Plasmon Coupling in Two-Dimensional Arrays of Silver Nanoparticles

Jan 25, 2010 - The effect of the interparticle distance on plasmon modes was also .... in suspension that were used to make the corresponding 2D array...
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Plasmon Coupling in Two-Dimensional Arrays of Silver Nanoparticles: II. Effect of the Particle Size and Interparticle Distance† Mark K. Kinnan and George Chumanov* 235 H.L. Hunter Laboratories, Clemson UniVersity, Clemson, South Carolina 29634 ReceiVed: December 1, 2009; ReVised Manuscript ReceiVed: January 5, 2010

The effect of the particle size and interparticle distance on the coherent plasmon coupling was studied in 2D arrays of silver nanoparticles. The plasmon coupling leads to the formation of a cooperative plasmon mode characterized by an intense and narrow peak in the blue spectral range. The arrays were fabricated via the self-assembly of the nanoparticles on poly(4-vinylpyridine)-modified glass substrates. Changing the ionic strength of the aqueous nanoparticle suspension prior to the self-assembly provided a possibility for controlling the interparticle distance in the arrays. The study revealed an optimum particle size around 86 nm and the corresponding optimum interparticle distance of about 107 nm for the strongest plasmon coupling, as determined from the most intense and sharpest resonance that was observed in water. Changing the dielectric medium will conceivably result in different values for the optimal particle size and interparticle distance. Introduction The effect of the particle size, shape, and the surrounding dielectric medium on plasmon resonances of silver nanoparticles (SNPs) is well-understood and intensively studied for particles in solution.1 These factors, together with the interparticle distance, also determine optical resonances in coupled systems of plasmonic particles. Plasmon coupling takes place in the nearfield when the particles are in close proximity to each other so that the electron oscillations in each particle are affected by the local field associated with the electron oscillations in neighboring particles. This paper explores the effect of the particle size together with the interparticle distance on the coherent plasmon coupling in 2D arrays of SNPs. The effect of the dielectric medium was previously discussed in the first paper of this series.2 An experimental study by Sung et al. compared two different sizes of L-shaped and V-shaped particles in 2D arrays prepared by electron beam lithography.3 Other groups have fabricated 2D arrays of triangular and cylindrically shaped particles by either electron beam lithography or nanosphere lithography and investigated the effect of size on plasmon coupling.4-6 These experimental studies found that the plasmon resonance shifted to longer wavelengths as the size of the particles increased due to the phase retardation. The effect of particle size on plasmon coupling in dimers7 and 1D arrays8 of nanoparticles has also been investigated theoretically, and the plasmon resonance was calculated to shift to longer wavelengths with the increase in the particle size. The effect of the interparticle distance on plasmon modes was also investigated for dimers,7,9-16 chains,5,17-19 and 2D arrays3-5,8,20-27 of silver and gold nanoparticles. It was found for two interacting nanoparticles (spherical, pyramidal, and rods) that the plasmon resonance shifted to either longer or shorter wavelengths as the interparticle distance decreased, depending on the polarization of incident light.9-11 For a side-by-side orientation, the polarization along the axis of two particles resulted in a large, red spectral shift, whereas polarization †

Part of the “Martin Moskovits Festschrift”. * Corresponding author. E-mail: [email protected].

