Article pubs.acs.org/JPCC
Plasmonic Coupling in Single Silver Nanosphere Assemblies by Polarization-Dependent Dark-Field Scattering Spectroscopy Xiangdong Tian,† Yadong Zhou,‡ Sravan Thota,† Shengli Zou,‡ and Jing Zhao*,§,† †
Department of Chemistry, University of Connecticut, 55 North Eagleville Road, Storrs, Connecticut 06269-3060, United States Department of Chemistry, University of Central Florida, 4104 Libra Drive, Orlando, Florida 32816-2366, United States § Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269-3136, United States ‡
S Supporting Information *
ABSTRACT: In this paper, we present an experimental and theoretical study of the plasmonic properties of single Ag nanospheres and the plasmon interactions in assemblies of Ag nanosphere dimers and trimers. High-quality Ag nanospheres with small size distribution are synthesized by etching prefabricated Ag nanocubes. We perform a 360° polarization-resolved scattering study on silver nanosphere dimers and trimers, and correlate the scattering anisotropy with nanoparticle structure through correlated dark-field spectroscopy and scanning electron microscopy (SEM) characterization. The polarization-resolved dimer scattering shows a dipolar pattern aligned with the long axis of the dimer. For single Ag nanosphere trimers assembled in an equilateral triangle geometry, we also observe the dipolar scattering pattern to a certain degree, although the dipolar pattern is not preferentially aligned with any sides of the triangle. Theoretical studies using the T-matrix method reveal that if the Ag nanospheres are perfectly spherical and are assembled in a trimer with D3h symmetry, the scattering spectra should be polarization independent, in contrast to the observed experimental results. The same phenomena are demonstrated in Ag nanopshere assemblies in D4h, D5h, and D6h symmetry as well. Using the discrete dipole approximation method, we find that slight elongation (5%) in one of the three axes of the Ag nanospheres can induce a significant anisotropy in the scattering pattern. We here have shown that even small variations in the nanoparticle geometry that are difficult to resolve with SEM can lead to significant effects in the plasmonic coupling, therefore affecting the scattering spectra of the assembled nanostructures.
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INTRODUCTION Nanostructures composed of Cu, Ag, and Au interact strongly with light.1−5 The optical response of these nanostructures is determined by plasmons. Plasmons are the collective oscillation of the surface conduction electrons of nanoparticles driven by incident photons. Excitation of plasmons leads to the following major optical responses of coinage metal nanoparticles: enhanced light absorption and scattering, as well as a strongly enhanced near field in the immediate vicinity of the nanoparticle surface.6,7 Plasmonics is the physical basis of many surface-enhanced spectroscopic techniques, such as surface-enhanced Raman spectroscopy,8−11 surface-enhanced infrared spectroscopy,12−14 and surface-enhanced fluorescence.15,16 Plasmonic materials also show great potential in applications such as plasmonic circuitry,17,18 three-dimensional optical holography,19,20 and surface plasmon amplification by stimulated emission of radiation.21,22 A single nanoparticle is the smallest functional unit to support plasmon-based applications. Single plasmonic nanoparticles have been used as nanoantennas, for chemical and biological sensing, and for optical labeling and tracking.23−26 Plasmonic nanoparticles can also be assembled into functional 2-D or 3-D structures with exotic optical properties, such as photonic crystals and metamaterials.27−30 There is increasing © XXXX American Chemical Society
interest over the past few years in assembled nanostructures because the interaction between the nanoparticle building blocks gives rise to interesting optical phenomena.31,32 In analogy to “bonding” or “anti-bonding” orbitals in molecular systems, plasmonic coupling may lead to distinct features in the optical spectrum of the assembled nanostructures.33 For example, Shegai et al. have demonstrated that single Ag nanoparticle trimers can effectively modulate the polarization of Raman scattering of molecules adsorbed to them.34 Fan and coworkers have shown that self-assembled clusters of metaldieletric Au@SiO2 spheres exhibit strong magnetic and Fano resonances.27 Chuntonov et al. have demonstrated that in trimeric “plasmonic molecules” composed of Ag nanoparticles, the symmetry of the trimers has strong impact on the degeneracy of their plasmon modes.35−37 Barrow et al. observed chain length-dependent plasmon resonance wavelength in DNA-facilitated gold nanoparticle assembly chains from monomer to hexamer.38 Although these studies have demonstrated the successful assembly of nanoparticles and manipulation of their optical properties, a systematic study of Received: April 9, 2014 Revised: May 23, 2014
