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Ag-Au-Ag Heterometal Nanowires: Synthesis, Diameter Control, and Dual Transversal Modes with Diameter Dependency Jongwook Jung,† Daeha Seo,† Garam Park,† Seol Ryu,*,‡ and Hyunjoon Song*,† Department of Chemistry, Korea AdVanced Institute of Science and Technology, Daejeon 305-701, Korea, and Department of Chemistry, Chosun UniVersity, Gwangju 501-759, Korea ReceiVed: May 25, 2010; ReVised Manuscript ReceiVed: June 22, 2010
We report on the synthesis of Ag-Au-Ag heterometal nanowires through an epitaxial seed-mediated growth of silver nanowires from gold decahedral seeds. The diameter of the Ag-Au-Ag nanowires is precisely controlled in a range of 60-150 nm by using gold decahedral seeds with corresponding edge sizes. The three transversally excited bulk, localized, and propagating modes exhibit optical signatures in the UV-vis spectra. In particular, the end-on incidence propagating mode is highly sensitive to the nanowire diameter. Such exact control of diameters and diameter-dependent surface plasmon modes are expected to be useful for photonic circuit elements and chemical and biosensors. Anisotropic nanostructures including nanorods and wires have attracted much attention because their physical properties are not averaged out by simple internal rotation.1,2 In particular, nanowires can be readily handled for device fabrication but do not lose their quantum properties due to directional anisotropy along long and short axes. Alongside the versatile use of semiconducting nanowires, metal nanowires have strong potential for various applications such as conductive lines, highresolution imaging tips, photonic waveguides, and transparent electrodes.3-6 Numerous works on the fabrication of metal nanowires have been reported, but there have been relatively few reports on reliable synthetic processes for attaining structurally welldefined, highly crystalline nanowires in a large scale.7 Xia et al. introduced a modified polyol process for high yield synthesis of silver nanowires,8-10 and some synthetic routes have also been studied in other face-centered-cubic metals, such as gold, platinum, and palladium.11-13 The fabricated nanowires basically have lengths larger than 1 µm and are thus practically regarded as infinite lines. As a result, their physical properties are significantly dependent upon their diameters (or thicknesses). For instance, different diameters lead to different light-surface plasmon coupling and out-coupling efficiencies to the far field, which affect their capacity to serve as subwavelength waveguides.14 However, to the best of our knowledge, there has yet to be any reports on rational methods to control nanowire diameters and corresponding physical properties. Multisegmented nanowires with different metals have mainly been synthesized through the template-assisted process.15,16 A number of applications for electronics, catalysis, and biology were suggested for these heterometal nanowires.15 On the other hand, template-free synthesis has been hardly reported, although this method has many advantages of the simple process with large-scale synthesis, excellent dispersion in solution, and wide structural variation. In the present study, we report on the synthesis of Ag-Au-Ag heterometal nanowires through an epitaxial seed-mediated * Corresponding authors. E-mail:
[email protected] (S.R.), hsong@ kaist.ac.kr (H.S.). † KAIST. ‡ Chosun University.
