Optical Fano Resonance in Self-Assembled MagneticâPlasmonic

Subscriber access provided by University of Otago Library

Article

Optical Fano Resonance in Self-Assembled Magnetic-Plasmonic Nanostructures Kai Liu, Xiaozheng Xue, Viktor Sukhotskiy, and Edward P. Furlani J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09473 • Publication Date (Web): 09 Nov 2016 Downloaded from http://pubs.acs.org on November 15, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Optical Fano Resonance in Self-Assembled Magnetic-Plasmonic Nanostructures Kai Liu1, Xiaozheng Xue2, Viktor Sukhotskiy1 and Edward P. Furlani1,2,* 1

Dept. of Electrical Engineering, University at Buffalo SUNY, NY 14260 Dept. of Chemical and Biological Engineering, University at Buffalo SUNY, NY 14260

2

ABSTRACT: We proposed a bottom-up approach

for the fabrication of magnetic-plasmonic nanostructures that exhibit tunable plasmon enhanced hots spots and Fano resonance behavior. The nanostructures are formed from the self-assembly of magnetic-plasmonic core-shell nanoparticles. The magnetic core enables magnetophoretic control of particles during assembly, while the plasmonic shell provides interesting and useful optical behavior. We demonstrate proof-of-concept using a combination of Monte Carlo analysis to predict self-assembly and full-wave computational analysis to study the optical properties of the assembled structures. Our analysis demonstrates that viable structures can be assembled using a magnetic template-assisted self-assembly protocol and that flexible tunability of the optical response can be achieved due to the strong sensitivity of nanogap hot spots and Fano resonance features on distinct geometric parameters and the surrounding medium. We demonstrate the self-assembly and Fano resonance response of a heptamer nanostructure formed from Fe3O4@Au nanoparticles and discuss its performance for biosensing. The ability to fabricate such nanostructures using bottom-up methods holds potential for numerous novel applications. Moreover, the photonic modeling approach demonstrated here broadly applies to arbitrary particle geometries, material properties and assemblies and can be used for the rational design of such applications.

INTRODUCTION Surface plasmons in metallic nanoparticles have attracted tremendous attention in recent years for diverse applications that span metamaterials with unprecedented optical properties to biomedical theranostics, e.g. enhanced spectroscopies, nanoscale sensing, and photothermal therapy. When

plasmonic nanoparticles are closely packed with subwavelength nanogaps, they exhibit interesting hybridized plasmon modes that can produce exotic optical line shapes such as those associated with a Fano resonance. A Fano resonance is typically induced by the interaction of a broadband superradiant mode and a narrowband subradiant mode. The superradiant mode is associated with optical scattering, while the subradiant mode preferentially traps the light energy and therefore suppresses scattering. As a result, a Fano resonance is typically characterized as a narrowband dip embedded within a broadband peak in the scattering spectrum. Fano resonant features can be observed in optical scattering, reflection and transmission spectra of various metamaterial designs including nanodisk arrays,1-2 nanoparticle clusters,3 and semiconductor nanostructres.4 In addition, Fano designs with more delicate morphologies, such as XI-shaped,5 dolmen-shaped,6 dual-resonator7 and oligomer arrays,8-10 have been proposed and their Fano resonance features have been well analyzed using both experimental and theoretical pathways. An in-depth discussion of hybridized modes and the energy exchange responsible for a Fano resonance is beyond the scope of this work and can be found in the literacture.11 Fano resonances are drawing intense interest because their features are highly sensitive to changes in geometric parameters and the refractive index of the ambient medium.12-13 This provides flexible tunability of optical properties, which is attractive for many applications including various biosensing and bioimaging modalities. For example, Fano minima can be used to dramatically enhance the sensitivity of localized surface plasmon resonance (LSPR) based sensing to detect biomolecules. As an example, a heptamer consisting of seven Au nanodisks, has been shown to exhibit a strong LSPR shift of 70 nm when functionalized by a monolayer of p-MA.13 Fano resonance has

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Magnetic-plasmonic nanostructures and the computational model (CD): (a) Fe3O4@Au core-shell particle, (b) CD showing the polarization and propagation direction of the incident field.