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orange color. The Ag nanocubes were isolated through centrifugation and stored in ethanol for further use. Synthesis of Ag Nanospheres. Ag nanospheres were synthesized through an etching method.42 In a typical synthesis, 200 μL of Ag nanocubes was added to 8 mL of 1% (w/v) PVPethanol solution. 800 μL of 5 mM ferric nitrate (Sigma-Aldrich) isopropanol solution was then added to the solution under vigorous stirring within 2 min. The stirring was continued for 2 h. The nanospheres were purified through centrifugation and redispersed in ethanol. Correlated SEM and Dark-Field Microscopy. To prepare the sample for single particle study, 10 μL of the Ag nanospheres in ethanol was dropped on a piece of No. 1 coverglass (Fisher Scientific). The solution was then quickly moved by a pipet tip to evenly spread the nanoparticles on the surface of the coverglass and the ethanol was evaporated in air. A Nikon Ti-u microscope was used for the collection of the scattering spectra of Ag nanospheres. The illumination light from a halogen lamp passes through a dark-field condenser (NA 0.85) and then forms an inverted hollow cone of light focused at the specimen plane on the coverglass. The light scattered by the nanoparticles was collected with a 100× NA 0.8 objective (variable NA 0.8−1.3). The scattered light was directed to the entrance slit of a spectrograph (IsoPlane SCT 320, Princeton Instruments) equipped with a CCD camera (PIXIS 1024BR, Princeton Instruments). The width of the entrance slit was manually adjusted to separate the scattered light of the target nanoparticle from that of the surrounding nanoparticles. To obtain linearly polarized illumination, a linear polarizer was placed between the field diaphragm and the condenser. The polarization angle of the illumination was changed by rotating the polarizer from 0° to 360° with an increment of 15°. Scattering spectra were corrected and normalized by signal collected from a nearby region without nanoparticles. Color images of the scattering spots associated with individual nanoparticles/nanoparticle assemblies were captured by the camera of a smartphone. The camera was first aimed at the eyepiece of the microscopy. By carefully adjusting the position of the camera, the targets were projected on the screen of the smartphone. The pictures were shot by keeping the smartphone still. The structure of each scattering spot was characterized by SEM after the collection of the spectra. To locate the scattering spots studied under the dark-field microscopy for SEM characterization, a reference nickel grid (Ted Pella, Inc.) was attached to the sample for guiding. It is also helpful to cooperatively use the color images taken by the smartphone to find the target nanoparticles.
single crystalline nanoparticle building blocks and assemblies with extremely uniform size and shape is still lacking. In assembled nanostructures, the geometry and monodispersity of the nanoparticle building blocks is critical to the control of the assembly process, and to the understanding of their optical properties from both theoretical and experimental aspects.39,40 The uniformity of geometry and smoothness of the surface of the nanoparticles are critical for the accurate comparison between experiments and theory. More importantly, uniform nanoparticles are highly desirable when used as building blocks to assemble into optical devices.41 Silver nanospheres are model objects and ideal building components in many theoretical and experimental studies of plasmonic nanoparticles and assemblies. However, due to the difficulty in synthesizing monodisperse samples, there is still a lack of study for the optical properties of high quality Ag nanospheres.42 Here, we synthesize high quality single crystalline Ag nanospheres by a two-step process: first, highly uniform Ag nanocubes are synthesized using a method developed by Tao et al.;43 second, the Ag nanocube precursors are etched with ferric nitrate following a slightly modified procedure developed by Cobley et al.42 Many studies have shown that the optical properties of single plasmonic nanoparticles are highly dependent on their structures.44,45 Correlating the structure and optical property of single plasmonic nanoparticles/assemblies is the ultimate tool to help us understand, and therefore control, the optical properties of them. In this work, we use correlated scanning electron microscopy (SEM) and single-particle dark-field scattering spectroscopy to characterize the structures and optical properties of single Ag nanospheres, dimers and trimers. Comparing the spectra of the nanosphere dimers and trimers to those of the single nanospheres, new features arise in the spectra of the assemblies due to plasmonic coupling between the nanospheres. Furthermore, we mapped full 360° polarization-dependent scattering of Ag nanosphere dimers and trimers in an equilateral triangle geometry. The scattering spectra of the Ag nanosphere assemblies are highly dependent on the polarization of the incident light relative to the dimer/ trimer axis. Theoretical studies using the T-matrix46 and the discrete dipole approximation (DDA)47 methods are performed to understand the phenomena. The comparison between the experimentally measured spectra with those from calculations shows the extraordinary sensitivity of their optical properties on the geometry of the individual nanoparticles within the Ag nanosphere assemblies, especially in a trimer assembly.
