Directed Assembly of Optoplasmonic Hybrid Materials with Tunable

Perspective pubs.acs.org/JPCL

Directed Assembly of Optoplasmonic Hybrid Materials with Tunable Photonic−Plasmonic Properties Yan Hong,†,‡ Wonmi Ahn,†,‡ Svetlana V. Boriskina,§ Xin Zhao,† and Björn M. Reinhard*,† †

Department of Chemistry and The Photonics Center, Boston University, Boston, Massachusetts 02215, United States Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States

§

ABSTRACT: Optoplasmonic materials are metallo-dielectric hybrid structures that combine metallic and dielectric components in defined geometries in which plasmonic and photonic modes synergistically interact. These beneficial interactions can be harnessed by integrating plasmonic nanoantennas into defined photonic environments generated, for instance, by discrete optical resonators or extended systems of diffractively coupled nanoparticles. Optoplasmonic structures facilitate photonic−plasmonic mode coupling and offer degrees of freedom for creating optical fields with predefined amplitude and phase in space and time that are absent in conventional photonic or plasmonic structures. This Perspective reviews the fundamental electromagnetic mechanisms underlying selected optoplasmonic approaches with an emphasis on materials available through template-guided self-assembly strategies.

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Unfortunately, the advantage of the small mode volume is partially compensated by low Q factors in noble metal nanostructures. High dissipative losses at optical frequencies and radiative losses limit Q to micrometers) and plasmonic nanoantennas (tens of nanometers), or the need to combine building blocks of different chemical composition, which can represent significant obstacles toward a rational realization of the materials. Template-guided selfassembly strategies have demonstrated great promise for combining metallic and dielectric components into intricate morphologies21−24 and are, therefore, of significant interest for implementing diverse optoplasmonic geometries. We focus in this Perspective on two representative examples. The first one is the implementation of the optoplasmonic superlens concept,25 which combines metallic antennas with WGM resonators into discrete hybrid resonators with relevance for photon-based information transfer and processing. The second integrates metallic nanoparticles into an extended array of diffractively coupled dielectric nanoparticles that create a morphology-dependent photonic environment.26 Both designs have in common that they provide a multitude of parameters, such as the chemical composition of the building blocks and their refractive index, size, and shape as well as the overall morphology of the optoplasmonic assembly, to tune the electromagnetic coupling and phase landscape.26,27

Figure 1. (a) FDTD simulated scattering spectrum of a 2 μm diameter PS sphere with mode assignments. The inset shows the E-field intensity enhancement, |E|2/|E0|2, map in the equatorial plane of the microsphere for the TM14,1 mode. (b) E-field intensity enhancement spectra for an optoplasmonic hybrid structure comprising a 2 μm diameter PS sphere and a single 148 nm diameter Au nanoparticle at different separations between the Au nanoparticle and the microsphere. The optoplasmonic structure is excited through a plane wave incident along the microsphere−nanoparticle axis. The inset displays the E-field intensity enhancement map at the microsphere−nanoparticle interface for the TM14,1 mode. The E-field intensity is evaluated at the marked (red dot) position located 5 nm away from the Au nanoparticle surface. For the isolated optical microcavity (OM, microsphere without nanoparticle), the E-field intensity is evaluated at the same location as in the hybrid structure (11 nm separation).

(l), and azimuthal (m) mode number. The radial number, n, gives the field maxima in the radial direction, l is a measure of the number of wavelengths that fit into the optical length of the equator, and l − m + 1 gives the number of field maxima around the meridian (perpendicular to the equatorial plane). The resonance wavelength is determined by n and l, and the fundamental WGMs with n = 1 and m = l are indexed as TE(M)m,1. The inset of Figure 1a contains a map of the E-field intensity enhancement associated with the TM14,1 mode in the equatorial plane of the microsphere under excitation through an incident linearly polarized plane wave. The mode generates a moderate peak E-field intensity enhancement, |E|2/|E0|2, of D2) is created through EBL in a PMMA layer to generate (b) a regular mask of assembly sites with two different diameters. The center-to-center separation in the D1 and D2 subarrays is Λ. (c) Dragging of a suspension of TiO2 nanoparticles with diameters too large to bind to D2 across the surface results in (d) an immobilization of TiO2 nanoparticles onto D1 sites. (e) Smaller Au nanoparticles are assembled onto vacant D2 binding sites from a colloidal solution. (f) After PMMA lift-off, the final optoplasmonic array is released. Reproduced with permission from ref 22. Copyright 2014, Wiley-VCH Verlag GmbH & Co. KGaA.