also been explored for various other novel applications, such as Raman signal enhancement,14 high harmonic generation,15-16 optical switch,17 color rendering,18 and colorimetric sensing19 etc. The magnetic-plasmonic heptamer design presented here holds potential for applications that utilize magneto-optical kerr rotation20-22 wherein an external magnetic field can be applied to modulate the optical responses in the magnetic-plasmonic clusters. To date, most metamaterial designs that support Fano resonances have been fabricated using costly and time-consuming top-down lithography and lift-off methods.13-14, 17 Recently, a heptamer nanostructure consisting of seven core-shell SiO2@Au nanoparticles have been shown to support Fano resonances.3 While such structures can in principle be fabricated using scalable bottomup methods such as self-assembly, such methods are problematic because competing nanoscale forces can interfere with a desired precision in particle placement, e.g. the random Brownian force, van der Waals' force and hydrodynamic interactions etc. Here, we address this challenge using multifunctional nanoparticles that enable controllable self-assembly while providing a tunable optical response, including hot spots and Fano resonance. In this paper, we propose the use of colloidal magnetic-plasmonic core-shell particles to form nanostructures that support nanogap hot spots and Fano resonance behavior. We further propose a bottom-up approach to fabricate such structures.

Page 2 of 16

The magnetic core enables magnetophoretic control of particle motion and gives rise to interparticle dipole-dipole forces that facilitate particle packing to achieve a desired assembly. The plasmonic shell produces a desired and tunable optical response. We adopt a novel pathway to identify and study Fano resonances of these structures by analyzing absorption spectra, instead of scattering spectra. The latter are sensitive to the angle of incident light, which is due to far-field interference between scattering pathways that create technical problems in experimental characterization of Fano lineshapes. In contrast, the absorption spectra are dominated by the near-field storage of the mode energy, so that Fano features in the absorption spectra remain nearly constant, exhibiting a superior advantage over scattering-based analysis.4 The absorption can be accessed experimentally using near-field optical techniques23 or nonlinear generation.24 The Fano features in the absorption spectra can be reasonably correlated with those in the scattering spectra. We determined that the subradiant mode corresponds to strong near-field trapping and enhanced absorption of light energy, which ∙ Im ∙ can be related by the equation: | | ∙ , where ω is the angular frequency of illumination, V is the volume of the assembled structure, Im (ε) is the imaginary part of the dielectric constant that determines intrinsic absorption of the lossy materials in the structure.11 The superradiant mode corresponds a broadband suppression of optical absorption. Therefore, the Fano resonance will manifest itself as a pronounced absorption peak which is embedded in a broadband absorption dip. METHODS In this paper, we use 3D full-wave computational analysis to systematically study the optical behaviour of a heptamer design that consists of seven Fe3O4@Au core-shell nanoparticles as shown in Figure 1. Figure 1a illustrates the geometry of an individual core-shell particle that consists of a Fe3O4 core with a radius Rc and a gold shell with a thickness ts. Each Fe3O4@Au nanoparticle simultaneously possesses plasmon-induced electromagnetic/optical responses and mangetic responses for magnetophoretic control of particles during the


Page 3 of 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Fano features of magnetic-plasmonic heptamers: (a) The line shape of a Fano resonance in the absorption spectrum. The red arrow identifies the Fano absorption peak. (b)-(c) Spatial profiles of the electric field (color plot) and the current flow (red arrows) for (b) subradiant mode at λ=1090 nm and (c) superradiant mode at λ=1500 nm.

self-assembly. It should be noted that while other magnetic metals (e.g. Co, Ni or Fe) and magnetic dielectrics (e.g. all phases of iron oxide) can be used for the core to enable magnetic-assisted selfassembly, we limit our analysis to an iron oxide in the form of Fe3O4 as a representative material. The reason for this selection is twofold. First, our numerical study is based on iron-oxide@Au coreshell nanoparticles that have been successfully synthesized and have desirable chemical stabilities.25-26 Second, iron oxide materials have been approved by FDA for clinical practice. It should also be noted that when comparing the heptamer design in this work with similar structures based on SiO2@Au particles as reported in previous literature, one needs to consider that iron oxide core materials can strongly absorb the incident near-infrared (NIR) light energy, while silica cores are transparent to the NIR light. Furthermore, the optical absorption in the iron oxide core is even more significant due to the plasmon-induced light trapping effect in Au nanoshells. Figure 1b shows the computational domain (CD) for the field analysis, which was performed using the finite element based RF module from Comsol (www.comsol.com). This model is used to investigate Fano resonant features and their dependence on geometric parameters and the surrounding medium. In the CD, seven core-shell particles are closely packed on a glass substrate (SiO2) with a uniform end-toend spacing distance S and immersed in water (H2O). The cluster is illuminated with a downward-directed plane wave with the E field polarized along the x-axis. A current source is used to generate the field as described in the literature.27-