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MATERIALS AND METHODS Synthesis of Ag Nanocubes. The Ag nanocubes were synthesized as reported previously.43 First two precursor solutions were prepared. One solution was prepared by dissolving 0.4 g of silver nitrate (Sigma-Aldrich) into 10 mL solution of copper(II) chloride (Sigma-Aldrich) in 1,5pentanediol (Sigma-Aldrich) with a concentration of 36 μg/ mL. The other solution was prepared by dissolving 0.2 g of poly(vinylpyrrolidinone) (PVP) (MW = 55 000, Sigma-Aldrich) in 10 mL of 1,5-pentanediol. The dissolution process for both solutions was assisted by sonication. Then 20 mL of 1,5pentanediol was heated under stirring for 10 min in an oil bath at 193 °C. The two precursor solutions were then simultaneously injected into the preheated 1,5-pentanediol through a syringe pump at a rate of 500 μL/min. The addition was stopped when the color of the solution turned into pinkish
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RESULTS AND DISCUSSION Synthesis of Ag Nanospheres. High-quality Ag nanospheres were synthesized by etching the precursor of Ag nanocubes using ferric nitrate as the etchant. The Ag nanocubes were synthesized through a polyol method.43 Figure 1a is the SEM image of the nanocubes. From the SEM image, we find that the Ag nanocubes are highly uniform in size with an edge length of ∼100 nm. The uniform Ag nanocubes are perfect precursors for the synthesis of single crystalline and uniform Ag nanospheres, as shown by Xia and co-workers in a series of papers.40,49 To obtain Ag nanospheres, ferric nitrate functions as an effective oxidant in the reaction to truncate the edges and corners of the nanocubes. It also triggers the assembly of Ag B
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Figure 2. Correlated dark-field scattering (a) and SEM (b) images of single Ag nanospheres/assemblies. The number beside the blue circle is an indication of the correlated scattering spots and SEM images. Figure 1. SEM and TEM micrographs of Ag nanoparticles. (a) Ag nanocube precusors with an edge length of 100 nm; (b) Ag nanospheres with a diameter of 85 nm; (c) TEM image of an Ag nanosphere; (d) TEM image of an Ag nanosphere dimer; (e) TEM image of an Ag nanosphere trimer; (f) zoomed in TEM image of the gap between two nanospheres.
No. 7 is also an Ag dimer. The correlation of SEM image with the dark-field scattering image allows for the further investigation of the optical properties of Ag nanosphere monomers, dimers and trimers, and the relative orientation of the nanospheres to the incident light polarization. In this study, we use the correlated dark-field microscopy and SEM technique to identify and study the scattering spectroscopy of 27 single Ag nanospheres, 20 Ag nanosphere dimers, and 18 Ag nanosphere trimers. Single Ag Nanosphere Spectroscopy. The scattering spectra of 11 individual Ag nanospheres are shown in Figure 3a. One major peak and a small side peak appeared in the scattering spectra of single Ag nanospheres. The major peak located at 453 ± 6 nm is originated from the interaction of electromagnetic waves with the free electrons of Ag nanospheres. The wavelengths of the major peak of 27 Ag nanospheres are plotted in Figure 3b. From Figure 3b, we find that the resonance wavelengths of the major peak are distributed in a narrow range, indicating that the Ag nanospheres are highly uniform. The side peak at 625 nm is due to the coupling between the Ag nanopshere and the glass substrate. It is well-known that the polarization charge of a supported nanoparticle can induce a charge distribution on a substrate, named image charge.50 The interaction between the single Ag nanosphere and its image gives rise to the resonance peak at 625 nm. Single Ag Nanosphere Dimer Spectroscopy. It has been shown that when plasmonic nanoparticles aggregate, the interaction between the particles lead to new optical phenomenon.32 To understand the plasmonic coupling, we perform a 360° polarization-dependent scattering study of single Ag nanosphere dimers. Figure 4a is the SEM image of the dimer, and Figure 4b shows the corresponding scattering spectra obtained when varying the polarization angle from 0 to 360 deg with a 15 degree increment. Two scattering peaks are observed in the scattering spectra, located at ∼460 nm and ∼644 nm. When the polarization direction of the illumination light is varied, the intensity of