growth from gold decahedral seeds. The diameter of the nanowires is precisely controlled from 60 to 150 nm by using gold seeds with distinct sizes. The dual transversal dipolar modes that include normal incidence and end-on incidence surface plasmon resonances (SPR) are observed, and the latter wavelength is highly sensitive to the nanowire diameter. Experimental Section Reagents. HAuCl4 (99.9+%, Aldrich), AgNO3 (99+%, Aldrich), ethylene glycol (EG, 96%, Aldrich), poly(vinylpyrrolidone) (PVP, Mw ) 55 000, Aldrich), and ethanol (99.9%, J.T. Baker) were used as received. Synthesis of Ag-Au-Ag Heterometal Nanowires from Gold Decahedrons. The gold decahedrons with mean edge sizes of 32 ( 2, 50 ( 4, and 88 ( 6 nm were synthesized according to the literature.17 The concentration of each decahedral seed solution was fixed to 0.014 M in EG with respect to the gold precursor concentration. The gold seed solution (0.50 mL) in EG was added to refluxing EG (5.0 mL) at 160 °C. Then, PVP (0.60 M, 3.0 mL) and AgNO3 (0.10 M, 3.0 mL) solutions in EG were periodically added every 30 s over 7.5 min, and the resulting mixture was allowed to reflux for 1 h. After cooling the mixture, precipitates were collected by centrifugation at 2000 rpm for 10 min. The product was washed with ethanol several times. Theoretical Simulation. For DDA calculations,18 we modeled a three-dimensional Ag-Au-Ag heterometal nanowire shape with a pentagonal cross section using cubic point dipoles. The nanowire structure was constructed with 36 625 point dipoles after the tip point apexes, and 5-fold edges of the halfdecahedral nanowire ends were snipped by 2 point dipoles at the planes perpendicular to their pointing directions. In the middle of the structure, a similarly snipped decahedron was defined as the gold-seed portion. The resultant structure having the aspect ratio of 20 was used to model the thin, medium, and thick nanowires by assgning the different effective volumes of 2.5255 × 10-3, 6.0874 × 10-3, and 4.0121 × 10-2 µm3, respectively. The spectra were averaged over 864 orientations of the nanowires with respect to the direction of incident light, and the illuminating wavelength was increased from 300 to 900
10.1021/jp104776w 2010 American Chemical Society Published on Web 06/30/2010
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Figure 1. (a) SEM and (b) TEM images of Ag-Au-Ag heterometal nanowires grown from Au decahedrons. (c) TEM image and SAED pattern of a single nanowire. (d) EDX line profile analysis along the long axis of a nanowire. (e) Elemental mapping of Au (red) and Ag (blue). (f) HRTEM image of a nanowire at the Ag-Au interface. The bars represent (a) 1 µm, (b) 500 nm, (c) 50 nm, and (f) 2 nm.
nm by 10 nm. The refractive index values for gold and silver were obtained from the literature.19,20 Results and Discussion Regular gold decahedrons were prepared by a one-pot polyol process in the presence of poly(vinylpyrrolidone) (PVP).17 The gold decahedrons were dispersed in refluxing ethylene glycol (EG). PVP and the silver precursor were periodically added to this seed dispersion, and the mixture was refluxed for 1 h. After the reaction, the product was readily precipitated by centrifugation. Figure 1a shows a representative sample quality of the nanowires. Each nanowire is uniform and straight, and the length is up to 5 µm. The average diameter is estimated to be 86 ( 7 nm (vide infra). The transmission electron microscopy (TEM) image in Figure 1b clearly shows that each nanowire has a dark spot in the middle of the structure. Statistically, 87% of the nanowires (257 out of 296) have distinguishable dark spots, indicating that heterogeneous seeded growth solely accounts for the formation of nanowires. A TEM image of a single nanowire (Figure 1c) shows a rhombic projection of the gold decahedron inside the straight nanowire structure. The symmetry axis (or C5 axis) of the decahedron lies on the long axis of the nanowire. The width of the nanowire projection also fits the gold decahedron diameter. A selected area electron diffraction (SAED) pattern corresponds to a superposition of 〈112〉 and 〈100〉 zone patterns, in good agreement with those of pentagonal rod structures (Figure 1c, inset).21 The nanowire growth direction is [110]. A line profile analysis of energy dispersive X-ray spectroscopy (EDX) along the long axis shows a simultaneous decrease of Ag and an increase of Au in the position of the gold decahedron (Figure 1d). Elemental mapping of gold and