perpendicular to the axis caused a small, blue spectral shift.7,12,13 For unpolarized light, the red shift was a dominant phenomenon because of much larger polarizability of the particles along the long axis as compared to that in the perpendicular direction. The two particles when coupled in the head-to-head and tailto-tail geometry (along the long axis) act more like a single, larger particle and resulted in the red shift of the combined plasmon resonance. A small, blue spectral shift in the perpendicular direction is also expected because the electron oscillations in this case are coupled in a head-to-tale geometry, causing mutual polarization enhancement and, consequently, the higher frequency of the combined plasmon mode. There are fewer examples in the literature for chains of nanoparticles, but the effect of the interparticle distance on the plasmon resonance was the same as that observed for dimers.5,17-19 For 2D arrays of nanoparticles, it was observed that the plasmon resonances shifted to shorter wavelengths upon the decrease of the interparticle distance.3,8,23-25 However, a red spectral shift of the plasmon resonance was observed for 10.5 nm gold nanoparticles in 2D arrays when the interparticle distance was decreased.21 Another report by Sung et al. showed the plasmon resonance of cylindrical particles resulted in a red spectral shift for 163 nm particles with decreased interparticle spacing, but a blue spectral shift was observed for 339 nm diameter particles.20 Presented here is a systematic study of the coherent plasmon coupling in 2D arrays of SNPs of seven different sizes and varying interparticle distance. The arrays were fabricated via particle self-assembly, and the interparticle distance was controlled by controlling the ionic strength of the solution. It was observed that the increase in the particle size caused a red spectral shift of the coherent plasmon mode. At the same time, the spectral width of the mode first decreased and then increased again. A more complete picture was revealed when the coherent plasmon coupling was studied as a function of both the particle size and interparticle distance. Experimental Section Details of experimental procedures were described previously in Part I.2 Briefly, SNPs of different sizes were synthesized by

10.1021/jp911411x  2010 American Chemical Society Published on Web 01/25/2010

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Figure 1. Electron microscopy images of about (a) 46, (b) 59, (c) 86, (d) 128, (e) 160, (f) 194, and (g) 287 nm diameter SNPs. Corresponding UV-vis extinction spectra of SNPs in water: A-G.

reduction of silver oxide in water with ultrahigh-purity hydrogen gas according to ref 28. The 2D arrays were fabricated via the self-assembly of SNPs on indium tin oxide (ITO)-coated unpolished float glass (Delta Technologies, Limited) slides with a sheet resistance of Rs ) 4-8 Ω. The ITO slides were further modified with poly(4-vinylpyridine) for the adsorption of SNPs.29 A known concentration of sodium sulfate was introduced to the SNP suspension to control the interparticle distance in the arrays. UV-vis extinction spectra and electron microscopy images were obtained using a Shimadzu UV-2501PC spectrophotometer and a Hitachi 4800 scanning electron microscope, respectively. The UV-vis extinction spectra are normalized to the same cross-section. Results and Discussion The UV-vis spectra in solution and corresponding electron microscope images of seven different sizes (ca. 46, 59, 86, 128, 160, 194, and 287 nm) of SNPs used for the fabrication of 2D arrays are illustrated in Figure 1. The spectra of small SNPs (Figure 1A-C) are dominated by the excitation of the dipole plasmon mode, whereas in the spectra of larger particles, quadrupolar, octupolar, and even hexadecapolar modes can be observed.30 Higher multipolar modes originate from the phase retardation of the incident field within the particle body as its dimensions of SNPs increases approaching the wavelength of light.1,30 When particles are small as compared to the wavelength of light, the entire particle experiences the same phase of the incident radiation, thereby favoring predominantly dipolar electron oscillations. As the particle dimensions become larger, the different areas of the particles experience different phases of the incident radiation, and the excitation of higher multipolar modes of the plasmon oscillations is possible. The higher the order of the plasmon mode, the larger is its frequency. The frequency of all plasmon modes shifts to the red spectral range as particle size increases; however, the frequency shifts more slowly for higher multipolar modes. The excitation of multipolar plasmon modes leads to the broad plasmon band and, consequently, to strong interaction of SNPs with light across the entire visible and near-IR spectral regions. It is important to emphasize that the excitation of plasmon modes in single crystal SNPs in the wavelength approximately above 450 nm leads to strong resonant scattering of light, whereas the excitation below this wavelength results in what is known as plasmon absorption. The relationship between the absorption and resonant scattering for different size SNPs can be viewed as the excitation of absorbing and radiative plasmon modes.31 When SNPs are assembled into 2D arrays in close proximity to each other so that the local electromagnetic field of each

Figure 2. UV-vis extinction spectra of coupled 2D SNP arrays fabricated with (a) 46, (b) 59, (c) 86, (d) 128, (e) 160, (f) 194, and (g) 287 nm particles.