31

Perfectly matched layers (PMLs) are applied at the top and bottom of the domain to reduce backscattering from these boundaries. Symmetry boundary conditions (BCs) are imposed perpendicular to E and H to simplify the analysis. It is important to note that with these BCs, the model mimics the response of an infinite 2D array of heptamers with the same center-to-center x and y lattice spacing equal to the spatial period P of the CD. A detailed description of computational model can be found in Supporting Information. The time-harmonic E field within the CD satisfies the equation:

 σ  ∇× ( μr−1∇× E ) − k02  ε r − j E = 0, ωε 0  

(1)

where μr, εr and σ are the relative permeability, permittivity and conductivity of the media, respectively. The dispersive optical constants of constituent materials Fe3O4 and Au in our model are extracted from previous literature32-35 and their expressions can be also found in Supporting Information. SiO2 and H2O have the constant refractive indices of =1.4 and =1.33. RESULTS & DISCUSSION FANO FEATURES We first perform an analysis to identify Fano resonance features of the heptamer design. The initial geometric parameters of the model are Rc=35 nm, ts=12 nm, S=1 nm, P=12×R where R=Rc+ts. In our analysis, the interparticle spacing S is not less than 1 nm in order to exclude the possibility of electron



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 16

Figure 3. Dependence of Fano resonance features on the dimensions of Fe3O4@Au particles: (a) Rc changes while with ts=12 nm. (b) ts changes while with Rc=35 nm.

Figure 4. Dependence of Fano resonance features on the interparticle and heptamer spacings S and P: (a) Fano absorption peak vs. interparticle spacing S. (b) Fano resonance absorption peak vs. heptamer-to-heptamer spacing P. Constituent Fe3O4 particles have dimensions of Rc=35 nm and ts=12 nm.

tunnelling within the nanogaps.36-37 Figure 2a is the absorption cross section spectrum of the heptamer σabs, which is defined by σabs = Qabs/Ilaser, where Qabs (W) is the absorbed power by the structure and Ilaser (W/m2) is the laser irradiance. The interference between superradiant and subradiant modes gives rise to a pronounced narrowband absorption peak at λ=1090 in the background of a broadband absorption dip. Unfortunately, since the dispersive optical constants of Fe3O4 are only available within the spectral range of 500-1500 nm, a full-scale plot of the broadband absorption dip can hardly be obtained. However, this does not affect the analysis of Fano absorption features. An alternative pathway to identify Fano resonance features is by observing the behavior of charge in the local region. Figures 2b and 2c are plots of local electric field (color plot) and the current flow (red arrows) of subradiant and superradiant modes in the heptamer design, respectively. Figure 2b indicates that the current flows in three

central particles are opposite to those of side particles, resulting in the destructive interference of the radiant fields and the enhancement of light absorption. In contrast, Figure 2c shows that the charge oscillations in the particles are in unison, causing constructive interference of their radiant fields. As a result, optical absorption is reduced, while scattering is enhanced. FANO RESONANCE vs. DIMENSIONS In this section, we explore the dependence of the heptamer Fano resonance on the dimensional parameters of the Fe3O4@Au nanoparticles. For the purpose of analysis, the end-to-end interparticle spacing distance and CD period are held fixed at S=1 nm and P=12×R, respectively. First, we explore the tunability of the Fano absorption peak as the core radius Rc is systematically increased from 25 nm to 45 nm with the shell fixed at ts=12 nm. There is a corresponding shift in the Fano resonance features from 940 nm to 1250 nm as shown


Page 5 of 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


in Figure 3a. This demonstrates the flexible tunability of the Fano features in the structural design. This is important for many biomedical applications in which it is desired to tune the Fano resonance to coincide with the source laser wavelength. For example, the largest reported SERS enhancement was obtained when the excitation and Stokes shifted wavelengths overlaps with the Fano resonance features, i.e. a dip in the scattering spectrum or a peak in the absorption spectrum.1 At the Fano absorption peak, the incident optical field is efficiently redirected into local regions, contributing to strongly concentrated local fields for enhancing SERS signals. Aside from SERS, other laser-based applications, such as CARS,14 fluorescence labeling,38 photothermal therapy,39 also require a flexible tunability of resonant features. Next, we study the correlation between the Fano resonance features and the shell thickness ts with the core fixed at Rc=35 nm. As plotted in Figure 3b, there is a slight blue-shift of the Fano absorption peak, as the shell thickness increases from 9 nm to 14 nm. More importantly, a heptamer with a thicker Au shell possesses a sharper Fano absorption peak with a better contrast with its broadband background. This is because a thicker Au shell has more oscillating electrons and therefore provides a stronger plasmonic response. FANO RESONANCE COUPLING

vs.

