Three-Dimensional Plasmonic Nanoclusters - Nano Letters (ACS

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3-D Plasmonic Nanoclusters Alexander Urban, Xiaoshuang Shen, Yumin Wang, Nicolas Large, Wang Hong, Mark W. Knight, Peter Nordlander, Hongyu Chen, and Naomi J. Halas Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl402231z • Publication Date (Web): 26 Aug 2013 Downloaded from http://pubs.acs.org on August 31, 2013

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3-D Plasmonic Nanoclusters AUTHOR NAMES Alexander S. Urban1,2, ‡, Xiaoshuang Shen3,†, ‡,Yumin Wang2,4, Nicolas Large1,2, Hong Wang3, Mark W. Knight1,2, Peter Nordlander1,2,4,*, Hongyu Chen3,*, Naomi J. Halas1,2,4,* AUTHOR ADDRESS 1

Department of Electrical and Computer Engineering, Rice University, Houston, TX 77005, USA 2

3

Laboratory for Nanophotonics, Rice University, Houston, TX 77005, USA

Divison of Chemistry, Nanyang Technological University, Singapore 637371, Singapore 4

Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA

KEYWORDS Plasmonics, Dark-field Spectroscopy, Metamolecule, Metafluid, Nanoclusters ABSTRACT Assembling nanoparticles into well-defined structures is an important way to create and tailor the optical properties of materials. Most advances in metamaterials research to date have been based on structures fabricated in two-dimensional planar geometries. Here, we show an efficient method for assembling noble metal nanoparticles into stable, three-dimensional clusters, whose optical properties can be highly sensitive or remarkably independent of cluster orientation,

1 ACS Paragon Plus Environment

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depending on particle number and cluster geometry. Some of the clusters, such as tetramers and icosahedra, could serve as the optical kernels for metafluids, imparting metamaterial optical properties into disordered media such as liquids, glasses, or plastics, free from the requirement of nanostructure orientation. TEXT The optical properties of subwavelength metallic structures arise from their ability to support surface plasmons, oscillations of the delocalized electrons in metals that couple with the electromagnetic field. A universal characteristic of these structures is their sensitive correspondence between structural geometry and optical properties, which has provided new approaches to the control and manipulation of light, particularly in the visible and near infrared regions of the spectrum1, 2. This has made it possible to engineer electric and magnetic responses over this wavelength range, enabling the construction of metamaterials with a negative refractive index at these wavelengths3, 4, along with a host of artificial media that manipulate light in ways that natural materials cannot5-7. It has also led to a deeper fundamental understanding of the properties of closely coupled metallic nanoparticles, which can now be seen as plasmonic “artificial molecules”8 or “meta-atoms”9. It has also stimulated many new and novel applications, such as electromagnetic cloaks10,

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, superlenses12, chemical sensors13,

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and color-sensitive

photodetectors15. The vast majority of efforts in this field have focused on planar twodimensional geometries, typically fabricated using self-assembly on a substrate or by lithographic methods16. Recently several approaches for the 3-D self-assembly of nanoparticlebased structures have been reported17-21. However, an inherent limitation in 3-D cluster assembly is cluster stability and robustness: they may disintegrate or deform if removed from the solution or substrate where they were formed or if they come into contact with other solvents or


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solutions18, 21, 22. Additionally, aging effects, e.g. contraction of DNA, can lead to stark changes in the cluster morphology. Here we report a fabrication method for highly regular and controllable, stable 3-D plasmonic nanoclusters, each encapsulated within a small polymer sphere that stabilizes their geometry and protects them against a wide range of solvents and solutions. The structural integrity of these clusters allows us to examine how the optical properties of individual 3-D clusters relate to nanoparticle number, geometry, and orientation of the cluster, and through quantitative theoretical simulations obtain close agreement between 3-D nanoscale structure and optical properties that support our observations. Moreover, while several groups have previously investigated globular aggregation of nanoparticles23-25, however mainly without control over size and positioning of the nanoparticles. Here, we prepare globular clusters with well-defined internal structure (icosahedron-based structures) of clusters ranging from dimers up to multi-icosahedral macroclusters, the method being very similar to previously reported studies26, 27. This approach for robust, defined cluster self-assembly paves the way for the development of novel isotropic metamaterials such as metafluids28, 29. Obtaining small clusters of close packed nanoparticles is quite challenging, due to the difficulty in tuning the balance between random aggregation and structural equilibration of the resulting aggregates. The constituent nanoparticles need to be mobile enough to achieve a minimal energy state, yet stable enough to prevent dissociation and aggregation during their isolation and subsequent study. We use an amphiphilic diblock copolymer, polystyrene-blockpoly(acrylic acid) (PS-PAA), to resolve this dilemma (Fig. 1a)30, 31. Citrate-stabilized gold (Au) nanoparticles (dAu = 15 nm) were functionalized with thiol-terminated polystyrene (PS115-SH, Mn = 12000) and then incubated with PS17-b-PAA83 (Mn = 1800 for PS and Mn = 6000 for PAA) in dimethylformamide (DMF) at 60 oC for 2 h. A small amount of water was added to this mixture,