the 460 nm peak is nearly constant, while that of the 644 nm peak changes drastically. A detailed analysis of the relative intensity of the 644 nm peak (divided by the intensity of the 460 nm peak and then normalized to 1) at different polarization angles reveals that scattering pattern is along the long axis of the Ag nanosphere dimer, as shown by the SEM image (Figure 4a) and the scattering profile (Figure 4c). To understand the origin of the scattering peaks, we simulated the scattering spectra of an Ag nanosphere dimer (schematic illustrated in Figure 4d) using the T-matrix method at different polarization angles, shown in Figure 4e. The effect of the glass substrate is treated using effective medium theory.51
nanospheres to some degree because it is well-known that the addition of electrolyte will cause instability of colloidal particles.48 Another possible reason for the assembly of Ag nanosphere is the hydrophobic effect.49 During the etching process, a fresh silver surface of the Ag nanocubes was exposed. The hydrophobic Ag surface has the tendency to aggregate to minimize its exposed area in the polar solvent of ethanol. From the SEM image of the Ag nanospheres (Figure 1b), we can see that more than 50% of the Ag nanospheres have assembled into clusters, forming dimers, trimers, tetramers, and so on. Figure 1c,d,e shows the transmission electron microscopy (TEM) images of a single Ag nanosphere, a dimer, and a trimer, respectively. It can be clearly seen from the TEM images that the Ag nanospheres are close to perfectly spherical in shape. The diameter of the Ag nanospheres is 85 (±5.9%) nm based on the measurement of 80 Ag nanospheres. Figure 1f shows that the gap between the nanospheres is smaller than 1 nm. Correlation between the Structure and Optical Property of Individual Ag Nanospheres. Due to the diffraction limit of dark-field microscopy, it is very difficult to identify the exact structure of each scattering spot in the scattering image. This limits the understanding of the light scattering phenomenon of plasmonic particles, which is highly dependent on their structures. To resolve this problem, we resort to a correlating approach, where the true-color scattering spots of interest can be further characterized by SEM. To realize the correlation of dark-field microscopy and SEM, we precisely locate the position of each scattering spots through the assistance of a reference nickle grid and a smartphone. The reference nickle grid functions as a two-dimensional coordinate system. Together with the dark-field picture shot by the smartphone camera, we can locate the nanoparticles with a ∼1 μm resolution. Figure 2a is a dark-field image of several single particles/clusters under linearly polarized illumination shot by a smartphone camera. The correlated SEM image is shown in Figure 2b. There are seven scattering spots shown in the darkfield image of Figure 2a where the polarization of the incident light is denoted by the white arrow. Through the SEM characterization, it is observed that the green scattering dots of Nos. 1, 2, and 6 are single Ag nanospheres, which are labeled with corresponding numbers in Figure 2b. The red scattering dots of Nos. 3 and 4 are two Ag trimers. The No. 5 scattering dot with dim red color is an Ag dimer. The red scattering dot of C
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Figure 3. Scattering spectroscopy of single Ag nanospheres. (a) Scattering spectra of the 11 single particles; (b) scattering peak wavelength of 27 single particles at 453 ± 6 nm. The red line denotes the average of the peak wavelength.
Figure 4. Experimental results of the polarization response of an Ag nanosphere dimer (a−c) and calculations of the polarization-dependent scattering spectra (d−f). (a) SEM image of the dimer. (b) Polarization-dependent scattering spectra of the dimer. (c) The relationship of the relative scattering intensity with the polarization angle. The relative scattering intensity is defined as the ratio of the peak intensity at 644 nm to peak intensity at 460 nm, normalized to 1. The red line simulates the experimental results (black squares) through the model Ipeak ∝ cos2 θ. (d) Schematic image of the dimer: the diameter of the Ag nanosphere is 85 nm, the gap is 1 nm. (e) Polarization dependent scattering spectra of the dimer calculated using T-matrix method. (f) The relative intensity profile based on the spectra in panel e.