PARTICLE

We now study the dependence of the Fano features on the interparticle spacing S and the spacing P between heptamers. In this analysis, the heptamer consists of Fe3O4@Au particles with dimensions of Rc=35 nm and ts=12 nm. Figure 4a shows the dependence of the Fano absorption peak on S. This distance can be well defined and manipulated by coating the nanoparticles with an additional polymeric layer during synthesis. We observe that the wavelength of the Fano absorption peak is very sensitive to the interparticle spacing because Fano resonance absorption is dominated by the nearfield profile. The change of the interparticle spacing strongly perturbs the local field profile, causing an obvious shift of the Fano absorption peak. In contrast, the position of the Fano absorption peak shows a negligible dependence on the distance P between neighboring heptamers, as shown

in Figure 4b, confirming the near-field nature of the Fano absorption. TEMPLATE-ASSISTED SELF-ASSEMBLY In this section, we discuss a bottom-up method for assembling magnetic-plasmonic core-shell (e.g. Fe3O4@Au) particles into structures that can support hot-spots and Fano resonances. Without loss of generality we demonstrate the method for the heptamer structure described above. We use a magnetic field-directed template-assisted self-assembly protocol that is described in detail in the literature.40-41 The template for the heptamer consists of a periodic patterned array of 4×4 nanostructured cylindrical permanent magnets embedded in a nonmagnetic substrate. Each magnet has a height Hm=200 nm and a radius Rm=140 nm. The magnets have a remanent magnetization Br = 1 T. The period of the array, i.e. the centerto-center distance between the magnets is Pm=500 nm. The template is designed to promote the formation of a heptamer particle structure over each magnet in the array. As a first step, we investigate the magnetic field of the magnet array. Specifically, we compute the z-component of the magnetic flux density Bz at different vertical distances, Sv = 50 nm, 100 nm and 150 nm, from the top surface of the template. This data is plotted in Figure 5a-5c and reveals the vertical gradient of the magnet field and its strong localization in proximity of the top surface of the magnets. A similar magnetic field profile has been proven to enable the rapid self-assembly of magnetic particles into particle superstructures with nanoscale precision in particle placement.40-41 Next, we use the Monte Carlo method with the Metropolis algorithm to determine the self-assembled equilibrium structure of monodisperse Fe3O4@Au core-shell particles with dimensions of Rc = 25 nm and ts = 12 nm. This analysis takes into account several competitive effects including induced magnetic dipole-dipole interactions, the electrostatic interparticle repulsion based on DLVO theory, Brownian dynamics, Van der Waals interaction, hydrodynamic interactions and a steric repulsive force caused by surfactant-surfactant contact as described in Supporting Information. In our analysis, the height of the computational domain is 500 nm and the carrier fluid is



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 16

Figure 5. Self-assembly of the heptamer particle structure using a template-assisted protocol based on an array of 4×4 nanostructured cylindrical permanent magnets: (a)-(c) Magnetic flux Bz of the 4×4 array at distances of Sv=50, 100, and 150 nm above the template. (d)-(e) Self-assembly of the heptamer at (d) the initial state and (e) the final equilibrium state. The insets are examples of the perfect heptamer and the heptamer with small defects in particle placement. The monodisperse coreshell particle has the dimensions of Rc=25 nm and ts=12 nm.

water. Figure 5d-5e illustrate the self-assembly process. In the initial state (Figure 5d), the nanoparticles are randomly distributed within the CD. In the final equilibrium state, the particles have self-assembled over the magnets. For 2 of the 16 magnets (12.5% of the templates), the particles assemble into defect-free heptamer structures. For an additional 3 of the 16 magnets (19% of the templates, Figure 5e) the particles assemble into heptamer structures with insignificant defects in particle placement, i.e. the structures exhibit hot spots and Fano resonance. Thus, in total, 31% of the assembled structures produce the desired optical response. This can be improved using more complex templates, a focus of our ongoing research. Our use of template-assisted bottom-up self-assembly to fabricate nanoparticle structures with a tunable Fano resonance is pioneering and superior to the conventional top-down approach. While the magnetic template itself is produced using lithograpic methods, it can be used repeatedly to create numerous material structures thereby greatly reducing fabrication time and cost. This bottom-up approach should spawn future research towards cost-