Extended Optoplasmonic Array: Fabrication. In the first step of the assembly procedure, nanoparticle assembly sites are created through a lithographic fabrication technique (e.g., electron beam lithography). In the second step, these sites are charged positively through incubation with poly lysine, and in the last step, negatively charged colloidal nanoparticles are immobilized and assembled into clusters on positively charged assembly sites through a charge-mediated assembly process.22,37 Although electron beam lithography makes it possible to define the separation, Λ, between individual assembly sites on the tens to hundreds of nanometer length scale, the number of nanoparticles on the sites, their separation, and the morphology of the assembly are determined by the size and shape of the binding site and the assembly conditions.37−39 It was shown recently that the interparticle separation in the nanoparticle clusters on the assembly sites can be systematically varied on lengths scales of a few nanometers and below through choice of the nanoparticle ligands and buffer conditions.39 Through combination of top-down and chemical assembly driven bottom-up fabrication, template-guided assembly strategies achieve control over interparticle separations ranging from

combined with clusters of (small) Au nanoparticles. First, two different binding sites are patterned in the electron beam resist. Then, the larger TiO2 nanoparticles are immobilized on the large diameter binding sites, leaving the small-diameter binding sites vacant for the binding of Au nanoparticles in a subsequent binding step. For details regarding the assembly process, please refer to ref 22. Figure 7 contains exemplary SEM images of different arrays obtained from 60 nm Au nanoparticles and 250 nm diameter TiO2 nanoparticles with binding sites containing diameters of D2 = 140 nm and D1 = 270 nm, respectively.22 The resulting arrays show a successful localization of TiO2 nanoparticles and Au nanoparticle cluster to distinct lattice sites. Although for the optoplasmonic array of interest in this Perspective Λ lies in the optical regime (hundreds of nanometers), the outlined fabrication approaches are compatible with much shorter separations. This is relevant because it allows the creation of novel metamaterials and metasurfaces. Electromagnetic interactions between a limited number of 2061

DOI: 10.1021/acs.jpclett.5b00366 J. Phys. Chem. Lett. 2015, 6, 2056−2064

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Perspective

nanoparticle clusters (see individual nanoparticle cluster spectrum). To experimentally test the predicted Λ-dependent E-field enhancement in the optoplasmonic array as shown in Figure 5c, we performed area-averaged surface-enhanced Raman scattering (SERS) measurements of the small test molecule paramercaptoaniline (pMA) chemisorbed onto optoplasmonic arrays with array periods between Λ = 700−1100 nm. Figure 9 contains the background corrected spectra in the important

Figure 7. SEM images of 2D arrays generated through templateguided self-assembly of 250 nm TiO2 nanoparticles and 60 nm Au nanoparticles. (a) Section of optoplasmonic array. (b) Complete optoplasmonic array with 20 × 20 TiO2 nanoparticles. (c) Side-view (30° tilt angle) of optoplasmonic array section. (d) Section of TiO2 nanoparticle array. (e) Section of Au nanoparticle cluster array. Λ is 1000 nm in (a)−(e). Scale bars are 500 nm in (a) and (c)−(e), and 2 μm in (b). Reproduced with permission from ref 22. Copyright 2014, Wiley-VCH Verlag GmbH & Co. KGaA.

building blocks (e.g., metallic and dielectric building blocks) in defined morphologies create new material properties that reach beyond those of the individual components. Electromagnetic simulations have demonstrated the utility of so-called “digital” metamaterials,40 which achieve an intricate molding of the permittivity through the spatial distribution of only two building blocks. Extended Optoplasmonic Arrays: Characterization through Elastic and Inelastic Scattering. Figure 8 shows the forward-scattering

Figure 9. SERS spectra of para-mercaptoaniline (pMA) in the range 1000−1180 cm−1 obtained on optoplasmonic arrays with different Λ values. The inset shows the filling-fraction corrected intensities of the 1077 cm−1 CS stretch mode as a function of Λ. Reproduced with permission from ref 22. Copyright 2014, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

1000−1180 cm−1 spectral window, which contains the CS stretch mode at 1077 cm−1. The SERS signal intensity initially increases with growing Λ until it peaks at Λ = 900 nm. If Λ is, however, further increased, the SERS signal decreases again. This effect was independent of the density of nanoparticle clusters as the filling fraction corrected data in the inset shows. As the SERS signal intensity scales as the product of the E-field intensities at the pump and Raman scattering wavelengths, the observed Λ-dependent SERS signal intensities confirm an additional enhancement of the E-field intensity due to an “array” effect. The interplay between delocalized photonic and plasmonic modes is illustrated in Figure 10 for linearly polarized, normal incident light. When the in-plane reradiated light by the dielectric nanoparticles (Figure 10a) overlaps with the plasmon resonance of the nanoparticle clusters (dashed line in Figure 10a), a multiplicative E-field enhancement is obtained in the hybrid array (Figure 10b). Importantly, this cascaded Efield enhancement is higher in the hybrid array than in square array of metal nanoparticle cluster with identical lattice period (Figure 10c), emphasizing the synergistic electromagnetic interactions in the optoplasmonic array. Concluding Remarks. Optoplasmonic materials that combine metallic and dielectric elements into morphologically welldefined hybrid structures can mitigate intrinsic material limitations of plasmonic and photonic materials through synergistic electromagnetic interactions between these complementary building blocks. Furthermore, photonic−plasmonic mode coupling in optoplasmonic hybrid materials provides new degrees of freedom for tuning optical near- and far-field responses, engineering uniformly “hot” electromagnetic surfaces, enhancing radiative rate and emission properties of closeby quantum emitters, and controlling the nanoscale optical energy flow by sculpturing the optical near-field phase landscape. Optoplasmonic materials generate new function-