Jung et al. silver indicates that the gold component is isolated well from the silver regions, as shown in Figure 1e. The high-resolution TEM (HRTEM) image in Figure 1f shows that the lattice fringe pattern is continuous throughout both the gold and silver regions, where the lattice spacings between neighboring fringes are identical as 0.20 nm, which corresponds to the distances of silver and gold {200} crystallographic planes. The clear spots of the SAED pattern and the continuous lattice fringe image confirm epitaxial growth of silver nanowires on the gold decahedron surface. This is mainly attributed to the close lattice constants of Au (407.83 pm) and Ag (408.63 pm). The growth mechanism of Ag-Au-Ag heterometal nanowires from gold decahedrons was investigated by taking samples during the reaction progress. SEM images of the samples showed morphological change at various stages. The nanowire lengths increased up to 2 µm, whereas the diameters (or thicknesses) remained almost constant (86 nm) throughout all stages of the reaction. The length changes could be assigned by three distinct stages. The gold particles were hardly grown from beginning to 12 min after AgNO3 addition in the reaction mixture. At this first stage, the silver precursor was slowly converted to reduced silver species (such as Ag42+) and tiny silver nanoparticles. Such an induction period was similarly observed in the formation of silver nanostructures.8 These silver species and nanoparticles were begun to deposit on the gold decahedral seeds either through a direct heterogeneous nucleation or via the Ostwald ripening process.22 Underpotential deposition of the silver species may be the main process to form epitaxial layers on the gold surface.23,24 Self-seeded silver particles were hardly observed in the reaction mixture, although pure silver nanowires were grown in the present reaction condition without the gold seeds, indicating that heterogeneous surface growth dominated the reaction kinetics at this stage. Figure 2a shows elongated decahedrons at 13 min after the reaction. The silver components were evenly deposited on the gold decahedral seeds and maintained half-decahedral facets that were composed of {111} faces on the top and bottom faces (inset). At 16 min after the reaction, the silver regions were sufficiently grown to produce nanorods with an aspect ratio of ∼2 (Figure 2b). Each nanorod had a bright stripe in the middle, which corresponded to the gold composition (inset). From 17 to 25 min after the reaction, the nanorods were rapidly grown into nanowires with a rate of ∼200 nm/min (Figure 2c-e). This linearity of the growth rate is ascribed to the selective directional growth of both ends of the nanowires. In spherical nanoparticles, the growth rates were generally reduced along the reaction progress due to rapid increase of their surface area. However, the growth sides of the nanowires were only {111} facets at the tips, where the active area remained constant during the entire growth stage. Consequently, the lengths increased at a constant rate until the metal precursor was totally consumed. After this stage, the Ag-Au-Ag nanowires approached their full lengths because of limited supply of the silver precursor. The nanowire diameters were nearly unchanged throughout the reaction (Figure 2f). Figure 2g shows three distinct stages, i.e., induction, linear growth, and diffusion limit of the length increments along the reaction progress. Previous studies of silver nanowire formation could not clearly observe such growth stages from the seeds because homogeneous nucleation and growth were difficult to be distinguished by ex-situ measurements.8,25 Instead, heterogeneous seeded growth in the present reaction condition was relatively clear enough to observe each mode in detail.
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Figure 2. SEM images of Ag-Au-Ag heterometal nanowires during the reaction progress. The samples were collected from the reaction mixture at (a) 13, (b) 16, (c) 19, (d) 22, and (e) 25 min after the first addition of PVP and AgNO3 solutions to the reaction mixture. (f) Diameter (O) and (g) length (b) changes from gold decahedrons (Au Dhs) to silver nanowires (Ag NWs) versus reaction periods. The bars represent 1 µm.
Such continuous anisotropic growth of heterometal nanowires relies on two main factors, as reported in the literature.25,26 The three-dimensional structure of the seeds was decahedral, which provided two distinct faces of {111} at the tips and {100} on the side walls of elongated decahedral shapes. Preferential interaction of PVP with the {100} facets leads to anisotropic growth. The {111} faces at the tips were relatively opened during the reaction, and incoming silver species were selectively deposited onto the tips to extend nanowires along their long axis. Some chemical treatments could prove high activity of the tip areas compared to the side walls in metal nanorods and wires.25,27 Figure 1c and the formation mechanism study indicate that Ag-Au-Ag heterometal nanowire diameters could be directly controlled by the edge sizes of the gold decahedral seeds. Ag-Au-Ag nanowires with an average diameter of 86 ( 7 nm were originally grown from gold decahedrons with an edge size of 50 ( 4 nm (Figure 1a,b). The relation between the edge