The dielectric constants of Ag are from Palik,52 and that of the environment is determined by matching the resonance wavelength of calculated single Ag nanosphere scattering spectrum with that from the experimental measurements. Details of the T-matrix calculations are available in the Supporting Information. Comparing the spectra in Figure 4b and 4e, the calculations agree well with the experimental measurements. From the theoretical study, the 435 nm peak rises due to the weakly coupled dipoles arranged shoulder by shoulder, excited with the electric field component perpendicular to the long axis of the dimer. The peak at 655 nm results from the strongly interacting dipoles arranged head to tail, excited with the electric field component parallel to the long axis. When varying the incident light polarization from parallel
to perpendicular to the long dimer axis, the relative intensity of the 655 nm peak decreases. Furthermore, the change in electric field intensity distribution around the dimer, when the incident polarization is varied, is consistent with the polarization dependence of scattering spectra, shown in Figure S1 in the Supporting Information. We have also observed polarization-dependent scattering in the scattering image in Figure 2a. Comparing scattering spots No. 5 and No.7, which are both produced by the Ag nanosphere dimers according to the SEM image in Figure 2b, we find No. 7 is much brighter than No. 5. This is because the electric-field vector denoted by the white arrow in Figure 2a is oriented along the long axis of the No. 7 Ag nanosphere dimer, while being nearly normal to the long axis of No. 5 dimer. D
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Figure 5. Experimental results of the polarization response of an Ag nanosphere trimer.(a) SEM image of the trimer. (b) Polarization dependent scattering spectra of the trimer. The peaks are indicated by dotted lines and numbered with 1, 2, and 3. (c) The relationship of the relative scattering intensity of peak 3 with the polarization angle.
Figure 6. Calculated scattering spectra of perfect Ag nanosphere trimers and slightly ellipsoidal-shaped Ag nanoparticles trimers. (a) Scheme of the Ag nanosphere trimers in a D3h geometry. In both cases, the gaps between the Ag nanospheres are kept at 1 nm. (b) The scattering spectra of a perfect Ag nanosphere trimer at polarization direction from 0° to 90° with a 15° increment. (c) The scattering spectra of an Ag nanoparticle trimer consisting of ellipsoidal particles with axes of 88:84:84 nm at polarization direction from 0 to 180° with 15° increment. (d) The relative intensity profile based on panel b for the red color curve, and on panel c for the black color curve.
again that the Ag nanospheres are uniform in size and shape. The detailed data are listed in Table S1 in the Supporting Information. Single Ag Nanosphere Trimer Spectroscopy. Recent studies by Chuntonov and Haran35,37 have shown that symmetry breaking in a series of Ag nanoparticle trimers in D3h, C2v, and D∞h symmetry can generate significant differences in the scattering spectra. However, the nanoparticles under study were visibly heterogeneous in size and shape, and only
Notice that from the SEM image, the individual nanostructures are at least 1 μm apart from each other. We do not expect there is much interaction between the neighboring nanostructures. Besides the scattering study of the single Ag nanosphere dimer described above, we have measured the scattering spectra of 20 single Ag nanosphere dimers with the electric field vector being parallel and perpendicular to the long dimer axis. All of the dimers show two scattering peaks at 458 ± 9 nm and 665 ± 16 nm. The small distribution in the scattering peaks demonstrates E
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method (spectra shown in Supporting Information, Figure S3). The calculations have confirmed that the scattering spectra of the clusters are independent of the incident polarization direction, when the order of principle axis of the nanoparticle clusters is higher than 2. We also notice that even though the scattering spectra of a perfectly arranged trimer with D3h symmetry did not depend on the incident polarization direction, the induced near fields do depend on the orientation of the incident light, as shown in Figure S4 in the Supporting Information. The calculated scattering spectra of perfect D3h trimers using the T-matrix method do not agree with the experimental measurements. We tried to vary the gap distances between the nanoparticles and found that the resonance peak at around 650 nm would split into two when a peak at 550 nm appears. However, the TEM and SEM images indicated that the gap distances between the nanoparticles in the trimer were similar and about 1 nm. Therefore, the difference in gap distance was not further considered as a cause of the polarization-dependent