nm belongs to the silver seeds. The LSPR bands are shape- and size-dependent. The changes in the LSPR band reflect the transformation of the nanoplates during the heat treatment. The in-plane dipole resonance peak gradually blue-shifted (from 655 to 472 nm in Figure 4A) with the heating time. Because the in-plane dipole resonance peak is very sensitive to the size and the aspect ratio,10 the blue shifts imply a reduction of the size and the aspect ratio of the nanoplates. It should be mentioned that the peak width at halfheight of the in-plane dipole resonance peak gets narrower (from 115 to 57 nm) with the heating time. The out-of-plane quadrupole resonance peak red shifts from 331 to 340 nm (Figure 4B). As a result, the aspect ratios of the nanoplates decrease in the shape evolution from triangular plates to circular disks. A remaining peak at 392 nm indicates that small nanoparticles appear in the reaction system. All of these representations in the LSPR spectra provide spectral evidence of the transformation from nanoprisms to nanodisks. They are coincident with the TEM results. The LSPR band can be effectively tuned in a certain range during the heat-aging process. The relationship of the in-plane dipole resonance peak to the heating time is shown in the Figure 4C. It has been mentioned that the in-plane dipole resonance
peak can be adjusted from 655 to 472 nm by the heat aging of the nanoplates. The tunable range of ∼200 nm for the LSPR peak can be obtained, which would be significant for LSPRrelated studies, such as SERS and surface-enhanced fluorescence (SEF). 3.4. Growth Mechanism. Through the TEM and UV-vis spectroscopy monitoring of the transformation of the nanoplates, we can summarize the process of reconstruction of the nanoplates during heat aging. As shown in Scheme 1, the dissociation and rearrangement of silver atoms simultaneously occur during the period of shape transformation from nanoprisms to nanodisks (as state I in Scheme 1). In the shape transformation, the silver atoms are dissociated from the vertexes of the nanoprisms and then rearranged on the sides of the nanoplates. In the subsequent heating period, the dissociation of silver atoms predominates, resulting in the size reduction of the nanodisks (as state II in Scheme 1). The evolution of the nanoplates does not terminate until the system reaches thermodynamic equilibrium. We can find a similar explanation for the growth mechanism in the literature.12 Chen et al.12b presented a well-defined selfassembled CTAB monolayer on the (111) plane that prevented further growth on this plane, leading to the rounding of the triangular particles. Yu et al.12d attributed the transformation to silver atoms migrating between crystalline planes in the interior of individual particles. In our experiments, the transformation process started with the corners of the triangular nanoplates being truncated. The electron diffraction pattern taken from an individual silver nanoprism by the perpendicular angle of the basal facets (Figure 2A) indicates that the nanoplate is a single
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Figure 5. HRTEM images of (A) triangular silver nanoplates and (B) nanodisks. The electron beam was directed perpendicular to the flat face of an individual nanoplate.
crystal. The hexagonal symmetry of these spots implies that the two basal facets of the nanoplates are bounded by {111} planes. The side facets of nanoprism are {110} planes.10b Just as for the nanoprisms, the hexagonal symmetry of the electron diffraction spots (Figure 2C) taken from an individual silver nanodisk indicates that the nanodisk is also a single crystal that is bounded by {111} planes at the two basal facets. It can be concluded that the remaining {111} facets were unchanged during heat aging. Furthermore, Figure 5 shows the HRTEM images of individual nanoprism and nanodisk particles recorded perpendicularly from their flat faces. The fringes of the silver nanoprism are separated by 2.49 Å, which can be ascribed to the (1/3){422} reflection, which is generally forbidden for a facecentered cubic lattice.10,17 The same fringes are also observed for the nanodisks. As can be seen, in the evolution of the nanoplates, the vertexes and the {110} side surface of the plates changes. Interestingly, the diameter of the nanodisks after 40 min of aging (57.2 nm) is longer than that of the inscribed circle of the initial nanoprisms. For heating times longer than 40 min, the final nanodisks become smaller and smaller. Thus, we can imagine that the changes of the