particle overlaps with neighboring particles, the system undergoes coherent plasmon coupling that is manifested as an intense and spectrally narrow plasmon mode in the blue spectral range.22 This coupled plasmon mode results from the phase coherence for electron oscillations in neighboring particles. The coupled plasmon modes shown in Figure 2 for each of the seven differently sized SNPs was defined as the sharpest resonance that could be obtained for each particle size by varying the concentration of the particles (the interparticle distance) on the surface. The combined effects of the particle size and the interparticle distance on the coherent plasmon coupling will be discussed further in more detail. As the particle size increases, the plasmon-coupled peak shifts to the red spectral region: the corresponding peak positions a-g are 416, 414, 414, 420, 441, 457, and 493 nm, respectively. This red shift echoes the shift of plasmon modes of individual SNPs in solution with the particle size. For SNPs smaller than about 86 nm, the coupled plasmon peak is less intense and broader, as compared with that for larger particles. The same observation was calculated by Zou et al. for 30 nm particles in 2D coupled arrays.8,25 For large SNPs, the coupled mode also broadened, albeit remaining fairly strong. This broadening was also attributed to the phase retardation of the incident light1,10,25 that was previously observed.5 It appeared that there is an optimum particle size that yielded the sharpest resonance.

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Figure 4. Spectral position of the plasmon-coupled mode of the 2D SNP arrays as a function of the particle size.

Figure 3. Electron microscopy images of 2D SNP arrays fabricated with (a) 86, (b) 128, and (c) 287 nm particles.

Electron microscope images were acquired to reveal the structure of the 2D arrays with different SNP size. To image nanoparticle arrays fabricated by methods based on selfassembly from solution, the arrangement of nanoparticles needs to be stabilized before the solvent is dried. Otherwise, drying results in particle aggregation due to the capillary forces associated with the solvent evaporation.22 To obtain an accurate representation of 2D arrays of SNPs, the particles were immobilized using a previously reported procedure based on spin-coating of a thin PMMA layer that fills the space between the particles.2 After the immobilization, the arrays were carboncoated to create a conductive layer that is needed for electron microscopy. Three representative arrays of SNPs with sizes (a) 86, (b) 128, and (c) 287 nm are shown in Figure 3. The halo that appears around the particles is due to the carbon coating, making the particles appear slightly larger. When the peak position of the coupled plasmon mode is plotted against the size of the particle, a near-linear fit is observed (Figure 4), with an R2 value of 0.95. It has also been previously reported that for electron-beam- and nanospherelithography-produced nanoparticle arrays, the peak position versus the particle size follows a linear trend.5,6 This linear trend is useful for a rational design of plasmon-coupled SNP arrays with a desired frequency by selecting an appropriate SNP size. The arrays with 46 and 59 nm SNPs are not included in the plot because the plasmon coupling, as defined by the sharpness and the intensity of the resonance, is much weaker, as compared with that of the arrays with larger SNPs. The coherent plasmon coupling in 2D SNP arrays simultaneously depends upon both the particle size and interparticle distance, necessitating the study of the two factors concurrently. The interparticle distance in such arrays was previously varied by stretching arrays embedded into a PDMS film.22 Even though the method is robust, the range of achieved interparticle distances was somewhat small. Here, we devised a different strategy based on controlling the electrostatic repulsion between SNPs in the