J. Phys. Chem. C, Vol. 114, No. 29, 2010 12531 size (a) of a regular decahedron and the diameter (d) of a circumscribed circle around the decahedron is d ) 1.70a, and hence the mean diameter of the resulting Ag-Au-Ag nanowires (86 nm) is identical to the size of the decahedrons (1.70 × 50 nm). When small and large gold decahedrons were used as seeds, thin (Figure 3a,b) and thick (Figure 3c,d) nanowires were yielded, respectively. The low-resolution SEM images show high yields of the nanowires of up to >80%. All nanowires are linear and straight with sharp tips at both ends. Each nanowire has a bright stripe in the middle, which indicates incorporation of a gold particle. The lengths of the thin nanowires are ∼1 µm, and those of the thick nanowires are up to several micrometers with wide size distributions. However, the diameters are very uniform with narrow distributions of less than 10%. When gold decahedrons with an edge size of 32 ( 2 nm were used, thin Ag-Au-Ag nanowires with a diameter of 63 ( 6 nm were successfully grown (Figure 3a,b). The large decahedral seeds with an edge size of 88 ( 6 nm generated thick nanowires with an average diameter of 146 ( 7 nm (Figure 3c,d). The diameters and the distributions of thin, medium, and thick nanowires can be distinctively separated, as shown in Figure 3e. All of the nanowire diameters are in good agreement with the sizes of circumscribed circles covering the gold decahedral seeds, which means that the nanowire diameter is solely dependent upon the edge size of the seeds. The nanowires with different diameters (or thicknesses) are expected to exhibit distinct physical properties. One of the important features of the silver and gold nanowires is a strong light extinction in the UV-vis region rooted from SPR.28,29 The optical properties of three samples of the thin, medium, and thick Ag-Au-Ag nanowires in this work were investigated both by UV-vis spectroscopy and by discrete dipole approximation (DDA) calculations.18 For calculation, the nanowires were modeled as snipped elongated decahedrons with an aspect ratio of 20, and the dielectric medium was chosen to be ethanol. To simulate randomly dispersed nanowires, the spectra were rotationally averaged over a sufficiently large number of orientations with respect to the direction of incident light. For thin nanowires with 63 nm diameter, the UV-vis spectrum in Figure 4a shows an asymmetric peak at 379 nm and a small peak at 351 nm. A broad shoulder is also observed at ∼440 nm. The theoretical spectrum by DDA, on the other hand, has a broad peak at the range of 330-550 nm, which can be deconvoluted into two distinct SPR transversal excitations at 370 and 440 nm. Note that transversal excitations can be induced as long as the electric-field polarization of light is perpendicular to the nanowire axis with no regard for the direction of light incidence (wave vector, k), whereas the longitudinal resonances are not feasible in the optical range since the nanowire is too long compared with light wavelength. The major peak at 370 nm, therefore, arises from the transversal dipolar SPR mode at the normal incidence of light (left of Figure 4a and green curve in Figure 4b).30 Since the diameter dimension is considerably smaller than incident wavelengths and the electron density is polarized across the diameter over the wire axis, this transversal surface plasmon has no preferred directions of propagation along the nanowire axis and may be considered as a localized dipolar mode as often observed for metal nanoparticles.31 The second transversal excitation at 440 nm stems from the end-on incidence (right of Figure 4a and blue curve in Figure 4b). In this case, the nanowire length scales are at least a few times larger than the illuminating wavelength, and hence the resulting surface plasmon may propagate along the nanowire axis. Combinations of these two modes excited
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Figure 3. High- and low-resolution SEM images of Ag-Au-Ag heterometal nanowires with average diameters of (a, b) 63 ( 6 nm and (c, d) 146 ( 7 nm, respectively. Insets are SEM images of original gold decahedral seeds. (e) Diameter distribution histograms of three samples. The bars represent (a, c) 500 nm, and (b, d) 2 µm.
at various incidence angles lead to the overall spectral shape agreement. The discrepancies between experimental and theoretical data are less than 15 nm and are mainly due to polymers present on the surface and idealized models different from the real nanowire structures. The very small minor peaks beyond 700 nm are artifacts from the fact that the modeled wires are shorter than real nanowires. The experimental 351 nm peak that is theoretically less obvious can be identified with a small bump at 340 nm in the normal-incidence green curve in Figure 4b. This small bump becomes much more noticeable with diameter thickening, as illustrated in Figure 4c,d, but shows little changes in peak position. Therefore, it is considered a bulk mode peak that has been reported for similar nanowire systems.8,9,28,32-34 The peaks are clearly separated in the medium and thick nanowires. For medium nanowires with a diameter of 86 nm, the peaks at 352, 384, and 490 nm appear in the UV-vis spectrum (Figure 4c). The theoretical analysis illustrates the normal-incidence localized and end-on-incidence propagating transversal modes at 400 and 490 nm as well as the weak normal-incidence bulk mode peak at 340 nm. For thick nanowires with 146 nm diameter, the experimental and theoretical UV-vis spectra match very well, both of which exhibit two transversal modes, at 440 and 650 nm, along with a bulk mode near 350 nm (Figure 4d).