effective fabrication of functional particle clusters for a variety of applications. APPLICATIONS Self-assembled structures such as the heptamer need to be transferred from the template to a desired substrate for a given application. We propose a simple process to transfer such structures to a glass substrate. Figure 6a illustrates a protocol for the transfer process. After self-assembly, the Fe3O4@Au nanoparticles in each heptamer have a specified interparticle spacing, which is welldefined by the thickness of their polymeric coating. The first step in the transfer process is to press a heated polymer film onto the template and then let it cool. In the second step, the polymeric film with the embedded heptamers is peeled off and transferred onto a precleaned glass substrate. In the last step, all the polymeric materials are dissolved using an acetone vapor to expose Au shells and make the array ready for the attachment of functional materials to exploit the nanogap hot spots and other biosensing modalities. A similar transfer process has been demonstrated by Henderson et


Page 7 of 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Biosensing applications of the magnetic-plasmonic heptamer: (a) The protocol of the pattern transfer: I. Immerse the heptamer array in a polymeric film; II. Transfer the polymeric film with heptamers onto the glass substrate; III. Remove polymers and expose Au surfaces of heptamers through acetone vapor. (b) Fano absorption peak as a function of the refractive index of the environment. (c) The extracted peak wavelengths vs. the surrounding refractive indices.

al.42 A key application of a Fano resonance is refractometric biosensing. Specifically, the shift of the Fano resonance features will be recorded as a function of the refractive index of the surrounding medium. In previous works, a strong sensitivity of Fano resonance features to the surrounding refractive index has been demonstrated based on the analysis of optical scattering spectra. As mentioned above, this is attributed to the influence of the environmental change on the far-field interference between different scattering pathways. However, the sensitivity of the Fano absorption peak on changes in the surrounding refractive index is unknown. This analysis is essential, especially considering the different natures of Fano scattering and absorption spectra. The latter is determined by the near field profile. We use our computational model to study this phenomenon by varying the surrounding refractive index within the range of 1.3-1.4. In doing so, we find a strong dependence of the Fano resonant peak on the background medium. As the refractive index increases, the Fano resonant peak redshifts, as shown in Figure 6b. Quantitatively, we extract the wavelength

of the Fano absorption peak and plot the trend in Figure 6c. The results shows a wavelength shift of the absorption peak from 1064 nm to 1132 nm which corresponds to a sensitivity of 680 nm/RIU. We found the sensitivity in our work is comparable with the value, i.e. 674 nm/RIU, reported in the literature, which is based on scattering analysis.11 The high sensitivity of the Fano resonance absorption peak in the heptamer justifies its strong potential for biosensing applications. CONCLUSIONS We have proposed a new pathway to realize nanostructures that support plasmon enhanced hot spots and Fano resonance behavior. The structures are fabricated using a bottom-up approach that involves the magnetic template–assisted self-assembly of magnetic-plasmonic core-shell nanoparticles. We demonstrate proof-of-concept for Fe3O4@Au particles using computational models that predict self-assembly of the particles and the optical behavior of the assembled structure. We analyze for the first time the Fano resonance of a Fe3O4@Au based heptamer nanostructure in the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

absorption spectrum. The narrowband Fano absorption peak in the background of the broadband absorption dip exhibits dependence on a variety of distinct factors including particle dimensions, interparticle spacing and coupling conditions, and the refractive index of the surrounding medium. The analysis demonstrates flexible spectral tunability of the optical response of the assembled particle structure and its potential for biosensing applications. The bottom-up approach that we propose is superior to costly and time-consuming topdown methods. It can be adapted to fabricate 2D planar arrays of different functional nanostructures using different template elements. Each structure could have distinct optical response. Our proposed approach to Fano resonant nanostructures and the demonstrated methods of rational design should stimulate the use of such structures in a wide range of applications. 1. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:

2. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. KL developed Comsol computational models and analyzed optical properties. XX developed computational models for selfassembly analysis and optimized properties of magnetic templates. VS participated in the discussion. EF directed and supervised the progress and quality of this work.

Notes The authors declare no competing financial interests.

3. ACKNOWLEDGMENT We acknowledge financial support from the U.S. National Science Foundation, through Award CBET-1337860.