solution and on the surface. It was previously suggested that hydrogen-reduced silver nanoparticles have a negative surface charge due to the silver hydroxide species present on their surface.28,31 Since the SNPs are synthesized in pure water that is a weak electrolyte with low ionic strength and the surface of SNPs is charged, the particles are surrounded by a thick, double, electric layer. This layer creates a repulsive shell that prevents the coalescence (aggregation) of particles and is the reason for the high stability and long-term (years) storage of hydrogenreduced SNPs in pure water. Manipulating the thickness of this shell provides a possibility for controlling the interparticle distance on the surface via changing the packing density of SNPs on the surface. Indeed, the addition of sodium sulfate to a suspension of SNPs prior to self-assembly resulted in higher densities of the particles on the surface and, correspondingly, the decrease of the interparticle distance in 2D arrays, as evident from the electron microscopy images. However, too much salt resulted in partial or complete irreversible aggregation of the particles in the suspension. The maximum limiting concentration of an electrolyte that can be tolerated by SNPs in aqueous suspension depends upon the size and absolute concentration of the particles as well as the chemical nature of the electrolyte. Smaller particles in higher concentrations can tolerate higher concentrations of an electrolyte. This principle was widely used in the past for the size separation of various colloidal particles. When an aqueous suspension of SNPs without sodium sulfate was used for the self-assembly, the packing density of the particles was somewhat low, and the interparticle distance, correspondingly large. Therefore, the plasmon coupling was weak, as illustrated by the red curves in Figure 5. As the number of particles on the slide increased, the interparticle distance decreased, and the plasmon coupling became stronger (from red to orange, to green, to light blue, to blue, and to violet curves in Figure 5). The dotted green line corresponds to the extinction spectra of SNPs in suspension that were used to make the corresponding 2D arrays. It was observed that for small particles (ca. 59 nm or less), the plasmon peak shifted to the red spectral range and broadened as more particles were self-assembled on the slides. In contrast, for SNPs larger than 128 nm, a blue shift and narrowing of the plasmon mode was evident. The SNPs of intermediate sizes

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Figure 5. UV-vis extinction spectra of the 2D SNP arrays with varying interparticle distance and different particle size: (a) 46, (b) 59, (c) 86, (d) 128, (e) 160, and (f) 287 nm. The color coding is explained in Table 1. The curves in the upper part of each panel correspond to the UV-vis extinction spectra of the corresponding SNPs in aqueous suspension.

TABLE 1: Statistical Analysis of the 2D SNP Arrays Shown in Figure 6 Figure 5b curve

Figure 5c

Figure 5d

ID meas ID calc salt ID meas ID calc salt ID meas ID calc salt λmax (nm) (nm) (mM) λmax(nm) AF (%) (nm) (nm) (mM) λmax(nm) AF (%) (nm) (nm) (mM) (nm) AF (%)

red 0.00 orange 1.00 green 3.00 lt blue 5.00 blue 9.00 maroon 15.00

417 417 418 419 422 423

15.9 29.8 39.4 43.6 48.1 49.7

134 ( 20 93 ( 14

118 85 77 79 66 66

0.00 0.50 1.00 3.00 5.00 7.00

445 413 415 413 414 415

14.6 24.5 36.5 40.4 44.0 46.2

218 ( 34 146 ( 24 129 ( 14 116 ( 13

194 134 122 106 101 107

0.00 0.25 0.50 0.75 1.00 7.00

446 436 430 427 419 417

17.6 27.9 32.6 35.0 37.8 60.1

249 ( 37 203 ( 28 190 ( 24

208 166 163 147 146 116

a

Column headings: Salt is the concentration of sodium sulfate in the SNP suspension, λmax is the maximum plasmon resonance, AF is the area fraction of SNPs on the substrate, ID meas is the distance between the center of particles as measured using ImageJ software, and ID calc is the calculated interparticle distance. The calculated interparticle distance was determined by taking a measured area, dividing it by the number of particles in that area, and taking the square root. The measured interparticle distances were determined from 100 measurements.

exhibited mixed behavior characterized by red and blue spectral shifts as well as spectral broadening and narrowing. From these experiments, it became evident that there is an optimal particle size with corresponding optimal interparticle distance that results in the most intense and the sharpest coherent plasmon mode. Attempts were also made to add more SNPs to a slide that already had adsorbed particles. First, a slide was rolled in a

SNP and sodium sulfate solution to produce 2D arrays. After adsorption of particles, the slide was rinsed with water, the UV-vis spectrum was acquired, and the slide was placed back into a vial containing a fresh solution of SNPs from the same batch and the same concentration of sodium sulfate. No more particles were adsorbed to the slide, as was concluded from the identity of the UV-vis spectra measured before and after the

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Figure 6. Electron microscopy images of 2D SNP arrays with different packing densities of the particles. The top, middle, and bottom rows correspond to the arrays in Figure 5b, c, and d, respectively.