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reducing the solubility of the PS-coated NPs and inducing their aggregation. In this high DMF content solution (VDMF/VH2O = 17.5:1), the long polystyrene chains are swollen, enabling the Au nanoparticles to move past each other, maximizing their packing within the cluster. The highDMF content is critical, because aggregation will not occur if the water content in the solution is too high (Fig. S1).The mixture was incubated at 40 oC for 10 min to achieve a suitable degree of aggregation. This time can also be prolonged, which leads to larger clusters containing more Au NPs. Subsequently a large amount of water was added, removing the DMF from the PS domains. These de-swollen domains solidified, trapping the enclosed clusters. The amphiphilic PS-PAA is soluble in the initial high DMF content solution, but in a solution of high water content, the polymer is adsorbed on the surface of the PS-coated clusters, endowing the surface with negative charges and preventing further aggregation of the clusters. Thus, the preserved clusters can be directly isolated by centrifugation and easily characterized by transmission electron microscopy (TEM) or scanning electron microscopy (SEM).


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Figure 1. Fabrication and characterization of 3-D plasmonic nanoclusters. (a) Schematic depicting the self-assembly of polystyrene-stabilized metallic nanoparticles (NPs) into clusters. The aggregated clusters of NPs quickly relax to an equilibrium configuration. After addition of a large amount of water, the growth is terminated and the cluster protected by encapsulation with PS-PAA. (b) TEM micrograph of 3-D nanoclusters comprised of small gold nanospheres (dAu = 15 nm). Marked in colored boxes are examples of nanoclusters ranging in size from 5 NPs to 10 NPs as well as an icosahedron (13 NPs) and a double icosahedron (19 NPs). Scale bar corresponds to 50 nm. Magnifications of these clusters are shown in (c) and (d) and compared with their 3-D models. Scale bars correspond to 20 nm. (e) SEM images of nanoclusters comprised of large silver nanospheres (60 nm). By using two detectors to detect both backscattered and secondary electrons, it becomes easier to distinguish between the polymer


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(colored in blue) and the nanoparticles (colored in yellow) and to visualize the threedimensionality of the structures. Scale bars correspond to 100 nm.

This fabrication process resulted in a variety of highly regular 3-D clusters containing predominantly 3 to 25 nanoparticles, with an interparticle spacing of nominally 10 nm (Fig. 1bd). The structures of the observed clusters are remarkably similar to those of Lennard-Jones clusters formed by noble gas atoms32,

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.While the interactions among PS-coated Au

nanoparticles are different from the van der Waals interactions among noble gas atoms, the fundamental packing principles should be similar. Most of the Au clusters have reached their minimal energy equilibrium structure. In small clusters (3≤N≤6), the structures formed were triangular, tetrahedral, trigonal bipyramidal, and octahedral, respectively (Fig. S2, a-d). For larger clusters (7≤N≤20), the structures roughly follow an essentially icosahedral growth scheme (Fig. S3), with the structures either assembling en route to, or growing onto, an icosahedral cluster (8). While virtually all of the N=13 clusters have the same icosahedral structure (total of 128 clusters surveyed), they appeared as several different projections in the TEM images (Fig. S4). Among them, the most recognizable is the ring-like pattern, with its 5-fold axis perpendicular to the substrate surface (Fig. 1d). Such ring-like or half-ring geometries can be frequently observed in the various sized clusters (Fig. 1b). Interestingly we note that the yield of icosahedral (N=13) and double-icosahedral clusters (N=19) is significantly larger than the yield of even slightly differently sized clusters (Fig. S5). This “magic number” behavior is directly reminiscent of atomic clusters32-34. Due to the small nanoparticle size and relatively large interparticle separation, the clusters formed from nanoparticles in this size range retain the optical properties of the individual


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constituent nanoparticles. However, this method is readily adapted to the fabrication of 3D clusters consisting of 60 nm diameter Ag nanoparticles with an interparticle spacing of 5-10 nm (Fig. 1e), whose properties are highly dependent upon interparticle coupling and cluster geometry and orientation. In the SEM images of these clusters one can distinguish the individual nanoparticles (yellow) and the polymer shell (blue) and clearly view the three-dimensional morphology and orientation of the clusters.