REFERENCES (1) Ye, J.; Wen, F.; Sobhani, H.; Lassiter, J. B.; Van Dorpe, P.; Nordlander, P.; Halas, N. J. Plasmonic Nanoclusters: Near Field Properties of the Fano Resonance Interrogated with SERS. Nano Lett. 2012, 12, 1660-1667. (2) Alegret, J.; Rindzevicius, T.; Pakizeh, T.; Alaverdyan, Y.; Gunnarsson, L.; Kall, M. Plasmonic Properties of Silver Trimers with Trigonal Symmetry Fabricated by ElectronBeam Lithography. J. Phys. Chem. C 2008, 112, 14313-14317. (3) Fan, J. A.; Wu, C.; Bao, K.; Bao, J.; Bardhan, R.; Halas, N. J.; Manoharan, V. N.; Nordlander, P.; Shvets, G.; Capasso, F.

Page 8 of 16

Self-Assembled Plasmonic Nanoparticle Clusters. Science 2010, 328, 1135-1138. (4) Fan, P.; Yu, Z.; Fan, S.; Brongersma, M. L. Optical Fano Resonance of an Individual Semiconductor Nanostructure. Nat. Mater. 2014, 13, 471-475. (5) Verellen, N.; Van Dorpe, P.; Huang, C. J.; Lodewijks, K.; Vandenbosch, G. A. E.; Lagae, L.; Moshchalkov, V. V. Plasmon Line Shaping Using Nanocrosses for High Sensitivity Localized Surface Plasmon Resonance Sensing. Nano Lett. 2011, 11, 391-397. (6) Gallinet, B.; Martin, O. J. F. Relation between near-Field and Far-Field Properties of Plasmonic Fano Resonances. Opt. Express 2011, 19, 22167-22175. (7) Lovera, A.; Gallinet, B.; Nordlander, P.; Martin, O. J. Mechanisms of Fano Resonances in Coupled Plasmonic Systems. ACS Nano 2013, 7, 4527-4536. (8) Verre, R.; Yang, Z. J.; Shegai, T.; Kall, M. Optical Magnetism and Plasmonic Fano Resonances in Metal-Insulator-Metal Oligomers. Nano Lett. 2015, 15, 1952-1958. (9) Liu, N.; Mukherjee, S.; Bao, K.; Li, Y.; Brown, L. V.; Nordlander, P.; Halas, N. J. Manipulating Magnetic Plasmon Propagation in Metallic Nanocluster Neworks. ACS Nano 2012, 6, 5482-5488. (10) Dregely, D.; Hentschel, M.; Giessen, H. Excitation and Tuning of Higher-Order Fano Resonances in Plasmonic Oligomer Clusters. ACS Nano 2011, 5, 8202-8211. (11) Fan, J. A.; Bao, K.; Wu, C.; Bao, J.; Bardhan, R.; Halas, N. J.; Manoharan, V. N.; Shvets, G.; Nordlander, P.; Capasso, F. Fano-Like Interference in Self-Assembled Plasmonic Quadrumer Clusters. Nano Lett. 2010, 10, 4680-4685. (12) Mirin, N. A.; Bao, K.; Nordlander, P. Fano Resonances in Plasmonic Nanoparticle Aggregates. J. Phys. Chem. A 2009, 113, 4028-4034. (13) Lassiter, J. B.; Sobhani, H.; Fan, J. A.; Kundu, J.; Capasso, F.; Nordlander, P.; Halas, N. J. Fano Resonances in Plasmonic Nanoclusters: Geometrical and Chemical Tunability. Nano Lett. 2010, 10, 3184-3189. (14) Zhang, Y.; Zhen, Y. R.; Neumann, O.; Day, J. K.; Nordlander, P.; Halas, N. J. Coherent Anti-Stokes Raman Scattering with Single-Molecule Sensitivity Using a Plasmonic Fano Resonance. Nat. Commun. 2014, 5, 4424. (15) Zhang, Y.; Wen, F.; Zhen, Y. R.; Nordlander, P.; Halas, N. J. Coherent Fano Resonances in a Plasmonic Nanocluster Enhance Optical Four-Wave Mixing. Proc. Natl. Acad. Sci. USA 2013, 110, 9215-9219. (16) Metzger, B.; Schumacher, T.; Hentschel, M.; Lippitz, M.; Giessen, H. Third Harmonic Mechanism in Complex Plasmonic Fano Structures. ACS Photonics 2014, 1, 471-476. (17) Chang, W. S.; Lassiter, J. B.; Swanglap, P.; Sobhani, H.; Khatua, S.; Nordlander, P.; Halas, N. J.; Link, S. A Plasmonic Fano Switch. Nano Lett. 2012, 12, 4977-4982. (18) Shen, Y. C.; Rinnerbauer, V.; Wang, I.; Stelmakh, V.; Joannopouos, J. D.; Soljacic, M. Structural Colors from Fano Resonances. ACS Photonics 2015, 2, 27-32. (19) King, N. S.; Liu, L.; Yang, X.; Cerjan, B.; Everitt, H. O.; Nordlander, P.; Halas, N. J. Fano Resonant Aluminum Nanoclusters for Plasmonic Colorimetric Sensing. ACS Nano 2015, 9, 10628-10636. (20) Moradi, M.; Mohseni, S. M.; Mahmoodi, S.; Rezvani, D.; Ansari, N.; Chung, S.; Akerman, J. Au/Nife Magnetoplasmonics: Large Enhancement of Magneto-Optical Kerr Effect for Magnetic Field Sensors and Memories. Electronic Mater. Lett. 2015, 11, 440-446. (21) Bonanni, V.; Bonetti, S.; Pakizeh, T.; Pirzadeh, Z.; Chen, J.; Nogues, J.; Vavassori, P.; Hillenbrand, R.; Akerman, J.; Dmitriev, A. Designer Magnetoplasmonics with Nickel Nanoferromagnets. Nano Lett. 2011, 11, 5333-5338.