second exposure (data not shown). This experiment provided further support that the double electric layer around SNPs controls the packing density of the particles that can be selfassembled on the slide. To obtain visual representations of the 2D SNP arrays and to compare these with the corresponding UV-vis spectra, electron microscopy images were acquired. The particles were immobilized using spin-coating of PMMA, as described above. For arrays exposed to SNP suspensions without sodium sulfate, the interparticle distance was approximately twice the nanoparticle diameter (Figure 6A, G, and M.). As expected, slides exposed to SNP suspensions with different concentrations of sodium sulfate had varied surface concentrations of particles. Higher concentrations of sodium sulfate resulted in an increased number of particles in the 2D arrays (Figure 6). As the surface concentration of particles increased, the likelihood for particle aggregation also increased especially during the PMMA spincoating procedure. The transfer of the slides with 2D arrays from water to 2-propanol and further to anisole (the solvent used for spin-coating of PMMA) disrupted the electric double layer, causing surface aggregation (clumping and stacking) of the particles. This explains why Figure 6E and F, for example, appears to have virtually the same packing densities. Table 1 summarizes the statistical analysis of the electron microscope images in Figure 6. Not all images report an ID meas value because the particles were partially aggregated on the surface, and the results from actual measurements would be ambiguous. It is important to note that the concentration of sodium sulfate that is required to achieve a desired packing is dependent on the concentration of SNPs in solution. For low concentrations of SNPs, lower concentrations of sodium sulfate are needed. To keep all samples consistent, the concentration of SNPs was kept constant and the concentration of the sodium sulfate was varied. The evidence of SNP surface aggregation was observed when high concentrations of sodium sulfate was used. In Figure 5a-c, the formation of a broad plasmon band in the red spectral range between 550 and 800 nm is due to aggregation that was observed from the UV-vis spectra before PMMA was used to immobilize the SNPs. The aggregation resulted in the formation of small clusters with a predominant fraction of dimers that exhibit longitudinal plasmon mode in the red spectral region due to the direct electronic coupling between adjacent particles (blue and violet curves in Figure 5a-c).

The optimum interparticle distance that resulted in the sharpest and most intense coherent plasmon mode (the strongest plasmon coupling) was different for each particle size. However, the surface-to-surface distance between particles was nearly identical for the films with different particle size and optimum interparticle distance. For example, the surface-to-surface distance corresponding to the strongest coupling for about 59, 86, and 128 nm sized particles was calculated from the values in Table 1 to be 20, 21, and 19 nm, respectively. It is not yet known whether this 20 ( 1 nm surface-to-surface distance is a peculiar coincidence or represents an optimal distance for the local electromagnetic field to interact with neighboring SNPs. In a theoretical study by Zhao et al.,32 the effect of interparticle distance on the plasmon coupling of ordered 2D arrays of 30 nm silver spherical particles was investigated. The authors observed that the intensity of the plasmon coupling decreased and the plasmon band shifted to the blue spectral region as the interparticle distance was decreased to 75 nm. Upon further decrease of the interparticle distance, the plasmon band shifted back to the red spectral range. The results from ref 32 are somewhat different from the experimental data presented here on random arrays. Even though our smallest SNPs were about 16 nm larger than those used in the calculations, both sizes can be considered close enough to compare the experimental results with the calculations. We did not observe the blue spectral shift, even though the interparticle distances were comparable to the values reported in the theoretical modeling. The suggested explanation in ref 32 for the red and blue spectral shifts is based on the retarded dipole sum. The blue or red spectral shift results when the real part of the retarded dipole sum is negative or positive, respectively.32 The discrepancy in the shifting of the plasmon band could also be attributed to the difference of an ordered array versus a random array of particles. In conclusion, the study of the combined effect of the particle size and interparticle distance on coherent plasmon coupling in 2D SNP arrays revealed an optimum particle size around 86 nm and the corresponding optimum interparticle distance of about 107 nm as measured from center to center that yielded the most intense and sharpest plasmon resonance. This specific result relates to the 2D SNP arrays in water; embedding the arrays in other dielectric media will change the condition for the plasmon coupling, most likely leading to different values

