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Communication

3-D Plasmonic NanoclustersAlexander Urban, Xiaoshuang Shen, Yumin Wang, Nicolas Large, Wang

Hong, Mark W. Knight, Peter Nordlander, Hongyu Chen, and Naomi J. HalasNano 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

1Department of Electrical and Computer Engineering, Rice University, Houston, TX 77005,

USA

2Laboratory for Nanophotonics, Rice University, Houston, TX 77005, USA

3Divison of Chemistry, Nanyang Technological University, Singapore 637371, Singapore

4Department 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,

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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, 11, superlenses12, chemical sensors13, 14 and color-sensitive

photodetectors15. The vast majority of efforts in this field have focused on planar two-

dimensional geometries, typically fabricated using self-assembly on a substrate or by

lithographic methods16. Recently several approaches for the 3-D self-assembly of nanoparticle-

based 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-block-

poly(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 high-

DMF 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 three-

dimensionality 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. 1b-

d). The structures of the observed clusters are remarkably similar to those of Lennard-Jones

clusters formed by noble gas atoms32, 33.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 three-

dimensional 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: halas@rice.edu (N.J.H.); hongyuchen@ntu.edu.sg (H.C.);

nordland@rice.edu (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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Nano Letters

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16

Page 16 of 16

ACS Paragon Plus Environment

Nano Letters

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