Page 9 of 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(22) Grimsditch, M.; Vavassori, P. The Diffracted Magneto-Optic Kerr Effect: What Does It Tell You? J. Phys. Condens. Matter. 2004, 16, R275-R294. (23) Ye, Z. L.; Zhang, S.; Wang, Y.; Park, Y. S.; Zentgraf, T.; Bartal, G.; Yin, X. B.; Zhang, X. Mapping The Near-Field Dynamics in Plasmon-Induced Transparency. Phys. Rev. B 2012, 86, 155148. (24) Walsh, G. F.; Dal Negro, L. Enhanced Second Harmonic Generation by Photonic-Plasmonic Fano-Type Coupling in Nanoplasmonic Arrays. Nano Lett. 2013, 13, 3111-3117. (25) Xu, Z.; Hou, Y.; Sun, S. Magnetic Core/Shell Fe3O4/Au and Fe3O4/Au/Ag Nanoparticles with Tunable Plasmonic Properties. J. Am. Chem. Soc. 2007, 129, 8698-8699. (26) Levin, C. S.; Hofmann, C.; Ali, T. A.; Kelly, A. T.; Morosan, E.; Nordlander, P.; Whitmire, K. H.; Halas, N. J. MagneticPlasmonic Core-Shell Nanoparticles. ACS Nano 2009, 3, 1379-1388. (27) Furlani, E. P.; Baev, A. Free-Space Excitation of Resonant Cavities Formed from Cloaking Metamaterial. J. Mod. Optic. 2009, 56, 523-529. (28) Alali, F.; Kim, Y. H.; Baev, A.; Furlani, E. P. PlasmonEnhanced Metasurfaces for Controlling Optical Polarization. ACS Photonics 2014, 1, 507-515. (29) Furlani, E. P.; Baev, A. Optical Nanotrapping Using Cloaking Metamaterial. Phys. Rev. E 2009, 79, 026607. (30) Furlani, E.; Jee, H.; Oh, H.; Baev, A.; Prasad, P. Laser Writing of Multiscale Chiral Polymer Metamaterials. Advances in OptoElectronics 2012, 2012, 861569. (31) Karampelas, I. H.; Liu, K.; Alali, F.; Furlani, E. P. Plasmonic Nanoframes for Photothermal Energy Conversion. J. Phys. Chem. C 2016, 120, 7256–7264. (32) Schlegel, A.; Alvarado, S. F.; Wachter, P. Optical Properties of Magnetite (Fe3O4). J. Phys. C: Solid State Phys. 1979, 12, 1157. (33) Johnson, P. B.; Christy, R. W. Optical Constants of the Noble Metals. Phys. Rev. B 1972, 4370-4379.