scattering spectra of the trimer. Since the nanospheres synthesized have some small variations in their geometry, the DDA method is used to include possible anisotropic effects of the particle shape. The DDA method is a numerical method that treats the studied particles as an array of polarizable dipoles.47 The method allows us to calculate the optical properties of particles of any shape. In the calculations, we used the environmental dielectric constant determined in the Tmatrix method. The particles in the DDA study are ellipsoidal instead of perfectly spherical to reproduce possible shape variations from the nanosphere synthesis. Specifically, one of the axes of the particle is elongated over 5% relative to the other two axes. The 5% elongation is close to the error bar observed from the TEM studies. The gap distances between the nanoparticles were kept at 1 nm as indicated by the TEM images, and it is the same as the grid length used in the DDA calculations. As shown in Figure 6c,d (black curve), when we use particles of ellipsoidal shape with diameters of 88:84:84 nm along three different axes, the calculated scattering spectra demonstrate dependence on the incident polarization direction as observed in the experiments. In addition, the scattering peak at ∼545 nm, which does not appear in the T-matrix calculations of perfect spheres in a D3h symmetry, is also observed in the spectra calculated using the DDA method. We attribute the peak at the wavelength of 545 nm to be due to the symmetry breaking of the system. Since the scattering spectra of the trimer are extremely sensitive to the particle shape and their relative positions, small deviations from the perfect equilateral triangle position and perfect spherical shape of the Ag nanosphere trimer lead to the resonance peak at the wavelength of 545 nm. These additional peaks and the polarization dependence of scattering spectra of the Ag nanosphere trimer reflect small geometric differences in the component nanoparticles that are not immediately visible by eye in the correlated SEM study. The extreme sensitivity of polarizationdependent optical scattering spectroscopy to the nanoparticle structure provides an alternative means to electron microcopies for the study of subtle differences in nanoparticle geometry. The light scattered by the dimers and trimers will propagate along different directions compared to the incident light; therefore the assembled Ag nanosphere structures may be used in plasmonic circuits to direct the propagation of light. The plasmonic coupling in the assembled structures also leads to
two orthogonal polarization components were studied. In this study, the Ag nanospheres are fairly homogeneous in size and plasmon resonance properties, based on our single nanosphere studies using both SEM and dark field scattering. During the synthetic process, some of the Ag nanospheres self-assemble into trimers with D3h symmetry. We explore whether the scattering behavior of Ag nanosphere trimers has a polarization dependence by changing the polarization direction of the incident light from 0 to 360°. Figure 5a shows the SEM image of a single Ag nanosphere trimer. Within the resolution of the SEM image, the three Ag nanospheres in the trimer assemblies appear spherical and uniform in size. Figure 5b is the corresponding scattering spectra of the trimer as the polarization angle of the incident light is varied from 0 to 360 deg. We observed three peaks (labeled as 1, 2, and 3 in Figure 5b) in the spectra, located at 486, 545, and 658 nm, respectively. Compared to the scattering spectra of the dimers, peaks 1 and 3 are at nearly the same wavelengths as the two peaks of the dimer, while a new peak (peak 2) appears at 545 nm in the scattering spectra of the trimer. We further analyze the relationship of the relative scattering intensity of peak 3 (divided by peak 1 intensity and then normalized to 1) and the polarization angle of the incident light. Figure 5c is the relative intensity profile of peak 3 as a function of the polarization angle. The relative scattering intensity is maximal at the direction of 15 and 195 degree as shown in Figure 5c. However, there is much less change in the scattering intensity of the trimer for different polarization directions, compared to that of the dimers, as shown in Figure 5b,c. For the trimer, the maximum relative intensity of peak 3 is only about 2.6 times of the minimum, unlike the dimer where the maximum relative intensity is about 17 times that of the minimum. We have collected scattering spectra of 18 individual Ag trimers assembled in D3h symmetry and observed polarizationdependent scattering behavior in all of them. Another example of a trimer scattering profile is shown in Figure S2 in the Supporting Information. Upon examination of the scattering spectra of all 18 trimers, peak 1 at 461 ± 6 nm and peak 3 at 652 ± 12 nm appear in