Two-Dimensional Arrays of Silver Nanoparticles for the optimal particle size and interparticle distance (see the first paper in this series2). It is likely that the optimal particle size corresponds to the maximum oscillator strength of individual SNPs that provides the strongest local field and, consequently, the strongest plasmon coupling. As the size of the plasmonic particle increases, more electrons are available to participate in the plasmon oscillations. At the same time, retardation effects in large plasmonic particles limit plasmon oscillation more to the surface electrons, thereby decreasing the oscillator strength per unit volume of metal. The efficiency for the interaction with light (oscillator strength) was experimentally measured for single SNPs in water, and the maximum was found to be for the particles around 60 nm.31 The discrepancy between the ∼60 nm and ∼86 nm that was concluded from the current study could be explained by the differences in the dielectric environment; specifically, the presence of the ITO substrate on which the 2D SNP arrays were self-assembled. The dielectric environment has a profound effect on how the plasmonic particles interact with light. Acknowledgment. The authors are grateful for the financial support from the United States Department of Energy, Grant No. DE-FG02-06ER46342. The authors also thank Zack Gosser for synthesizing some of the silver nanoparticles used in these studies. References and Notes (1) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, New York, 1995. (2) Kinnan, M. K.; Kachan, S.; Simmons, C. K.; Chumanov, G. Plasmon Coupling in Two-Dimensional Arrays of Silver Nanoparticles: I. Effect of the Dielectric Medium. J. Phys. Chem. C 2009, 113 (17), 7079– 7084. (3) Sung, J.; Hicks, E. M.; Van Duyne, R. P.; Spears, K. G. Nanoparticle Spectroscopy: Dipole Coupling in Two-Dimensional Arrays of L-Shaped Silver Nanoparticles. J. Phys. Chem. C 2007, 111 (28), 10368– 10376. (4) Felidj, N.; Grand, J.; Laurent, G.; Aubard, J.; Levi, G.; Hohenau, A.; Galler, N.; Aussenegg, F. R.; Krenn, J. R. Multipolar Surface Plasmon Peaks on Gold Nanotriangles. J. Chem. Phys. 2008, 128 (9), 094702. (5) Bouhelier, A.; Bachelot, R.; Im, J. S.; Wiederrecht, G. P.; Lerondel, G.; Kostcheev, S.; Royer, P. Electromagnetic Interactions in Plasmonic Nanoparticle Arrays. J. Phys. Chem. B 2005, 109 (8), 3195–3198. (6) Huang, W. Y.; Qian, W.; El-Sayed, M. A. The Optically Detected Coherent Lattice Oscillations in Silver and Gold Monolayer Periodic Nanoprism Arrays: The Effect of Interparticle Coupling. J. Phys. Chem. B 2005, 109 (40), 18881–18888. (7) Gluodenis, M.; Foss, C. A. The Effect of Mutual Orientation on the Spectra of Metal Nanoparticle Rod-Rod and Rod-Sphere Pairs. J. Phys. Chem. B 2002, 106 (37), 9484–9489. (8) Zou, S. L.; Janel, N.; Schatz, G. C. Silver Nanoparticle Array Structures That Produce Remarkably Narrow Plasmon Lineshapes. J. Chem. Phys. 2004, 120 (23), 10871–10875. (9) Gunnarsson, L.; Rindzevicius, T.; Prikulis, J.; Kasemo, B.; Kall, M.; Zou, S. L.; Schatz, G. C. Confined Plasmons in Nanofabricated Single Silver Particle Pairs: Experimental Observations of Strong Interparticle Interactions. J. Phys. Chem. B 2005, 109 (3), 1079–1087. (10) Su, K. H.; Wei, Q. H.; Zhang, X.; Mock, J. J.; Smith, D. R.; Schultz, S. Interparticle Coupling Effects on Plasmon Resonances of Nanogold Particles. Nano Lett. 2003, 3 (8), 1087–1090. (11) Jain, P. K.; El-Sayed, M. A. Surface Plasmon Coupling and Its Universal Size Scaling in Metal Nanostructures of Complex Geometry: Elongated Particle Pairs and Nanosphere Trimers. J. Phys. Chem. C 2008, 112 (13), 4954–4960.

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