(34) Etchegoin, P. G.; Le Ru, E. C.; Meyer, M. Erratum: "An Analytic Model for the Optical Properties of Gold" [ J. Chem. Phys. 125 , 164705 ( 2006 )]. J. Chem. Phys. 2007, 127, 189901-189901-1. (35) Etchegoin, P. G.; Le Ru, E. C.; Meyer, M. An Analytic Model for The Optical Properties of Gold. J. Chem. Phys. 2006, 125, 164705. (36) Perez-Gonzalez, O.; Zabala, N.; Borisov, A. G.; Halas, N. J.; Nordlander, P.; Aizpurua, J. Optical Spectroscopy of Conductive Junctions in Plasmonic Cavities. Nano Lett. 2010, 10, 3090-3095. (37) Zuloaga, J.; Prodan, E.; Nordlander, P. Quantum Description of The Plasmon Resonances of a Nanoparticle Dimer. Nano Lett. 2009, 9, 887-891. (38) Ayala-Orozco, C.; Liu, J. G.; Knight, M. W.; Wang, Y.; Day, J. K.; Nordlander, P.; Halas, N. J. Fluorescence Enhancement of Molecules inside a Gold Nanomatryoshka. Nano Lett. 2014, 14, 2926-2933. (39) Ayala-Orozco, C., et al. Au Nanomatryoshkas as Efficient near-Infrared Photothermal Transducers for Cancer Treatment: Benchmarking against Nanoshells. ACS Nano 2014, 8, 6372-6381. (40) Xue, X.; Wang, J.; Furlani, E. P. Self-Assembly of Crystalline Structures of Magnetic Core-Shell Nanoparticles for Fabrication of Nanostructured Materials. ACS Appl. Mater. Interfaces 2015, 7, 22515-22524. (41) Xue, X.; Furlani, E. P. Analysis of the Dynamics of Magnetic Core−-Shell Nanoparticles and Self-Assembly of Crystalline Superstructures in Gradient Fields. J. Phys. Chem. C 2015, 119, 5714-5726. (42) Henderson, J.; Shi, S.; Cakmaktepe, S.; Crawford, T. M. Pattern Transfer Nanomanufacturing Using Magnetic Recording for Programmed Nanoparticle Assembly. Nanotechnology 2012, 23, 185304.



TOC GRAPHIC 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Page 10 of 16

Page 11 of 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Magnetic-plasmonic nanostructures and the computational model (CD): (a) Fe3O4@Au core-shell particle, (b) CD showing the polarization and propagation direction of the incident field. 609x415mm (96 x 96 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. Fano features of magnetic-plasmonic heptamers: (a) The line shape of a Fano resonance in the absorption spectrum. The red arrow identifies the Fano absorption peak. (b)-(c) Spatial profiles of the electric field (color plot) and the current flow (red arrows) for (b) subradiant mode at λ=1090 nm and (c) superradiant mode at λ=1500 nm. 1371x480mm (96 x 96 DPI)


Page 12 of 16

Page 13 of 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Dependence of Fano resonance features on the dimensions of Fe3O4@Au particles: (a) Rc changes while with ts=12 nm. (b) ts changes while with Rc=35 nm. 1244x491mm (96 x 96 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. Dependence of Fano resonance features on the interparticle and heptamer spacings S and P: (a) Fano absorption peak vs. interparticle spacing S. (b) Fano resonance absorption peak vs. heptamer-toheptamer spacing P. Constituent Fe3O4 particles have dimensions of Rc=35 nm and ts=12 nm. 1248x492mm (96 x 96 DPI)


Page 14 of 16

Page 15 of 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Self-assembly of the heptamer particle structure using a template-assisted protocol based on an array of 4×4 nanostructured cylindrical permanent magnets: (a)-(c) Magnetic flux Bz of the 4×4 array at distances of Sv=50, 100, and 150 nm above the template. (d)-(e) Self-assembly of the heptamer at (d) the initial state and (e) the final equilibrium state. The insets are examples of the perfect heptamer and the heptamer with small defects in particle placement. The monodisperse core-shell particle has the dimensions of Rc=25 nm and ts=12 nm. 499x341mm (200 x 200 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. Biosensing applications of the magnetic-plasmonic heptamer: (a) The protocol of the pattern transfer: I. Immerse the heptamer array in a polymeric film; II. Transfer the polymeric film with heptamers onto the glass substrate; III. Remove polymers and expose Au surfaces of heptamers through acetone vapor. (b) Fano absorption peak as a function of the refractive index of the environment. (c) The extracted peak wavelengths vs. the surrounding refractive indices. 777x523mm (96 x 96 DPI)


Page 16 of 16

Optical Fano Resonance in Self-Assembled MagneticâPlasmonic

Recommend Documents

Optical Fano Resonance in Self-Assembled MagneticâPlasmonic