each spectrum. Sixteen out of 18 trimers show peak 2 at 521 ± 10 nm. Some of the Ag trimers (12 out of 18) have an additional peak at 551 ± 8 nm. The appearance of peak 2 and the additional peak in the scattering spectra can be used as a hallmark to differentiate Ag dimers and trimers. Detailed data of the scattering peaks are available in Table S2 in the Supporting Information. In order to understand the scattering spectra of trimers, we calculate their spectra using both the T-matrix and the DDA methods, and the results are shown in Figure 6. Figure 6a is the schematic of the Ag nanosphere trimers modeled in the simulations. Figure 6b is the calculated scattering spectra of a D3h symmetry trimer excited with light at different linear polarization angles. As shown in Figure 6b, there are only two peaks in the spectra when three identical spherical particles are arranged in a D3h symmetry according to the T-matrix calculation. In addition, the spectra in Figure 6b completely overlap with each other, showing no dependence on the polarization of the incident light. Analysis of the relative scattering peak intensity profile further confirms the lack of dependence on polarization, as shown by the red curves in Figure 6d. The calculated results are consistent with the observations made by Chuntonov and Haran.35 In order to understand the phenomenon, scattering spectra of clusters with D4h, D5h, and D6h symmetries are calculated using the T-matrix F
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ACKNOWLEDGMENTS We thank Dr. Katherine Willets for useful discussions. J.Z. thanks the UCONN Startup Fund and Faculty Large Grant. S.Z. thanks the National Science Foundation (NSF ECCS1238738) and the Office of Naval Research (ONR N00014-01-1118) for the support of the research.
polarization-dependent enhancement for surface-enhanced spectroscopies.
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CONCLUSION
In conclusion, we have prepared high-quality Ag nanospheres by etching Ag nanocubes with Fe3+. The plasmonic properties of single Ag nanospheres, dimers, and trimers are studied through the correlation of dark-field microscopy and SEM characterization. The scattering peaks of single Ag nanospheres are dispersed in a narrow wavelength range of 453 ± 6 nm, indicating high uniformity of the Ag nanospheres; this is confirmed by TEM characterization. For the Ag nanosphere dimers, the maximal scattering intensity can be reached only when the polarization angle is aligned with the long dimer axis. For Ag nanosphere trimers, three peaks are observed in the scattering spectra. Although the wavelengths of the two major peaks are very similar to those of dimers, the profiles of the scattering intensity with polarization angle for the trimers and dimers are different. Theoretical studies using T-matrix method demonstrate that, among the nanoparticle assemblies consisting of nanoparticles with perfect spherical shape surveyed, only structures with C2v symmetry strongly couples with the polarization of the electric field of the incident light, while structures with D3h, D4h, D5h, and D6h symmetry do not. Calculations using the DDA method show that slight deviations in the nanoparticle shape away from perfect spheres results in extra peaks and polarization-dependence of the scattering spectra, in good agreement with the experimental results. In these nearly perfect nanoparticle assemblies, slight breaking of the symmetry of the system leads to significant changes in their optical behavior. With the fundamental understanding of their optical properties, the Ag nanosphere dimers and trimers are promising plasmonic structures to manipulate light propagation, and potential building blocks to construct plasmonic circuits and metamaterials.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
Table S1 lists the scattering peak positions of 20 individual Ag dimers. Table S2 lists the scattering peak positions of 18 individual Ag trimers. Figure S1 is the maps of near-field electric field intensity distribution of an Ag dimer with varying the wavelengths and the polarization direction. Figure S2 is the polarization-dependent scattering of an Ag nanosphere trimer. Figure S3 is the calculated scattering spectroscopy of Ag nanosphere clusters with D4h, D5h, and D6h symmetry, respectively. Figure S4 is the maps of near-field electric field intensity distribution of an Ag trimer with varying the wavelengths and the polarization direction. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Address Correspondence to Department of Chemistry, University of Connecticut, 55 North Eagleville Rd., Storrs, CT, 06269-3060, USA. Phone 860-486-2443; E-mail: jing.
[email protected]. Notes
The authors declare no competing financial interest. G
dx.doi.org/10.1021/jp503505x | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
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