In order to shed light on the two different transversal modes, we calculated the normalized squared electric field distribution, |E|2/|E0|2, around a single nanowire with 86 nm diameter at the resonance wavelengths found in Figure 4c when the illuminating electric field, E0, was provided by the plane-wave light of wave vector k. For the normal incidence localized mode at 400 nm, the surface electric field is polarized across the short axis of the nanowire. For instance, when the wave vector of the incident light is perpendicular to a side of the pentagonal cross section (Figure 5a), a nodal plane forms at the perpendicular bisector of the side and the electric near-field due to localized surface plasmon is intensified on the nanowire circumference. The field at the middle gold seed of the nanowire is slightly attenuated because the imaginary part of permittivity of the gold is larger than that of silver. However, the entire field distribution is not significantly influenced by the gold content (Figure 5b). The end-on-incidence transversal excitation at 490 nm polarizes the electron density in a similar fashion. However, with the wave vector along the long wire axis, the developing surface plasmon becomes a propagating mode with momentum transferred from the incident light via scattering processes at the tapering halfdecahedral nanowire end.32 Figure 5c shows the electric field distribution at the cross section when the incident light electric field is perpendicular to a side of the pentagon. In the broad
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Figure 6. Green and blue lines illustrate red-shiftings of the peaks induced by (a) localized and (b) propagating surface plasmon modes at normal incidence with the electric field polarization perpendicular to the Ag-Au-Ag nanowire axis.
Figure 4. (a) Scheme of relative orientations between the incident light and the nanowire axis. Experimental and theoretical UV-vis spectra of (b) thin, (c) medium, and (d) thick Ag-Au-Ag heterometal nanowires containing gold decahedrons on the center. Each theoretical spectrum is composed of the normal-incidence localized (green) and end-on-incidence propagating (blue) transversal modes, along with a bulk mode at normal incidence.
Figure 5. Electric field distribution |E|2/|E0|2 of the medium Ag-Au-Ag heterometal nanowires containing gold decahedrons on the center: (a, b) normal incidence localized mode at 400 nm; (c, d) end-on-incidence propagating mode at 490 nm; (e, f) bulk mode at 340 nm. (a, c, e) The pentagonal cross sections are at the positions of 150 nm distant from the center of the nanowire. The x and y axes in each map are in nm.
side view (Figure 5d), the electric field starts to lose its intensity at the input end of the nanowire and continues to show much more extensive lateral intensity depletions all the way to the distal end. Notably, the field depletions away from the distal end form a conical surface region, which holds the intensified
electric field extended from the distal end to the far field. In other words, Figure 5c,d illustrates that the nanowire behaves as a subwavelength waveguide.14,35-37 Note that the plasmon wavelength is about 280 nm and the two closely spaced (∼80 nm) heterojunctions due to the gold seed in the middle has little effects on the plasmon propagation. Interestingly, this end-on propagating mode produces local fields stronger than the normalincidence localized mode (compare their best enhancements shown in Figure 5a,c), which reflects better light-plasmon couplings at the half-decahedral ends than at the decahedral wire edges. However, the contribution of propagating modes to the total averaged spectrum is statistically limited because end-on or grazing incidences are less likely in the randomly oriented nanowire samples, which results in a broad shoulder in the real UV-vis spectrum. A separate Mie-type analytical calculation38 in which an infinitely long cylindrical nanowire with the same cross-sectional area is illuminated at normal incidence shows no shoulders in the spectrum, proving that this propagating mode can only be obtained when the wave-vector component parallel to the wire axis is available (Figure