vertexes and {110} facets can be attributed to two states (Scheme 1): state I, shape transformation, including both the dissociation of the silver atoms from the vertexes and the rearrangement of the dissociated silver atoms toward more stable positions, and state II, size reduction, mainly including the dissociation of the silver atoms from the sides of the nanoplates. It is known that several physical and chemical factors can affect the shape evolution of nanoplates. According to the Gibbs-Thomson effect, a convex structure has a higher surface energy than a flat one. In contrast, a concave surface has a lower surface energy than a flat surface of the same material in the same phase.18 Less stability might lead to the truncation. It has been reported that the {110} facets have a higher surface energy and are less stable than the {111} facets because of the lower coordinating number of surface atoms.19 It is suggested that the difference in surface energy between the corners and the {110} facets is the pivotal driving force determining the shape transformation from prisms to disks during the heating process. As a result, the evolution of the nanoplates takes place first on the {110} facets. The citrate ions are preferentially adsorbed on the {111} facets20 to maintain the planar morphology and thickness of the plates during the shape evolution. The silver atoms are transferred from the unstable sites to the stable ones,
which is regarded as rearrangement through the migration of silver atoms. The migration of silver atoms in a planar structure might be triggered by a small difference in interaction energy between surfactant molecules and facets (Ag{110} and Ag{100}).12d These results indicate that the sharp vertexes of prisms, because of the instability in their energy and geometry, are easily truncated as the active sites at high temperature. The influence of heating on the vertexes of nanoprisms is more intense than that on the sides. The side faces ({110} facets) can become the sites where the silver atoms grow. In our experiments, the shape transformation from triangles to disks was caused by the difference in surface energy and the selective adsorption of citrate. The diameter of the nanodisks could be further reduced if the colloidal system was continuously heated. During this period, the thickness remained unchanged, and the total volume of the nanodisks decreased. This phenomenon is apparently different from that reported in a previous work.12d The silver atoms continued to dissolve during the whole heating process. However, in the shape transformation process (state I), the dissociated silver atoms began to rearrange at the same time, because the geometric anisotropy and energy of different facets of the nanoprisms both changed. As a result, the nanoparticles were isotropic, which meant that the effect of heating was the same for every direction. Therefore, in the next step, the rearrangement of dissociated silver atoms stopped, and the dissociation of the silver atoms was overwhelming. The concentration of citrate in the silver nanoplate colloid was critical in determining the evolution of the silver nanoplates. 3.5. Role of Citrate. To explore the effect of the citrate, we changed the concentration of citrate (molar ratio of Na3C6H5O7 to AgNO3) at the beginning of the synthesis of the triangular nanoplates. Citrate has two roles in the reaction: reducing Ag+ and stabilizing and directing the structure (shape). In fact, we found that the initial concentration of citrate affects the formation and stability of the nanodisks during heat treatment of the nanoplates. We monitored the process of the heat aging by UV-vis spectroscopy. Three ratios of Na3C6H5O7 to AgNO3 were tested: (1) For [Na3C6H5O7]/[AgNO3] ) 10:1, the LSPR band changed slightly at the beginning. The side and basal planes of the silver nanoprisms underwent no change after heating for 60 h. (2) For [Na3C6H5O7]/[AgNO3] ) 2:1, the LSPR band of the silver nanoprisms clearly changed after heating (Figure 4A). (3) For [Na3C6H5O7]/[AgNO3] ) 1:1, the rate of the evolution process increased, and the diameter of the final
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Figure 8. UV-vis spectra of (a) the fresh silver nanoplate colloid and the corresponding silver colloid after (b) 2 and (c) 3 months at room temperature.
Figure 6. TEM image of the final silver nanodisks with a 1:1 molar ratio of citrate to AgNO3 after heating.
Figure 7. UV-vis spectra of the evolution of the silver colloid with a 1:1 ratio of citrate to AgNO3 after heat aging.