Dark-field scattering spectra of specific individual monomers, dimers, trimers, and tetramers of the Ag clusters were acquired using a hyperspectral dark field microscope (Fig. 2a), then imaged by scanning electron microscopy (Fig. 2b,c). Among these small 3-D clusters, three primary characteristics emerged. First, the localized surface plasmon resonance of the monomer was strongly red-shifted and broadened relative to Mie theory for a polymer-encapsulated monomer of equivalent size, geometry, and orientation. This is a critical observation, since a quantitative understanding of the monomer resonance is crucial to our analysis of the optical responses of the substantially more complex multiparticle clusters. Second, the 3-D spatial orientation of the dimer and trimer clusters has a dominant and dramatic influence on the shape of their respective spectra. Third, in contrast to the dimers and trimers, the tetrahedral clusters appeared to have remarkably isotropic optical properties. To better assess these observations, we used the finite element method (FEM) to calculate the scattering properties of these clusters. Geometrical parameters were obtained directly from the SEM images, and shape, size, substrate, and symmetry were adjusted to account for experimental observations. The observed spectral redshift of the monomer was shown to be consistent with the presence of a thin silver oxide shell around the nanoparticles, which one would anticipate should form readily around each nanoparticle 35, 36.


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However, the presence of an Ag oxide shell accounted for only 20% of the experimentally observed broadening of the monomer plasmon resonance. An additional contribution to the broadening is likely the diffusion of silver ions and impurities into the surrounding PS-PAA polymer during the cluster formation process. Such a “doping” of the polymer capsule is likely to introduce loss37. Indeed, a close agreement with the experimental scattering spectra for the monomer resulted (Fig. 2b-d) when the PS-PAA polymer was modeled as a lossy medium with ñ=n+iκ, where n=1.6 and κis in the range 0.04 to 0.38 38, 39.

Figure 2. Optical properties of small (n≤4) nanoclusters. (a) To investigate their optical properties, nanoclusters are dropcast onto ITO-coated glass substrates patterned with gold microgrids. Hyperspectral imaging is performed on 100x100 µm squares, yielding a dark-field scattering spectrum for each individual nanocluster. (b) Dark-field scattering spectra were


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recorded for small (n≤4) silver nanoclusters. Corresponding SEM images are shown in (c). (d) Finite element method calculations of the scattering spectra of the individual nanoclusters show a very strong agreement with the experimental results. (e) Polarization-dependent scattering spectra of a dimer are highly anisotropic, varying between predominantly scattering from the longitudinal mode at 0° to mainly scattering from the transverse mode at 45° and back again. (f) The tetramer, in contrast, possesses highly isotropic scattering properties, with nearly no variation with the polarization. Scale bars correspond to 50 nm.

Using this observation as a starting point, we were able to obtain excellent agreement between the calculated and measured scattering spectra for the clusters shown in Fig. 2b-d. The threedimensional orientation of each structure induces profound changes in the observed optical properties. This can be seen quite dramatically in the scattering spectrum of the dimer and trimer clusters, where any change in orientation of the cluster with respect to incident light polarization reveals large spectral shifts and the appearance of additional modes for certain specific orientations. Extensive angle-dependent calculations, incorporating slight asymmetries due to nanoparticle position and shape within the cluster, were needed to obtain this level of quantitative agreement, even for the simplest cluster geometries. We examined the polarization dependence of the dimer and tetrahedral clusters in greater detail. Based on previous theoretical studies that examined two-dimensional orientation of these clusters on a flat substrate, we would expect the dimer to exhibit a pronounced anisotropic optical response, while the optical response of an ideal tetrahedron should be virtually independent of cluster orientation18, 28. Here the clusters, while supported on a substrate for the purposes of optical characterization, retain a random three dimensional out-of-plane orientation