S1, Supporting Information). Figure 5e,f shows the field distribution of 340 nm bulk mode at the normal incidence. Interestingly, the bulk mode is longitudinal to the wave vector in contrast with Figure 5a,b, albeit with the same illumination and polarization conditions.28,34 The field distribution appears to be a little bit quadrupolar, particularly when the pentagonal shape effects are eliminated, because the bulk mode is quite close to a short-wavelength quadrupole at an embryonic stage (Figure S2, Supporting Information). The weaker bulk mode peaks in theory than in experiment, on the other hand, seem to be the consequences of the use of empirical permittivity data not well-behaved in the UV range of wavelength. It is interesting to note that the peak positions of both localized and propagating modes are highly sensitive to the nanowire diameters. The peak positions of both modes are monotonically increasing by the increase of diameters (Figure 6). In particular, the average increment (∆λ/∆d ) 2.6) of the propagating mode is 3.7 times larger than that (0.70) of the localized mode. Therefore, if we align the nanowires in a particular direction and then have the propagating mode selectively excited at the end-on incidence, it is feasible to use this mode as a probe for detecting (axially uniform) diameter or dielectric constant change of the nanowires by surface deposition or adsorption. Conclusion We have successfully synthesized Ag-Au-Ag heterometal nanowires from gold decahedral seeds through epitaxial seed-
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mediated growth. The nanowire diameters were precisely controlled in a range of 60-150 nm by using gold seeds with corresponding edge sizes. The three transversally excited bulk, localized, and propagating modes, in the order of increasing red-shifting tendency with diameter thickening, exhibit optical signatures in the UV-vis spectra. Such exact control of diameters and clear observation of different surface plasmon modes are expected to be useful for designing optimal photonic circuit elements such as nanoscale waveguides14,35,36,39 and ultrasensitive chemical and biosensors.40-44 The high sensitivity of the propagating mode upon diameter thickening, for instance, could be exploited in a probe for detecting diameter or dielectric constant change of nanowires by surface deposition or adsorption when nanowires are aligned and selectively excited at the end-on incidence. Acknowledgment. This work was supported by the Pioneer Research Program under Contract 2008-05103 and by the National Research Foundation (NRF) grant funded by the Korean Government (MEST) (Grant R11-2007-050-00000-0). This study was also supported by the research fund from Chosun University 2009. Supporting Information Available: Extinction efficiency of the infinitely long circular cylinder at normal incidence and electric field distribution of a medium silver cylinder with a spherical cross section. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353–389. (2) Lieber, C. M.; Wang, Z. L. MRS Bull. 2007, 32, 99–108. (3) Wiley, B. J.; Wang, Z.; Wei, J.; Yin, Y.; Cobden, D. H.; Xia, Y. Nano Lett. 2006, 6, 2273–2278. (4) Motoyama, M.; Dasgupta, N. P.; Prinz, F. B. J. Electrochem. Soc. 2009, 156, D431–438. (5) Li, Z.; Hao, F.; Huang, Y.; Fang, Y.; Nordlander, P.; Xu, H. Nano Lett. 2009, 9, 4383–4386. (6) Lee, J.-Y.; Connor, S. T.; Cui, Y.; Peumans, P. Nano Lett. 2008, 8, 689–692. (7) Chen, J.; Wiley, B. J.; Xia, Y. Langmuir 2007, 23, 4120–4129. (8) Sun, Y.; Yin, Y.; Mayers, B. T.; Herricks, T.; Xia, Y. Chem. Mater. 2002, 14, 4736–4745. (9) Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Nano Lett. 2002, 2, 165– 168. (10) Sun, Y.; Xia, Y. AdV. Mater. 2002, 14, 833–837. (11) Kim, F.; Sohn, K.; Wu, J.; Huang, J. J. Am. Chem. Soc. 2008, 130, 14442–14443. (12) Chen, J.; Herricks, T.; Geissler, M.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 10854–10855. (13) Huang, X.; Zheng, N. J. Am. Chem. Soc. 2009, 131, 4602–4603. (14) Akimov, A. V.; Mukherjee, A.; Yu, C. L.; Chang, D. E.; Zibrov, A. S.; Hemmer, P. R.; Park, H.; Lukin, M. D. Nature 2007, 450, 402–406.
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