nanodisks decreased. The diameter of the resultant nanodisks after 16 min of aging was reduced to 23.5 ((4.2) nm (Figure 6). A series of extinction spectra of silver colloid is shown in Figure 7. These contrasting experiments reveal that the citrate plays a crucial role in the size and LSPR tuning of silver nanoplates. At high concentration, citrate stabilizes the crystal facets of the nanoprisms and weakens the heat-aging effect. The dissociation and rearrangement of silver atoms in the nanoplates are correlated with the concentration of citrate and the aging time. The initial concentration of citrate is of importance for the aging rate, as well as the size and LSPR property of the resultant nanodisks. In other words, the range in the size of nanodisks can be wider. 3.6. Stability of Silver Nanodisks. The stability of the silver nanodisks in the colloids was evaluated. UV-vis spectra of the silver nanodisks (Figure 8) were obtained before and after they had been left to stand for 2 and 3 months at room temperature. The main band and the profile of LSPR are almost unchanged, which indicates that the silver nanoplates are stable for a period of time. However, the 392-nm band attributed to the plasmon resonance of the small spherical nanoparticles decreased in intensity, which might be due to the oxidation of small silver nanoparticles from the dissolved oxygen in the solution. In addition, the UV-vis spectrum (curve c in Figure 8) of the silver
Figure 9. UV-vis spectra of (A) the silver nanoprism colloid into which additional citrate was added and (B) the same silver nanoprism colloid without more citrate added after heating periods of 0, 15, 30, and 45 min.
nanoplates after 3 months also confirms that the silver nanoplates did not aggregate after a longer period. The stability of the silver nanoplates was quite high, which makes them promising for applications in LSPR sensing fields. Moreover, the silver nanoplates can be stable even at high temperature (95 °C), on the condition that additional citrate is added to the silver colloid in a certain phase (nanoprism or nanodisks). We added additional citrate (0.16 mL, 100 mM) to the as-synthesized nanoprisms (20 mL, 0.1 mM). Figure 9A shows UV-vis spectra of the silver nanoprisms with the additional citrate after being aged at 95 °C. It was found that the UV-vis spectra changed slightly at the beginning but then exhibited no shifts after heat aging for a long period (45 min). In contrast, the UV-vis spectra of the same silver nanoprisms without the additional citrate clearly changed after heating (Figure 9B). The LSPR band blue-shifted from 654 to 574 nm within 45 min of heating. These results indicate that addition
Silver Nanodisks with Tunable Size by Heat Aging of citrate can help silver nanoplates hold their intrinsic structure at high temperature and might extend the scope of applications derived from the functional modification of silver nanoplates. 4. Conclusion We have successfully prepared, in a large quantity, uniform and stable disk-like silver nanoplates by heat aging of silver nanoprisms. The silver nanodisks with dimensions controllable in the range of 57-39 nm can be tuned by adjusting the heating time at a certain molar ratio of citrate to Ag. The same simple approach can be used to finely control the LSPR band in the range of 655-472 nm. The shape transformation from nanoprisms to nanodisks is caused by the difference in surface energy of the facets of the nanoplates and the facet-selectivity of the citrate. In addition, the concentration of citrate is a crucial factor for determining the size reduction of the nanoplates. The silver nanoplates with LSPR optical property have potential LSPRrelated applications, such as the SERS, surface-enhanced IR (SEIR) spectrscopy, and SEF. Moreover, they can be applied as biological labels and chemical sensors because the plasmonic nanoparticles in this work are easy to synthesize and their LSPR optical property is easy to control. Acknowledgment. This work was supported by the National Natural Science Foundation of China (NSFC20773045 and 20627002). We thank Prof. Guangdi Yang (Jilin Univerisity, Changchun, China) for proofreading the manuscript. References and Notes (1) (a) Feldstein, M. J.; Keating, C. D.; Liau, Y. H.; Natan, M. J.; Scherer, N. F. J. Am. Chem. Soc. 1997, 119, 6638. (b) Haynes, C. L.; McFarland, A. D.; Zhao, L.; Van Duyne, R. P.; Schatz, G. C.; Gunnarsson, L.; Prikulis, J.; Kasemo, B.; Kall, M. J. Phys. Chem. B 2003, 107, 7337. (2) (a) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690. (b) Li, Y.; Wu, Y.; Ong, B. S. J. Am. Chem. Soc. 2005, 127, 3266. (3) Shi, J.; Gider, S.; Babcock, K.; Awschalom, D. D. Science 1996, 271, 937. (4) (a) Street, S. C.; Xu, C.; Goodman, D. W. Annu. ReV. Phys. Chem. 1997, 48, 43. (b) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (c) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (5) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van.Duyne, R. P. Nat. Mater. 2008, 7, 442.
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