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due to their round polymer capsule. We inserted a linear polarizer into the emission path of the microscope and acquired polarization dependent scattering spectra of these clusters. The highly orientation-dependent scattering spectrum of the dimer, comprised of linear combinations of transverse and longitudinal plasmon modes, could be clearly observed for this randomly oriented case (Fig. 2e). In contrast, the scattering spectrum of the tetrahedron is nearly isotropic (Fig. 2f). This observed isotropy provides direct evidence that a plasmonic tetrahedron does indeed behave like an isotropic “metamolecule”: such clusters can be dispersed in a liquid, and result in a metafluid28. As previously mentioned, determining the structure of larger nanoclusters is quite challenging. However, one can extract geometrical information by comparing their measured optical properties to the calculated optical properties of modeled nanoclusters. To illustrate this possible approach, we chose a nanocluster which appears to be an icosahedron in SEM images, another nanocluster with isotropic optical properties (Fig. 3a). Indeed, the polarization-dependent scattering spectra of this far more complex cluster vary only weakly with polarization angle (Fig. 3b) and are in good agreement with theoretical simulations for a perfect icosahedron (Fig. 3c).


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Figure 3. Optical properties of icosahedral clusters. (a) SEM image (left) and 3-D model (right) of an icosahedron (13 NPs). Scale bar corresponds to 100 nm. (b) Polarization-dependent scattering spectra of the icosahedron show an isotropic behavior with only a slight spectral variation for varying polarization angles of the scattered light. (c) The experimental scattering spectra show strong agreement with FEM calculations. Five localized surface plasmon (LSP) modes can be identified from the calculations (arrows), which can be found at the same position in the experimental spectrum. (d) Charge plots of the modes identified in the scattering spectrum of the icosahedron. The positions of each are marked in the spectra. The two modes dominating the scattering spectrum are a magnetic mode at 765 nm and a dark mode at 715 nm. For the three other modes, one nanoparticle has been removed to reveal the higher order nature of these modes.


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Five main localized surface plasmon modes contribute to the calculated spectrum of the icosahedron, as can be seen in the charge plots calculated for each mode (Fig. 3d). The two main peaks are associated with a magnetic plasmon mode at 765 nm and a dark plasmon mode at 715 nm. The other three modes, located between 495 and 625 nm, are higher order modes, which can be seen from the calculated charge distributions (Fig. 3d). All of these modes show up in the experimental spectrum at approximately the same positions as in the calculations. The observed variations in the relative peak intensities between the experiment and the simulations can be explained by slight structural imperfections in the fabricated structure, mostly arising from variations in shape of the constituent nanoparticles. From this direct comparison, we conclude that this nanocluster is indeed an icosahedron.

The nanoparticle clusters we report open up access to a completely new dimension of optically active materials. Clusters with highly orientation-independent optical properties, such as tetrahedra and icosahedra, could enable polarization-independent and non-directional negative index media like fluids, free-form solids and isotropic films. The universality of the fabrication method reported here can extend the use of plasmonic nanoclusters to other regions of the spectrum by incorporating either different materials, e.g. Aluminum for the UV, or spherical core/shell nanoparticles, e.g., nanoshells for the IR. These 3-D nanoparticle clusters can lead to easily applicable material coating methods, such as aerosols, for the realization of materials with transparency windows at specific frequencies and with constant ratios and linewidths. These 3-D structural components can enable electromagnetic characteristics not yet achievable in current types of metamaterials, as well as new approaches to current technological challenges, such as high-throughput chemical and biological sensing.


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ASSOCIATED CONTENT Supporting Information. Materials and methods and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Correspondence

to:

[email protected]

(N.J.H.);

[email protected]

(H.C.);

[email protected] (P.N.) Present Addresses †Department of Chemistry, Florida University, Gainesville, Florida 32611, USA Author Contributions A.S.U. and X.S. contributed equally to this work. X.S. and H.W. prepared the samples, A.S.U. performed the dark-field measurements, A.S.U and M.W.K contributed to the SEM measurements, X.S. and H.W. contributed to the TEM measurements, Y.W., N.L. and A.S.U. contributed to the FEM calculations. A.S.U., N.L and N.J.H wrote the manuscript. All the authors contributed to revising the manuscript and Supplementary Information, and participated in discussions about this work. Funding Sources NJH and PN acknowledge financial support from the Robert A. Welch Foundation (C-1220 and C-1222) and the the U.S. Army Research Laboratory and Office under contract/grant number


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WF911NF-12-1-0407. This work was supported in part by the Cyberinfrastructure for Computational Research funded by NSF under Grant CNS-0821727.

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