Figure 8. (a) Normalized elastic scattering spectra of optoplasmonic hybrid array with periodicities of Λ = 1000 nm, 900 nm, 800 nm, 700 nm (top to bottom). A single cluster spectrum was included for comparison below the array spectra. All spectra contain the diffractive grating orders (1, 0) (B2) and (1,1) (B1). (b) Fitted peak wavelengths (Exp.) of B1 and B2 are in excellent agreement with the predicted spectral range for (1,1) and (1,0) grating orders for NA = 1.2−1.4 based on grating formula mechanisms and numerical simulations. Adapted with permission from ref 22. Copyright 2014, Wiley-VCH Verlag GmbH & Co. KGaA.

spectra for the optoplasmonic arrays described in Figure 7 as a function of grating period, Λ. The spectra contain two bands (B1 and B2) that systematically red-shift with increasing Λ and whose spectral positions are in excellent agreement with the predicted diffraction bands of a square array under oblique illumination.22 The arrays also contain a spectral band between approximately 690 and 820 nm associated with the LSPR of the 2062

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Figure 10. Simulated E-field enhancement spectra at the hottest spot in (a) an array of 20 × 20 TiO2 nanoparticles (solid) and a single trimer of Au nanoparticles (dashed), (b) an optoplasmonic array comprising 20 × 20 TiO2 nanoparticles with Au nanoparticle trimers in the interstitial sites, (c) a rectangular array of 20 × 20 Au nanoparticle trimers. The arrays were illumined with a plane wave under normal incidence as shown in (b). The unit cell geometries of corresponding arrays are shown as insets. Adapted with permission from ref 22. Copyright 2014, Wiley-VCH Verlag GmbH & Co. KGaA.

alities that reach beyond those of the constituent photonic and plasmonic elements, and we believe that this gain in functionality will be extremely useful for the development of next generation integrated optical circuits for information and light processing as well as in future sensing and spectroscopy applications. The recent demonstration of an optoplasmonic amplifier, which synergistically combines photon recycling in a WGM resonator and E-field enhancement through plasmonic nanostructures, underlines the great potential of optoplasmonic devices.20 To realize the full potential of optoplasmonics, fabrication approaches are required that provide control over the relative location of metallic and dielectric building blocks in a rational and scalable fashion. Template-guided self-assembly strategies show great promise in this regard, and we anticipate that this versatile fabrication strategy will be highly instrumental in exploring this emerging class of electromagnetic materials.

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University, China. She is currently developing and characterizing metallo-dielectric hybrid structures with enhanced optical properties in the Nano-Bio Interface Laboratory. Björn M. Reinhard is an Associate Professor of Chemistry and faculty member of the Boston University Photonics Center, where he is the director of the Nano-Bio Interface Laboratory (http://www.bu.edu/ reinhardlab/). His current research interests include fundamental aspects of photonic and plasmonic nanomaterials as well as their integration into enabling tools for investigating living systems and cellular processes.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering under Award DOE DE-SC0010679 (electromagnetic material design and fabrication) and the National Science Foundation under Award CBET-1159552 (sensing applications).

AUTHOR INFORMATION

Corresponding Author

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*E-mail: [email protected].

Author Contributions ‡

(Y.H., W.A.) These authors contributed equally.

REFERENCES

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Notes

The authors declare no competing financial interest. Biographies Yan Hong received his B.S. in Chemistry from Beihang University in Beijing, P.R. China (2010). He is a Ph.D. candidate in the Nano-Bio Interface Laboratory at Boston University. His work focuses on the fabrication, characterization, and implementation of dielectric-metallic hybrid arrays. Wonmi Ahn is a postdoctoral researcher at the Department of Chemistry, Boston University. She received the Ph.D. degree in Materials Science and Engineering from the University of Utah. Her current research interests include discrete optoplasmonic materials for efficient energy and information processing, and microcavity resonators for biosensing. Svetlana V. Boriskina is a Research Scientist at MIT. She obtained her M.S. and Ph.D. degrees from Kharkiv National University. Svetlana’s research focuses on nanoscale light−matter interactions. Her awards include a Joint ICO-ICTP Award, a NATO-UK Royal Society Fellowship, a SUMMA Graduate Fellowship, and Senior Memberships in IEEE and OSA. Website: http://www.mit.edu/~sborisk/. Xin Zhao is a Ph.D. candidate in the Department of Chemistry at Boston University. She received her B.S. in Chemistry from Wuhan 2063

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